TRUING UP THE FUSELAGE
Before an airplane is assembled for the first time after leaving the
factory, and especially after it has made its first few “breaking-in”
flights, the fuselage or basic framework should be carefully examined
and checked up. This is done in order to determine whether or not the
fuselage became distorted from rough usage during shipment (which is
always likely) or from taking sets due to the flying stresses to which
it was subjected for the first time during the “breaking-in” flights.
It frequently happens that rough landings and “stunt” flying cause
distortions of the fuselage frame and other parts of the airplane so
that it is very necessary to make a careful inspection immediately
after to ascertain not only what twists, bows and stretching of vital
parts have resulted, but also to detect fittings, wires, etc., which
may have been pulled loose or broken. The extreme importance of having
your airplane adjusted correctly and carefully, and to know that it is
in the proper condition can not be reiterated too often. And, since
the fuselage is the foundation from which, so to speak, the entire
apparatus is built up, it is doubly important that it should always be
in correct adjustment.
When the fuselage is built in the factory it is placed on a long table
whose surface is perfectly horizontal and which has metal strips
inlaid. This table in reality is a big face plate especially arranged,
as described, for fuselage truing in the factory. The fuselage, of
course, has had none of its coverings applied when it is placed on the
table, nor are the accessories such as controls and engine in place.
On this table then the builders begin to do the necessary adjusting
and this is no simple or quick job. Working from a perfectly smooth
horizontal surface it is, of course, easy to detect warpings, twists,
etc., of the framework. These are first remedied by tightening or
loosening of cross wires, etc., as the case may be. Then, when the
fuselage is reasonably square and level, lengthwise and crosswise, as
determined by the eye, check measurements are taken by rule, trams and
level and final adjustments made to bring the various parts in final
proper relation to one another. For instance, the rudder post must be
perfectly vertical, as determined by a plumb line, when the engine
bearers or the top longerons are level. The various fittings such as
those for horizontal and vertical stabilizers and the engine sections
and side panels must all conform accurately to one another so that
the airplane as a whole, when it is assembled, will not contain any
inherent defects such as tail planes with slightly distorted angles of
incidence, left main panels ahead of right or over or under right main
panels, fittings so located that an initial strain must be imposed
upon them by forcing them together, etc.
After the fuselage has been lined up in the factory as described
briefly above, it is permitted to set for a week or so and then it is
checked up again and such additional slight corrections made which
would be necessitated by the sets which had occurred. The additional
fittings required are then applied and the fuselage finally covered and
sent away to have the engine and instruments applied.
When checking and truing a fuselage on the flying field after the
airplane has been assembled and flown the process is not quite so
simple as when the fuselage is checked up and trued in the factory,
largely owing to the lack of ideal factory facilities and also because
so many fittings, coverings, etc., are in the way which one must
always be cautious about removing. In general, the method of procedure
may be outlined as follows, but it must be obvious that one can not
in a series of written notes touch upon all the possible queries and
combinations of fuselage distortions which may occur and the ways for
detecting and correcting them. A certain amount of experience in the
field accompanied with some fixed habits of inspection, and everlasting
curiosity about the perfections of your machine, and a willingness and
readiness always to pitch in and help correct the defects found, will
soon develop in you the ability to diagnose easily and quickly and
remedy intelligently whatever trouble you may run across.
For satisfactory fuselage checking and truing let us say in the field
shop, a certain minimum equipment of tools is necessary. This equipment
is:
At least two sawhorses about 3 to 4 ft. high for mounting the fuselage
in flying position.
Several wooden wedges (show taper) for easy adjustment of fuselage for
cross and lengthwise level.
About 25 yd. of strong linen line for checking center lines.
2 carpenter levels about 2 to 3 ft. long.
4 perfectly formed steel cubes about 1¼ to 1½ in. in size.
1 plumb bob.
1 small screw jack.
1 pair of wood clamps.
1 straight edge about 12 ft. long.
Several small Crescent adjustable wrenches.
Several pliers with wire-cutting attachment.
Pins for manipulating turnbuckles.
1 steel tape.
1 foot rule, 6 ft. long.
1 small brass hammer.
A small work bench equipped with a 3-in. or 4-in. vise.
The fuselage which is to be trued is mounted on the horse with the
wedges between the top horse rails and the lower longerons. These
horses or trestles should be so arranged that about three-fourths of
the fuselage toward the tail sticks out unsupported. In this way it
will take, as near as possible, its normal flying position. It is
always desirable, in fact quite necessary, especially when checking
a fuselage for the first time, to have the airplane’s specifications
as well as a detailed drawing of the fuselage and an assembly of the
airplane as a whole available. The reason for this, of course, is
quite obvious.
The engine bearers and the top longerons are the basic parts from
which the fuselage as a whole is lined up. Consequently the first
thing which is done, when inspecting the fuselage for alignment, is
to test the truth of these parts. This is done by sighting the top
longerons lengthwise to see if they are bowed downward, upward, inward
or outward. As near as possible the fuselage is made level on the
trestles. The steel blocks or cubes referred to in the tool list above
are placed on the longerons and the straight edge and level placed on
these, first crosswise and then lengthwise. A string is stretched over
the top of the fuselage touching the top cross braces and brought as
close as possible to the center of these pieces. This string should
stretch from the rudder post as far forward as possible. Then the
cross wires or diagonal brace wires are sighted to see how close their
intersections agree with this center-line string. Furthermore, the
level is placed on the engine bearers and they are tested for cross
level and longitudinal level. If the engine is mounted in place, but
one point on the bearers will be available for this purpose, but the
check should nevertheless be made. It may also be found that the
longitudinal level of the engine bearers can be tested from underneath
by placing the steel cubes mentioned above on the top of the level
and then holding the level up against the bottom of the bearers. As a
rule, if the fuselage is warped it should be possible to detect this
with the eye, but when engine bearers are out of line this can only be
detected with certainty by the use of the level.
Let it be assumed that the fuselage is out of true. The first parts
to tackle are, of course, the engine bearers. If they should not be
in line they must first be brought so, and afterward kept in this
condition. The diagonal wires at the front of the fuselage should be
adjusted to make this correction. If the bearers are badly out of line
it will, perhaps, be wisest to remove the engine, or at least loosen
it up from the bearers before doing any adjusting for the reason that
it may become strained by serious pulling on the bearers. After the
bearers are in place, it will be safe to bolt the engine fast again.
With the engine bearers temporarily disposed of, the fuselage proper is
tackled. Here the first thing to do is to get the top surfaces of the
longerons level crosswise. Use the spirit level and the two steel cubes
mentioned in the tool list for this purpose. Start at the front of the
fuselage in the cock pit. Adjust the internal diagonal wires until the
level bubble is in its proper place. Then measure these first two sets
of diagonal wires, getting them of equal length. Continue this process
throughout the length of the fuselage until the rear end is reached,
always working from the front.
Lastly, before proceeding to the next operation, try the engine bearers
for level again. If out, make the proper adjustments.
If the centers of the crosswise struts are not marked, this should
first be done before going further. Then stretch a string from No.
1 strut, or as far forward as possible to the center of the rudder
post. All center points on the cross struts, if the fuselage is true
lengthwise, should lie exactly on this string. If not, adjust the
horizontal diagonal wires, top and bottom, working from the front,
until the center-line points all agree. Always check by measuring
diagonal wires which are mates. These should be of equal length. If
not, some wire in the series may be overstressed. In order to pull
the center points on the cross struts over, always stop to analyze
the situation carefully, determining which are the long diagonals and
which the short ones from the way the fuselage is bowed. Then shorten
the long ones and ease off on the short ones, being careful never to
overstress any of the wires.
The last thing to do is to bring the longerons or the center line
of the fuselage into level lengthwise. For this purpose a long
straight-edge, the two cubes, and a spirit level are of advantage,
although simply stretching a string closely over the top of the
longeron may suffice. Then as in the case of removing a crosswise
bow in the fuselage, here too, we manipulate the outside up and down
diagonal wires in bringing the top longerons into their proper level
position lengthwise, always working from the front.
After all this is done it is well to make some overall checks with
steel tape or trams to see how various fittings located according to
the drawings, agree with one another. Since there is a right and a
left side, distance between fittings on these sides may be compared.
And, finally, the engine bearers should be tried again. In short no
opportunity should be neglected to prove the truth of the fuselage as a
whole and in detail.
It might be pointed out that an excellent time to check the fuselage is
when engine is being removed or changed. In fact this time in general
is a good one to give the airplane as a whole, a careful inspection.
After all the necessary corrections have been made and all the parts
of the fuselage brought into correct relation with one another, the
turnbuckles are safety wired and then served with tape to act as a
final protection. The linen covering is reapplied if it had previously
to be removed and the level, empennage wires, panels etc., are placed
in position and aligned as pointed out in the notes on assembly and
alignment.