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Chapter 16
Propulsion
The propulsion group is a set of components and engine groups, connected by a drive system. The
engine model describes a particular engine, used in one or more engine group. The components (rotors)
define the power required. The engine groups define the power available.
16–1 Drive System
The drive system defines gear ratios for all the components it connects. The gear ratio is the ratio
of the component rotational speed to that of the primary rotor. There is one primary rotor per propulsion
group (for which the reference tip speed is specified); other components are dependent (for which a gear
ratio is specified). There can be more than one drive-system state, in order to model a multiple-speed or
variable-speed transmission. Each drive-system state corresponds to a set of gear ratios.
For the primary rotor, a reference tip speed Vtip−ref is defined for each drive-system state. By
convention, the “hover tip speed” refers to the reference tip speed for drive state #1. If the sizing task
changes the hover tip speed, then the ratios of the reference tip speeds at different engine states are kept
constant. By convention, the gear ratio of the primary rotor is r =1. For dependent rotors, either the
gear ratio is specified (for each drive-system state) or a tip speed is specified and the gear ratio calculated
(r =Ωdep/Ωprim, Ω=Vtip−ref/R). For the engine group, either the gear ratio is specified (for each drivesystem
state) or the gear ratio calculated from the specification engine turbine speed Ωspec =(2π/60)Nspec
and the reference tip speed of the primary rotor (r =Ωspec/Ωprim, Ωprim = Vtip−ref/R). The latter option
means the specification engine turbine speed Nspec corresponds to Vtip−ref for all drive-system states.
To determine the gear ratios, the reference tip speed and radius are used, corresponding to hover.
The flight state specifies the tip speed of the primary rotor and the drive-system state, for each
propulsion group. The drive-system state defines the gear ratio for dependent rotors and the engine
groups. From the rotor radius the rotational speed of the primary rotor is obtained (Ωprim = Vtip/R);
from the gear ratios, the rotational speed of dependent rotors (Ωdep = rΩprim) and the engine groups
(N = (60/2π)rengΩprim) are obtained; and from the rotor radius, the tip speed of the dependent rotor
(Vtip =ΩdepR) is obtained. The flight-state specification of the tip speed can be an input value; the
reference tip speed; a function of flight speed or a conversion schedule; or one of several default values.
These relationships between tip speed and rotational speed use the actual radius of the rotors in the flight
state, which for a variable-diameter rotor may not be the same as the reference, hover radius.
An optional conversion schedule is defined in terms of two speeds: hover and helicopter mode for
speeds below VChover, cruise mode for speeds above VCcruise, and conversion mode for speeds between
VChover and VCcruise. The tip speed is Vtip−hover in helicopter and conversion mode, and Vtip−cruise in
airplane mode. Drive-system states are defined for helicopter, cruise, and conversion-mode flight. The