trailing edges
 

Hi everyone. I have a pretty basic question that I need answered. Do you all leave the trailing edges of your ailerons, rudder, and elevator square or do you round or bevel them off when you build kits? An old time kit builder that I go to for advice on building says to always leave them square. What do you guys reccommend? Thanks

RE: trailing edges
 

I have been building for 40+ years and have always rounded my trailing surfaces, unless the plans called for a sharp taper. Last year I was told by a very competent source that flutter can result from the shape of the trailing edge. The worst being those that are rounded, followed by sharp tapered, and lastly (least to produce flutter) is the squared off TE.
I'm sure others will chime in on this, but I have only experieced flutter with one plane that I have built, and I believe that was due to choice of wood for the ailerons. (may have been a little too soft) [:-]

RE: trailing edges
 

When I see a plane built with a squared off trailing edge, the first thing that comes to mind is that the builder is lazy. It looks unfinished and just plain WRONG. The majority of ARFs are built that way. Do you think they are concerned with aerodynamics? Hell no! It's the good old $$$$$$$$$$. It's cheaper and faster to leave the control surface square. I'm sorry to say that this is the way that many modelers view it also. Slap it together, cover it fast with plastic wrap and get to the flying field. No sense of pride in workmanship any more. When was the last time you saw a full size plane with rectangular shaped control surfaces? NEVER! We fly in the same air that full size planes use, under the same air flow conditions and surface loads. It has to do with smooth air flow and aerodynamics. Before you know it, there will be larger models with nothing but a plank for a wing, like the foam electrics use. They don't fly, they are dragged through the air by the power of the electric motor.

RE: trailing edges
 

While rounded or sharp taper trailing edges LOOK nice, on a model it makes no difference.

I have found though, that by leaving the trailing edge squared off it does make covering easier and neater (at least for me ).

RE: trailing edges
 

Hi!
Blunt or square trailing edges???
A trailing edge should be razor sharp!;)

Regards!
Jan K

RE: trailing edges
 

I agree with Jan. Full scale wings have sharp trailing edges. Ever walk into a Cessna wing?? Look for a corregated crease in your forehead! Lately I have been sandwiching the trailing edge with balsa and thin G-10 phenolic sheet. That way you can get the sharp edge required. And it's bullet-proof and will not ding or fold over as unsupported edges do. Regards.

RE: trailing edges
 

So, what you all are saying is that it really doesn't matter what shape the trailing edges are? If I decide to taper the edges, what is the best way to do it to make sure they aren't uneven? How do you guys do it? Thanks

RE: trailing edges
 

Here's whay I do.

Note, the drawing shows how to mark a piece of TE stock for beveling the Hinge Line, but the routine is the same for the other end.

Lay the stock on a flat surface.

Take a straight-sided ink pen, and lay it flat next to the wood.

Slide the wood along the tip of the pen (The pen doesn't need to be centered on the wood, just close)

Flip the wood over, and repeat on the other side,

Now you have two lines. Sand each side down to the line and you're even,

RE: trailing edges
 

Aerodynamically, the rounded trailing edge causes much more turbulence than the squared off TE. tapered TE, is aerodynamically cleaner than both. I remember one of the X rocket planes had a very thick squared off rudder TE, but I imagine it was for supersonic reasons.

RE: trailing edges
 

The X-15 has very thick and squared off control surfaces... I know that squared off provide less turbulence and I like the look better so I leave mine squared off... They're also easier to cover! ;)

RE: trailing edges
 

Squared off... actually triangular, with the pointy end at the front are the bomb at Mach 5.
At Mach .05, the air could care less...

RE: trailing edges
 

Tapered to a sharp edge will have less drag, but will be less effective. Squared off will create more drag, weigh a little more, will be a little stronger but will be more effective. How much/less drag and how much/less effective depends on a whole bunch of factors. Definitely do not round them. If you're going to make them sharp like the rudder of a glider, then laminate a thin strip of carbon in the middle of the structure.

-Ben

RE: trailing edges
 

Hello!
After all these answers...may I ask why you ask??
When i build my models (scale)I try to duplicate the real stuff...and I have never (except for the F-15) seen a blunt or squared rudder or aileron on any full scale airplane.
For me, a model with a blunt or square trailing edge is sloopy workmanship...

Regards!
Jan K
Sweden

RE: trailing edges
 

Squared off is seen as sloppy or lazy. If you build a model to sell it will get pointed out right away.
If you leave them square on a Jet or scale subject they Will talk about you behind your back for the above. Never see that at TOP GUN do you.
Now on a sport design airplane , (trainer etc) nobody would really care as that is normal for newbies to do. Does it effect the sport plane, Not really, you'll never notice.

And yes all you guys that leave them square and have been doing this hobby a long time....

WE ARE TALKING BEHIND YOURE BACK

RE: trailing edges
 

For sport models, it makes NO difference what the trailing edge is.
For competition models, it can make a difference.
But even full scales fail to comprehend the horrors of a rounded trailing edge... imagine that...
Watching this silliness, I have to conclude that those warning against trailing edge shapes have never taken the time to find out for themselves how little it affects our model airplanes, and are merely passing on urban legends.

RE: trailing edges
 

Everybody would laugh at me if I put a tapered trailing edge on my coroplast pizza box:D

RE: trailing edges
 

All of this talk about rounded TEs causing turbulance is 100% true..... It has been tested on full scale and it is proven to cause flutter. I personally can not tell you exactly why.


Does this apply to modeling is the real question. I am not so sure that it does. Most of these models have a relatively thin airfoil AND typically are not flown extremely fast. For these reasons, I think the RC world can get away with rounded TEs. I mostly build giant scale aerobatic planes and leave the trailing edges squared off...... everyone is entitled to their own opinions, but it is not due to laziness and it does finish the plane off very well.

RE: trailing edges
 

I agree that rounded trailing edges are the worst of the three configurations, but the issue of relevance is real enough. For most model applications it won't matter much -- but speed planes & pylon racers are an entirely different issue -- in these cases, rounded trailing edges are a bad idea.

RE: trailing edges
 

Personally, I never worry about it.

There are many factors that can contribute to flutter, BUT there are MANY more factors that contribute to a control surface NOT fluttering.

The fact that you have a rounded edge CAN contribute to flutter, but it can also be a factor in preventing a surface from fluttering under different circumstances.

Build your plane however you want.

Fly it.

IF you are one of the rare people who experiences flutter, THEN it's time to make a few changes

(Yes, I know, flutter has happened to a lot of people, but there are many, MANY more people that it HASN'T happened to)

RE: trailing edges
 

To answer your question, I've always made ailerons per plan. Some are tapered, some are square, none are sharp (too much chance to ding the sharp edge.)

Did the Old Time Kit builder who told you to leave 'em square say why? The reason I ask is because the Old Timer I know said that is one way to reduce sensitivity around neutral. The effect is as if you had a computer radio with exponential. I've heard or read it often enough elsewhere that it sticks in my memory.

(Some posts in this thread sure show opinion - do we really need trailing edge snobbery?)

Best wishes,
Dave Olson

RE: trailing edges
 

I think that any trailing edge influence on flutter is due to small
fluctuating vortices that arise there and happen to occur at a
frequency that 'excites' the structure. A rounded trailing edge is
probably the worst in this regard, because the separation point
in a rounded trailing edge will be able to move around. A squared
off trailing edge will always separate at the corners, which tends
to minimize one source of the fluctuations. A sharp trailing edge
is the least likely to give rise to the little fluctuating vortices.

I would have to guess that control surfaces that flutter due to
the trailing edge treatment probably have a different problem,
like too much slop in the linkage.

banktoturn

RE: trailing edges
 

I have to agree with scar. The tapered control surface is less sensitive around nuetral. Plus they just plain look finished.

RE: trailing edges
 

If you search the literature, you will find that rounded edges are the worst case airodynamically. For minimum drag, very sharp. Most versitile and nearly as drag free as sharp are the squared off edges. Strongest, squared off edges. Easy to cover, squared off. Least labor, squared off. On small models, doesn't make much difference but the larger ones can be effected.

RE: trailing edges
 

I agree with Jan as well. A trailing edge should be as thin as you can make it and still be structurally sound. Rounded trailing edges are a bad idea. I only round a trailing edge on a scale plane that has surfaces made of tube. It's not laziness, it's just that round is wrong.

RE: trailing edges
 

My foam F-86 Sabre has razor sharp trailing edges.:D