A helicopter performance analysis is made to answer the questions:
The results of the analysis may be used in design tradeoff studies, in a pilot’s handbook, in a set of military Standard Aircraft Characteristic charts (SAC charts), or in a sales brochure. Before the analysis can be done, it is necessary to collect the individual items of information that are required: the performance of the individual rotors, the installed engine performance, the power losses in transmissions and accessories, the vertical drag in hover, the tail rotor-fin interference, and the parasite drag in forward flight. Methods of estimating rotor performance have been described in the preceding chapters. The other items will be discussed in the following paragraphs.
Almost all of the performance characteristics of the helicopter depend on engine performance. Engine manufacturers describe the performance of their engines in a specification. Among other things, these documents present the engine ratings and fuel consumptions under various conditions, based on the engine performance as measured in a test cell and then modified by standard and accepted techniques to account for the effects of altitude, temperature, and forward flight that cannot be duplicated in the test cell. In general, there are three types of ratings of interest to the helicopter engineer, which apply to both reciprocating and turboshaft engines:
Emergency, takeoff, or contingency Military or intermediate Maximum continuous or normal
On turboshaft engines, the rating is limited by the maximum allowable turbine inlet temperature, torque, or fuel flow. On reciprocating engines the limits are usually based on maximum intake manifold pressure and rpm. The ratings are a function of altitude, temperature, and forward speed. Figure 4.1 shows the zero
forward speed engine ratings for a typical turboshaft engine corresponding to the one installed in the example helicopter. Figure 4.2 shows how the ratings are affected by forward speed due to the ram effect on the performance of the compressor.
. The fuel flow at a given power is also a function of altitude and temperature. Fuel flow o? specific fuel consumption values in the engine specification apply only to new engines..The military requires that when preparing SAC charts, all fuel flow figures be raised 5 percent to account for possible engine deterioration. Figure 4.3 shows typical fuel flow curves incorporating the 5 percent increase.