*Types of Ribs*. The rib first used by the Wright Brothers consisted of
two spruce strips separated by a series of small pine blocks.
Practically the same construction was used by Etrich in Austria. With
the coming of the monoplane, and its deep heavy spars, the old Wright
rib was no longer suitable for the blocks were not efficient in thick
wing sections. The changes in the wing form then led to the almost
universal adoption of the "I" type rib in which an upper and lower
flange strip are separated by a thin vertical web of wood. At present
the "I" rib is used on nearly every well known machine. It is very
strong and light, and is capable of taking up the end thrust of the drag
wires, as well as taking care of the bending stresses due to the
vertical loading or lift.
Fig. 1 shows the original Wright rib with the "Battens" or flanges (g)
and the spacing blocks B. The front spar is at the leading edge (F), and
the rear spar at S. An "I" beam, or "Monoplane" type is shown by Fig. 2,
and as will be seen is more suitable for deep spars such as (F’) and S’.
The upper and lower flanges (g) are separated by the thin perforated web
(w), the sectional view at the right showing the connection between the
flanges and the web. Lightening holes (h) reduce the weight of the web,
with enough material left along the center of the web to resist the
horizontal forces. The web is glued into a slot cut in the flanges, and
the flanges are then either tacked to the web with fine nails, or bound
to it by turns of thread around the flange.
On the average machine, the web is about 3/16 thick, while the flanges
are from 3/4 to 1 inch wide, and from 3/16 to 1/4 inch thick. On the
very large machines, the dimensions of course are materially increased.
At the strut locations in biplanes, and the point of cable bracing
attachment in monoplanes, the ribs are increased in strength unless the
end thrust of the stay wires is taken up by a separate strut. At the
point of stay connection in the old Nieuport monoplane the rib was
provided with a double web thus making a hollow box form of section
great enough to account for the diagonal pull of the stays.
[Illustration: Fig. 1. Wright Type Rib with Battens and Block
Separators. Fig. 2. Monoplane or "I" Type of Rib with Solid Web.]
Fig. 3 shows the Nieuport monoplane ribs, which are good examples of box
ribs. The sections at the left are taken through the center of the ribs.
The wing chord tapers from the body to the wing tip, while the thickness
of the wing section is greatest at the middle, and tapers down both
toward the tips and toward the body. The upper section is located at the
body, the second is located midway between the body and the tip, and the
other two are near the tips, the bottom being the last rib at the outer
end. The ribs shown are of box form as they are at points of connection,
but the intermediate ribs are of the "I" type shown by Fig. 2.
[Illustration: Fig. 3. Nieuport Monoplane Ribs. This Wing Is Thickest in
the Center and Washes Qut Toward Either End, Thus Making All of the Ribs
Different in Curvature and Thickness. At the Point of Stay Wire
Attachment Double Webbed Box Ribs Are Used.]
*Rib Material*. In American aeroplanes, the flanges of the ribs are
generally made of spruce. The webs are of poplar, whitewood, cottonwood
or similar light material. There is not a great deal of stress on a rib,
and the strongest material is not necessary, but as there are a great
many ribs in a wing assembly lightness is a primary consideration. A few
ounces difference on each rib makes a great deal of difference in the
total weight, especially when there are 80 or more ribs in a complete
machine. Exception to the above materials will be found in the Curtiss
"Super-American" Flying Cruiser which has ribs with pine webs and birch
flanges. European aeroplane practice makes use of hardwood in the ribs.
*Web Stiffeners*. The webs being thin and deep, and cut for lightening
as well, need bracing at the points where concentrated loads are placed,
such as at the front and rear spars, and at points between the
lightening holes. By gluing thin strips to the webs (in a vertical
direction), and so that the tops and bottoms of the strips come tight
against the upper and lower flanges, a great deal of the strain on the
web can be avoided. The stiffening blocks are shown by (x) in Fig. 4,
and are placed on both sides of the front and rear spars F and S, and
also between the lightening holes H.
[Illustration: Fig. 4. Details of "I" Type Rib, Showing Lightening Holes
and Stiffeners.]
*Flange Fastenings*. In the section at the right of Fig. 4 it will be
seen that the web is inserted into a groove cut in the flanges and is
then glued into place. It would be unsafe to trust entirely to the glue,
owing to the effects of aging, moisture and heat, and consequently some
additional means of fastening must be had. It has been customary to nail
through the flange into the web, but as the web is only about 3/16 inch
thick it is likely to split.
An approved method is shown in Fig. 4 in which Irish linen thread wraps
(d) are passed through the lightening holes (H), and over the flanges
(G). The thread is coated with glue before wrapping and after, and when
dry it is thoroughly varnished for protection against moisture. The
bands are spaced from 3 to 4 inches apart. If nails are used they should
be brass nails—never steel or iron.
At the points (F) and (S) where the spars pass through the web, the web
is entirely cut out so that the flanges ordinarily lie directly on the
spars. In this case it is necessary to bevel the spar so that it at
least approximately fits the curve of the flange. Sometimes when a full
size spar is impossible, as in cases where the spar tapers toward the
tips, wood packing pieces may be placed between the flange and the spar;
tapered to make up for the curve. The flanges in any case must be
securely fastened to the spar by brass wood screws as at (e), and the
edges of the web should fit tightly against the sides of the spars.
*Wing Battens*. The wing battens run along the length of the wings, from
end to end, and between the spars, and serve to brace the ribs sideways
as shown in some of the general views of the wing assembly. To
accommodate the battens, the openings (f) are cut directly under the
flange. Usually the battens are thin spruce strips from 3/16 to 1/4 inch
thick and 1/2 inch wide, and should be run through the web at a point
near the stiffeners. The thickness from the top of the flange to the
under side of the lightening hole is from 1/2 to 3/4 inch as indicated
by (C).
*Strength of Wood Ribs*. The strength of a rib for any individual case
can be found by the method used in computing beams, the rib usually
being assumed to have a uniformly distributed load, although this is not
actually the case, as before explained. The greater part of the load in
normal flight is near the front spar, but this shifts back and forth
with the angle of incidence so that there is no real stationary point of
application, and the rib must be figured for the maximum condition. The
total load carried by one of the intermediate ribs is due to the area
between the ribs, or to the unit loading multiplied by the rib spacing
and chord. The portion of the rib between the spars can be calculated as
a uniformly loaded beam, supported at both ends. The entering edge in
front of the spar, and the trailing edge to the rear may be taken as
uniformly loaded beams supported at one end. The proportion of the loads
coming on the ends and center position can be taken from the pressure
distribution diagrams as shown under "Aerofoils."
A number of tests were made on ribs by Mr. Heinrich of the Heinrich
Aeroplane Company, the ribs being built up on short pieces of spar so
that actual conditions were approached. Instead of using a distributed
load, such as usually comes on the rib, a concentrated load was placed
at the center. If the rib were uniform in section the equivalent
uniformly distributed load could be taken as one-half the concentrated
load, but because of the lightening holes this would not be very exact.
It would be on the safe side, however, as such a test would be more
severe than with a uniform load. The ribs were of the same type as shown
in Fig. 5, and were placed 32.5 inches apart. The front spar was 2 7/16
inches deep, the rear spar 2 1/16 inches deep, and the overall depth of
the rib at the center was 3 1/8 inches. The rib flanges were of white
wood 3/4 inch wide, and 3/16 inch thick. In rib No. 1 the web was solid
whitewood, 3/16 inch thick, and in ribs Nos. 2 and 3, the webs were
mahogany three-ply veneer (5/32 inch thick).
*Test of Rib No. 1*. There are 5 lightening holes between spars, with 2
inches of material left between the holes, and 1/2 inch between first
hole and front spar opening. With 95 pounds concentrated load at the
center, the first rupture appeared as a split between the first hole and
spar opening. At 119 pounds, the flanges had pulled away from one side
of the spar, and 1/8 inch away from the web. Full failure at 127.5
pounds. Web was split between each lightening hole with a complete cross
break at center of web, the latter being caused by a brad hole in the
web.
*Test of Rib No. 2*. Laminated web, with no brads driven opposite
lightening holes. At 140 pounds rib deflected 5/16 inch, and when
relieved sprang back only 3/16 inch. With 175 pounds, the deflection
again was 5/16 inch, but the rib continued to bend slowly, the flanges
pulling away from the web and spars. The wood was not broken anywhere,
the failure being in the brads and glue.
*Test of Rib No. 3*. Same materials as No. 2, but web was fitted inside
of the "I" beam spar and the rib flanges were screwed to the spar. At
175 pounds there was no sign of rupture anywhere, and the deflection was
5/16 inch. At 185 pounds the rib broke very suddenly and cleanly, and in
such a way as to indicate that this was the true strength of the rib.
The normal loading on the rib in flight was 17.5 pounds, uniformly
distributed, so that with a concentrated load of 370 pounds equivalent,
the safety factor was 21.1.
The conclusions to be arrived at from this test are as follows:
(1) When a solid soft wood web is used, there should be at least 2 1/2
to 3 inches between lightening holes.
(2) A laminated or three-ply web is the best.
(3) No brads should be driven opposite lightening holes.
(4) The web should fit closely to the spar sides and the flange of the
rib should be tightly screwed to the top and bottom of spar.
The above gives an idea as to the strength of the usual form of wood
rib, and can be used comparatively for other cases if the reader is not
familiar with strength calculations.
[Illustration: Fig. 7. Rib Bending Press for Curving the Rib Flanges.]
*Making the Rib*. Wooden webs are cut out on the band saw, and the webs
are so simple that there is not much more to be said on the subject. The
flanges, however, must be steamed and bent to the nearly correct form
before assembly. After planing to size and cutting the groove for the
reception of the web, the ribs are placed in the steamer and thoroughly
steamed for at least an hour. A rib flange press shown by Fig. 7
consists of two heavy blocks with the inner faces cut approximately to
the rib outline. The steamed ribs are then placed between the blocks,
the bolts are screwed down tight, and is left for 24 hours so that the
strips have ample time to cool and dry. For the amateur or small
builder, the steamer can be made of a galvanized "down-spout" connected
with an opening cut in the top of an ordinary wash boiler. One end of
the spout is permanently sealed, while the other is provided with a
removable cover so that the strips can be inserted. A hole cut near the
center of the spout is connected to the opening in the boiler cover by a
short length of spouting or pipe. The spout should be made large enough
in diameter to contain all of the ribs that can be pressed at one time,
and should be long enough to accommodate longer pieces such as the
fuselage longerons, etc.
When removed from the press, the rib flanges can be glued to the webs
taking care that the glue is hot, and that it thoroughly covers the
groove surface. The rib must now be held accurately in place in a second
form, so that the true rib outline will be retained until the glue drys.
A great deal depends upon the accuracy of the second form, and the
accuracy with which the web outline is cut. The larger manufacturers use
metal rib forms or "jigs," but the small builder must be content with a
wooden form consisting of a board fitted with suitable retaining cleats,
or lugs. The outline of the aerofoil is drawn on the board, the tips of
the cleats are brought to the line, and are screwed to the board so that
they can be turned back and forth for the admission and release of the
ribs. The strip bending press in Fig. 7 is only intended to bend the
flanges approximately to form, and hence two layers may be put in the
press at one time without much error.
*Wing Spars*. In American aeroplanes these members are usually of the
solid "I" form for medium size exhibition and training machines, but for
small fast aeroplanes, where every ounce must be saved, they are
generally of the built up type, that is, made up of two or more members.
In Europe, built up construction is more common than in this country,
and is far preferable for any machine that justifies the additional time
and expense. The wing spars are the heaviest and most important members
in the wing and no trouble should be spared to have them as light as the
strength and expense will permit. They are subjected to a rather severe
and complex series of stresses; bending due to the load carried between
supports, compression due to the pull of the stay wires, bending due to
the twist of non-central wire fittings, stresses due to drag and those
caused by sudden deviations in the flight path and by the torque of the
motors. These should be accurately worked out by means of stress
diagrams if the best weight efficiency is to be obtained.
[Illustration: Fig. 9. Types of Wing Spars. (A) Is the "I" Beam Type.
(B) Box Spar. (C) Is Composite Wood and Steel, Wrapped with Tape.]
A number of different wing spar sections are shown by Figs. 9, 10, 11.
Spar (A) in Fig. 9 is the solid one piece "I" type (generally spruce),
channeled out along the sides to remove the inefficient material at the
center. The load in this case is assumed to be in a vertical direction.
In resisting bending stresses, it should be noted that the central
portion of the material is not nearly as effective as that at the top
and bottom, and that the same weight of material located top and bottom
will produce many times the results obtained with material located along
the center line. At points of connection, or where bolts pass through
the spar, the channeling is discontinued to compensate for the material
cut away by the bolt and fittings.
Spar (B) is of the hollow type, made in two halves and glued together
with hardwood dowel strips. The doweling strips may be at the top and
bottom as shown, or on the horizontal center line as shown by Spar (J).
The material of the box portion is generally of spruce. This is a very
efficient section as the material lies near the outer edge in every
direction, and offers a high resistance to bending, both horizontally
and vertically. Unfortunately a great deal depends upon the glued
joints, and these require careful protection against moisture. There is
absolutely no means of nailing or keying against a slipping tendency or
horizontal shear. The best arrangement to insure against slipping of the
two halves is to tape around the outside as shown by spar (E). This is
strong linen tape and is glued carefully to the spar, and the whole
construction is proofed against moisture by several coats of spar
varnish and shellac. In addition to the strengthening effect of the
tape, it also prevents the wood from splintering in accidents.
[Illustration: Fig. 10. Four Types of Wing Spars, the Spar D Being a
Simple Steel Tube as Used in the Caudron and Breguet Machines.]
Spar (C) consists of a central ash "I" section, with steel strips in the
grooves. Two spruce side strips are placed at either side as stiffeners
against lateral flexure, and the entire construction is taped and glued.
This is very effective against downward stresses, and for its strength
is very compact. Since spruce is much stiffer than either the thin steel
strip, or the ash, it is placed on the outside. Spar (D) in Fig. 10 has
been described before.
Fig. (F) consists of two spruce channels placed back to back, with a
vertical steel strip between them. Again the spruce is used as a side
stiffener, and in this case probably also takes a considerable portion
of the compression load. Spar (G) is a special form of box spar used
when the spar is at the entering edge of the wing, the curved nose being
curved to the shape of the aerofoil nose. In Fig. 11 (H), a center ash
"I" is stiffened by two spruce side plates, the ash member taking the
bending moment, and the spruce the compression. Spar (I) has a compound
central "I," the upper and lower flanges being of ash and the center web
of three ply veneer. The two outer plates are of spruce. This should be
a very efficient section, but one that would be difficult and costly to
build. Fig. (J) is the same as (B), except that the parting lies in a
horizontal plane. Spar (K) has ash top and bottom members, and spruce or
veneer side plate. The resistance of this shape to side thrust or twist
would be very slight. The sides are both screwed and glued to the top
and bottom members.
[Illustration: Fig. 11. Built-Up Wooden Wing Spars, Commonly Used with
European Aeroplanes.]
The front end of a Hansa-Brandenburg wing is shown by Fig. 12, the box
spar and its installation being drawn to scale and with dimensions in
millimeters. The top and bottom are sloped in agreement with the rib
flange curve, and the rib web is strengthened by stiffeners at either
side of the spar. The hardwood dowel strips are at top and bottom as in
Fig. B, and when placed in this position the glued joint is not
subjected to the horizontal shear forces. The walls are thicker at top
and bottom than at the sides, in order to resist the greater vertical
forces. For the same reason is deeper than it is wide. As will be
remembered, the drag is very much less than the lift, and again, the
drag stress is greatly reduced by the internal drag wire bracing.
*Leading Edge Construction*. In the early Bleriot monoplanes the leading
edge was of sheet aluminum, bent into "U" form over the nose of the rib.
In modern biplanes, this edge is generally of "U" form hollow spruce,
about 3/16 inch thick. Another favorite material is flattened steel
tubing, about 1/2" x 1/4", and of very light gauge, the long side being
horizontal. The tube has the advantage of being much thinner and much
stiffer than the other forms, and the thin edge makes it very suitable
for certain types of aerofoils. The wing tip bows are generally of
hollowed ash and are fastened to the spar ends, leading edges, and
trailing edges with maple dowels, the joint being of a long scarfed
form. When the scarfed joint is doweled together, it is wrapped with one
or two layers of glued linen tape. In some types of machines the top
surface of leading edge is covered with thin two ply wood from the
extreme front edge to the front spar. This maintains the aerofoil curve
exactly at the most critical point of lifting, and also stiffens the
wing against the drag forces.
[Illustration: Fig. 12b. An Old Type of Curtiss Biplane Strut
Socket, at Left. At the Right Is a More Modern Type in Which the
Bolts Do Not Pierce the Spar.]
*Trailing Edges*. These are either of thin beveled ash or Steel tube. On
army machines, the rear part of the trailing edge fabric is pierced with
holes about 3/16 inch diameter, the holes being provided with rust proof
eyelets. This relieves an excess of pressure due to rips or tears; one
opening being located between each rib and next to the body.
[Illustration: Fig. 12c. A Standard H-3 Interplane Strut Socket
Is Shown at the Left, the Bolts in This Case Passing on Either
Side of the Spar. Note the Stay Wire Attachment Clips and Pinned
Strut Connection. A German Strut Socket at Right. Courtesy
"_Aerial Age_."]
*Protection of Wing Wood Work*. In protecting the wood framework of the
wings from the effects of moisture, at least three coats of good spar
varnish should be carefully applied, with an extra coat over the glued
surfaces and taping. Shellac is not suitable for this purpose. It cracks
with the deflection of the wings and finally admits water. The steel
parts of the wing should be given two coats of fine lead paint, and then
two coats of spar varnish over the paint. Wires are treated with some
flexible compound, as the vibration of the thin wires, or cables, soon
cracks off any ordinary varnish. The use of shellac cannot be too
strongly condemned; it is not only an indifferent protection, but it
causes the fabric to rot when in contact with the doped surface.
*Monoplane Wing Spars*. A few representative monoplane wing spars are
shown by Fig. 13, the R. E. P., Bleriot XI, and the Nieuport. Except at
points where the stay wires are connected, the Bleriot spar is channeled
out into "I" beam form as indicated in the figure. It will be noted that
the top and bottom faces of the spar are slanted to agree with the
curvature of the ribs. A. steel connection plate is bolted to the sides
of the spar by through bolts, and with a lug left top and bottom for the
top and bottom guy wires. The R. E. P. is also an "I" type, the section.
"A" being taken through the channeled portions, while "B" is taken
through one of the connection points where the beam is a solid
rectangle. The channeling should always stop at connection points;
first, so that the plates have a good bearing surface, and second, to
allow for the material removed by the bolt holes.
[Illustration: Fig. 13. Typical Monoplane Wing Spar
Construction.]
Probably the most interesting of all the spars is the Nieuport, which is
a combined truss and girder type. This spar tapers down from the center
to both ends, being thickest at the points where the guys are attached.
The top flange (J), and the bottom flange (L), are ash, while the side
plates and diagonals (H) are spruce. The diagonals resist the shear, and
are held in place by the tie bolts (I). At the left, the spruce cover
plate is removed, while at the right it is in place with the interior
construction shown in dotted lines. The dimensions are in millimeters.
[Illustration: Fig. 14. Plate Connection for Monoplane Stay Wire
Connection to Spar. A Compression Member or Drag Strut Is Shown
in the Center of the Spar Which Takes Up the Thrust Due to the
Angularity of the Stays and Also the Drag Stress.]
*Location of Spars*. There are a number of items that affect the
location of the spars in regard to the leading edge. The most important
factors in the choice of location are: (1) Shape and depth of wing
section, (2) Center of pressure movement, (3) Drag bracing requirements,
(4) Width of ailerons, (5) Method of attaching the interplane struts.