The FVA-15 was a Grunau Baby, modified to test lateral force control with compressed air instead of ailerons.
On the Origin and testing of bladder control was reported by Dr.- Ing. H. Stone member of the FVA in "Research and fly " in January 1954 , Issue 2:
" In terms of time the blow-through control created during the last years of the war in Aachen Aerodynamic Institute, whose leadership even then Prof. Seewald held . At the time, I got the research was to examine together with my staff , control of Lippisch . This involved a jet of compressed air , let escape the Lippisch from inside the fully enclosed blade by a plane parallel to the blade axis gap to achieve lateral force differences. the effect was good especially the strongly curved upper surface of a wing , a practical application of this control method is , however, to carry a relatively heavy compressor unit in aircraft contrary . (... )
Advantages of the through Breath Control
The hinge moments that can change with the different flight modes and speeds strong despite compensation , are variously as large dasss it no longer dominates the pilot with his physical strength , especially large aircraft must be equipped with a rowing machine . Motivated by these experiments , I made the suggestion to use a controller that is also to achieve the lateral force change with a Ausblasestrahl , but the requisite amount of air takes through a crack in the wing leading edge of the free flow of air . After some preliminary experiments that confirm the effectiveness of the arrangement , a Meßsreihe was to determine
• the best sizes for the inlet and outlet gap
• The best location and gap
• the appropriate blowout down.
It was found that the inlet cross-section is greater than the Auslaßsquerschnitt be selected so that the Ausblasestrahl get a sufficiently large intensity. From the leading edge of the measured Ausblasspalt is about 2/3 chord with a Ausblasewinkel by 60 ° with respect to the outflow direction. Pressure distribution measurements in experimental profiles with and without sparging resulted in a reduction of the Unierdruckgebietes at the edge disturbed . Immediately before the Ausblasespalt pressure is reached, the pressure distribution on the undisturbed edge does not change much . As a result, the diversity is achieved by the pressure distributions at the profile flanks a corresponding shear force . (...) "
Developed by Dr.- Ing. H. Stein, the FVA began the practical testing. After several attempts to model clockwork motor models a Grunau Baby III was equipped with this control. The opinions of eminent experts also were very divided, it was feared a flow separation at the leading edge would endanger control via any small leaks on the valve bodies.
Despite the gloomy prophecies of the conversion of normal surfaces, between the outer ribs, 7 inlet slots were cut out, of which led plywood lined nozzle-shaped channels for the upper wing surface. The regulation of the amount of air happened by hinged flaps, which were based on a gasket with a thin sheet of rubber. The operation was parallel to the beam to steel pipe. For safety reasons, the normal aileron assembly had to be additionally maintained. Below the instrument panel, therefore, a hand lever was also attached to the cable pulls and bumpers, the tubes were operated in the area.
Underwriting procedures required for testing the control above a safe altitude of 500 m, so that a tug was required for the initial launch. After load testing, the test flights were made in the fall of 1953. Flight instructor Thoemssen (FVA) reported the aircraft was flying in every normal cornering without the aid of the rudder. The intensity of the built-in slots was sufficient. With simultaneous activation of aileron and blow-control, no discrimination of normal roll was revealed. Due to the low intensity of the experimental arrangement, it was even possible to cancel the effect of the fully open slots.
The flight tests proved that the new control may be superior in many cases, than conventional controllers. Mainly space and weight savings.