Engine matching

In multi-engined installations each engine will typically operate to an individual droop law due to different mechanical tolerances within the engines and their control systems. To ensure that the maximum power is available from all the engines a means of power matching is usually provided. The pilot will normally match the engine torques but engine temperatures may also be used if they become the limiting variable. Pilots are sensitive to torque mismatches at all power settings as this can be the initial indication of an engine failure or governor malfunction. Consequently some automatic alerting systems use torque mismatches, above a certain threshold, as the trigger for activation of an engine failure warning. These have not proved popular due to the difficulty of eliminating false warnings.

Testing of static power matching can be combined with static droop testing by matching engine torques at a datum power setting, normally minimum power for rotors at pre-take off NR on the ground or maximum continuous torque in flight, and then noting the mismatch throughout the power range. Mismatches during rapid power changes are also documented during transient overswing and engine accelera­tion tests. The additional workload placed on the crew in dealing with power matching will determine if a deficiency exists. This will depend not only on the size of the mismatches but also on the characteristics of the manual power matching system. For example, an aircraft which suffers from a large power mismatch on take-off and has the manual matching control mounted off the flight controls may significantly increase the crew workload. For obvious reasons the engines must be correctly adjusted before these tests.

7.4.3 Transient droop, transient torque response and overswing

Once the static variation of NR with power setting has been established tests are made to determine the dynamic characteristics of the engine(s) and governing systems. Transient droop, transient torque response and overswing testing involves assessing how the power supply system reacts to sudden changes in power required.

7.4.6.1 Transient droop

The amount by which the NR droops following a rapid collective pitch increase depends on the rate of collective increase, the rotor inertia, the acceleration capabilities of the engine(s) and the speed of reaction of the engine(s) and rotor governing system. For the operational pilot a large amount of rotor droop following a collective lever pull could have serious consequences; it might not be possible to arrest a rate of descent as quickly as required and may also lead to problems with control response at low rotor speeds.

Testing transient droop involves collective lever pulls from a low power position to a high power position at incrementally increasing rates until a limiting factor is reached or a satisfactory result consistent with the aircraft role is obtained. The low power position should ideally equate to zero torque but depending on the design of the gearbox it may be necessary to ‘join the needles’ by matching the speeds of the power turbine(s) and the rotor to avoid damage to the transmission system. The higher collective position is chosen such that any torque spikes remain within the transient limits; typical values are collective positions that equate to 90% or 95% of maximum continuous torque. Some means is used to block the co-pilot’s collective to prevent the maximum test value being exceeded; this can either be the flight test engineer’s hand or a fixture held by him or her.

The test is conducted by setting the maximum collective position and establishing the block. The collective is then lowered to the low power position previously established. A slow collective ramp input is made over a period of at least 5 seconds until the collective contacts the block. The timing is achieved by one of the crew members conducting a cadence count. It is important for the control to be moved at a constant rate throughout the pull and no attempt should be made to vary the rate of application simply to meet the target time. In addition to noting the Nr droop, the maximum torque value and the maximum engine temperatures are recorded. The crew must also be alert for any sign of instability within the power system and for other effects such as large yaw rates occurring. If the test team conclude that it is safe to increase the rate of collective application then the test is repeated using a count reduced by one second. Although this incremental approach is the safest way of approaching rapid lever rates experience has shown that large variations in peak torque may occur with even small increases in collective rate.

Once the ‘academic’ tests have been completed a series of mission tasks should be flown which are designed to identify any problems with transient droop. These may include jump take-offs, baulked landings, recovery from flight idle glide and the final stages of a quick stop. For naval aircraft, landings are made onto a heaving deck or this task is simulated. Aircraft with collective anticipators fitted may have no droop problems with collective inputs, however, droop may occur with inputs made with the cyclic or yaw pedals. In these cases tests are also made of rapid rolling in forward flight and of rapid yaw inputs in the hover.