When I tell people that I fly helicopters the comment that I hear a lot is, “Isn’t that dangerous? If something happens to the engine you can’t glide like an airplane.” Well, I explain that is not true, helicopters do glide, it’s called autorotation. Without going into too much detail about the aerodynamics, I describe how it works with the concept of stored energy.
For discussion purposes, a helicopter on the ramp switched Off contains zero energy. However, when a pilot starts the engine, the fuel is converted to energy that is used to start spinning the rotor system. The rotor rpm is brought up to 100 percent; the pilot then lifts off and begins accelerating and climbing. Once established at cruise altitude and airspeed, the helicopter has two kinds of stored energy—Potential energy (energy because of position) in altitude, and kinetic energy (energy do to motion) in airspeed and rotor rpm. Essentially, this is money in the bank to be used in an emergency. It is the successful manipulation of this energy that will bring the helicopter and its occupants to a safe landing during a loss of power.
When a helicopter’s engine stops in flight, a freewheeling unit disconnects the engine from the rotor system to prevent the engine drag from slowing the rotor rpm. In addition, the pilot must immediately lower the collective pitch allowing the helicopter to start descending and forcing the airflow up through the rotor system. Basically, the helicopter begins consuming altitude energy to maintain rotor rpm. This is a very important step because waiting too long to lower the collective will allow drag to slow the rotor system and stall the blades. If this happens we destroy the helicopter’s ability to manipulate energy and it will simply fall out of the sky with fatal results.
Once established in autorotation the descent rate is normally 1,200 to 1,500 feet per minute and the pilot should maintain about 60 knots and maneuver the helicopter to the best landing area available. Approaching 50 to 75 feet agl, the pilot begins to rapidly decrease airspeed with a flare. Airspeed energy is used to arrest the descent rate. If timed correctly, the helicopter should momentarily end up about five feet above the ground with little to no airspeed. With all the altitude and airspeed energy gone, the only energy left is in the rotor system. The helicopter will start descending and the pilot should then raise the collective pitch control and use the rotor rpm energy for a gentle touchdown. As the rotor system slows to a stop the helicopter returns to a state of zero energy.
Of course all this assumes ideal conditions. We all know that in the real world it doesn’t always work that way. There are certain combinations of airspeed and altitude that simply do not have enough stored energy to make a safe landing. For example, hovering at 150 feet the pilot must rely mainly on rotor rpm to cushion the landing. The vast majority of helicopters do not have enough energy stored in the rotor system to completely stop the descent rate. Most likely the landing will damage the helicopter and injure its occupants. If a pilot is hovering higher, say 500 feet, there is enough altitude energy to trade for airspeed and complete a successful autorotation.
This explains why helicopter pilots prefer to take off by moving forward to gain airspeed first, instead of going straight up.
Tags: Tim McAdams