FVA 15 - “Blow-through control system” of a Grunau Baby

Dr.-Ing. H. Stein, a member of the FVA, reports on the development and testing of the blow-through control system in “Forschen und Fliegen” January 1954, issue 2:

“From a chronological point of view, the blow-through control system was developed during the last years of the war in the Aachen Aerodynamic Institute, which was already headed by Prof. Seewald at that time. At that time, I was given the research assignment to investigate a control system by Lippisch together with my colleagues. It was a jet of compressed air which Lippisch let escape from the inside of the wing, which was closed on all sides, through a gap parallel to the wing axis in order to achieve differences in lateral forces. The effect was particularly good on the strongly curved upper surface of an airfoil, but the practical applicability of this control method is precluded by the fact that a relatively heavy compressor unit is carried in the airplane. (…)

The rudder moments, which can change greatly in the various flight conditions and speeds despite compensation, are sometimes so great that the pilot can no longer control them with his body forces; in particular, large aircraft must be equipped with rudder machines. Inspired by these experiments, I proposed the use of a control system which would also achieve the change in lateral force by means of a blow-out jet, but which would take the necessary air volume from the free air flow through a gap in the wing leading edge. After orienting preliminary tests, which confirmed the effectiveness of the arrangement, a series of measurements was run to determine it:

  • the most favorable sizes for inlet and outlet gaps
  • the most favorable gap position and
  • the most suitable discharge direction

It was found that the inlet cross-section must be selected larger than the outlet cross-section so that the discharge jet has a sufficiently large intensity. Measured from the wing nose, the discharge gap is located at about 2/3 wing depth with a discharge angle inclined by 60° against the discharge direction. Pressure distribution measurements on test profiles with and without blow-through showed a reduction of the unier pressure region at the disturbed flank. Immediately before the blowout gap, overpressure is reached, while the pressure distribution on the undisturbed flank hardly changes. As a result, a corresponding transverse force is obtained due to the diversity of pressure distributions on the profile flanks. (…)“

The FVA began practical testing of this type of screen after glider flight was reinstated. After several model tests on clockwork-controlled motor models, a Grunau-Baby III was equipped with this control system. The opinions of even well-known experts were very divided. There were fears that the flow would break off at the leading edge of the wing, that vibration would occur, and that the control capability would be endangered by possible small leaks at the valve organs. Despite the gloomy prophecies, the reconstruction of normal surfaces began, based on the results of the investigation. Between the outer ribs, 7 inlet slots were cut out, from which plywood-lined nozzle-shaped ducts led to the top of the wing. The air volume was regulated by pivoting flaps, which were covered with thin sheet rubber for sealing. They were operated by steel tubes parallel to the spar. For safety reasons, the normal aileron arrangement had to be retained in addition. Therefore, an additional hand lever was mounted below the instrument panel, with which the tubes in the wings could be operated via cables and pushrods.

Insurance regulations required testing of the control system above a safety altitude of 500 m, so that a towing machine was necessary for the first takeoffs. After a load test proved the durability of the modified strength structure, test flights were made in the fall of 1953. Flight instructor Thoemssen (FVA) reported the details after his launch, which went off without a hitch, and brought back some important measurement results. In fact, the aircraft could be flown in any normal turn attitude without the aid of the rudder. The intensity of the built-in slots proved to be sufficient, but without reser- ves. Any critical phenomena did not occur. When aileron and blow-through control were operated simultaneously, there was no disadvantage to the normal rudders; in fact, because of the low intensity of the experimental arrangement, it was possible to cancel out the effect of the fully open slots by the twisting flaps.

The flight tests proved that the new approach is viable; the aircraft designer has a new control element which in many cases can be superior to conventional controls. In commercial aircraft of the largest design, for example, the control system would be eliminated, saving space and weight and increasing safety. The control system also offers advantages for certain shipbuilding applications, since a water flow can be rerouted for directional changes.