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Chapter 11
Rotor
The aircraft can have one or more rotors, or no rotors. In addition to main rotors, the rotor component
can model tail rotors, propellers, proprotors, ducted fans, thrust vectoring rotors, and auxiliary-thrust
rotors. The principal configuration designation (main rotor, tail rotor, or propeller) is identified for
each rotor component, and in particular determines where the weights are put in the weight statement
(summarized in table 11-1). Each configuration can possibly have a separate performance or weight
model, which is separately specified. Antitorque rotors and auxiliary-thrust rotors can be identified, for
special sizing options. Other configuration features are variable diameter, and ducted fan.
Multi-rotor systems (such as coaxial or tandem configuration) are modeled as a set of separate rotors,
in order to accommodate the description of the position, orientation, controls, and loads. Optionally the
location of the center of the rotor system can be specified, and the rotor locations calculated based on
input separation parameters. The performance calculation for twin rotor systems can include the mutual
influence of the induced velocity on the power.
The main rotor size is defined by the radius R or disk loading W/A, thrust-weighted solidity σ,
hover tip speed Vtip, and blade loading CW /σ = W/ρAV 2
tipσ. With more than one main rotor, the disk
loading and blade loading are obtained from an input fraction of design gross weight, W = fW WD.
The air density ρ for CW /σ is obtained from a specified takeoff condition. If the rotor radius is fixed
for the sizing task, three of (R or W/A), CW /σ, Vtip, σ are input, and the other parameters are derived.
Optionally the radius can be calculated from a specified ratio to the radius of another rotor. If the
sizing task determines the rotor radius (R and W/A), then two of CW /σ, Vtip, σ are input, and the other
parameter is derived. The radius can be sized for just a subset of the rotors, with fixed radius for the
others.
For antitorque and auxiliary-thrust rotors, three of (R or W/A), CW /σ, Vtip, σ are input, and the
other parameters are derived. Optionally the radius can be calculated from a specified ratio to the radius
of another rotor. The disk loading and blade loading are based on fT, where f is an input factor and T
is the maximum thrust from designated design conditions. Optionally the tail-rotor radius can be scaled
with the main-rotor radius: R = fRmr(0.1348 + 0.0071W/A), where f is an input factor and the units of
disk loading W/A are pound/feet 2 . Figure 11-1 is the basis for this scaling.
Table 11-1. Principal configuration designation.
configuration weight statement weight model performance model
main rotor rotor group rotor rotor
tail rotor empennage group tail rotor rotor
propeller propulsion group rotor, aux thrust rotor