I wonder if you could accurately measure airspeed acoustically, instead of using pitot tubes and pressure? For instance, suppose you placed microphones near the trailing edge of the front wings, and near the leading edge of the rear wings.

Compare the sound of the engines as picked up by the front microphones and the rear microphones, and figure out how far behind the latter is shifted in time compared to the former. The faster the plane is going relative to the air, the shorter that time should be.

If the engine sound is too uniform to provide any way to match up the sound as recorded from the two locations, some kind of sound source could be added.