The first aircraft sound you hear when the fast moving aircraft is overhead is from the sound waves that left the aircraft before it was overhead.
Ah OK so am I drawing a right angle triangle with the vertical height, vt of plane, ct of sound But then the ct of sound would be on the hypotenuse but it can't be cause the plane is moving faster therefore covering a greater distance
I'm trying to show the point in time when the plane (arrowhead) is directly overhead of the observer, X is the point on the wavefront behind and below the plane that's heading towards the observer.
and we need to know how long it will take the wave at point x to reach the observer travelling at 340m/s