I’m considering which flap to build onto a home built aircraft, but not exactly sure which flap would best fit the aircraft and which one would be more beneficial. The aircraft has a fighterlike look to it, but I’m mostly worried about how the flaps will effect airflow during landing and takeoff.

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    $\begingroup$ We probably need a little more basis than "a fighterlike look to it" to give anything approaching meaningful advice. Split flaps were mostly used on fighters with very high power to weight ratios, so the high drag of the flap wasn't a major problem. If you don't have that kind of power, you might want to consider slotted flaps instead (they also increase lift, but generally don't increase drag nearly as much as plain or split flaps). $\endgroup$ Feb 6, 2018 at 17:02
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    $\begingroup$ Me personally, I would try to approach building an aircraft with some engineering-like methodology (being an engineer sort of biases me here I guess). That would include calculations on expected mission and flying performance (do I need flaps, and how much additional lift do I need/drag can I afford) as well as thoughts on structural integration and construction. Such reasoning would help influence my design decisions like which type of flap to use. There’s many good books on aircraft design. Why not start there? $\endgroup$ Feb 6, 2018 at 18:40
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    $\begingroup$ It's great that you're building an aircraft, but it's a huge task and especially if you want to design it yourself rather than buy a kit or plans. In other words, you're going to have questions that require detailed discussions and this site isn't a good format for those. If you haven't already done so, I strongly suggest you join the EAA (or local equivalent) and connect with other builders who can be a source of advice and help. You're always very welcome to ask questions here, but keep in mind the limitations of the Q&A format. $\endgroup$
    – Pondlife
    Feb 6, 2018 at 19:44

1 Answer 1


As usual, the answer is: It depends.

A short comparison would read like this:

  • Plain flaps have better L/D when deployed at small deflection angles.
  • Split flaps have a higher maximum lift at the same deflection angle.

Therefore, a plain flap can be used for take-off or slow flight, as well. Split flaps are only good for landing, but in this they are better than plain flaps because the increased drag makes the landing easier.

Airflow on the bottom of the airfoil is very similar, and on the top the plain flap will exhibit flow separation somewhere along the flap chord while the split flap has a well defined separation point at the trailing edge. This should produce a more steady wake and more linear characteristics of the split flap when compared to a plain flap.

Proof: Figure 7 from Chapter 5 of Sighard Hoerner's "Fluid Dynamic Lift":

Comparison between plain and split flaps

  • $\begingroup$ I’m glad people are willing to share their knowledge with others. This acquired understanding could help amateur pilots/ home builders to build and fly safer planes but also to enjoy the experience. I’m currently studying the FAA-H-8083-3B Airplane Flying Handbook to learn the roots. I’ve also constructed a 3-D model of aircraft control systems to benefit in a better comprehension of how things work, which way the parts move, and what to look for during construction. $\endgroup$ Feb 7, 2018 at 2:11

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