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The Junkers Flap: Lesser Known Flap Designs

Header - Junkers

You've probably heard of the four main flap designs used on airplanes: plain, split, slotted, and fowler. However, over the past 100 years, many more flap designs have been successfully tested and used. One of the those is the Junkers flap.

Junkers Flap History

The Junkers flap was invented in Germany in the 1920s by O. Mader Junkers. The flap was designed to decrease takeoff and landing distance, as well as increase roll responsiveness in flight. The design was a huge success in the 1920s and 30s, and was used on many Junkers aircraft designs. The most successful design implementation was on the Ju-52, which was one of the most widely used passenger and cargo planes in Europe at the time.

Junkers 2
Junkers 3

Flap Operation

The Junkers design is a slotted plain flap, where the flap is fixed below the trailing edge of the wing. This design allows airflow to pass between the wing and flap, even when the flap is retracted. Because of the increase in airflow, a significant increase in lift is added to the wings, even at slow speeds.

Junkers Retracted Junkers Extended
Junkers Flap Advantages
  • Creates more lift than a plain or split flap
  • Mechanically simple
Junkers Flap Disadvantages
  • Creates more parasitic and induced drag than other flaps at cruise speed, because it cannot be retracted into the wing
  • Creates high amounts of adverse yaw

Junkers Flap Today

Variations of the Junkers flap can still be found on aircraft today. While the design doesn't perform well at high speeds, it provides excellent lift and control at low speeds. Because of this, it's found on many Short Takeoff and Landing (STOL) and kit-build aircraft, like the STOL CH 701 pictured below. The flap performance impressive. At max gross weight, the STOL CH 701 can takeoff in as little as 120 feet on grass and 90 feet on a hard surface.

STOL

Colin Cutler

Colin is a Boldmethod co-founder, pilot and graphic artist. He's been a flight instructor at the University of North Dakota, an airline pilot on the CRJ-200, and has directed development of numerous commercial and military training systems. You can reach him at colin@boldmethod.com.

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