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82 Rotor
11-3.2 No-Feathering Plane Command
No-feathering plane command is characterized by control of rotor cyclic pitch angles and direct
control of the rotor thrust magnitude. This control mode requires calculation of rotor collective pitch
angle and tip-path plane tilt from the thrust magnitude and cyclic control, including the influence of
inflow.
a) Collective: magnitude of the rotor thrust T or CT /σ (shaft axes).
b) Cyclic: tilt of the no-feathering plane, usually producing tilt of the thrust vector;
longitudinal cyclic pitch angle θs (positive aft) and lateral cyclic pitch angle θc
(positive toward retreating side).
c) Shaft tilt: shaft incidence (tilt) and cant angles, acting at a pivot location.
11-3.3 Aircraft Controls
Each control can be connected to the aircraft controls cAC: c = c0+STcAC, with c0 zero, constant, or
a function of flight speed (piecewise linear input). The factor S can be introduced to automatically scale
the collective matrix: S = a/6 =1/60 if the collective control variable is CT /σ; S = ρV 2
tip Ablade(a/6)
if the collective control variable is rotor thrust T . For cyclic matrices, S =1with no-feathering plane
command, and S = −1 for tip-path plane command.
11-3.4 Rotor Axes and Shaft Tilt
The rotor hub is at position zF hub , where the rotor forces and moments act; the orientation of the rotor
shaft axes relative to the aircraft axes is given by the rotation matrix CSF . The pivot is at position zF pivot.
The hub or shaft axes S have origin at the hub node; the z-axis is the shaft, positive in the positive thrust
direction; and the x-axis downstream or up. The rotor orientation is specified by selecting a nominal
direction in body axes (positive or negative x, y,orz-axis) for the positive thrust direction; the other two
axes are then the axes of control. For a main rotor the nominal direction would be the negative z-axis;
for a tail rotor it would be the lateral axis (ry-axis, depending on the direction of rotation of the main
rotor); and for a propeller the nominal direction would be the positive x-axis. This selection defines a
rotation matrix W from F to S axes. The hub and pivot axes have a fixed orientation relative to the body
axes:
hub incidence and cant: C HF = UθhVφh pivot dihedral, pitch, and sweep: C PF = XφhYθh Zψp
where U and V depend on the nominal direction, as described in table 11-2. The shaft control consists
of incidence and cant about the pivot axes, from reference angles iref and cref:
Ccont = Ui−iref Vc−cref
For a tiltrotor aircraft, one of the aircraft controls is the nacelle angle, with the convention αtilt =0for
cruise, and αtilt =90degree for helicopter mode. The rotor-shaft incidence angle is then connected to
αtilt by defining the matrix Ti appropriately. For the locations and orientation input in helicopter mode,
iref =90. Thus the orientation of the shaft axes relative to the body axes is:
C SF = WC HF C FP CcontC PF