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132 Engine Group<br />
the drag coefficient is nonzero. The drag area or drag coefficient is defined for forward flight and for<br />
vertical flight. The drag coefficient at angle-of-attack α is<br />
CD = CD0 +(CDV − CD0)| sin α| Xd<br />
typically using Xd =2. In sideward flight, CD = CD0 is used. The nacelle drag is Dnac = qSnacCD.<br />
17–4 Engine Scaling<br />
The parameters of the engine model can be defined for a specific engine, but it is also necessary to<br />
scale the parameters as part of the aircraft sizing task, in order to define an engine for a specified power.<br />
In addition, advanced technology must be represented in the model. Scaling and advanced technology<br />
are handled in terms of specific power and specific fuel consumption (at SLS static conditions, MCP, and<br />
Nspec). Figures 17-1 through 17-3 present historical data for engine specific fuel consumption, weight,<br />
and specific power, respectively; the variation at a given power reflects technology insertion with time.<br />
The engine model includes reference values of the engine performance parameters: P0R, SP0R,<br />
PmechR, sfc0C, SF0C, Nspec, and Nopt0C. Mass flow and fuel flow are obtained from ˙m0R = P0R/SP0R and<br />
˙w0C = sfc0CP0C. The reference power at each engine rating R defines a ratio to MCP: rp0R = P0R/P0C.<br />
Similarly for specific power and mechanical limits: rs0R = SP0R/SP0C and rm0R = PmechR/P0C. These<br />
ratios are kept fixed when the engine is scaled.<br />
The engine size is specified as takeoff power Pto = Peng: power at rating R, for SLS static conditions<br />
and specification turbine speed Nspec. Hence the MCP is P0C = Pto/rp0R, and the power at all other<br />
ratings follows.<br />
The actual (perhaps scaled) values of the performance parameters are available for the engine group:<br />
P0R, SP0R, PmechR, sfc0C, Fg0C, Nspec, Nopt0C.<br />
17–5 Weights<br />
The component weight consists of engine system, engine section or nacelle group, and air induction<br />
group. The engine system consists of engine, exhaust system, and accessories. The engine section or<br />
nacelle group consists of engine support, engine cowling, and pylon support. These weights are for the<br />
engine group, consisting of Neng engines.