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There is a part on DA40's and also DA42's flap. I cannot find the technical document and I couldn't decide what is the purpose of that part. Is it a mass balance part or what? You may see the part at picture. (Note: First picture is not showing the part I meant to ask. The picture was wrong but I keep the picture not to make comments meaningless about it. Correct part is marked at second picture.) enter image description here

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Sorry everybody, I drew wrong part before by mistake. The part marked at first picture is the aileron control rod. All previous answers and comments are correct for aileron. I add a new picture for the correct part and I found out it functions as a mass balance horn. We see housings from pictures but the related part is inside the housing. Edit time: 11th Nov, 2016 10:30 UTC enter image description here

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  • $\begingroup$ This picture or this other one have more favorable perspectives. This looks like this when the flaps are extended. Very similar, but more obvious, on the Cirrus SR20. $\endgroup$
    – mins
    Commented Nov 8, 2016 at 21:28

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It is an actuator fairing. It covers the flap actuator so to minimize the drag created by it.

An actuator directly exposed to the airflow would create more drag, increasing fuel consumption and reducing the flight range.

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  • $\begingroup$ No the flap actuator linkage is located in the wing root on a DA-40. $\endgroup$
    – Romeo_4808N
    Commented Nov 8, 2016 at 18:35
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These are hinges, pivot points for the flaps, and some of them also probably contain the extension mechanism. The flaps on the DA-40 are slotted flaps, which are similar to plain and split flaps in that they typically pivot on hinges. In the case of slotted flaps the pivot points are below the wing creating a gap, or slot, between the wing and the flap when extended. These mechanisms are covered by a lightweight fairing which allows air to pass by cleanly, reducing drag.

You don't see these on airplanes with fowler flaps, where the flap is extended out the back of the wing rather than swung down on a hinge. This is a more complex type of flap, but is typically more effective than other flap types.

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  • $\begingroup$ The comparison with other types is wrong. Plain and split flaps rotate around hinge in the wing and don't have these (there is nothing below the wing of c172 (simple flaps) or dc-3 (split flaps)). On the other hand Fowler flaps and their double-slotted variants do have flap tracks below the wing. They are even more complicated than hinge, but they are below the wing, and in jet aircraft, they are usually covered with very conspicuous fairings that double as anti-shock bodies! $\endgroup$
    – Jan Hudec
    Commented Nov 8, 2016 at 22:40
  • $\begingroup$ You are right about the single and split flaps, that didn't come out well and I've edited. However, the c172 does not have simple flaps, they are a type of fowler flap. $\endgroup$
    – GdD
    Commented Nov 9, 2016 at 8:08
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Given the position on the wing, that appears to be the linkage for the aileron control rod which actuates the ailerons, not flaps. The outboard flap hinge can be seen to the left of that.

With the new edit, yes, that points to the aerodynamic fairing for the outboard flap hinge.

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  • $\begingroup$ From the picture links I posted under the question, you seem to be right, the nearby flap linkage fairing has a more vertical edge. But the perspective is deceptive. $\endgroup$
    – mins
    Commented Nov 8, 2016 at 21:36
  • $\begingroup$ Your answer is correct for my wrong marked part so I accept this as the correct answer. I have edited the question and found out the correct part functions as a mass balance horn. Thanks! $\endgroup$
    – aiternal
    Commented Nov 9, 2016 at 12:42

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