ERECTING AIRPLANES
Airplanes shipped from the manufacturer or from another field almost
always suffer more or less from shipment or packing. Care must be
exercised in unpacking in order not to do any more damage. Boxes should
be placed with the part marked “Top” uppermost. Cables and wires must
be handled carefully in order not to bend or twist them. Every bent or
kinked wire or damaged turnbuckle must be replaced, or at least brought
to the attention of an inspector.
The order of erection is as follows:
1. Assemble landing gear to fuselage and align landing gear before
putting on main panels.
2. Assemble tail.
3. Assemble engine section and align before attaching main panels.
4. Assemble main panels.
1. =Landing Gear Assembly to Fuselage.=—The landing gear is assembled
by mounting the wheels on the axle, and bolting wheels in place. The
fuselage should now be elevated to receive the landing gear. This
may be accomplished in one of two ways—either by tackle or by shims
and blocking. For either method, first connect up the tail skid.
This is accomplished by pinning up the front end of the skid to the
spring-fitting, and then pinning in the other end to the tail-post
socket.
If block and tackle are used to raise the fuselage, pass a line under
the engine-bed supports or sills just to the rear of the radiator.
To this line attach hook of block. =To avoid damaging or crushing
some part do not attach lifting device to any other point.= With the
fuselage now resting on its attached tail skid, lift the front end
until the lower longeron clips clear the landing gear. When the clips
on the longerons line up with the clips on the ends of the struts of
the landing gear the bolts are passed down through the holes thus
aligned. This places the nuts _on the down side_ of the connection thus
facilitating assemblies and inspection of connections. The castellated
nuts are then put on the bolts and drawn up tight, until the drilled
hole in the bolt is visible through the castle of the nut. Then insert
cotter-pin and spread the two leaves backward over the nut. This locks
the nut in place. When the landing gear has been completely assembled
to the fuselage, the tail of the machine should be elevated and
supported by a horse and blocking until the upper longeron is level.
This can be determined by placing a spirit level on the upper longeron
at the tail or on the two engine-bed sills in machines where these
sills are parallel to the top longeron, as in Curtiss JN-4B.
2. =Horizontal Stabilizer.=—After the upper longeron is levelled up,
the horizontal stabilizer is assembled to the tail of the fuselage.
The horizontal stabilizer is fastened by means of bolts in the top
longeron and the tail post. The nuts are all drawn up tight and
cotter-pinned. The vertical stabilizer is next erected in place.
=3. Vertical Stabilizer.=—The vertical stabilizer is now fastened to
the horizontal stabilizer, first by means of the bolt which passes
up through the forward part of the horizontal stabilizer and then by
means of the flexible stay lines running from the top of the vertical
stabilizer. The forward bolt passes through the clip at the lower front
point of the vertical stabilizer. Draw the nuts up tight and lock with
cotter-pins. Flexible wire cables are attached to vertical stabilizer,
and turnbuckles are used to align and tighten cables. The vertical
stabilizer is further aided in its alignment by the bolt clip at its
toe and by the double clip at its heel. This rear double clip passes
over the two bolts which are attached to the tail post and which hold
down the horizontal stabilizer.
=4. Rudder.=—The control braces are first attached to the rudder.
These braces are so placed that the upper tips point toward the hinge
line. In this fashion the holes will match up. The rudder is mounted
on the tail post and vertical stabilizer by means of the hinges. The
hinge pins are inserted in the hinges, and cotter-pins passed through
the drilled holes in the bottom of the pins. The cotter-pins should be
spread backward as usual.
=5. Elevators or Flaps.=—These are first equipped with the control
braces which are also arranged so that the upper tips point toward the
hinge line. The elevators are mounted to the horizontal stabilizer by
means of the hinges and hinge pins. The hinge pins are kept in their
bearings by the cotter-pins, inserted through the drilled holes in the
bottom of the hinge pins.
=6. Panel Assembly.=—The panels are now to be assembled. Before the
main panels can be connected to the fuselage, the engine section panel
must be erected.
_Engine Section Panel._—The engine section struts are first set into
place in their sockets on the engine section. Then the whole thing
is lifted up to place and the four struts are set into their sockets
on the upper longeron. The bracing wires are attached and the engine
section aligned by means of them (see alignment).
=7. Main Panels.=—The main panels are now to be assembled to the
machine. There are two methods for accomplishing this: first, assemble
panels, struts and wires, before attaching to fuselage; second,
assemble the upper plane to the engine section, and complete assembly.
The first method is the most advantageous, since it permits the setting
of the main panels at the correct stagger and dihedral, and does not
require as much adjustment as the second method, which will be omitted.
=Assembling Panels Together Before Fastening Them to Fuselage.=—All the
main struts will be found to bear a number. These numbers run from 1
to 8, on Curtiss JN-4. The numbers on the Standard run from 1 to 12
including the center section struts. The method used in numbering the
posts is as follows: Starting at post No. 1, with the outer post, on
the left-hand side of the pilot, as he faces his direction of travel,
the posts are numbered successively from No. 1 to No. 4; Nos. 1 and 2
being on the left side and Nos. 3 and 4 being on the right side. The
rear posts are similarly numbered from No. 5 to No. 8, Nos. 5 and 6
being on the left and Nos. 7 and 8 being on the right. This system of
numbering does not include the engine section struts. The plan shows
the system graphically (see Fig. 39).
The system of marking also insures that the struts are not inverted
in their sockets. This is accomplished by painting the number on the
strut, so that when viewed from the pilot’s seat, all numbers can
be read, _i.e._, the numbers are painted on that side of the strut
intended to face the fuselage. If a strut is inverted by mistake, it
can thus be quickly detected. The procedure of assembling panels is as
follows:
1. The upper left-wing panel is first equipped with mast, by inserting
the mast into its socket on the upper surface of the wing. The mast
wire is then connected up to the clips to the right and left of the
mast. Adjust the tension in this wire, by means of turnbuckles, until
the spar becomes straight.
2. Stand the upper left-wing panel and lower left-wing panel on their
“leading” or “entering” edges, properly supporting the panels in
cushioned blocks to prevent damage to the nose. Space the panels apart,
at a distance approximately equal to the length of the struts.
3. Next connect up the diagonal cross wires. These must be loosely
connected up, to permit the easy entering of the posts into the
sockets. The wires are connected before the posts or struts are set in
place, since with the latter in place, the connecting of the wires to
the lugs of the sockets is accomplished only with difficulty. After
these wires are thus connected, insert the posts and bolts into place.
4. Connect up closely the “landing” (single) wires, and “flying”
(double) wires of the outer bay to hold the wings together as a unit.
The outer bay is thus completely wired, though but loosely.
5. The posts that are used for this left side are, according to the
diagram, No. 1, No. 2, No. 5, No. 6. No. 1 is the outer front; No. 2 is
the inner front; No. 5 is the outer rear; No. 6 the inner rear.
6. The wings, as above assembled, are now erected to the fuselage.
Extreme care should be exercised in transferring the wings to the
fuselage, not to strain or break them. In carrying the wings, use
wooden boards placed under the wings, and block up under the wing
beams (which can be easily located), so that these take the strain of
the load. Do not attempt handling assembled wings, using the posts
as carriers; or by attachments to the trailing or leading edges. The
wings should be suitably supported temporarily by suitable sling at
the outer upper post point (not beyond this point) or by a horse,
properly blocked under lower wing at outer lower post point (not beyond
this point) during fitting of wing to machine. The wings will have
the approximate stagger if assembled as above, since the posts are
in place, and the tension cross wires are adjusted to almost correct
length when shipped. Insert the hinge pins through the hinges as now
coupled up, lower hinges first.
The machine is now ready for alignment, perhaps the most important of
the rigger’s duties.
=Alignment of Airplanes.=—The proper alignment of a machine largely
determines the flying qualities of that machine.
The alignment of the fuselage should be done at the factory or in the
repair shop. However, the alignment of the whole machine depends upon
the correctness of the fuselage. Directions for aligning and checking
fuselage are, therefore, given.
The order in which the different parts of a machine should be aligned
is as follows:
1. Alignment of landing gear.
2. Alignment of center section.
3. Alignment of leading edge.
4. Getting both wings the same height.
5. Dihedral angle, if any.
6. Alignment of trailing edge (angle of incidence).
7. Stagger.
8. Droop.
9. Tightening and safetying all wires.
10. Length of struts, positions and fittings, warp in planes.
11. Alignment of ailerons.
12. Alignment of stabilizer.
13. Alignment of elevator flaps.
14. Alignment of rudder.
The tail of the machine should be raised until the fuselage is nearly
horizontal before starting the alignment.
=1. Alignment of Landing Gear.=—When a machine is being assembled, it
is easier to align the landing gear before the wings are put on.
Take the weight off the landing gear by supporting the fuselage on
sawhorses.
The axle should be parallel with the lateral axis of the machine.
The center of the axle should be directly under the center of the
fuselage. This can be secured by either of two methods:
(_a_) _By Measuring Cross Distances._—Loosen and tighten the cross
wires until the cross distances are exactly the same. Take all
measurements from similar points on the fittings to which the wires are
attached.
(_b_) _With Level and Plumb Bob._—Level the fuselage crosswise. Mark
the exact center of the fuselage and drop a plumb bob. Mark the exact
center of the axle. Adjust the cross wires until the plumb bob is over
the center of the axle. Tighten the wires until fairly tight, and
safety them.
=2. Alignment of the Center Section.=—When assembling a machine, the
center section should be aligned before the wings are put on.
When a machine is already assembled, the first thing to do is to loosen
all wires except the landing wires. This is very important, for if one
wire is tightened against another wire, an unnecessary and possibly
a dangerous strain may be put upon some member. The bracing wires
connecting tops of center section struts should be tight enough to hold
the shape of the center section when bracing wires are tightened up.
[Illustration: FIG. 38.—Center section and undercarriage alignment.]
(_a_) _Machines Having No Stagger._—In machines having no stagger, the
struts of the center section should be perpendicular to the propeller
axis. As the upper longerons are usually parallel to the propeller
axis, they may be used as a base line.
Align one side of the center section first, then the other side, and
lastly the front.
From a point at the lower end of one of the front center section
struts (the center of a bolt head for example), measure forward on the
longeron a certain distance. From the same point (center of bolt head)
measure back on the longeron exactly the same distance.
Move the upper end of the strut forward or backward by loosening one of
the bracing wires and tightening the other, until the distance from the
two points on the longerons to some point on the center line at the top
of the strut (center of bolt head) are exactly the same. The strut will
then be perpendicular to the propeller axis. Tighten both wires evenly
until fairly tight. Measure the cross distances (the diagonal distances
between similar points at the upper and lower ends of the front and
rear struts), and align the other side of the center section until its
cross distances are the same as those on the opposite side.
Align the front of the center section by loosening one cross wire and
tightening the other, until one cross distance is exactly the same as
the other cross distance.
(_b_) _Machines Having Stagger._—In machines having stagger, the shape
and position of the center section strut fittings usually determines
the amount of stagger the machine was designed to have (Fig. 38-_a_).
The JN-4 has 10⅝-in. stagger, _i.e._, a plumb line dropped from the
leading edge of upper panel should be 10⅝-in. from leading edge of
lower panel.
Adjust the wires on one side of the center section until the struts
and that side are in their correct positions as shown by the shape
of the fittings. Tighten the wires, measure the cross distances, and
adjust the wires on the other side of the center section until the
cross distances are exactly similar to the first set.
A more accurate method is to drop a plumb line from the leading edge of
the center section and adjust until the line is at the correct distance
ahead of the point on the fuselage where the leading edge of the lower
wing meets it. This point may be determined by measuring the distance
from the inside of the front hinge to the leading edge of the lower
wing and then laying off this distance on the body from the front of
the hinge on the lower longeron. Better still, if the hinges are at
the same distance from the leading edge on both top and bottom wings,
the plumb line may be dropped from the front side of the hinge on the
center section and the stagger measured back to the hinge on the lower
longeron (Fig. 38-_b_). This has the advantage of setting the plumb
line out far enough to clear the fuselage. Also the measurements are
easily made.
Next, adjust the two front wires until one cross distance is exactly
the same as the other cross distance (Fig. 38-_c_).
=3. Alignment of Leading Edge.=—(_a_) _Upper Plane._—The leading
edges of the upper and lower planes of one wing should next be made
perfectly straight. By standing on a step ladder, placed 15 to 20 ft.
to one side, and sighting along the leading edge of the upper plane,
any bow or warp can be easily seen. This should be straightened out by
loosening or tightening the front landing wires. The edge should be
brought in exact line with the leading edge of the center section.
(_b_) _Lower Plane._—After the leading edge of the upper plane has been
made straight, sight along the leading edge of the lower plane. If
there is no warp in the plane, this edge should also be straight.
(_c_) _Align the opposite wing in the same manner._
=4. Getting Both Wings the Same Height.=—Place a small tack exactly in
the middle of the leading edge of the center panel.
Measure from this tack to similar points at the lower ends of the
intermediate and outer struts (Fig. 39). Make these distances the
same on each side by raising or lowering one wing or the other, or by
raising one wing and lowering the other wing, all the while keeping the
leading edges of both wings perfectly straight.
=5. Dihedral.=—The method of setting the wings of a machine at a
dihedral angle is as follows:
Place two tacks in the leading edge of the upper plane, one tack near
the tip of each wing and exactly the same distance out from the tack
in the center section. Stretch a string tightly between the two outer
tacks, until there is no sag in the string.
A dihedral angle of 178° means that each wing has been raised 1°. To
set the wings of a machine at a dihedral angle of 178° for example:
(_a_) Find the natural sine of 1° (0.0175).
(_b_) Multiply this by the distance in inches between the center tack
and one of the outer tacks. The result will give the rise, in inches,
of the string over the tack in the center section.
Raise the wings equally, keeping the leading edges perfectly straight,
until the proper rise shows over the center section.
[Illustration: FIG. 39.—Alignment diagrams.]
=6. Alignment of Trailing Edge (Angle of Incidence).=—(_a_) _Lower
Plane._—The trailing edge should be brought parallel to the leading
edge. This can be done by bringing the rear spar in line with the
leading edge.
Stand squarely in front of the center of the machine 15 to 20 ft.
away. Sight under the leading edge of the lower plane; move forward or
backward until the fittings under the rear spar are just visible. Raise
or lower the trailing edge by loosening or tightening the rear landing
wires, until all of the fittings on the rear spar appear equally under
the leading edge.
(_b_) _Upper Plane._—After aligning the trailing edge of the lower
plane, place a ladder in front of the center of the machine, and sight
under the leading edge of the upper plane. If there is no warp in this
plane, the trailing edge should align with the leading edge.
The objection to this method is that since there are no fittings next
the body on the rear spar, there is room for considerable error in the
angle of incidence.
Reversing the process and finding the angle of incidence at each set
of struts secures the alignment of the trailing edge and removes
the liability to error. To set wings at correct angle of incidence
proceed as follows (Fig. 39): Place the airplane in rigging position,
_i.e._, level up the top longeron or engine bearers. Set the corner
of the straight-edge against the center of the rear spar, level up
the straight-edge, and measure from the top of the straight-edge to
the center of the front spar or to the lowest point of the leading
edge. This must be done next the body and under each set of struts.
(It is useless to make such a measurement between the struts because
of possible warping of the wings.) Unless the wings have a washout or
washin the measurements must agree, thus making the angle of incidence
the same all along the wing. Then the trailing edge must necessarily be
parallel to the leading edge.
=7. Stagger.=—The stagger should be the same all along the wing as it
is for the center section. With the machine in rigging position drop a
plumb line from the leading edge of the upper wing in front of each set
of struts. The distance from the plumb line to the lower edge should
equal the stagger. If there is too much, tighten the diagonal wire
running from the lower rear socket to the upper front socket, being
sure that the other diagonal wire is loosened somewhat. For too little
stagger tighten the latter and loosen the former wire.
Check up the dihedral and alignment of the trailing edges to see if
these have been disturbed while setting the stagger. If not, the droop
may be put in.
=8. Droop.=—To correct for the torque of the propeller, one wing of a
machine is slightly drooped.
In single-motored tractor types, if the propeller turns to the right,
when looking from the rear, the left wing is drooped, and _vice versa_.
The outer rear landing wire of the wing to be drooped should be
loosened until the trailing edge, between the outer and intermediate
struts, appears to be about an inch (for machines of not more than 100
hp.) lower than the rest of the trailing edge. The practice with the
Curtiss JN-4B is to loosen the inner rear landing wire on the left
wing ¼ in. and loosen the outer rear landing wire ½ in. after the angle
of incidence and stagger have been adjusted so that corresponding wires
on the right and left wings are the same length.
=9. Tightening and Safetying All Wires.=—(_a_) After the wing is
drooped, all flying wires should be tightened to the same tension, and
just taut enough to take out all sag.
(_b_) Next tighten all drift or cross wires between the front and rear
struts to the same tension.
(_c_) Drift wires from the wings to the fuselage, and from the wings to
the landing gear, if any, should be tightened last.
(_d_) Safety all turnbuckles. A wire too loose will vibrate when the
machine is in the air.
The flying and drift wires should be so tightened that when they take
the weight of the machine in the air, there will be no sag in the
landing wires.
=10. Length of Struts, Positions of Fittings, Warp in Planes.=—The
above instructions are given for machines that are true, that is,
machines having no bends, warps, or bows in the spars and leading or
trailing edges.
(_a_) Similar struts should be of the same length.
(_b_) Similar fittings occupying similar positions should be spaced
the same. If difficulties are encountered in getting the measurements
to tally, check up the lengths of the struts and the positions of the
fittings.
(_c_) If the planes of a machine are warped, the machine should be so
aligned that the warp is equally divided between both planes.
=11. Alignment of Ailerons.=—Before aligning ailerons, place the
shoulder yoke or wheel controlling the ailerons in the center of its
path of movement.
(_a_) _Trailing-edge Ailerons._—Trailing-edge ailerons should be set
¾ inch lower than the trailing edge of the plane to which they are
attached.
(_b_) _Interplane Ailerons._—Interplane ailerons should be set so that
they are both in the same plane, when in neutral position.
In machines having interplane ailerons, nose heaviness and tail
heaviness may be corrected by setting the trailing edges of the
ailerons up or down.
The proper amount to raise or lower the trailing edges can be
determined only by experimenting with each particular type of machine.
(_c_) The control wires should be just tight enough to eliminate any
lost motion.
=12. Alignment of Stabilizer.=—Support the weight of the tail on the
tail skid.
The rear edge of the stabilizer should be perfectly straight, and
should be parallel with lateral axis of the machine.
Stand behind the center of the stabilizer, and align its rear edge on
the leading edge of the upper plane by sighting. Tighten wires and
safety turnbuckles.
=13. Alignment of Elevator Flaps.=—Set the elevator control in its
mid-position. Adjust the elevator control wires until the flaps are in
their neutral position and both are in the same plane. The wires should
be just tight enough to eliminate any lost motion. Safety turnbuckles.
=14. Alignment of Rudder.=—Set the rudder control (wheel, foot pedals,
or foot bar) in its mid-position. Adjust the rudder control wires until
the rudder is in its neutral position. The control wires should be just
tight enough to eliminate any lost motion. Safety the turnbuckles.
=15. General.=—All connections having been made, carefully go over
each shackle, pin, and turnbuckle, and see that all pins are properly
in place, all nuts on bolts tight and all cotter-pinned. Try out all
controls for action and freedom of movement. See that no brace wires
are slack, yet not so taut that when plucked they “sing.”
=16. Overall Adjustments.=—As a final check, the following overall
measurements should be taken (see Fig. 39).
The straight lines _AC_ and _BC_ should be equal to within ⅛ in. The
point _C_ is the center of the propeller, or in the case of the pusher
the center of the nacelle. _A_ and _B_ are points on the main spar
and must be at the same distance from the butt of the spar. They must
not be merely the sockets of the outer struts as these may not be
accurately placed. _AC_ and _BC_ must be taken from both top and bottom
spars; two measurements on each side of the airplane.
Similarly _FD_ and _FE_ should be equal to within ⅛ in. _F_ is the
center of the fuselage or rudder post. _D_ and _E_ are points marked on
both top and bottom rear spars just as _A_ and _B_ were marked on front
spars.
If these measurements are not correct, it is probably due to some of
the drift or antidrift wires being too tight or too slack. These must
then be located and corrected.
WING COVERING AND PATCHING
The wings are covered with best quality Irish linen which must have a
tensile strength of at least 50 lb. per inch width, undoped, and 70 lb.
when doped.
The linen strips are sewed together on a sewing machine in such a way
that when folded together they form a sort of bag which just slips over
the wing frame. The seams then run diagonally across the wing. The
bag is stretched up loosely and tacked temporarily along the leading
edge. The edges are folded under a little and sewed together along the
leading edge of the wing and the temporary tacks are removed. To hold
the covering up to the ribs, thread is looped through from one side
of the panel to the other around the ribs. The rough surfaces made by
the thread along the ribs and the edges are covered over with strips
of linen pasted on with dope. To make a smooth job, the edges of these
strips are frayed out ⅛ in.
Three or more coats of dope are applied and rubbed down after each
coating is dry. This is then covered over with one or two coats of
varnish to make it more weatherproof and smooth. Varnish also prevents
the dope from peeling off.
Dope shrinks the linen and makes it fit up tight to the framework.
Breaks in the fabric are patched by first removing the dope around the
break with dope remover and then sticking on a patch with dope. This
is applied with a rag instead of a brush in order to prevent the patch
from becoming white. Ten to sixteen coats of dope are then applied over
the patch, each coat being allowed to dry before the next is applied.
FAULTS IN FLIGHT, DUE TO IMPROPER ALIGNMENT AND HOW TO CORRECT THEM
An airplane pilot may experience difficulty with the flying
qualities of his machine. Consequently he should know something
about the conditions which are responsible for the various kinds of
unsatisfactory flying qualities which are more or less characteristic
of airplanes.
In the chapter on “Principles of Flight” the reader has been made
acquainted with such terms as stability, instability, longitudinal
stability, etc. For the purposes of rigging, however, it will be well
to review these terms again.
_Stability_ is a condition whereby an object disturbed has a natural
tendency to return to its first and normal position. Example: a weight
suspended by a cord.
_Instability_ is a condition whereby an object disturbed has a natural
tendency to move as far as possible away from its first position, with
no tendency to return. Example: a stick balanced vertically on your
finger.
_Neutral stability_ is a condition whereby an object disturbed has
no tendency to move farther than displaced by the force of the
disturbance, and no tendency to return to its first position.
Now in order that an airplane may be reasonably controllable, it is
necessary for it to possess some degree of stability longitudinally,
laterally and indirectionally.
_Longitudinal stability_ is its stability about an axis transverse to
the direction of normal horizontal flight, and without which it would
pitch and toss.
_Lateral stability_ is its stability about its longitudinal axis, and
without which it would roll sideways.
_Directional stability_ is its stability about its vertical axis, and
without which it would have no tendency to keep its course.
Whenever an airplane does not fly properly, aside from conditions
arising from engine or propeller trouble, either its longitudinal,
lateral, or directional stability is affected. When its longitudinal
stability is affected we call this condition longitudinal instability;
likewise, regarding lateral stability and directional stability,
referring to these conditions respectively as lateral and as
directional instability. The effect of alignment errors will be
treated under the foregoing respective heads.
=Alignment Errors, Longitudinal.=—
1. _The Stagger May Be Wrong._—The top surface or wing 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
amount, 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 airplane will then be said to be nose-heavy. A ¼-in. error in the
stagger will make a very considerable difference in the longitudinal
stability.
2. _The Angle at Which the Main Surfaces Are Set Relative to the
Fuselage May Be Wrong._—This will have a bad effect especially in the
case of an airplane with a lifting tail plane or horizontal stabilizer.
If the angle of incidence is too great, the machine will have a
tendency to fly “tail-high.” If the angle is too small the airplane may
have a tendency to fly “tail-down.”
3. _The Fuselage May Have Become Warped Upward or Downward._—This would
give the tail plane or horizontal stabilizer an incorrect angle of
incidence. If it has too much angle, it will lift too much, and the
airplane will be “nose-heavy.” If it has too little angle, it will not
lift enough and the airplane will be “tail-heavy.”
4. _The Tail Plane May Be Mounted upon the Fuselage at a Wrong Angle of
Incidence._—If this condition exists, it must be corrected by making a
change at the fittings. If nose-heavy, the tail plane should be given a
smaller angle of incidence. If tail-heavy, it should be given a greater
angle of incidence; 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 smaller angle of incidence than is the main
surface, and if that difference is decreased too much, the airplane
will become uncontrollable longitudinally. Sometimes the tail plane
is mounted on the airplane at the same angle as the main surface, but
it actually engages the air at a lesser angle, owing to the air being
deflected downward by the main surfaces.
=Alignment Errors, Lateral.=—The machine manifests a tendency to fly
one wing down. The reason for such a condition is a difference in the
lifts of the right and left wings, assuming the motor torque is already
taken care of by washout. That may be caused as follows:
1. _The Angle of Incidence of One Wing May Be Wrong._—If it is too
great, it will produce more lift than on the other side of the
airplane; and if too small, it will produce less lift than on the other
side—with the result, in either case, the airplane will try to fly one
wing down.
2. _Distorted Surfaces._—If some part of the surface is distorted, the
lift will not be the same on both sides of the airplane, which, of
course, will again cause it to fly one wing down.
3. _The Ailerons May Be Set Slightly Wrong._—This may be due to one
control cable being longer than the other, or one of the aileron horns
being bent or twisted. This condition can easily be detected by setting
the aileron control—in neutral and checking up the position of the
ailerons.
=Alignment Errors, Directional.=—If there is more resistance on one
side of the airplane than on the other the airplane will, of course,
tend to turn to the side having the most resistance. This may be caused
by the following conditions:
1. _The Angle of Incidence of the Right and Left Surfaces May Be
Unequal._—The greater the angle of incidence, the greater the
resistance. The less the angle, the less the resistance.
2. _If the Alignment of the Fuselage, Vertical Stabilizer, the Struts
or Stream-line Wires Is Not Absolutely Correct._—That is to say, if
they are turned a little to the right or left instead of being in line
with the direction of flight—then they will act as a rudder and cause
the airplane to turn off its course.
3. _If Any Part of the Surface Is Disturbed It Will Cause the Airplane
to Turn off Its Course._—If, owing to the leading edge, spars, or
trailing edge becoming bent, curvature is spoiled, that will result in
changing the amount of resistance on one side of the airplane, which
will then develop a tendency to turn off its course.
=Additional Flight Defects.=—In addition to the foregoing the following
conditions may also exist which cause trouble when flying as well as
when landing:
_Airplane Climbs Badly._—Such a condition, apart from engine or
propeller trouble, is probably due to excess resistance somewhere.
_Flight Speed Poor._—This condition apart from engine or propeller
trouble, is probably due to (1) distorted surfaces, (2) wrong angle of
incidence, or (3) dirt or mud, resulting in excessive skin friction and
weight.
_Inefficient Control._—This is probably due to (1) wrong setting of the
control surfaces, (2) distortion of control surfaces, or (3) control
cables being badly tensioned.
_Will Not Taxi Straight._—If the airplane is uncontrollable on the
ground it is probably due to (1) alignment of the undercarriage being
wrong, (2) unequal tension of shock absorbers, (3) tires unequally
inflated, (4) axle bent, (5) tight wheel and axle, (6) loose spokes
causing wheel to wobble.