Concept
Convinced of the advantages of electrically powered model helicopters, and stimulated by the idea to increase their flight time, I bought myself a Kalt Whisper helicopter kit some years ago. After discovering its wonderful characteristics and following a lot of experiments with rotorblades, engines, gears, controllers, batteries and chargers, I was tempted to go even further.
Wanting to ever improve flight time I challenged myself to realise a helicopter with 2 coaxial main rotors. This counterrotating concept would benefit from the absence of a tail rotor, which absorbs up to 10 % of the engine power without generating any lift. The weightload per rotor would also be reduced because the machine should have 2 main rotors, 2 battery packs, 2 engines, but only one radio control system and one frame. Analysis of the performance of the real size Russian Kamov 27 ( www.kamov.ru ) coaxial seemed to confirm this approach and the blueprint of the construction had already matured in my mind.
Construction
The machine was build using mainly Kalt Whisper parts. The main rotor axis for instance was already hollow and I had to slightly modify just a few of the other parts. I managed to find a simple mechanism to drive the 2 rotors in opposite directions and mounted everything on a glued frame, constructed out of 1 mm Triplex, which made the project progress rapidly.
Wanting to keep the mechanism of the rotorheads simple too, I first tried a system with direct controls. I hoped to resolve the problem of the low inherent stability of a small rotor without a stabilisation bar, by using a gyro on the two swashplate commands.
The results however were not satisfying enough to allow easy flight. This left me no choice but to go for the more complicated concept with a Bell/Hiller stabilisation bar on each of the rotors. In the end this solution proved however to be less difficult as expected. The stabilisation bar of the lower rotor for instance could be realised in two parts enabling the passage of the upper rotor axis.
Luckily I had developed right from the start a very simple differential collective rotorpitch control, for the yaw command of the helicopter.
The superposition of two conventional model helicopter rotors worked well and allowed the first successful flights of the electric coaxial helicopter model.
Flight
The stability and control of the coaxial machine are comparable to those of the Kalt Whisper helicopter. The yaw command is precise and stable thanks to a gyro. The flight characteristics in curves are however different because of the absence of a tail rotor and the counterrotation of the two rotors. During fast forward flight one can notice the opposite lateral tilting of the two rotordiscs. This can cause the colliding of rotorblade tips between the upper and lower rotor if the distance between the two rotordiscs is not sufficient.
The flight time of about 9 minutes while hovering above ground effect is also comparable to what the Kalt Whisper performs. This does however not meet my initial expectations. The efficiency of the lower rotor proved to be poor because of its operation in the strong downswash of the upper rotor. I underestimated this effect which neutralises the positive influence of the reduced relative weight and absence of a tail rotor. The phenomenon is apparently of less importance in real size coaxial helicopters.
I have tried to overcome this downswash problem and tested a configuration with a large upper rotor turning slowly above a small and fast lower rotor.
Regretfully this did not improve the flight time significantly.
The mechanical concept of the coaxial, which is original and impressive, is never the less a success.