The next challenge is to establish the centre of gravity for the vehicle. It is sometimes necessary to find a way to shift the centre of gravity, when changing from flight mode to car mode, and vice versa.

Basic engineering for the automobile calls for a center of gravity (and the driver) to be located half way between the front and rear wheels. The purpose, of course, is that this is where the car provides the best ride.

The locating of the center of gravity in the airplane must be done for different reasons. It is usually slightly ahead of the center of lift. The reason for this is that in a power-off situation, the craft will gently nose down and keep flying. If it were behind the center of lift, the aircraft would nose up and eventually stall. It is usual to place the pilot at the center of gravity too, not to give him a better ride, but because the weight of the pilot is a variable. When located at the center of gravity, he or she may be light or heavy and still be accommodated with a minimum of trim adjustment. The center of gravity must also be slightly ahead of the main gear (on a tricycle landing gear) to keep the tail of the aircraft from falling on the ground when parked. As well, the center of gravity may not be too far ahead of the main gear. If it is, the aircraft will not rotate for takeoff. As you can see, there are some basic incompatibilities here that must be dealt with.

The dimensions of the vehicle must also be considered. The automobile has limitations on size depending, first of all, on an allowable road width of 8 feet. After that, size depends on whether it is a luxury automobile, a compact car, or something in between.

It is the span of the wings that normally takes up most of the width of an aircraft, but actually, it is possible to build an aircraft with no wings at all. Such an aircraft could be built using vectored thrust, ducted fans, and rotors. These are, of course, Vertical Take-Off and Landing aircraft.

If you insist on having a wing, it is possible to build one with a span of only eight feet to accomodate the width limitation imposed on road vehicles. However, performance may be affected in a variety of ways. The span can be increased by using folding or telescoping wings that may be extended, or by using attachable wings. There are advantages and disadvantages to all of these possibilities.