Answers by Paul Moller and Bruce Calkins

{Headings and formatting added by Henry Lahore}

Your Web presentation of the Skycar is very impressive and very convincing. However, I have some reservation regarding the Skycar's claim of 20mpg fuel efficiency as well as its ability to operate in the high altitude of 20-30 thousand feet. Unless one is very intimately familiar with the intermittent combustion engine or with aerodynamics, the following cannot become obvious.

First of all, the Skycar sports eight engines with combined output of 960hp, and that is a lot of power, which consumes a lot of fuel during the take off and landing phase. However, for cruising at 300mph at 30,000 ft. only 200 hp is projected to be needed, thus mpg can be calculated as follow: For the Freedom Motors with a known peak rated specific fuel consumption of 0.5 lb of fuel/hour per horse power, at 200 hp, 100 lbs of fuel will be consumed. 100lbs of fuel is equivalent to about 15 gallons of fuel, and at speed of 300mph will give a mpg of about 20. (300mi/15gal=20mpg). The catch here is that during cruise, the Skycar engines produces too little power (1/5 of maximum power output) at too high an engine speed (required to maintain a fast 300 mph cruise, given fixed pitch fan), thus combustion would be terribly inefficient.

To maintain efficiency in an intermittent combustion engine at part throttle requires that the engine speed be slowed down significantly, proportionally to the horsepower reduction, so that the combustion process has sufficient time to complete, given a thinner fuel-air concentration. For example, an engine produces 500 hp at 5000rpm when power is reduced to 100 hp, the engine speed should decrease to no more than 1500 rpm if efficient combustion is to take place, and that's why automakers put overdrive gears on car transmission to slow down the engine at cruising hence maintaining efficiency. Even then, specific fuel consumption would still suffer at low throttle power. In the Skycar, there is no overdrive gearing between the engine and the fan blades, to maintain a 300mph cruise requires almost near maximum engine rpm. This is also hard on the engine. In a propeller aircraft, the propeller pitch will be increased in cruise, thus lower propeller rpm will be needed, thus slow down the engine. Typical piston engine airplanes cruises at 60-70% of maximum power, thus allowing the engine to achieve peak specific fuel consumption of 0.43 lb of fuel/hr per hp produced. Wankel rotary such as in the Skycar is not as fuel efficient as a piston engine, and the peak specific fuel consumption will be around 0.5 to 0.6 lb of fuel/hr per hp produced. What if Dr. Moller says that he would shut down half of the engines, thus allowing the rest to produce 40% of horse power, thus improve fuel efficiency?

I wish such would work, but it would not! At 30,000 ft, the air is so thin that a normally aspirated engine can only produce 1/4 to 1/5 of its maximum sea-level power, so all engines will need to be run at full throttle plate opening if one want 1/4 to 1/5 of maximum sea level power. What about turbocharging to increase mixture density in order to improve fuel efficiency? I see no proposal for turbocharging in the Skycar illustration nor in the description. But, What if Dr. Moller says he will turbocharge 2 to four of the engines at high altitude and shut down the rest in order to improve fuel efficiency?

I wish this would work, but it still will not work. This is because at high altitude, the air is so thin that the thrust from each fan blade turning at the same maximum rpm is reduced to 1/4 to 1/5 of its maximum sea-level thrust. Due to Mach number limitation which will decrease with thinner air at higher altitude, the engine speed cannot be much increased in order to increase thrust. Thus, all fan blades and thus all engines will need to be run at near maximum rpm if adequate thrust is to be available to keep the Skycar flying. Ah, if only the Skycar would sprout a pair of long and slender wings as in a typical airliner or a business jet, then the thrust require for cruising flight would be reduced by half, hence half of the engines may be shut down. But too bad, the does not have much wings to speak of. The Aeronautical engineers from your Boeing must have known something when they design them Boeing airliners with long slender wings, for them long slender wings are crucial for efficient high altitude cruising with minimum power available, and if one wonders about the importance of efficiency, just ask any airline executives who must now searching for ways to fight those escalating fuel bills.

The next major hurdle will be life support system at 30,000ft. It is not easy, it is complex and will add significant amount of weight and cost to the aircraft, as well as maintenance cost. It will at least require turbocharging of the engines in order for the cabin to be pressurize, and no proposal for turbocharging of the Skycar was ever mentioned. The pressure ratio of the skycar's ducted fans is simply too low for cabin pressurization, given the size of the fan with respect to the level of thrust that the fan produces.

Bird strike is another consideration. Even the low time Maverick Air kit built turbojet power test aircraft suffered from bird strike, but luckily, this is a twin engined winged aircraft which landed safely. The skycar has no wing, if one nacelle got knocked out because of bird strike, parachute must be deployed which may save the occupants but will destroy the entire vehicle. Just ask Cirrus Design when they put parachute in the legendary SR-20 aircraft.

Mr. Pham's comments are well thought out and address legitimate issues regarding most of the information we have released to date. Unfortunately for proprietary reasons, I cannot answer all of his questions as I would like to. I will however address most of those issues he raised. The M400 prototype, as currently shown, has a mileage capability of about 15 to 16 MPG at 25,000 ft. with variable pitch fans, as stated in the M400 data sheets.

Turbo-charging at least two of the engines is planned and is part of the required pressurization.

Total weight penalty for pressurization of the Lancair 400 was 100 lbs.

With turbo-charging both RPI, Curtiss-Wright and ourselves have seen SFC values of less than .4lbs./HP-Hr. on rotary engines.

Mr. Pham is wrong is discussing the relationship of altitude to thrust capability of a propeller. For example, at take-off the lift coefficient on each fan blade is approximately .7 and the thrust generated from each nacelle is 600 lbs. At 25,000 ft. the thrust required for optimum cruise speed is only 66 lbs. from each nacelle or if only two are used 132 lbs. each.

A lift coefficient is inversely related to air density and directly related to thrust generated, hence at 25,000 ft. the lift coefficient would be .34 for two nacelles, which is certainly not a problem. In fact one would slow the fans down to bring the lift coefficient up. Therefore, Lift coefficient = .7 @25,000 ft. would require an RPM of 5000 assuming sea level RPM = 7200. Finally, through proprietary configuration changes we have been able to increase the Lift/Drag ratio from 9 to 13 (i.e., higher aspect ratio).

We hope that this information clarifies the projected capabilities of our aircraft and reassures Mr. Pham of the soundness of our design. It is difficult to provide all of the information an informed person needs to be confident while maintaining a level of security appropriate for this stage of the process. Hopefully, we have struck a balance here that is sufficient.

Regards, Paul Moller

Paul and I will review and provide you with at least a partial response. Some of the more obvious areas which can be addressed immediately is our intent to use variable pitch vanes for the fans in the production aircraft, the aerodynamic lift obtained from the current design and our typical suggested flight profile of <10,000 feet.

Also, the increased weight on the Cirrus aircraft for pressurization was only about 100 lbs. We have more untapped power than we could possibly use at cruise from the un-loaded engines for any number of functions including running a compressor.

Although we have many options for boosting power, turbo charging is under consideration, although not specified at this time.

Our analysis of FOD is that we could loose the front engine in all four engine nacelles and still cruise around looking for an appropriate parachute-assisted landing site or (much more difficult) come in for a high-speed landing at an airfield. With both parachutes and the undercarriage air-bags we do not anticipate massive damage to the entire aircraft in an emergency landing. Our current thinking is that we will deliberately bias the attitude during parachute decent so that the impact will be absorbed (probably at the tail) with a "crush-zone" and result in a relatively low cost repair.

Let me get with Paul and see if we want to comment on fuel usage. We are doing a lot in this area right now, but some of the details are still sensitive.