Turns on one control

As discussed above the standard test for lateral and directional static stability is the steady heading sideslip (SHSS). The problem with this test technique is that it only indicates cockpit stability, that is how the stability appears to the pilot. A technique that may be of use in determining the relationship between stability and control power is that of turns on one control or ‘TO1C’. Although a co-ordinated turn normally involves both cyclic and pedal movements and possibly collective control, consider what happens if an attempt is made to turn the helicopter using only one control, cyclic stick or pedals, while the other is fixed.

From a condition of steady, trimmed level flight, consider the effects of initiating a turn to starboard using yaw pedals only. Firstly, as right pedal is pushed forward the helicopter will yaw to the right and sideslip to the left. The sideslip will cause the rotor disk to flap away from the relative airflow and this will produce a rolling moment to starboard. The subsequent bank to starboard will reduce the sideslip to the left and eventually cause a sideslip to the right. Finally, the helicopter will settle into a steady turn with the yaw pedal deflection adjusted as necessary to maintain the roll attitude of the aircraft. It will then normally have adopted an attitude in which it is yawing to starboard, sideslipping to starboard, banked to starboard. The amount of sideslip required to generate the roll response will indicate the strength of Lv since the test has not involved any contribution from LM. This, of course, assumes that the rolling moment due to pedal L0tr is negligible. Consideration of the stabilized turn leads to:

Assume a starboard turn (r positive) with starboard sideslip (v positive). The turn is opposed by the yaw damping (Nr being negative) and assisted by the directional stability
(Nv being positive). Thus, Nr. r is negative and Nv. v is positive so the sign of the term (Nr. r + Nv. v) depends on the relative magnitudes of the two terms. As N9tr is negative by defintion, if (Nr. r + Nv. v) is negative then 9tr will be positive and vice versa. The pedal deflection required can therefore be summarized as follows:

• The yaw pedals are deflected to yaw the helicopter into the turn (right pedal forward, 9tr is negative), then Nr. r > Nv. v and the yaw damping term is dominant.

• The yaw pedals are deflected to yaw the helicopter out of the turn (left pedal forward, 9tr is positive), then Nr. r < Nv. v and the directional stability term is dominant.

• No pedal movement from the trim position is necessary to maintain the turn (9tr is zero), then Nr. r = Nv. v and neither term is dominant.

Thus it is possible during the stabilized portion of a turn on one control-pedal (TO1C – P) to determine the relative magnitudes of the directional stability and the yaw damping. Equally the strength of the Lv effect can be gauged by the readiness with which the helicopter responds in roll to the sideslip generated by a TO1C-P. Assuming there is little or no contribution from L9a, strong lateral stability will be present if the helicopter rolls smartly to the right following the application of right pedal and the onset of left sideslip. Pedal-only turns are generally more difficult to perform than turns on cyclic alone (see later discussion) as some sideslip must be generated before the aircraft responds, as there will be a lag between yaw pedal application and the helicopter rolling into a turn. Consequently the input is usually in the form of a steady ramp with the rate of application varied to establish whether this has an effect on the subsequent response of the aircraft. Care must be exercised, as it is relatively easy to exceed sideslip limits during this test. Longitudinal cyclic is used as necessary to maintain the airspeed.

Now consider the effects of initiating a turn to starboard using lateral cyclic only. Firstly, as right cyclic is applied the helicopter will roll to the right and commence sideslipping to the right as the aircraft descends. The sideslip will cause the tail rotor and fin to generate a yaw moment starboard. Eventually, the helicopter will settle into a steady turn with the lateral cyclic deflection adjusted as necessary to maintain the roll attitude of the aircraft. The amount of sideslip required to generate the yaw response will indicate the strength of Nv since the test has not involved any contribution from N9a. This, of course, assumes that the yawing moment due to lateral cyclic NAl is negligible. Consideration of the stabilized turn yields:

Ai = – L~(Lr. r + Lv. v)

LAl

In a turn to starboard (r positive) with starboard sideslip (v positive), Lr. r will be positive (act to starboard) whilst Lv. v will be negative (act to port). As LA is negative, by definition Al will be positive if (L. r>Lv. v) and negative if (Lr. r < Lv. v). The control deflection can thus be summarized as: [10]

• Once established in the turn no cyclic movement from the position required for trim is necessary therefore neither term dominates as Lr. r = Lv. v.

With the aircraft established in a steady constant bank angle turn inspection of a sideslip gauge, skid-ball or suitably mounted string will confirm the strength of Nv. Weak directional stability will be evident if large values of sideslip are observed. Alterna­tively, strong directional stability will be evident if little sideslip is recorded.

Turns on one control-cyclic (TO1C-C) testing begins by stabilizing the helicopter at the required balanced flight condition and recording the trimmed control positions, aircraft heading, sideslip and bank angle. The rate of application and the magnitude of the lateral cyclic displacement is chosen to achieve a bank angle which is related to the requirements of the role, although the bank angle chosen should be approached incrementally (20° is a good initial condition). At the desired bank angle longitudinal cyclic is used as necessary to maintain constant IAS. The cyclic is then returned to the initial trim condition before the test is repeated in the opposite direction.