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78 Rotor
R tr /R mr
0.26
0.24
0.22
0.20
0.18
0.16
0.14
aircraft
equation
0.12
0. 2. 4. 6. 8. 10. 12. 14. 16.
disk loading (lb/ft 2)
Figure 11-1. Tail-rotor radius scaling.
11–1 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