Flypaper2,
There's an excellent chance that the rocket plane needed to be draggy for it's flight after burnout. The plane would have been under power and into MACH from the gitgo and the shapes behind the shock wave wouldn't have mattered much, but when it flamed out, it would have been fast as.... well, a rocket, right? And they would have preferred that it wouldn't be real slippery, so they'd have done whatever they could to build some drag into it. And if the plane had been really fast, they would have had a heat problem on any surfaces or skin that didn't have some "heat sink" behind it, like the TE for example. So to solve that problem, don't have a thin, easily heat damaged, TE. Back then, they didn't have relatively cheap exotic materials. So they designed for what they had.

I used to race catamaran sailboats and the rudders that came from the factory would break into a high frequency vibration at high speeds. This would make a sound similar to the humming of power lines in a strong wind. The cure for this was to file the trailing edge of these rudders to a squared off taper instead of the rounded trailing edge that they came with. You couldn't file them to a point because you would file through the fiberglass skin and expose the foam core.
Theoretically, a razor sharp wedge shaped trailing edge is ideal but a truncated wedge is nearly as good while being far more practical.

This was right out at the edge of space too, so I don't think they were worried about RE. This was one of the X planes. Should see if I can find a PIC.

The X plane you are thinking about was the X-15

It is my understanding that the rules of good aerodynamic design change at the speed of sound.

B.L.E.
Yup, everything has a flutter speed. Even a bowling ball will flutter at the right speed.

Those statements were what I often heard the AEs muttering at Lockheed.

Your example is true for the rudder you had on that boat. You saw it happen. But if the rudder's shape had been changed, the flutter speed would probably have been different. If they'd used mass to balance it differently, it's flutter speed would have changed and it'd been "cured". If they'd changed the surface texture, that probably would have been "the solution". I'm guessing that fileing the TE took a very short time and zero extra cost. And of course it worked. But it really was just the solution for that rudder at that speed. All the skippers of all those boats had no real way to tell that the change actually slowed the boat's speed because that change was so small and so very, very hard to prove or disprove on something that encounters so very many variables. And that blunt TE might have actually fluttered at lower speeds and slower intervals where the oscillation went unnoticed or where the linkage had sufficient power to hold or the slop of the steering linkage hid the slower movement within the linkage or whatever.....

Squaring off TEs or sharpening them simply changes the performance envelope. Either way may fix flutter or cause it in the part of the performance envelope we're actually using, but truth is, "fix" and "cause" are only what we're seeing at our operating speeds. In our cases, we're looking for a fix and don't care about where the envelope expanded or contracted or if there was more drag or less etc.

In any case, it's almost certainly true that making a TE thicker will definitely increase the drag and any increase is usually considered to be wasteful. (Unless of course you're designing a rocket plane that you want to have some extra drag after burnout.)

My Mirage sailboat has this same treatment on both the rudder and fin keel. I suspect it allows a stronger trailing edge with minimal increase in drag and less possibility for damage when backing up. Lest we forget, flutter causes drag, also! My Funtana has tapered ailerons with approximately a 3/32nd flat at the T.E. The rudder and elevator are 1/4th thick balsa. They are flat with no taper. The L.E. is rounded, the T.E.s are not. The airplane is designed for 3D manouvers and slower speeds. I'm not about to peel back the covering and re-shape the T.E.s, based on input from this thread. I'm going to go with the designers knowing what they're doing. After all, they are famous for their abilities and knowledge about design and who am I, to second guess their designs.

////////// darock: we are in the same channel !!! ////////////

darock we are in the same channel

sorry, dont konw what happend with my replies

I hate windows

Flutter and resonance -sorta the same thing-
I had the pleasure of working for years with a guy who had the ability to cut thru any BS theory.
We had a machine design problem -resonance.
I asked "how do we fix it?"
His reply "oh, we just do something."
I found out that there was really no single fix -but many ways of fixing resonance.
We could change material thickness slightly
or bend the material -slightly
or coat the material with something - a little bit.
etc..
perfect airfoil design is another area where "perfection" can be acheived in various means.- cut it short bend it change materials coat it etc..
On models most of the theory on sharp edges is just plain ol whimsey

I'm with Lou and Paul on this one. The shape has much less to do with flutter than the slop of the linkage, the gap at the hingeline, or the location of the surfaces.

Plus, squared edges are really ugly

I agree with daven square edges look ugly, if I am all that concerned about flutter I use a pull pull set up, very rare to get flutter then. Just use a good stong servo if it flutters you are probably at the VNE for the aircraft.

Cool! Now, who are Lou and Paul?

Check the posts above.

another one of the reasons people are happy to have thick square trailing edges is that modern rc aircraft have an excess of power to weight. Drag is not an issue unless the engine flames out - and most people don't seem to worry much about glides back to the strip dead stick. (model) Gliders don't have have thick squared off trailing edges - so why build them into powered planes?

Also, the flutter argument sounds pretty weak to me. The only reason I see a square trailing edge being better than a tapered one for flutter is that the squared off trailing edge is likely to contribute to a stronger/stiffer control surface. Having said that I taper all my control surfaces and don't get flutter and get way better glide performance gliding deadstick than anyone at my club flying artfs, rtfs or any kit. Can't help thinking eliminating drag during design helps with this.

And: The argument that full size is not the same as models is also pretty spurious. OK yes they are different - but you know what? most of the difference is that because reynolds numbers are smaller, model performance is worse - worse L/D, more unpredictable stalls, more flow separatation, hysteresis loops - non-linear lift/alpha. So the thing to do is to take aerodynamics just as seriously for models as for full size. Even with scale effect considerations, models are more similar to full size than they are different. I can't help feeling the luddite contingent of this forum don't want to talk about what is actually happening with airflow around our models because it makes their brains hurt.

Des, it's when the ideas go past Mach 1 to get things to use on models that absurdities creep in.
The X-15 triangular vertical isn't applicable to any model.
Or even lower Machs.. The "tadpole" verticals on the A-4 and F4J
Nor is "area rule"..
On the first builds of the U2R, after some years in service the tails began to fall off.
The fix was these external ribs, top and bottom.
The later builds had stiffer structures.
Flutter is an arcane science, and everything about the plane can contribute to it; the shape, the structure, the speed.. change any of these and the flutter mode excitation point changes with it.

Paul - yes of course if when people mean full size they mean jets capable of transonic and supersonic speeds then of course aerodynamics of these aircraft differs from models. No argument there! When I think full size - I'm thinking full size light aircraft - going up to WWII warbirds rather than jets, stealth aircraft, really high altitude reconnisance. X-15! Didn't that virtually fly into space?