Trailing Edges In Wood By Arthur W. J. G. Ord-Hume, EAA 8579 Lake, Sandown, Isle of Wight, England

P

IONEER BUILDERS — and they were homebuilders in the truest sense of the term — treated the trailing edges of their wings with surprising scorn. Again, it could have been a surprising stroke of brilliance at the time — times change and with them ideas. The prime purpose of this part of the wing, thought our homebuilding predecessors, was to bring the chord to an end and orovide something to hook the canvas to. The answer was a piece of wire. The resultant wing presented an attractive scalloped rear edge around which the cloth was stretched in not so attractive wrinkles. A wire trailing edge is simple, fairly effective and light in weight. It does, however, depend to a far greater extent than any other type of member on the resistance of the rib to buckling and transmitted loads. As can readily be appreciated, a knock on the trailing edge wire can be transferred to all the ribs and the ham-fisted mechanic who clouts the hangar door with your wing can probably ruin the entire panel instead of just goofing up a foot or two of trailing edge wood and a rib. Again, the wire trailing edge has no inherent resistance to bending and can deflect downwards — or upwards — under the tension of the fabric, producing a highly unaerodynamic effect which will make itself apparent on anything intended to fly faster than 40 mph. For the same reasons, although not so liable to their effects, the thin tubular metal trailing edge, fixed to the wooden ribs with little wrapper plates, is also to be frowned upon. It is light, simple and effective — but it warps, distorts, buckles and, once dented, proudly proclaims its condition regardless of any action other than Divine Providence or a new section of member. Why use wood? To begin with, it can be glued to the wood ribs and at once forms part of a homogenous structure. It is easy and positive to work, is capable of simple, effective and quick repair and is far, far stronger than any other equivalent trailing edge for a fabric-covered wing. To paraphrase this, the old C.3 Aeronca had Vi in. square capstrips and the trailing edge comprised a flanged "V" section of light alloy which was nailed to the ribs and bound with tape. The whole issue gradually worked loose and one unconsciously found oneself relying on the cotton fabric to keep the wing together. We can consider four basic types of wooden trailing edge member and each is depicted in the accompanying illustrations. Needless to say, there can be variants and mutations on each of them. Figs. 1 and 2 can be thought of as mainly suitable only for sailplanes and gliders, combining extreme lightness with ample strength. Figs. 3 and 4 are suited to the average homebuilt airplane. Let's take an analytical look at each one. Fig. 1 comprises a thin spruce strip, suitably shaped to the streamlined form dictated by the rib. In itself, this strip has little inherent strength, being perhaps only % in.

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by l/2 in. However, by slitting it on a power saw and inserting a long-grained plywood strip into the resultant slot, the effective strength of the little spruce strip is vastly improved. Furthermore, the ply strip provides an adequate method of attaching the member to the ribs and, as it forms its own gusset, no additional plywood reBUILT-UP MEMBER

RIB GUSSET TO KEEP CAPSTRIPS IN CORRECT POSITION

1/16" LONGGRAINED PLY STRIP SLOTTED INTO SPRUCE T/E MEMBER

619 E-lth Ave.

EDGE MEMBER

1

inforcement of the joint is needed. The ply strip should be inset into the actual trailing edge member by about >/4 in. and should protrude between 1 in. and 1V4 in., depending on the rib spacing and the actual size of the spruce member. Normal 12 in. rib spacing dictates about 1 in. protrusion. LOZENGE-SHAPED MEMBER RIB GUSSET TO KEEP CAPSTRIPS IN

CORRECT POSITION

PLYWOOD

GUSSET

__^ J

TV

NAILED FROM

RIB TO T/E MEMBER (CHORDWISE PLY GRAIN)

FIG. 2

Fig. 2 is an all-spruce variation on the design shown in Fig. 1. This is more tricky to prepare but requires no slitting and consequent glueing in of the ply strip. It forms a stronger joint to the rib but requires one gusset on the undersurface (always gusset underneath unless special rea(Continued on next page)

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FIG.

New & Used

D. F. DeLong

SLOTS BETWEEN >•*' CAPSTRIPENDS

EXPERIMENTAL AIRCRAFT ASSN. BOX 229

HALES CORNERS, WIS.

SPORT AVIATION

45

TRAILING EDGES IN WOOD . . . (Continued from preceding page)

MEMBER FITS

UNDER TOP .^CAPSTRIP

FLY SHEAR GUSSET

RECTANGULAR MEMBER SHAPED INTO FINE TRIANGLE

PRIOR TO ASSEMBLY

PLYWOOD GUSSET NAILED TO UNDERSIDE (CHORDWISE PLY GRAIN)

FIG.

3

sons dictate otherwise so as to preserve the top surface) to make a good fixing. This is a light edge member and can be used to advantage on tail surfaces. Fig. 3 is a very robust member and is designed so as to fit flush with the undersurface of the wing. Attachment is with a gusset. For normal 12 in. rib spacing, a 1 in. or ly 4 in. by ¥4 in. member, suitably shaped, is extremely strong. The gusset should be approximately a 3 in. base by 3 in. high triangle with the grain running in the direction of the chord-line of the wing. Fig. 4 is again very strong and has the merit of needing no gussets. It is ideally suited to the high camber MEMBER SLOTS BETWEEN CAPSTRIP ENDS

PLY SHEAR

GUSSET ACROSS CAPSTRIP ENDS

wing where the trailing edge of the rib will accommodate a fairly thick-sectioned member. Conversely, it is unsuitable for a thin wing and should not be used on sections less than about 17 percent. The member can be about IVi in. by J/4 in. and suitably shaped to suit the airfoil section. Why use such heavy sections for the trailing edge of the wing? One of the first things that the "old school" of aero-mechanics was taught was that you never pushed an

airplane by the rear edge of the wing. If you de-rigged an

airplane, you never stood the wing on its trailing edge. Today, you find just about everybody pushes an airplane by the trailing edge. This is possibly sheer indifference to the commercially-built tin flying machine — or just plain pig-headedness! Quite often the wing leading edge is so cluttered up with pitot pipes, fairings, fences, stall warning indicators, buffet strips, landing lamps, sophisticated cow-catchers and so on that the only edge you can stand the wing on is the fragile trailing edge. With these tough, yet light members in your wings, you can be sure that no ham-fisted mechanic is going to push your trailing edge flush with the rear spar, no trailing edge standing will make your wing look like a roller coaster. In conclusion, these four basic design forms can be used equally well on curved wings. Fig. 1 is simply slotted and then glued to a curved strip of ply, the pieces held in a simple jig while the glue sets. Fig. 2 is man enough to be laminated in two strips and so are both Figs. 3 and 4. There is a saying that you should build certain features into an airplane. For the same reason that aspirin is only to ease the discomfort of a common cold — it does not prevent it — I feel that building features into an airplane is only second best. They should be designed in right at the start. It is, after all, the details which count. #

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RECTANGULAR SPRUCE MEMBER SANDED TO SHAPE AFTER ASSEMBLY

Experimental

Aircraft

Assn.

Box 229 — Hales Corners, Wis.

FIG.

4

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