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The force on the actuator disc caused by the pressure drop, given by Eq. (3.9), can also be non‐dimensionalised to give a coefficient of thrust C_T

(3.15)

(3.16)

A problem arises for values of because the wake velocity, given by (1 − 2a)U_∞, becomes zero, or even negative: in these conditions the momentum theory, as described, no longer applies, and an empirical modification has to be made (Section 3.7).

The variation of power coefficient and thrust coefficient with a is shown in Figure 3.3. The solid lines indicate where the theory is representative and the dashed lines where it is not.

Figure 3.3 Variation of C_P and C_T with axial induction factor a.