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to which they are attached also should be investigated for pertinent loads. A minimum limit factor of safety of 1.0 and a minimum ultimate factor of safety of 1.5 should be used unless otherwise specified. Also, see table 1-III for factors of safety required in certain
Loads on fittings.-A limit load of 900 pounds or 2.0 times the gross weight, whichever is greater, should be assumed to act in the following separate cases:
• Forward at the towing and launching fitting (or mechanism),
and aft at the rear holding fitting.
upward at the maximum angle which will afford adequate
downward at an angle of 75 degrees with the longitudinal axis. • At the towing and launching fitting, and directed forward and
sideward at an angle of 30 degrees with the longitudinal axis. The above loadings are shown in fig. 1-XXI. The effects of these loads need not be investigated aft of the front wing spar.
Wing truss strength.—Unless the strength of the wing in resisting rearward acting chord loads is equal to or greater than the strength in resisting forward acting chord loads, provisions should be made to provide adequate strength of wing drag trusses to resist chord inertia loads developed in glider launching and towing.
It can be assumed that a limit rearward acting chord load factor of 3.0 is developed in shock cord/winch launches.
MULTIPLYING FACTORS OF SAFETY In addition to the minimum factors of safety specified for each loading condition, the multiplying factors specified in table 1-III and the following paragraphs should be incorporated in the structure. The total factor of safety required for any structural component or part equals the minimum factor of safety for the loading condition in question multiplied by the factors of safety hereinafter specified, except that certain multiplying factors may be included in others, as indicated in table 1-III. The use of a fitting factor is not considered necessary in cases wherein certain special factors are used, such as those covered by items la, 2, 6, 7 and 8 of table 1-III.
Note: P = 900 pounds or 2.0 times the gross weight, whichever is greater (limit).
Figure 1-XXI. Launching and towing loads.
Fittings.—All fittings in the primary structure should incoporate the multiplying factor of safety specified in table 1-III. For this purpose, fittings are defined as parts used to connect one primary member to another and should include the bearing of these parts on the members thus connected. Continuous joints in metal plating and welded joints between primary structural members are not classified as fittings. (See Chap. 3.)
Castings.-All castings in the primary structure should incorporate a multiplying factor of safety not less than that specified in table 1-III.
Parallel double wires.—When parallel double wires are used in wing lift trusses, each wire should incorporate a multiplying factor of safety as specified in table 1-III.
Wires at small angles.-Wire or tie-rod members of wing or tail surface external bracing should incorporate a multiplying factor of safety as follows: K =L/2R (except that K shall not be less than 1.0), where
K = the additional factor
the wing or tail surface plane, and
1 For bearing stresses only.
Double drag trusses.—Whenever double drag trussing is employed, all drag wires should incorporate a multiplying factor of safety varying linearly from 3.0, when the ratio of overhang to root chord of overhang is 2.0 or greater, to 1.20 when such ratio is 1.0 or less, assuming an equal division of drag load between the two systems.
Torques tubes used as hinges.—When steel torques tubes are employed in direct bearing against strap-type hinges they should incorporate a multiplying factor of safety at the hinge point not less than that specified in table 1-III. (Also, see Chap. 3.)
Control surface hinges and control system joints.—Control surface hinges and control system joints subjected to angular motion, excepting ball and roller bearings and AN standard parts used in cable control systems, should incorporate multiplying factors of safety not less than those specified in table 1-III with respect to the ultimate bearing strength of the softest material used for a bearing. For ball or roller bearings a yield factor of safety of 1.0 with respect to the manufacturer's non-Brinell rating is considered sufficient to provide an adequate ultimate factor of safety.
Wire sizes.-(See Chap. 3.)
Wing lift trusses.—Lift wires should be in duplicate and when landing loads are carried through a portion of the lift truss the lift truss members so affected should incorporate a multiplying factor of safety not less than that specified in table 1-III.
Chapter 2-PROOF OF STRUCTURE Proof of compliance with the loading recommendations suggested in Chapter 1 consists of structural analyses, load tests, flight tests, references to previously approved structures, or combinations of the above. Any condition shown to be noncritical need not be further investigated. Also, the glider applicant is free to develop and present any method he chooses for showing compliance with the specified recommendations.
DETERMINATION OF LOADINGS
CONTROL SYSTEMS Flight controls.—The minimum loads to be used for the design of flight control systems are covered on pp. 33, 34.
Secondary controls.—The minimum loads to be used for the design of secondary control systems are covered on pp. 34, 35.
FUSELAGES Weight distribution.—All major items of weight affecting the fuselage should be distributed to convenient panel points so that the original center of gravity of the fuselage and its contents is maintained. A suitable vertical division of loads should be included. The following rules should be followed in computing the panel point loads for conventional gliders.
• The weight of an item located between two adjacent panel
points of the side trusses should be divided between those panel points in inverse porportion to the distance from them
to the center of gravity of the item. • The weight of an item supported at three or more panel
points should be divided between those points by the aid of an investigation and analysis of the method of support, if practicable. Otherwise, a conservative distribution should be
item about an origin near the nose of the fuselage should be