Effect of changes in collective pitch
The concept of using turning flight as a method of assessing manoeuvre stability can be extended to encompass the more role-relatable case of a level turn at constant speed. As with the descending turn, since the load factor will be related to the bank angle the severity of the manoeuvre can be incrementally increased. The complication introduced by level turns is that in order to maintain height collective pitch will be required and the pitch change with power effect will require cyclic pitch to compensate. Assuming that an increase in collective pitch causes a nose-up pitching moment it is clear that a level turn will require less aft cyclic than a descending turn at the same load factor (angle of bank). Therefore, to the pilot, the helicopter will appear to be less manoeuvre stable. In order to make the distinction between these two cases the following terminology is used: collective fixed manoeuvre stability (assessed either during pull-ups/push-overs or during descending turns at constant speed and fixed collective) and apparent manoeuvre stability (assessed during level turns at constant speed).
18.104.22.168 Test techniques
Although apparent static stability tests are role relatable in that most steep turns are conducted in level flight (albeit not necessarily at constant IAS), they are of limited use in determining the true characteristics of the aircraft since, as stated above, the increased collective necessary to sustain level flight at high angles of bank is usually destabilizing. Additionally the Ministry of Defence Standard 00-970 [5.1] requires manoeuvre stability to be evaluated in a turn initiated from steady straight and level flight conditions. Consequently both forms of stability testing are conducted.
Descending turns at fixed collective, or wind-up turns, are commenced by first establishing a trim condition in level unaccelerated, ball-centred flight at the datum altitude. The aircraft is then climbed a suitable increment above datum altitude, without re-trimming, and the trim control positions (collective and longitudinal) are re-established. A constant airspeed, ball-centred, fixed-collective descending turn is then entered aiming to be on condition in stable flight at the desired bank angle, and therefore load factor, as the datum altitude is passed. Data is normally recorded continuously in an altitude band +1000 ft from datum, the most stable 10 seconds or so being selected for analysis after the flight. Data typically includes airspeed, load factor, longitudinal cyclic stick position and fuel state. The test is then repeated at incrementally increasing bank angle until a limiting condition (angle of bank or load factor) is reached. At small bank angles it is often possible to complete several test points within the test altitude band before having to climb back up.
Manoeuvre stability during symmetric manoeuvres can be evaluated by studying the normal acceleration time history following a pull-up or push-over. Since a key parameter in manoeuvre stability testing is airspeed it should be maintained essentially constant. This exposes a complication with symmetric manoeuvres since the change in flight path angle results in a variation in the ‘X’ component of gravity causing a change in airspeed and an attitude-related change in normal acceleration as measured in the ‘Z’ axis of the aircraft. The corollary of these two effects is that data must be taken during the short period when forward velocity is essentially equal to the datum airspeed and when the aircraft is in the level pitch attitude. The type of manoeuvre necessary to comply with these constraints is difficult to fly accurately. The Ministry of Defence Standard 00-970 [5.1] attempts to get around these problems by requiring only that the peak increment in normal acceleration should be ‘substantially proportional to the magnitude of the control input’ and that it should ‘increase progressively with increasing initial airspeed ’, the implication being that testing can be achieved by incremental step inputs from trimmed, level, unaccelerated flight. Nevertheless, in order to allow enough time to develop peak load factor in an agile aircraft, it is necessary to commence the manoeuvre from a nose-down attitude so that the recovery can be made before an excessive nose-up condition develops. A control fixture can be used to provide some means of incrementally increasing the severity of the manoeuvre. Push-overs are always approached with the utmost caution in helicopters with teetering rotors or with articulated rotors featuring low hinge offset. The technique is the reverse of that for the pull-ups: the aircraft is accelerated/descended from the trim condition, pulled up to a nose-high attitude and then bunted using the appropriate control input as it decelerates towards the trim speed.