I'm not talking about anything unrealistic (like Moller's skycar, etc).
But what would prevent a design (even low performance/ultra light to start)
that would allow a pilot to fly into an airport, then fold up his wings and
cruise down surface streets at 45mph?

What is the difference between the skycar and what you describe?

Nothing prevents you from designing something like that, but what will
result is an airplane that doesn't fly very well, and a car that
doesn't drive very well. All the stuff you'll need to carry around in
order to be able to safely drive the vehicle on roads after landing
would need to be carried as part of the airplane, making it heavier
and lowering its payload. This includes the extra weight used to make
the vehicle crashworthy (assuming you would like to survive a battle
between you and a Toyota). And all the stuff you need to fly with
would need to be carried while driving on the ground - this means
you'd be dragging the wings and tail all over the place. You would
probably not be carrying fuel in the (detachable) wings, so the fuel
tank would be in the empenage, which has weight and balance consequences.

The tradeoffs would make the engineering much more difficult than you
suspect, and the result would be sub-optimal in all motion regemes.
Put another way, for the same performance, you'd spend lots more
money, and you'd be better off having a cab waiting for you.

Further, every time you wanted to take off, you'd have to reassemble
the airplane - at the very least, you'd have to put the wings back on.
The fasteners would be a weak point, and the time spent doing that
would reduce the time savings of commuting by air this way. Looking
(cynically) into the future, with all the TSA stuff coming down, such
a vehicle would probably be seen as a threat to security, and you
might end up on the receiving end of hours of security questioning
every time you wanted to take off or drive away, especially at a new
airport.

If you are thinking of doing this for commuting, you'd be better off
leaving a car in Reno for your commute, and flying in a regular plane
from Minden. If you are thinking of it for the coolness factor,
what's wrong ith Moller's aircar?

Jose
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