Pilot action and rate of descent

Whilst the simplistic analysis detailed above gives a basic guide to the factors affecting the time taken for the rotor speed to decay to the minimum permissible value it does not take account of important additional factors. These include the action of the pilot in attempting to contain the rotor speed within limits and the effect of rate of descent on the angle of attack of the rotor blades. To investigate these effects further it is necessary to construct a more detailed model of a rotor entering a vertical autorotation. As before consider the three-quarter radius as being representative of the conditions on the complete rotor blade. Now:

T = L cos ф — D sin ф

Q = 0.75R(D cos ф + L sin ф)

Using basic aerofoil theory it can be shown that: L =1 pV0275 abcRa015

D = 2 PV2.15 bcRCD

Now the angle of attack and airflow velocity will depend on the blade pitch, the vertical velocity and the rotational velocity at the three-quarter radius. Assuming a linear twist :

00.75 = 00 + °.7501

and:

V0275 = (0.75R ))2 + (V + vih) 2

The effect of changes in thrust developed and torque required on the vertical velocity and the rotor speed can be determined by assuming constant acceleration over a short time interval, St:

[Vc ]t+st = [Vc ]t +[T ~mgl St m

[)]t + St = [)]t – Q St

A more detailed study of the situation following an engine failure (see Fig. 2.25) highlights the role of the rate of descent in changing the magnitude and direction of the forces acting on the rotor blades and thus the unbalanced torque causing the rotor speed to decay. It can be seen that the speed of a rotor initially operating at 35 rad/s, with an inertia of 6000 kg m2, stabilizes at 21.5 rad/s once the vertical rate of descent has reached 1500 ft/min. It should be remembered that the analysis presented ignores the effect of entry into the vortex-ring condition. In reality the pilot will attempt to arrest the rotor speed decay and contain the NR within narrower limits by rapid reductions in collective pitch. This has the effect of increasing the rate of descent and unloading the rotor blades thereby reducing, and ultimately reversing, the decelerating torque applied to the rotor. Since prompt action following an engine failure is often vital to success it is important to know if the pilot has sufficient time to identify a problem with the powerplant(s) and take corrective action. This is determined by measuring the delay time.