Lets start at the beginning, shall we? For airplanes, that's range and payload.
Minimum requirement out of the box is 500 nautical miles. Targeted objective is 1,000 nm. We need to be able to get to the larger airports from small rural fields; at this point I'm not interested in transcontinental performance (I've flown from Hawaii to California in a turboprop, seriously, get a ticket on a jet for the long haul).
The intent is to allow the operator to make two round trips before having to refuel, each round trip to be approximately 200 nautical miles in length on average.
Initial range and performance to be done with Jet A, or the current commercial equivalent. Once we have that, then we can do a trade study to switch to ethanol or bio-diesel. From what I've read in the trades, hydrogen is a long way down the line, but I can buy ethanol today.
Twenty paying passengers with two air-crew. I'm not familiar enough with part 135 operations to know if a cabin crewmember will be required.
Remember, this is an air-taxi and not a scheduled operation we are supporting. Therefore, twenty seats may well be at the large end. My thought is that at ten seats and below, we are competing with Cessna and over forty seats, I think we go head-to-head with Bombardier. I don't have any real data that supports my assertion that there is a hole in the market at twenty seats, but that's my feel. Clearly, we want to stay out of the hundred seat (give or take twenty seats) market.
Building a four to ten seat personal airplane is very do-able. Getting a twenty-seat airplane built for commercial use is a very different project, but still in the range of quite feasible. We aren't talking billions of dollars here. In fact, with good academic support we may well be able to build the prototype for very little indeed.
Tentatively planned to be a Pratt and Whitney PT-6 of some form or other. The reason I am specifying a turboprop instead of a small turbofan is because of the short, soft field that I expect to be a significant part of air taxi operations all around the world. Turbofans have a bit of a lull in acceleration, which can be operationally significant. I'm willing to trade performance at altitude for better short runway performance. As for other engines, I'm perfectly open to whatever you guys have data for.