Flettner oar

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A Flettner rudder , also known as a Flettner flap , is a trim flap on the rear part of a rudder in aircraft , which automatically moves in the opposite direction with every deflection of the rudder and, through its aerodynamic effect, supports the steering force to be applied by the pilot.

The German engineer Anton Flettner applied for a patent in 1918 for the Flettner flap .

Mode of action

How the Flettner valve works. Explanation in the text

If a control surface (rudder surface) is moved on an aircraft in flight, then this - except for pendulum rudders - must be done against the flow pressure of the air, which "pushes back" the rudder. Depending on the speed and rudder area, this can require very large forces.

The Flettner flap also experiences the flow pressure. But since it is deflected in the opposite direction to the actual rudder surface, the restoring force of the flowing air also acts in the opposite direction - and thus supports the deflection of the actual rudder surface.

Example: The rudder in the picture should be moved downwards, which must be done against the air flowing from right to left, which wants to keep the rudder up. The upward-facing Flettner flap, however, is in turn pressed down by the air flow and thereby presses the rear end of the main rudder to which it is attached down - the desired steering movement is supported, the force required for this is much less.


The Flettner flap is easiest to control indirectly. Then it is connected to a stationary part of the aircraft fuselage via a linkage. If the rudder is knocked out, this rod automatically turns the Flettner flap in the opposite direction.

However, it can also be operated directly via the control instead of the actual rudder. This option was used in large aircraft where the pilot could no longer provide the necessary control forces. That was z. B. the case with Blohm & Voss BV 222 , where when pulling or pushing the control column, only the two Flettner flaps on the innermost parts of the three-part elevator halves were operated by the pilot. These in turn pushed the associated rudder parts in the desired direction. To pull , the steering column was really pulled backwards, the flettner flap that was actuated deflected downwards, but the entire rudder moved upwards as a result. The Junkers Ju 290 were also controlled in a similar way.


The Flettner flap allows the designer to adjust the rudder forces according to the results of the flight tests in order to achieve a balanced control behavior. Changes can be made by modifying the linkage without changing the tail unit itself. In addition to acting as a "servo" to reduce the steering forces to be applied by the pilot, reversing the direction of articulation can also increase the manual force and increase the restoring forces of the rudder (so-called anti-flettner, e.g. with the Grob G 110 ). Higher restoring forces can be improved the flight stability when the controls are released and give the pilot improved feedback about his control inputs. Flettner flaps on the elevator to increase hand force are used on some gliders and are also the trim rudder there. (e.g. ASK 13 , K 8 )


Since the center of gravity of the Flettner flap is far behind its pivot point, it tends to vibrate and flutter, which means a considerable load on the material and requires the installation of vibration dampers. Furthermore, the Flettner flap reduces the rudder effect.


The Flettner flap has lost its importance in modern large aircraft, since the rudder is adjusted with the help of technical actuators ( e.g. hydraulic cylinders ). In smaller machines it is still used today, whereby it mostly serves for trimming at the same time by keeping the rudder in the desired trim position as a "permanent setting".

Flettner also developed this invention further for shipbuilding. On ships, the Flettner rudder was a rudder with an auxiliary rudder on the trailing edge of the rudder blade. But the rudder could not prevail in shipbuilding.


  • Hasenfuß, Dorner-Müller: Investigation report CX010-098 from the BFU, Kaman K-1200 helicopter, crash in Hindelang , March 2000
  • Peter Kämpf: Manual forces when designing an aircraft control system , August 1, 2000
  • E. Foerster: Praktischer Stahlschiffbau 1929, from p. 223 explanations on the Flettner rudder ( limited preview in the Google book search)

Individual evidence

  1. Anton Flettner: "Main rudder that can be operated by remote adjustment of an auxiliary steering rudder.", Austrian patent AT000000094396B, June 6, 1918 (pdf)
  2. Walther Parey: The Flettner oar. In: Polytechnisches Journal . 339, 1924, pp. 29-32.