Servo tab

A Flettner rudder, also Flettner flap, for aircraft is an auxiliary flap on the rear part of a rudder, which runs along automatically opposed at every stroke of the oar and supported by its aerodynamic effect to be applied by the pilot control force.

The Flettner flap was filed in 1918 by the German engineer Anton Flettner for a patent.

Operation

If on an aircraft in flight a control surface ( rudder ) moves, then it must - except for pendulum Rowing - done against the flow pressure of the air that " pushes back " the helm. Depending on the speed and rudder area very large forces may be needed.

The Flettnerklappe learns the flow pressure. Since it is opposite but deflected to the actual rudder surface, the restoring force of the flowing air also acts opposite - and so supportive to the actual deflection of the rudder area.

Example: The rudder in the picture is to be moved down, which must be done against the right to the left flowing air that wants to keep the rudder above. The upwardly oriented Flettnerklappe however is in turn pressed by the air flow downwardly and thereby presses the rear end of the main rudder, to which it is attached, down - the desired control movement is supported, the force required for this is much lower.

Control

The Flettner flap can most easily be controlled indirectly. Then it is connected by a linkage to a stationary part of the aircraft fuselage. If the rudder knocked out, then automatically turns this linkage the Flettnerklappe in the opposite direction.

But it can also be operated directly on the control instead of the actual rudder. This possibility was used for large aircraft in which the necessary control forces from the pilot were no longer apply. That was the case 222, where the pulling or pushing the control column, only the two Flettner valves were operated at the innermost parts of the three split elevator halves by the pilot, for example, at the Blohm & Voss BV. This in turn expressed its rudder parts in the desired direction. To pull the control column was pulled back so really that it operated Flettnerklappe struck out down the entire rudder but moved by upward. The Junkers Ju 290 was similarly controlled.

Benefits

The Flettner flap allows the designer to adjust the rudder forces according to the results of flight testing to achieve a balance control behavior. Changes can be achieved by modification of the linkage, without affecting the tail itself. In addition to the action as a " power " to reduce the steering forces applied by the pilot may also be achieved by reversing the Anlenkungsrichtung a manual force increase and an increase in the restoring forces of the rudder. Higher restoring forces improve the flight stability control if you let go and give the pilot an improved feedback on its control inputs. Flettner tabs on the elevator to hand power increase are used in some gliders and are there at the same time the trim tab. ( for example, ASK13, Ka8 )

Disadvantages

Since the focus of the Flettner flap is far behind its center of rotation, it tends to vibrate and flutter, which is a considerable burden material means and requires the installation of vibration dampers. Furthermore, the valve has a reduction flettner the rudder effect result.

Importance

The Flettnerklappe lost in modern large aircraft their importance, as is done here, the adjustment of the rudder with the help of technical actuators (for example, hydraulic cylinders). For smaller machines but it is also still used today, and they usually also have the trim is used by the rudder stops as' permanent setting " in the desired trim position.

Flettner developed this invention further also for shipbuilding. The Flettner rudder was a freely rotatable by 360 ° rudder, so also a pendulum rudder, with an auxiliary rudder on the trailing edge of the rudder blade in ships. The rudder but could not prevail in shipbuilding.