There are a number of unique characteristics that are featured in the Challis Heli-Plane. The main intention of the design is to combine the stable hovering capabilities of a helicopter, with the higher cruising speed of an airplane.

As discussed on the Helicopter Limitations page, the retreating blade stall has been a major barrier for helicopter and heli-plane designs to over come. The Challis Heli-Plane addresses this fundamental limitation in a number of ways. First and foremost is the realization and implementation of one single fixed wing on the retreating blade side of the aircraft. This is, with out a doubt, the first aircraft design to have a fixed wing on only one side of an aircraft. Having only one small fixed wing on the retreating blade side of the aircraft is one of the most crucial characteristics in allowing a Challis Heli-Plane to break the theoretical maximum speed of a helicopter. This design presents a relationship between the lift that is present on the advancing blade side of the aircraft and the lift produced from the one small fixed wing on the retreating blade side of the aircraft.

Prototype #83

The second crucial characteristic addressing retreating blade stall is; the implementation of a tractor propeller. Typically the thrust for a helicopter is provided through the main rotor system. The forward thrust on a conventional helicopter is accomplished by increasing the pitch of the main rotor and applying extra power to the rotor blades, this actually compounds the retreating blade stall problem. The Challis Heli-Plane design utilizes a tractor propeller to provide forward thrust. This allows the collective pitch of the main rotor to be reduced, further minimizing the effect of retreating blade stall. The reason for the implementation of a tractor propeller mounted on the nose of the aircraft, as opposed to a pusher propeller mounted on the tail, is because of the need to address instability problems associated with compound helicopters such as oscillation. By utilizing a tractor propeller on the nose of the aircraft it is much easier to incorporate into the tail structure the third crucial characteristic of the Challis Heli-Plane, which is an oversized horizontal stabilizer, angled significantly downward.

Through the development stages of this design it has been demonstrated that in order to eliminate pitch oscillation, as well as correct the tendency for the heli-plane to climb during forward flight, an oversized horizontal stabilizer must lift the rear of the aircraft. The aerodynamic forces on the rotor blade react in a different manner when the rotor is used only for lift, as opposed to lift and thrust like in a typical helicopter.

Further teachings in the patent pending design, relate to the critical relationship between the direction of rotation of the propeller and that of the main rotor. After many different experiments involving various thrust line settings of the tractor propeller, as well as experimenting with various types of propellers, came the realization that there is a relationship between the propeller and rotor. If the highest portion of the propellers rotation is moving in a direction opposite to that of the main rotor blade, the effect of applying power to the propeller will cause the aircraft to pitch downward. The adjacent photo shows prototype # 65 with a thrust line or propeller axis, tilted back about 14 degrees. This particular prototype had the propeller spinning in the opposite direction to the main rotor. In order for the propeller to apply a thrust force parallel to the longitudinal axis of the aircraft, the thrust line of the propeller needed to be tilted back in an unusual manner. When the highest point of the propeller turns in a common direction to that of the main rotor blade, the effect to the aircraft when applying power to the propeller is; the aircraft well begin to climb or pitch upward. The result or effect of the heli-plane pitching up with the propeller rotating in a common direction, or the heli-plane pitching down with the propeller rotating in the opposite direction to the main rotor is much more pronounced at lower speeds.

Challis Heli-Plane displaying incorrect rotatation

Challis Heli-Plane displaying correct rotatation

Prototype #65

After test flying both types of prototypes it was clearly demonstrated that the correct way to proceed is; the design that has a propeller turning in a common direction of the main rotor. The best method of transition from a hover to forward flight of a heli-plane is to apply extra power and collective in a typical manner and as the cyclic is tilting the aircraft forward, power can then be applied to the propeller to assist in forward thrust in a safe and predictable manner. Once moderate forward speed has been obtained, the cyclic can be eased back to a more neutral position. The power and collective setting of the main rotor may be reduced with an increase of power being applied to the propeller.

The aerodynamic forces reacting on the rotor blade of heli-plane aircraft are different than experienced on a conventional helicopter or a conventional gyrocopter. All three types of rotorcraft; helicopter, gyrocopter, or heli-plane, each have a unique aerodynamic circumstance or characteristic.

An unusual effect to the heli-plane that was clearly apparent from testing many different prototypes using a tractor propeller, at moderate to high speeds, the aircraft will have the tendency to climb. Regardless, if a tractor propeller was turning in the correct manner which is in the common direction of the rotor, (the initial effect is to make the aircraft climb) or incorrectly, with the propeller turning in the opposite direction to the main rotor (the initial effect is to make the aircraft nose down) however, once moderate speed is achieved, the aircraft would start to climb. This happens, even with forward cyclic and the collective pitch at the usual hover setting. Many attempts were made to fly the heli-plane at a consistent altitude by applying forward cyclic, which typically produced a condition inciting oscillation, sending the aircraft violently out of control.

To correct the problems of the aircraft’s tendency to climb and oscillate, the third crucial characteristic of the Challis Heli-Plane is; an oversized, horizontal stabilizer, which extents at the rear of the aircraft and is angled significantly downward. This has proven to be a workable solution.