FVA 21 - Automatic plain flap

On the initiative of Prof. Dr.-Ing. A.-W. Quick, honorary member and tireless supporter of the FVA, the FVA was awarded a research contract in 1972 for an “Automatic flap adjustment for gliders”, which was financially supported by the German Research Association DFG.

Airfoils with flaps provide higher gliding performance compared to airfoils without flaps only if the flaps are adjusted correctly according to the ca requirement of the airplane. The pilot must therefore keep the flap setting angle at an optimum value, depending on the wing loading and airspeed. For cross-country glider flights, it is necessary to vary the airspeed depending on the average climb of the thermal updrafts and the instantaneous sink or rise of the air encountered. The constant adjustment of the flaps in this way imposes an additional burden on the pilot. If the adjustment of the flaps were left to an automatic system, the pilot would be relieved and the handling of the flap aircraft would be equal to that of an easier to fly glider without flaps. The project manager was Heiner Oberdörster, who took over the supervision of the work until its successful completion in 1975.

After extensive preliminary studies, it was decided to make the control of the flap angle of attack dependent on the angle of attack of the glider. An outwardly projecting wind vane was mounted at a suitable point in the fuselage side wall, which adjusted itself in flight without delay according to the respective angle of attack of the aircraft. The angle measured at the wind vane was converted into an electrical signal by a field plate differential sensor and fed to a controller. This controlled the adjustment of the flap linkage by a hydraulic power cylinder via electric directional control valves until the actual value corresponded to the specified set value.

When the flap lever was adjusted manually, the control loop switched off automatically, as it did when the landing gear was extended and during takeoff and landing approaches.

The FVA was also not at a loss for a test carrier for the automatic flaps, because thanks to the generous support of the VFW-Fokker company, it was possible to acquire an FK-3 (an all-metal glider of the open class with an 18 m wingspan and flaps) on very favorable terms in 1971.

The automatic flap system was installed in the FK-3 in the spring of 1974, and the first flight of the complete automatic system took place on August 22, 1974. During the rest of 1974, the system was tested in more than 50 flying hours by about 20 different pilots.

“Flying the FK-3A (A = automatic) was very impressive. The flap lever set itself to the optimum value, as if moved by magic. Expectations were confirmed by longer series of measurements and, to everyone’s surprise, we also obtained optimum values for turning flight. The automatic flap adjustment enables a flight style according to the McCready theory particularly well during cross-country flight. It is observed that the automatic system takes forces from the control stick in the manner of a trim when adjusting. In torn and bucking thermals, the control proves to be too sensitive, according to the pilots’ initial assessment. It also reacts too quickly to jerky stick movements. In the meantime, this disadvantage has been eliminated with the help of a switchable damping system, which ensures that only longer lasting speed changes cause the flaps to be adjusted.” (from FVA Annual Report 1973/74)

The entire system still weighed about 20kg in the FK-3A. However, in discussions with some manufacturers of hydraulic equipment, it soon became clear that in reducing the weight, a compromise would have to be made between the weight savings and the resulting increase in the cost of the system. In the years from 1976 to 1978, Liebherr succeeded in developing a hydraulic unit for this plant, which was installed in the LS 3a of the FVA.

Thus, the automatic flap system fully met the demands placed on it and was therefore successfully tested in other aircraft. For this reason, the only criticism of the automatic system was not of a technical but of an aeronautical nature: Critics were of the opinion that an electric control device had no place in a glider. Nevertheless, the technical achievement of the FVA was recognized.