Summary Remarks on Momentum Theory
The place of momentum theory is that it gives a broad understanding of the functioning of the rotor and provides basic relationships for the induced velocity created and the power required in
Figure 2.24 Effective flat plate drag
producing a thrust to support the helicopter. The actuator disc concept, upon which the theory is based, is most obviously fitted to flight conditions at right angles to the rotor plane, that is to say the hover and axial flight states we have discussed. Nevertheless, further reference to the theory will be made when discussing forward flight (Chapter 5).
Momentum theory brings out the importance of disc loading as a gross parameter; it cannot, however, look into the detail of how the thrust is produced by the rotating blades and what design criteria are to be applied to them. For such information we need additionally a blade element theory, corresponding to aerofoil theory in fixed-wing aerodynamics. We shall turn to this in Chapter 3.