Control surface trailing edges
 

What is the current thinking on control surface trailing edges? Traditional thought was they should taper down to knife edge. In reality I always put some type of full radius on the tapered edges because of the ease of damaging small fine edges. However, my FuntanaS .40 3D, has rounded leading edges and squared trailing edges. Does this help alleviate flutter, or is there some other reason for this?

RE: Control surface trailing edges
 

I typically just square off the trailing edges. My ailerons are usually tapered because the leading edge is so thick to begin with. The T.E. ends up being about 1/8"-3/32" thick. Aerodynamaically, you get better, cleaner air separation using squared edges vs rounded. Clean separation helps eliminate flutter.

Dr.1

RE: Control surface trailing edges
 

Square corners help prevent flutter and rounded corners promote flutter.

If you want to taper you can slice and inlay 1/64 ply into the edge and taper down to it. Make a cutter by screwing a #11 blade down with a shim underneath that is equal to half the thickness of the TE before trimming. Slide the TE across the blade and you have a slot for the 1/64 ply. You can easily taped down to it. Seal it in with thin CA.

Another thing is to taper to a sharp edge, then soak the edge with thin CA to harden the wood.

Personally, I leave mine square.

Here's a photo of the TE of the aileron of a Morris TopCap 3D plane.

RE: Control surface trailing edges
 

what do you think about this picture??
it s absolutely new for me, what s the reason for this TE shape

RE: Control surface trailing edges
 

Ed, The only reason I would taper is if I was building a scale airplane and that's the way the real airplane was. Now that it has been identified that square corners help alleviate flutter, I'm good with leaving them square! Why add the added effort? I think rounded corners look better, however.

Thanks to DR1 and Ed for this information!!

RE: Control surface trailing edges
 

Flutter is more due to a loose control system.. Excess weight hanging on the hinges, or slop at the servo arm or the control surface horn, or a wimpy pushrod; than the shape of the trailing edges of the control surface.
The fancy trailing edge shown is intended to make that surface more effective. Whether it works or not only the user could say.
There is no MUST BE THUSLY on anything on a model.

RE: Control surface trailing edges
 

Tall Paul,

Control surface shape contributes more than you realize. Turbulence begins at the TE and travels forward. Any shape that helps make a clean break when the air passes the surface helps prevent flutter. Flutter can be caused by the conditions you mention, AND by the airflow separating from the surface due to turbulence.

Dr.1

RE: Control surface trailing edges
 

Torino,

Looks like they're trying to achieve a clean air separation at the TE. Many shapes can do that. Rounded is NOT one of them.

Dr.1

RE: Control surface trailing edges
 

Tapering a surface is a really good idea aerodynamically. Having a uniform TE is also a very good idea aerodynamically.

People got into leaving the TE square for a couple of reasons.

1. It's a lot easier to create a uniform squared off TE than any other shape.... and a uniform TE is good.
2. A squared TE is a lot easier to maintain. It's strength compared to a sharp TE is considerable and normal hangar rash is nothing to a squared TE but ruins a sharp one.
3. A lot of people are dead lazy and have little or no building skills. OK.... that's harsh :D but it's true, isn't it.
4. Those same people have little or no aerodynamic knowledge other than what they got from the loudest experts at their flying field.
5. The loud experts are described in #3 and #4 above. (Like I said above, this is harsh, but it's true also, isn't it.)

So the way square TEs came about:
Long ago when only kits or plans were available to the hobby, some of the newbies who hadn't any knowledge of aerodynamics didn't know to taper all the surfaces. And when they showed up at the field with these squared off things, the lazy guys noticed them and after seeing them fly, realized that they didn't have to go to all that extra work. Now, when those same lazy guys showed up with THEIR squared off ugly things and were questioned as to why they'd done such aerodynamic foolishness, they stuttered and spluttered and then bs'd something about Reynolds numbers and how the decalage of the empennage and the wing volume all contributed to an imbalance that could only be cured by moving the abscission point aft of the TE and that's why they had square TEs. And all the lousy builders who hadn't thought of cutting corners (pun intended) by sluffing past the effort of tapering, saw that they could actually come closer to making square TEs look halfway good and they started doing it too. And it caught on. ;)

RE: Control surface trailing edges
 

With our models, the best TE shape for a combination of aerodynamics and maintenance of the TE is a tapered surface to a relatively thin but square TE.

BTW, a thick, square TE is considerably more apt to contribute to flutter than any TE on a tapered surface, even a modestly tapered surface. Yes, flutter is lot's more probable with slop in the rigging, but it originates from the aerodynamics of the surface.

RE: Control surface trailing edges
 

I round the trailing edges on those surfaces I don't taper.
I've played with square and round and found zero difference.
Here's the rudder and elevator on my Morris Top Cap...
Nicely rounded.
Same on my Fazer, Wacky Waco, Ultimate....
On my 3D foamies, the foam is cut square in the kit, and I leave it that way.
Any flutter is due to excessive speed with very large chord surfaces.
Stiffening the control system to move the point of flutter excitation beyond the speed capability of the airplane works for models just as it does for full-scale, when flutter is encountered.

RE: Control surface trailing edges
 

As the originator of this thread, I feel my questions have been sufficiently answered. Statements that people "have little, or no building skills" are "Dead lazy" and "Loud experts" etc. is far beyond constructive help! Being "Harsh" is unnecessary!

Personally, I have found the R/CUniverse forum to be a place where a lot of very boring people like to get into little "Cock fights" over esoteric minutia!

RE: Control surface trailing edges
 

And those statements in that one post by that one person were salted with happy faces to help everyone understand that they were in jest. The laughing happy face suggests laughter. The winking happy face says, "this is just funning around". If there had been a "tongue in cheek" happy face I would have used that too.

They were in jest. Humor makes fun of ourselves often. I realized that some wouldn't get it, and that some wouldn't think it was funny even if they noticed that it was humor, but decided to give it a shot anyway. And having to be politically correct has never been my strong suit. [&:] Sorry to offend you.

RE: Control surface trailing edges
 

Yumping Yimminy..... talk about being harsh! Chuckle... chuckle....

I've found it to be a place where a lot of very sharp people help a lot of other people. (And some of the posters try to throw in a little humor every once in awhile.) Guess one's perspective really matters.

But I've been doing this model flying stuff since hmmmm.... the '50s.... and the first square TEs I saw really were on models that had been built by guys who didn't know much about aero and had made a mistake. Really. Honest. And darned if it really didn't happen that the next guys to embrace the practice were the ones who only painted 2 coats of paint, didn't fuel proof the insides, didn't cut off bolts that stuck WAY out unnecessarily, wouldn't have ironed a wrinkled ARF if their life depended on it, etc. Honest. but whatever....
"but it's the truth, right?" It is.

RE: Control surface trailing edges
 

I agree with darock, squared and tapered. It gives the cleanest look, best aerodynamics, and best finish. Some decent sandpaper tacked to a piece of 1x4 and you can have a completed taper job in short order. I call it therapy!

Bedford

RE: Control surface trailing edges
 

Sorry Darock, I find that I don't pay attention to smileys, I feel they clutter-up the dialog! Also, I find it a hassle to go to the smiley box an determine a definition for the one posted; so, I've gotten to the point where I don't even see them.

If we correct my mistake here, I am still going to go with my statement "I have found the R/CUniverse forum to be a place where a lot of very boring people like to get into little "Cock fights" over esoteric minutia"! However, in this statement, maybe it would be more correct to change "a lot" to "some".

Getting back to squared corners: In my example (The Hanger 9 FuntanaS .40 3D) this airplane was designed, I believe by Mike McConville in co-operation with Sebastiano Silvresti. These two folks know so very much more then I do about R/C planes, I had no doubt that the reason for the squared T.E.s was sound. I only wanted to know what the reason was. I took a stab at it being to reduce flutter. Dr1 and Ed Moorman confirmed my suspicions.

RE: Control surface trailing edges
 

Dr1Driver and Ed are correct about squared T.E. corners helping to reduce flutter problems. However a squared off T.E. is not an insurance against flutter. For that one need to mass balance the surface. Other methods like sealing control gaps do help to some extent by adding "deflection stiffness" that rises the natural resonance frequency of the moving surface, thus delaying the onset of flutter towards higher airspeeds.

There are other more subtle effects of a square trailing edge as opposed to a radiused one. For example, developers of hang gliders and trikes that make extensive use of streamlined aluminum profiles have found that the drag of streamlined profiles with a squared off T.E islower than for profiles with a radiused T.E. There are also reports on square T.E. having an effect of the reponsiveness of controls in full size airobatic aircraft. There seem to be some agreement in that community that a square T.E. offer higher control forces (which can be compensated for by increasing the area of spades or aerodynamic balancing horns) but at the same time the controls are perceived to be "crisper" at small deflections.

As others have posted, for modelling purposes, IMHO the most important reason for a square T.E. is to simplify construction and to provide a strong T.E. that stands up well to "hangar rash"

/Red B.

RE: Control surface trailing edges
 

Tall Paul is right on with this one. T.E. shape is pretty insignificant on our models.

To explain further, flutter requires two factors. There must be a forcing frequency. This occurs when the air leaves the trailing edge and is always there whatever the shape. The frequency depends on the speed and its intensity may be influenced slightly by shape.

Then the resonant frequency of the surface must be in the same range as forcing frequency. The resonant frequency depends on rigidity of the control surface and the linkage driving it. With a reasonably rigid surface and not much slop in the linkage, the resonant frequency is so far from the forcing frequency that flutter just won’t happen.

Rather than being overly concerned with the forcing frequency, which can only be affected slightly by configuration, pay most attention to system rigidity.

Use whatever trailing edge shape looks good to you, use good hinges and rigid surfaces, be careful to eliminate slop in the linkage, and you don’t have to worry about flutter.

RE: Control surface trailing edges
 

with my modest experience and knowledge of building and aerodynamics, im totally disagree with the idea that square TE avoids flutter...A square surface is absolutely the opposite to an aerodynamic surface!

if you use a square TE is because you are a begginer, a lazy builder or youre trying to prevent the airplane from the hangar accidents
its suppose that the air should flow softly and the airplane must have accurate edges deppending if it is the LE or the TE, of course the uses are different
have you ever seen a full scale aiplane with any square edge or surface? i dont think so, and the guys who desing them konw something about aerodynamics...( the word I use is ENGINEER )

with the exeption of the F 117 stealth of course, but the objective is another one in that ship
there must be lots of reasons that produces flutter
but try to see ANY TE of a wing profile.....they re not square!

ok...lets take a look to the masters of flight; have you seen a bird with square feathers???

i havent
well, sorry if this message became i bit rude, but its my opinion , if any engineer say it its true and proves it on a wing tunnel, MY APOLOGIES
sorry for the english

RE: Control surface trailing edges
 

It's already been proven, Torino, look it up. No one has ever said a squared T.E. AVOIDS flutter on R/C models. It HELPS PREVENT flutter, especially at higher speeds. Other things that help prevent flutter are stiff linkage, mechanical advantage, stiff surfaces, and dynamic balancing of the surface. Would you believe it may be beneficial for the L.E. of the surface to be slightly thicker than the T.E. of what it's attached to? Its' true.

Take a look at a full size plane close up. There are bumps and edges all over it, some of them square. It's very NON-aerodynamic in micro. Remember, what works in full size doesn't always work in models, and vice-versa. Although the size of the airplane changes, the size of the air molecules doesn't.

Birds are not airplanes. If an R/C airplane could be built TOTALLY like a bird, THEN you could compare them.

'nuff said.

Dr.1

RE: Control surface trailing edges
 

Torino!
NACA research has shown that the perfect trailing edge is a razor sharp trailing edge. The NACA research also showed the next best was a square trailing edge, as much as 3/16" on a 12" section. The worst is a rounded off section, the way many models are done!

For one example of a square trailing edge an a full size aircraft have a look at this (rather poor quality) photo of the trailing edge of a Saab Draken fighter aircraft.

/Red B.

RE: Control surface trailing edges
 

There was a rocket plane back in the 50s or 60s that a very thick t.e. on the rudder and nice and pointy on the front. Looked like someone took an axe to it:D This was one of multi thousand MPH jobbies so I think it depends on the circumstances. I chop them off square on fun planes and taper them on scale ones.

RE: Control surface trailing edges
 

Red B.:

Do you know the NACA paper number? I would be interested in reading that.

L

RE: Control surface trailing edges
 

For our models, the best TE is on a surface that tapers back to the smallest squared off TE that leaves enough material to have sufficient strength to stand up to hangar rash.

Elevators that haven't even been tapered at all work ok when the elevator wasn't thick to begin with. If the surface is thick and untapered there will be induced drag as a result. And the drag is wasted drag. There is no return of value for it's cost. Taper the surface to a thin, square TE and you don't have the "surplus" drag. BTW, this surplus drag can be the source of flutter. It's not biased up or down and certainly doesn't uniformly pull the surface in a stable direction, like straight back. It is basically an unknown that will just as easily introduce a flow that actually pulls the surface up or down, and sometimes rapidly and most certainly unstably.

RE: Control surface trailing edges
 

Snitch,
I believe that there is a section on TEs in AEROSPACE VEHICLE DESIGN, AIRCRAFT DESIGN by K.D.Wood that says exactly what NACA says about this topic, that sharp was best, small but squared was next, round wasn't good and thick was bad. Obviously, not in those words.

I tried to find it today but the book has a lousy Table of Contents and worse Index. But the author is an Aeronautical Engineer after all, so you'd expect that right? ;)