*General Wing Frame Layout*. In many ways, the frame of the wing is one
of the most important structural parts of the aeroplane. It not only
maintains the proper aerodynamic form of the aerofoil, but also
transmits the air pressure and lift to the body of the machine, and
therefore carries the entire weight of the aeroplane when in flight. In
spite of the heavy loading on this frame it has been brought to a
remarkable degree of strength and lightness. Not only is "Brute"
strength necessary, but it must also be rigid enough to properly retain
the outlines of the aerofoil with the heaviest loadings, hence the
efficiency of the aeroplane greatly depends upon the stiffness as well
as strength. The contour of the entering edge must be particularly
accurate and well supported since it is at this point that the greater
part of the lift is obtained, and where a slight deviation in form will
materially affect the lift and drag.
The fabric surface, on which the air pressure is exerted, must transmit
the pressure and lift to the main structural members through the parts
that give form to the surface. The fabric surfacing, being flexible and
pliant, must be supported at frequent intervals by the forming members
which in effect are similar to the joists of a floor system. The forming
members are then supported in turn by longitudinal beams, or girders,
that transmit the pressure to the point where the load is applied. The
girders not only carry the lifting force, but must also take care of the
drag which acts at right angles to the lift. To pass girders that are
sufficiently strong, and yet within the limits of weight, through the
narrow space between the top and bottom surfaces of the wing is not
always the simplest of problems.
Figs. 1 and 2 show typical wing frames in diagrammatic form, the upper
views are the plans, while at the bottom are sections taken through the
wing. The outlines of the sections are curved to the outlines of the
aerofoil adopted for the wings, and after this outline is drawn out to
scale, we must maneuver our structural members so that they will lie
entirely between the surfaces.
In Fig. 1, the forming ribs are indicated by R, these being the members
curved to the aerofoil form. They are spaced along the length of the
wing at intervals of about one foot and the fabric is applied to the top
and bottom edges of the rib. The ribs are fastened to the front spar F,
and the rear spar S. The spars are equivalent to beams, and are for the
purpose of transmitting the lift of the ribs to the body. A thin strip E
(nosing) running along the entering edge of the wing, serves to hold the
fabric taut at this point and also forms it to the shape of the aerofoil
entering edge. The thin trailing edge strip (T) performs the same
purpose, and the wing outline is completed by the "End bow" (A) which
retains the fabric at the wing tips. Between the front spar F and the
rear spar S is the trussed "Drag bracing," which binds the two spars
into a truss in a horizontal direction, and against the drag of the
surfaces. This consists of the "Drag" wires or cables (d) and the short
wood struts (e), although in many cases the ribs are strengthened at the
point of attachment of the drag wires and serve as struts. The aileron G
is located at the outer tip and is hinged to the rear spar or to an
extension of the rear spar. Between the spars are thin strips known as
"battens" which stiffen the ribs sideways, these are shown by (F).
[Illustration: Fig. 1. (Left) Wing Assembly with Spar to the Rear of the
Entering Edge. Fig. 2. (Right) Assembly with the Front Spar at the
Entering Edge.]
Metal connection clips C, at the end of the wing spars, are for
attaching the wings to the body, or for connection of the two halves of
the upper wing of a biplane. Looking at the lower sectional view we see
the interplane struts of a biplane attached to the front and rear spars
as at (m) and (n). Referring to the plan view, the location of the
struts is indicated by * * * at the points where the drag-bracing is
attached to the spars.
Fig. 2 is a form of wing in which the spar F’ also forms the entering
edge, thus eliminating one part of the wing. One objection to this
construction is that the front spar must necessarily be shallower than
the spar shown in Fig. 1. The rear spar is in the usual location at S’,
the two spars being connected through the usual end bow A’. The trailing
edge T’ may be either a thin strip, or it may be a thin cable as
indicated. This wing is similar to the wing used on the early Wright
machines, and is still used by Farman, Voisin and other European
manufacturers of biplanes. Usually the trailing ends of the ribs
overhang the rear spar for quite a distance, in this type of wing,
giving a flexible trailing edge. The front and rear interplane struts
(m) and (n) are shown, the former connecting with the front spar at a
point near the entering edge.
[Illustration: Fig. 3. Sub-Rib Construction, the Sub-Ribs (r) Are Placed
at the Entering Edge.]
Fig. 3 shows the usual construction except that short "Sub-ribs" marked
(r) are placed between the main ribs R at the entering edge. These short
ribs increase the support and accuracy of the curve at the entering
edge, or else allow wider spacing of the main ribs R. The fabric must be
well supported at this point, not only to maintain the best efficiency
of the aerofoil, but to relieve the stress on the fabric, as it is here
(Top surface) that the greatest suction pressure comes. Should there be
a rip or tear near the entering edge, in the lower surface, the upper
fabric will be subjected to both the pressure underneath and the vacuum
above. This adds fully 25 per cent to the load on the upper facing.
The main spars may be of wood or steel tubing, although the former
material is generally used. They are of a variety of forms, the "I" beam
section, solid rectangular, hollow box, or a combination of plate and I
sections, the total object being to obtain the greatest strength with
the least possible weight. When made up of several pieces of wood they
are known as "Built up" spars.
[Illustration: Fig. 4. Effects of C.P. Movement on Spar Loading]
The load on the spars varies with the total weight carried, and also
with the movement of the center of pressure due to changes in the angle
of incidence. When the center of pressure moves to any extent, the loads
on the two spars may vary between wide limits, and in extreme cases,
either spar may carry the full load. This is shown clearly by Fig. 4, a
section taken through the wing. The front spar F and the rear spar S are
spaced by the distance L, the respective spar loads being indicated by Y
and Z. As before explained, the center of pressure moves forward at
large angles (CP), while at small angles it moves back say to position
(CP’). Should it move back as far as CP-2, the load will come directly
under the rear spar and this member will therefore carry the entire
load. When at the forward position CP, the greater part of the load will
come on the front spar, and only a small portion will now come on S. In
the same way, when at a small angle of incidence, the center of pressure
will be at CP’, a distance (K) from the rear spar. The greater part of
the load will now be on S. The action is the same as if the entire
weight W or lift, were concentrated at the center of pressure.
When intermediate between the two spars, the center of pressure causes a
bending moment in the rib R, and is at a maximum when the CP is midway
between the two spars. It will be seen that the C. P. movement has an
important effect outside of the question of stability, and this travel
must be taken into careful consideration when the strength of the spars
is calculated. To find the load on the rear spar, for example, with the
center of pressure at CP, multiply the lift W by the distance P, and
divide by the spar spacing L. This will give the load Z. With the C. P.
in the same position, the load on the front spar will be the difference
between the total lift W and the load on the rear spar, or Y = W-Z. With
the load at CP’, the load on the front spar will be: Y = WxK/L, and the
load on S will be Z = W - Y.
For example, we will assume that the lift W = 1000 pounds, and the
distance P = 12 inches. The spar spacing L = 30 inches and the center
pressure is at CP. The load Z on the rear spar, will be: Z = WxP/L =
1000 x 12/30 = 400 pounds. The load on the front spar can be found from
the formula, Y = W – Z = 1000–400 = 600 pounds.
[Illustration: Fig. 5. Perspective View of Wing Construction (Rear Spar
Omitted), Showing Hollowed Entering Edge and Built-Up Spar. Rib Is of
the "I" Beam Type. Courtesy "_Flight_."]
Fig. 5. shows a typical form of wing construction (rear spar omitted).
The front spar is of the "Built up type," and the trailing edge is a
flattened steel tube. The rear spar is simply a solid rectangular beam.
A central ash "I" beam is used as the front spar, with vertical spruce
plates on either side. The spruce entering edge, or "nosing," is formed
to the shape of the entering edge and is hollowed out for lightness. The
rib is also of the built up type, the upper and lower flanges are of
spruce and the middle portion (Web) is cotton-wood. At the point where
the spar passes through the rib, the rib flanges pass over, and are
tacked to the spar. The spruce nosing fits closely over the front web of
the rib. The rib flanges are cut away so that the outside of the nosing
will come flush with the flange line of the rib.
[Illustration: Fig. 6. Caudron Monoplane Wing with Steel Tube Spars and
Flexible Trailing Edge. A Slot in the Rear of the Rib Web Permits the
Deflection of the Trailing Edge. Drag Wire Bracing Is Used Between the
Front and Rear Spars. Courtesy "_Flight_."]
A wing of decidedly different construction is the Caudron monoplane wing
shown by Fig. 6, the front and rear spars of this wing being steel tubes
with an entering edge of thin wood. The drag bracing wires may be seen
connected at alternate ribs by small steel plates and the latter also
serve to attach the ribs to the spars. Instead of being cut out
entirely, the webs of the ribs are hollowed out between the spars.
Probably the most unique feature is the construction of the long
flexible trailing ends of the rib at the right. The trailing rib edge is
divided into an upper and lower section by a long slot, the upper
sections being rigid, while the lower edge is thin and flexible. The
flexible edges allow the lower ends of the ribs to give locally, and
reduce the camber when struck by a heavy gust. This aids in the lateral
stability, since the lift is thus considerably reduced at the point of
impact, and it also relieves the wing of unnecessary stresses. The rigid
upper section of the rib acts as a limit stop to the lower half, and
prevents the flexure from exceeding a certain amount. Owing to the
flexibility of the trailing edges a steel wire or cable must be used for
the trailing edge.
[Illustration: Fig. 7. Standard H-3 Wing Construction. Spar, Rib and
Drag Strut Connections at Left. Body Connection Fitting or Hinge at
Right. Note Drag Wire Fittings. Courtesy "_Aerial Age_."]
Details of the framing of the Standard H-3 are shown by Fig. 7. The
figure at the left gives a clear idea of the connections between the
drag struts and spar, while the view at the right shows the body
connection at the end of the spar. I am indebted to "Aerial Age" for
these sketches. The main spar is in a solid piece, channeled out to "I"
beam form, except at the point where the spruce drag strut is attached.
At the end of this strut is attached a sheet steel fitting that affords
a means of connecting the drag wires, and for fastening the strut to the
main wing spar. At the point of attachment, wooden plates are fastened
to either side of the spar. These prevent the fitting bolts and the
fitting from sliding along the spar when subjected to an uneven pull in
the wires. A veneer top and bottom plate still further strengthen this
joint and hold the sub-rib in place. The main ribs are strengthened, at
the point where the spar passes through the rib web, by small vertical
blocks. In the right hand figure the steel clevis is shown bolted to the
spar. A lug for the wing drag wire is brought out from the fitting. The
clevis on the wing engages with a similar clevis on the body of the
machine, and the two are fastened together with a bolt or pin.
[Illustration: Fig. 8. Typical Biplane Wing. Gap for Aileron Shown at
Right End of Wing. Left End Rests Against Fuselage, the Observation Port
Being Cut Out at the Upper Left Hand Corner. Drag Wire Bracing Clearly
Shown. Courtesy "_Aerial Age_."]
Fig. 8 is a photograph of a biplane wing with the framing members
completed and ready for the application of the fabric. At the right is
the opening left for the aileron, and at the left is the observation
port, the latter coming next to the body. As this is a lower wing, the
sockets for the attachment of the interplane struts can be seen on the
upper and near edges of the main spar. Between the spars are very thin
wood strips running with the length of the wing. These are the "Battens"
used for stiffening the ribs between the points of support at the spars.
As the distance between the spars is comparatively great, in respect to
the thickness of the rib flanges, some sidewise support of this kind is
necessary. The drag-bracing cables cross three rib spaces, or are
fastened to every fourth rib. Between the front spar (at the bottom),
and the entering edge, are the small strips that serve as sub-ribs.
Double cross bracing is used at the inner end of the wing (left), while
additional knee braces are placed at the aileron opening, and at the
outer tips. This is necessary to withstand the stresses due to
assembling and handling, rather than for the flight stresses.
Fig. 9 is a Standard Wing ready for covering. Before the fabric is
applied, a narrow cloth strip is wrapped over the trailing edge, as
shown, and is stitched to the frame. This forms a means of stitching the
main covering at the rear edge, where the ends of the upper and lower
surfaces meet.
*Wing Fabric or Covering*. At the present time unbleached Irish linen is
used almost exclusively for covering the wing structure, although in the
early days of flying rubberized fabrics were used to a great extent.
[Illustration: Fig. 9. Standard H-3 Wing Ready for Covering. Opening for
Aileron Flap Shown at Upper Left Hand Edge (Trailing Edge).]
After the linen is stretched on the wing frame, it is given several
coats of a special preparation commonly known as "Dope" to proof the
fabric against moisture. In addition to waterproofing, the dope adds
considerably to the strength of the fabric and shrinks it tightly on the
ribs—much more evenly than could be done by hand. When completely
"Doped," the linen should be proof against the effects of salt water,
moisture, or extreme dryness, and the fabric must be "Drum tight" at
every point on the surface of the wing.
The linen should have a tensile strength of at least 75 pounds per inch
of width in any direction, and weigh from 3.75 to 4.4 ounces per square
yard. It must be wet spun, free from filling matter and uncalendered. As
a usual thing, the width should not be less than 36 inches, although the
width can be altered to meet conditions of rib spacing, etc. The U.S.A.
seaplane specifications (1916) require a minimum strength along the warp
of 75 pounds per inch width, and 85 pounds per inch of width along the
weft. The following table gives the properties of well known wing
fabrics:
*Wing Dope*. Wing dopes are in nearly every case based on
cellulose—either cellulose acetate or nitrate being the most common
base. This has proven far superior to the resin, copal, gum or oil bases
contained in ordinary varnish, since the cellulose of the dope seems to
amalgamate with the cellulose of the flax fiber and bond the whole into
an integral structure. The fact that the dope must be elastic bars the
use of shellac or other hard resin solutions. The solvents used for the
cellulose dopes vary with the makers, some using amyl-acetate,
tetrachlorethane, etc., while others use special secret compounds that
are best adapted for their bases. Many of the solvents give off
poisonous gases in drying, and this must be guarded against by good
ventilation. The vapor of tetrachlorethane is particularly dangerous,
and has resulted in many deaths.
[Illustration: Fig. 10. Complete Framing Plan of Typical Monoplane
Structure. (A) Pilot. (B) Passenger. (M) Motor. (S) Stabilizer. (E)
Elevator. (R) Vertical Rudder.]
Doped surfaces have from 10 to 25 per cent greater tensile strength and
resistance to tearing than the undoped linen, and increases the weight
of the fabric by about 0.7 ounce per square yard for each coat applied.
Under ordinary weather conditions, dope will require from 20 to 40
minutes per coat for drying, and at least one-half hour should be
allowed between each coat. Weather conditions have a great effect on the
action of dope, and with cellulose compounds the best results are
obtained in a clean dry room, well warmed, and without drafts. On rainy
days the linen is damp and the dope does not set well, and this trouble
is not greatly helped by artificial heat. Drafts cause white spots and
streaks, especially if cold air is allowed to enter directly upon the
warm wing surface. To prevent drafts the ventilating ducts should be
near the floor, and as the vapor is heavier than the air, and flows
downwards, this means of ventilation is entirely practicable.
*Applying the Dope*. The number of coats depend upon the character of
the job, but at least five coats should be applied, and preferably
seven. On the best grade of work, the dope is generally covered with
three or more final coats of spar varnish, although this is not
absolutely necessary. For ordinary work, dope alone on Irish linen has
proved very satisfactory for land machines, five coats being the usual
amount applied on exhibition aeroplanes and planes for amateur use.
Seven coats of dope with three coats of spar varnish are specified for
military machines that are to be used on salt water. Seaplanes are
subjected to conditions that are particularly hard on fabric and must be
protected accordingly.
In applying the dope, at least one-half hour should be allowed for
drying between each coat, and more if possible. The first two coats
should be painted on lightly, the purpose being simply to fill the pores
of the fabric and to prevent the succeeding coats from sinking through.
If the first two coats are too heavy, the dope filters through the mesh
of the linen and drops on the lower surface, causing spots and a waste
of a very expensive material. Dope is expensive even with the greatest
care exercised in its application, and the writer has seen cases where
the first two coats were so heavily applied that fully 50 per cent of
the fluid ran through and caked in among the structural parts of the
machine. This ran the doping expense up to a terrific figure. The cloth
should be dry, and the work performed, if possible, on a dry day. To
save dope, never take out of the supply drum more than can be used for
one coat, for the dope soon becomes tacky on exposure to the air, and a
satisfactory job is hard to obtain if it gets in this condition.
[Illustration: Fig. 10a. Testing the Wing Structure of a Curtiss Biplane
by Means of Sand Bag Loading. The Wings Are Turned Upside Down and a
Sand Load Is Laid Uniformly Over the Wings. So That They Produce a Load
Equal to, or Greater Than, the Flight Load.]
In placing the fabric on the wings, particular care must be taken in
stretching so as not to have it too tight when cellulose dope is used.
The dope shrinks the linen to a very considerable extent, and if it is
too tight to begin with, the stress due to shrinkage will place an
excessive stress on both the fabric and the structure. When of the
proper tautness, the fabric should sound like a drum when snapped with
the finger. Any less tension than this will permit the fabric to sag
badly when under the air pressure and reduce the efficiency of the
wings. In fastening the cloth it should be just stretched taut and no
more. In damp weather the cloth can be stretched a little tighter than
in dry weather.
*Transparent Coverings*. In some types of battle-planes and scouts, a
part of the wing section directly above the pilot is covered with a
transparent fireproof cellulose sheet, much resembling celluloid. This
permits the pilot to see above him through the overhanging wing, and is
of great value in action. In some cases, a strip is placed on the lower
wings along the sides of the body so that the ground is also easily
visible. These cellulose sheets will not crack nor splinter, and are
nearly as flexible as rubber. Celluloid of the ordinary variety must not
be used, for this is easily ignited and is likely to start a disastrous
fire.
*Placing the Fabric on the Wings*. In some aeroplanes the seams of the
fabric are run parallel to the ribs, and are tacked or sewed directly to
them, while in other cases the seams are run diagonally across the plane
or on the "bias." Diagonal seams are most satisfactory, and if care is
taken there is no more waste of linen than with the straight seam. The
seams should be of the double-lapped or "English welt" type, and this of
course necessitates sewing before the fabric is placed on the wings. The
seams used on overcoats are satisfactory for this purpose, and give a
covering that will not stretch nor bag. Some use linen thread and others
use silk, but the linen is preferable, since dope often causes silk to
rot. The seams should be covered with linen to protect them from the
weather and to prevent the entrance of water to the interior woodwork.
Ordinarily, the wing is turned upside down for covering, with the
concave side uppermost. The seams are sewed together so that the
completed fabric is wider than the length of the wing and is a little
longer than is necessary to wrap entirely around the width of the wing.
The fabric is then temporarily fastened along the trailing edge, is
passed under the wing to the front edge, and over the concave upper side
back to the trailing edge. At this point the excess material will hang
down over the rear edge. With the wing in its upside down position, the
convex side will be at the bottom, and if a weight is hung on the
overhanging material at the rear edge, the cloth will be pulled tight
against the lower convex side and straight across and above the concave
side. The fabric at the top is then stretched along the cordal line of
the ribs. By laying a narrow board on top of the fabric, and near the
entering edge, the fabric can be brought down uniformly along the
concave edge of the ribs, and by tacking or sewing as the board is moved
back the concave face can be covered without further trouble. After the
concave face is disposed of, the wing can be turned over and the fabric
is then fastened to the convex side of the ribs.
[Illustration: Fig. 11. Method of Stretching Fabric on Wings. Fabric
Passes Under and Then Over Concave Side and Is Pressed Down into Hollow
by a Board as Shown.]
One method of fastening the linen is to lay tape over the ribs, and then
drive tacks through the tape and fabric into the rib. The tape keeps the
tacks from tearing through the linen. The tape should be heavy linen of
from 3/4 to 1 1/4 inches wide, and laid in cellulose before tacking, so
that the tape will be cemented to the fabric and the solution will be
driven into the tack holes. After the tape is in place, it should be
covered with not less than three coats of cellulose dope before the main
surface is treated. This gives an additional three coats over the tape
where it is most needed for protection against moisture. In any case,
the seam or tacking strip should be pressed down so that it projects as
little as possible above the general surface of the wing.
Tacking is not desirable, for tacks and nails always tend to split the
thin members of the rib, and very often corrode the cloth and weaken the
fabric. This has resulted in the whole fabric being ripped off while in
flight. Iron or steel tacks should never be used, as they destroy the
strength of the fabric very rapidly by the formation of rust, and
particularly with sea-planes used on salt water. Sewing the fabric to
the ribs with linen thread is the most satisfactory method and is in
general use among high-grade builders.
The fabric can be stitched to bands of thread or tape, the latter being
wrapped about the rib. Stitches can also be taken from one surface
straight through to the other. The thread or tape bands on the ribs are
merely wrappings taken around the rib flange, and through the lightening
holes, these bands usually being about 4 inches apart. They at once tie
the flange to the web and form a soft surface into which to take the
stitches. Fig. 4 in Chapter X shows the thread bands (d) wrapped around
the rib flange (G), and through the lightening hole, the fabric lying
above and below the rib flanges as shown. A section view through the rib
and fabric is shown at the right.
*Varnishing*. When varnish is to be used over the dope, only the best
grade of spar varnish should be used, since any other kind is soon
destroyed by moisture. From two to three varnish coats will be
sufficient, and each coat should be thoroughly dried and sandpapered
before the next is applied, care being taken not to injure the fabric
with the sandpaper. Sandpapering between the dope layers is not
necessary, since each successive coat partially dissolves the preceding
coat, and thus welds the layers together. Varnish, however, does not act
in this way, and the coats must be roughened. Shellac rots the linen and
should not be used.
The Government specifies one coat of flexible white enamel in which a
small quantity of lead chromate is mixed. This is applied over the last
coat of varnish. The lead chromate filters out the actinic rays of the
sun, thus reducing the injurious effect of the sunlight on the covering.
If coloring matter is added to the dope, it should be in liquid form, as
powders destroy the strength and texture of the dope deposit.
[Illustration: Fig. 12. Wing Structure of Handley-Page Giant Biplane.
Courtesy "_Aerial Age_."]
*Patching Fabric*. The majority of dopes can be used as cement for
patching, but as dope will not stick to varnish, all of the varnish
around the patch must be thoroughly removed with some good varnish
remover. The varnish must be thoroughly cleaned off or there will be no
results. Before applying the dope, the patch must be well stitched all
around the edges, then cemented with the dope. The patch must now be
covered with at least five coats of thin dope as in finishing the
surface. Particular attention must be paid to filling the dope in the
stitching.