Re: How do 'ax-heads' improve effiency on helicopters?

Greetings:

The EH101 is fitted with special helicopter blades made out of glass and 
carbon fibers which have been woven and bonded together. The blades are 
manufactured with new shaped tips, which were developed from the British 
Experimental Rotor Program (BERP). The helicopter has five main blades, 
which provide a great deal more lift than conventional blades. You can see 
a close up photograph of BERP developed blades at the following URL under 
helicopter systems:

http://www.dra.hmg.gb/html/products 

 Helicopter rotor blades experience a number of complex flow phenomena such 
as transonic flows on advancing blades, dynamic stall on retreating blades 
and various interactions of the blades with the rotor wake. 

A major limitation in high speed flight is the transonic flows on the 
advancing blade which cause high vibration levels, power divergence and 
noise.  Air flowing off the tips of the rotor blades forms a trailing 
vortex which rotates clockwise when viewed from the rear on the right-hand 
side of the helicopter. The generation of this vortex  consumes a 
significant amount of wasted power from the helicopter engines. 

Jet transports also have the wing tip vortex problem and so do birds. 
Modern jet transports add a winglet on the wing tips to significantly 
reduce the generation of these vortices  which adds as much as 5% greater 
range to the aircraft. 

Ducks fly in Vee formations so that the vortex from the wing tip of the 
leading duck rotates in the opposite direction of the vortex from the wing 
tip of the trailing duck. If the ducks fly close enough, (wing tip to wing 
tip), which they try to do,  it is estimated that it takes 30 to 50 percent 
less energy for a duck to fly behind the lead duck. The lead duck has to 
use maximum energy to fly and this is why on ocassion the lead duck will 
change position in the formation because the work of flying lead is much 
harder!

Helicopter blades are much more complex than aircraft wings because they 
are moving more slowly near the hub and while moving backward and they 
reach near sonic speeds at the tips when they are advancing ahead of the 
helicopter. You can see plots of this complex action generated by NASA at 
the following URL;

http://science.nas.nasa.gov/Pubs/TechSums/9293/6.html

The BERP in the United Kingdom was set up to address the complex vortex, 
vibration and aerodynamic problems encountered by helicopter blades and 
they developed the ax-head shape for the rotor tips to reduce the vortex 
and vibration problems while maintaining a compromise between the 
requirement for a blade speed to range from near stalling to near the speed 
of sound as the rotor rotates around a fast moving helicopter. It is 
claimed that the ax-head improves the efficiency of the EH 101 by as much 
as 30%.

Best regards, your Mad Scientist
Adrian Popa