Explanation.
(1) For rotorcraft equipped with auxiliary rotors, normally called tail rotors, an
endurance test is required by § 27.923, and structural strength substantiation is
required. Section 27.401(b) specifically refers to structural strength substantiation of
detachable blade systems for centrifugal loads resulting from maximum design rotor
RPM.
(2) The rotor blade structure must have sufficient strength to withstand not only
aerodynamic loads generated on the blade surface, but also inertial loads arising from
centrifugal, coriolis, gyroscopic, and vibratory effects produced by this blade movement.
Sufficient stiffness and rigidity must be designed into the blades to prevent excessive
deformation and to ensure that the blades will maintain the desired aerodynamic
characteristics. As a design objective, the structural strength requirements should be
met with the minimum material. Excess blade weight imposes extra centrifugal loads
that may increase the operating stress levels. Blade weight and strength should be
optimized. Even though a structural strength analysis for the blade design loads is
required, a flight load survey and fatigue analysis are also required by § 27.571.
(3) Section 27.1509 defines the design rotor speed as that providing a
5 percent margin beyond the rotor operating speed limits.
b. Procedures.
(1) The endurance tests prescribed by §§ 27.923 and 27.927 require achieving
certain speeds, power, and control displacement for the auxiliary (tail) rotor as well as
the main rotor. The parts must be serviceable at the conclusion of the tests.
(2) Structural substantiation of the auxiliary (tail) rotor is required to ensure
integrity for the minimum and maximum design rotor speeds and the maximum design
rotor thrust in the positive and negative direction. Thrust capability of the rotor should
offset the main rotor torque at maximum power as required by § 27.927(b).
(i) The maximum and minimum operating rotor speed, power-off, is
95 percent of the maximum design speed and 105 percent of the minimum design
speed, respectively.
(ii) The rotor operating speed limits shown during the official
FAA/AUTHORITY flight tests must include the noted 5 percent margin with respect to
the design speeds.
(iii) The auxiliary rotor generally has a positive and negative pitch limit that
ensures adequate directional control throughout the operating range of the rotorcraft.
The power-off rotor speed limits are generally broader than the power-on rotor speed
limits because of the required autorotational rotor speed characteristics. Thus, the
auxiliary rotor design conditions concern the maximum and minimum design rotor
speeds in conjunction with the maximum positive or negative pitch thrust, as
appropriate. Thrust capability and precone angle of the rotor, if any, will significantly
influence the rotor design loads. The variations in rotor design features and an example
of substantiation would be too lengthy to include here. However, ANC-9, “Aircraft
Propeller Handbook” contains principles that may be applied to tail rotor designs. Tail
rotors may be considered a special propeller design.
(iv) Bearings are generally used in the tail rotor installation to allow
flapping and feathering motion of the blades. The bearing manufacturer’s ratings of
these bearings must not be exceeded. Bearings generally used in main and tail rotors
are classified as ABEC Class 3, 5, or 7. Class 7 is the highest quality presently
available. Satisfactory completion of the endurance tests of §§ 27.923 and 27.927 is a
means of proving that use of a particular bearing is satisfactory.
AC 27-1B 9/30/99
(v) The analysis must include appropriate special factors, casting factors,
bearing factors, and fitting factors prescribed by §§ 27.619, 27.621, 27.623, and 27.625,
respectively. The fitting factor of 1.15 must be applied in the analysis of the tail rotor
installation.
