Ludwieg tubes offer a convenient and inexpensive way to provide high Reynolds number test flows; their primary defect is the very short run time available for flow measurement. Even during this period, the test section Mach number is liable to be affected by the boundary-layer growth along charge tube walls and shows gradual incease. In order to decrease this Mach number creep, a Ludwieg tube equipped wiith a quick-moving choker flap is developed. Through use of the flap the effective area ratio of the test section to the sonic throat is kept essentially constant for some period during the nominal run time. An appreciable time lag is observed between the tunnel start and the establishment of the constant Mach number flow. The finite inertia of the flap and the flap drive mechanism as well as the finite pressure signal propagation speed are the main causes of the delay and several means to eliminate the lag are discussed.