Accurate Performance Prediction

The ability to deploy computer methods in performance calculation has been a major factor in the rapid development of helicopter technology since the Second World War. Results may often not be greatly different from those derived from the simple analytical formulae, but the fact that the feasibility of calculation is not dependent upon making a large number of challengeable assumptions is important in pinning down a design, making comparisons with flight tests or meeting guarantees. So it is that commercial organizations and research centres are equipped nowadays with computer programs for use in all the principal phases of performance calculation – hover characteristics, trim analysis, forward flight performance, rotor thrust limits and so on.

It may be noted en passant that performance calculation is generally not the primary factor determining the need for numerical methods. The stressing of rotor blades makes a greater demand for complexity in calculation. Another highly important factor is the need for quantification of handling characteristics, as for example to determine the behaviour of a helicopter flying in an adverse aerodynamic environment.

Within the realm of performance prediction are contained many sub-items, not individually dominant but requiring detailed assessment if maximum accuracy is to be achieved. One such sub-item is parasite drag, in toto an extensive subject, as with fixed-wing aircraft, about which not merely a whole chapter but a whole book could be written. For computation purposes the total drag needs to be broken down into manageable groupings, among which are streamlined and non-streamlined components, fuselage angle of attack, surface roughness, leakage and cooling-air loss. Maximum advantage must be taken of the review literature, as compiled by Hoerner [2], Keys and Wiesner [3] and others, and of background information on projects similar to the one in hand.

Once a best estimate of parasite drag has been made, the accuracy problem in power calculation devolves upon the induced and profile items, as Equation 7.12 shows, together with the additional sub-items of tail rotor power, transmission loss and power to auxiliaries. Improving the estimation of induced and profile power comes down to the ability to use a realistic distribution of induced velocity over the disc area and the most accurate blade section lift and drag characteristics, including dynamic effects. This information has to be provided separately; the problem in the rotor is then to ascertain the angles of attack and Mach numbers of all blade sections, these varying from root to tip and round the azimuth as the blade rotates. That is basically what the focal computer programs do. Iterative calculations are normally required among the basic equations of thrust, collective and cyclic pitch and the flapping angles. A primary difficulty with helicopter rotor analysis is that one cannot solve a subset of the rotor equations – one has to address the interactions all together. Starting with, say, values of thrust and the flapping coefficients, corresponding values of the pitch angle, collective and cyclic, can be calculated; the information then allows the blade angles and local Mach numbers to be determined, from which the lift forces can be integrated into overall thrust for comparison with the value initially assumed. When the iterations have converged, the required output data – power requirement, thrust limits, and so on – can be ascertained.

These brief notes provide an initial examination. Going more deeply into the subject would immerse one immediately into more detail, so a calculation strategy using spread­sheets is provided in the Appendix. In addition, an excellent and thorough exposition of the total process of performance prediction is available in Stepniewski and Keys (Vol. II), to which the reader who wishes to come to grips with the whole computational complex is also referred.

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