Wing fittings. In designing wing-root fittings, care should be taken to box or brace the extending flat ears at the attachment bolts or pins so that the drag loads will not induce appreciable bending stresses in the ears. This also applies to fittings at strut, or wire attachment fittings that may have side load components as well as compression and tension loads. This can usually be accomplished by welding on a shear plate to form a three-sided box, or by bracing with an external web on one or more sides of the fitting. If clevis pins are used in place of bolts to connect the two parts of a fitting, the fitting should be designed accordingly. Particular care should be made to provide the extra rigidity in fittings that would normally be provided by the clamping action of bolts and nuts. It should be pointed out that the load distribution in a particular fitting may differ when the attaching bolts and nuts are replaced by clevis pins and safety pins.

Bolts for attaching fittings to spars are passed directly through the spar, or through suitable bushings which have for their purpose to increase the bearing area in the wood. Care should be taken not to weaken the spar by too close spacing of bolts, or reduction of the effective section moment of inertia below the critical value. If necessary, the spar can be padded out locally by laminating to increase the spar width. Ash or maple is sometimes used for this when the stress is high as it gives greater bearing strength under the bolts. Unless ash or maple is used under fittings, soft wood should always be built up locally with a layer of birch plywood to prevent crushing under the fittings and bolts, and to prevent splitting of the spar.

Care should be taken to have all bolts in wood spars stressed only axially and/or in shear, wherever possible, as bending on a bolt is more likely to split the wood. Bolts passing through wood should always be provided with the large bearing washers where there is no fitting to serve this purpose. For grouped bolts, it is desirable to provide a single plate on the back side to distribute the load over a large area, rather than to provide separate washers for each bolt. Examples of good and bad fitting designs are given in fig. 3-VII. Fabric covering.

The fabric should be well finished with dope to provide a smooth surface essential to high performance.

. A rough surface is conducive to high skin-friction drag. Since the drag of a well designed high performance glider is mostly of this type and form drag is reduced to a minimum, it is important to get the best surface finish possible.

. All handholes through the fabric and holes where controls, et cetera, enter should be properly reinforced by inspection rings or frames.

• When covering a wing with fabric, it is common practice to pass the fabric around the leading edge.

Metal covered wings.-Metal covered wings should be free from buckling or wrinkling of the metal covering. Deflections or deformations at low load factors which may result in fatigue failures also should be avoided.

CONTROL SURFACES DESIGN

a. Trailing edges of control surfaces should be as substantial as the trailing edges of wings. They should hold their shape when subjected to fabric tension loads.

b. The covering of control surfaces should be provided with holes for drainage and "breathing."

c. An improvement in control and reduction in drag can be accomplished by covering the gap between control surfaces. Thin metal or celluloid on the outside of the gap, or fabric inside, is frequently used for this purpose. On some designs a circular leading edge on the moving surface fits snugly into the other surface so that the resulting small gap needs no other seal. Care should be taken that the gap covers do not obstruct free drainage, or create undue friction, and that interference cannot occur.

d. The stabilizer on experimental designs should be arranged so that the incidence can be changed if necessary to obtain the proper balance.

e. On wings which have no means of adjustment for twist, a small metal tab fastened to the trailing edge of one aileron can be bent as necessary to trim the wing laterally. In extreme cases this may be necessary on the rudder also.

f. If the elevator is unusually low so that it drags in high grass, it may be advisable to cover it with the heavier Grade A fabric instead of the light glider fabric.

g. The leading edge of wood fins and stabilizers usually is covered with plywood to hold the airfoil contour. Spars may be built up or solid. Ribs are trussed as in wings unless the surface is thin, then ribs are usually built with solid plywood webs.

h. Wood tail ribs are trussed in the usual way with diagonals in compression under normal flight. A minimum practical size for caps and diagonals is about 1 by 1/4 inches, when using gussets. When the trussing is glued directly to the caps without gussets the material must be wider and thinner to provide sufficient gluing area. joints should be made as concentric as is practical.

All

i. It will be found very convenient for construction purposes to design the wood wing ribs and aileron or flap ribs together so that the two surfaces can be built up as a unit with their respective spars in place and parted after completion. This avoids tedious lining up and independent jigging.

j. Special care should be taken to provide sufficient strength to carry concentrated loads from aileron and flap hinges into the main structure. A separate structure is usually not necessary to carry these loads, but the ribs at these points should be somewhat oversize with respect to the hinge reactions because of the redundancy between aileron and wing structure. The control horn reactions in a fore and aft direction also should be considered. It is desirable to have the control horn at a hinge so that the control load can be reacted through the hinge without bending the beam in the control surface. This also eliminates undesirable flexibility in the control system.

k. Control surface horns are generally constructed of plywood or metal, preferably the latter. They should be attached to the control surface spars near a hinge to avoid bending the spar, and in such a manner as to distribute the loads into the spar without tending to split it.

7. Aileron and flap surfaces, as well as tail surfaces, are designed to carry the loads from the control horn either by adopting a torsiontube construction, or using diagonal ribs. The first method is carried out by boxing in the leading edge of the surface with plywood, providing cap strips at the corners of the box for gluing. In the latter method, the surface ribs are laid out in a continuous zigzag truss from the horn.

m. Long ailerons often have two control horns in order to provide extra torsional stiffness and keep the surface from feeling "rubbery" under load.

n. Although many high performance ships have continued to use external control surface horns, it is believed that internal horns are a worthwhile attempt to reduce drag.

Control surface stops.-Stops are advisable for all control surfaces, particularly adjustable stabilizers and elevator trailing edge tab systems. For these, the stops should be positioned so as to limit the travel to the approved range. In general, stops are advisable for all surfaces in order to avoid interferences and possible damage to the parts concerned, particularly in the case of large surfaces where the deflections in the control system may permit the surface to exceed the design range of travel. Stops should be installed at the control systems in the cockpit and also at the control surfaces.

Hinges.

a. Hinges of the strap type bearing directly on torque tubes are advisable only in the case of steel torque tubes which have a multiplying factor of safety as specified in Chap. 1. In other cases sleeves of suitable material should be provided for bearing surfaces.

b. Clevis pins may be used as hinge pins provided that they are made of suitable material and are properly locked.

c. The following points have been found of importance in connection with hinges:

1. Provisions for lubrication should be made if self-lubricated

or sealed bearings are not used.

2. The effects of deflection of the surfaces, such as in bending, should be allowed for, particularly with respect to misalignment of the hinges. This may also influence spacing of the hinges.

3. Sufficient restraint should be provided in one or more brackets to withstand forces parallel to the hinge center line. Rudders, for instance, may be subjected to high vertical accelerations in ground operation.

4. Hinges welded to elevator torque tubes or similar components may prove difficult to align unless kept reasonably short and welded in place in accurate jigs.

5. Piano type hinges are acceptable with certain restrictions. In general, only the "closed" type should be used, that is, the hinge leaf should fold back under the attachment means. The attachment should be made with some means other than wood screws, and this attachment should be as close as possible to the hinge line to reduce flexibility. Piano hinges should not be used at points of high loading, such as exist at control horns, unless the reaction is satisfactorily distributed. Due to the difficulty in inspecting or replacing a worn hinge wire, it is better to use several short lengths than one long hinge. Installation.

a. Movable tail surfaces should be so installed that there is no interference between the surface or their bracing when any one is held in its extreme position and any other is operated through its full angular movement.

b. It is very important that control surfaces have sufficient torsional rigidity. No specific limits of permissible maximum deflection of the surface alone are offered, since these may vary widely with the type, size and construction of the surface. However, the behavior of the surface during proof tests should be closely observed. In addition the effect of the control system "stretch" on the total surface deflection under limit maneuvering loads should be considered from the standpoint of "surface usefulness."

c. Clearances, both linear and angular, should be sufficient to prevent jamming due to deflections or to wedging by foreign objects, particularly safety pins. It is common practice in the design stage to incorporate an angular clearance of 5 degrees beyond the full travel limit. Surfaces and their bracing should have sufficient ground clearance to avoid damage in operation, or when one wing tip is resting on the ground.

d. External wire bracing on tails is subject to vibration and the design of the wire assembly and end connections should be such as to