Contradictions define flying car design. Longer wings are more efficient (see gliders) but need to be stored within a normal car’s width (max. 2.2m / 7'2"). Lift must switch to downforce, the CG needs to move aft, and the motor must stay cool in traffic and comply with strict particle emissions standards:
Carplane® solves these issues by:
employing a twin-hull layout, limiting drag by housing its roadwheels within the hulls and protecting and storing the wings between the hulls at an angle which creates downforce; and
combining this with a wing-extension mechanism which stores the longest possible wings, and an extendable empennage which both shifts the center-of-gravity and lengthens the empennage moment-arm.
Among the 2,400 known attempts to build a roadable aircraft, 300 have flown. However, none have achieved certification. Among the many reasons they failed are:
they stored the wings in a manner which negatively affected driving performance;
they made the wings so small that it negatively affected flight performance;
they segmented the wing-spar, thus weakening the structure or making it too heavy;
they positioned the empennage, undercarriage and center-of-lift in such a way that it blocked rotation and/or limited control effectiveness;
they positioned the center-of gravity in such a way that it caused cornering instability.