recompute airspeed with a different ARSPD_RATIO

def airspeed_ratio(VFR_HUD):
    '''recompute airspeed with a different ARSPD_RATIO'''
    import mavutil
    mav = mavutil.mavfile_global
    airspeed_pressure = (VFR_HUD.airspeed**2) / ratio
    airspeed = sqrt(airspeed_pressure * ratio)
    return airspeed