Vertical climbing flight
The problem of vertical climbing flight is identical to that studied in Section 9.1, with the thrust equal to the helicopter weight plus the air resistance of the fuselage etc., to the vertical motion, and with the oncoming stream speed V equal to the rate of climb of the helicopter.
9.5.2 Slow, powered, descending flight
In this case, the air approaches the rotor from below and has its momentum decreased on passing through the disc. The associated loss of kinetic energy of the air appears as a power input to the ideal actuator, which therefore acts as a windmill. A real rotor will, however, still require to be driven by the engine, unless the rate of descent is large. This case, for the ideal actuator disc, may be treated by the methods of Section 9.1 with the appropriate changes in sign, i. e. V positive, Vs < Vo < V, p > pi and the thrust T = – W.