Gliding flight in a helicopter?

Many of us know about the gliding capacity of airplanes, the fixed-wing aircraft can glide without the engine power until landing safely with appropriate conditions and well aerodynamic airplane performance. However, how do the helicopters overcome the lack of engine power in mid-flight? That is somewhat disturbing to know that there is an engine failure.

Enstrom Helicopter Corp. Enstrom Piston 280FX
Source: aerocontact.com

It is intuitive that the helicopter, when its blades stop rotating, descends in free fall like any other object, unlike an airplane that glides. But there’s something up its sleeve. A state of flight or process along with a mechanism that allows the helicopter descends like a leaf.

Merlin HC.3 Blade
Source: b-domke.de

Autorotation

The first one is called Autorotation, the process when the rotor or blades of the helicopter work and rotate by the action of the air and without any input of engine power. And here comes the best and the smart use of this process in case of engine failure or tail-rotor failure, the helicopter can safely land with the rotation of the main rotor blades.

Straight-in autorotation
Source: FAA.gov

The autorotation process is a common procedure in helicopters in emergency cases. Therefore, the helicopter pilots must be trained in this events or scenarios when flight characteristics are affected by lack of engine power.

During an autorotation, the upward flow of relative wind permits the main rotor blades to rotate at their normal speed. In effect, the blades are “gliding” in their rotational plane. Source: FAA.gov

Freewheeling Unit

What about the mechanism? It is a freewheeling unit, a bearing roller that transfers directly the power output of the engine to the main transmission and finally to the main rotor, but this freewheeling unit in case of engine failure disconnects, allowing the main rotor and blades to continue turning as the helicopter descends. Why does this unit disconnect of the engine? Because, in such a way, the main rotor does not drag the weight of the dead engine, subtracting energy to the autorotation.

Drive system of a Bell 206B. The Freewheel assembly is the freewheeling unit. Source: tsb.gc.ca

So, the next time you may think about the engine failure of the helicopter, take it for granted that the certification basis of the approved type certificate of helicopters have the autorotation process in their designs.


Sources:

Article: https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/helicopter_flying_handbook/media/hfh_ch11.pdf

Cover photo: http://www.flyingmag.com/Courtesy Lockheed Martin