How a Helicopter Autorotation Works - Easy to Understand
Helicopters have the amazing capability of landing safely on a small area free of obstacles even if engine power is lost. In the following, the phases, types and aerodynamics of an autorotation are explained in detail and hopefully easy to understand.
Table of Contents
Normal Flight vs Autorotation
In normal flight, the rotor of a helicopter is powered by the engine and produces lift due to its aerodynamics and rotational speed. In contrast, in an autorotation the rotor is not powered by the engine. It is solely powered by the air flowing upwards through the rotor when the helicopter is descending. It does not need any engine power to keep rotating and therefore produces lift also during the descent.
To make it more clear, you may think of the rotor in an autorotation working as a wind wheel, windmill or wind turbine. The airflow produces the rotational speed and again the aerodynamics of the rotor blades then produce lift.
In order for this to work, the engine and the rotor must be separated by a free wheel. Hence, the engine can power the rotor at any time but if the engine stops, the rotor is not inhibited by the non-working engine and can rotate freely.
Autogyro vs Helicopter
For an autogyro this is the normal state of flight, because the rotor is not powered by an engine by design. Hence, an autogyro always needs air flowing through the rotor from bottom to top in order to keep it turning and producing lift. This is achieved by a forward motion of the autogyro, which is produced by a propeller. Therefore, an autogyro cannot stop in the air and hover like a helicopter, because this would stop the airflow through the rotor disc and the lift would disappear. It can, however, fly quite slow compared to fixed-wing aircraft.
For a helicopter, an autorotation is solely an emergency procedure which becomes necessary:
- When the engine quits, becomes erratic or has to be shut off (i.e. due to an engine fire)
- When there is a complete tail rotor loss
- To get out of a vortex ring state
- In order to sink faster than a normal powered flight state would allow.
During an autorotation, the helicopter stays fully maneuverable. The helicopter can be accelerated or decelerated, can be flown with no forward speed or even backwards. Curves or circles are also possible. The only restriction is that the helicopter will be descending (except for a short period before landing) during this manoeuver.
Phases of an Autorotation
An autorotation consists of four distinct phases which are explained in detail below: the entry phase, the established glide phase, the flare and the landing.
This is the most critical phase of an autorotation, especially if it suddenly becomes necessary due to loss of power. This can be due to either an engine or transmission failure. In such an event, the rotor rpm decreases suddenly and fast. Now the pilot only has a limited time available to enter into the autorotation before too much rotor rpm is lost, which then cannot be recovered. Depending on the type of helicopter and especially the inertia of the rotor and the weight of the helicopter, more or less time is available for the pilot to react. Typically it’s within the range of several seconds only.
The entry phase consists of three actions:
- Immediately lowering the collective to lower the air resistance of the rotor, therefore stopping the loss of rotor rpm
- Use left/right pedal to keep the nose straight.
- Slightly pulling the stick (cyclic control) back, therefore flaring the helicopter slightly. This increases the airflow through the rotor and recovers already lost rpm.
2. Established Glide
During the descent, it is very important to monitor these three parameters closely:
- Rotor rpm. The rotor rpm needs to stay within a specific envelope. First, it needs to be fast enough to produce enough lift. Second, it also must not be too fast because this could cause overstress to the structure and could result in blade separation. Usually there is a rotor rpm warning system which warns the pilot both visually and audibly of either too high or too low rpm.
- Airspeed. There is a recommended airspeed during an autorotation, but it can be varied according to the circumstances during the beginning and the middle of the descent phase. During the end of the descent phase, as the ground approaches, it is important to go back to the recommended airspeed. This is because the airspeed determines how easy or difficult the following phases of flaring and landing will be. Sufficient airspeed makes landing easier because only then will the flare break the sink rate and increase the time available for a smooth landing.
- Landing Field. The best autorotation is not worth much if you are heading straight into trees, buildings, or other obstacles. The pilot must scan the area for an appropriate and reachable landing site and make a decision on where to land as soon as possible. Also, the wind direction needs to be considered. Landing into the wind is always preferred as it makes landing easier. Once that decision is made, the pilot can vary the airspeed, fly curves or circles to make sure that he hits the landing field. The good thing about a helicopter, even in an autorotation, is that the landing site can be relatively small but needs to be free of obstacles.
Approximately 60 ft above ground, the pilot pulls the stick back to flare the helicopter. When done with enough airspeed, this reduces the sink rate and also increases rotor rpm. The flare is held until the helicopter is only a few feet above ground. It is important to stop the flare before the tail rotor hits the ground.
Only very shortly before the ground approaches, the pilot levels the helicopter and then must raise the collective upward. This increases the lift produced by the rotor blades and therefore cushions the impact with the ground. At the same time rotor rpm is decreasing due to the increased drag. Basically the rotational force stored in the rotor is converted into lift. After this action, the rotor rpm is very low and cannot be recovered anymore. So the pilot has only one chance to perform this manoeuver. Hence it is important not to pull the collective upward too much in a way that the helicopter will start to climb. Because then the helicopter would be away from the ground with very low rotor rpm and rapidly decreasing lift. A consequent crash would be inevitable.
Autorotation Practice in a Cabri G2
In the video below, you can see an autorotation training in the Cabri G2 helicopter. The different phases, especially the flare, are clearly visible. Also you can see how the rotor rpm increases during the flare (see large white needle on the flight display). The engine is not actually shut off but put into idle during the practice.
Aerodynamics of Autorotation
Zero Speed Autorotation
For practical purposes, it is best to look at the aerodynamics of a zero speed autorotation first. Here the helicopter descents straight down with zero airspeed. Therefore the airflow is straight up through the rotor disc.
In order to understand the aerodynamics of an autorotation, it is important to understand the following:
- The effective wind direction v(eff) results from combining the upward airflow through the rotor disc (caused by the descent) and the speed of the rotor element (caused by the rotation). Since a rotor element far away from the rotor center needs to travel a larger distance in the same time as a rotor element close to the rotor center, its velocity against the air is larger the farther away the rotor element is from the center. See the graphic below for better understanding. Keep in mind that the rotor velocity increases towards the tips and therefore the effective wind direction is tilted more and more towards the horizontal.
- The dynamic lift of a rotor element is always perpendicular to the effective wind direction. The drag is always in the same direction as the effective wind direction, i.e. perpendicular to the lift. Combining lift and drag gives the total aerodynamic force of the rotor element.
- Since the total aerodynamic force is tilted forward from the vertical, there is a force acting forward and therefore driving the rotor element. See the graphic below for better understanding.
It must be added that the effective wind direction varies from the center of the rotor to the tip. Therefore, the total aerodynamic force also varies in direction:
- Close to the center, the effective angle of attack is too large. Therefore the rotor element is stalled. This is called the stalled region.
- In the middle of the rotor disc the total aerodynamic force is tilted forward and drives the rotor. This is called the driving region.
- Towards the tip of the rotor, the total aerodynamic force is tilted backwards and slows the rotor down. This is called the driven region.
Since the force of the driving region is greater than the force acting in the driven region, the rotor is driven forward.
Every helicopter flight manual contains a height-velocity- or short HV-diagram. This tells the pilot at which combinations of height (above terrain) and air speed an autorotation can be carried out safely in case of an emergency. In general, the helicopter needs either enough air speed or sufficient height. Otherwise there may not be enough energy (kinetic or potential) in the system to keep the rotor rpm in a safe range while entering the autorotation. So operation at low speed and low height should be avoided whenever possible, especially with single engine helicopters. At the same time, this is exactly the area in which helicopters can show their unique capabilities and a lot of commercial operations are within this range (i.e. power line inspections). As a private pilot you should avoid operating in this range.
Types of Autorotations
There are different types of autorotations. All have the goal of reaching a safe landing spot as best as possible.
- Straight-in Autorotation: This is the basic type of an autorotation. The direction of the helicopter nose is kept the same during the entire manoeuver.
- 360° Autorotation: If a suitable landing field is directly in front of the helicopter, but the helicopter is still too high to reach the landing field with a straight-in autorotation (i.e. would overshoot), a 360° autorotation can be used. The pilot just flies a 360° curve during the established glide phase. After the curve, the helicopter is still in the same position over ground but has lost height during the curve. The landing field is still in front.
- 180° Autorotation: If a suitable landing field has just been overflown and/or the wind is coming from behind, a 180° autorotation can be used. The pilot flies a 180° curve during the established glide phase.
- S-Curves: Another pilot manoeuver that can be used to precisely hit a nearby landing spot is to fly s-curves. This manoeuver maintains the airspeed while reducing the speed over ground, depending on the size of the s-curves flown.
- Zero Speed Autorotation: If a suitable landing spot is directly in front of the helicopter, a zero speed autorotation can be used as well. Here the airspeed is decelerated all the way to zero or even backwards during the established glide phase. It is important to regain enough airspeed before the flare and landing phase.
- Power Recovery Autorotation: This is a manoeuver used to practice autorotations. The engine is intentionally put into idle state to simulate a loss of power and then reengaged shortly before landing. So when the pilot raises the collective at the very end, the helicopter does not touch the ground but stays hovering. Compared to a real autorotation, this manoeuver additionally requires the use of pedals to counteract the engine induced torque to keep the nose straight.
- Hover Autorotation: This is not really an autorotation because the helicopter never enters an autorotational state. Instead, it refers to the procedure for a power loss during hover a few feet above ground. Here the pilot does not lower the collective, as one is already in the landing phase a few feet from the ground. So the right procedure is to keep the collective up, use the pedals to keep the nose straight and then raise the collective shortly before contact with the ground to cushion the landing.
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