Design > Stunt design
Flap design - Flat surface or contoured to follow the airfoil?
Ken Culbertson:
This is one that can go either way. I have flown both on the same design and found little if any difference. However, the ones you are looking at do not extend far enough into the wing TE to be effective. For them to work properly the hinge point should be as close to the TE as possible which means that the flap will extend into the wing as much as half of the thickness of the flap. Endgame IV which I started last week will use these hinges for both the flaps and elevator even though they provide very little aerodynamic advantage. I think they are cool and it is a hobby so the time making them is not wasted. Their main advantage is to electric - Zero binding. I am not 100% sold on the benefit of sealing on electric if it adds any stiffness.
The problem(s) that Brett pointed out are real. If the TE is not shaped to allow the flap to rotate at a point not directly on the TE it will create a huge distortion in the airfoil when the flaps move.
I have flown them sealed and unsealed and to me there was not enough difference to make it worth sealing them. I have never seen wind tunnel tests on imbedded flaps, so I don't know if they "leak" the same as traditional hinging. My *guess* is that they don't.
Using this method does not preclude using flat flaps to which I am officially a convert.
Ken
Jim - I was flying with Al just after he finished with his famous car hood experiments. If I remember properly, he discarded the imbedding due to the distortion I mentioned earlier but he was trying a very thick version that basically made the flap part of the airfoil. To my knowledge, he never used flat flaps. His were more teardrop shaped. Long skinny teardrops.
Dennis Toth:
The flat flap was something that Lou Andrews did on the Barnstomer series with the fixed flap. The square edge kinda acts like it was deflected a bit as the ship turns, faster than a tapper/contour flap. Some have used it on coupled flaps, you need to adjust the amount of deflection since it comes in faster and can cause a little jumping around neutral if to quick. On the elevator this seems OK some have even added Gurney strips. Maybe Bert can elaborate on them and how/why they are used.
Al did a lot of development and particularly for the Sea Fury. Here is an article where he describes the different airfoils he tested and also the "Super Airfoil" that as Bert said didn't work out like he thought it would.
https://www.airplanesandrockets.com/airplanes/sea-fury-march-1973-american-aircraft-modeler.htm
Best, DennisT
Bill Schluckbier:
Dennis,
Thank you for posting the article, I will definitely take a look. Inset flaps are more difficult to build if you go through the effort of offsetting the hinges and creating a cove on the trailing edge so that the flap's LE can nest within while keeping a small gap between the two. And as Brett pointed out, you cannot seal the gap either.
In my rather limited experience I have noticed that either smaller flaps or smaller flap deflections appear to create smaller wakes when the aircraft is maneuvering. This is just an observation, I have a model with relatively narrow cord flaps which corners just fine but seems to suffer less from wake encounters in dead calm conditions. I also have another model with more traditionally sized flaps in which an accidental wake encounter will get your attention. Since both models appear to be able to fly tight corners, it does not appear to me that the smaller flaps are necessarily a hinderance or that maybe large deflections are a must. I should also point out that the first model also has a relatively long tail moment of 19" between hinge lines along with a generous stab and elevator. The model was built off a Sig Fazer kit and after even after adding wing tips its wingspan is of only 51". I also fly it with a 13x5 prop (electric). I will admit that it is goofy looking but it flies well. Go figure.
So in a run about way I am also wondering if inset flaps, combined with smaller deflections would work if you we willing to go with a longer tail moment. The reason for all of the questions is that I am getting ready to build another experiment and wanted to see what people though about airfoiled flaps.
Thank you,
Bill
Jim Svitko:
It would be nice to see a real experiment, in a wind tunnel, with both set ups. I am not the fluid dynamics expert so I am having problems with understanding this "discontinuity" aspect. It looks to me like either one would produce a discontinuity.
I like simple, and easy to make, so I stay with the flat flap concept. I make them with a slight taper to the trailing edge, mainly for looks, because I doubt if it matters much to simply round them off or leave them square. One thing I have gone to is reducing the flap chord, particularly at the root. Some planes I built in the past had close to 3 inches of flap chord at the root. I go for 20% flap chord, as a percentage of total wing chord, which is wing box chord plus flap chord. Maybe a bit less than 20% at the tip. Control forces are pretty light, and planes set up that way turn very well. No stalling due to insufficient flap chord or insufficient deflection amount.
Some time ago, I searched the web for actual aircraft flap arrangements. I should have saved that search. On one design, which was close to that European model version, I remember seeing a seal, a type of accordion seal, between the flap leading edge and the wing trailing edge box. So, the guys who designed that plane saw some benefit in sealing that gap. Trying to do something like that on a CL model would be quite an effort.
Ken Culbertson:
--- Quote from: Bill Schluckbier on December 16, 2023, 07:27:34 PM --- The reason for all of the questions is that I am getting ready to build another experiment and wanted to see what people though about airfoiled flaps.
Thank you,
Bill
--- End quote ---
Bill, just for reference, the design I have been referring to in my comments has a 19 1/2-inch tail and a large elevator. The current one I am flying only has 25 degrees of total flap movement of which I only use about 15, but the flaps are pretty normal size. If you are seriously interested in the flap wing union, you might look up some of the postings by Mark Wood. He is doing (0n and off) some rather radical testing of just what we are discussing here.
Ken
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