## High Speed Helicopter Variable Sweep Rotor Design

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In the world of helicopters, very high speed is the Holy Grail. The aerodynamics of helicopter rotors limit the top speed of a conventional helicopter to about 285 MPH.

The primary reason is the advancing rotor blade tips airspeed must not exceed about 1,000 feet per second (Mach .9) which is the sum of the tangential speed of the rotor tip plus the forward airspeed of the helicopter, and the minimum airspeed of the retreating blade must not fall below its stall speed, which is about 120 MPH, which is the sum of the tangential speed of the rotor tip MINUS the forward airspeed of the helicopter.

Raising or lowering rotor RPM will cause one side or the other to violate the min/max tip speeds, as will just pushing the helicopter faster with added thrust. Lift must balance side to side.

To date, there is no demonstrated way to get past that limitation in any meaningful way. Tilt-rotors, co-axial or tandem rotors suffer the same limitations but at a somewhat higher airspeed.

A novel way to avoid both high and low tip speed is to somehow slow down the advancing blade tip speed while increasing the retreating blade tip speed and not necessarily change rotor RPM is the solution to the problem.

It is done by changing the sweep angle of the advancing blade to a high sweep back angle while sweeping the retreating blade forward to compensate, all during a single rotation of the rotor.

Increasing forward sweep of the retreating blade has the effect of speeding up the blade tip.

Mathematical and CAD simulations of this effect show that the theoretical maximum airspeed will exceed 500 MPH, and may be as high as 575+ MPH. And that is faster than any tilt-rotor, co-axial or tandem rotor flying or proposed.

It does require a very significant change to the rotor design to allow a maximum variable sweep of about 110 degrees, but only a variable sweep of up to 80 degrees is needed in flight. The basic change is the rotors blade hub diameter, from about 1 to 2 feet to about 10 feet for a usual 35 foot diameter rotor. The extra sweep angle allows the rotor to self-stow to about 1/3 its diameter.

For takeoff, landing (VTOL) and slow airspeeds the curved blades are maintained straight out as in a conventional helicopter. As airspeed increases the variable blade sweep starts in conjunction with airspeed.

The mechanism to accomplish this is proprietary (patent reasons) but is relatively simple, consisting in part of just 9 gears and 3 small actuator motors for five or more blade rotors. This will entail 5 patents.

Manufacturing uses conventional machines and materials. Only 3 components are considered difficult to make.

The replacement market for high speed helicopters is estimated at well over 3,200 units (military and civil) over the next 5 to 10 years, averaging \$10,000,000 each.

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