AMC No 1 to CS 27.351 Yawing conditions

ED Decision 2016/024/R

(a)     Definitions:

(1)     Suddenly. For the purpose of this AMC, ‘suddenly’ is defined as an interval not to exceed 0.2 seconds for a complete control input. A rational analysis may be used to substantiate an alternative value.

(2)     Initial Trim Condition. Steady, 1G level flight condition with zero bank angle or zero sideslip.

(3)     ‘Line’. The rotorcraft’s sideslip envelope, defined by the rule, between 90° at 0.6V_NE and 15° at V_NE or V_H whichever is less (see Figure 1).

(4)     Resulting Sideslip Angle. The rotorcraft’s stabilised sideslip angle that results from a sustained maximum cockpit directional control deflection or as limited by pilot effort in the initial level flight power conditions.

(b)     Explanation: The rule requires a rotorcraft’s ‘structural’ yaw or sideslip design envelope that must cover a minimum forward speed or hover to V_NE or V_H whichever is less. The scope of the rule is intended to cover structural components that are primarily designed for the critical combinations of tail rotor thrust, inertial and aerodynamic forces. This may include but is not limited to fuselage, tailboom and attachments, vertical control surfaces, tail rotor and tail rotor support structure.

(1)     The rotorcraft’s structure must be designed to withstand the loads in the specified yawing conditions. The standard does not require a structural flight demonstration. It is a structural design standard.

(2)     The standard applies only to power-on conditions. Autorotation need not be considered.

(3)     This standard requires the maximum allowable rotor revolutions per minute (RPM) consistent with each flight condition for which certification is requested.

(4)     For the purpose of this AMC, the analysis may be performed in international standard atmosphere (ISA) sea level conditions.

(5)     Maximum displacement of the directional control, except as limited by pilot effort (27.397(a)), is required for the conditions cited in the rule. A control-system-limiting device may be used, however the probability of failure or malfunction of these system(s) should be considered (See AMC No 2 to CS 27.351 Interaction of System and Structure).

(6)     Both right and left yaw conditions should be evaluated.

(7)     The air loads on the vertical stabilisers may be assumed independent of the tail rotor thrust.

(8)     Loads associated with sideslip angles exceeding the values of the ‘line’, defined in Figure 1, do not need to be considered. The corresponding points of the manoeuvre may be deleted.

(c)      Procedure: The design loads should be evaluated within the limits of Figure 1 or the maximum yaw capability of the rotorcraft whichever is less at speeds from zero to V_H or V_NE whichever is less for the following phases of the manoeuvre (see Note 1):

(1)     With the rotorcraft at an initial trim condition, the cockpit directional control is suddenly displaced to the maximum deflection limited by the control stops or by the maximum pilot force specified in 27.397(a). This is intended to generate a high tail rotor thrust.

(2)     While maintaining maximum cockpit directional control deflection, within the limitation specified in (c)(1) of this AMC allow the rotorcraft to yaw to the maximum transient sideslip angle. This is intended to generate high aerodynamic loads that are determined based on the maximum transient sideslip angle or the value defined by the ‘line’ in Figure 1 whichever is less (see Note 1).

(3)     Allow the rotorcraft to attain the resulting sideslip angle. In the event that the resulting sideslip angle is greater than the value defined by the ‘line’ in Figure 1, the rotorcraft should be trimmed to that value of the angle using less than maximum cockpit directional-control deflection by taking into consideration the manoeuvre’s entry airspeed (see Note 2).

(4)     With the rotorcraft yawed to the resulting sideslip angle specified in (c)(3) of this AMC, the cockpit control is suddenly returned to its initial trim position. This is intended to combine a high tail rotor thrust and high aerodynamic restoring forces.

Figure 1 — YAW/FORWARD SPEED DIAGRAM

NOTE:

(1)     When comparing the rotorcraft’s sideslip angle against the ‘line’ of Figure 1, the entry airspeed of the manoeuvre should be used.

(2)     When evaluating the yawing condition against the ‘line’ of Figure 1, sufficient points should be investigated in order to determine the critical design conditions. This investigation should include the loads that result from the manoeuvre, specifically initiated at the intermediate airspeed which is coincident with the intersection of the ‘line’ and the resultant sideslip angle (point A in Figure 1).

(d)     Another method of compliance may be used with a rational analysis (dynamic simulation), acceptable to the Agency/Authority, performed up to V_H or V_NE whichever is less, to the maximum yaw capability of the rotorcraft with recovery initiated at the resulting sideslip angle at its associated airspeed. Loads should be considered for all portions of the manoeuvre.

[Amdt 27/4]