1.   But approaching or reaching supersonic speeds presents a new set of aerodynamic problems that raise risks in areas that are unknown.

2.   For the Department of Defense, flapping offers a way around aerodynamic problems that make it hard to miniaturize fixed-wing planes.

3.   The contract had been held up pending agreement on a design fix to remedy aerodynamic problems.

4.   Then the Navy held up release of the funds pending resolution of what officials described as an aerodynamic problem called wing drop.