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256 

14 CFR Ch. I (1–1–24 Edition) 

§ 25.529 

n

C V

W

K

r

w

S

x

=

×

+

(

)

1

0

1

2

2

2
3

1
3

2
3

1

Tan

β

(b) The following values are used: 
(1) 

n

W

= water reaction load factor 

(that is, the water reaction divided by 
seaplane weight). 

(2) 

C

1

= empirical seaplane operations 

factor equal to 0.012 (except that this 
factor may not be less than that nec-
essary to obtain the minimum value of 
step load factor of 2.33). 

(3) 

V

S0

= seaplane stalling speed in 

knots with flaps extended in the appro-
priate landing position and with no 
slipstream effect. 

(4) 

= angle of dead rise at the longi-

tudinal station at which the load fac-
tor is being determined in accordance 
with figure 1 of appendix B. 

(5) 

W= 

seaplane design landing 

weight in pounds. 

(6) 

K

1

= empirical hull station weigh-

ing factor, in accordance with figure 2 
of appendix B. 

(7) 

r

x

= ratio of distance, measured 

parallel to hull reference axis, from the 
center of gravity of the seaplane to the 
hull longitudinal station at which the 
load factor is being computed to the ra-
dius of gyration in pitch of the sea-
plane, the hull reference axis being a 
straight line, in the plane of sym-
metry, tangential to the keel at the 
main step. 

(c) For a twin float seaplane, because 

of the effect of flexibility of the attach-
ment of the floats to the seaplane, the 
factor 

K

1

may be reduced at the bow 

and stern to 0.8 of the value shown in 
figure 2 of appendix B. This reduction 
applies only to the design of the carry-
through and seaplane structure. 

[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as 
amended by Amdt. 25–23, 35 FR 5673, Apr. 8, 
1970] 

§ 25.529

Hull and main float landing 

conditions. 

(a) 

Symmetrical step, bow, and stern 

landing.  For symmetrical step, bow, 
and stern landings, the limit water re-
action load factors are those computed 
under § 25.527. In addition— 

(1) For symmetrical step landings, 

the resultant water load must be ap-

plied at the keel, through the center of 
gravity, and must be directed per-
pendicularly to the keel line; 

(2) For symmetrical bow landings, 

the resultant water load must be ap-
plied at the keel, one-fifth of the longi-
tudinal distance from the bow to the 
step, and must be directed perpendicu-
larly to the keel line; and 

(3) For symmetrical stern landings, 

the resultant water load must be ap-
plied at the keel, at a point 85 percent 
of the longitudinal distance from the 
step to the stern post, and must be di-
rected perpendicularly to the keel line. 

(b) 

Unsymmetrical landing for hull and 

single float seaplanes. Unsymmetrical 
step, bow, and stern landing conditions 
must be investigated. In addition— 

(1) The loading for each condition 

consists of an upward component and a 
side component equal, respectively, to 
0.75 and 0.25 tan 

times the resultant 

load in the corresponding symmetrical 
landing condition; and 

(2) The point of application and di-

rection of the upward component of the 
load is the same as that in the sym-
metrical condition, and the point of ap-
plication of the side component is at 
the same longitudinal station as the 
upward component but is directed in-
ward perpendicularly to the plane of 
symmetry at a point midway between 
the keel and chine lines. 

(c) 

Unsymmetrical landing; twin float 

seaplanes.  The unsymmetrical loading 
consists of an upward load at the step 
of each float of 0.75 and a side load of 
0.25 tan 

at one float times the step 

landing load reached under § 25.527. The 
side load is directed inboard, per-
pendicularly to the plane of symmetry 
midway between the keel and chine 
lines of the float, at the same longitu-
dinal station as the upward load. 

§ 25.531

Hull and main float takeoff 

condition. 

For the wing and its attachment to 

the hull or main float— 

(a) The aerodynamic wing lift is as-

sumed to be zero; and 

(b) A downward inertia load, cor-

responding to a load factor computed 
from the following formula, must be 
applied: 

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257 

Federal Aviation Administration, DOT 

§ 25.533 

n

C

V

W

TO

S

=

1

2

2
3

1
3

tan

β

where— 

= inertia load factor; 
C

TO

= empirical seaplane operations factor 

equal to 0.004; 

V

S1

= seaplane stalling speed (knots) at the 

design takeoff weight with the flaps ex-
tended in the appropriate takeoff posi-
tion; 

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

grees); and 

= 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

S1

= 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

S1

= seaplane stalling speed at the design 

water takeoff weight with flaps extended 
in the appropriate takeoff position; and 

= 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— 
= 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

S0

= seaplane stalling speed (Knots) with 

landing flaps extended in the appropriate 
position and with no slipstream effect; 
and 

V

S0

= seaplane stalling speed with landing 

flaps extended in the appropriate posi-
tion and with no slipstream effect; and 

= 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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