The Aeroplane Speaks
by
H. Barber
Part 2 out of 3
``Ah! my boy. You do a bit more flying and you'll
discover that things are not always as they appear from a
distance!''
``There she is, sir!'' cries the Flight-Sergeant. ``Just a
speck over the silvery corner of that cloud.''
A tiny speck it looks, some six miles distant and three
thousand feet high; but, racing along, it rapidly appears
larger and soon its outlines can be traced and the sunlight
be seen playing upon the whirling propeller.
Now the distant drone of the engine can be heard,
but not for long, for suddenly it ceases and, the nose of
the Aeroplane sinking, the craft commences gliding downwards.
``Surely too far away,'' says a subaltern. It will be
a wonderful machine if, from that distance and height, it
can glide into the Aerodrome.'' And more than one express
the opinion that it cannot be done; but the Designer smiles
to himself, yet with a little anxiety, for his reputation is
at stake, and Efficiency, the main reward he desires, is perhaps,
or perhaps not, at last within his grasp!
Swiftly the machine glides downwards towards them,
and it can now be seen how surprisingly little it is affected
by the rough weather and gusts; so much so that a little
chorus of approval is heard.
``Jolly good gliding angle,'' says someone; and another,
``Beautifully quick controls, what?'' and from yet another,
``By Jove! The Pilot must be sure of the machine. Look,
he's stopped the engine entirely.''
Then the Aeroplane with noiseless engine glides over
the boundary of the Aerodrome, and, with just a soft soughing
sound from the air it cleaves, lands gently not fifty yards from
the onlookers.
``Glad to see you,'' says the Squadron Commander to
the Pilot. ``How do you like the machine?'' And the
Pilot replies:
``I never want a better one, sir. It almost flies itself!''
And the Designer turns his face homewards and towards
his beloved drawing-office; well satisfied, but still dreaming
dreams of the future and . . . looking far ahead whom should
he see but Efficiency at last coming towards him! And to
him she is all things. In her hair is the morning sunshine;
her eyes hold the blue of the sky, and on her cheeks is the
pearly tint of the clouds as seen from above. The passion
of speed, the lure of space, the sense of power, and the
wonder of the future . . . all these things she holds for him.
``Ah!'' he cries. ``You'll never leave me now, when
at last there is no one between us?''
And Efficiency, smiling and blushing, but practical as
ever, says:
``And you will never throw those Compromises in my
face?''
``My dear, I love you for them! Haven't they been
my life ever since I began striving for you ten long years
ago?''
And so they walked off very happily, arm-in-arm together;
and if this hasn't bored you and you'd like some more of the
same sort of thing, I'd just love to tell you some day of the
wonderful things they accomplish together, and of what
they dream the future holds in store.
And that's the end of the Prologue.
CHAPTER I
FLIGHT
Air has weight (about 13 cubic feet = 1 lb.), inertia, and
momentum. It therefore obeys Newton's laws[[14]] and resists
movement. It is that resistance or reaction which makes
flight possible.
[[14]] See Newton's laws in the Glossary at the end of the book.
Flight is secured by driving through the air a surface[[15]]
inclined upwards and towards the direction of motion.
[[15]] See ``Aerofoil'' in the Glossary.
S = Side view of surface.
M = Direction of motion.
CHORD.--The Chord is, for practical purposes, taken to
be a straight line from the leading edge of the surface to its
trailing edge.
N = A line through the surface starting from its trailing
edge. The position of this line, which I call the Neutral
Lift Line, is found by means of wind-tunnel research, and it
varies with differences in the camber (curvature) of surfaces.
In order to secure flight, the inclination of the surface must
be such that the neutral lift line makes an angle with and
ABOVE the line of motion. If it is coincident with M, there is
no lift. If it makes an angle with M and BELOW it, then
there is a pressure tending to force the surface down.
I = Angle of Incidence. This angle is generally defined
as the angle the chord makes with the direction of motion,
but that is a bad definition, as it leads to misconception.
The angle of incidence is best described as the angle the
neutral lift line makes with the direction of motion relative
to the air. You will, however, find that in nearly all rigging
specifications the angle of incidence is taken to mean the
angle the chord makes with a line parallel to the propeller
thrust. This is necessary from the point of view of the
practical mechanic who has to rig the aeroplane, for he could
not find the neutral lift line, whereas he can easily find the
chord. Again, he would certainly be in doubt as to ``the
direction of motion relative to the air,'' whereas he can
easily find a line parallel to the propeller thrust. It is a
pity, however, that these practical considerations have
resulted in a bad definition of the angle of incidence becoming
prevalent, a consequence of which has been the widespread
fallacy that flight may be secured with a negative
inclination of the surface. Flight may conceivably be
secured with a negative angle of chord, but never with a
negative inclination of the surface. All this is only applicable
to cambered surfaces. In the case of flat surfaces the neutral
lift line coincides with the chord and the definition I have
criticised adversely is then applicable. Flat lifting surfaces
are, however, never used.
The surface acts upon the air in the following manner:
As the bottom of the surface meets the air, it compresses
it and accelerates it DOWNWARDS. As a result of this definite
action there is, of course, an equal and opposite reaction
UPWARDS.
The top surface, in moving forward, tends to leave the
air behind it, thus creating a semi-vacuum or rarefied area
over the top of the surface. Consequently the pressure of
air on the top of the surface is decreased, thus assisting the
reaction below to lift the surface UPWARDS.
The reaction increases approximately as the square of
the velocity. It is the result of (1) the mass of air engaged,
and (2) the velocity and consequent force with which the
surface engages the air. If the reaction was produced by
only one of those factors it would increase in direct proportion
to the velocity, but, since it is the product of both factors,
it increases as V<2S>.
Approximately three-fifths of the reaction is due to the
decrease of density (and consequent decrease of downward
pressure) on the top of the surface; and only some two-
fifths is due to the upward reaction secured by the action
of the bottom surface upon the air. A practical point in
respect of this is that, in the event of the fabric covering the
surface getting into bad condition, it is more likely to strip
off the top than off the bottom.
The direction of the reaction is approximately at right-
angles to the chord of the surface, as illustrated above; and
it is, in considering flight, convenient to divide it into two
component parts or values, thus:
1. The vertical component of the reaction, i.e., Lift,
which is opposed to Gravity, i.e., the weight of the
aeroplane.
2. The horizontal component, i.e., Drift (sometimes
called Resistance), to which is opposed the thrust of the
propeller.
The direction of the reaction is, of course, the resultant
of the forces Lift and Drift.
The Lift is the useful part of the reaction, for it lifts the
weight of the aeroplane.
The Drift is the villain of the piece, and must be overcome
by the Thrust in order to secure the necessary velocity to
produce the requisite Lift for flight.
DRIFT.--The drift of the whole aeroplane (we have considered
only the lifting surface heretofore) may be conveniently
divided into three parts, as follows:
Active Drift, which is the drift produced by the lifting
surfaces.
Passive Drift, which is the drift produced by all the rest
of the aeroplane--the struts, wires, fuselage, under-carriage,
etc., all of which is known as ``detrimental surface.''
Skin Friction, which is the drift produced by the friction
of the air with roughnesses of surface. The latter is practically
negligible having regard to the smooth surface of the
modern aeroplane, and its comparatively slow velocity
compared with, for instance, the velocity of a propeller
blade.
LIFT-DRIFT RATIO.--The proportion of lift to drift is
known as the lift-drift ratio, and is of paramount importance,
for it expresses the efficiency of the aeroplane (as distinct
from engine and propeller). A knowledge of the factors
governing the lift-drift ratio is, as will be seen later, an
absolute necessity to anyone responsible for the rigging of an
aeroplane, and the maintenance of it in an efficient and safe
condition.
Those factors are as follows:
1. Velocity.--The greater the velocity the greater the
proportion of drift to lift, and consequently the
less the efficiency. Considering the lifting surfaces
alone, both the lift and the (active) drift, being
component parts of the reaction, increase as the
square of the velocity, and the efficiency remains
the same at all speeds. But, considering the
whole aeroplane, we must remember the passive
drift. It also increases as the square of the
velocity (with no attendant lift), and, adding
itself to the active drift, results in increasing
the proportion of total drift (active + passive) to
lift.
But for the increase in passive drift the efficiency
of the aeroplane would not fall with increasing
velocity, and it would be possible, by doubling
the thrust, to approximately double the speed
or lift--a happy state of affairs which can never
be, but which we may, in a measure, approach
by doing everything possible to diminish the passive
drift.
Every effort is then made to decrease it by
``stream-lining,'' i.e., by giving all ``detrimental''
parts of the aeroplane a form by which they will
pass through the air with the least possible drift.
Even the wires bracing the aeroplane together are,
in many cases, stream-lined, and with a markedly
good effect upon the lift-drift ratio. In the case
of a certain well-known type of aeroplane the
replacing of the ordinary wires by stream-lined
wires added over five miles an hour to the flight
speed.
Head-resistance is a term often applied to passive
drift, but it is apt to convey a wrong impression,
as the drift is not nearly so much the result of
the head or forward part of struts, wires, etc.,
as it is of the rarefied area behind.
Above is illustrated the flow of air round two
objects moving in the direction of the arrow M.
In the case of A, you will note that the rarefied
area DD is of very considerable extent; whereas
in the case of B, the air flows round it in such a
way as to meet very closely to the rear of the
object, thus DECREASING DD.
The greater the rarefied area DD. then, the less
the density, and, consequently, the less the pressure
of air upon the rear of the object. The less such
pressure, then, the better is head-resistance D
able to get its work in, and the more thrust will
be required to overcome it.
The ``fineness'' of the stream-line shape, i.e.,
the proportion of length to width, is determined
by the velocity--the greater the velocity, the
greater the fineness. The best degree of fineness
for any given velocity is found by means of wind-
tunnel research.
The practical application of all this is, from a
rigging point of view, the importance of adjusting
all stream-line parts to be dead-on in the line of
flight, but more of that later on.
2. Angle of Incidence.--The most efficient angle of
incidence varies with the thrust at the disposal
of the designer, the weight to be carried, and the
climb-velocity ratio desired.
The best angles of incidence for these varying
factors are found by means of wind-tunnel research
and practical trial and error. Generally
speaking, the greater the velocity the smaller
should be the angle of incidence, in order to preserve
a clean, stream-line shape of rarefied area
and freedom from eddies. Should the angle be
too great for the velocity, then the rarefied area
becomes of irregular shape with attendant turbulent
eddies. Such eddies possess no lift value,
and since it has taken power to produce them,
they represent drift and adversely affect the lift-
drift ratio.
From a rigging point of view, one must presume
that every standard aeroplane has its lifting
surface set at the most efficient angle, and the
practical application of all this is in taking the
greatest possible care to rig the surface at the
correct angle and to maintain it at such angle.
Any deviation will adversely affect the lift-drift
ratio, i.e., the efficiency.
3. Camber.--(Refer to the second illustration in this
chapter.) The lifting surfaces are cambered, i.e.,
curved, in order to decrease the horizontal component
of the reaction, i.e., the drift.
The bottom camber: If the bottom of the surface
was flat, every particle of air meeting it would do
so with a shock, and such shock would produce a
very considerable horizontal reaction or drift. By
curving it such shock is diminished, and the curve
should be such as to produce a uniform (not
necessarily constant) acceleration and compression
of the air from the leading edge to the trailing
edge. Any unevenness in the acceleration and
compression of the air produces drift.
The top camber: If this was flat it would produce
a rarefied area of irregular shape. I have already
explained the bad effect this has upon the lift-
drift ratio. The top surface is then curved to
produce a rarefied area the shape of which shall
be as stream-line and free from attendant eddies
as possible.
The camber varies with the angle of incidence,
the velocity, and the thickness of the surface.
Generally speaking, the greater the velocity, the
less the camber and angle of incidence. With
infinite velocity the surface would be set at no
angle of incidence (the neutral lift line coincident
with the direction of motion relative to the air),
and would be, top and bottom, of pure streamline
form--i.e., of infinite fineness. This is, of
course, carrying theory to absurdity as the surface
would then cease to exist.
The best cambers for varying velocities, angles
of incidence, and thicknesses of surface, are found
by means of wind-tunnel research. The practical
application of all this is in taking the greatest
care to prevent the surface from becoming distorted
and thus spoiling the camber and consequently
the lift-drift ratio.
4. Aspect Ratio.--This is the proportion of span to
chord. Thus, if the span is, for instance, 50 feet
and the chord 5 feet, the surface would be said to
have an aspect ratio of 10 to 1.
For A GIVEN VELOCITY and A GIVEN AREA of surface,
the greater the aspect ratio, the greater the reaction.
It is obvious, I think, that the greater
the span, the greater the mass of air engaged,
and, as already explained, the reaction is partly
the result of the mass of air engaged.
Not only that, but, PROVIDED the chord is not
decreased to an extent making it impossible to
secure the best camber owing to the thickness of
the surface, the greater the aspect ratio, the better
the lift-drift ratio. The reason of this is rather
obscure. It is sometimes advanced that it is
owing to the ``spill'' of air from under the wing-
tips. With a high aspect ratio the chord is less
than would otherwise be the case. Less chord
results in smaller wing-tips and consequently less
``spill.'' This, however, appears to be a rather
inadequate reason for the high aspect ratio producing
the high lift-drift ratio. Other reasons
are also advanced, but they are of such a contentious
nature I do not think it well to go into them
here. They are of interest to designers, but this
is written for the practical pilot and rigger.
5. Stagger.--This is the advancement of the top surface
relative to the bottom surface, and is not, of course,
applicable to a single surface, i.e., a monoplane.
In the case of a biplane having no stagger, there
will be ``interference'' and consequent loss of
Efficiency unless the gap between the top and bottom
surfaces is equal to not less than 1 1/2 times the
chord. If less than that, the air engaged by the
bottom of the top surface will have a tendency
to be drawn into the rarefied area over the top
of the bottom surface, with the result that the
surfaces will not secure as good a reaction as would
otherwise be the case.
It is not practicable to have a gap of much
more than a distance equal to the chord, owing
to the drift produced by the great length of struts
and wires such a large gap would necessitate.
By staggering the top surface forward, however,
it is removed from the action of the lower surface
and engages undisturbed air, with the result that
the efficiency can in this way be increased by
about 5 per cent. Theoretically the top plane
should be staggered forward for a distance equal
to about 30 per cent. of the chord, the exact
distance depending upon the velocity and angle
of incidence; but this is not always possible to
arrange in designing an aeroplane, owing to difficulties
of balance, desired position, and view of
pilot, observer, etc.
6. Horizontal Equivalent.--The vertical component of
the reaction, i.e., lift, varies as the horizontal
equivalent (H.E.) of the surface, but the drift
remains the same. Then it follows that if H.E. grows
less, the ratio of lift to drift must do the same.
A, B, and C are front views of three surfaces.
A has its full H.E., and therefore, from the point
of view from which we are at the moment considering
efficiency, it has its best lift-drift ratio.
B and C both possess the same surface as A,
but one is inclined upwards from its centre and
the other is straight but tilted. For these reasons
their H.E.'s are, as illustrated, less than in the
case of A. That means less vertical lift, and,
the drift remaining the same (for there is the
same amount of surface as in A to produce it),
the lift-drift ratio falls.
THE MARGIN OF POWER is the power available above
that necessary to maintain horizontal flight.
THE MARGIN OF LIFT is the height an aeroplane can gain
in a given time and starting from a given altitude.
As an example, thus: 1,000 feet the first minute,
and starting from an altitude of 500 feet above
sea-level.
The margin of lift decreases with altitude, owing
to the decrease in the density of the air, which
adversely affects the engine. Provided the engine
maintained its impulse with altitude, then, if we
ignore the problem of the propeller, which I will
go into later on, the margin of lift would not
disappear. Moreover, greater velocity for a given
power would be secured at a greater altitude, owing
to the decreased density of air to be overcome.
After reading that, you may like to light your pipe
and indulge in dreams of the wonderful possibilities
which may become realities if some brilliant genius
shows us some day how to secure a constant power
with increasing altitude. I am afraid, however,
that will always remain impossible; but it is probable
that some very interesting steps may be taken in
that direction.
THE MINIMUM ANGLE OF INCIDENCE is the smallest
angle at which, for a given power, surface (including
detrimental surface), and weight, horizontal flight
can be maintained.
THE MAXIMUM ANGLE OF INCIDENCE is the greatest
angle at which, for a given power, surface (including
detrimental surface), and weight, horizontal flight
can be maintained.
THE OPTIMUM ANGLE OF INCIDENCE is the angle at
which the lift-drift ratio is highest. In modern
aeroplanes it is that angle of incidence possessed by the
surface when the axis of the propeller is horizontal.
THE BEST CLIMBING ANGLE is approximately half-way
between the maximum and the optimum angles.
All present-day aeroplanes are a compromise between
Climb and horizontal Velocity. We will compare
the essentials for two aeroplanes, one designed for
maximum climb, and the other for maximum velocity.
ESSENTIALS FOR MAXIMUM CLIMB:
1. Low velocity, in order to secure the best lift-drift
ratio.
2. Having a low velocity, a large surface will be
necessary in order to engage the necessary mass
of air to secure the requisite lift.
3. Since (1) such a climbing machine will move
along an upward sloping path, and (2) will climb
with its propeller thrust horizontal, then a large
angle relative to the direction of the thrust will be
necessary in order to secure the requisite angle
relative to the direction of motion.
The propeller thrust should be always horizontal, because
the most efficient flying-machine (having regard to climb OR
velocity) has, so far, been found to be an arrangement of an
inclined surface driven by a HORIZONTAL thrust--the surface
lifting the weight, and the thrust overcoming the drift.
This is, in practice, a far more efficient arrangement than
the helicopter, i.e., the air-screw revolving about a vertical
axis and producing a thrust opposed to gravity. If, when
climbing, the propeller thrust is at such an angle as to tend
to haul the aeroplane upwards, then it is, in a measure,
acting as a helicopter, and that means inefficiency. The
reason of a helicopter being inefficient in practice is due to
the fact that, owing to mechanical difficulties, it is impossible
to construct within a reasonable weight an air-screw of the
requisite dimensions. That being so, it would be necessary,
in order to absorb the power of the engine, to revolve the
comparatively small-surfaced air screw at an immensely
greater velocity than that of the aeroplane's surface. As
already explained, the lift-drift ratio falls with velocity on
account of the increase in passive drift. This applies to a
blade of a propeller or air-screw, which is nothing but a
revolving surface set at angle of incidence, and which it is
impossible to construct without a good deal of detrimental
surface near the central boss.
4. The velocity being low, then it follows that for
that reason also the angle of incidence should be
comparatively large.
5. Camber.--Since such an aeroplane would be of
low velocity, and therefore possess a large angle
of incidence, a large camber would be necessary.
Let us now consider the essentials for an aeroplane of
maximum velocity for its power, and possessing merely
enough lift to get off the ground, but no margin of lift.
1. Comparatively HIGH VELOCITY.
2. A comparatively SMALL SURFACE, because, being
of greater velocity than the maximum climber,
a greater mass of air will be engaged for a given
surface and time, and therefore a smaller surface
will be sufficient to secure the requisit lift.
3. A small angle relative to the propeller thrust, since
the latter coincides with the direction of motion.
4. A comparatively small angle of incidence by reason
of the high velocity.
5. A comparatively small camber follows as a result
of the small angle of incidence.
SUMMARY.
Essentials for Maximum Essentials for Maximum
Climb. Velocity
1. Low velocity. High velocity.
2. Large surface. Small surface.
3. Large angle relative to Small angle relative to
propeller thrust. propeller thrust.
4. Large angle relative to Small angle relative to direction
direction of motion. of motion.
5. Large camber. Small camber.
It is mechanically impossible to construct an aeroplane
of reasonable weight of which it would be possible to very
the above opposing essentials. Therefore, all aeroplanes
are designed as a compromise between Climb and Velocity.
As a rule aeroplanes are designed to have at low altitude
a slight margin of lift when the propeller thrust is horizontal.
ANGLES OF INCIDENCE (INDICATED APPROXIMATELY) OF AN AEROPLANE
DESIGNED AS A COMPROMISE BETWEEN
VELOCITY AND CLIMB, AND POSSESSING A SLIGHT MARGIN OF LIFT AT A LOW
ALTITUDE AND WHEN THE THRUST IS HORIZONTAL
MINIMUM ANGLE.
This gives the greatest velocity
during horizontal flight at a low
altitude. Greater velocity would
be secured if the surface, angle,
and camber were smaller and designed
to just maintain horizontal
flight with a horizontal thrust.
Also, in such case, the propeller
would not be thrusting downwards,
but along a horizontal line
which is obviously a more efficient
arrangement if we regard
the aeroplane merely from one
point of view, i.e., either with
reference to velocity OR climb.
OPTIMUM ANGLE
(Thrust horizontal)
The velocity is less than at the
smaller minimum angle, and, as
aeroplanes are designed to-day, the
area and angle of incidence of the
surface is such as to secure a
slight ascent at a low altitude. The
camber of the surface is designed
for this angle of incidence and
velocity. The lift-drift ratio is
best at this angle.
BEST CLIMBING ANGLE
The velocity is now still less by
reason of the increased angle
producing increase of drift. Less
velocity at A GIVEN ANGLE produces
less lift, but the increased angle
more or less offsets the loss of lift
due to the decreased velocity, and
in addition, the thrust is now hauling
the aeroplane upwards.
MAXIMUM ANGLE
The greater angle has now produced
so much drift as to lessen
the velocity to a point where the
combined lifts from the surface
and from the thrust are only just
able to maintain horizontal flight.
Any greater angle will result in a
still lower lift-drift ratio. The lift
will then become less than the
weight and the aeroplane will
consequently fall. Such a fall is
known as ``stalling'' or ``pancaking.''
NOTE.--The golden rule for beginners: Never exceed the Best Climbing Angle.
Always maintain the flying speed of the aeroplane.
By this means, when the altitude is reached where the margin
of lift disappears (on account of loss of engine power), and
which is, consequently, the altitude where it is just possible
to maintain horizontal flight, the aeroplane is flying with
its thrust horizontal and with maximum efficiency (as distinct
from engine and propeller efficiency).
The margin of lift at low altitude, and when the thrust
is horizontal, should then be such that the higher altitude at
which the margin of lift is lost is that altitude at which most
of the aeroplane's horizontal flight work is done. That
ensures maximum velocity when most required.
Unfortunately, where aeroplanes designed for fighting
are concerned, the altitude where most of the work is done
is that at which both maximum velocity and maximum
margin of lift for power are required.
Perhaps some day a brilliant inventor will design an
aeroplane of reasonable weight and drift of which it will be
possible for the pilot to vary at will the above-mentioned
opposing essentials. Then we shall get maximum velocity,
or maximum margin of lift, for power as required. Until
then the design of the aeroplane must remain a compromise
between Velocity and Climb.
CHAPTER II
STABILITY AND CONTROL
STABILITY is a condition whereby an object disturbed
has a natural tendency to return to its first and normal
position. Example: a weight suspended by a cord.
INSTABILITY is a condition whereby an object disturbed
has a natural tendency to move as far as possible away from
its first position, with no tendency to return. Example:
a stick balanced vertically upon your finger.
NEUTRAL INSTABILITY is a condition whereby an object
disturbed has no tendency to move farther than displaced
by the force of the disturbance, and no tendency to return
to its first position.
In order that an aeroplane may be reasonably controllable,
it is necessary for it to possess some degree of stability
longitudinally, laterally, and directionally.
LONGITUDINAL STABILITY in an aeroplane is its stability
about an axis transverse to the direction of normal horizontal
flight, and without which it would pitch and toss.
LATERAL STABILITY is its stability about its longitudinal
axis, and without which it would roll sideways.
DIRECTIONAL STABILITY is its stability about its vertical
axis, and without which it would have no tendency to keep
its course.
For such directional stability to exist there must be,
in effect,[[16]] more ``keel-surface'' behind the vertical axis
than there is in front of it. By keel-surface I mean every-
thing to be seen when looking at an aeroplane from the side
of it--the sides of the body, undercarriage, struts, wires, etc.
The same thing applies to a weathercock. You know what
would happen if there was insufficient keel-surface behind
the vertical axis upon which it is pivoted. It would turn
off its proper course, which is opposite to the direction of
the wind. It is very much the same in the case of an aeroplane.
[[16]] ``In effect'' because, although there may be actually the greatest proportion
of keel-surface In front of the vertical axis, such surface may be much nearer to
the axis than is the keel-surface towards the tail. The latter may then be actually
less than the surface in front, but, being farther from the axis, it has a greater
leverage, and consequently is greater in effect than the surface in front.
The above illustration represents an aeroplane (directionally
stable) flying along the course B. A gust striking it
as indicated acts upon the greater proportion of keel-surface
behind the turning axis and throws it into the new course.
It does not, however, travel along the new course, owing to
its momentum in the direction B. It travels, as long as
such momentum lasts, in a direction which is the resultant
of the two forces Thrust and Momentum. But the centre
line of the aeroplane is pointing in the direction of the new
course. Therefore its attitude, relative to the direction of
motion, is more or less sideways, and it consequently receives
an air pressure in the direction C. Such pressure, acting
upon the keel-surface, presses the tail back towards its first
position in which the aeroplane is upon its course B.
What I have described is continually going on during
flight, but in a well-designed aeroplane such stabilizing
movements are, most of the time, so slight as to be imperceptible
to the pilot.
If an aeroplane was not stabilized in this way, it would
not only be continually trying to leave its course, but it would
also possess a dangerous tendency to ``nose away'' from the
direction of the side gusts. In such case the gust shown in
the above illustration would turn the aeroplane round the
opposite way a very considerable distance; and the right
wing, being on the outside of the turn, would travel with
greater velocity than the left wing. Increased velocity
means increased lift; and so, the right wing lifting, the
aeroplane would turn over sideways very quickly.
LONGITUDINAL STABILITY.--Flat surfaces are longitudinally
stable owing to the fact that with decreasing angles of
incidence the centre line of pressure (C.P.) moves forward.
The C.P. is a line taken across the surface, transverse
to the direction of motion, and about which all the air forces
may be said to balance, or through which they may be said
to act.
Imagine A to be a flat surface, attitude vertical, travelling
through the air in the direction of motion M. Its C.P. is
then obviously along the exact centre line of the surface
as illustrated.
In B, C, and D the surfaces are shown with angles of
incidence decreasing to nothing, and you will note that the
C.P. moves forward with the decreasing angle.
Now, should some gust or eddy tend to make the surface
decrease the angle, i.e., dive, then the C.P. moves forward
and pushes the front of the surface up. Should the surface
tend to assume too large an angle, then the reverse
happens--the C.P. moves back and pushes the rear of the
surface up.
Flat surfaces are, then, theoretically stable longitudinally.
They are not, however, used, on account of their poor
lift-drift ratio.
As already explained, cambered surfaces are used, and
these are longitudinally unstable at those angles of incidence
producing a reasonable lift-drift ratio, i.e., at angles below:
about 12 degrees.
A is a cambered surface, attitude approximately vertical,
moving through the air in the direction M. Obviously the C. P.
coincides with the transverse centre line of the surface.
With decreasing angles, down to angles of about 30 degrees,
the C.P. moves forward as in the case of flat surfaces (see B),
but angles above 30 degrees do not interest us, since they produce
a very low ratio of lift to drift.
Below angles of about 30 degrees (see C) the dipping front part
of the surface assumes a negative angle of incidence resulting
in the DOWNWARD air pressure D, and the more the angle of
incidence is decreased, the greater such negative angle and its
resultant pressure D. Since the C.P. is the resultant of all
the air forces, its position is naturally affected by D, which
causes it to move backwards. Now, should some gust or
eddy tend to make the surface decrease its angle of incidence,
i.e., dive, then the C.P. moves backwards, and, pushing up
the rear of the surface, causes it to dive the more. Should
the surface tend to assume too large an angle, then the reverse
happens; the pressure D decreases, with the result
that C.P. moves forward and pushes up the front of the surface,
thus increasing the angle still further, the final result
being a ``tail-slide.''
It is therefore necessary to find a means of stabilizing
the naturally unstable cambered surface. This is usually
secured by means of a stabilizing surface fixed some distance
in the rear of the main surface, and it is a necessary condition
that the neutral lift lines of the two surfaces, when projected
to meet each other, make a dihedral angle. In other words,
the rear stabilizing surface must have a lesser angle of
incidence than the main surface--certainly not more than
one-third of that of the main surface. This is known as the
longitudinal dihedral.
I may add that the tail-plane is sometimes mounted upon
the aeroplane at the same angle as the main surface, but,
in such cases, it attacks air which has received a downward
deflection from the main surface, thus:
{illust.}
The angle at which the tail surface attacks the air (the.
angle of incidence) is therefore less than the angle of incidence
of the main surface.
I will now, by means of the following illustration, try
to explain how the longitudinal dihedral secures stability:
First, imagine the aeroplane travelling in the direction
of motion, which coincides with the direction of thrust T.
The weight is, of course, balanced about a C.P., the resultant
of the C.P. of the main surface and the C.P. of the stabilizing
surface. For the sake of illustration, the stabilizing surface
has been given an angle of incidence, and therefore has a
lift and C.P. In practice the stabilizer is often set at no
angle of incidence. In such case the proposition remains
the same, but it is, perhaps, a little easier to illustrate it
as above.
Now, we will suppose that a gust or eddy throws the
machine into the lower position. It no longer travels in
the direction of T, since the momentum in the old direction
pulls it off that course. M is now the resultant of the Thrust
and the Momentum, and you will note that this results in a
decrease in the angle our old friend the neutral lift line makes
with M, i.e., a decrease in the angle of incidence and therefore
a decrease in lift.
We will suppose that this decrease is 2 degrees. Such decrease
applies to both main surface and stabilizer, since both are
fixed rigidly to the aeroplane.
The main surface, which had 12 degrees angle, has now only
10 degrees, i.e., a loss of ONE-SIXTH.
The stabilizer, which had 4 degrees angle, has now only 2 degrees,
i.e., a loss of ONE-HALF.
The latter has therefore lost a greater PROPORTION of its
angle of incidence, and consequently its lift, than has the
main surface. It must then fall relative to the main surface.
The tail falling, the aeroplane then assumes its first position,
though at a slightly less altitude.
Should a gust throw the nose of the aeroplane up, then
the reverse happens. Both main surface and stabilizer
increase their angles of incidence in the same amount, but
the angle, and therefore the lift, of the stabilizer increases
in greater proportion than does the lift of the main surface,
with the result that it lifts the tail. The aeroplane then
assumes its first position, though at a slightly greater
altitude.
Do not fall into the widespread error that the angle of
incidence varies as the angle of the aeroplane to the horizontal.
It varies with such angle, but not as anything approaching it.
Remember that the stabilizing effect of the longitudinal
dihedral lasts only as long as there is momentum in the direction
of the first course.
These stabilizing movements are taking place all the
time, even though imperceptible to the pilot.
Aeroplanes have, in the past, been built with a stabilizing
surface in front of the main surface instead of at the rear of
it. In such design the main surface (which is then the tail
surface as well as the principal lifting surface) must be set
at a less angle than the forward stabilizing surface, in order
to secure a longitudinal dihedral. The defect of such design
lies in the fact that the main surface must have a certain angle
to lift the weight--say 5 degrees. Then, in order to secure a
sufficiency of longitudinal stability, it is necessary to set the
forward stabilizer at about 15 degrees. Such a large angle of incidence
results in a very poor lift-drift ratio (and consequently great
loss of efficiency), except at very low velocities compared with
the speed of modern aeroplanes. At the time such aeroplanes
were built velocities were comparatively low, and this defect
was; for that reason, not sufficiently appreciated. In the end
it killed the ``canard'' or ``tail-first'' design.
Aeroplanes of the Dunne and similar types possess no
stabilizing surface distinct from the main surface, but they
have a longitudinal dihedral which renders them stable.
The main surface towards the wing-tips is given a
decreasing angle of incidence and corresponding camber. The
wing-tips then act as longitudinal stabilizers.
This design of aeroplane, while very interesting, has
not proved very practicable, owing to the following
disadvantages: (1) The plan design is not, from a mechanical
point of view, so sound as that of the ordinary aeroplane
surface, which is, in plan, a parallelogram. It is, then,
necessary to make the strength of construction greater than
would otherwise be the case. That means extra weight.
(2) The plan of the surface area is such that the aspect ratio
is not so high as if the surface was arranged with its leading
edges at right angles to the direction of motion. The lower
the aspect ratio, then, the less the lift. This design, then,
produces less lift for weight of surface than would the same
surface if arranged as a parallelogram. (3) In order to secure
the longitudinal dihedral, the angle of incidence has to be
very much decreased towards the wing-tips. Then, in order
that the lift-drift ratio may be preserved, there must be a
corresponding decrease in the camber. That calls for surface
ribs of varying cambers, and results in an expensive and
lengthy job for the builder. (4) In order to secure directional
stability, the surface is, in the centre, arranged to dip down
in the form of a V, pointing towards the direction of motion.
Should the aeroplane turn off its course, then its momentum
in the direction of its first course causes it to move in a
direction the resultant of the thrust and the momentum. It
then moves in a more or less sideways attitude, which results
in an air pressure upon one side of the V, and which tends to
turn the aeroplane back to its first course. This arrangement
of the surface results in a bad drift. Vertical surfaces at
the wing-tips may also be set at an angle producing the same
stabilizing effect, but they also increase the drift.
The gyroscopic action of a rotary engine will affect the
longitudinal stability when an aeroplane is turned to right
or left. In the case of a Gnome engine, such gyroscopic
action will tend to depress the nose of the aeroplane when it
is turned to the left, and to elevate it when it is turned to
the right. In modern aeroplanes this tendency is not sufficiently
important to bother about. In the old days of crudely
designed and under-powered aeroplanes this gyroscopic action
was very marked, and led the majority of pilots to dislike
turning an aeroplane to the right, since, in doing so, there
was some danger of ``stalling.''
LATERAL STABILITY is far more difficult for the designer
to secure than is longitudinal or directional stability. Some
degree of lateral stability may be secured by means of the
``lateral dihedral,'' i.e., the upward inclination of the surface
towards its wing-tips thus:
Imagine the top V, illustrated opposite, to be the front
view of a surface flying towards you. The horizontal equivalent
(H.E.) of the left wing is the same as that of the right
wing. Therefore, the lift of one wing is equal to the lift
of the other, and the weight, being situated always in the
centre, is balanced.
If some movement of the air causes the surface to tilt
sideways, as in the lower illustration, then you will note that
the H.E. of the left wing increases, and the H.E. of the right
wing decreases. The left wing then, having the greatest
lift, rises; and the surface assumes its first and normal
position.
Unfortunately however, the righting effect is not proportional
to the difference between the right and left H.E.'s.
In the case of A, the resultant direction of the reaction
of both wings is opposed to the direction of gravity or weight.
The two forces R R and gravity are then evenly balanced,
and the surface is in a state of equilibrium.
In the case of B, you will note that the R R is not directly
opposed to gravity. This results in the appearance of M,
and so the resultant direction of motion of the aeroplane
is no longer directly forward, but is along a line the resultant
of the thrust and M. In other words, it is, while flying
forward, at the same time moving sideways in the direction M.
In moving sideways, the keel-surface receives, of course,
a pressure from the air equal and opposite to M. Since
such surface is greatest in effect towards the tail, then the
latter must be pushed sideways. That causes the aeroplane
to turn; and, the highest wing being on the outside of the
turn, it has a greater velocity than the lower wing. That
produces greater lift, and tends to tilt the aeroplane over
still more. Such tilting tendency is, however, opposed by
the difference in the H.E.'s of the two wings.
It then follows that, for the lateral dihedral angle to
be effective, such angle must be large enough to produce,
when the aeroplane tilts, a difference in the H.E.'s of the
two wings, which difference must be sufficient to not only
oppose the tilting tendency due to the aeroplane turning,
but sufficient to also force the aeroplane back to its original
position of equilibrium.
It is now, I hope, clear to the reader that the lateral
dihedral is not quite so effective as would appear at first
sight. Some designers, indeed, prefer not to use it, since its
effect is not very great, and since it must be paid for in loss
of H.E. and consequently loss of lift, thus decreasing the lift-
drift ratio, i.e., the efficiency. Also, it is sometimes advanced
that the lateral dihedral increases the ``spill'' of air from the
wing-tips and that this adversely affects the lift-drift ratio.
The disposition of the keel-surface affects the lateral
stability. It should be, in effect, equally divided by the
longitudinal turning axis of the aeroplane. If there is an
excess of keel-surface above or below such axis, then a side
gust striking it will tend to turn the aeroplane over sideways.
The position of the centre of gravity affects lateral stability.
If too low, it produces a pendulum effect and causes the
aeroplane to roll sideways.
If too high, it acts as a stick balanced vertically would
act. If disturbed, it tends to travel to a position as far as
possible from its original position. It would then tend,
when moved, to turn the aeroplane over sideways and into
an upside-down position.
From the point of view of lateral stability, the best
position for the centre of gravity is one a little below the
centre of drift.
Propeller torque affects lateral stability. An aeroplane
tends to turn over sideways in the opposite direction to which
the propeller revolves.
This tendency is offset by increasing the angle of incidence
(and consequently the lift) of the side tending to fall; and it
is always advisable, if practical considerations allow it, to
also decrease the angle upon the other side. In that way
it is not necessary to depart so far from the normal angle
of incidence at which the lift-drift ratio is highest.
Wash-in is the term applied to the increased angle.
Wash-out is the term applied to the decreased angle.
Both lateral and directional stability may be improved
by washing out the angle of incidence on both sides of the
surface, thus:
The decreased angle decreases the drift and therefore the
effect of gusts upon the wing-tips which is just where they
have the most effect upon the aeroplane, owing to the distance
from the turning axis.
The wash-out also renders the ailerons (lateral controlling
services) more effective, as, in order to operate them, it is
not then necessary to give them such a large angle of incidence
as would otherwise be required.
The less the angle of incidence of the ailerons, the better
their lift-drift ratio, i.e., their efficiency. You will note
that, while the aileron attached to the surface with washed-out
angle is operated to the same extent as the aileron illustrated
above it, its angle of incidence is considerably less. Its efficiency
is therefore greater.
The advantages of the wash-in must, of course be paid for
in some loss of lift, as the lift decreases with the decreased angle.
In order to secure all the above described advantages,
a combination is sometimes effected, thus:
BANKING.--An aeroplane turned off its course to right
or left does not at once proceed along its new course. Its
momentum in the direction of its first course causes it to
travel along a line the resultant of such momentum and the
thrust. In other words, it more or less skids sideways and
away from the centre of the turn. Its lifting surfaces do
not then meet the air in their correct attitude, and the lift
may fall to such an extent as to become less than the weight,
in which case the aeroplane must fall. This bad effect is
minimized by ``banking,'' i.e., tilting the aeroplane sideways.
The bottom of the lifting surface is in that way opposed to
the air through which it is moving in the direction of the
momentum and receives an opposite air pressure. The
rarefied area over the top of the surface is rendered still more
rare, and this, of course, assists the air pressure in opposing
the momentum.
The velocity of the ``skid,'' or sideways movement, is
then only such as is necessary to secure an air pressure equal
and opposite to the centrifugal force of the turn.
The sharper the turn, the greater the effect of the centrifugal
force, and therefore the steeper should be the ``bank.''
Experentia docet.
The position of the centre of gravity affects banking. A low
C.G. will tend to swing outward from the centre of the turn,
and will cause the aeroplane to bank--perhaps too much, in
which case the pilot must remedy matters by operating the
ailerons.
A high C.G. also tends to swing outward from the centre
of the turn. It will tend to make the aeroplane bank the
wrong way, and such effect must be remedied by means of
the ailerons.
The pleasantest machine from a banking point of view is
one in which the C.G. is a little below the centre of drift.
It tends to bank the aeroplane the right way for the turn,
and the pilot can, if necessary, perfect the bank by means
of the ailerons.
The disposition of the keel-surface affects banking. It
should be, in effect, evenly divided by the longitudinal axis.
An excess of keel-surface above the longitudinal axis will,
when banking, receive an air pressure causing the aeroplane
to bank, perhaps too much. An excess of keel-surface below
the axis has the reverse effect.
SIDE-SLIPPING.--This usually occurs as a result of over-
banking. It is always the result of the aeroplane tilting
sideways and thus decreasing the horizontal equivalent, and
therefore the lift, of the surface. An excessive ``bank,''
or sideways tilt, results in the H.E., and therefore the lift,
becoming less than the weight, when, of course, the aeroplane
must fall, i.e., side-slip.
When making a very sharp turn it is necessary to bank
very steeply indeed. If, at the same time, the longitudinal
axis of the aeroplane remains approximately horizontal,
then there must be a fall, and the direction of motion will be
the resultant of the thrust and the fall as illustrated above
in sketch A. The lifting surfaces and the controlling surfaces
are not then meeting the air in the correct attitude,
with the result that, in addition to falling, the aeroplane
will probably become quite unmanageable.
The Pilot, however, prevents such a state of affairs from
happening by ``nosing-down,'' i.e., by operating the rudder
to turn the nose of the aeroplane downward and towards
the direction of motion as illustrated in sketch B. This
results in the higher wing, which is on the outside of the turn,
travelling with greater velocity, and therefore securing a
greater reaction than the lower wing, thus tending to tilt
the aeroplane over still more. The aeroplane is now almost
upside-down, but its attitude relative to the direction of
motion is correct and the controlling surfaces are all of them
working efficiently. The recovery of a normal attitude
relative to the Earth is then made as illustrated in sketch C.
The Pilot must then learn to know just the angle of bank
at which the margin of lift is lost, and, if a sharp turn
necessitates banking beyond that angle, he must ``nose-down.''
In this matter of banking and nosing-down, and, indeed,
regarding stability and control generally, the golden rule
for all but very experienced pilots should be: Keep the
aeroplane in such an attitude that the air pressure is always
directly in the pilot's face. The aeroplane is then always
engaging the air as designed to do so, and both lifting and
controlling surfaces are acting efficiently. The only exception
to this rule is a vertical dive, and I think that is
obviously not an attitude for any but very experienced
pilots to hanker after.
SPINNING.--This is the worst of all predicaments the
pilot can find himself in. Fortunately it rarely happens.
It is due to the combination of (1) a very steep spiral
descent of small radius, and (2) insufficiency of keel-surface
behind the vertical axis, or the jamming of the rudder
end or elevator into a position by which the aeroplane is forced
into an increasingly steep and small spiral.
Owing to the small radius of such a spiral, the mass of
the aeroplane may gain a rotary momentum greater, in effect,
than the air pressure of the keel-surface or controlling surfaces
opposed to it; and, when once such a condition occurs,
it is difficult to see what can be done by the pilot to remedy
it. The sensible pilot will not go beyond reasonable limits
of steepness and radius when executing spiral descents.
GLIDING DESCENT WITHOUT PROPELLER THRUST.--All
aeroplanes are, or should be, designed to assume their gliding
angle when the power and thrust is cut off. This relieves
the pilot of work, worry, and danger should he find himself
in a fog or cloud. The Pilot, although he may not realize
it, maintains the correct attitude of the aeroplane by observing
its position relative to the horizon. Flying into a
fog or cloud the horizon is lost to view, and he must then rely
upon his instruments--(1) the compass for direction; (2) an
inclinometer (arched spirit-level) mounted transversely to
the longitudinal axis, for lateral stability; and (3) an inclinometer
mounted parallel to the longitudinal axis, or the airspeed
indicator, which will indicate a nose-down position
by increase in air speed, and a tail-down position by decrease
in air speed.
The pilot is then under the necessity of watching three
instruments and manipulating his three controls to keep the
instruments indicating longitudinal, lateral, and directional
stability. That is a feat beyond the capacity of the ordinary
man. If, however, by the simple movement of throttling
down the power and thrust, he can be relieved of looking
after the longitudinal stability, he then has only two instruments
to watch. That is no small job in itself, but it is,
at any rate, fairly practicable.
Aeroplanes are, then, designed, or should be, so that the
centre of gravity is slightly forward of centre of lift. The
aeroplane is then, as a glider, nose-heavy--and the distance
the C.G. is placed in advance of the C.L. should be such as
to ensure a gliding angle producing a velocity the same as
the normal flying speed (for which the strength of construction
has been designed).
In order that this nose-heavy tendency should not exist
when the thrust is working and descent not required, the
centre of thrust is placed a little below the centre of drift
or resistance, and thus tends to pull up the nose of the
aeroplane.
The distance the centre of thrust is placed below the
centre of drift should be such as to produce a force equal
and opposite to that due to the C.G. being forward of the
C.L.
LOOPING AND UPSIDE DOWN FLYING.--If a loop is desired,
it is best to throttle the engine down at point A. The C.G.
being forward of the C.P., then causes the aeroplane to nose-
down, and assists the pilot in making a reasonably small
loop along the course C and in securing a quick recovery.
If the engine is not throttled down, then the aeroplane may
be expected to follow the course D, which results in a longer
nose dive than in the case of the course C.
A steady, gentle movement of the elevator is necessary.
A jerky movement may change the direction of motion so
suddenly as to produce dangerous air stresses upon the surfaces,
in which case there is a possibility of collapse.
If an upside-down flight is desired, the engine may, or
may not, be throttled down at point A. If not throttled
down, then the elevator must be operated to secure a course
approximately in the direction B. If it is throttled down,
then the course must be one of a steeper angle than B, or
there will be danger of stalling.
Diagram p. 88.--This is not set at quite
the correct angle. Path B should slope
slightly downwards from Position A.
CHAPTER III
RIGGING
In order to rig an aeroplane intelligently, and to maintain
it in an efficient and safe condition, it is necessary to possess
a knowledge of the stresses it is called upon to endure, and
the strains likely to appear.
STRESS is the load or burden a body is called upon to
bear. It is usually expressed by the result found by dividing
the load by the number of superficial square inches contained
in the cross-sectional area of the body.
Thus, if, for instance, the object illustrated above contains
4 square inches of cross-sectional area, and the total load
it is called upon to endure is 10 tons, the stress would be
expressed as 2 1/2 tons.
STRAIN is the deformation produced by stress.
THE FACTOR OF SAFETY is usually expressed by the result
found by dividing the stress at which it is known the body
will collapse, by the maximum stress it will be called upon to
endure. For instance, if a control wire be called upon to endure
a maximum stress of 2 cwts., and the known stress at which
it will collapse is 10 cwts., the factor of safety is then 5.
[cwts. = centerweights = 100 pound units as in cent & century.
Interestinly enough, this word only exists today in abbreviation
form, probably of centreweights, but the dictionary entries, even
from a hundred years ago do not list this as a word, but do list
c. or C. as the previous popular abbreviation as in Roman Numerals]
The word listed is "hundredweight. Michael S. Hart, 1997]
COMPRESSION.--The simple stress of compression tends
to produce a crushing strain. Example: the interplane and
fuselage struts.
TENSION.--The simple stress of tension tends to produce
the strain of elongation. Example: all the wires.
BENDING.--The compound stress of bending is a combination
of compression and tension.
The above sketch illustrates a straight piece of wood of
which the top, centre, and bottom lines are of equal length.
We will now imagine it bent to form a circle, thus:
The centre line is still the same length as before being
bent; but the top line, being farther from the centre of the
circle, is now longer than the centre line. That can be due
only to the strain of elongation produced by the stress of
tension. The wood between the centre line and the top
line is then in tension; and the farther from the centre,
the greater the strain, and consequently the greater the
tension.
The bottom line, being nearest to the centre of the circle,
is now shorter than the centre line. That can be due only
to the strain of crushing produced by the stress of compression.
The wood between the centre and bottom lines is
then in compression; and the nearer the centre of the circle,
the greater the strain, and consequently the greater the
compression.
It then follows that there is neither tension nor compression,
i.e., no stress, at the centre line, and that the wood
immediately surrounding it is under considerably less stress
than the wood farther away. This being so, the wood in
the centre may be hollowed out without unduly weakening
struts and spars. In this way 25 to 33 per cent. is saved in
the weight of wood in an aeroplane.
The strength of wood is in its fibres, which should, as far
as possible, run without break from one end of a strut or
spar to the other end. A point to remember is that the
outside fibres, being farthest removed from the centre line,
are doing by far the greatest work.
SHEAR STRESS IS such that, when material collapses under it,
one part slides over the other. Example: all the locking pins.
Some of the bolts are also in shear or ``sideways'' stress,
owing to lugs under their heads and from which wires are
taken. Such a wire, exerting a sideways pull upon a bolt,
tries to break it in such a way as to make one piece of the bolt
slide over the other piece.
TORSION.--This is a twisting stress compounded of compression,
tension, and shear stresses. Example: the propeller shaft.
NATURE OF WOOD UNDER STRESS.--Wood, for its weight,
takes the stress of compression far better than any other
stress. For instance: a walking-stick of less than 1 lb. in
weight will, if kept perfectly straight, probably stand up to
a compression stress of a ton or more before crushing; whereas,
if the same stick is put under a bending stress, it will probably
collapse to a stress of not more than about 50 lb. That is
a very great difference, and, since weight is of the greatest
importance, the design of an aeroplane is always such as to,
as far as possible, keep the various wooden parts of its
construction in direct compression. Weight being of such vital
importance, and designers all trying to outdo each other in
saving weight, it follows that the factor of safety is rather
low in an aeroplane. The parts in direct compression will,
however, take the stresses safely provided the following
conditions are carefully observed.
CONDITIONS TO BE OBSERVED:
1. All the spars and struts must be perfectly straight.
The above sketch illustrates a section through an
interplane strut. If the strut is to be kept straight,
i.e., prevented from bending, then the stress of
compression must be equally disposed about the
centre of strength. If it is not straight, then
there will be more compression on one side of the
centre of strength than on the other side. That
is a step towards getting compression on one side
and tension on the other side, in which case it
may be forced to take a bending stress for which
it is not designed. Even if it does not collapse
it will, in effect, become shorter, and thus throw
out of adjustment the gap and all the wires attached
to the top and bottom of the strut, with the result
that the flight efficiency of the aeroplane will be
spoiled.
The only exception to the above condition is
what is known as the Arch. For instance, in the
case of the Maurice Farman, the spars of the centre-
section plane, which have to take the weight of
the nacelle, are arched upwards. If this was not
done, it is possible that rough landings might
result in the weight causing the spars to become
slightly distorted downwards. That would produce
a dangerous bending stress, but, as long as
the wood is arched, or, at any rate, kept from
bending downwards, it will remain in direct
compression and no danger can result.
2. Struts and spars must be symmetrical. By that I mean
that the cross-sectional dimensions must be correct,
as otherwise there will be bulging places on the
outside, with the result that the stress will not be
evenly disposed about the centre of strength, and
a bending stress may be produced.
3. Struts, spars, etc., must be undamaged. Remember
that, from what I have already explained about
bending stresses, the outside fibres of the wood are
doing by far the most work. If these get bruised
or scored, then the strut or spar suffers in strength
much more than one might think at first sight;
and, if it ever gets a tendency to bend, it is likely
to collapse at that point.
4. The wood must have a good, clear grain with no cross-
grain, knots, or shakes. Such blemishes produce
weak places and, if a tendency to bend appears,
then it may collapse at such a point.
5. The struts, spars, etc., must be properly bedded into
their sockets or fittings. To begin with, they must
be of good pushing or gentle tapping fit. They
must never be driven in with a heavy hammer.
Then again, a strut must bed well down all over its
cross-sectional area as illustrated above; otherwise
the stress of compression will not be evenly disposed
about the centre of strength, and that may
produce a bending stress. The bottom of the strut
or spar should be covered with some sort of
paint, bedded into the socket or fitting, and then
withdrawn to see if the paint has stuck all over the
bed.
6. The atmosphere is sometimes much damper than at
other times, and this causes wood to expand and
contract appreciably. This would not matter but
for the fact that it does not expand and contract
uniformly, but becomes unsymmetrical, i.e., distorted.
I have already explained the danger of that in
condition 2. This should be minimized by WELL
VARNISHING THE WOOD to keep the moisture out of it.
FUNCTION OF INTERPLANE STRUTS.--These struts have to
keep the lifting surfaces or ``planes'' apart, but this is only
part of their work. They must keep the planes apart, so
that the latter are in their correct attitude. That is only so
when the spars of the bottom plane are parallel with those of
the top plane. Also, the chord of the top plane must be
parallel with the chord of the bottom plane. If that is not
so, then one plane will not have the same angle of incidence
as the other one. At first sight one might think that all
that is necessary is to cut all the struts to be the same length,
but that is not the case.
Sometimes, as illustrated above, the rear spar is not so
thick as the main spar, and it is then necessary to make
up for that difference by making the rear struts correspondingly
longer. If that is not done, then the top and
bottom chords will not be parallel, and the top and bottom
planes will have different angles of incidence. Also, the
sockets or fittings, or even the spars upon which they are
placed, sometimes vary in thickness owing to faulty manufacture.
This must be offset by altering the length of the
struts. The best way to proceed is to measure the distance
between the top and bottom spars by the side of each strut,
and if that distance, or ``gap'' as it is called, is not as stated
in the aeroplane's specifications, then make it correct by
changing the length of the strut. This applies to both front
and rear interplane struts. When measuring the gap, always
be careful to measure from the centre of the spar, as it may
be set at an angle, and the rear of it may be considerably
lower than its front.
BORING HOLES IN WOOD.--It should be a strict rule that
no spar be used which has an unnecessary hole in it. Before
boring a hole, its position should be confirmed by whoever
is in charge of the workshop. A bolt-hole should be of a size
to enable the bolt to be pushed in, or, at any rate, not more
than gently tapped in. Bolts should not be hammered in, as
that may split the spar. On the other hand, a bolt should not
be slack in its hole, as, in such a case, it may work sideways and
split the spar, not to speak of throwing out of adjustment
the wires leading from the lug or socket under the bolt-head.
WASHERS.--Under the bolt-head, and also under the nut,
a washer must be placed--a very large washer compared
with the size which would be used in all-metal construction.
This is to disperse the stress over a large area; otherwise
the washer may be pulled into the wood and weaken it,
besides possibly throwing out of adjustment the wires
attached to the bolt or the fitting it is holding to the spar.
LOCKING.--Now as regards locking the bolts. If split
pins are used, be sure to see that they are used in such a way
that the nut cannot possibly unscrew at all. The split pin
should be passed through the bolt as near as possible to the
nut. It should not be passed through both nut and bolt.
If it is locked by burring over the edge of the bolt, do not
use a heavy hammer and try to spread the whole head of
the bolt. That might damage the woodwork inside the
fabric-covered surface. Use a small, light hammer, and gently
tap round the edge of the bolt until it is burred over.
TURNBUCKLES.--A turnbuckle is composed of a central
barrel into each end of which is screwed an eye-bolt. Wires
are taken from the eyes of the eye-bolt, and so, by turning
the barrel, they can be adjusted to their proper tension.
Eye-bolts must be a good fit in the barrel; that is to say,
not slack and not very tight. Theoretically it is not neces-
sary to screw the eye-bolt into the barrel for a distance
greater than the diameter of the bolt, but, in practice, it is
better to screw it in for a considerably greater distance than
that if a reasonable degree of safety is to be secured.
Now about turning the barrel to secure the right adjustment.
The barrel looks solid, but, as a matter of fact, it
is hollow and much more frail than it appears. For that
reason it should not be turned by seizing it with pliers, as
that may distort it and spoil the bore within it. The best
method is to pass a piece of wire through the hole in its centre,
and to use that as a lever. When the correct adjustment
has been secured, the turnbuckle must be locked to prevent
it from unscrewing. It is quite possible to lock it in such a
way as to allow it to unscrew a quarter or a half turn, and
that would throw the wires out of the very fine adjustment
necessary. The proper way is to use the locking wire so
that its direction is such as to oppose the tendency of the
barrel to unscrew, thus:
WIRES.--The following points should be carefully observed
where wire is concerned:
1. Quality.--It must not be too hard or too soft. An
easy practical way of learning to know the approximate
quality of wire is as follows:
Take three pieces, all of the same gauge, and each about a
foot in length. One piece should be too soft, another too hard,
and the third piece of the right quality. Fix them in a vice,
about an inch apart and in a vertical position, and with the light
from a window shining upon them. Burnish them if necessary,
and you will see a band of light reflected from each
wire.
Now bend the wires over as far as possible and away from
the light. Where the soft wire is concerned, it will squash
out at the bend, and this will be indicated by the band of
light, which will broaden at that point. In the case of the
wire which is too hard, the band of light will broaden very
little at the turn, but, if you look carefully, you will see some
little roughnesses of surface. In the case of the wire of the
right quality, the band of light may broaden a very little
at the turn, but there will be no roughnesses of surface.
By making this experiment two or three times one can
soon learn to know really bad wire from good, and also learn
to know the strength of hand necessary to bend the right
quality.
2. It must not be damaged. That is to say, it must be
unkinked, rustless, and unscored.
3. Now as regards keeping wire in good condition. Where
outside wires are concerned, they should be kept WELL GREASED
OR OILED, especially where bent over at the ends. Internal
bracing wires cannot be reached for the purpose of regreasing
them, as they are inside fabric-covered surfaces. They should
be prevented from rusting by being painted with an anti-rust
mixture. Great care should be taken to see that the wire
is perfectly clean and dry before being painted. A greasy
finger-mark is sufficient to stop the paint from sticking to
the wire. In such a case there will be a little space between
the paint and the wire. Air may enter there and cause the
wire to rust.
4. Tension of Wires.--The tension to which the wires are
adjusted is of the greatest importance. All the wires should
be of the same tension when the aeroplane is supported in
such a way as to throw no stress upon them. If some wires
are in greater tension than others, the aeroplane will quickly
become distorted and lose its efficiency.
In order to secure the same tension of all wires, the aeroplane,
when being rigged, should be supported by packing
underneath the lower surfaces as well as by packing underneath
the fuselage or nacelle. In this way the anti-lift wires
are relieved of the weight, and there is no stress upon any
of the wires.
As a general rule the wires of an aeroplane are tensioned
too much. The tension should be sufficient to keep the
framework rigid. Anything more than that lowers the factor
of safety, throws various parts of the framework into undue
compression, pulls the fittings into the wood, and will, in
the end, distort the whole framework of the aeroplane.
Only experience will teach the rigger what tension to
employ. Much may be done by learning the construction
of the various types of aeroplanes, the work the various
parts do, and in cultivating a touch for tensioning wires by
constantly handling them.
5. Wires with no Opposition Wires.--In some few cases
wires will be found which have no opposition wires pulling
in the opposite direction. For instance, an auxiliary lift
wire may run from the bottom of a strut to a spar in the top
plane at a point between struts. In such a case great care
should be taken not to tighten the wire beyond barely taking
up the slack.
Such a wire must be a little slack, or, as illustrated above,
it will distort the framework. That, in the example given,
will spoil the camber (curvature) of the surface, and result
in changing both the lift and the drift at that part of the surface.
Such a condition will cause the aeroplane to lose its
directional stability and also to fly one wing down.
I cannot impress this matter of tension upon the reader
too strongly. It is of the utmost importance. When this,
and also accuracy in securing the various adjustments, has
been learned, one is on the way to becoming a good
rigger.
6. Wire Loops.--Wire is often bent over at its end in the
form of a loop, in order to connect with a turnbuckle or
fitting. These loops, even when made as perfectly as possible,
have a tendency to elongate, thus spoiling the adjustment
of the wires Great care should be taken to minimize this
as far as possible. The rules to be observed are as
follows:
(a) The size of the loop should be as small as possible
within reason. By that I mean it should not be
so small as to create the possibility of the wire
breaking.
(b) The shape of the loop should be symmetrical.
(c) It should have well-defined shoulders in order to
prevent the ferrule from slipping up. At the same
time, a shoulder should not have an angular place.
(d) When the loop is finished it should be undamaged,
and it should not be, as is often the case, badly scored.
7. Stranded Wire Cable.--No splice should be served with
twine until it has been inspected by whoever is in charge of
the workshop. The serving may cover bad work.
Should a strand become broken, then the cable should be
replaced at once by another one.
Control cables have a way of wearing out and fraying
wherever they pass round pulleys. Every time an aeroplane
comes down from flight the rigger should carefully examine
the cables, especially where they pass round pulleys. If
he finds a strand broken, he should replace the cable.
The ailerons' balance cable on the top of the top plane
is often forgotten, since it is necessary to fetch a high pair
of steps in order to examine it. Don't slack this, or some
gusty day the pilot may unexpectedly find himself minus the
aileron control.
CONTROLLING SURFACES.--The greatest care should be
exercised in rigging the aileron, rudder, and elevator properly,
for the pilot entirely depends upon them in managing the
aeroplane.
The ailerons and elevator should be rigged so that, when
the aeroplane is in flight, they are in a fair true line with the
surface in front and to which they are hinged.
If the surface to which they are hinged is not a lifting
surface, then they should be rigged to be in a fair true line
with it as illustrated above.
If the controlling surface is, as illustrated, hinged to the
back of a lifting surface, then it should be rigged a little below
the position it would occupy if in a fair true line with the
surface in front. This is because, in such a case, it is set
at an angle of incidence. This angle will, during flight,
cause it to lift a little above the position in which it has been
rigged. It is able to lift owing to a certain amount of slack
in the control wire holding it--and one cannot adjust the
control wire to have no slack, because that would cause it
to bind against the pulleys and make the operation of it too
hard for the pilot. It is therefore necessary to rig it a little
below the position it would occupy if it was rigged in a fair
true line with the surface in front. Remember that this
only applies when it is hinged to a lifting surface. The
greater the angle of incidence (and therefore the lift) of the
surface in front, then the more the controlling surface will
have to be rigged down.
As a general rule it is safe to rig it down so that its trailing
edge is 1/2 to 3/4 inch below the position it would occupy if in
a fair line with the surface in front; or about 1/2 inch down for
every 18 inches of chord of the controlling surface.
When making these adjustments the pilot's control levers
should be in their neutral positions. It is not sufficient
to lash them. They should be rigidly blocked into position
with wood packing.
The surfaces must not be distorted in any way. If
they are held true by bracing wires, then such wires must be
carefully adjusted. If they are distorted and there are no
bracing wires with which to true them, then some of the
internal framework will probably have to be replaced.
The controlling surfaces should never be adjusted with
a view to altering the stability of the aeroplane. Nothing
can be accomplished in that way. The only result will be
to spoil the control of the aeroplane.
FABRIC-COVERED SURFACES.--First of all make sure
that there is no distortion of spars or ribs, and that they are
perfectly sound. Then adjust the internal bracing wires
so that the ribs are parallel to the direction of flight. The
ribs usually cause the fabric to make a ridge where they occur,
and, if such ridge is not parallel to the direction of flight,
it will produce excessive drift. As a rule the ribs are at
right angles to both main and rear spars.
The tension of the internal bracing wires should be just
sufficient to give rigidity to the framework. They should
not be tensioned above that unless the wires are, at their
ends, bent to form loops. In that case a little extra tension
may be given to offset the probable elongation of the
loops.
The turnbuckles must now be generously greased, and
served round with adhesive tape. The wires must be rendered
perfectly dry and clean, and then painted with an anti-rust
mixture. The woodwork must be well varnished.
If it is necessary to bore holes in the spars for the purpose
of receiving, for instance, socket bolts, then their places
should be marked before being bored and their positions
confirmed by whoever is in charge of the workshop. All is
now ready for the sail-maker to cover the surface with
fabric.
ADJUSTMENT OF CONTROL CABLES.--The adjustment of
the control cables is quite an art, and upon it will depend to
a large degree the quick and easy control of the aeroplane
by the pilot.
The method is as follows:
After having rigged the controlling surfaces, and as far
as possible secured the correct adjustment of the control
cables, then remove the packing which has kept the control
levers rigid. Then, sitting in the pilot's seat, move the
control levers SMARTLY. Tension the control cables so that
when the levers are smartly moved there is no perceptible
snatch or lag. Be careful not to tension the cables more than
necessary to take out the snatch. If tensioned too much
they will (1) bind round the pulleys and result in hard work
for the pilot; (2) throw dangerous stresses upon the controlling
surfaces, which are of rather flimsy construction; and (3)
cause the cables to fray round the pulleys quicker than would
otherwise be the case.
Now, after having tensioned the cables sufficiently to
take out the snatch, place the levers in their neutral positions,
and move them to and fro about 1/8 inch either side of such
positions. If the adjustment is correct, it should be possible
to see the controlling surfaces move. If they do not move,
then the control cables are too slack.
FLYING POSITION.--Before rigging an aeroplane or making
any adjustments it is necessary to place it in what is known
as its ``flying position.'' I may add that it would be better
termed its ``rigging position.''
In the case of an aeroplane fitted with a stationary engine
this is secured by packing up the machine so that the engine
foundations are perfectly horizontal both longitudinally and
laterally. This position is found by placing a straight-edge
and a spirit-level across the engine foundations (both
longitudinally and laterally), and great care should be taken to
see that the bubble is exactly in the centre of the level. The
slightest error will assume magnitude towards the extremities
of the aeroplane. Great care should be taken to block up
the aeroplane rigidly. In case it gets accidentally disturbed
while the work is going on, it is well to constantly verify the
flying position by running the straight-edge and spirit-level
over the engine foundations. The straight-edge should be
carefully tested before being used, as, being generally made of
wood, it will not remain true long. Place it lightly in a vice,
and in such a position that a spirit-level on top shows the
bubble exactly in the centre. Now slowly move the level
along the straight-edge, and the bubble should remain exactly
in the centre. If it does not do so, then the straight-edge
is not true and must be corrected. THIS SHOULD NEVER BE
OMITTED.
In the case of aeroplanes fitted with engines of the rotary
type, the ``flying position'' is some special attitude laid
down in the aeroplane's specifications, and great care should
be taken to secure accuracy.
ANGLE OF INCIDENCE.--One method of finding the angle
of incidence is as follows:
First place the aeroplane in its flying position. The
corner of the straight-edge must be placed underneath and
against the CENTRE of the rear spar, and held in a horizontal
position parallel to the ribs. This is secured by using a
spirit-level. The set measurement will then be from the
top of the straight-edge to the centre of the bottom surface
of the main spar, or it may be from the top of the straight-
edge to the lowest part of the leading edge. Care should be
taken to measure from the centre of the spar and to see that
the bubble is exactly in the centre of the level. Remember
that all this will be useless if the aeroplane has not been placed
accurately in its flying position.
This method of finding the angle of incidence must be
used under every part of the lower surface where struts
occur. It should not be used between the struts, because,
in such places, the spars may have taken a slight permanent
set up or down; not, perhaps, sufficiently bad to make any
material difference to the flying of the machine, but quite bad
enough to throw out the angle of incidence, which cannot
be corrected at such a place.
If the angle is wrong, it should then be corrected as follows:
If it is too great, then the rear spar must be warped up
until it is right, and this is done by slackening ALL the wires
going to the top of the strut, and then tightening ALL the
wires going to the bottom of the strut.
If the angle is too small, then slacken ALL the wires going
to the bottom of the strut, and tighten ALL the wires going to
the top of the strut, until the correct adjustment is secured.
Never attempt to adjust the angle by warping the main spar.
The set measurement, which is of course stated in the
aeroplane's specifications, should be accurate to 1/16 inch.
LATERAL DIHEDRAL ANGLE.--One method of securing
this is as follows, and this method will, at the same time,
secure the correct angle of incidence:
The strings, drawn very tight, must be taken over both
the main and rear spars of the top surface. They must run
between points on the spars just inside the outer struts.
The set measurement (which should be accurate to 1/16 inch
or less) is then from the strings down to four points on the
main and rear spars of the centre-section surface. These
points should be just inside the four centre-section struts;
that is to say, as far as possible away from the centre of the
centre-section. Do not attempt to take the set measurement
near the centre of the centre-section.
The strings should be as tight as possible, and, if it can
be arranged, the best way to accomplish that is as shown in
the above illustration, i.e., by weighting the strings down to
the spars by means of weights and tying each end of the strings
to a strut. This will give a tight and motionless string.
However carefully the above adjustment is made, there is
sure to be some slight error. This is of no great importance,
provided it is divided equally between the left- and right-
hand wings. In order to make sure of this, certain check
measurements should be taken as follows:
Each bay must be diagonally measured, and such measurements
must be the same to within 1/16 inch on each side of
the aeroplane. As a rule such diagonal measurements are
taken from the bottom socket of one strut to the top socket
of another strut, but this is bad practice, because of possible
inaccuracies due to faulty manufacture.
The points between which the diagonal measurements
are taken should be at fixed distances from the butts of the
spars, such distances being the same on each side of the
aeroplane, thus:
It would be better to use the centre line of the aeroplane
rather than the butts of the spars. It is not practicable
to do so, however, as the centre line probably runs through
the petrol tanks, etc.
THE DIHEDRAL BOARD.--Another method of securing
the dihedral angle, and also the angle of incidence, is by
means of the dihedral board. It is a light handy thing to
use, but leads to many errors, and should not be used unless
necessary. The reasons are as follows:
The dihedral board is probably not true. If it must be
used, then it should be very carefully tested for truth before-
hand. Another reason against its use is that it has to be
placed on the spars in a position between the struts, and
that is just where the spars may have a little permanent
set up or down, or some inaccuracy of surface which will,
of course, throw out the accuracy of the adjustment. The
method of using it is as follows:
The board is cut to the same angle as that specified for
the upward inclination of the surface towards its wing-
tips. It is placed on the spar as indicated above, and it
is provided with two short legs to raise it above the flanges
of the ribs (which cross over the spars), as they may vary
in depth. A spirit-level is then placed on the board, and the
wires must be adjusted to give the surface such an inclination
as to result in the bubble being in the centre of the level.
This operation must be performed in respect of each bay
both front and rear. The bays must then be diagonally
measured as already explained.
YET ANOTHER METHOD of finding the dihedral angle,
and at the same time the angle of incidence, is as follows:
A horizontal line is taken from underneath the butt of
each spar, and the set measurement is either the angle it makes
with the spar, or a fixed measurement from the line to the
spar taken at a specified distance from the butt. This operation
must be performed in respect of both main and rear
spars, and all the bays must be measured diagonally afterwards.
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