In the event of an engine failure, airplanes have a distinct advantage of being able to glide farther than helicopters, hopefully far enough for a safe forced landing. While helicopters can’t glide as far, they do have the great benefit of being able to land with little to no groundspeed, greatly improving the survivability of those on board. The best of both worlds would be to be able to glide efficiently and then land without any groundspeed. While a helicopter is able touch down with little to no groundspeed, a glide angle of about 18 degrees won’t garner much range. However, there is a technique, not included in the FAA Rotorcraft Flying Handbook, that can be used to extend the range and in some cases help reach a more desirable landing area.
During an autorotation there are many things a pilot can do when too high; turns, slips/skids, or airspeed adjustments. However, when faced with the situation of being too low, there is only one thing a pilot can do. Extend the autorotation glide.
There are four factors in an autorotation that affect the descent rate: density altitude, gross weight, rotor RPM, and forward airspeed. While we can’t control density altitude or gross weight (unless jettisoning an external load), we can control rotor RPM and forward airspeed. In a forward flight autorotation one can significantly increase the range by increasing airspeed and lowering rotor RPM, while still complying with the Rotorcraft Flight Manual limitations for that particular helicopter.
Let’s take airspeed first, using the Agusta Westland 139 as an example, referring to the RFM limitations and emergency procedures sections. The AW139 has a minimum autorotational airspeed of 40 knots, a minimum rate of descent autorotational airspeed of 80 knots, and a best range autorotational airspeed of 100 knots. Some helicopters may not have a best range airspeed published, but instead a maximum autorotational airspeed. In any case, one would use the autorotational maximum airspeed or best range airspeed to extend the power-off glide. Note, the minimum rate of descent airspeed also corresponds with the best rate of climb airspeed (Vy), as this is the airspeed where the coefficient of the lift to drag ratio is highest. The farther one deviates from Vy the greater the descent rate. Know that exceeding the maximum or best range autorotation speed can cause rotor RPM decay and put undue stress on the rotor blades, and we certainly don’t want to decay rotor RPM because we want to use it for lift to extend the range in an autorotation.
We have increased speed to the maximum allowable, but are now faced with an increased descent rate as the helicopter is now faster than Vy. However, this can be compensated for by increasing collective and reducing rotor RPM to minimum allowable, which decreases the descent rate. The AW139 has a power-off rotor RPM limitation of 95-110 percent, not counting transient limitations. In order to maximize the autorotational range of the AW139, fly at 100 knots and 95 percent RPM. At some point with ample altitude remaining, the pilot would reduce the airspeed to the recommended 80 knots and increase the RPM to the recommended 110 percent. This provides as much energy as possible to the rotor system for landing. Aircraft vary as to how quickly they can recover rotor RPM, and while a low inertia rotor system will recover quicker it will also have less energy during pitch pull before touchdown.
There is really just one reason to fly an autorotation at the maximum airspeed and lower range of rotor RPM, and that is to increase the range of the helicopter. As we increase collective, pitch is increased and the L/D (lift drag ratio) of the rotor system becomes more efficient and more lift is produced.
Next training session, try a couple of extended range autorotations and compare them to the standard ones. Results vary with the type of helicopter, but the difference is very significant in the aircraft I have flown. ou will likely find the rate of descent at high speed/low RPM to be even less than the rate at Vy/normal RPM. A lower descent rate coupled with a higher airspeed greatly enhances the range, which could make all the difference someday.