In applying § 4a.137 it shall be assumed that the ailerons are loaded in opposite directions. The control force specified in Table 4a-6 and Fig. 4a-9 shall be assumed to act in a lateral direction at the grip of a control stick, or shall be assumed to act as part of couple equal to the specified force multiplied by the diameter of a control wheel. The following assumptions shall be made:

(a) For nondifferential ailerons, 75 percent of the stick force or couple shall be assumed to be resisted by a down aileron, the remainder by the other aileron; also, as a separate condition, 50 percent shall be assumed to be resisted by an up aileron, the remainder by the other aileron.

(b) For differential ailerons, 75 percent of the stick force or couple shall be assumed to be resisted by each aileron in either the up or down position, or rational assumptions based on the geometry of the system shall be made.

§ 4a.142 Flap and tab control systems.

In applying § 4a.137 suitable minimum manual forces shall be assumed to act on flap and tab control systems and other similar controls.

The conditions set forth in §§ 4a.1484a.156 represent the minimum amount of investigation required for conventional (tail down type) landing gear. For unconventional types it may be necessary to investigate other landing attitudes, depending on the arrangement and design of the landing gear members. Consideration will be given to a reduction of the specified limit load factors when it can be proved that the shock absorbing system will positively limit the acceleration factor to a definite lower value in the drop test specified in § 4a.148 (b). The minimum factors of safety are specified for each loading condition. See also §§ 4a.207 through 4a.216 for multiplying factors of safety required in certain cases.

§ 4a.148 Level landing.

The minimum limit load factor is specified in Fig. 4a-10. The resultant of the ground reaction shall be assumed to be a force lying at the intersection of the plane of symmetry and a plane

in which are located the axles and the center of gravity of the airplane less chassis. The propeller axis (or equivalent reference line) shall be assumed horizontal and the basic value of the vertical component of the resultant of the ground reaction shall be equal to the gross weight of the airplane minus chassis and wheels. The horizontal component shall be of the magnitude required to give the resultant force the specified direction except that it need not be greater than 25 percent of the vertical component. The resultant of the ground reaction shall be assumed to be divided equally between wheels and to be applied at the axle at the center of the wheel. The shock-absorber unit and tires shall be assumed to be deflected to half their total travel, unless it is apparent that a more critical arrangement could exist. The minimum ultimate factor of safety shall be 1.5.

(a) Sliding element. If a sliding element instead of a rolling element is used for the landing gear, a horizontal component of one-half of the vertical component shall be used to represent the effect of ground friction, except that ski gear which is designed and used only for landing on snow and ice may be designed for the same horizontal component as wheel gear.

The level

(b) Energy absorption. landing condition specified in § 4a.148 shall be assumed to be produced by a free drop, in inches, equal to 0.36 times the calculated stalling speed (V.) in miles per hour, except that the height of free drop shall not be less than 18 inches for airplanes employing devices which increase the normal sinking speed, but need not exceed 18 inches when such devices are not employed. The height of free drop is measured from the bottom of the tire to the ground, with the landing gear extended to its extreme unloaded position. (See §§ 4a.278, 4a.475.) § 4a.151 Three-point landing.

The minimum limit load factor is specified in Fig. 4a-10. The value of the sum of the static ground reactions shall be the gross weight of the airplane less chassis. The total load shall be divided between the chassis and tail skid or wheel in inverse proportion to the distances, measured parallel to the ground line, from the center of gravity of the airplane less chassis to the points of contact with the ground. The load on the

chassis shall be divided equally between wheels. Loads shall be assumed to be perpendicular to the ground line in the three-point landing attitude, with all shock absorbers and tires deflected to the same degree as in level landing. The tail wheel or skid installation shall also be investigated for this condition. The minimum ultimate factor of safety shall be 1.5.

§ 4a.152 Energy absorption.

The three-point landing condition specified in § 4a.151 shall be assumed to be produced by a free drop as specified under § 4a.148(b). This requires shock absorption by both main wheels and tail wheel (or skid). (See §§ 4a.278, 4a.475.) § 4a.153 Side load.

The minimum limit load factor shall be 0.667. The weight of the airplane shall be assumed to act on one wheel in a direction perpendicular to the ground. In addition, a side component of equal magnitude shall be assumed to act inward and normal to the plane of symmetry at the point of contact of the wheel, and an aft component equal to 0.55 times the vertical component shall be assumed to act parallel to the ground at such point. The airplane shall be assumed to be in a three-point attitude with the shock absorbers deflected to their static position and the tires deflected one-quarter the nominal diameter of their cross section. The minimum ultimate factor of safety shall be 1.5. § 4a.154

One-wheel landing.

An investigation of the fuselage structure is required for a one-wheel landing, in which only those loads obtained on one side of the fuselage in the level landing condition are applied. The resulting load factor is therefore one-half of the level landing load factor. (This condition is identical with the level landing condition insofar as the landing gear structure is concerned.) The minimum ultimate factor of safety shall be 1.5.

§ 4a.155 Braked landing.

The minimum load factor shall be 1.33. Airplanes equipped with brakes shall be investigated for the loads incurred when a landing is made with the wheels locked and the airplane is in an attitude such that the tail skid or wheel just clears the ground. The weight of the airplane less chassis shall be assumed to act on the wheels in a direction perpendicular to

the ground line in this attitude. In addition, a component parallel to the ground line shall be assumed to act at the point of contact of the wheels and the ground, the magnitude of this component being equal to the weight of the airplane less chassis times a coefficient of friction of 0.55. The tire in all cases shall be assumed to have deflected not more than one-quarter the nominal diameter of its cross section, and the deflection of the shock absorbers shall be the same as in level landing. The minimum ultimate factor of safety shall be 1.5.

§ 4a.156 Side loads on tail wheel or skid.

Suitable assumptions shall be made to cover side loads acting on tail skids or tail wheels which are not free to swivel or which can be locked or steered by the pilot.

§ 4a.161

The

WATER LOADS General.

requirements set forth in §§ 4a.162-4a.177 shall apply to the entire airplane, but have particular reference to hull structures, wings, nacelles, and float supporting structure. The requirements for certification of floats as individual items of equipment are specified in Part 15 of this subchapter (14 F.R. 4190, July 16, 1949). The minimum factors of safety are specified for each loading condition.

CROSS REFERENCES: For multiplying factors of safety required in certain cases, see §§ 4a.207-4a.216. For detail design requirements for hulls and floats, see §§ 4a.4884a.492.

FLOAT SEAPLANES

§ 4a.162 Landing with inclined reactions (float seaplanes).

The vertical component of the limit load factor shall be 4.20 except that it need not exceed a value given by the following formula:

n=3.0+0.133 W/S.

The propeller axis (or equivalent reference line) shall be assumed to be horizontal and the resultant water reaction to be acting in the plane of symmetry and passing through the center of gravity of the airplane less floats and float bracing, but inclined so that its horizontal component is equal to one-quarter of its vertical component. The forces representing the weights of and in the airplane shall be assumed to act in a direction par

The weight

allel to the water reaction. of the floats and float bracing may be deducted from the gross weight of the airplane.

§ 4a.163 Float attachment members.

For the design of float attachment members, including the members necessary to complete a rigid brace truss through the fuselage, the minimum ultimate factor of safety shall be 1.85. For the remaining structural members the minimum ultimate factor of safety shall be 1.50.

§ 4a.164 Landing with vertical reactions (float seaplanes).

The limit load factor shall be 4.33, acting vertically, except that it need not exceed a value given by the following formula:

n=3.0+0.133 W/S.

The propeller axis (or equivalent reference line) shall be assumed to be horizontal, and the resultant water reaction to be vertical and passing through the center of gravity of the airplane less floats and float bracing. The weight of the floats and float bracing may be deducted from the gross weight of the airplane.

§ 4a.165 Safety factors.

The minimum factors of safety shall be the same as those specified in § 4a.163.

§ 4a.166 Landing with side load (float seaplanes).

The vertical component of the limit load factor shall be 4.0, to be applied to the gross weight of the airplane less floats and float bracing. The propeller axis (or equivalent reference line) shall be assumed to be horizontal and the resultant water reaction shall be assumed to be in the vertical plane which passes through the center of gravity of the airplane less floats and float bracing and is perpendicular to the propeller axis. The vertical load shall be applied through the keel or keels of the float or floats, and evenly divided between the floats when twin floats are used. A side load equal to one-fourth of the vertical load shall be applied along a line approximately half way between the bottom of the keel and the level of the water line at rest. When built-in struts are used, check calculations shall be made for the built-in struts with the side load at the

The

(b) Variation in local pressure. local pressures to be applied to the hull bottom shall vary in accordance with Figure 4a-11. No variation from keel to chine (beamwise) shall be assumed, except when the chine flare indicates the advisability of higher pressures of the chine.

(c) Application of local pressure. The local pressure determined from § 4a.167 (a) and Figure 4a-11 shall be applied over a local area in such a manner as to cause the maximum local loads in the hull bottom structure.

[Amdt. 48, 5 F. R. 1836, May 22, 1940] § 4a.168 Distributed bottom pressures.

(a) For the purpose of designing frames, keels, and chine structure, the limit pressures obtained from § 4a.167 (a) and Figure 4a-11 shall be reduced to one-half the "local" values and simultaneously applied over the entire hull bottom. The loads so obtained shall be carried into the side-wall structure of the hull proper, but need not be transmitted in a fore-and-aft direction as shear and bending loads. The minimum ultimate factor of safety shall be 1.5.

(b) Unsymmetrical loading: Each floor member or frame shall be designed for a load on one side of the hull centerline equal to the most critical symmetrical loading, combined with a load

on the other side of the hull center line equal to one-half of the most critical symmetrical loading.

[Amdt. 48, 5 F. R. 1836, May 22, 1940]

§ 4a.169 Step loading condition.

(a) Application of load. The resultant water load shall be applied vertically in the plane of symmetry so as to pass through the center of gravity of the airplane (in full load condition).

(b) Acceleration. The limit acceleration shall be 4.33.

(c) Hull shear and bending loads. The hull shear and bending loads shall be computed from the inertia loads produced by the vertical water load. To avoid excessive local shear loads and bending moments near the point of water load application, the water load may be distributed over the hull bottom, using pressures not less than those specified in § 4a.168 (a). The minimum ultimate factor of safety shall be 1.5. [Amdt. 48, 5 F. R. 1836, May 22, 1940] § 4a.170 Bow loading condition.

(a) Application of load. The resultant water load shall be applied in the plane of symmetry at a point one-tenth of the distance from the bow to the step and shall be directed upward and rearward at an angle of 30 degrees from the vertical.

(b) Magnitude of load. The magnitude of the limit resultant water load shall be determined from the following equation:

where

Po load in pounds,

n, step landing load factor, We effective weight which is assumed equal to one-half the design weight of the airplane.

(c) Hull shear and bending loads. The hull shear and bending loads shall be determined by proper consideration of the inertia loads which resist the linear and angular accelerations involved. To avoid excessive local shear loads, the water reaction may be distributed over the hull bottom, using pressures not less than those specified in § 4a.168 (a). The minimum ultimate factor of safety shall be 1.5.

[Amdt. 48, 5 F. R. 1836, May 22, 1940] § 4a.171

Stern loading condition.

(a) Application of load. The resultant water load shall be applied vertically in the plane of symmetry and shall be

distributed over the hull bottom from the second step forward with an intensity equal to the pressures specified in § 4a.168 (a).

(b) Magnitude of load. The limit resultant load shall equal three-quarters of the design weight of the airplane.

(c) Hull shear and bending loads. The hull shear and bending loads shall be determined by assuming the hull structure to be supported at the wing attachment fittings and neglecting internal inertia loads. This condition need not be applied to the fittings or to the portion of the hull ahead of the rear attachment fittings. The minimum ultimate factor of safety shall be 1.5. [Amdt. 48, 5 F. R. 1836, May 22, 1940]

§ 4a.172 Side loading condition.

(a) Application of load. The resultant water load shall be applied in a vertical plane through the center of gravity. The vertical component shall be assumed to act in the plane of symmetry and the horizontal component at a point half way between the bottom of the keel and the load water line at design weight (at rest).

(b) Magnitude of load. The limit vertical component of acceleration shall be 3.25 and the side component shall be equal to 15 percent of the vertical component.

(c) Hull shear and bending loads. The hull shear and bending loads shall be determined by proper consideration of the inertia loads or by introducing couples at the wing attachment points. To avoid excessive local shear loads, the water reaction may be distributed over the hull bottom, using pressures not less than those specified in § 4a.168 (a). The minimum ultimate factor of safety shall be 1.5.

[Amdt. 48, 5 F. R. 1836, May 22, 1940]

SEAPLANE FLOAT LOADS

§ 4a.173 Seaplane float loads.

Each main float of a float seaplane shall be capable of carrying the following loads when supported at the attachment fittings as installed on the airplane. The minimum ultimate factor of safety shall be 1.5.

(a) A limit load, acting upward, applied at the bow end of the float and of magnitude equal to one-half of that portion of the airplane gross weight normally supported by the particular float.

(b) The limit load specified in paragraph (a) of this section, acting upward at the stern.

(c) A limit load, acting upward, applied at the step and of magnitude equal to 1.33 times that portion of the airplane gross weight normally supported by the particular float.

[Amdt. 5, 4 F. R. 1171, Mar. 9, 1939]

§ 4a.174 Seaplane float bottom loads.

Main seaplane float bottoms shall be designed to withstand the following loads. The minimum ultimate factor of safety shall be 1.5.

(a) A limit load of at least 5.33 pounds per square inch over that portion of the bottom lying between the first step and a section at 25 percent of the distance from the step to the bow.

(b) A limit load of at least 2.67 pounds per square inch over that portion of the bottom lying between the section at 25 percent of the distance from the step to the bow and a section at 75 percent of the distance from the step to the bow.

(c) A limit load of at least 2.67 pounds per square inch over that portion of the bottom lying between the first and second steps. If only one step is used, this load shall extend over that portion of the bottom lying between the step and a section at 50 percent of the distance from the step to the stern.

WING-TIP FLOAT LOADS

§ 4a.175 Wing-tip float loads.

Wingtip floats and their attachment, including the wing structure, shall be analyzed for each of the following conditions, using a minimum ultimate factor of safety of 1.5:

(a) A limit load acting vertically up at the completely submerged center of buoyancy and equal to three times the completely submerged displacement.

(b) A limit load inclined upward at 45 degrees to the rear and acting through the completely submerged center of buoyancy and equal to three times the completely submerged displacement.

(c) A limit load acting parallel to the water surface (laterally) applied at the center of area of the side view and equal to one and one-half times the completely submerged displacement.

§ 4a.176 Wing structure.

The primary wing structure shall incorporate sufficient extra strength to in

sure that failure of wing-tip float attachment members occurs before the wing structure is damaged.

MISCELLANEOUS WATER LOADS

§ 4a.177 Sea wing loads.

Special rulings shall be obtained from the Administrator for the strength requirements for sea wings.

SPECIAL LOADING CONDITIONS

§ 4a.187 Engine torque.

In the case of engines having five or more cylinders the stresses due to the torque load shall be multiplied by a limit load factor of 1.5. For 4-, 3-, and 2cylinder engines the limit load factors shall be 2, 3, and 4, respectively. The torque acting on the airplane structure shall be computed for the take-off power desired and the propeller speed corresponding thereto (see § 4a.727). The engine mount and forward portion of the fuselage and nacelles shall be designed for this condition. The minimum ultimate factor of safety shall be 1.5 unless higher factors are deemed necessary by the Administrator in order to make special provision for conditions such as vibration, stress concentration, and fatique.

[Amdt. 5, 4 F. R. 1171, Mar. 9, 1939, as amended by Amdt. 75, 5 F. R. 3946, Oct. 8, 1940] § 4a.188

torque.

High angle of attack and

The limit loads determined from § 4a.187 shall be considered as acting simultaneously with 75 percent of the limit loads determined from condition I (§ 4a.79). The engine mount, nacelles, and forward portion of the fuselage (when a nose engine is installed) shall be designed for this condition. The minimum ultimate factor of safety shall be 1.5.

§ 4a.189 Engine mounts, nacelles, etc.

The engine mounts, nacelles, and forward portion of the fuselage (when a nose engine is installed) shall be investigated for the limit loads determined from condition I (see §§ 4a.79 and 4a.94) acting simultaneously with the limit loads due to the engine torque determined in accordance with § 4a.187, except that the engine power and the propeller speed shall correspond to the design power (§ 4a.38 (b)) or the output specified for climbing flight (see