I think 20mpg sounds ok.
The Engine
VTOL aircraft require a great deal of power to obtain lift-off and perform a safe landing. Since the engine must lift its own weight along with the weight of the craft it must be lightweight. Economy is another area that requires much attention. Purchase price, operating costs, and maintenance costs are all factors which, if too high, can make the engine impractical. Lastly, to be truly efficient, an engine must be environmentally friendly. Ideally, clean burning engines capable of using the most readily available fuels would provide the best option.
The need for a moderately high disc loading results in a relatively high installed power in order to hover. For the installed powerplant weight not to become excessive, the engines must be light for their power output. The key element in determining the VTOL aircraft cost may then become the powerplant. If, for example, one uses 200 lb/ft< disc loading, it can be shown that:
(Installed Horsepower / Gross Vehicle Weight) ~ 0.5
In this case, a VTOL aircraft with a modest payload requires installed power in the 1000 HP range and an engine HP to weight ratio near 2. A turbine can meet this weight requirement; however, a small 100 HP turbo-shaft can cost $100,000, while a single 1000 HP turbo-shaft can cost $300,000. The smaller turbo-shaft gives a poor specific fuel consumption, while the single engine provides no back up. Any design using turbo-fan engines will expect even higher engine costs. A turbo-fan using disc loading like the Harrier generates only one half pound of lift for every horsepower. Hence, the Harrier requires approximately 40,000 HP with little payload capability in the VTOL mode.
To meet the 2 HP/lb requirement in a small fuel-efficient form, only two engine alternatives appear to be possible at an economical cost:
A turbo-charged or super-charged fuel injected 2-cycle engine. This engine would need to be developed.
A rotary engine that employs aluminum housings, peripheral porting and an air-cooled rotor. Engines of this design are in existence.
Moller rotary engines were developed from technology obtained from Outboard Marine Corporation (OMC) and are of the Wankel-Type. During each rotation of the rotor a four-stroke spark ignition combustion process occurs in each of the three pockets of a triangular rotor. After one full rotation of the rotor the engine has completed the four-stroke process three times. They therefore provide a high power-to-weight ratio at a reasonable cost and are very small for their power output. The 150 HP model used in the M400 can be easily carried by one person. Eight Rotapower engines are used in the production model volantor.
Wankel-type rotary engines in general are very reliable as a result of their simplicity. The number of moving parts in a Moller rotary engine (dual-rotor) is approximately seven percent of those in a four-cylinder piston engine. The rotapower engine is also multi-fuel capable. Moller rotary engines in the volantor are typically configured to run on unleaded gasoline however, we have recently demonstrated the engine's ability to run on diesel to the Army and on Natural Gas to another organization.
