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Author Topic: Flap design - Flat surface or contoured to follow the airfoil?  (Read 2443 times)

Online Bill Schluckbier

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Flap design - Flat surface or contoured to follow the airfoil?
« on: December 16, 2023, 06:52:13 AM »
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
« Last Edit: December 16, 2023, 09:17:31 AM by Bill Schluckbier »

Offline Jim Svitko

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Re: Flap design - Flat surface or contoured to follow the airfoil?
« Reply #1 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?

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.

Offline Brett Buck

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Re: Flap design - Flat surface or contoured to follow the airfoil?
« Reply #2 on: December 16, 2023, 04:51:47 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?


   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

Offline Jim Svitko

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Re: Flap design - Flat surface or contoured to follow the airfoil?
« Reply #3 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? 

Offline Brett Buck

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Re: Flap design - Flat surface or contoured to follow the airfoil?
« Reply #4 on: December 16, 2023, 06:02:55 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?


      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

Offline Ken Culbertson

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Re: Flap design - Flat surface or contoured to follow the airfoil?
« Reply #5 on: December 16, 2023, 06:26:35 PM »
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.
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Offline Dennis Toth

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Re: Flap design - Flat surface or contoured to follow the airfoil?
« Reply #6 on: December 16, 2023, 06:38:21 PM »
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

Online Bill Schluckbier

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Re: Flap design - Flat surface or contoured to follow the airfoil?
« Reply #7 on: December 16, 2023, 07:27:34 PM »
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

Offline Jim Svitko

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Re: Flap design - Flat surface or contoured to follow the airfoil?
« Reply #8 on: December 16, 2023, 07:45:51 PM »
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.

Offline Ken Culbertson

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Re: Flap design - Flat surface or contoured to follow the airfoil?
« Reply #9 on: December 16, 2023, 08:13:26 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
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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Offline Brett Buck

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Re: Flap design - Flat surface or contoured to follow the airfoil?
« Reply #10 on: December 16, 2023, 10:57:55 PM »
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.

   There's a picture in the Mustunt article, showing the top surface of the wing blending smoothly with the flap at about 25 degree deflection. If it was conformal/"fair" with the airfoil before you deflected it, there would be a sharp angle at the hinge line. I also note that the Mustunt, all of Al's later airplanes, and Igor's airplane, all have lots of curvature in the rear half of the airfoil to accomoplish the same purpose. Doing that, along with trying to limit the LE radius to something that looked good to him, resulted in airfoils that were vaguely like laminar flow airfoils.

    I am curious what you would want to learn about with a wind tunnel. Al's car hood tests were geared to maximizing the lift, which might have been important in 1969,  is probably not the most critical factor in the era of modern powerplants. The observation from almost everyone who ever tried a "conformal" flap was that the control loads were/negative pitching moment was astronomical and led us all to running the CG back to radical aft CGs. Same airplanes with conventional flaps, no problem.

    Brett

     

Offline Trostle

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Re: Flap design - Flat surface or contoured to follow the airfoil?
« Reply #11 on: December 17, 2023, 03:21:59 PM »

       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 -

     Brett

Al Rabe tested the "conformal flap" arrangement with his experiments using wing/flap configurations mounted on the hood of his car at different speeds.  That led to the airfoil/flap arrangements on his semi-scale designs.  The success he had cannot be argued.

The test he did with my airfoil/flap design placed the hinge line of the flap behind the flap leading edge radius so that as soon as the flap started to deflect, the leading edge of the flap would start to protrude (as in a bump) above the surface of the wing trailing edge.  His reasoning to do this was as the flap would deflect, the flap LE would start to seal the entire hinge line, this restricting the flow of air from beneath the wing to the lower pressure above the wing/flap.  The greater the flap deflection, the greater the protrusion (bump).  The airflow over the aft part of the wing is already transitioned from laminar to turbulent.  As soon as this turbulent airflow goes over the flap LE protrusion (bump), it will likely become turbulent, destroying much of the lift over the top of the flap that it could have had without the bump at the LE of the flap.  Al performed an interesting experiment and obtained results that should have been expected.  It would have been interesting if he had continued the testing with a similar wing flap configuration.  One test with the flap hinge line on the center line of the flap LE and maybe a few other tests with the flap hinge line in different positions ahead of the flap LE.

I have had some success over the years with this wing/flap arrangement where the flap is part of the airfoil, not the essential flat sheet most often seen on stunt ships.  I place the flap hinge line approximately 1/32" in front of the flap LE radius and will continue to do so.

Keith

Offline Howard Rush

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Re: Flap design - Flat surface or contoured to follow the airfoil?
« Reply #12 on: December 18, 2023, 02:01:38 AM »
Jive Combat Team Investigative Services is in possession of a Yatsenko Shark Evolution.  It flies very well, but its flap is different than ours.  It is hinged from three points and is not sealed.  The inboard hinge is at 12.5% flap chord.  The flap leading edge is an arc centered at or near the hinge.  The trailing edge of the wing is slightly concave.
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Online Lauri Malila

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Re: Flap design - Flat surface or contoured to follow the airfoil?
« Reply #13 on: December 18, 2023, 04:40:38 AM »
I have many Sharks, and of course at one point I tried to seal the hinge lines with a kind of a lip seal.
I canít say why, but the results were not at all good like they usually are with other, US-style designs. Part of the problems were of course due to increased friction, but clearly the seals also kicked the model somehow Ēoff the balanceĒ aerodynamically. Especially the round maneouvres suffered.
The Yatsenko etc. models are structurally much more sophisticated  than an average US-model; because of the molded construction they can reach a better stiffness, accuracy and form stability with less material. They also basically always end up in their ideal flying weight. That all leads to less surprises during trimming and flying.
One possible reason why they donít need seals could be the much thicker wing t.e. It could be so that the bigger volume shadowed between wing and flap acts like a buffer during more sharp pressure changes, like when flying corners. Kind of same idea when we use a small groove milled around exhaust port to stop the used gases from entering the crankcase bottom end. Itís called a labyrinth seal but itís not really a labyrinth, just a groove with correct volume. Works very well. L

Offline Ken Culbertson

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Re: Flap design - Flat surface or contoured to follow the airfoil?
« Reply #14 on: December 18, 2023, 07:49:39 AM »
Jive Combat Team Investigative Services is in possession of a Yatsenko Shark Evolution.  It flies very well, but its flap is different than ours.  It is hinged from three points and is not sealed.  The inboard hinge is at 12.5% flap chord.  The flap leading edge is an arc centered at or near the hinge.  The trailing edge of the wing is slightly concave.
I have flown one of the European/Russian designs and one in our flying group uses one for practice.  Both have the pocket hinge and a rather large (by US standards) gap between the flaps and wing TE without sealing.  The ones I flew were different from Howards in that they had rounded wing T/E.  What is common to them is the consistency of the gap.  We use wide hinges on ours, they use narrow.
Could the hinges be the culprit since we seal over them as well?  I recently removed all of the controls and surfaces from Endgame III to balance the movements on the logarithmic and rudder.  When I finished, I forgot to reseal the hinge line.  I was two sessions into retrimming when I noticed and resealed them.  I did not see any change in how it flew.  I am planning to use the European style hinges on IV and I would like to know more about this.  I hope there are some that know why ours appear to need sealing and theirs don't.

ken

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Online Bill Schluckbier

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Re: Flap design - Flat surface or contoured to follow the airfoil?
« Reply #15 on: December 18, 2023, 08:20:16 AM »
Howard,

Thank you for posting the photos.  The cove for the flap in that particular model is quite shallow; there is a Polish (or Czech) builder out there that builds his wings with a much deeper cove.  Glad to hear that the model flies well.

Thanks!

Offline Howard Rush

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Re: Flap design - Flat surface or contoured to follow the airfoil?
« Reply #16 on: December 30, 2023, 03:32:39 AM »
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?

Level flight lift would equal the airplane weight.

I hope you didn't put those orange blobs on your wing just to show us where the hinges are.  Be careful taking them off.  Alternatively, I guess you could work them into the paint scheme.  I don't see how, but I'm no artist. 
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Offline Chris Wilson

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Re: Flap design - Flat surface or contoured to follow the airfoil?
« Reply #17 on: March 19, 2024, 04:10:47 AM »
I thought that the point of the 'ideal'  flap was to have the least drag when deflected and the greatest lift, when compared to level flight.
A flat plate flap creates a reasonable undercamber whereas a hinged airfoil does not.
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Offline Istvan Travnik

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Re: Flap design - Flat surface or contoured to follow the airfoil?
« Reply #18 on: March 19, 2024, 07:18:40 PM »
I build from 1987 "pollywog" shaped airfoils. It means that the flap part of airfoil is fully concave.
In other words it can be considered as a modification of Wortmann FX-71 L 150/30 (in the root) and /20 at the wingtip.
It is distorted in 3 parameters:
-maximal thickness moved forwards: from 35% of full chord to 25%
-maximal thickness increased from 15% to 16,7%  (in the root only)
-trailing edge of flap got some 1.5% thickness, with sharp corners (no rounded)
Total movement never exceeds  + - 20į
"Leading edge" of flap is rounded, and the turning axle is in the center.
This construction gives same lift as conventional wings, however has 10% narrower chord.
Drag is a bit lower.
Turning ability, and tolerance against turbulences is better.
Istvan

Offline Air Ministry .

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Re: Flap design - Flat surface or contoured to follow the airfoil?
« Reply #19 on: March 29, 2024, 05:22:56 AM »
To be annoying ? The ' traditional ' SEWN hinges ,
Theflap initially rolls out into the airflow at thge leading edge ,
and the sheet flap is further into the airflow , moved , than conventional ore other ' hinges ' .

The zig zag thread ( dual upholsterers thread ) is like a turbulator , there . Also .
The Rolling - on a straight vertical , or radiused , trailing edge , is as a moving center of rotation .

My observation is they give great initial bite for fine corrections in bumpy air . If your under 40 .  S?P

Dunno if its possable to get these with other hinge systems . Dunno what APPEARANCE would do for the score there , either .
With the thread external on the surface . But well done ( a evening / 4 hours work ,) a non irregular pattern could enhance the score ! ? .
Even if ya dont wantem glossy .

Online Bob Hunt

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Re: Flap design - Flat surface or contoured to follow the airfoil?
« Reply #20 on: April 04, 2024, 07:18:05 AM »
This is an interesting thread and topic. Through the years I have tried both the thin flap attached to the rear of the wing and also the continuous airfoil type. I have settled on the continuous airfoil type (integral to the shape of the airfoil) as my preferred arrangement. But, the reasons that I like it better may be something that has not been discussed in the thread.

Thin sheet flaps do not have much torsional resistance. And, since we are driving them from the root end, the tendency is for the flap to twist under airload when deflected. My take on this is that we are not getting the full effect of the flap as it washes-out towards the tip. And, if your flaps are not made from similar density wood, the flaps will probably twist differently to each other, causing a roll tendency. Under heavy wind conditions this differential in "twisting" gets even worse, causing all sorts of problems (IMHO).

By making the flap integral to the airfoil, the leading edge of the flap is much thicker (deeper...?). And when you increase that dimension of the triangle (flap shape as viewed from the end...), the torsional rigidity (stiffness) goes up tremendously, and the flaps are far less likely to twist under load when being driven from one end. For me this fact trumps any theoretical aerodynamic advantage of the flat plate or thin flap. I've found that my planes fly more predictably in both calm and severe wind conditions since I went to the integral flap concept.

And, the way the leading edge of the flap is beveled or rounded on the thicker flap may be another area of performance enhancement. In fact, I used to just bevel the leading edges to allow enough flap travel, but lately I've been rounding off the upper and lower edges of that bevel. I cannot give empirical evidence that one is better than the other, but the newer planes just seem to turn and lock in a much more "friendly" manner, so I'll continue doing that as well.

The attached photo kind of shows the integral flap to the airfoil on my Wildfire twin. This model also featured the rounded edge on the flap leading edge bevel, although that is difficult to see in this photo. This has become one of my all time favorite models and it is extremely easy and fun to fly. The flaps are mongo rigid...

Later - Bob Hunt   

Offline Howard Rush

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Re: Flap design - Flat surface or contoured to follow the airfoil?
« Reply #21 on: April 05, 2024, 05:33:03 PM »
Bbbbbbbbbbb flat flaps,
When I put Ďem on Iím a rariní to go.

Hope this helps.
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Online Bob Hunt

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Re: Flap design - Flat surface or contoured to follow the airfoil?
« Reply #22 on: April 06, 2024, 05:51:04 AM »
Left Twix, right Twix...

Offline Ken Culbertson

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Re: Flap design - Flat surface or contoured to follow the airfoil?
« Reply #23 on: April 06, 2024, 08:34:41 AM »
I am not sure about the "twix" choice since my flaps are adjustable independently so I can "twix" either the left or right.  What I do prefer is what Bob has outlined.  I don't like flat flaps.  I was forced to use them on Endgame III because I used a Geo-Bolt airfoil with a thin wing TE.  Tapering 1/4" flaps is not helpful.  However, I had to move the CG forward somewhat to kill the hunting.  Flaps may not have been the cause but it was an easy one to blame it on.  This may well be an age issue.  Hunting is harder to control as the reflexes deteriorate.  Tapered flaps from sanding tend to warp so I build mine up and use a lot of "X" pattern ribs and use a .5mm CF strip in the TE to keep them from warping.  Endgame IV under construction uses an XL airfoil so I will be able to use my favorite 3/8" tapered to 1/8" flaps.  Still using the "X".  Takes a while to build but I don't mind.  I love building (hate finishing!)

Ken
« Last Edit: April 06, 2024, 08:56:14 AM by Ken Culbertson »
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Offline doug coursey

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Re: Flap design - Flat surface or contoured to follow the airfoil?
« Reply #24 on: April 06, 2024, 09:44:17 AM »
This is an interesting thread and topic. Through the years I have tried both the thin flap attached to the rear of the wing and also the continuous airfoil type. I have settled on the continuous airfoil type (integral to the shape of the airfoil) as my preferred arrangement. But, the reasons that I like it better may be something that has not been discussed in the thread.

Thin sheet flaps do not have much torsional resistance. And, since we are driving them from the root end, the tendency is for the flap to twist under airload when deflected. My take on this is that we are not getting the full effect of the flap as it washes-out towards the tip. And, if your flaps are not made from similar density wood, the flaps will probably twist differently to each other, causing a roll tendency. Under heavy wind conditions this differential in "twisting" gets even worse, causing all sorts of problems (IMHO).

By making the flap integral to the airfoil, the leading edge of the flap is much thicker (deeper...?). And when you increase that dimension of the triangle (flap shape as viewed from the end...), the torsional rigidity (stiffness) goes up tremendously, and the flaps are far less likely to twist under load when being driven from one end. For me this fact trumps any theoretical aerodynamic advantage of the flat plate or thin flap. I've found that my planes fly more predictably in both calm and severe wind conditions since I went to the integral flap concept.

And, the way the leading edge of the flap is beveled or rounded on the thicker flap may be another area of performance enhancement. In fact, I used to just bevel the leading edges to allow enough flap travel, but lately I've been rounding off the upper and lower edges of that bevel. I cannot give empirical evidence that one is better than the other, but the newer planes just seem to turn and lock in a much more "friendly" manner, so I'll continue doing that as well.

The attached photo kind of shows the integral flap to the airfoil on my Wildfire twin. This model also featured the rounded edge on the flap leading edge bevel, although that is difficult to see in this photo. This has become one of my all time favorite models and it is extremely easy and fun to fly. The flaps are mongo rigid...

Later - Bob Hunt   
BOB HOW DO YOUR INSTALL YOUR HINGES...DO YOU RECESS THEM IN THE FLAP ONLY OR PUT THE HINGE PIVIOT IN THE MIDDLE..HALF IN THE FLAP AND HALF IN THE TRAILING EDGE
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Online Bob Hunt

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Re: Flap design - Flat surface or contoured to follow the airfoil?
« Reply #25 on: April 06, 2024, 09:59:33 AM »
Hi Doug:

Up to this point I have been installing the hinges as I always have with the barrel recessed just into the leading edge of the flap and elevators. To be clear, this puts the forward edge of the barrel right at the hinge line, and allows the center of the horn wire to be on the same pivot point as the hinge pins. This allows for sealing the hinge line. However, I have flown a few models with the hinge pin recessed to the center of the radius of the flap/elevator, and I have seen the value of this type of arrangement as well. I realized that the flap horn also has to be moved somewhat aft of the trailing edge of the wing to allow it to pivot and not arc... Also, this type of hinging does not allow sealing of the flaps/elevator hinge lines. In the model that I flew, this did not seem to be a problem as those models had amazing cornering ability even without the sealed gaps. Still lots to be learned...

Later - Bob

Offline Ken Culbertson

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Re: Flap design - Flat surface or contoured to follow the airfoil?
« Reply #26 on: April 06, 2024, 10:58:48 AM »
For what it is worth, I have been using this method for making recessed flaps and elevators for a couple of years.  Sorry the pictures are a bit fuzzy but here is the method.  Make the elevator with the desired round LE.  This one uses pocket hinges with the pivot at the center of the LE radius.  Sand it to finished shape and wrap it with 100 grit sandpaper.  Use it as a sanding block to carve out the Stab/wing TE to just over the desired depth to make the hinge line match the TE.  Take off the sandpaper and mold a strip of 1/32" balsa around it.  After it dries take the molded piece off and put three layers of blue tape on the stab/flap.  Put the molded 1/32 over the tape and glue it into the stab/wing TE.  Perfect fit, even gap.

Ken
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Offline doug coursey

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Re: Flap design - Flat surface or contoured to follow the airfoil?
« Reply #27 on: April 06, 2024, 11:24:08 AM »
Hi Doug:

Up to this point I have been installing the hinges as I always have with the barrel recessed just into the leading edge of the flap and elevators. To be clear, this puts the forward edge of the barrel right at the hinge line, and allows the center of the horn wire to be on the same pivot point as the hinge pins. This allows for sealing the hinge line. However, I have flown a few models with the hinge pin recessed to the center of the radius of the flap/elevator, and I have seen the value of this type of arrangement as well. I realized that the flap horn also has to be moved somewhat aft of the trailing edge of the wing to allow it to pivot and not arc... Also, this type of hinging does not allow sealing of the flaps/elevator hinge lines. In the model that I flew, this did not seem to be a problem as those models had amazing cornering ability even without the sealed gaps. Still lots to be learned...

Later - Bob
I RECESS THE HINGES IN BOTH THE WING TRAILING EDGE AND THE FLAP LEADING EDGE ( SAME FOR THE ELEVATOR ) AND MOUNT THE CONTROL HORN SO THE PIVOT POINT IS IN LINE WITH THE HINGE PIVOT POINT..I END UP WITH A VERY SMALL GAP BETWEEN THE FLAP AND WING EVEN WITH UP TO 40 DEGREES TRAVEL
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Offline Howard Rush

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Re: Flap design - Flat surface or contoured to follow the airfoil?
« Reply #28 on: April 06, 2024, 06:05:35 PM »
For what it is worth, I have been using this method for making recessed flaps and elevators for a couple of years.  Sorry the pictures are a bit fuzzy but here is the method.  Make the elevator with the desired round LE.  This one uses pocket hinges with the pivot at the center of the LE radius.  Sand it to finished shape and wrap it with 100 grit sandpaper.  Use it as a sanding block to carve out the Stab/wing TE to just over the desired depth to make the hinge line match the TE.  Take off the sandpaper and mold a strip of 1/32" balsa around it.  After it dries take the molded piece off and put three layers of blue tape on the stab/flap.  Put the molded 1/32 over the tape and glue it into the stab/wing TE.  Perfect fit, even gap.

Ken

That is cool.
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Offline Ken Culbertson

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Re: Flap design - Flat surface or contoured to follow the airfoil?
« Reply #29 on: April 06, 2024, 07:47:34 PM »
That is cool.
I love to tinker!  Some of them actually work.

Ken 
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Online Bob Hunt

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Re: Flap design - Flat surface or contoured to follow the airfoil?
« Reply #30 on: April 07, 2024, 06:23:27 AM »
I ran across a photo that somewhat illustrates the point I made above about thicker flap leading edges (top to bottom). This photo shows just how thick (deep?) the trailing edge of the wing on my Wildfire twin is. Naturally the corresponding flap follows the airfoil, and it's leading edge is just as thick or deep.. Again, this makes for a very rigid flap that will not twist when driven from one end as is the tendency for a thinner flap.   

Later - Bob Hunt

« Last Edit: April 08, 2024, 03:06:29 PM by Bob Hunt »

Offline doug coursey

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Re: Flap design - Flat surface or contoured to follow the airfoil?
« Reply #31 on: April 07, 2024, 08:19:00 PM »
I ran across a photo that somewhat illustrates the point I made above about thicker flap leading edges (top to bottom). This photo shows just how thick (deep?) the trailing edge of the wing on my Wildfire twin is. Naturally the corresponding flap follows the airfoil, and it's leading edge is just as thick or deep.. Again, this makes for a very rigid flap that will not twist when driven from one end is the tendency for a thinner flap.   

Later - Bob Hunt
   is that a foam wing or a lost foam version like your tucker
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Re: Flap design - Flat surface or contoured to follow the airfoil?
« Reply #32 on: April 08, 2024, 06:30:05 AM »
That's a foam wing that was done in what I call the "reverse composite construction" method. It features a 3/4-inch swept forward trailing edge; something I firmly believe in.

Later - Bob 
« Last Edit: April 08, 2024, 03:28:06 PM by Bob Hunt »


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