MANOEUVRE STABILITY TESTING

The manoeuvre stability of a helicopter will manifest itself to the pilot as the amount of aft stick required to maintain an elevated load factor. The pilot can achieve such a load factor either in a pull-up manoeuvre or during turning flight. Therefore manoeuvre stability can be assessed in either situation. (Push-overs can be used to assess stability at reduced load factor.) The pros and cons of each method are:

(1) Pull-ups. The pull-up manoeuvre is easily role-relatable and does not give false’ data since the radius of the circular flight path is in line with the normal axis of the helicopter. The test technique which requires achievement of the elevated load factor in level flight, at zero pitch attitude, on speed and with the collective fixed at PFLF is, however, difficult to fly accurately and repeatably. It is also not possible to assess manoeuvre instability using PUPOs as it will be impossible to establish the correct balance of flight parameters.

(2) Turning flight. Descending turns, at constant speed with fixed collective (at PFLF), are, perhaps, less easy to role-relate. This manoeuvre is, however, easier to fly accurately and provides a method of incrementally increasing the load factor by simply increasing the angle of bank. The data obtained will be ‘false’ since for a given load factor a greater pitch rate will be required than for a pull – up because the centrifugal acceleration is not aligned with the normal axis of the helicopter (except at 90° of bank). Despite the increased workload it is possible to temporarily stabilize a manoeuvre unstable aircraft during quantita­tive data gathering.

5.3.1 Theoretical treatment of test methods

5.3.1.1 Symmetrical pull-ups

During pull-up testing, as described earlier, the pilot endeavours to achieve the desired load factor with the aircraft at zero pitch attitude, wings level, on speed and with no
yaw rate developing. The tests are conducted with fixed collective and it is also assumed that a steady pitch rate is achieved. Therefore:

и = и = w = p = p = r = q = 0e = 80c = 0

Thus the linearized form of the longitudinal equations of motion reduce to:

– mqUe = w. Zw + q. Zq + SB,. ZBi

0 = w. Mw + q. Mq + SB,. MBi

Applying the concept of circular motion to the pull-up manoeuvre yields:

g

qPun-uP = и(n -1)

Ue

Substituting and solving simultaneously:

SB, = g ГMqZw – mUeMw – ZqMw (n – 1) Ue ZB, Mw – MB, Zw

Assuming that Zq = 0:

dB, = g MqZw – mUeMw dn = Ue ZB, Mw – MB, Zw

5.3.1.2 Steady turns

Consider a helicopter performing a steady turn with fixed collective. Assuming that the flight path angle and bank angle are small then it is accurate to assume that the thrust vector acts parallel to the rotor shaft. For equilibrium:

mg = Tcos ф CF = T sin ф CF = mRturn m2

Now:

T Vmg2 + CF2 Г Zcf 2 1

ntur n = = = 1 + ( I = Г

mg mg ^ mg I cos ф

The linear and angular velocities can be related by:

CF = mRturn m2 = mVm

If the flight path angle is small and w, the vertical velocity, is small then V = Ue. Thus:

nt2urn = 1 + j and m = g Vntum – 1

Noting that the pitch rate, q, is related to the rate of turn (in a vertical bank they would be equal) by qturn = m sin ф, gives:

g (nt2urn – 1)

Un

It is evident from the above equation that the pitch rate required in a turn is greater than a pull-up for the same load factor. Since:

n2 + 1

then: