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136 Referred Parameter Turboshaft Engine Model
The inlet ram air temperature ratio and pressure ratio are obtained then from M and the inlet ram
recovery efficiency ηd:
γ − 1 2
θM = 1+ M =
2 1+0.2M 2
γ − 1
δM = 1+ ηdM
2
2
γ
γ−1
= 1+0.2ηdM 23.5 where the ratio of specific heats γ =1.4.
18–2 Engine Ratings
The power available from a turboshaft engine depends on the engine rating. Each engine rating
has specific operating limitations, most importantly an operating time limit intended to avoid damage
to the engine. Typical engine ratings are given in table 18-1. Engine power is generally specified in
terms of SLS static MCP. Takeoff typically uses MRP. CRP or ERP are restricted to use in one-engine
inoperative (OEI) emergencies.
Table 18-1. Typical engine ratings.
rating description time limit
MCP maximum continuous power ∞
IRP intermediate rated power 30 min
MRP maximum rated power 10 min
CRP contingency rated power 2.5 min
ERP emergency rated power 1.0 min
18–3 Performance Characteristics
The engine performance is described by: the power available Pa, at each engine rating and the
specification engine-turbine speed Nspec; the mass flow ˙m and fuel flow ˙w required to produce power
required Pq at engine-turbine speed N; and the gross jet thrust F at a given power required Pq. Then the
specific power is SP = P/ ˙m, and the specific fuel consumption is sfc = ˙w/P.
The reference performance is at sea-level-standard static conditions (subscript 0), and MCP (subscript
C). Referred or corrected engine parameters are used in the model:
power
mass flow
specific power
P
δ √ θ
˙m
δ/ √ θ
P/ ˙m
θ
fuel flow
thrust
turbine speed
For each rating R, the performance is characterized by the following quantities for sea-level-standard
static conditions: power P0R, specific power SP0R, and mechanical power limit PmechR. The mass flow
˙w
δ √ θ
F
δ
N
√ θ