The Aeroplane Speaks
by
H. Barber
Part 3 out of 3
Whichever method is used, be sure that after the job is
done the spars are perfectly straight.
STAGGER.--The stagger is the distance the top surface
is in advance of the bottom surface when the aeroplane
is in flying position. The set measurement is obtained as
follows:
Plumb-lines must be dropped over the leading edge of
the top surface wherever struts occur, and also near the
fuselage. The set measurement is taken from the front of the
lower leading edge to the plumb-lines. It makes a difference
whether the measurement is taken along a horizontal line
(which can be found by using a straight-edge and a spirit-
level) or along a projection of the chord. The line along
which the measurement should be taken is laid down in the
aeroplane's specifications.
If a mistake is made and the measurement taken along
the wrong line, it may result in a difference of perhaps 1/4
will, in flight, be nose-heavy or tail-heavy.
After the adjustments of the angles of incidence, dihedral,
and stagger have been secured, it is as well to confirm all of
them, as, in making the last adjustment, the first one may
have been spoiled.
OVER-ALL ADJUSTMENTS.--The following over-all check
measurements should now be taken.
The straight lines AC and BC should be equal to within
1/8 inch. The point C is the centre of the propeller, or, in the
case of a ``pusher'' aeroplane, the centre of the nacelle.
The points A and B are marked on the main spar, and must
in each case be the same distance from the butt of the spar.
The rigger should not attempt to make A and B merely the
sockets of the outer struts, as they may not have been placed
quite accurately by the manufacturer. The lines AC and BC
must be taken from both top and bottom spars--two measurements
on each side of the aeroplane.
The two measurements FD and FE should be equal to
within 1/8 inch. F is the centre of the fuselage or rudder-
post. D and E are points marked on both top and bottom
rear spars, and each must be the same fixed distance from
the butt of the spar. Two measurements on each side of the
aeroplane.
If these over-all measurements are not correct, then it
is probably due to some of the drift or anti-drift wires being
too tight or too slack. It may possibly be due to the fuselage
being out of truth, but of course the rigger should have made
quite sure that the fuselage was true before rigging the rest
of the machine. Again, it may be due to the internal bracing
wires within the lifting surfaces not being accurately adjusted,
but of course this should have been seen to before covering the
surfaces with fabric.
FUSELAGE.--The method of truing the fuselage is laid
down in the aeroplane's specifications. After it has been
adjusted according to the specified directions, it should then
be arranged on trestles in such a way as to make about three-
quarters of it towards the tail stick out unsupported. In
this way it will assume a condition as near as possible to
flying conditions, and when it is in this position the set
measurements should be confirmed. If this is not done it
may be out of truth, but perhaps appear all right when
supported by trestles at both ends, as, in such case, its
weight may keep it true as long as it is resting upon the
trestles.
THE TAIL-PLANE (EMPENNAGE).--The exact angle of
incidence of the tail-plane is laid down in the aeroplane's
specifications. It is necessary to make sure that the spars
are horizontal when the aeroplane is in flying position and
the tail unsupported as explained above under the heading
of Fuselage. If the spars are tapered, then make sure that
their centre lines are horizontal.
UNDERCARRIAGE.--The undercarriage must be very carefully
aligned as laid down in the specifications.
1. The aeroplane must be placed in its flying position
and sufficiently high to ensure the wheels being off the ground
when rigged. When in this position the axle must be hori-
nontal and the bracing wires adjusted to secure the various
set measurements stated in the specifications.
2. Make sure that the struts bed well down into their
sockets.
3. Make sure that the shock absorbers are of equal
tension. In the case of rubber shock absorbers, both the
number of turns and the lengths must be equal.
HOW TO DIAGNOSE FAULTS IN FLIGHT, STABILITY, AND CONTROL.
DIRECTIONAL STABILITY will be badly affected if there is
more drift (i.e., resistance) on one side of the aeroplane than
there is on the other side. The aeroplane will tend to turn
towards the side having the most drift. This may be caused
as follows:
1. The angle of incidence of the main surface or the tail
surface may be wrong. The greater the angle of incidence,
the greater the drift. The less the angle, the less the drift.
2. If the alignment of the fuselage, fin in front of the
rudder, the struts or stream-line wires, or, in the case of
the Maurice Farman, the front outriggers, are not absolutely
correct--that is to say, if they are turned a little to the
left or to the right instead of being in line with the direction
of flight--then they will act as a rudder and cause the aeroplane
to turn off its course.
3. If any part of the surface is distorted, it will cause
the aeroplane to turn off its course. The surface is cambered,
i.e., curved, to pass through the air with the least possible
drift. If, owing perhaps to the leading edge, spars, or trailing
edge becoming bent, the curvature is spoiled, that will
result in changing the amount of drift on one side of the aeroplane,
which will then have a tendency to turn off its course.
LATERAL INSTABILITY (FLYING ONE WING DOWN).--The only possible
reason for such a condition is a difference in the lifts
of right and left wings. That may be caused as follows:
1. The angle of incidence may be wrong. If it is too
great, it will produce more lift than on the other side of the
aeroplane; and if too small, it will produce less lift than on
the other side--the result being that, in either case, the aeroplane
will try to fly one wing down.
2. Distorted Surfaces.--If some part of the surface is
distorted, then its camber is spoiled, and the lift will not be
the same on both sides of the aeroplane, and that, of course,
will cause it to fly one wing down.
LONGITUDINAL INSTABILITY may be due to the following reasons:
1. The stagger may be wrong. The top surface may have
drifted back a little owing to some of the wires, probably
the incidence wires, having elongated their loops or having
pulled the fittings into the wood. If the top surface is not
staggered forward to the correct degree, then consequently
the whole of its lift is too far back, and it will then have a
tendency to lift up the tail of the machine too much. The
aeroplane would then be said to be ``nose-heavy.''
A 1/4-inch area in the stagger will make a very considerable
difference to the longitudinal stability.
2. If the angle of incidence of the main surface is not right,
it will have a bad effect, especially in the case of an aeroplane
with a lifting tail-plane.
If the angle is too great, it will produce an excess of lift,
and that may lift up the nose of the aeroplane and result in
a tendency to fly ``tail-down.'' If the angle is too small,
it will produce a decreased lift, and the aeroplane may have a
tendency to fly ``nose-down.''
3. The fuselage may have become warped upward or
downward, thus giving the tail-plane an incorrect angle of
incidence. If it has too much angle, it will lift too much,
and the aeroplane will be ``nose-heavy.'' If it has too little
angle, then it will not lift enough, and the aeroplane will be
``tail-heavy.''
4. (The least likely reason.) The tail-plane may be
mounted upon the fuselage at a wrong angle of incidence,
in which case it must be corrected. If nose-heavy, it should
be given a smaller angle of incidence. If tail-heavy, it should
be given a larger angle; but care should be taken not to give
it too great an angle, because the longitudinal stability
entirely depends upon the tail-plane being set at a much
smaller angle of incidence than is the main surface, and if
that difference is decreased too much, the aeroplane will
become uncontrollable longitudinally. Sometimes the tail-
plane is mounted on the aeroplane at the same angle as the
main surface, but it actually engages the air at a lesser angle,
owing to the air being deflected downwards by the main
surface. There is then, in effect, a longitudinal dihedral
as explained and illustrated in Chapter I.
CLIMBS BADLY.--Such a condition is, apart from engine
or propeller trouble, probably due to (1) distorted surfaces,
or (2) too small an angle of incidence.
FLIGHT SPEED POOR.--Such a condition is, apart from
engine or propeller trouble, probably due to (1) distorted
surfaces, (2) too great an angle of incidence, or (3) dirt or
mud, and consequently excessive skin-friction.
INEFFICIENT CONTROL is probably due to (1) wrong setting
of control surfaces, (2) distortion of control surfaces, or
(3) control cables being badly tensioned.
WILL NOT TAXI STRAIGHT.--If the aeroplane is uncontrollable
on the ground, it is probably due to (1) alignment
of undercarriage being wrong, or (2) unequal tension of shock
absorbers.
CHAPTER IV
THE PROPELLER, OR ``AIR-SCREW''
The sole object of the propeller is to translate the power
of the engine into thrust.
The propeller screws through the air, and its blades, being
set at an angle inclined to the direction of motion, secure
a reaction, as in the case of the aeroplane's lifting surface.
This reaction may be conveniently divided into two
component parts or values, namely, Thrust and Drift.
The Thrust is opposed to the Drift of the aeroplane, and
must be equal and opposite to it at flying speed. If it falls
off in power, then the flying speed must decrease to a velocity,
at which the aeroplane drift equals the decreased thrust.
The Drift of the propeller may be conveniently divided
into the following component values:
Active Drift, produced by the useful thrusting part of the propeller.
Passive Drift, produced by all the rest of the propeller,
i.e., by its detrimental surface.
Skin Friction, produced by the friction of the air with
roughnesses of surface.
Eddies attending the movement of the air caused by
the action of the propeller.
Cavitation (very marked at excessive speed of revolution).
A tendency of the propeller to produce a
cavity or semi-vacuum in which it revolves, the
thrust decreasing with increase of speed and
cavitation.
THRUST-DRIFT RATIO.--The proportion of thrust to drift
is of paramount importance, for it expresses the efficiency
of the propeller. It is affected by the following factors:
Speed of Revolution.--The greater the speed, the greater
the proportion of drift to thrust. This is due to
the increase with speed of the passive drift, which
carries with it no increase in thrust. For this
reason propellers are often geared down to revolve
at a lower speed than that of the engine.
Angle of Incidence.--The same reasons as in the case of
the aeroplane surface.
Surface Area.--Ditto.
Aspect Ratio.--Ditto.
Camber.--Ditto.
In addition to the above factors there are, when it comes
to actually designing a propeller, mechanical difficulties to
consider. For instance, the blades must be of a certain
strength and consequent thickness. That, in itself, limits
the aspect ratio, for it will necessitate a chord long enough
in proportion to the thickness to make a good camber possible.
Again, the diameter of the propeller must be limited, having
regard to the fact that greater diameters than those used
to-day would not only result in excessive weight of construction,
but would also necessitate a very high undercarriage
to keep the propeller off the ground, and such undercarriage
would not only produce excessive drift, but would also tend
to make the aeroplane stand on its nose when alighting.
The latter difficulty cannot be overcome by mounting the
propeller higher, as the centre of its thrust must be approximately
coincident with the centre of aeroplane drift.
MAINTENANCE OF EFFICIENCY.
The following conditions must be observed:
1. PITCH ANGLE.--The angle, at any given point on the
propeller, at which the blade is set is known as the pitch
angle, and it must be correct to half a degree if reasonable
efficiency is to be maintained.
This angle secures the ``pitch,'' which is the distance the
propeller advances during one revolution, supposing the air
to be solid. The air, as a matter of fact, gives back to the
thrust of the blades just as the pebbles slip back as one
ascends a shingle beach. Such ``give-back'' is known as
Slip. If a propeller has a pitch of, say, 10 feet, but actually
advances, say, only 8 feet owing to slip, then it will be said
to possess 20 per cent. slip.
Thus, the pitch must equal the flying speed of the
aeroplane plus the slip of the propeller. For example,
let us find the pitch of a propeller, given the following
conditions:
Flying speed .............. 70 miles per hour.
Propeller revolutions ..... 1,200 per minute.
Slip ...................... 15 per cent.
First find the distance in feet the aeroplane will travel
forward in one minute. That is--
369,600 feet (70 miles)
------------------------ = 6,160 feet per minute.
60 `` (minutes)
Now divide the feet per minute by the propeller revolutions
per minute, add 15 per cent. for the slip, and the result
will be the propeller pitch:
6,160
----- + 15 per cent. = 5 feet 1 3/5 inches.
1,200
In order to secure a constant pitch from root to tip of
blade, the pitch angle decreases towards the tip. This is
necessary, since the end of the blade travels faster than its
root, and yet must advance forward at the same speed as
the rest of the propeller. For example, two men ascending
a hill. One prefers to walk fast and the other slowly, but they
wish to arrive at the top of the hill simultaneously. Then
the fast walker must travel a farther distance than the slow
one, and his angle of path (pitch angle) must be smaller
than the angle of path taken by the slow walker. Their
pitch angles are different, but their pitch (in this case altitude
reached in a given time) is the same.
In order to test the pitch angle, the propeller must be
mounted upon a shaft at right angles to a beam the face of
which must be perfectly level, thus:
First select a point on the blade at some distance (say
about 2 feet) from the centre of the propeller. At that
point find, by means of a protractor, the angle a projection
of the chord makes with the face of the beam. That angle
is the pitch angle of the blade at that point.
Now lay out the angle on paper, thus:
The line above and parallel to the circumference line must
be placed in a position making the distance between the
two lines equal to the specified pitch, which is, or should be,
marked upon the boss of the propeller.
Now find the circumference of the propeller where the
pitch angle is being tested. For example, if that place is
2 feet radius from the centre, then the circumference will
be 2 feet X 2 = 4 feet diameter, which, if multiplied by
3.1416 = 15.56 feet circumference.
Now mark off the circumference distance, which is
represented above by A-B, and reduce it in scale for convenience.
The distance a vertical line makes between B and the
chord dine is the pitch at the point where the angle is being
tested, and it should coincide with the specified pitch. You
will note, from the above illustration, that the actual pitch
line should meet the junction of the chord line and top
line.
The propeller should be tested at several points, about
a foot apart, on each blade; and the diagram, provided the
propeller is not faulty, will then look like this:
At each point tested the actual pitch coincides with the
specified pitch: a satisfactory condition.
A faulty propeller will produce a diagram something
like this:
At every point tested the pitch angle is wrong, for nowhere
does the actual pitch coincide with the specified pitch.
Angles A, C, and D, are too large, and B is too small. The
angle should be correct to half a degree if reasonable efficiency
is to be maintained.
A fault in the pitch angle may be due to (1) faulty manufacture,
(2) distortion, or (3) the shaft hole through the boss
being out of position.
2. STRAIGHTNESS.--To test for straightness the propeller
must be mounted upon a shaft. Now bring the tip of one
blade round to graze some fixed object. Mark the point it
grazes. Now bring the other tip round, and it should come
within 1/8 inch of the mark. If it does not do so, it is due to
(1) faulty manufacture, (2) distortion, or (3) to the hole
through the boss being out of position.
3. LENGTH.--The blades should be of equal length to
inch.
4. BALANCE.--The usual method of testing a propeller
for balance is as follows: Mount it upon a shaft, which must
be on ball-bearings. Place the propeller in a horizontal
position, and it should remain in that position. If a weight
of a trifle over an ounce placed in a bolt-hole on one side of
the boss fails to disturb the balance, then the propeller is
usually regarded as unfit for use.
The above method is rather futile, as it does not test for
the balance of centrifugal force, which comes into play as
soon as the propeller revolves. It can be tested as follows:
The propeller must be in a horizontal position, and then
weighed at fixed points, such as A, B, C, D, E, and F, and
the weights noted. The points A, B, and C must, of course,
be at the same fixed distances from the centre of the propeller
as the points D, E, and F. Now reverse the propeller and
weigh at each point again. Note the results. The first
series of weights should correspond to the second series,
thus:
Weight A should equal weight F.
`` B `` `` `` E.
`` C `` `` `` D.
There is no standard practice as to the degree of error
permissible, but if there are any appreciable differences the
propeller is unfit for use.
5. SURFACE AREA.--The surface area of the blades should
be equal. Test with callipers thus:
The points between which the distances are taken must,
of course, be at the same distance from the centre in the
case of each blade.
There is no standard practice as to the degree of error
permissible. If, however, there is an error of over 1/8 inch,
the propeller is really unfit for use.
6. CAMBER.--The camber (curvature) of the blades should
be (1) equal, (2) decrease evenly towards the tips of the blades,
and (3) the greatest depth of the curve should, at any point
of the blade, be approximately at the same percentage of
the chord from the leading edge as at other points.
It is difficult to test the top camber without a set of
templates, but a fairly accurate idea of the concave camber
can be secured by slowly passing a straight-edge along the
blade, thus:
The camber can now be easily seen, and as the straight-
edge is passed along the blade, the observer should look for
any irregularities of the curvature, which should gradually
and evenly decrease towards the tip of the blade.
7. THE JOINTS.--The usual method for testing the glued
joints is by revolving the propeller at greater speed than it
will be called upon to make during flight, and then carefully
examining the joints to see if they have opened. It is not
likely, however, that the reader will have the opportunity
of making this test. He should, however, examine all the
joints very carefully, trying by hand to see if they are quite
sound. Suspect a propeller of which the joints appear to
hold any thickness of glue. Sometimes the joints in the
boss open a little, but this is not dangerous unless they extend
to the blades, as the bolts will hold the laminations together.
8. CONDITION OF SURFACE.--The surface should be very
smooth, especially towards the tips of the blades. Some
propeller tips have a speed of over 30,000 feet a minute,
and any roughness will produce a bad drift or resistance
and lower the efficiency.
9. MOUNTING.--Great care should be taken to see that
the propeller is mounted quite straight on its shaft. Test in
the same way as for straightness. If it is not straight, it
is possibly due to some of the propeller bolts being too slack
or to others having been pulled up too tightly.
FLUTTER.--Propeller ``flutter,'' or vibration, may be due
to faulty pitch angle, balance, camber, or surface area. It
causes a condition sometimes mistaken for engine trouble,
and one which may easily lead to the collapse of the propeller.
CARE OF PROPELLERS.--The care of propellers is of the
greatest importance, as they become distorted very easily.
1. Do not store them in a very damp or a very dry place.
2. Do not store them where the sun will shine upon them.
3. Never leave them long in a horizontal position or
leaning up against a wall.
4. They should be hung on horizontal pegs, and the
position of the propellers should be vertical.
If the points I have impressed upon you in these notes
are not attended to, you may be sure of the following results:
1. Lack of efficiency, resulting in less aeroplane speed
and climb than would otherwise be the case.
2. Propeller ``flutter'' and possible collapse.
3. A bad stress upon the propeller shaft and its bearings.
TRACTOR.--A propeller mounted in front of the main
surface.
PUSHER.--A propeller mounted behind the main surface.
FOUR-BLADED PROPELLERS.--Four- bladed propellers are
suitable only when the pitch is comparatively large.
For a given pitch, and having regard to ``interference,''
they are not so efficient as two-bladed propellers.
The smaller the pitch, the less the ``gap,'' i.e., the distance,
measured in the direction of the thrust, between the
spiral courses of the blades.
If the gap is too small, then the following blade will
engage air which the preceding blade has put into motion,
with the result that the following blade will not secure as
good a reaction as would otherwise be the case. It is very
much the same as in the case of the aeroplane gap.
For a given pitch, the gap of a four-bladed propeller is
only half that of a two-bladed one. Therefore the four-
bladed propeller is only suitable for large pitch, as such
pitch produces spirals with a large gap, thus offsetting the
decrease in gap caused by the numerous blades.
The greater the speed of rotation, the less the pitch for
a given aeroplane speed. Then, in order to secure a large
pitch and consequently a good gap, the four-bladed propeller
is usually geared to rotate at a lower speed than would be
the case if directly attached to the engine crank-shaft.
CHAPTER V
MAINTENANCE
CLEANLINESS.--The fabric must be kept clean and free
from oil, as that will rot it. To take out dirt or oily patches,
try acetone. If that will not remedy matters, then try
petrol, but use it sparingly, as otherwise it will take off an
unnecessary amount of dope. If that will not remove the
dirt, then hot water and soap will do so, but, in that case,
be sure to use soap having no alkali in it, as otherwise it may
injure the fabric. Use the water sparingly, or it may get
inside the planes and rust the internal bracing wires, or cause
some of the wooden framework to swell.
The wheels of the undercarriage have a way of throwing
up mud on to the lower surface. This should, if possible, be
taken off while wet. It should never be scraped off when
dry, as that may injure the fabric. If dry, then it should
be moistened before being removed.
Measures should be taken to prevent dirt from collecting
upon any part of the aeroplane, as, otherwise, excessive skin-
friction will be produced with resultant loss of flight speed.
The wires, being greasy, collect dirt very easily.
CONTROL CABLES.--After every flight the rigger should
pass his hand over the control cables and carefully examine
them near pulleys. Removal of grease may be necessary
to make a close inspection possible. If only one strand is
broken the wire should be replaced. Do not forget the aileron
balance wire on the top surface.
Once a day try the tension of the control cables by smartly
moving the control levers about as explained elsewhere.
WIRES.--All the wires should be kept well greased or
oiled, and in the correct tension. When examining the wires,
it is necessary to place the aeroplane on level ground, as
otherwise it may be twisted, thus throwing some wires into
undue tension and slackening others. The best way, if there
is time, is to pack the machine up into its ``flying position.''
If you see a slack wire, do not jump to the conclusion
that it must be tensioned. Perhaps its opposition wire is
too tight, in which case slacken it, and possibly you will
find that will tighten the slack wire.
Carefully examine all wires and their connections near
the propeller, and be sure that they are snaked round with
safety wire, so that the latter may keep them out of the way
of the propeller if they come adrift.
The wires inside the fuselage should be cleaned and regreased
about once a fortnight.
STRUTS AND SOCKETS.--These should be carefully examined
to see if any splitting has occurred.
DISTORTION.--Carefully examine all surfaces, including
the controlling surfaces, to see whether any distortion has
occurred. If distortion can be corrected by the adjustment
of wires, well and good; but if not, then some of the internal
framework probably requires replacement.
ADJUSTMENTS.--Verify the angles of incidence; dihedral,
and stagger, and the rigging position of the controlling-
surfaces, as often as possible.
UNDERCARRIAGE.--Constantly examine the alignment and
fittings of the undercarriage, and the condition of tyres and
shock absorbers. The latter, when made of rubber, wear
quickest underneath. Inspect axles and skids to see if
there are any signs of them becoming bent. The wheels
should be taken off occasionally and greased.
LOCKING ARRANGEMENTS.--Constantly inspect the locking
arrangements of turnbuckles, bolts, etc. Pay particular
attention to the control cable connections, and to all moving
parts in respect of the controls.
LUBRICATION.--Keep all moving parts, such as pulleys,
control levers, and hinges of controlling surfaces, well greased.
SPECIAL INSPECTION.--Apart from constantly examining
the aeroplane with reference to the above points I have made,
I think that, in the case of an aeroplane in constant use
it is an excellent thing to make a special inspection of every
part, say once a week. This will take from two to three
hours, according to the type of aeroplane. In order to carry
it out methodically, the rigger should have a list of every part
down to the smallest split-pin. He can then check the parts
as he examines them, and nothing will be passed over. This,
I know from experience, greatly increases the confidence of
the pilot, and tends to produce good work in the air.
WINDY WEATHER.--The aeroplane, when on the ground,
should face the wind; and it is advisable to lash the control
lever fast, so that the controlling surfaces may not be blown
about and possibly damaged.
``VETTING'' BY EYE.--This should be practiced at every
opportunity, and, if persevered in, it is possible to become
quite expert in diagnosing by eye faults in flight efficiency,
stability and control.
The aeroplane should be standing upon level ground, or,
better than that, packed up into its ``flying position.''
Now stand in front of it and line up the leading edge
with the main spar, rear spar, and trailing edge. Their
shadows can usually be seen through the fabric. Allowance
must, of course, be made for wash-in and wash-out; otherwise,
the parts I have specified should be parallel with each other.
Now line up the centre part of the main-plane with the
tail-plane. The latter should be horizontal.
Next, sight each interplane front strut with its rear
strut. They should be parallel.
Then, standing on one side of the aeroplane, sight all
the front struts. The one nearest to you should cover all
the others. This applies to the rear struts also.
Look for distortion of leading edges, main and rear spars,
trailing edges, tail-plane and controlling surfaces.
This sort of thing, if practiced constantly, will not only
develop an expert eye for diagnosis of faults, but will also
greatly assist in impressing upon the memory the characteristics
and possible troubles of the various types of aeroplanes.
MISHANDLING OF THE GROUND.--This is the cause of a
lot of unnecessary damage. The golden rule to observe is:
PRODUCE NO BENDING STRESSES.
Nearly all the wood in an aeroplane is designed to take
merely the stress of direct compression, and it cannot be bent
safely. Therefore, in packing an aeroplane up from the
ground, or in pulling or pushing it about, be careful to stress
it in such a way as to produce, as far as possible, only direct
compression stresses. For instance, if it is necessary to
support the lifting surface, then the packing should be
arranged to come directly under the struts so that they may
take the stress in the form of compression for which they are
designed. Such supports should be covered with soft packing
in order to prevent the fabric from becoming damaged.
When pulling an aeroplane along, if possible, pull from
the top of the undercarriage struts. If necessary to pull
from elsewhere, then do so by grasping the interplane struts
as low down as possible.
Never lay fabric-covered parts upon a concrete floor.
Any slight movement will cause the fabric to scrape over the
floor with resultant damage.
Struts, spars, etc., should never be left about the floor,
as in such position they are likely to become scored. I
have already explained the importance of protecting the outside
fibres of the wood. Remember also that wood becomes
distorted easily. This particularly applies to interplane
struts. If there are no proper racks to stand them in, then
the best plan is to lean them up against the wall in as near a
vertical position as possible.
TIME.--Learn to know the time necessary to complete
any of the various rigging jobs. This is really important.
Ignorance of this will lead to bitter disappointments in civil
life; and, where Service flying is concerned, it will, to say the
least of it, earn unpopularity with senior officers, and fail to
develop respect and good work where men are concerned.
THE AEROPLANE SHED.--This should be kept as clean and
orderly as possible. A clean, smart shed produces briskness,
energy, and pride of work. A dirty, disorderly shed nearly
always produces slackness and poor quality of work, lost
tools and mislaid material.
GLOSSARY
Aeronautics--The science of aerial navigation.
Aerofoil--A rigid structure, of large superficial area relative to its
thickness, designed to obtain, when driven through the air at an
angle inclined to the direction of motion, a reaction from the air
approximately at right angles to its surface. Always cambered
when intended to secure a reaction in one direction only. As the
term ``aerofoil'' is hardly ever used in practical aeronautics,
I have, throughout this book, used the term SURFACE, which,
while academically incorrect, since it does not indicate thickness,
is a term usually used to describe the cambered lifting surfaces,
i.e., the ``planes'' or ``wings,'' and the stabilizers and the
controlling aerofoils.
Aerodrome--The name usually applied to a ground used for the
practice of aviation. It really means ``flying machine,'' but is
never used in that sense nowadays.
Aeroplane--A power-driven aerofoil with stabilizing and controlling
surfaces.
Acceleration--The rate of change of velocity.
Angle of Incidence--The angle at which the ``neutral lift line'' of
a surface attacks the air.
Angle of Incidence, Rigger's--The angle the chord of a surface makes
with a line parallel to the axis of the propeller.
Angle of Incidence, Maximum--The greatest angle of incidence at
which, for a given power, surface (including detrimental surface),
and weight, horizontal flight can be maintained.
Angle of Incidence, Minimum--The smallest angle of incidence at
which, for a given power, surface (including detrimental surface),
and weight, horizontal flight can be maintained.
Angle of Incidence, Best Climbing--That angle of incidence at which
an aeroplane ascends quickest. An angle approximately halfway
between the maximum and optimum angles.
Angle of Incidence, Optimum--The angle of incidence at which the
lift-drift ratio is the highest.
Angle, Gliding--The angle between the horizontal and the path along
which an aeroplane at normal flying speed, but not under engine
power, descends in still air.
Angle, Dihedral--The angle between two planes.
Angle, Lateral Dihedral--The lifting surface of an aeroplane is said to
be at a lateral dihedral angle when it is inclined upward towards
its wing-tips.
Angle, Longitudinal Dihedral--The main surface and tail surface are
said to be at a longitudinal dihedral angle when the projections
of their neutral lift lines meet and produce an angle above them.
Angle, Rigger's Longitudinal Dihedral--Ditto, but substituting
``chords'' for ``neutral life lines.''
Angle, Pitch--The angle at any given point of a propeller, at which
the blade is inclined to the direction of motion when the propeller
is revolving but the aeroplane stationary.
Altimeter--An instrument used for measuring height.
Air-Speed Indicator--An instrument used for measuring air pressures
or velocities. It consequently indicates whether the surface is
securing the requisite reaction for flight. Usually calibrated in
miles per hour, in which case it indicates the correct number of
miles per hour at only one altitude. This is owing to the density
of the air decreasing with increase of altitude and necessitating
a greater speed through space to secure the same air pressure
as would be secured by less speed at a lower altitude. It would
be more correct to calibrate it in units of air pressure.
Air Pocket--A local movement or condition of the air causing an
aeroplane to drop or lose its correct attitude.
Aspect-Ratio--The proportion of span to chord of a surface.
Air-Screw (Propeller)--A surface so shaped that its rotation about
an axis produces a force (thrust) in the direction of its axis.
Aileron--A controlling surface, usually situated at the wing-tip, the
operation of which turns an aeroplane about its longitudinal axis;
causes an aeroplane to tilt sideways.
Aviation--The art of driving an aeroplane.
Aviator--The driver of an aeroplane.
Barograph--A recording barometer, the charts of which can be calibrated
for showing air density or height.
Barometer--An instrument used for indicating the density of air.
Bank, to--To turn an aeroplane about its longitudinal axis (to tilt
sideways) when turning to left or right.
Biplane--An aeroplane of which the main lifting surface consists
of a surface or pair of wings mounted above another surface or
pair of wings.
Bay--The space enclosed by two struts and whatever they are fixed to.
Boom--A term usually applied to the long spars joining the tail of a
``pusher'' aeroplane to its main lifting surface.
Bracing--A system of struts and tie wires to transfer a force from
one point to another.
Canard--Literally ``duck.'' The name which was given to a type of
aeroplane of which the longitudinal stabilizing surface (empennage)
was mounted in front of the main lifting surface. Sometimes
termed ``tail-first'' aeroplanes, but such term is erroneous,
as in such a design the main lifting surface acts as, and is, the
empennage.
Cabre--To fly or glide at an excessive angle of incidence; tail down.
Camber--Curvature.
Chord--Usually taken to be a straight line between the trailing and
leading edges of a surface.
Cell--The whole of the lower surface, that part of the upper surface
directly over it, together with the struts and wires holding them
together.
Centre (Line) of Pressure--A line running from wing-tip to wing-tip,
and through which all the air forces acting upon the surface may
be said to act, or about which they may be said to balance.
Centre (Line) of Pressure, Resultant--A line transverse to the
longitudinal axis, and the position of which is the resultant of the
centres of pressure of two or more surfaces.
Centre of Gravity--The centre of weight.
Cabane--A combination of two pylons, situated over the fuselage,
and from which anti-lift wires are suspended.
Cloche--Literally ``bell.'' Is applied to the bell-shaped construction
which forms the lower part of the pilot's control lever in
a Bleriot monoplane, and to which the control cables are
attached.
Centrifugal Force--Every body which moves in a curved path is
urged outwards from the centre of the curve by a force termed
``centrifugal.''
Control Lever--A lever by means of which the controlling surfaces
are operated. It usually operates the ailerons and elevator. The
``joy-stick".
Cavitation, Propeller--The tendency to produce a cavity in the air.
Distance Piece--A long, thin piece of wood (sometimes tape) passing
through and attached to all the ribs in order to prevent them from
rolling over sideways.
Displacement--Change of position.
Drift (of an aeroplane as distinct from the propeller)--The horizontal
component of the reaction produced by the action of driving
through the air a surface inclined upwards and towards its direction
of motion PLUS the horizontal component of the reaction produced
by the ``detrimental'' surface PLUS resistance due to
``skin-friction.'' Sometimes termed ``head-resistance.''
Drift, Active--Drift produced by the lifting surface.
Drift, Passive--Drift produced by the detrimental surface.
Drift (of a propeller)--Analogous to the drift of an aeroplane. It is
convenient to include ``cavitation'' within this term.
Drift, to--To be carried by a current of air; to make leeway.
Dive, to--To descend so steeply as to produce a speed greater than the
normal flying speed.
Dope, to--To paint a fabric with a special fluid for the purpose of
tightening and protecting it.
Density--Mass of unit volume, for instance, pounds per cubic foot.
Efficiency--Output
Input
Efficiency (of an aeroplane as distinct from engine and propeller)--
Lift and Velocity
Thrust (= aeroplane drift)
Efficiency, Engine--Brake horse-power
Indicated horse-power
Efficiency, Propeller-- Thrust horse-power
Horse-power received from engine
(= propeller drift)
NOTE.--The above terms can, of course, be expressed in foot-
pounds. It is then only necessary to divide the upper term by
the lower one to find the measure of efficiency.
Elevator--A controlling surface, usually hinged to the rear of the tail-
plane, the operation of which turns an aeroplane about an axis
which is transverse to the direction of normal horizontal flight.
Empennage--See ``Tail-plane.''
Energy--Stored work. For instance, a given weight of coal or petroleum
stores a given quantity of energy which may be expressed
in foot-pounds.
Extension--That part of the upper surface extending beyond the
span of the lower surface.
Edge, Leading--The front edge of a surface relative to its normal
direction of motion.
Edge, Trailing--The rear edge of a surface relative to its normal
direction of motion.
Factor of Safety--Usually taken to mean the result found by dividing
the stress at which a body will collapse by the maximum stress
it will be called upon to bear.
Fineness (of stream-line)--The proportion of length to maximum width.
Flying Position--A special position in which an aeroplane must be
placed when rigging it or making adjustments. It varies with
different types of aeroplanes. Would be more correctly described
as ``rigging position.''
Fuselage--That part of an aeroplane containing the pilot, and to which
is fixed the tail-plane.
Fin--Additional keel-surface, usually mounted at the rear of an
aeroplane.
Flange (of a rib)--That horizontal part of a rib which prevents it
from bending sideways.
Flight--The sustenance of a body heavier than air by means of its
action upon the air.
Foot-pound--A measure of work representing the weight of 1 lb.
raised 1 foot.
Fairing--Usually made of thin sheet aluminum, wood, or a light
construction of wood and fabric; and bent round detrimental
surface in order to give it a ``fair'' or ``stream-like'' shape.
Gravity--Is the force of the Earth's attraction upon a body. It
decreases with increase of distance from the Earth. See ``Weight.''
Gravity, Specific--Density of substance
Density of water.
Thus, if the density of water is 10 lb. per unit volume, the same
unit volume of petrol, if weighing 7 lb., would be said to have a
specific gravity of 7/10, i.e., 0.7.
Gap (of an aeroplane)--The distance between the upper and lower
surfaces of a biplane. In a triplane or multiplane, the distance
between a surface and the one first above it.
Gap, Propeller--The distance, measured in the direction of the thrust,
between the spiral courses of the blades.
Girder--A structure designed to resist bending, and to combine lightness
and strength.
Gyroscope--A heavy circular wheel revolving at high speed, the effect
of which is a tendency to maintain its plane of rotation against
disturbing forces.
Hangar--An aeroplane shed.
Head-Resistance--Drift. The resistance of the air to the passage of
a body.
Helicopter--An air-screw revolving about a vertical axis, the direction
of its thrust being opposed to gravity.
Horizontal Equivalent--The plan view of a body whatever its attitude
may be.
Impulse--A force causing a body to gain or lose momentum.
Inclinometer--A curved form of spirit-level used for indicating the
attitude of a body relative to the horizontal.
Instability--An inherent tendency of a body, which, if the body is
disturbed, causes it to move into a position as far as possible away
from its first position.
Instability, Neutral--An inherent tendency of a body to remain in the
position given it by the force of a disturbance, with no tendency
to move farther or to return to its first position.
Inertia--The inherent resistance to displacement of a body as distinct
from resistance the result of an external force.
Joy-Stick--See ``Control Lever.''
Keel-Surface--Everything to be seen when viewing an aeroplane from
the side of it.
King-Post--A bracing strut; in an aeroplane, usually passing through
a surface and attached to the main spar, and from the end or ends
of which wires are taken to spar, surface, or other part of the
construction in order to prevent distortion. When used in connection
with a controlling surface, it usually performs the additional
function of a lever, control cables connecting its ends with the
pilot's control lever.
Lift--The vertical component of the reaction produced by the action
of driving through the air a surface inclined upwards and towards
its direction of motion.
Lift, Margin of--The height an aeroplane can gain in a given time and
starting from a given altitude.
Lift-Drift Ratio--The proportion of lift to drift.
Loading--The weight carried by an aerofoil. Usually expressed in
pounds per square foot of superficial area.
Longeron--The term usually applied to any long spar running length-
ways of a fuselage.
Mass--The mass of a body is a measure of the quantity of material
in it.
Momentum--The product of the mass and velocity of a body is known
as ``momentum.''
Monoplane--An aeroplane of which the main lifting surface consists
of one surface or one pair of wings.
Multiplane--An aeroplane of which the main lifting surface consists
of numerous surfaces or pairs of wings mounted one above the
other.
Montant--Fuselage strut.
Nacelle--That part of an aeroplane containing the engine and
pilot and passenger, and to which the tail plane is not fixed.
Neutral Lift Line--A line taken through a surface in a forward direction
relative to its direction of motion, and starting from its
trailing edge. If the attitude of the surface is such as to make
the said line coincident with the direction of motion, it results
in no lift, the reaction then consisting solely of drift. The position
of the neutral lift line, i.e., the angle it makes with the chord,
varies with differences of camber, and it is found by means of
wind-tunnel research.
Newton's Laws of Motion--1. If a body be at rest, it will remain at
rest; or, if in motion, it will move uniformly in a straight line
until acted upon by some force.
2. The rate of change of the quantity of motion (momentum) is
proportional to the force which causes it, and takes place in the
direction of the straight line in which the force acts. If a body
be acted upon by several forces, it will obey each as though the
others did not exist, and this whether the body be at rest or in
motion.
3. To every action there is opposed an equal and opposite
reaction.
Ornithopter (or Orthopter)--A flapping wing design of aircraft intended
to imitate the flight of a bird.
Outrigger--This term is usually applied to the framework connecting
the main surface with an elevator placed in advance of it. Sometimes
applied to the ``tail-boom'' framework connecting the
tail-plane with the main lifting surface.
Pancake, to--To ``stall ''
Plane--This term is often applied to a lifting surface. Such application
is not quite correct, since ``plane'' indicates a flat surface,
and the lifting surfaces are always cambered.
Propeller--See ``Air-Screw.''
Propeller, Tractor--An air-screw mounted in front of the main lifting
surface.
Propeller, Pusher--An air-screw mounted behind the main lifting surface.
Pusher--An aeroplane of which the propeller is mounted behind the
main lifting surface.
Pylon--Any V-shaped construction from the point of which wires
are taken.
Power--Rate of working.
Power, Horse--One horse-power represents a force sufficient to raise
33,000 lbs. 1 foot in a minute.
Power, Indicated Horse--The I.H.P. of an engine is a measure of the
rate at which work is done by the pressure upon the piston or
pistons, as distinct from the rate at which the engine does work.
The latter is usually termed ``brake horse-power,'' since it may be
measured by an absorption brake.
Power, Margin of--The available quantity of power above that necessary
to maintain horizontal flight at the optimum angle.
Pitot Tube--A form of air-speed indicator consisting of a tube with
open end facing the wind, which, combined with a static pressure
or suction tube, is used in conjunction with a gauge for measuring
air pressures or velocities. (No. 1 in diagram.)
Pitch, Propeller--The distance a propeller advances during one revolution
supposing the air to be solid.
Pitch, to--To plunge nose-down.
Reaction--A force, equal and opposite to the force of the action producing
it.
Rudder--A controlling surface, usually hinged to the tail, the operation
of which turns an aeroplane about an axis which is vertical in
normal horizontal flight; causes an aeroplane to turn to left or
right of the pilot.
Roll, to--To turn about the longitudinal axis.
Rib, Ordinary--A light curved wooden part mounted in a fore and aft
direction within a surface. The ordinary ribs give the surface
its camber, carry the fabric, and transfer the lift from the fabric
to the spars.
Rib, Compression--Acts as an ordinary rib, besides bearing the stress
of compression produced by the tension of the internal bracing
wires.
Rib, False--A subsidiary rib, usually used to improve the camber of
the front part of the surface.
Right and Left Hand--Always used relative to the position of the
pilot. When observing an aeroplane from the front of it, the
right hand side of it is then on the left hand of the observer.
Remou--A local movement or condition of the air which may cause
displacement of an aeroplane.
Rudder-Bar--A control lever moved by the pilot's feet, and operating
the rudder.
Surface--See ``Aerofoil.''
Surface, Detrimental--All exterior parts of an aeroplane including
the propeller, but excluding the (aeroplane) lifting and (propeller)
thrusting surfaces.
Surface, Controlling--A surface the operation of which turns an aeroplane
about one of its axes.
Skin-Friction--The friction of the air with roughness of surface. A
form of drift.
Span---The distance from wing-tip to wing-tip.
Stagger--The distance the upper surface is forward of the lower surface
when the axis of the propeller is horizontal.
Stability--The inherent tendency of a body, when disturbed, to return
to its normal position.
Stability, Directional--The stability about an axis which is vertical
during normal horizontal flight, and without which an aeroplane
has no natural tendency to remain upon its course.
Stability, Longitudinal--The stability of an aeroplane about an axis
transverse to the direction of normal horizontal flight, and without
which it has no tendency to oppose pitching and tossing.
Stability, Lateral--The stability of an aeroplane about its longitudinal
axis, and without which it has no tendency to oppose sideways
rolling.
Stabilizer--A surface, such as fin or tail-plane, designed to give an
aeroplane inherent stability.
Stall, to--To give or allow an aeroplane an angle of incidence greater
than the ``maximum'' angle, the result being a fall in the lift-
drift ratio, the lift consequently becoming less than the weight of
the aeroplane, which must then fall, i.e., ``stall'' or ``pancake.''
Stress--Burden or load.
Strain--Deformation produced by stress.
Side-Slip, to--To fall as a result of an excessive ``bank'' or ``roll.''
Skid, to--To be carried sideways by centrifugal force when turning
to left or right.
Skid, Undercarriage--A spar, mounted in a fore and aft direction, and
to which the wheels of the undercarriage are sometimes attached.
Should a wheel give way the skid is then supposed to act like the
runner of a sleigh and to support the aeroplane.
Skid, Tail--A piece of wood or other material, orientable, and fitted
with shock absorbers, situated under the tail of an aeroplane in
order to support it upon the ground and to absorb the shock of
alighting.
Section--Any separate part of the top surface, that part of the bottom
surface immediately underneath it, with their struts and wires.
Spar--Any long piece of wood or other material.
Spar, Main--A spar within a surface and to which all the ribs are
attached, such spar being the one situated nearest to the centre
of pressure. It transfers more than half the lift from the ribs
to the bracing.
Spar, Rear--A spar within a surface, and to which all the ribs are
attached, such spar being situated at the rear of the centre of
pressure and at a greater distance from it than is the main spar.
It transfers less than half of the lift from the ribs to the bracing.
Strut--Any wooden member intended to take merely the stress of
direct compression.
Strut, Interplane--A strut holding the top and bottom surfaces apart.
Strut, Fuselage--A strut holding the fuselage longerons apart. It
should be stated whether top, bottom, or side. If side, then it
should be stated whether right or left hand. Montant.
Strut, Extension--A strut supporting an ``extension'' when not in
flight. It may also prevent the extension from collapsing upwards
during flight.
Strut, Undercarriage--
Strut, Dope--A strut within a surface, so placed as to prevent the
tension of the doped fabric from distorting the framework.
Serving--To bind round with wire, cord, or similar material. Usually
used in connection with wood joints and wire cable splices.
Slip, Propeller--The pitch less the distance the propeller advances
during one revolution.
Stream-Line--A form or shape of detrimental surface designed to
produce minimum drift.
Toss, to--To plunge tail-down.
Torque, Propeller--The tendency of a propeller to turn an aeroplane
about its longitudinal axis in a direction opposite to that in which
the propeller revolves.
Tail-Slide--A fall whereby the tail of an aeroplane leads.
Tractor--An aeroplane of which the propeller is mounted in front of
the main lifting surface.
Triplane--An aeroplane of which the main lifting surface consists of
three surfaces or pairs of wings mounted one above the other.
Tail-Plane--A horizontal stabilizing surface mounted at some distance
behind the main lifting surface. Empennage.
Turnbuckle--A form of wire-tightener, consisting of a barrel into each
end of which is screwed an eyebolt. Wires are attached to the
eyebolts and the required degree of tension is secured by means
of rotating the barrel.
Thrust, Propeller--See ``Air-Screw.''
Undercarriage--That part of an aeroplane beneath the fuselage or
nacelle, and intended to support the aeroplane when at rest, and
to absorb the shock of alighting.
Velocity--Rate of displacement; speed.
Volplane--A gliding descent.
Weight--Is a measure of the force of the Earth's attraction (gravity)
upon a body. The standard unit of weight in this country is
1 lb., and is the force of the Earth's attraction on a piece of platinum
called the standard pound, deposited with the Board of Trade
in London. At the centre of the Earth a body will be attracted
with equal force in every direction. It will therefore have no
weight, though its mass is unchanged. Gravity, of which weight
is a measure, decreases with increase of altitude.
Web (of a rib)--That vertical part of a rib which prevents it from
bending upwards.
Warp, to--To distort a surface in order to vary its angle of incidence.
To vary the angle of incidence of a controlling surface.
Wash--The disturbance of air produced by the flight of an aeroplane.
Wash-in--An increasing angle of incidence of a surface towards its
wing-tip.
Wash-out--A decreasing angle of incidence of a surface towards its
wing-tip.
Wing-tip--The right- or left-hand extremity of a surface.
Wire--A wire is, in Aeronautics, always known by the name of its
function.
Wire, Lift or Flying--A wire opposed to the direction of lift, and used
to prevent a surface from collapsing upward during flight.
Wire, Anti-lift or Landing--A wire opposed to the direction of gravity,
and used to sustain a surface when it is at rest.
Wire, Drift--A wire opposed to the direction of drift, and used to
prevent a surface from collapsing backwards during flight.
Wire, Anti-drift--A wire opposed to the tension of a drift wire, and
used to prevent such tension from distorting the framework.
Wire, Incidence--A wire running from the top of an interplane strut to
the bottom of the interplane strut in front of or behind it. It
maintains the ``stagger'' and assists in maintaining the angle
of incidence. Sometimes termed ``stagger wire.''
Wire, Bracing--Any wire holding together the framework of any part
of an aeroplane. It is not, however, usually applied to the wires
described above unless the function performed includes a function
additional to those described above. Thus, a lift wire, while
strictly speaking a bracing wire, is not usually described as one
unless it performs the additional function of bracing some well-
defined part such as the undercarriage. It will then be said to
be an ``undercarriage bracing lift wire.'' It might, perhaps,
be acting as a drift wire also, in which case it will then be de-
scribed as an ``undercarriage bracing lift-drift wire.'' It should
always be stated whether a bracing wire is (1) top, (2) bottom,
(3) cross, or (4) side. If a ``side bracing wire,'' then it should be
stated whether right- or left-hand.
Wire, Internal Bracing--A bracing wire (usually drift or anti-drift)
within a surface.
Wire, Top Bracing--A bracing wire, approximately horizontal and
situated between the top longerons of fuselate, between top tail
booms, or at the top of similar construction.
Wire, Bottom Bracing--Ditto, substituting ``bottom'' for ``top.''
Wire, Side Bracing--A bracing wire crossing diagonally a side bay
of fuselage, tail boom bay, undercarriage side bay or centre-section
side bay. This term is not usually used with reference to incidence
wires, although they cross diagonally the side bays of the
cell. It should be stated whether right- or left-hand.
Wire, Cross Bracing--A bracing wire, the position of which is diagonal
from right to left when viewing it from the front of an aeroplane.
Wire, Control Bracing--A wire preventing distortion of a controlling
surface.
Wire, Control--A wire connecting a controlling surface with the pilot's
control lever, wheel, or rudder-bar.
Wire, Aileron Gap--A wire connecting top and bottom ailerons.
Wire, Aileron Balance--A wire connecting the right- and left-hand top
ailerons. Sometimes termed the ``aileron compensating wire.''
Wire, Snaking--A wire, usually of soft metal, wound spirally or tied
round another wire, and attached at each end to the framework.
Used to prevent the wire round which it is ``snaked'' from becoming,
in the event of its displacement, entangled with the
propeller.
Wire, Locking--A wire used to prevent a turnbuckle barrel or other
fitting from losing its adjustment.
Wing--Strictly speaking, a wing is one of the surfaces of an ornithopter.
The term is, however, often applied to the lifting surface of
an aeroplane when such surface is divided into two parts, one being
the left-hand ``wing,'' and the other the right-hand ``wing.''
Wind-Tunnel--A large tube used for experimenting with surfaces and
models, and through which a current of air is made to flow by
artificial means.
Work--Force X displacement.
Wind-Screen--A small transparent screen mounted in front of the
pilot to protect his face from the air pressure.
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