As you will see on the link,(click the heading!) DARPA is asking for submissions for a 4 seat offroad capable VTOL vehicle for the Marines. This links in some new technology such as computer navigation and hybrid power with specs something like that flying car people have been asking about ever since we learned how to fly. Since I have already made an Aircar or two, I thought I'd have a go at predicting what this thing will look like when it arrives (planned for 2015).
Here are my first sketches. I made ducted fans since this looks to be the way to go from the specs, and they are as big as will fit the size limits, so that's one key limitation right there.
Side elevation - here you can see the first design problem clearly: for strength, the pair of lift fans and their mounting bridge are one piece which pivots on a vertical axis to fold them on land - but as seen here there is a bit of an overhang past the wheels which may get them caught in trees or other obstacles in offroad mode - I tried moving the wheels further apart but then you need six wheels and that pushes the weight up.
Front elevation above.
Ortho view above.
Critical issues for the design definitely include the weight of the ducted fans at the top of the vehicle - this could make it hard to get good offroad performance, but then the offroad vs. VTOL performance is really the crunch with this project: how will they deal with water crossings, steep hills and mud while still keeping the flying parts operational? A ductfan covered in mud will not work well, and too much mud on the wheels and underpan could push up the weight so that VTOL is a lot harder, even if you have clean fans.
I can't wait to see what the big aircraft companies come up with.