Code of Federal Regulations - Title 14

239

Federal Aviation Administration, DOT

§ 23.629

§ 23.627

Fatigue strength.

The structure must be designed, as

far as practicable, to avoid points of
stress concentration where variable
stresses above the fatigue limit are
likely to occur in normal service.

§ 23.629

Flutter.

(a) It must be shown by the methods

of paragraph (b) and either paragraph
(c) or (d) of this section, that the air-
plane is free from flutter, control re-
versal, and divergence for any condi-
tion of operation within the limit V-n
envelope and at all speeds up to the
speed specified for the selected method.
In addition—

(1) Adequate tolerances must be es-

tablished for quantities which affect
flutter, including speed, damping, mass
balance, and control system stiffness;
and

(2) The natural frequencies of main

structural components must be deter-
mined by vibration tests or other ap-
proved methods.

(b) Flight flutter tests must be made

to show that the airplane is free from
flutter, control reversal and divergence
and to show that—

(1) Proper and adequate attempts to

induce flutter have been made within
the speed range up to V

or V

for jets;

(2) The vibratory response of the

structure during the test indicates
freedom from flutter;

(3) A proper margin of damping exists

at V

, or V

for jets; and

(4) As V

(or V

for jets) is

approached, there is no large or rapid
reduction in damping.

dict freedom from flutter, control re-
versal and divergence must cover all
speeds up to 1.2 V

/1.2 M

, limited to

Mach 1.0 for subsonic airplanes.

(d) Compliance with the rigidity and

mass balance criteria (pages 4–12), in
Airframe and Equipment Engineering
Report No. 45 (as corrected) ‘‘Sim-
plified Flutter Prevention Criteria’’
(published by the Federal Aviation Ad-
ministration) may be accomplished to
show that the airplane is free from
flutter, control reversal, or divergence
if—

(1) V

for the airplane is less than

260 knots (EAS) and less than Mach 0.5,

(2) The wing and aileron flutter pre-

vention criteria, as represented by the
wing torsional stiffness and aileron
balance criteria, are limited in use to
airplanes without large mass con-
centrations (such as engines, floats, or
fuel tanks in outer wing panels) along
the wing span, and

(3) The airplane—
(i) Does not have a T-tail or other un-

conventional tail configurations;

(ii) Does not have unusual mass dis-

tributions or other unconventional de-
sign features that affect the applica-
bility of the criteria, and

(iii) Has fixed-fin and fixed-stabilizer

surfaces.

(e) For turbopropeller-powered air-

planes, the dynamic evaluation must
include—

(1) Whirl mode degree of freedom

which takes into account the stability
of the plane of rotation of the propeller
and significant elastic, inertial, and
aerodynamic forces, and

(2) Propeller, engine, engine mount,

and airplane structure stiffness and
damping variations appropriate to the
particular configuration.

(f) Freedom from flutter, control re-

versal, and divergence up to V

must be shown as follows:

(1) For airplanes that meet the cri-

teria of paragraphs (d)(1) through (d)(3)
of this section, after the failure, mal-
function, or disconnection of any single
element in any tab control system.

(2) For airplanes other than those de-

scribed in paragraph (f)(1) of this sec-
tion, after the failure, malfunction, or
disconnection of any single element in
the primary flight control system, any
tab control system, or any flutter
damper.

(g) For airplanes showing compliance

with the fail-safe criteria of §§ 23.571
and 23.572, the airplane must be shown
by analysis to be free from flutter up
to V

after fatigue failure, or obvi-

ous partial failure, of a principal struc-
tural element.

(h) For airplanes showing compliance

with the damage tolerance criteria of
§ 23.573, the airplane must be shown by
analysis to be free from flutter up to
V

with the extent of damage for

which residual strength is dem-
onstrated.

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