RV-8 Rudder Stability

On the second flight today, cruising along at high power to seat the rings, I did a control stability check. Ailerons and elevator seem very stable but when I kicked the rudder, it went into an oscillation that did not want to quit by itself. It feels like a dutch roll in a big airplane.

After a few cycles I stopped it with the rudder pedals. I kicked the rudder again, this time I let it go to 5 or 6 cycles and it seemed to be diminishing, but I stopped it anyhow. Once excited, it does not get worse, it just moves back and forth through center at a constant rate.

I called Vans. Ken Scott has never heard of such a phenomena. We talked for some time and he suggested installing a trim tab to put a load on the rudder see what happens. Also, I need to check the trailing edge. Maybe the bend is not tight enough although I will not squeeze it just yet.

Anyone here have any thoughts on a rudder that will cycle if excited. It seems like it has neutral stability, it doesn't want to quit but it does not get worse. (It might quit after more than 6 cycles as it was slowing down the on the second check)

The -8s are known to wag the tail in turbulence (mine does), but I've never noticed the rudder displacing. I can put my feet on the pedals (creating a fixed surface) which helps, but does not always eliminate, the issue.

David-aviator said:

Also, I need to check the trailing edge. Maybe the bend is not tight enough although I will not squeeze it just yet.

Click to expand...

Yaw stability...

This is the very first thing you should check. It can have a significant effect.

Refer to Chapter 5.7 in Section 5 HERE

Actually, builders should check the skin shape of all control surfaces that have
bent trailing edges. It can have a significant effect on control harmony, stability, etc.

Also check for friction in the rudder hinging. It should move very freely

rvbuilder2002 said:

Yaw stability...

This is the very first thing you should check. It can have a significant effect.

Refer to Chapter 5.7 in Section 5 HERE

Actually, builders should check the skin shape of all control surfaces that have
bent trailing edges. It can have a significant effect on control harmony, stability, etc.

Also check for friction in the rudder hinging. It should move very freely

Click to expand...

Thanks Scott.

I just checked all the flight control surfaces. The ailerons are perfect but the elevators and rudder are not with regard to the trailing edge bend. That will be corrected before the next flight.

All surfaces do swing free, there is no binding or stiffness.

Quality control?? We home builders need to tighten up on that subject. Also, EAA tech coucilers need to know what to check. I had 5 inspections on this airplane and theses surfaces were not checked for correct bend because the two EAA guys are RV builders.

Dutch Roll

If you are kicking a rudder pedal and then allowing it to swing free in flight, you will definitely excite the Dutch roll mode, and it will be very poorly damped. This is pretty true in all the RV's. I don't think the -8 is any worse than others, although it is certainly more noticeable to a back-seat occupant.

When you put both feet on the pedals and prevent the rudder from swinging, you effectively double the fin area, so of course the Dutch roll mode is better damped, but still not particularly well.

Accepting poor Dutch roll damping is a consequence of wanting to have some dihedral (lateral stability). More dihedral would improve the spiral mode stability but make the Dutch roll mode very poor. Less dihedral and the lateral control would feel like trying to balance a stick on your finger. I think we have it about as good as you can hope for given the short coupling of our airplanes.

Not evident in RV7A

Just finished phase I today in my 7A I kicked in a boot full of rudder at 130knt IAS and oscillation self dampened after 2 swings back and forth. I was impressed with how quickly it recovered. I do notice some Dutch roll with turbulence but it's not yaw instability in my plane. I hope this single data point helps.

Rudder

I know of one RV4 years ago that had this problem. The rudder skin near the trailing edge bulged out badly. Squeezing the trailing edge fixed the problem. This is a problem on many airplanes and not just the rudder. The ailerons can be even worse.
Barnaby Wainfan has written about this in great detail in Kitplanes.

Yaw Stability Confirmed

Getting the rudder trailing edge bend closer the the straight edge specification did the trick. Kicked the rudder today and it dampened out in 2-3 cycles.

squeeze the trailing edge without cracking paint?

Anyone have a trick to squeezing the trailing edge without cracking the paint? Heat? I think my rudder trailing edge is too "rounded", but I'd like to avoid damaging the paint if possible. I have a wagging tail under some circumstances, and the trailing edge is probably the problem.

https://youtu.be/Lybx-l0BMMQ

rv8ch said:

Anyone have a trick to squeezing the trailing edge without cracking the paint? Heat? I think my rudder trailing edge is too "rounded", but I'd like to avoid damaging the paint if possible. I have a wagging tail under some circumstances, and the trailing edge is probably the problem.

https://youtu.be/Lybx-l0BMMQ

Click to expand...

Before you go squeezing the rudder, I will point out that the RV-8 definitely likes to wag its tail in bumps if you don't have your feet on the peddles -= put your feet on them evenly and the wagging goes away....

rvbuilder2002 said:

Yaw stability...

This is the very first thing you should check. It can have a significant effect.

Refer to Chapter 5.7 in Section 5 HERE

Actually, builders should check the skin shape of all control surfaces that have
bent trailing edges. It can have a significant effect on control harmony, stability, etc.

Also check for friction in the rudder hinging. It should move very freely

Click to expand...

This popped up when I clicked on your link:

"NOT FOUND
Sorry, but the page you were trying to view does not exist."

Moved

PCHunt said:

This popped up when I clicked on your link:

"NOT FOUND
Sorry, but the page you were trying to view does not exist."

Click to expand...

It's moved here, along with gobs of other very helpful files.

https://www.vansaircraft.com/downloads/

https://www.vansaircraft.com/service-information-and-revisions/5/

Anyone have a trick to squeezing the trailing edge without cracking the paint? Heat? I think my rudder trailing edge is too "rounded", but I'd like to avoid damaging the paint if possible. I have a wagging tail under some circumstances, and the trailing edge is probably the problem.

https://youtu.be/Lybx-l0BMMQ

Click to expand...

I hope someone has a better answer to your problem... right now I would:
- prior to any action, give the paint shop a call and ask them what they think of it
- remove the rudder
- protect the jaws of the bending brake to be used with paper tape
- like you suggest, try to warm up the TE, therefore hopefully softening the paint enough to permit the slight stretching required... the problem will be heat transfer back to the brake. The protecting tape might act as insulator, and having someone with a hot gun (lo setting ) on the other side of the brake might help...

In any case, good luck!