The Art of the Helicopter John Watkinson - Karatunov.net

92 The Art of the Helicopter
(a)
(b)
(c)
Fig. 3.24 (a) Given the pressure difference across a rotor in the hover, surrounding air will tend to move
upwards. In translational flight the edge of the rotor may actually encounter upwash. (b) In forward flight the
rotor acts like a low aspect ratio wing. Vortices curl over the tips and down through the rear of the disc, reducing
the angle of attack there. Loss of lift at the rear precesses into inflow roll. (c) As the rotor blades are coned, in
forward flight the air approaches at a different angle as shown here. The component perpendicular to the blade
is greater for blades at the rear, where the angle of attack is reduced.
Whilst coning causes no undue aerodynamic problems to a hovering helicopter, it
has an unwanted effect in forward flight. Figure 3.24(c) shows that the effect of coning
is to reduce the angle of attack of the blade passing across the tail compared with
that of the blade passing across the nose. As for the inflow roll, this results in a taildown
moment applied to the disc, which likewise manifests itself as a roll towards the
advancing side because of the phase lag of the rotor. The two effects take place in
parallel, but as different functions of airspeed. Inflow roll commences on leaving the
hover but reduces significantly as speed increases, whereas coning roll slightly increases