257

**Federal Aviation Administration, DOT **

**§ 25.533 **

*n*

*C*

*V*

*W*

*TO*

*S*

=

⎛

⎝

⎞

⎠

1

2

2

3

1

3

tan

β

where—

*n *= inertia load factor; *C*

*TO*

= empirical seaplane operations factor

equal to 0.004;

*V*

*S*1

= seaplane stalling speed (knots) at the

design takeoff weight with the flaps ex-

tended in the appropriate takeoff posi-

tion;

b

= angle of dead rise at the main step (de-

grees); and

*W *= design water takeoff weight in pounds.

[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as

amended by Amdt. 25–23, 35 FR 5673, Apr. 8,

1970]

**§ 25.533**

**Hull and main float bottom **

**pressures. **

(a)

*General. *The hull and main float

structure, including frames and bulk-

heads, stringers, and bottom plating,

must be designed under this section.

(b)

*Local pressures. *For the design of

the bottom plating and stringers and

their attachments to the supporting

structure, the following pressure dis-

tributions must be applied:

(1) For an unflared bottom, the pres-

sure at the chine is 0.75 times the pres-

sure at the keel, and the pressures be-

tween the keel and chine vary linearly,

in accordance with figure 3 of appendix

B. The pressure at the keel (psi) is

computed as follows:

*P*

*C*

*K V*

*k*

*S*

*k*

=

×

2

2

1

2

tan

β

where—

*P*

*k*

= pressure (p.s.i.) at the keel;

*C*

2

= 0.00213;

*K*

2

= hull station weighing factor, in accord-

ance with figure 2 of appendix B;

*V*

*S*1

= seaplane stalling speed (Knots) at the

design water takeoff weight with flaps

extended in the appropriate takeoff posi-

tion; and

b

K

= angle of dead rise at keel, in accordance

with figure 1 of appendix B.

(2) For a flared bottom, the pressure

at the beginning of the flare is the

same as that for an unflared bottom,

and the pressure between the chine and

the beginning of the flare varies lin-

early, in accordance with figure 3 of ap-

pendix B. The pressure distribution is

the same as that prescribed in para-

graph (b)(1) of this section for an

unflared bottom except that the pres-

sure at the chine is computed as fol-

lows:

*P*

*C*

*K V*

*ch*

*S*

=

×

3

2

1

2

tan

β

where—

*P*

*ch*

= pressure (p.s.i.) at the chine;

*C*

3

= 0.0016;

*K*

2

= hull station weighing factor, in accord-

ance with figure 2 of appendix B;

*V*

*S*1

= seaplane stalling speed at the design

water takeoff weight with flaps extended

in the appropriate takeoff position; and

b

= angle of dead rise at appropriate station.

The area over which these pressures

are applied must simulate pressures oc-

curring during high localized impacts

on the hull or float, but need not ex-

tend over an area that would induce

critical stresses in the frames or in the

overall structure.

(c)

*Distributed pressures. *For the de-

sign of the frames, keel, and chine

structure, the following pressure dis-

tributions apply:

(1) Symmetrical pressures are com-

puted as follows:

*P*

*C*

*K V*

*S*

=

×

4

2

0

2

tan

β

where— *P *= pressure (p.s.i.); *C*

4

= 0.078

*C*

1

(with

*C*

1

computed under

§ 25.527);

*K*

2

= hull station weighing factor, deter-

mined in accordance with figure 2 of ap-

pendix B;

*V*

*S*0

= seaplane stalling speed (Knots) with

landing flaps extended in the appropriate

position and with no slipstream effect;

and

*V*

*S*0

= seaplane stalling speed with landing

flaps extended in the appropriate posi-

tion and with no slipstream effect; and

b

= angle of dead rise at appropriate sta-

tion.

(2) The unsymmetrical pressure dis-

tribution consists of the pressures pre-

scribed in paragraph (c)(1) of this sec-

tion on one side of the hull or main

float centerline and one-half of that

pressure on the other side of the hull or

main float centerline, in accordance

with figure 3 of appendix B.

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258

**14 CFR Ch. I (1–1–24 Edition) **

**§ 25.535 **

These pressures are uniform and must

be applied simultaneously over the en-

tire hull or main float bottom. The

loads obtained must be carried into the

sidewall structure of the hull proper,

but need not be transmitted in a fore

and aft direction as shear and bending

loads.

[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as

amended by Amdt. 25–23, 35 FR 5673, Apr. 8,

1970]

**§ 25.535**

**Auxiliary float loads. **

(a)

*General. *Auxiliary floats and their

attachments and supporting structures

must be designed for the conditions

prescribed in this section. In the cases

specified in paragraphs (b) through (e)

of this section, the prescribed water

loads may be distributed over the float

bottom to avoid excessive local loads,

using bottom pressures not less than

those prescribed in paragraph (g) of

this section.

(b)

*Step loading. *The resultant water

load must be applied in the plane of

symmetry of the float at a point three-

fourths of the distance from the bow to

the step and must be perpendicular to

the keel. The resultant limit load is

computed as follows, except that the

value of

*L *need not exceed three times

the weight of the displaced water when

the float is completely submerged:

*L*

*C V*

*W*

*r*

*S*

*s*

*y*

=

+

(

)

5

2

0

2

2

3

2

3

2

3

1

tan

β

where—

*L *= limit load (lbs.); *C*

5

= 0.0053;

*V*

*S*0

= seaplane stalling speed (knots) with

landing flaps extended in the appropriate

position and with no slipstream effect;

*W *= seaplane design landing weight in

pounds;

b

*S*

= angle of dead rise at a station

3

⁄

4

of the

distance from the bow to the step, but

need not be less than 15 degrees; and

*r*

*y*

= ratio of the lateral distance between the

center of gravity and the plane of sym-

metry of the float to the radius of gyra-

tion in roll.

(c)

*Bow loading. *The resultant limit

load must be applied in the plane of

symmetry of the float at a point one-

fourth of the distance from the bow to

the step and must be perpendicular to

the tangent to the keel line at that

point. The magnitude of the resultant

load is that specified in paragraph (b)

of this section.

(d)

*Unsymmetrical step loading. *The re-

sultant water load consists of a compo-

nent equal to 0.75 times the load speci-

fied in paragraph (a) of this section and

a side component equal to 0.25 tan

b

times the load specified in paragraph

(b) of this section. The side load must

be applied perpendicularly to the plane

of symmetry of the float at a point

midway between the keel and the

chine.

(e)

*Unsymmetrical bow loading. *The re-

sultant water load consists of a compo-

nent equal to 0.75 times the load speci-

fied in paragraph (b) of this section and

a side component equal to 0.25 tan

b

times the load specified in paragraph

(c) of this section. The side load must

be applied perpendicularly to the plane

of symmetry at a point midway be-

tween the keel and the chine.

(f)

*Immersed float condition. *The re-

sultant load must be applied at the

centroid of the cross section of the

float at a point one-third of the dis-

tance from the bow to the step. The

limit load components are as follows:

vertical

aft

side

=

=

⎛

⎝

⎞

⎠

=

⎛

⎝

⎞

⎠

ρ

ρ

ρ

*g*

*x*

*y*

*V*

*C*

*V*

*KV*

*C V*

*KV*

*S*

*S*

2

2

2

2

2

3

0

2

3

0

where—

r

= mass density of water (slugs/ft.

2

);

*V *= volume of float (ft.

2

);

*C*

*x*

= coefficient of drag force, equal to 0.133;

*C*

*y*

= coefficient of side force, equal to 0.106;

*K *= 0.8, except that lower values may be used

if it is shown that the floats are incapa-

ble of submerging at a speed of 0.8

*V*

*S*0

in

normal operations;

*V*

*S*0

= seaplane stalling speed (knots) with

landing flaps extended in the appropriate

position and with no slipstream effect;

and

*g *= acceleration due to gravity (ft./sec.

2

).

(g)

*Float bottom pressures. *The float

bottom pressures must be established

under § 25.533, except that the value of

K

2

in the formulae may be taken as 1.0.

The angle of dead rise to be used in de-

termining the float bottom pressures is

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