§ 4a.190

Side load on engine mount.

The limit load factor for this condition shall be equal to one-third of the limit load factor for flight condition I (§ 4a.79) but shall in no case be less than 1.33. The engine mount and forward section of the fuselage and nacelles shall be analyzed for this condition, considering the limit load to be produced by inertia forces. The minimum ultimate factor of safety shall be 1.5.

§ 4a.191 Up load on engine mount.

For engine mounts the limit load in each member shall be arbitrarily assumed as 50 percent of that in the level landing condition but of opposite sign. The minimum ultimate factor of safety will be 1.5.

most severe combinations have been investigated.

CROSS REFERENCE: For standard weights, see § 48.771.

§ 4a.195 Rigging loads.

Structures braced by wires (or tierods) shall be capable of developing an ultimate factor of safety of 1.5 with respect to the limit loads due to rigging the wires to 20 percent of their rated strength (strength of wire, not terminal). When the structure is such that all wires cannot be simultaneously rigged to 20 percent of their rated loads, a rigging condition shall be assumed in which the average of the rigging loads, expressed in percent, equals 20. (See also § 4a.211.) The above condition need not be superimposed on other loading conditions, but the Administrator may require additional investigation for residual rigging loads when such investigation appears necessary. (See also § 4a.253.)

§ 4a.196 Air loads on struts.

External wing-brace struts which are at an angle of more than 45 degrees with the plane of symmetry and which have a cross-sectional fineness ratio of more than 3 shall be assumed to act as lifting air foils and shall be designed to carry the resultant transverse loads in combination with the specified axial loads. In computing the limit loads the strut sections shall be assumed to have a normal force coefficient equal to 1.0 and the total air load shall be based on the exposed area of the strut. The chord components and vertical reactions of such air load and the lift contributed by the strut shall not be considered in the analysis of the wing.

K the additional factor

R the reaction resisted by the wire in a direction normal to the wing or tail surface plane, and

L=the load required in the wire to balance the reaction R.

§ 4a.212 Double drag trusses.

Whenever double drag trussing is employed, all drag wires shall 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.

§ 4a.213 Torque tubes used as hinges.

When steel torque tubes are employed in direct bearing against strap-type hinges they shall incorporate a multiplying factor of safety at the hinge point not less than that specified in Table 4a-7. (See also § 4a.448.)

excepting ball or roller bearings and Army-Navy standard parts used in cable control systems, shall incorporate multiplying factors of safety not less than those specified in Table 4a-7 with respect to the ultimate bearing strength of the softest material used as 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.

§ 4a.215 Wire sizes.

(See §§ 4a.319, 4a.322, 4a.335.) § 4a.216 Wing lift truss system.

All structural members of the wing lift truss system which transmit direct loads from the landing gear shall, in the landing conditions, incorporate a multiplying factor of safety not less than that specified in Table 4a-7.

[Amdt. 5, 4 F. R. 1170, Mar. 9, 1939]

Subpart D-Proof of Structure

SOURCE: § 4a.227 to 4a.299 contained in Civil Air Regulations, May 31, 1938, as amended by Amendment 75, 5 F. R. 3946, Oct. 8, 1940, except as otherwise noted. § 4a.227 General.

Proof of compliance with the loading requirements outlined in Subpart C shall be made in a manner satisfactory to the Administrator and may consist of structural analyses, load tests, flight tests, references to previously approved structures, or combinations of the above. Any condition which can be shown to be noncritical need not be further investigated.

§ 4a.228 Proof of structural analysis.

(a) Structural analyses will be accepted as complete proof of strength only in the case of structural arrangements for which experience has shown such analyses to be reliable. References shall be given for all methods of analysis, formulas, theories, and material properties which are not generally accepted as standard. The acceptability of a structural analysis will depend to some extent on the excess strength incorporated in the structure.

(b) The structural analysis shall be based on guaranteed minimum mechanical properties of the materials specified on the drawings, except in cases where exact mechanical properties of the materials used are determined.

(c) The effects of welding, form factors, stress concentrations, discontinuities, cutouts, instability, end fixity of columns and vibration shall be accounted for when such factors are present to such an extent as to influence the strength of the structure.

§ 4a.229 Combined structural analysis and tests.

In certain cases it will be satisfactory to combine structural analysis procedure with the results of load tests of portions of the structure not subject to accurate analysis. In such cases test results shall be reduced to correspond to the mechanical properties of the materials actually used in the airplane. When a unit other than the specific one tested is incorporated in the airplane presented for certification, test results shall be reduced to correspond to the minimum guaranteed mechanical properties of the materials specified on the drawings.

§ 4a.230 Load tests.

Proof of compliance with structural loading requirements by means of load tests only is acceptable: Provided, That strength and proof tests (see § 4a.43 (h) and (i)) are conducted to demonstrate compliance with §§ 4a.61, 4a.62, respectively: And further provided, That the following paragraphs of this section are complied with:

(a) The tests shall be supplemented by special tests or analyses to prove compliance with multiplying factor of safety requirements. (See §§ 4a.207-4a.216.)

(b) When a unit other than the specific one tested is incorporated in the airplane presented for certification, the results of strength tests shall be reduced to correspond to the minimum guaranteed mechanical properties of the materials specified on the drawings, unless test loads are carried at least 15 percent beyond the required values.

(c) The determination of test loads, the apparatus used, and the methods of conducting the tests shall be satisfactory to the Administrator.

(d) The tests shall be conducted in the presence of a representative of the Administrator unless otherwise directed by the Administrator.

test methods are proved suitable to the satisfaction of the Administrator.

§ 4a.232 Load tests required.

The following load tests are required in all cases and shall be made in the presence of a representative of the Administrator unless otherwise directed by the Administrator:

(a) Strength tests of wing ribs. (See § 4a.248.)

(b) Pressure tests of fuel and oil tanks. (See § 4a.608.)

(c) Proof tests of tail and control surfaces. (See § 4a.263, 4a.264.)

(d) Proof and operating tests of control systems. (See §§ 4a.269, 4a.271.) WINGS

§ 4a.242 Webs.

The strength of shear webs shall be proved.

§ 4a.243 Axial load.

When axial load is present tests are required to determine the effective "EI" in the case of truss-type beams and beams having unconventional web construction.

§ 4a.244 Joint slippage in wood beams.

When a joint in a wood beam is designed to transmit bending from one section of the beam to another or to the fuselage, the stresses in each part of the structure shall be calculated on the assumption that the joint is 100 percent efficient (except in mid-bay for which see § 4a.334) and also under the assumption that the bending moment transmitted by the joint is 75 percent of that obtained under the assumption of perfect Continuity. Each part of the structure shall be designed to carry the most severe loads determined from the above assumptions.

In computing the area, moment of inertia, etc., of wood beams pierced by bolts, the diameter of the bolt hole shall be assumed to be one-sixteenth inch greater than the diameter of the bolt. § 4a.246 Box beams.

In computing the ability of box beams to resist bending loads only that portion of the web with its grain parallel to the beam axis and one-half of that portion of the web with its grain at an angle of 45 degrees to the beam shall be considered. The more conservative method of neglecting the web entirely may be employed.

satisfaction of the Administrator that it is impractical to simulate the actual loading conditions in a static test. Such analyses shall, on the basis of guaranteed minimum material properties, show proof of strength at 125 percent of the required ultimate loads. The following points shall also apply in proving the strength of ribs.

§ 4a.249 Load distribution.

The load shall be suitably distributed between upper and lower wing surfaces unless a more severe distribution is used. § 4a.250 Ailerons and high-lift devices.

The effects of ailerons and high-lift devices shall be properly accounted for. § 4a.251 Rib tests.

Rib tests shall simulate conditions in the airplane with respect to torsional rigidity of spars, fixity conditions, lateral support, and attachment to spars.

COVERING

§ 4a.252 Covering.

Proof of strength of fabric covering is not required when standard grades of cloth and methods of attaching and doping are employed: Provided, however, That the Administrator may require special tests when it appears necessary to account for the effects of unusually high design air speeds or slipstream velocities, or similar factors. When metal covering is employed its ability to perform its structural function shall be demonstrated by tests of typical panels or by other means acceptable to the Administrator. In particular, compliance with § 4a.62 requires demonstration of the behavior of the covering under load in order to determine the effects of temporary deformations (wrinkles).

load requirements only when the structure conforms with conventional types for which reliable analytical methods are available. Proof tests as defined in § 4a.43 (i) are required to prove compliance with yield load requirements.

(a) Control surface tests shall include the horn or fitting to which the control system is attached.

(b) In the analysis of control surfaces proper allowance shall be made for rigging loads in brace wires in cases where the counter wires do not go slack before the ultimate load is reached.

(c) Analyses or individual load tests shall be conducted to demonstrate compliance with the multiplying factor of safety requirements outlined in §§ 4a.207-4a.216 for control surface hinges and brace wires.

§ 4a.264 Vibration tests.

The natural frequencies of vibration of the wings, fuselage, and control surfaces shall be within such ranges of values as are satisfactory for the prevention of flutter. Compliance with this requirement shall be demonstrated by vibration tests or other methods acceptable to the Administrator.

[Amdt. 98, 6 F. R. 1145, Feb. 26, 1941]

CONTROL SYSTEMS

§ 4a.269 Proof of control systems.

Structural analyses of control systems will be accepted as complete proof of compliance with ultimate load requirements only when the structure conforms with conventional types for which reliable analytical methods are available. Proof tests as defined in § 4a.43 (i) are required to prove compliance with yield load requirements.

HULLS AND FLOATS

§ 4a.283 Proof of hulls and floats.

Structural analyses of hulls and auxiliary floats will be accepted as complete proof of compliance with load requirements only when the structure conforms with conventional types for which reliable analytical methods are available. The strength of the structure as a whole and its ability to distribute water loads from the bottom plating into the main structural members shall be demonstrated. See Part 15 of this subchapter (14 F. R. 4190, July 16, 1949) for the requirements for main floats.