Flaps vs Aileron radius

I'm getting ready to bend the ailerons. The plans call for 3/32 to 1/8 radius. They don't specify inside or outside radius, but a measurement from the drawings clearly indicates a 3/32 inside radius. I measured the flap radius and it is 5/32 inside. This exactly matches the drawings.

Questions:

1) Why would the flap trailing edge radius be different than the aileron?

2) I read that a larger radius results in a lighter control feel. Anyone have specific experience flying planes with different radius? I'm finding it hard to beleive that it makes any significant difference.

Robert,

Aileron trailing edges have recently become a topic of conversation among our local group, so yours is a timely post. We were talking about control feel on various planes, and I mentioned (to a well known airshow/record-breaking pilot) that I found the RV-8 to have a very "solid" feel compared to the RV-6, which felt very light. He looked at me like was from Mars, and said that he thought the -8 was so light in roll that it was down-right squirrelly! I have flown a few -8's and most felt solid, just like mine, but one was unexpectedly light.

It turns out that the only -8 this particular guy had flow was sitting right there, so we looked at the ailerons, and sure enough - they have a convexity to their surface. Most planes (and the plans) have flat upper and lower surfaces that continue straight right until they become tangent to the actual trailing edge radius. This one gently curved before joining the radius. I then took a look at Louise's -6, sitting right there, and sure enough - it has a slight convexity to the surfaces as well.

Since I have slightly squeezed my right aileron to relieve a heavy left wing, I actually have a little concavity to mine- which probably adds to the slightly more solid feel. I think that there are actually two factors that contribute to the "lightness" of the roll feel in these planes - the radius PLUS how the surface approaches the radius - and I am not sure that the second factor is not more critical than the first.

This is all recent speculation based on observation and reading over the years - I wouldn't take it as fact. But it would be very interesting to build several sets of ailerons with different shapes and try them on the same airplane, just to find out....

Paul

Ailerons are more subject to flutter than flaps and a smaller radius on the trailing edge increases flutter margin. Plus, and correct me if I'm wrong, the flap skin is thicker than the aileron.

Yep

I've experienced this same phenomena Paul.

A couple of '8 that I've flown from the rear seat feel (in roll only) that you're seating on the head of a pin. The roll never wants to settle and ailerons feel really light.

My '8 (an another I've flown) feels rock solid in roll. I spent an evening, before painting, with a straight edge and a pair of hand seamers to make sure the trailing edges were consistent and squeezed tight enough.

I just don't think that you can get the trailing edge tight enough with the two boards bending brake.

Ironflight said:

Robert,

Aileron trailing edges have recently become a topic of conversation among our local group, so yours is a timely post. We were talking about control feel on various planes, and I mentioned (to a well known airshow/record-breaking pilot) that I found the RV-8 to have a very "solid" feel compared to the RV-6, which felt very light. He looked at me like was from Mars, and said that he thought the -8 was so light in roll that it was down-right squirrelly! I have flown a few -8's and most felt solid, just like mine, but one was unexpectedly light.

It turns out that the only -8 this particular guy had flow was sitting right there, so we looked at the ailerons, and sure enough - they have a convexity to their surface. Most planes (and the plans) have flat upper and lower surfaces that continue straight right until they become tangent to the actual trailing edge radius. This one gently curved before joining the radius. I then took a look at Louise's -6, sitting right there, and sure enough - it has a slight convexity to the surfaces as well.

Since I have slightly squeezed my right aileron to relieve a heavy left wing, I actually have a little concavity to mine- which probably adds to the slightly more solid feel. I think that there are actually two factors that contribute to the "lightness" of the roll feel in these planes - the radius PLUS how the surface approaches the radius - and I am not sure that the second factor is not more critical than the first.

This is all recent speculation based on observation and reading over the years - I wouldn't take it as fact. But it would be very interesting to build several sets of ailerons with different shapes and try them on the same airplane, just to find out....

Paul

Click to expand...

What is your radius?

Paul,

I bent the elevator and rudder skins using traditional methods (set a 1/8 gap in the bending boards in the closed position). The outcome was fine with a 3/32" inside radius on the skins. Using the same equipment, I expect to get the same results on the ailerons.

You report that you're aileron feel is is not overly light. What is your inside trailing edge radius? What is the radius of the plane you mentioned that had a lighter control feel?

It sounds like I'm good to go using a 3/32 inside radius but I thought it was unusual that is is quite a bit smaller that the Vans radius provided on the flaps.

Thanks.

DaAV8R said:

Paul,

I bent the elevator and rudder skins using traditional methods (set a 1/8 gap in the bending boards in the closed position). The outcome was fine with a 3/32" inside radius on the skins. Using the same equipment, I expect to get the same results on the ailerons.

You report that you're aileron feel is is not overly light. What is your inside trailing edge radius? What is the radius of the plane you mentioned that had a lighter control feel?

It sounds like I'm good to go using a 3/32 inside radius but I thought it was unusual that is is quite a bit smaller that the Vans radius provided on the flaps.

Thanks.

Click to expand...

The ailerons on my plane are stock Quickbuilds - probably built exactly to plans. I'd have to go on a "profile hunt" for the numbers on the other planes. Again, I don't think the actual trailing edge radius is as big a culprit as how the surface approaches the radius. But that is a theory based on a couple of observations and a recollection from either the building instructions or an RVator article that stresses the importance of having the surfaces straight and tangent to the final radius.

Paul

Has anyone considered a squared off trailing edge? I have in mind, making it from the AEX wedge or similar. I understand that this is also drag reducing.

Any thoughts from the brain trust?

My data agree with Paul D. and David E.

I have built and flown two RV-8's, which behaved very differently on first flights due to aileron trailing edge configuration. I built the wings and control surfaces on the first 8, paying careful attention to bending the trailing edges of the ailerons so that a straight edge on the aileron surface extends right to the bend, even between stiffners in the ailerons. This plane is rock solid and stable and if the stick is pushed hard to one side, it immediately returns to center by itself. Aileron control force increases according to deflection.

The second RV-8 is a quickbuild. As the ailerons came from Van's, they had significant convex "bulges" between stiffner locations, so that a straight edge would sit tangent to only a half inch or so on the aileron surface. I thought that maybe the QB folks knew something that I didn't, so I did not alter the ailerons before the first flight. Wow. What a difference. The airplane actually exhibited aileron "snatch." If I pushed the stick sideways, it actually accelerated in that direction, with no resistance and no tendency to return to neutral. This is a very unpleasant characteristic. Before the next flight, I used a hand seamer (with two pieces of thin lath) to carefully squeeze the trailing edges of both ailerons to remove the convex shape. After several flights and careful squeezing of the aileron trailing edges between flights, the aileron snatch disappeared and the aileron behavior became more and more normal and airplane became stable and solid like my first plane. I feel that this is actually a safety issue and that builders should adjust aileron trailing edge configuration according to plans before the first flight to avoid a disconcerting and dangerous control behavior.

Dan Miller
RV-8 N3TU 765 hours

Ironflight said:

Robert,

Aileron trailing edges have recently become a topic of conversation among our local group, so yours is a timely post. We were talking about control feel on various planes, and I mentioned (to a well known airshow/record-breaking pilot) that I found the RV-8 to have a very "solid" feel compared to the RV-6, which felt very light. He looked at me like was from Mars, and said that he thought the -8 was so light in roll that it was down-right squirrelly! I have flown a few -8's and most felt solid, just like mine, but one was unexpectedly light.

It turns out that the only -8 this particular guy had flow was sitting right there, so we looked at the ailerons, and sure enough - they have a convexity to their surface. Most planes (and the plans) have flat upper and lower surfaces that continue straight right until they become tangent to the actual trailing edge radius. This one gently curved before joining the radius. I then took a look at Louise's -6, sitting right there, and sure enough - it has a slight convexity to the surfaces as well.

Since I have slightly squeezed my right aileron to relieve a heavy left wing, I actually have a little concavity to mine- which probably adds to the slightly more solid feel. I think that there are actually two factors that contribute to the "lightness" of the roll feel in these planes - the radius PLUS how the surface approaches the radius - and I am not sure that the second factor is not more critical than the first.

This is all recent speculation based on observation and reading over the years - I wouldn't take it as fact. But it would be very interesting to build several sets of ailerons with different shapes and try them on the same airplane, just to find out....

Paul

Click to expand...

Actually Paul you are spot on. The same interaction exists with the rudder and elevators as well. If the trailing edges of them are not finished correctly it will also effect the stick force gradient in pitch and the feed back force on the rudder. It can also have an impact on pitch and yaw stability.

If anyone ever asks me to do a pre first flight inspection on an RV, one of the tools I use during the inspection is a 12" straight edge. In my opinion, all tech. councilors and DAR's should do the. same.

rvbuilder2002 said:

Actually Paul you are spot on. The same interaction exists with the rudder and elevators as well. If the trailing edges of them are not finished correctly it will also effect the stick force gradient in pitch and the feed back force on the rudder. It can also have an impact on pitch and yaw stability.

If anyone ever asks me to do a pre first flight inspection on an RV, one of the tools I use during the inspection is a 12" straight edge. In my opinion, all tech. councilors and DAR's should do the. same.

Click to expand...

I finished the aileron bends tonight and used a 3/32" radius. I built my break with 1/8 gap in the closed position and it seems to yield a consistent 3/32" radius and a flat skin to the tangent point. Based upon the information provided in this thread I also wondered about the elevator and rudder bends. The more acute angle of those bends makes it more difficult to prevent pillowing between the stiffeners.

I did call Vans today and they reinforced the instruction that 1/8" is the max radius and that anything greater will result in a control that wants to "snatch" as described by Dan. They did not mention the importance of a flat skin.

Thanks for all the help. You guys are a wealth of information.