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Aerodynamics

bjdecker

Well Known Member
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I was thinking about the multitude of trailing edge configurations now deployed in the RV series, and was kind of wondering "why?"

Examples:
Single sheet forms 3/16" radius edge - Seen on RV-3,4,6,7(old),8... Round Trailing Edge on flight control surfaces

Two skins formed with "AEX" wedge -- Pointy Trailing Edge (RV-7 New Rudder, RV-9 Rudder, RV-10 Rudder, RV-14 Rudder)

Two skins formed with "Rail Road" wedge -- Concave & Squared off Trailing Edge (RV-14 elevator, RV-10 elevator...)

Honorable Mention - Czech's on the F-1 Rocket, squared off trailing edges..

So the question -- Steve S., Scott D. , others, what's the deal here? How does a squared off trailing edge compare to an indeterminate trailing edge (Round)

Cheers!
 
I was thinking about the multitude of trailing edge configurations now deployed in the RV series, and was kind of wondering "why?"

Examples:
Single sheet forms 3/16" radius edge - Seen on RV-3,4,6,7(old),8... Round Trailing Edge on flight control surfaces

Two skins formed with "AEX" wedge -- Pointy Trailing Edge (RV-7 New Rudder, RV-9 Rudder, RV-10 Rudder, RV-14 Rudder)

Two skins formed with "Rail Road" wedge -- Concave & Squared off Trailing Edge (RV-14 elevator, RV-10 elevator...)

Honorable Mention - Czech's on the F-1 Rocket, squared off trailing edges..

So the question -- Steve S., Scott D. , others, what's the deal here? How does a squared off trailing edge compare to an indeterminate trailing edge (Round)

Cheers!

My hunch is, within the RV series at least, the different empennage trailing edge shapes have more to do with manufacturing considerations than they do for aerodynamic reasons. That is not to say that the different trailing edges don't have different aerodynamic qualities, but just that it may not have been the primary consideration in design selection. The RV-9/Late RV-7 rudder was the first in the RV series to use the AEX wedge trailing edge, but I suspect that this was incorporated due to the desire to make the larger skin in 2 pieces instead of 1. There were also probably some structural reason for doing this as well, and maybe that was actually the primary consideration.

Skylor
 
It’s my understanding that the rounded, folded trailing edges produce lighter, more pleasant control forces, while the sharp squared edges take a little more force. Improper formed folded edges (radius too big and not squeezed enough) produce really light, almost twitchy control forces.
 
I can't explain the aerodynamics, but the ailerons on the early Lancairs (like mine) went from thin trailing edges to 1/4" thick to reduce stick forces in roll. This was based upon flight testing. That's all I know.
 
For drag, you really would like to have square-edged trailing edge. Depending on a few subtle factors, the minimum drag might not be the sharpest (the finest square edge you can make) but it usually is.

A rounded trailing edge has some aerodynamic problems, but as long as the radius is fairly small (like most RVs) the disadvantages are not too pronounced, and the trade-off for ease/simplicity of manufacturing is worth it.

For control feel (stick force) some bluntness is beneficial. This can be especially true if the aft face is not perpendicular - lets call it a trapezoid edge instead of a square edge. But the blunt trailing edge should be square-edged, not rounded, if you can.

Large radius rounded trailing edges can have an unstable point at zero deflection, and want to seek a small deflecton on either side of neutral. On a rudder, this can cause hunting. On an elevator, it can give a rather vague stick force feel.
 
I would assume that Steve's post has a typo and that what he might have meant to say was - For low drag, you really wouldn't like to have square-edged trailing edge. Purple adjustments made by me.

Skylors' hunch is pretty much correct.

The RV-9(A) was the first RV model to be introduced with riveted control surface trailing edges.
The primary design motivation was to simplify the production and assembly process's. The secondary benefit was a slightly heavier control feel which with the RV-9 being design biased more towards a cross country travel airplane, would be beneficial. An outlier benefit would be that they might help a bit with drag reduction but since an RV-9 has never been tested with the other style trailing edges we have to just rely on the theoretical for that.

Some may say that the simpler build idea is debatable.
One thing that is not debatable is that the designed and expected shape of the riveted trailing edge control surfaces is more consistently repeatable by amateur builders in their home shops.

The bent trailing edge is wonderful in its simplicity, but it is fairly common for these finished control surfaces to have an improper shape.

The problem with this is that the shape can be quite influential in the effectiveness of the control surface, its tendency to return to a specific neutral point with no control input, and the amount of feedback force (control resistance) that it exerts on the control system.

Put more simply, there are a lot of RV's flying around with control systems that are far from ideal, though in most instances the airplanes still handle so well that I think people are in a sort of "ignorance is bliss" state.

This is why there is an entire subsection in Section 5 of the construction manual showing how to evaluate bent trailing edge control surfaces, and describing what to do if they are found to be out of spec.

Edit: Forgot about the RV-14... The prototype originally had elevators with AEX wedge trailing edges. It flew fine, but for reasons never entirely understood, it had a very slight dead band at the neutral point of elevator travel that made the pitch control feel a bit lighter than desired for a primarily cross country and potentially IFR airplane. The reverse trailing edge ended up providing the desired change in the control response and force of the pitch control circuit.

Coincidently, right after the change was made, Barnaby Wainfan had a great article published in Kitplanes that discussed different trailing edge shapes and the effects they can have on a control surface. His article described the exact influence that we chose the change for. It is very informative and would be a good read for anyone interested in learning more about the subject.
 
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No! Scott's assumption is incorrect. For low drag, you want the finest trailing edge you can make, but whatever fineness it is, you want the corners sharp, square-edged, not a radius!

With a radius trailing edge, you get ill-defined, and often unsteady separation points where the flow on each surface leaves and merges with the flow from the other side. The resulting wake is not only unsteady, but thicker than the actual trailing edge thickness. With sharp corners, you get a crisp, well-defined separation point from each surface, and the boundary layers from each surface merge happily together to form the thinnest possible wake.

I would assume that Steve's post has a typo and that what he might have meant to say was - For low drag, you really wouldn't like to have square-edged trailing edge. Purple adjustments made by me.

Skylors' hunch is pretty much correct.

The RV-9(A) was the first RV model to be introduced with riveted control surface trailing edges.
The primary design motivation was to simplify the production and assembly process's. The secondary benefit was a slightly heavier control feel which with the RV-9 being design biased more towards a cross country travel airplane, would be beneficial. An outlier benefit would be that they might help a bit with drag reduction but since an RV-9 has never been tested with the other style trailing edges we have to just rely on the theoretical for that.

Some may say that the simpler build idea is debatable.
One thing that is not debatable is that the designed and expected shape of the riveted trailing edge control surfaces is more consistently repeatable by amateur builders in their home shops.

The bent trailing edge is wonderful in its simplicity, but it is fairly common for these finished control surfaces to have an improper shape.

The problem with this is that the shape can be quite influential in the effectiveness of the control surface, its tendency to return to a specific neutral point with no control input, and the amount of feedback force (control resistance) that it exerts on the control system.

Put more simply, there are a lot of RV's flying around with control systems that are far from ideal, though in most instances the airplanes still handle so well that I think people are in a sort of "ignorance is bliss" state.

This is why there is an entire subsection in Section 5 of the construction manual showing how to evaluate bent trailing edge control surfaces, and describing what to do if they are found to be out of spec.

Edit: Forgot about the RV-14... The prototype originally had elevators with AEX wedge trailing edges. It flew fine, but for reasons never entirely understood, it had a very slight dead band at the neutral point of elevator travel that made the pitch control feel a bit lighter than desired for a primarily cross country and potentially IFR airplane. The reverse trailing edge ended up providing the desired change in the control response and force of the pitch control circuit.

Coincidently, right after the change was made, Barnaby Wainfan had a great article published in Kitplanes that discussed different trailing edge shapes and the effects they can have on a control surface. His article described the exact influence that we chose the change for. It is very informative and would be a good read for anyone interested in learning more about the subject.
 
The AEX trailing edge wedge does have a squared off (though very thin) aft trailing edge.

I was thinking your comment was regarding thick squared off trailing edges being less desirable for low drag.
 
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The AEX trailing edge wedge does have a squared off (though very thin) aft trailing edge.

I was thinking your comment was regarding thick squared off trailing edges being less desirable for low drag.

The main point is that whatever trailing edge thickness you have, thick or thin, it is better to be squared off than rounded for low drag.

Of course it is (almost) always better to make the trailing edge thinner (sharper). The AEX trailing edge should be lower drag than the old-style folded skin, both because it is thinner, and because it is squared off.
 
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