Design > Stunt design
Flap design - Flat surface or contoured to follow the airfoil?
Bill Schluckbier:
Hello,
I have noticed that European builders tend to use flaps which generally follow the shape of the airfoil whereas in the US the preference is to use a simpler, flat and un-airfoiled section. Many years ago I remember reading that having the flaps disrupt the airfoil either more effective, or that by having some play at the elevator made it possible to make minor changes in altitude during the level laps without giving it away by having the model either pitch up or down.
I have enclosed a photo of a set of wings off an European model. As you can see the hinge line is offset relative to the wing's trailing edge. The trailing edge of the wing normally has a cove which allows the rounded leading edge of the flap to fit within.
It seems to me that in level flight, that a wing airfoil would generate more lift (assuming a small angle of attack) if using an airfoiled flap. Is one better than the other or are they basically the same?
Thank you,
Bill
Jim Svitko:
I have also wondered the same thing. Which is better? I never messed with that European set up but I like the looks of it. It looks like it would be better, but I have no personal experience to prove that. I have only done the "flat flap." The flat flap system seems to work pretty well so I am not so sure it can be improved.
One thing about that particular European set up is the number of hinges. Are three hinges enough? And, how much more work is involved to produce that set up? Is there enough performance improvement to justify it?
As far as elevator play goes, I never deliberately build play into the system. Normal wear will produce some play in regular use. Years ago, I had a bushing come out of the elevator horn. The pushrod had plenty of slop then and it caused flutter under certain conditions.
Brett Buck:
--- Quote from: Jim Svitko on December 16, 2023, 02:30:05 PM ---I have also wondered the same thing. Which is better? I never messed with that European set up but I like the looks of it. It looks like it would be better, but I have no personal experience to prove that. I have only done the "flat flap." The flat flap system seems to work pretty well so I am not so sure it can be improved.
One thing about that particular European set up is the number of hinges. Are three hinges enough? And, how much more work is involved to produce that set up? Is there enough performance improvement to justify it?
--- End quote ---
If you just blend the flap into the airfoil, the experience has been that it is a detriment to the performance, rather than an improvement, presumed to be due to the discontinuity when it is deflected.
I do not like the hinge line treatment at all. It looks good, but it also means you cannot seal it, which is more important.
Brett
Jim Svitko:
I am not sure of this, but I remember reading something about Al Rabe's flap experiments. Was it Al who determined that the flat flap, attached to the trailing edge but not blended into the wing contour, was the way to go?
Brett Buck:
--- Quote from: Jim Svitko on December 16, 2023, 05:37:43 PM ---I am not sure of this, but I remember reading something about Al Rabe's flap experiments. Was it Al who determined that the flat flap, attached to the trailing edge but not blended into the wing contour, was the way to go?
--- End quote ---
That was the first I know about, but hardly the only one. Al was specifically trying to prove that the conformal flap using Keith Trostle's hinge line treatment (very close to the Euro RTF models) was better, in his experiment, it was worse. There was plenty of argument over that, and he may have jumped to a conclusion - which is very tempting - but plenty of other people tried variants of the same idea since. Al, using his data, ended up with the aft-high-point airfoils that faired the flaps into the aft end of the airfoil better when deflected.
Brett
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