Helicopter Limitations

The general characteristics and principles of helicopter flight have greatly restricted the maximum cruising speed of a helicopter. In order to achieve a significant increase in speed, the flight characteristics or flight principles of a helicopter have to be substantially altered.

The maximum theoretical speed for a helicopter is 250 MPH. The world speed record for a helicopter was set in the U K in 1986 by a company called Westland.  Their Westland Lynx helicopter flew at a recorded speed of 249 mph.

The main limiting factor for a helicopter to reach a speed beyond 250 MPH is a problem referred to as retreating blade stall. This term “retreating blade” stall is a well-known critical factor relating to the dissymmetry of lift of the main rotor blades in forward flight. When a helicopter is in a hover  (stationary above ground) the main rotor blades are in a balanced state or symmetrical lift is produced on both sides of the aircraft.  However, when the helicopter begins to fly forward, the lift becomes unbalanced, this dissymmetry of lift is the result of an advancing blade that is increasing with air speed and a retreating blade that is decreasing with air speed.  The advancing blade is moving in the direction of flight of the helicopter and into the oncoming air, thus increasing in the  amount of lift. The retreating blade, which is moving toward the tail, is moving in the same direction of the oncoming air, so the amount of lift decreases.           

At a relatively low speed (perhaps 150 MPH on a typical 6 passenger civilian helicopter) the helicopter will approach the retreating blade stall. This retreating blade stall will begin with a buffeting or shaking of the helicopter and if the pilot continues to try to make the helicopter fly faster the result of a complete retreating blade stall will occur.  At this time the helicopter will pitch up and then roll into the direction of the retreating blade side. 


This is an excellent illustration relating to retreating blade stall:
http://www.dynamicflight.com/aerodynamics/retreating/



Click here to learn more about the Challis Heliplane Design