LANDING GEAR DESIGN

a. It is undesirable and unnecessary to mount wheels on an external landing gear structure as is done on power planes for ground clearance. One landing wheel located near the center of gravity (preferably aft) supplemented by a nose skid and either a tail skid or a skid directly aft of the wheel is customary. Some designs dispense entirely with the wheel, and use skids only.

b. A tail-skid may be desirable on some designs to protect the bottom of the fuselage and rudder. If the skid takes much shock on takeoff or landing, the spring leaf type is desirable.

Landing skids.

a. Main skids.-Landing skids of ash or similar material are used on most gliders to take and distribute nose-down landing loads. Skids must be sprung by rubber blocking or other means when pneumatic wheels are not provided to absorb the major part of the shock. Even with wheels, the skid is often sprung.

b. Shock absorption for skids.-Whether "rubber doughnuts" blocks, tennis balls, or other springing is supplied, there should be two or more points of support besides the front anchorage, capable of taking side as well as direct vertical length into the fuselage by means of a flexible boot.

c. Skid design.-Skids should be reasonably easy to replace especially on gliders not provided with a wheel. A minimum size of 1⁄2 x 2 inches is recommended. For use on coarse-surfaced airports, a metal-faced skid should be used.

MAIN GEAR

The wheels of the main landing gear are most satisfactory for all varieties of operations if located close underneath the average CG of the glider. This enables the pilot to hold the tail either high or low as may be desired. When braking, the nose bears on the ground and helps to slow the glider down as well as to kill the lift on the wings. The attitude of the glider when held over on the nose should not allow the tail to rise higher than necessary to kill most of the wing lift, as it would turn over more easily in a tail wind.

Shock absorption.

a. The wheel should be located so as to project enough below the forward skid so that practically full tire deflection is available at a speed slightly above the stall. More than this amount is undesirable if the skid is to protect the tire from rough obstructions.

b. When using a small wheel, with limited tire deflection for shock absorption, it is desirable to spring the skid. Large wheels can take all the loads. Wheels and tires need not be of special glider type, unless a wheel or tire failure will prove dangerous to the particular glider in question.

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c. Regardless of the type or extent of the shock absorbing qualities of the glider, it is advisable, from service considerations, to design a weak and easily replaceable part into the system, the partial failure of which will not damage the remaining landing gear structure. Such procedure will prevent unduly high stresses from being transmitted to the main fuselage or wing structure, and will greatly simplify repairs.

Wheel support structure.—The wheel support structure should be kept as independent as possible of the lift truss system. Otherwise an incipient or unnoticed failure in the wheel or attachment from a bad landing might cause a failure of the lift truss system in flight.

Brakes.-Brakes on landing wheels are necessary to give control over the landing run, such as when landing down wind or down hill. Glider type brakes built into the wheel are available on some size wheels which might be suitable for the heavy two-seaters. For smaller and lighter wheels, a simple brake consisting of a shoe of metal or composition materials pressed down on the circumference of the tire is satisfactory. A control wire is carried forward from the brake to a handle in the pilot's cockpit.

Dual wheels.-Although not as common, landing wheels are sometimes used in pairs. If spaced so that the CG of the glider cannot fall outside of the wheel base, the glider will not lie on the wing tip as with a single wheel.

FUSELAGE DESIGN

a. Purpose. The fuselage is designed to carry the pilot, support the tail surfaces and landing gear with as little weight and drag as is consistent with the general purpose of the glider.

b. Nose. The nose of the fuselage should be made sufficiently strong to give the pilot reasonable protection in case of a crash, as well as carry the launching and towing loads from the towing hook.

c. Landing gear.—The landing wheel and skid, if any, should be well supported structurally. The former should be boxed around so that snow, mud or sand cannot pack into the interior of the fuselage.

d. Ground angle and clearance.-The bottom of the fuselage aft of the wheel should provide sufficient ground angle and clearance so that the wing can be held at a high enough angle of attack for takeoff and landing. An angle on the wing of at least 10 or 12 degrees is advisable, part of which can be provided by setting the wing at an angle of incidence on the fuselage.

e. Tail skid.-The tail end should be provided with a skid or so arranged that the bottom of the rudder is protected from obstructions when landing.

f. Provisions for turn-over.-The fuselage and cabins should be designed to protect the passengers and crew in the event of a complete turnover and adequate provision should be made to permit egress of passengers and crew in such event.

Pilot and passenger compartments.—

a. Ventilation and visibility. The pilots compartment should be so constructed as to afford suitable ventilation and adequate vision to the pilot under normal flying conditions. In cabin gliders the windows should be so arranged that they may be readily cleaned or easily opened in flight to provide forward vision for the pilot.

b. Seats. Seats for passengers should be securely fastened in place in both open and closed gliders, whether or not the safety belt load is transmitted through the seat. (See the applicable TSO for safety belt requirements.) Consideration should be given in the seat installation, to the fact that under some types of operations parachutes will be required. Provisions therefor should be made for parachutes. Shoulder harness may also be necessary in certain operations.

c. Pilot and passenger enclosures.-Removable "scoops" around the pilot should be securely attached to carry the air loads encountered at the maximum gliding speed, but must be easy to release and push off in case the pilot has to bail out. They should be so designed that their removal in flight at high speeds will not injure or inconvenience the pilot or passengers, or block the exits. The nose may be built up around the pilot and only a local portion be removable. Sufficient room should be provided for exit wearing the type parachute for which the seat is designed. A clear fore and aft opening of not less than 24 inches is desirable. The above recommendations also apply to the hinged, sliding or removable canopy types of enclosures.

Steel tube fuselages.

a. General. Steel fuselages are built up by welding round or square tubing into a rigid truss structure. Sections through the fuselage are usually combinations of rectangular, triangular, and diamond shape. The basic shape is extended up to carry the wing and down for the wheel and landing skid. The portion forward of the wing is frequently cantilevered out under the pilot, the top and side fairing being removable as a whole for exit.

b. Load distribution.-Loads from the pilot's seat, belt, wing, and strut attachments, wheel and tail surfaces should be properly distributed into the fuselage trussing. Eccentricities and bending of the truss members should be avoided wherever possible. Usually, the stress analysis can be simplified by elimination of redundant members.

c. Diagonal braces.-Diagonal braces are necessary for stability in rectangular or diamond shaped bulkheads having unsymmetrical loads applied on them. Where one side of a rectangular truss is broken, as for a cockpit opening, the adjacent bulkheads usually require a diagonal to carry the shear around the open rectangle. This is not necessary when an extra over or under truss serves the same purpose. Many gliders have an extra bottom "V" structure which serves to support the skid and wheel under the main structure. It is

not necessary that all the bulkheads in a rectangular fuselage have a diagonal, but an occasional diagonal between the tail and rear wing attachment bulkhead will increase the torsional rigidity and provide means for transferring the load if a member is damaged.

d. Joints.—Wherever, possible joints should be designed for simplicity. Wherever possible, not over six members should intersect at a joint. This is to reduce the likelihood of strain cracks after welding. Joints must not be butted square unless there is no possibility of tension or bending in the member. On members designed for tension rather than for column loads, it is beneficial to make the end joints at a flat angle to the tube axis so as to develop the full tensile capacity of the member.

e. Splices. Splices in tubing should be made by telescoping, the outer tube being cut at an angle or by butting at an angle of 20 to 30 degrees to the tube axis and supporting with an inserted sleeve. External sleeves may also be used. All welds should be at an angle rather than straight around the tube, unless the member is loaded in pure torsion only.

f. Fairing. Truss fuselages can be faired out for better shape which results in lower aerodynamic drag. Strips of wood, tubing, or aluminum alloy sections are used for this purpose. These members should be strong enough to resist the fabric tension and handling loads. The fairing strips are supported by clips on the structure or plywood formers built out from the bulkheads.

Plywood monocoque fuselages.—

a. General.-Plywood monocoque fuselages are either of simplified type with flat faces, or of full curved form. The latter type has the best aerodynamic efficiency. The simple type is built up on four main longerons with flat sides. The top "deck" is flat, round, or "V" shaped, faired down to the tail from the neck carrying the wing. The bottom surface is carried down in "V" shape to support the keelson above the landing skid. Pneumatic wheels are carried between main fuselage bulkheads, and recessed up into the fuselage body. Since the skids are usually sprung on rubber or other means for shock absorption, heavier bulkheads are provided locally to carry the loads into the main fuselage shell.

b. Plywood sizes.—In general, the lightest practical sizes of plywood will be thick enough to carry the design shear loads in the side panels so that diagonals will not be required. Diagonals may be needed at panels where the external loads are high as between the main wing bulkheads, and in the pilot's bay. A minimum thickness of 1 mm. for birch and %4 inch for spruce or mahogany is recommended for fuselage covering.

c. Bulkheads. The bulkheads in this type of fuselage can be made of straight struts fastened at the corners by blocking notched for the longerons and gusseted with plywood.

d. Curved monocoque type.-The curved monocoque type of construction necessitates laying the plywood on in smaller panels where there is compound curvature. Longitudinal plywood seams should be supported by light internal stiffeners. These may run through the bulkheads and serve also as longerons, or be laid only in between as local (intercostal) stiffeners. The rear part of curved fuselages is often made straight conical with oval sections so that there is curvature in only one plane. The plywood then may be laid in long lengthwise panels.

e. Rigidity. Care should be taken that the minimum section just forward of the fin attachment is not so small as to be too flexible under torsional loads from the vertical tail surfaces, causing flutter in rough air or at high speed. This precaution also applies to all kinds of fuselages in which the rear part is necked down to form a long thin "boom." In such cases the stiffness in bending about both axes is important. No definite limits for rigidity can be set down; however, the natural periods of vibration in torsion and both bending directions should all be of different periods to prevent interaction. These vibration rates can be measured on the ground by vibration tests conducted with the glider on the ground.

f. Concentrated loads.-The loads from main fittings should be well distributed to the bulkheads by means of suitable blocking. Birch plywood or ash pads should be provided under fittings. Wood corner blocks which carry shear through the glue joint should be laminated pie-fashion if necessary to avoid gluing at an angle of more than 30 degrees off the grain direction. Butt glue joints on end grain will not carry shear or tension loads. For various methods of installing corner blocks, see fig. 3-IX.

Crash protection.

a. General. The fuselage should be designed to give reasonable assurance that each occupant provided with the proper belts and harness will not suffer serious injury during minor crash conditions as a result of contact of any vulnerable part of his body with any penetrating or relatively solid object.

If the characteristics of the glider make a turnover reasonably probable, the fuselage, in combination with other portions of the structure, should be designed to afford protection of the occupants in a complete turnover.

Compliance Suggestion

PERSONNEL COMPARTMENT

Relatively rigid structural members or rigid mounted items of equipment which might be struck by the head, arm, knees, et cetera, should be padded. Padding preferably should be of foam polystryrene or unicellular polyvinal chloride material rather than ordinary foam or sponge rubber, which are ineffective. Heavy transversal