January 6, 2009 Conventional helicopters are incredibly useful vehicles in many short-range scenarios - but their asymmetrical aerodynamics enforce a fairly low terminal speed limit of around 150mph, making them less than ideal for longer-range missions. Tilt-rotor aircraft like the Falx and Osprey, and coaxial 'copters like the Sikorsky X2 are tackling the problem from different angles, but both result in complicated and expensive solutions - which is what makes the new Challis Heliplane concept quite remarkable. Still in early stages, the Challis uses a very simple design to balance the lift forces of a helicopter and bring top speeds of over 300mph into reach. And wait 'til you see this thing accelerate!

Helicopters have a built-in speed limit - imagine it this way: the rotor wings on the top are spinning at high speed to provide lift. At a stationary hover, these blades are moving through the air at the same speed and constantly providing equal lift - but when the helicopter moves forward, one side of the rotor has the blades moving forward into the air, and the other has the blades moving backwards. This means that significantly more air is passing over the blades on the advancing side of the rotor than the retreating side, and because lift increases with airspeed over a wing, the lift forces on the helicopter become more and more unbalanced toward the advancing blade side of the aircraft as horizontal speeds increase.


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Helicopter rotor blades are designed to 'flap' up and down to counter this effect - the blades on the advancing side are tilted more toward the horizontal to catch less air, and on the retreating side they flap downward to catch more air and equalize the lift.

At a certain speed (theoretically around 250mph, but in practice closer to 150mph for most regular helicopters) the lift imbalance becomes so large that the flapping effect can no longer balance the aircraft, and if the pilot tries to fly above this speed, a state of "retreating blade stall" occurs and the helicopter begins to roll over - which is as dangerous as you'd imagine.

Several solutions are fighting to provide the best solution for an aircraft that has the hovering advantages of a chopper but can achieve high speed flight. Sikorsky's X2 coaxial helicopter mounts a second rotor above the normal one, and rotates it in the opposite direction. A prototype achieved its first 30-minute low-speed flight test late last year. Then there's the tilt-rotor concept, as seen on the V-22 Osprey currently in use by the U.S. military, in which twin wing-mounted counter-rotating rotors slowly tilt forward to become forward propellers, giving the aircraft two distinct modes of flight.

The new Challis Heliplane concept provides perhaps the least mechanically complicated solution of the three. It uses a single main rotor and a tail rotor like a conventional helicopter, but adds a single fixed wing on the retreating blade side and a tail wing behind the tail rotor. There's also a propeller at the front of the aircraft.

In a hover, the Challis design acts exactly the same as a helicopter - but when forward airspeed increases, the asymmetrical design begins to show its worth. The fixed wing on the retreating blade side develops lift as speed increases, balancing the disproportionate lift on the advancing wing side and eliminating the retreating blade stall effect. The front propeller can then be used to provide additional forward drive with no fear of unbalanced high speed aerodynamics, and the rear wing provides lift to the back of the aircraft. Challis claims that top speeds over 300mph will be possible using the design.

The miniature radio-controlled flight demonstrator shown in this video on the Challis Heliplane website shows the platform in operation, although it can't act as a proof of concept until we see the heliplane at very high speeds. It does give us a glimpse of how quickly the concept can accelerate from a hovering start once the front propeller kicks in - full throttle might be a case of "hold onto your lunch, lads!"

Importantly, the Challis Heliplane will be able to execute an emergency landing with no power - which is a key requirement if the design is to be accredited for civilian use. The Sikorsky coaxial helicopter also meets this requirement - but the Challis design's strength is its simplicity and relatively inexpensive construction.

Challis expects to have a 740-pound small unmanned version of the Heliplane available as early as 2010, for around half a million Canadian dollars. The 320mph, 1200 horsepower, 3350-pound version (still unmanned) is expected in 2012 for around five and a half million.

The website leaves a few questions unanswered; does the heliplane use a fixed rotor or one with flapping hinges? Is the rotor speed modulated at high speeds to account for the sonic boom effect that can cause shock waves and turbulence when the wing tips exceed the speed of sound? And what is the path and expected date for a manned prototype or civillian version?

We look forward to seeing the Challis design tested and proven against the other prominent high-speed chopper designs over the next few years. There's no question such a vehicle would be extremely useful.

Loz Blain

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