title-14-part-27-sec27-339

499

Federal Aviation Administration, DOT

§ 27.339

§ 27.307

Proof of structure.

(a) Compliance with the strength and

deformation requirements of this sub-
part must be shown for each critical
loading condition accounting for the
environment to which the structure
will be exposed in operation. Struc-
tural analysis (static or fatigue) may
be used only if the structure conforms
to those structures for which experi-
ence has shown this method to be reli-
able. In other cases, substantiating
load tests must be made.

(b) Proof of compliance with the

strength requirements of this subpart
must include—

(1) Dynamic and endurance tests of

rotors, rotor drives, and rotor controls;

(2) Limit load tests of the control

system, including control surfaces;

(3) Operation tests of the control sys-

tem;

(4) Flight stress measurement tests;
(5) Landing gear drop tests; and
(6) Any additional test required for

new or unusual design features.

(Secs. 604, 605, 72 Stat. 778, 49 U.S.C. 1424,
1425)

[Doc. No. 5074, 29 FR 15695, Nov. 24, 1964, as
amended by Amdt. 27–3, 33 FR 14105, Sept. 18,
1968; Amdt. 27–26, 55 FR 7999, Mar. 6, 1990]

§ 27.309

Design limitations.

The following values and limitations

must be established to show compli-
ance with the structural requirements
of this subpart:

(a) The design maximum weight.
(b) The main rotor r.p.m. ranges

power on and power off.

each main rotor r.p.m. within the
ranges determined under paragraph (b)
of this section.

(d) The maximum rearward and side-

ward flight speeds.

(e) The center of gravity limits cor-

responding to the limitations deter-
mined under paragraphs (b), (c), and (d)
of this section.

(f) The rotational speed ratios be-

tween each powerplant and each con-
nected rotating component.

(g) The positive and negative limit

maneuvering load factors.

LIGHT

OADS

§ 27.321

General.

(a) The flight load factor must be as-

sumed to act normal to the longitu-
dinal axis of the rotorcraft, and to be
equal in magnitude and opposite in di-
rection to the rotorcraft inertia load
factor at the center of gravity.

(b) Compliance with the flight load

requirements of this subpart must be
shown—

(1) At each weight from the design

minimum weight to the design max-
imum weight; and

(2) With any practical distribution of

disposable load within the operating
limitations in the Rotorcraft Flight
Manual.

[Doc. No. 5074, 29 FR 15695, Nov. 24, 1964, as
amended by Amdt. 27–11, 41 FR 55468, Dec. 20,
1976]

§ 27.337

Limit maneuvering load fac-

tor.

The rotorcraft must be designed for—
(a) A limit maneuvering load factor

ranging from a positive limit of 3.5 to
a negative limit of

1.0; or

(b) Any positive limit maneuvering

load factor not less than 2.0 and any
negative limit maneuvering load factor
of not less than

0.5 for which—

(1) The probability of being exceeded

is shown by analysis and flight tests to
be extremely remote; and

(2) The selected values are appro-

priate to each weight condition be-
tween the design maximum and design
minimum weights.

[Amdt. 27–26, 55 FR 7999, Mar. 6, 1990]

§ 27.339

Resultant limit maneuvering

loads.

The loads resulting from the applica-

tion of limit maneuvering load factors
are assumed to act at the center of
each rotor hub and at each auxiliary
lifting surface, and to act in directions,
and with distributions of load among
the rotors and auxiliary lifting sur-
faces, so as to represent each critical
maneuvering condition, including
power-on and power-off flight with the
maximum design rotor tip speed ratio.
The rotor tip speed ratio is the ratio of
the rotorcraft flight velocity compo-
nent in the plane of the rotor disc to

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500

14 CFR Ch. I (1–1–24 Edition)

§ 27.341

the rotational tip speed of the rotor
blades, and is expressed as follows:

μ =

V cos a

where—

V = The airspeed along flight path (f.p.s.);
a = The angle between the projection, in the

plane of symmetry, of the axis of no
feathering and a line perpendicular to
the flight path (radians, positive when
axis is pointing aft);

omega = The angular velocity of rotor (radi-

ans per second); and

R = The rotor radius (ft).

[Doc. No. 5074, 29 FR 15695, Nov. 24, 1964, as
amended by Amdt. 27–11, 41 FR 55469, Dec. 20,
1976]

§ 27.341

Gust loads.

The rotorcraft must be designed to

withstand, at each critical airspeed in-
cluding hovering, the loads resulting
from a vertical gust of 30 feet per sec-
ond.

§ 27.351

Yawing conditions.

(a) Each rotorcraft must be designed

for the loads resulting from the maneu-
vers specified in paragraphs (b) and (c)
of this section with—

(1) Unbalanced aerodynamic mo-

ments about the center of gravity
which the aircraft reacts to in a ration-
al or conservative manner considering
the principal masses furnishing the re-
acting inertia forces; and

(2) Maximum main rotor speed.
(b) To produce the load required in

paragraph (a) of this section, in unac-
celerated flight with zero yaw, at for-
ward speeds from zero up to 0.6 V

—

(1) Displace the cockpit directional

control suddenly to the maximum de-
flection limited by the control stops or
by the maximum pilot force specified
in § 27.397(a);

(2) Attain a resulting sideslip angle

or 90

, whichever is less; and

(3) Return the directional control

suddenly to neutral.

paragraph (a) of this section, in unac-
celerated flight with zero yaw, at for-
ward speeds from 0.6 V

up to V

, whichever is less—

(1) Displace the cockpit directional

control suddenly to the maximum de-
flection limited by the control stops or

by the maximum pilot force specified
in § 27.397(a);

(2) Attain a resulting sideslip angle

or 15

, whichever is less, at the lesser

speed of V

or V

;

(3) Vary the sideslip angles of para-

graphs (b)(2) and (c)(2) of this section
directly with speed; and

(4) Return the directional control

suddenly to neutral.

[Amdt. 27–26, 55 FR 7999, Mar. 6, 1990, as
amended by Amdt. 27–34, 62 FR 46173, Aug. 29,
1997]

§ 27.361

Engine torque.

(a) For turbine engines, the limit

torque may not be less than the high-
est of—

(1) The mean torque for maximum

continuous power multiplied by 1.25;

(2) The torque required by § 27.923;
(3) The torque required by § 27.927; or
(4) The torque imposed by sudden en-

gine stoppage due to malfunction or
structural failure (such as compressor
jamming).

(b) For reciprocating engines, the

limit torque may not be less than the
mean torque for maximum continuous
power multiplied by—

(1) 1.33, for engines with five or more

cylinders; and

(2) Two, three, and four, for engines

with four, three, and two cylinders, re-
spectively.

[Amdt. 27–23, 53 FR 34210, Sept. 2, 1988]

ONTROL

URFACE AND

YSTEM

OADS

§ 27.391

General.

Each auxiliary rotor, each fixed or

movable stabilizing or control surface,
and each system operating any flight
control must meet the requirements of
§§ 27.395, 27.397, 27.399, 27.411, and 27.427.

[Amdt. 27–26, 55 FR 7999, Mar. 6, 1990, as
amended by Amdt. 27–34, 62 FR 46173, Aug. 29,
1997]

§ 27.395

Control system.

(a) The part of each control system

from the pilot’s controls to the control
stops must be designed to withstand
pilot forces of not less than—

(1) The forces specified in § 27.397; or
(2) If the system prevents the pilot

from applying the limit pilot forces to
the system, the maximum forces that