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.
[Illustration: Cross Sectional area]
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.
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.
[Illustration]
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:
[Illustration]
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.
[Illustration]
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._
[Illustration]
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.
[Illustration: Strut straight. Wires and gap correctly adjusted. Strut
bent throwing wires and gap out of adjustment.]
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.
[Illustration: Strut bedded properly. Strut bedded badly.]
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.
[Illustration]
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 necessary
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:
[Illustration]
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 roughness 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 roughness 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.
[Illustration: Distortion of upper wing caused by auxiliary lift wire
being too tight.]
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 much as possible. The rules to be observed are as
follows:
[Illustration: Wrong shape. Result of wrong shape. Right Shape.]
(_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.
[Illustration: Position in which controlling surface must be rigged. It
will be its position during flight.]
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.
[Illustration: Position during flight. Position in which controlling
surface must be rigged.]
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:
[Illustration]
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:
[Illustration: FRONT ELEVATION and PLAN.]
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 their ends to struts. 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:
[Illustration: Points A, B, and C, must be the same fixed distances
from the butt as are Points D, E, and F. Distances 1 and 2 must equal
distances 3 and 4.]
The above applies to both front and rear bays.
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 beforehand. 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:
[Illustration]
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.
[Illustration]
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:
[Illustration]
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 inch or more to the stagger,
with the certain result that the aeroplane 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.
[Illustration: The dotted lines on the surface represent the spars
within it.]
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 horizontal 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.