Part 3
AUTOROTATION: THEORY & TECHNIQUE
by Chuck Meager

I would like to conclude this series on aerodynamics with autorotation. Autorotation occurs when the rotor becomes free wheeling. In other words, aerodynamic forces affect the rotor in such a way as to keep it turning. The primary reason for the rotor becoming free-wheeling is that mechanical power (the engine) is no longer driving the blades. This can happen if the engine fails or perhaps if the drive shaft fails. In either case, quick action is required of the pilot.

Every helicopter pilot knows that the first thing that you do, when you experience this power loss, is lower the collective. The action of lowering the collective changes the angle of attack on the rotor blades. If the pilot did not lower the collective, the rotor RPM would decrease rapidly. Without rotor RPM, the helicopter flies about like this computer when I get mad at it. Decreasing collective allows the blades to be driven by the inflow of air from underneath the aircraft as it is descending. Many books describe this as an exchange of energy. Altitude is exchanged for rotor RPM.

During autorotation, the rotor disk is separated into three areas or regions. The diagrams and table below describes these three regions.

All this really means is that if the pilot acts correctly, the forces in the rotor will balance and keep the rotor at a constant RPM.

Now that we all understand why the rotor turns during an autorotation, let's move on to different techniques for aircraft control.

The first step is to identify the need to enter autorotation. There are several indications available to the pilot that the rotor is no longer being driven by the engine. The pilot may experience a yaw in the aircraft with a change in engine noise a decrease in rotor RPM plus some other failed instruments. Many a pilot has been fooled by getting only an instrument failure and misdiagnosing that as an engine failure. Always verify this emergency with several indications.

After the pilot enters the autorotation by lowering the collective, he must think about selecting a landing area, rates of descent, and of course aircraft control. As the pilot approaches the ground, he/she now must think of slowing down. Normally about 100 feet or so, (depending on the aircraft, density altitude, gross weight) the pilot applies aft cyclic to begin a deceleration. This will slow the rate of descent and forward airspeed. As the helicopter settles (hopefully vertically) the pilot applies collective to increase the pitch in the rotor blade. This creates an increased lift vector so termination can be like a normal landing. The trade off is that both of these control movements will dramatically effect rotor RPM. You only have one chance to use the controls like this so you have to make it count.

Airspeed! Watch your airspeed! I can not over-emphasize the importance of airspeed. Remember, we have to put this helicopter down on the ground in a small area and calmly but completely exit the aircraft (we want our bodily fluid loss to exit the appropriate orifice at the appropriate time). We want to be successful at landing this aircraft with no further damage to the people in it, the aircraft, or anything on the ground.

The optimum airspeed varies with the type of helicopter. If the pilot does not have enough airspeed then he might descend faster than he wants. If the pilot has too much airspeed, then the pilot will again descend faster than he wants. Each type of helicopter has what is known as a minimum rate of descent airspeed and a maximum glide airspeed. The minimum rate of descent airspeed is exactly what it sounds like. It's the airspeed at which the helicopter descends the slowest. The problem with this airspeed is that the aircraft is going to have a rather steep rate of descent. This can be beneficial if the landing area is close or is very small or you need more time to think about what landing area you will select. Many people teach to automatically adjust to this airspeed in the event of an engine failure. I consider this ignorance. The technical difficulty for completing the autorotation at minimum rate of descent is very high. The tolerance for error is small. In addition getting lost airspeed back is very difficult. For an inexperienced pilot, walking the line at maintaining that airspeed can be difficult. And, when you finally get to the deceleration part of the autorotation, decel and collective application are almost simultaneous.

The preferred method is to maintain situational awareness and adjust your airspeed as necessary to arrive at your destination. (Sometimes easier said than done.) Never give up too much airspeed because getting it back can be costly. It is much easier to shorten your path using maneuvering than trying to increase airspeed to extend your glide.

Maneuvering brings up another point to remember about autorotation and rotor RPM. Turns can create too much rotor RPM. This also depends on the type of rotor system being used. Remember the driving and driven region? Well G forces and turns may cause these regions to adjust so the Driving region is larger. More lift forward of the axis of rotation causes the rotor speed to increase. If this happens, a slight increase in collective is necessary.

Just remember there is always a trade off when it comes to using all this inertia in the form of rotor RPM and altitude loss. Attempt to stay calm, control your aircraft, look for a place to land, watch out for wires, and do what you've been taught. Sounds simple enough anyway.