have gotten some debatable answers on this so here goes,

at low speed such as landing a fairly fast design,
the elevator has most efficient flying ability when the TE is blunt or sharp?

Believe it or not, a tapered TE can generate turbulence that creates drag and loss of lift. The airflow must break cleanly from the surface. At the speeds our planes fly, a squared-off TE is the best.

yeah that's what's confusing....talk to pattern guys and they say square as it creates less effect which is what they want to do their manuevers.

talk to speed guys and they say sharp as it creates less flutter amd has more effect at low speeds............

There's the difference. Speed. The air molecules are the same size whether you're flying a 6' R/C trainer or a 50' commuter jet. The aerodynamics are drastically different. IMO, from slow speed 3D to sport planes, squared off is best. For pylon racing...probably slightly tapered.

For sport flying, it makes no difference.
The most adamant about the shapes will never try the other, as they KNOW what the best is.
And strangely, the shape they approve is usually the one that was on their first airplane.

I believe that pattern flyers use a blunt shape for an entirely different reason than drag. If I'm remembering this right they use it because it promotes a soft center response where they can smoothly make minor corrections which hopefully are not easily noticed by the judges. But once they use larger inputs then the shape makes no really big difference.

But you're talking about using it to flair a model during landing. Under that sort of use the trailing edge shape isn't the issue. Instead it's the location of the CG of the model. A model with a more forward CG location is going to put more load requirement on the tail surface. Because of this to generate the downwards tail lift at low speed the elevator needs to be more strongly deflected to generate the lift required to hold the nose up. At that point it won't matter what the trailing edge is shaped like because it'll be a case of how much peak lift at what optimum angle of elevator can the stabilizer and elevator shape provide. Moving the CG rearward puts less load on the tail so it doesn't require as much elevator angle to achieve the amount of negative lift required to achieve the flare angle. And at this point a sharp or Kamm like squared off trailing edge really won't matter compared to the airfoil used by the stabilizer, the location of the hinge line and how it affects the performance of the "variable camber airfoil" that the stabilizer and elevator form and the angle of elevator displacement.

For pure low drag such as on a racer there is only one option. A fully and properly shaped airfoil that comes to a razor like trailing edge. This applies to wings as well as was shown by Dr Michel Selig when he did his early wind tunnel testing at model speed reynolds numbers. A trailing edge that was dubbed off square and of 1/16 height had a measureable increase in drag over the same airfoil with a razor like trailing edge. It wasn't much but for a world class glider running the speed task it could be the couple of tenths needed to beat the next fellow. Note that I'm not talking about a flat surface that you sort of stick on a sanded down rounded to sharp shape over the last 1/4 to 1/2 inch. Rather a proper airfoil over the whole surface.

One thing is for sure, the last thing you want is to round over a dubbed off trailing edge. A rounded trailing edge promotes oscillating airflow that can lead to flutter. It's too easy to get that now without encouraging it.

thx for explaining that as you did. i have a much better understanding of what i'm doing and trying to accomplish.

thumbs up to you BMatthews[sm=thumbs_up.gif][sm=shades_smile.gif]

A company I used to work for had 6 Grumman Tigers that we flew in formation. One of them had a modification where they cut the taper off of the trailing edge of the ailerons leaving a squared off TE. Out of the 6 planes this plane had the worst flying qualities. For one it was noticably slower than the others. And two the controll pressure to get the plane to roll was incredibly more than any other Tiger that I've flown. This made one of the nicest flying Low Performance singles unpleasant to fly.

With a model? I don't know, I imagine there might be an increase in pressure to move the ailerons and possible a speed decrease.

A flat slab control surface takes less deflection to get into the airstream from center. This should contribute to responsiveness and detract from smoothness.

A tapered surface has aerodynamic exponential. I think this is covered in a book written by Dick Mathis.

I doubt I could tell the difference on a RC sport model, but I'll bet a C/L Stunt Guru could tell the difference..

This is simply not true. The flat slab and the airfoiled surfaces are fully wetted which means control inputs will be felt regardless of how small. The comment above implies that the free airstream around the wing or stab somehow is separated from the surface and it takes distance of travel before the control surface gets into the airstream.

Of the two, the airfoiled is more efficient in terms of lower drag

MattK

Tall Pall is correct. I've designed and built pattern planes for 30 years or more. I've heard the blunt TE thesis (1/4" to as much as 3/4") from some people who I consider to be knowledgeable and at the top of the heap. For them, it may make some difference in terms of how the response feels but the rest of us mere mortals, it doesn't amount to much.

I've looked at both blunt TEs and sharper TEs (less than 1/8" and in many cases 1/16") in my designs...I prefer sharper TE's since these fly better to my taste. One thing that should not be overlooked is how accurately these same people build their surfaces. The surfaces must be made with great care and precision such that a fair assessment can be made. Very few of us are as fussy as necessary regarding surface accuracy and precision

One thing certain, for pure drag reduction, bringing the airstrem together at the TE with no disruption and no turbulence is about as good as you can hope to do. Razor sharp TE's do that. Consider how propellers are designed...the sharper the TE the better. APC props have extremely sharp TE's for this specific reason. The resultant drag is minimized which means more engine power is converted to thrust

Pattern planes are not about drag reduction, far from it. Racers are all about drag reduction

MattK

However the current plane I am flying (a large corporate type jet) does have squared off trailing edge at about 1/2 inch for a 60 ft wingspan. This plane also is not sensitive/responsive to control inputs. This plane however is Fast! Not sure what conclusion to draw from it.

A squared-off trailing edge places unneeded material some distance back from hinge line.
With solid slabs, the opportunity for flutter are seriously increased.
My Hobby Hangar Ricochet had a slab elevator with a few large lightening holes.
It fluttered easily and at very low speeds.
Changing to a stick structure got rid of that problem, while keeping the untapered surface.
Changing the structure to sticks lightens all the stuff hanging on the hinge, reducing the chance for flutter.
Tapering the surface also reduces the materials aft of the hinge.
Look at full scales, like Cubs and Aeroncas.. flat surfaces with tubing for the edges.
These don't flutter.

I've built and flown 6 nearly identical planes with slight changes made mostly to the elevons, fin area, airfoil....but essentially the same plane.
These planes have been powered with .32 to .50 sized engines and have ranged in top speed from 120 to 200 mph. They are hand launched, so having "fully wetted" control surfaces are a must with the low static thrust props utilized. One of the planes recently built had airfoiled elevons that tapered to a sharp edge. At low speeds the control authority was marginal, final approaches ran out of elevator prematurely..and this was with a lighter payload than what this style / size airframe has been flown with. I removed the tapered elevons that were obviously NOT operating in "totally wetted" mode and replaced them with flat slab elevons [no taper] and with slightly more chord.
Problem solved, now the model is rock solid 'till it hits the grass and flares as beautifully as the Space Shuttle. It also has made the launches much more reliable, the plane is quicker to respond and is flying under control as soon as it leaves my hand now. A little extra mass aft of the hinge line is a non issue here.
The 200 mph plane uses a .40 sized engine, 7x7.4 prop, has a 6% thick airfoil with elevons that taper to a 1/16" thick trailing edge. I doubt that sharpening the TE further would make a noticable difference in top speed. With this plane I do not attempt slow landings and the launches are dicey.
Is this textbook theory? Full scale anecdotes? Or is this real life, been there, done that testimony?

But you altered two things at the same time. It's possible that fully tapered elevons with the same wide chord would have worked just as well for landing.

That's true Bruce. The item I failed to mention is that my first model of this series had stubby, blunt TE elevons and handled like a champ both during launch and landing. This is why I was surprised that a later model with the same flap chord, [but tapered to a point at the TE] was the source of that planes' problems.........but adding extra chord does throw a wrench into the findings.
Of the 6 planes, it is safe to say that none of the flat plank elevons has had any handling problems...can't say the same for one of the models with tapered flaps.