Glide Distance

Following a power failure, the pilot has a limited zone in which to select a suitable landing spot. The radius of this zone is equal to the maximum glide distance from the altitude at which he enters autorotation. This altitude is either the altitude he had at the time of the power failure or the altitude to which he can zoom. The

zoom maneuver is possible when the failure occurs at a forward speed, V0i higher than the autorotational speed, Vv The altitude gained can be related to the kinetic energy made available by decreasing the air speed modified by the power dissipated during the time required by the maneuver:

G-v-(K-F?)-550 j .р.<//, ft

4____________________ J‘Q

G. W.

(Although some kinetic energy might be available beause of a decrease in rotor speed, this should not be included, since the pilot will want to get the rotor speed back up to normal before landing.) For this calculation, the power is assumed to correspond to the average level flight power for the two conditions and the time of the maneuver is the time required to decelerate from V0 to Vx using, the component of gravity along the flight path. Using these assumptions, the equation becomes:

where the climb angle, yn is:

CJo

(CT/o),

A conservative value of (CT/o)max for this calculation is 0.12. The extra glide distance due to the zoom maneuver is:

The analysis has been done for the example helicopter assuming a complete power failure at 160 knots, and the results are plotted in Figure 5.6. The power required was taken from Figure 4.24 and the rate of descent from Figure 5.5. It may be seen that the best autorotative speed is about 87 knots, which is closer to the speed for minimum rate of descent than to the speed for minimum angle of descent.