Choice of referred grouping
The choice of referred grouping is partly dependent on the test aircrew’s ability to adjust rotor speed in flight. Another factor to consider is the test objectives. If there is a requirement to assess the magnitude of compressibility and retreating blade stall effects (tip effects) then it is prudent to use the W/am2 method regardless. Consider the referred power required by a typical conventional helicopter as a function of referred true airspeed. If test data is plotted using the P/am3 grouping the absence of any tip effects, implied by a constant drag coefficient, will be immediately apparent by the coincidence of the curves. This is because all the referred parameters in this grouping contain RRPM (m) and so any increase in power in proportion to the increased rotor speed will be removed by the referring process. However, any increase in power not in proportion to RRPM (due in reality to an increased average drag coefficient) will be unaffected by the referring process and will be evident when plotted, see Fig. 3.14.
1.4.1 Engine characteristics
The manufacturer’s predictions of engine performance will be based on test bed data. It is important, therefore, during a comprehensive performance trial that engine
Fig. 3.14 Effect of referred rotor speed on power required.
parameters are recorded on every flight so that these predictions can be confirmed. Additionally, the effects of inlet losses and air bleeds and of any deterioration throughout a prolonged flying period can be ascertained. If manufacturer’s engine performance data is not available then the effects of varying m/^0 and any other engine configuration parameter, such as the IGV or BCV position (A), on power available should be considered. Although experience has shown that these parameters do not generally alter the unique relationship between P/§V0, G/SV0, and N/-J0 over the working range of the engine they cannot be ignored in the absence of reliable data. They should be dealt with as follows. If m/^0 is critical then curves of referred power against referred N will vary with m/^0 due to changes in the efficiency of the free power turbine (FPT). Tests can be made at high, low and intermediate values of m/V0; interpolation will then be possible between them. If rotor speed does not vary, sorties can be flown at extremes of OAT to establish the required spread of values of 1/V0. If A is critical, generally because the IGVs are not fully open over the working range of the engine, differential corrections will have to be calculated to obtain the correct values of power and fuel flow.
When undertaking an evaluation of the level flight performance accurate calibration of the engine, the power instruments and the fuel flow measurement system is essential. Usually the engine will be bench tested at the manufacturer before and after the tests so that the results can be compared with the manufacturer’s predictions based on accurate test bed data. Some deterioration of engine condition is inevitable during a prolonged trial and so the post-trial engine bench tests are as important as the pretrial bench tests. Test bed data is also used for the correction of the effects of engine bleeds, although the bleed ports can be blanked off to ensure that they do not affect the results. Roots and Blake [3.6] give more details on the use of calibrated engines and the importance of gathering engine data whilst conducting evaluations of aircraft performance.