Flight test techniques
Prior to take-off the helicopter loading is checked and the correct graphs, tables or hand-held computer program made available showing height to fly versus AUM or fuel gone and OAT. Care needs to be taken in selecting the combination of altitude and AUM used to obtain a given target referred weight as they both may limit the maximum and minimum speed that can be tested. Equally, consideration must be given to the order in which the speeds are flown so that the maximum coverage is obtained. It may therefore be necessary to either reballast or refuel the aircraft between a series of runs to contain the test height band within reasonable limits. It is worth remembering that provided the relevant referred parameters are kept constant, data gathered from several flights may be plotted together. This is one of the main advantages of the experimental method.
Before a test run is commenced, 1013 mbar is set on the altimeter sub-scale to ensure that it reads pressure altitude, and the position of all relevant secondary controls checked. The likely AUM of the aircraft at the start of the data run is calculated and a climb made to the altitude required to target the desired referred weight based on that estimated AUM. At this altitude the correct rotor RPM is set, if applicable, for the observed OAT. AUM, altitude, OAT and rotor RPM should then be re-checked and adjusted as necessary. Note that the target referred weight should be achieved approximately halfway through the conduct of the test point. If altimeter PECs are significant then due allowance will have to be made when reading the altimeter. If the atmospheric conditions are stable and the W/am2 method is being used time can be saved by first climbing to the top of the anticipated altitude band required for the complete sortie whilst documenting the outside air temperature. This provides the test aircrew with a complete air density profile thereby reducing the iteration required to obtain the target referred weights.
The following parameters are recorded when the helicopter is stabilized in level flight at the required airspeed:
• outside air temperature;
• rotor RPM;
• engine parameters (temperature and speed);
• fuel state;
• fuel flow;
The test condition should be maintained for a minimum of one minute, ideally taking further readings of the above parameters at the middle and end of the period to determine a mean value of each parameter; the accuracy of the test condition itself and the fuel flow in the absence of appropriate instrumentation. In a given aircraft, the accuracy with which a test run can be flown will depend on the atmospheric conditions, the airspeed and the altitude. The following accuracies are typical:
• airspeed – no worse than +1 kt
• altitude – no worse than +20 ft of desired altitude with no perceptible change during the run
• rotor speed – essentially fixed
• slipball – central
• referred weight – within 0.5% of target.
In this context, the vertical speed indicator (VSI) rarely gives sufficiently accurate indications of zero climb or descent. On suitably equipped rotorcraft; maximum fuel flow measurement accuracy will be obtained by starting the stopwatch as a fuel counter clicks over and stopping timing, after approximately one minute, as click over of the final reading occurs, see Roots and Blake [3.6] for alternative methods.