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

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

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

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

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.

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

     

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.

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

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. 

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  + - 20°
"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

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   

Left Twix, right Twix...

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

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

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.

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

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