by Dave Jackson

Created on: May 15, 2008

Achieving heavier than air flight was a triumph in engineering and aerodynamics. But it was only a baby step down the path toward a successful helicopter. Thirty-six years would pass before Igor Sikorski made the first tethered flight in his VS 300. Sikorski had an uncanny ability to solve the problems of the rotary wing aircraft. He insisted on personally flight testing each of his breakthroughs. Being the first, Sikorski had to learn to fly what is still the most difficult and dangerous type of aircraft.

Designing the rotor blade was the easy part, as earlier research into airfoils (wings) and angle of attack directly applied. Sikorski only needed to make them smaller and devise the mechanism of the rotor head that would spin them. Understanding how the spinning rotor disk behaved and how to control it was a very different matter.

Airflow moves past a wing in one direction. Not so for a helicopter rotor, because this is only true for part of the path the blade follows. As the blade turns toward the rear, it then faces backwards. This changes the lift production of each blade, causing the blades to rise and fall each rotation. As drag increases with increased lift, each blade also retreats and advances with each rotation. The tendency of each blade to rise, fall, advance and retreat hundreds of times per minute was mechanically disastrous. Only the invention of "drag" hinges and "flap" hinges would solve these issues.

Sikorski had to solve all these problems before his machine could be flown and controlled. Each lesson was learned the hard way, with several crashes. It's amazing (and lucky for us) that he wasn't killed.

As if these weren't enough problems, the gyroscopic effect has to be taken into account. Any change in input to the spinning rotor disk happens 90 degrees later. Sikorski had to devise his control system to adjust the rotor head 90 degrees earlier to account for this.

The spinning rotors cause torque to be applied to the airframe. The tail rotor cancels this effect but adds weight, complexity and uses about one quarter of the horsepower. Failure of this system guarantees disaster. Twin, counter rotating rotor helicopters like the CH-47 Chinook avoid wasting this power. Some Russian helicopters stack two counter-acting rotors. More efficient Fennistrom rotors are contained in a shroud, reducing wasteful tip vortexes. Notar (no tail rotor) helicopters use directed engine thrust. A helicopter pilot lets his machine turn on the vertical axis by adjusting

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