withstand this condition. Leading edges and struts should have adequate strength to withstand handling loads if handles or grips are not provided.

e. Direct welding of control horns to torque tubes (without the use of a sleeve) should be done only when a large excess of strength is indicated.

Elevators.

a. When separate elevators are installed they should be rigidly interconnected.

b. When dihedral is incorporated in the horizontal tail the universal connection between the elevator sections should be rugged and free from play.

Tabs.

a. Control surface trailing-edge tabs should be statically balanced about their hinge lines, unless an irreversible nonflexible tab control system is used. The installation should be such as to prevent development of any free motion of the tab.

b. If trailing-edge tabs are installed, care should be taken in proportioning areas and relative movements so that the main surface is not aerodynamically overbalanced at any time.

c. Minimum deflections and play are of primary importance in the installation of tabs. Strength of the surface and anchorages should be sufficient to prevent damage or misalignment from handling, particularly thin sheet tabs which are set by bending to the proper position.

CONTROL SYSTEMS DESIGN

a. Rigidity.—It is essential that control systems, when subjected to limit load and operation tests, indicate no signs of excessive deflection or permanent set. In order to insure that the surfaces to which the control system attaches will retain their effectiveness in flight, the deflection in the system should be restricted to a reasonable limit. As a guide for conventional control systems, the average angular deflection of the surface, when both the control system and surface are subjected to limit loads as computed for the maneuvering condition neglecting the minimum limit control force but including tab effects, should not exceed approximately one-half of the angular throw from neutral to the extreme position.

b. Dual controls.-Dual control systems should be checked for the effects of opposite loads on the wheel or stick. This may be critical for some members such as aileron bell crank mountings as an "open" system, that is, no return except through the balance cable between the ailerons. In addition, the deflections resulting from this long load path may slack off the direct connection sufficiently to cause jamming of cables or chains unless smooth close-fitting guards and fair leads are used.

c. Control system locks.-When a device is provided for locking a control surface while the aircraft is on the ground or water, compliance with the following requirements should be shown:

1. The locking device should be so installed as to positively prevent taxiing or taking off, either intentionally or inadvertently, while the lock is engaged.

2. Means should be provided to preclude the possibility of the lock becoming engaged during flights.

Installation.

The predominating type of cockpit controls is the stick and pedal system. Wheel control is sometimes used for high performance gliders with restricted cockpit size.

a. Travel.—It is suggested that the total travel at the top of the stick should be approximately 12 inches or more in both planes to avoid undue sensitivity of the elevators and provide sufficient leverage on the aileron. When a wheel control is used the angular motion should be not less than 60 degrees either side of neutral. Rudder pedals should have at least 2 inches travel either way.

b. Positioning. In the layout and positioning of a control consideration should be given to its relative importance and to its convenient placement for the usual sequence of operations. Thus for landing, it is desirable that flap control and brakes be operable without changing hands on the wheel or stick. Likewise, secondary controls should be so located that the possibility of accidental or mistaken operation is remote.

c. Centering.-A point sometimes overlooked is the effect of the weight of a control member or of a pilot's arm or leg on the centering characteristics of the control.

Stops.

a. All control systems should be provided with stops that positively limit the range of motion of the control surfaces. Stops should be capable of withstanding the loads corresponding to the design conditions for the control system.

b. Although the location of stops within the control system is not specified, they should preferably be located close to the operating force in order to avoid a "springy" control. Additional stops also should be installed at the surfaces. Stops should be adjustable where production tolerances are such as to result in appreciable variation in range of motion.

Hinges, bearings, and joints.—

a. Hinges. Control surface hinges of aircraft standard normally are available at the supply houses. In general, hinge pins should be inch or more in diameter. If clevis pins are used, a washer should be placed under the cotter pin. Standard A-N bolts or clevis bolts are preferable to clevis pins in the rotating joints of hinges or controls.

b. Bearings.-Bearings should be arranged so that they can be readily inspected and lubricated.

c. Friction.-Excessive friction should be especially avoided in the aileron control systems of large high performance gliders. This may sometimes dictate the use of special anti-friction bearings at heavily loaded pivots.

d. Locking devices.-Bolts, straight pins, taper pins, studs, and other fastening means should be secured with approved locking devices. The assembly of universal, and ball and socket joints should be insured by positive locking means, rather than by springs. Woodruff keys should not be used in tubing unless provision is made against the key dropping through an oversize or worn seat.

e. Cockpit controls.—Stick pivots and other similar joints in the control system that tend to wear rapidly should be constructed with a spacer tube on the through bolt to take the wear in the bearing and allow the bolt to be clamped down tight. See fig. 3-IV.

Cables, pulleys and fairleads.—

a. Cables.-Control cables should be of the 6 x 19 or 7 x 19 extraflexible type, except that 6 x 7 or 7 x 7 flexible cable is acceptable in the 2 inch diameter size and smaller. For properties see table 8.212 of MIL-HDBK-5. The 7 x 7 construction is found satisfactory where slight cable bends around pulleys of only 30° or less are encountered. Cable and splices should be made by an approved tuck method such as that of the Army and Navy, except that standard served and soldered splices are acceptable for cables not over 2 inch in diameter. However, cables of other sizes with served and soldered fastenings will be satisfactory provided they are not stressed above 50 percent of their rated strength. Approved swaged type terminals are also acceptable. Dimensions for approved splices are given in fig 3-V. It should be noted that cable sizes are governed by deflection conditions as well as by strength requirements, particularly when a long cable is used. Some acceptable types of cable joints are given in fig. 3-VI.

b. Turnbuckles.-Turnbuckles should be located so as to be accessible for adjustment and preferably not in the center of long unsupported spans where they can slap around too freely. Examples of turnbuckle installations are given in fig. 3-VII.

c. Spring connecting links.-Spring type connecting links for chains have not been found to be entirely satisfactory in service. It is advisable that a more reliable means, such as peening or cotter pins, be employed.

d. Fairleads.-Fairleads of non-metalic material, such as the phenolic plastic compounds, should be used to prevent cables, chains and links from chafing or slapping against parts of the glider, but should not be used to replace pulleys as a direction-changing means. However, where the cable load is small, and the location is open to easy

visual inspection, direction changes (through fairleads) not exceeding 3° are satisfactory in primary control systems. A somewhat greater value may be used in secondary control systems.

e. Pulleys. Standard pulleys of the A-N type usually are lighter than metal pulleys. Pulleys that carry a 180-degree wire bend should be carefully mounted so that there is no danger of their leaning over and binding under load. They should also line up accurately with the plane of the cables or the flanges will wear out quickly. See fig. 3-VIII. All pulleys should be provided with annular guards so located that a slack wire cannot get off the pulley in any manner and jam it.

Figure 3-IV. Adjustable elevator bungee, control system details.