The first successful vertical powered flight was achieved in 1939 by Igor Sikorsky and it remains an important milestone of our history. With the ability to hover and rotate, helicopters provide unique maneuverability which cannot be achieved with a fixed-wing aircraft. This ability to stay stationary in the air and fly in any direction makes them crucial for uses such as search and rescue, transporting troops to active battlefields, and flying to remote inaccessible areas.
However, when it comes to safety, the fixed-wing airplanes are considered safer owing to their ability to glide in case of engine failure, provided there is sufficient airflow over wings. One may imagine what will happen to a helicopter if its engine fails, and whether it will be able to glide!
So, can a helicopter glide?
Helicopters can glide in case of an emergency (engine failure) albeit in a manner much different than fixed-wing airplanes. Though helicopters do not have any fixed wings, their rotor blades can act like rotating wings, making sure sufficient lift is available for the helicopter to glide till safe landing.
It is a common misconception that helicopters may not be able to glide due to their lack of fixed lift-generating wings. There is a simple answer to why this is, namely autorotation.
What is autorotation, you may ask?
Let’s find out!
Table of Contents
What is Autorotation and Why is it Important?
For aircraft, both fixed-wing and rotary-wing (helicopters), the need to have maneuverability despite a loss of power is quite significant. If a helicopter is unable to glide, it may just fall to the ground like a rock. It would, therefore, be dangerous to fly not just for the onboard crew but for any population on the ground as well.
To ensure the safe landing of a helicopter in case of engine failure, autorotation plays a vital role.
So, what is autorotation?
Autorotation is the ability of a helicopter to in some situations have its rotors powered by the flow of air rather than the engine. In short, as the helicopter descends, the upward directed flow of air pushes through the rotors, makes them rotate faster, which in turn provides lift.
This lift is enough to ensure that a helicopter won’t fall to the ground if handles properly. However, the
How does a Helicopter Glide?
In the normal flight of a helicopter, its rotor blades generate thrust by pushing the air downwards, thus lifting the aircraft.
The lift being generated depends on two key parameters:
- the rotational velocity (speed) of rotor blades
- their angle of attack.
The angle of attack (or pitch) of a helicopter rotor is commonly termed as collective pitch.
The speed of the rotor blades, together with their angle of attack, determines the amount of air being displaced downwards, and how much lift is generated.
In case a helicopter loses engine power during flight, the pilot can change the pitch of the rotor blades so that they start pushing air in the upward direction. This allows the rotors to be powered and rotated by the air passing through the rotors, as the helicopter descends.
What are the flight controls used in autorotation?
Autorotation is carried out by using the collective pitch lever of most helicopters. By carefully lifting or lowering the collective pitch lever, a helicopter pilot can change the pitch of all rotor blades at once.
During autorotation, the collective pitch directly impacts the airspeed of the rotor blades. In other words, the collective lever of a helicopter controls the RPM of the main rotor or its descent velocity during glide.
This is based on the fact that:
- A higher pitch will take a bigger scoop of the air, causing a loss of rotational speed.
- a lower pitch means that the rotors move less air, which increases their rotational speed.
The role of the helicopter pilot during autorotation is to adjust the pitch and flight speed so that the rotors maintain enough speed to continue to provide lift.
In other words, the control stick and the collective pitch lever become the two key controls to attain safe flight.
Just like any other flying technique, autorotation requires theoretical understanding as well as practice to be mastered.
Let us have a quick overview of the distinct phases of autorotation.
The stages (phases) of autorotation
Autorotation can be dissected into a few distinct stages:
It begins with the pilot putting the helicopter into autorotation as soon as power is lost. During this stage, the pilot uses the collective lever to ensure that the helicopter does not stall and has the necessary forward velocity for glide. The collective pitch must be reduced quickly to ensure that the speed of the rotors doesn’t drop below the safe threshold, below which the lift is insufficient to keep the helicopter flying.
As the helicopter descends in the forward direction, it gains velocity. At this stage, the angle of descent is typically around 20 degrees.
Just before touchdown, at around 100ft above ground, pilots apply flare to gain some altitude and reduce the vertical velocity of the aircraft. Applying flare simply means that the nose is lifted, whereby speed is traded for lift.
The image below shows an example of an airplane applying flare.
By reducing the velocity, a helicopter can land safely and that is the last and final phase of autorotation.
In theory, autorotation seems quite simple but practical complexities encountered by a pilot make it one of the most difficult techniques to master for helicopter pilots.
The need for precise timing in autorotation
Precise and accurate control of the collective pitch and speed of the helicopter enables a pilot to land safely in case of an emergency. As soon as engine failure is identified, the helicopter must be put into autorotation immediately. Timing is quite important as collective pitch must be adjusted according to the airspeed of the helicopter or else, the rotor blades would stall, and it would become impossible to glide.
As we discussed earlier, another important stage is at the end, when the forward speed of the helicopter is traded in for a slight gain in altitude. During this phase, the pilot must accurately adjust the attitude of the helicopter so just enough velocity is left to have a safe sliding impact on the ground.
What Is the Glide Ratio of Helicopters?
While discussing the glide of an aircraft, the glide ratio is an important parameter to investigate.
The glide ratio merely indicates the distance an aircraft travels in the forward direction as compared to the altitude being lost. The higher this ratio, the better and farther a helicopter can glide.
During autorotation, this ratio is being controlled by the pilot and a 4:1 glide ratio is considered safe. It is entirely possible to land a helicopter in autorotation with a 3:1 glide ratio as well but the steeper it gets, the more difficult the flare-out stage becomes.
The safe range of glide ratio for autorotation is defined by manufacturers based on the helicopter characteristics. Before obtaining type rating for any helicopter, learning, and understanding important parameters such as the one being discussed, are part of their theoretical and practical training
While pilots learn and practice autorotation routinely, they must be aware of the risks involved as well.
What are the risks involved in autorotation?
Autorotation is an emergency maneuver and is therefore inherently perilous. As a helicopter loses power, handling a helicopter (well, nearly) plummeting towards the ground does not just sound hazardous, it is!
The most common risks involved in autorotation are:
- Not being able to instantly put the helicopter in autorotation by adjusting collective pitch and attitude can result in a rotor stall, which is extremely hard to recover from.
- If the helicopter nose is lowered more than the required amount, it can lead to a nosedive
- Not being able to maintain adequate rotor RPM during descent can stall the helicopter
- Not applying flare-out timely, a loss of heading during flare-out, or improper leveling of the helicopter can lead to a hard landing or a crash.
- If the speed of the helicopter is greater than the recommended number or a pilot loses track of ground during touch-down, the helicopter can tip over.
Knowing the hazards associated with autorotation, it is understandable if people question the safety of helicopters in general. Why don’t we examine the statistics a little?
Are helicopters safe?
Short answer? Yes, helicopters are safe. When we look at the figures collected by NTSB, it does show that helicopters crash at a rate of 9.84 per 100,000 flight hours which is slightly more than airplanes (i.e. 7.28 crashes per 100,000 flight hours).
But while we review these statistics, it is important to understand that fixed-wing airplanes operate to and from controlled aerodromes and on pre-determined flight paths, whereas helicopters operate in more concentrated urban or challenging remote locations for various purposes such as search and rescue.
Summary and Conclusion
- Helicopters have the crucial ability to glide in case of any powerplant failure though a technique known as autorotation. Unlike fixed-wing aircraft, that are stable and can glide through the air smoothly for extended distances, helicopters need to be piloted precisely for them to glide to safety.
- Autorotation requires instant and accurate pilot actions so that rotor blades of a helicopter keep rotating through reverse airflow and can provide some maneuverability for the helicopter to land.
- Pilots hone their skills for these emergency scenarios during their initial and recurrent training where they learn the risks involved in autorotation and how to mitigate them.
- Therefore, autorotation plays a key role in ensuring the safety of helicopter flight. For when a helicopter loses engine power, it can glide and bring the crew and passengers to the ground safely.
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