Full text.

High disc loading (up to a point of course) leads to higher propulsive efficiency at higher speeds. For example, typical tilt-rotor aircraft (V22, BA609) have a disc loading of less than 50 lbs./ft2 compared to approximately 150 lbs/ft2 for the M400. The V22 should be 75 miles per hour slower than the M400, based on the historic relationship between speed and disc-loading, but by using more power in cruise than in hover it is able to approach the M400’s cruise speed. However to do so it burns over three times as much fuel per passenger mile in cruise. The volantor’s higher disc loading with its small efficient ducted fans account for this reduced fuel consumption.

Building a VTOL aircraft with low disc loading is a very modest achievement. It has been done many times, but has not resulted in a practical VTOL commuter vehicle. If VTOL is not achieved with disc loading between 100 lbs/ft2 and 200 lbs/ft2, one can have serious doubts as to the general usefulness of the resulting vehicle. It will be slow, mechanically complex, (i.e. require variable pitch for control) and user unfriendly. This description sounds not surprisingly similar to that for the helicopter. So one wants a relatively high disc loading at cruise speed where the Skycar volantor will spend 99%+ of its operating time. High disc loading in cruise means high disc loading in hover and therein lies the rub. The problem is simply that the amount of power to fly efficiently at cruise speed is insufficient to hover the volantor. Therefore it is necessary to generate significantly more power from the engines for a short time during hover (about twice as much as required for high speed cruise). That way one can get the best of both worlds.

The classical problem with generating much more power from a given engine is the increased heat or more precisely, how to get rid of this heat. The Rotapower engine used in the Skycar has rotor, bearing, and crankshaft that can easily handle double the power (even at double the power, one is talking about 90 HP per liter at 6,000 RPM, which is reasonable.)

Moller International has developed a proprietary power boost technology that while doubling the power actually cools the engine internally. Fuel consumption is significantly higher during the process but with the M400 being able to accelerate from zero to 100 MPH in less than nine seconds, the time spent in or near hover is trivial and the fuel consumed is correspondingly low. If one needs to spend a lot of time in hover then a better choice of aircraft is the helicopter. The benefit of boosted power/increased internal cooling is that the cooling system can be sized for cruise where the high velocity and lower power require a much smaller radiator, etc.

One argument presented against higher disc loading is that it leads to a higher jet exit velocity which might impact the take-off area. However, we have made many flights in the M200X with similar disc loading to the M400 where the take-off was from grass. We saw no damage either from the air-stream or from heat. Perhaps of greater importance is the fact that much less total air is moved by the volantor at a disc loading 150 lbs./ft2 than a similar payload helicopter at 10 lbs/ft2 (about 1/4th as much) as a consequence the disturbance to ground personnel at 50 ft. from the M400 will be dramatically less than with a helicopter or V22.

The final justification for the higher loading is that it is a natural consequence of eliminating any gearboxes. Gearboxes account for over 70% of all helicopter failures and also their weight would significantly reduce payload. Without a gearbox and limited by tip-speed for low noise, etc. the diameter or area of the fan is limited which leads to a high disc loading.

I would welcome any comments that counter those I have provided here.

Sincerely,

Paul S. Moller, Ph.D. President, Moller International