A wetted area to frontal area ratio under 15 is achievable with an efficient airframe design. A high aspect ratio is also hard to achieve in a light vehicle where the fuselage becomes a lifting body, due to its size relative to the small wing area required for efficient high-speed flight with a modest payload. We have our own 250 mph wind tunnel in which we have performed over 1000 hours of detailed flight testing using both powered and un-powered models to ensure that we have chosen an optimum design. As seen from the example above, VTOL aircraft requires 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.