I got a Bushnell Speedster speed gun that will measure from 6 to 200 mph + or - 1 mph and if you hold the trigger down the digital reading will be in real time, if the plane slows the gun will show it. It was made for clocking base ball pitches or race cars but will work on anything. When you let off the trigger the display shows the speed when you let off. Uses 6 AA batteries. Check around, the cost should be under 175.00
