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Chapter 17<br />
Engine Group<br />
The engine group consists of one or more engines of a specific type. For each engine type an engine<br />
model is defined. The Referred Parameter Turboshaft Engine Model (RPTEM) is implemented.<br />
The engine size is described by the power Peng, which is the sea-level static power available per<br />
engine at a specified takeoff rating. The number of engines Neng is specified for each engine group.<br />
If the sizing task determines the engine power for a propulsion group, the power Peng of at least one<br />
engine group is found (including the first engine group). The total power required is PPG = r NengPeng,<br />
where r = max(PreqPG/PavP G). The sized power is Psized = PPG − <br />
fixed NengPeng. Then the sized<br />
engine power is Peng = fnPsized/Neng for the n-th engine group (with f1 = <br />
n=1,sized fn for the first<br />
group).<br />
17–1 Engine Performance<br />
17-1.1 Power Available<br />
Given the flight condition and engine rating, the power available Pa is calculated (from the specific<br />
power SPa and mass flow ˙ma). The flight-condition information includes the altitude, temperature,<br />
flight speed, and primary rotor speed; a power fraction fP ; and the states of the engine, drive system,<br />
and IR suppressor. The engine turbine speed is N = (60/2π)rengΩprim, where Ωprim is the current rotor<br />
speed and reng is the gear ratio (depending on the drive-system state). If the reference primary rotor<br />
speed Ωprim corresponds to the specification turbine speed Nspec, then reng =Ωspec/Ωprim; alternatively,<br />
the engine gear ratio can be a fixed input.<br />
In the engine model, installation losses Ploss are subtracted from Pa (Pav = Pa − Ploss), and then<br />
the mechanical limit applied: Pav = min(Pav,PmechR).<br />
The engine model gives the performance of a single engine. The power available of the engine<br />
group is obtained by multiplying the single-engine power by the number of engines operational (total<br />
number of engines less inoperable engines):<br />
PavEG =(Neng − Ninop)Pav<br />
The propulsion group power available is obtained from the sum over the engine groups: PavP G =<br />
fP PavEG, including a specified power fraction fP .<br />
The drive-system rating at the flight condition is rPDSlimit, where r =Ωprim/Ωref. Optionally<br />
this limit is applied to the propulsion group power: PavP G = min(PavP G,rPDSlimit). Similarly the<br />
engine-shaft limit at the flight condition is optionally applied to the engine group power.