Motions and clearances. a. Tension changes.-The movements of horns, cables and other components with respect to each other should be such that there is no excessive change in system tension throughout the range. Elevators mounted on adjustable stabilizers, in particular, should be checked for this possibility. Pulley guards should be close fitting to prevent jamming from slack cables since wide temperature variations may cause rigging loads to vary appreciably. b. Aerodynamic balancing.-When using extreme values of differential motion in the aileron control system or a high degree of aerodynamic balance of the ailerons, the friction in the system must be kept low; otherwise the ailerons will not return to neutral and the lateral stability characteristics will be adversely affected. This is particularly true when the ailerons are depressed as part of a flap system, in which case there may even be definite over-balance effects. c. Creeping.-Adjustable stabilizer controls should be free from "creeping" tendencies. When adjustment is secured by means of a screw or worm, the lead angle should not exceed 4 degrees unless additional friction, a detent, or equivalent means is used. In general, some forms of irreversible mechanism should be incorporated in the system, particularly if the stabilizer is hinged near the trailing edge. d. Interference.-Proper precautions should be taken with respect to control systems to eliminate the possibility of jamming, interference from cargo, passengers or loose objects, and chafing or slapping of cables against parts of the glider. All pulleys should be provided with satisfactory guards. A control column or stick located between a pilot and a passenger should not be used unless a throwover type of wheel control is incorporated. The cockpit controls should be protected by a flexible boot or similar means, if necessary, to preclude any possibility of any objects becoming fouled in the air controls. e. Clearance. At the control surfaces themselves ample clearances (5°) should be left beyond the normal deflections, to prevent interferences and possible damage when the surface is slammed over by wind on the ground. f. Nose wheel.-It is essential when a nose wheel steering system is interconnected with the flight controls that care be taken to prevent excessive loads from the nose wheel over-stressing the flight control system. This objective may be attained by springs, a weak link, or equivalent means incorporated in the nose wheel portion of the control system. Single cable controls. Single cable controls refer to those systems which do not have a positive return for the surface or device being controlled. In general, service experience shows that their use has been satisfactory. Rudder control systems without a balance cable at the pedals are considered satisfactory if some means such as a spring is used to maintain cable tension and to hold the pedals in the proper position. If should be noted that it is not the intent of the specified requirement to require a duplication of cables performing the same function. Spring devices.-The use of springs in the control system either as a return mechanism or as an auxiliary mechanism for assisting the pilot (bungee device) is discouraged except under the following conditions: a. The glider should be satisfactorily maneuverable and controllable and free from flutter under all conditions with and without the use of the spring device. b. In all cases the spring mechanism should be of a type and design which can be demonstrated to be satisfactory by actual flight tests. c. Rubber cord should not be used for this purpose. Flap controls. a. The flap operating mechanism should be such as to prevent sudden inadvertent or automatic opening of the flap at speeds above the design speed for the extended flap conditions. Means should be provided to retain flaps in their fully retracted position and to indicate such position to the pilot. b. Undesirable flight characteristics, such as loss of lift and consequent settling, may result from too rapid operation of flaps which give appreciable lift. When the prime function of the flap is to act as a brake, however, slow operation is not so important. When flaps extend over a large portion of the span, the control and means of interconnection should be such as to insure that the flaps on both sides function simultaneously. Tab controls. a. Position indicator.-When adjustable elevator tabs are used for the purpose of trimming the glider, a tab position indicator should be installed and means should be provided for indicating to the pilot a range of adjustment suitable for safe takeoff and the directions of motion of the control for nose-up and nose-down motions of the glider. b. Reversibility.-Tab controls should be irreversible and nonflexible, unless the tab is statically balanced about its hinge line. c. Wear and vibration.-In addition the the air loads, consideration should be given in the design to the lapping effect of dust and grease on fine threads, deflections of the tab due to the small effective arm of the horn or equivalent member, and vibration common to the trailing edge portion of most movable surfaces. d. Degree of travel. It is advisable to avoid a tab control with small travel because of the resulting abrupt action of the tab. a. Direction of operation.-Proper precautions should be taken against the possibility of inadvertent or abrupt tab operation and operation in the wrong direction. |