What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

yaw instability

jcbarker

Well Known Member
Ok, so this might be a bit long and esoteric but this forum is the best place to discuss such things... I flew with a friend in his -8 to OSH last week. We're from Oregon. The airplane flew beautifully. However, I did notice a disconcerting lack of yaw stability. Here's the setup. I'm in back (200lbs) with about 25 lbs. luggage behind the seat. The pilot is 180 and there's about 40 lbs in the forward baggage compartment. The airplane flies straight with feet off the rudder pedals until some turbulence is encountered. This induces a yaw divergence which gets worse with each oscillation. This is a bad thing. Both feet on the rudder pedals and it stops. Hmmm. So, all the way to OSH and back with feet on the pedals. Kind of tiring. On the last day, in calm air, we tried a couple of stability tests. (apologies to all the aero engineers out there) First: Auto pilot on (two axis Tru-Trak) alt hold. Feet off the pedals. Pulse rudder pedal about 1-2". Oscillation begins with each divergence being a little larger than the one before. After 6 or 7 cycles we stop the oscillations. Ok, next test: AP off, stick fixed, same pulse, same result. Finally: AP off, stick free, same pulse, same result. I'm sure we were a bit aft loaded but within limits. At least in pitch it did not fly as though it were close to neutral or outside the aft limit. Having not actually weighed the aircraft in this load condition I can't say for sure but it did not fly like an airplane with the cg out the aft limit. I've flown aircraft in that condition. Negative pitch stability is not fun.
Anyway, have others encountered this? What's the thinking?
 
It is true that yaw stability will improve when your feet are on the pedals. However, it should still be adequate with the rudder 'floating'.

My guess is that there is friction in the rudder cable system. Many folks run the cables through plastic tubing through the cockpit and there may be some binding on the cables due to this.

A ground check of the entire rudder and rudder controls should be done, I think.

Vern
 
Seems like friction would improve "feet free" stability in that feet on the pedals is the ultimate in system "friction".
 
DUTCH ROLL

This oscillating motion is called Dutch Roll. It is not really a sign of simply poor directional stability, but rather a complex interaction of dihedral and fin size and c.g. location.

While not harmful, a poorly damped Dutch roll mode is rather annoying, especially for the back seat passengers. If you ride in a King-air, don't sit in the back! RV's are not the only airplanes that suffer from this, to varying degrees.

Putting your feet firmly on the pedals nearly doubles the effective fin area by preventing the rudder from staying 'force neutral'. With it floating, it can not contribute much, because any stabilizing force it would produce would just make it swing. So, yeah, this is the most effective solution.

From the designer's standpoint, the only solution short of a yaw damper is to make the fin bigger. This, like everything else, is a compromise for other factors, not the least of which may be asthetics. But if you ever wondered why WWII bombers had such big fins, now you know why. Imagine trying to do a bomb drop looking through the bomb sight mounted way up in the nose, swinging back and forth with a dutch roll.

Adding a few inches to the height of the fin (taller tip fairings) may help - but should be done with a revised structural calculation for the added fin area AND increased lift-curve slope of the higher aspect-ratio fin.

The straightforward solution is to just put both feet on the pedals
 
That makes sense. And with the cg close to neutral I can see how that makes the mass of the aircraft more easily displaced. Tru-Trak makes a yaw damper for one of their auto pilots but I think it's expensive. As for modifying the airframe; not an option.
 
Seems like friction would improve "feet free" stability in that feet on the pedals is the ultimate in system "friction".

Increased but equal static friction on the cables would produce what you describe.

If the cables are free and you hit turbulence the rudder deflects some then goes back to neutral as far as the wind is concerned. If the cable has increased friction on it then it may not return to neutral when the turbulence subsides, instead holding for extra time and increasing the yaw until the yaw swings the rudder in the opposite direction. This time the rudder has even more deflection than before just in the opposite direction. This continues until you neutralize the rudder with your feet.

I have never felt this in the -4. I find in turbulence in cruise I put my feet flat on the floor and make a diamond with my hands around the control stick. This way I don't make the turbulence worse by over controlling. It will dutch roll 2-3 times then straighten out.
 
Hmm, maybe. I think our rudder system is free but just to make sure we'll jack it up and test it just to eliminate that from the argument.
 
An aeroplane with less stability is "fun to fly", with more stability is "easy to fly". Apart from aircraft design, moving CG aft towards the limit decreases the stability... and if solo in the front seat is to be tolerably fun, then dual has to be less stable.

IMHO it is not a problem in the design, but in the application i.e. expecting a smooth stable ride in turbulence, and/or easily controllable by AP, with an aft CG, is not the essence of the RV-8... If it was smooth and stable, then it would be a boring aeroplane :(

Andy
 
I agree with you. Less stable = more responsive = more fun. However, divergent stability is never fun or safe. I understand "dutch roll" tendency but this is a condition that gets worse with every cycle. In other words, the yaw amplitude increases instead of damping out. That's what bothers me about this.
 
more description please

You've described the character of the vertical axis (yaw) but can you tell us more about the longitudinal axis (roll)? As a previous post points out, this is most likely a dutch-roll mode and I suspect it is as not many configurations produce pure yaw moments. Would you say the yaw divergence grows faster than the roll divergence?

Also, you need to test the spiral stability - tendency for the airplane to recover, maintain, or increase AOB during a stick free turn.

As stated Dutch Roll is a complex combination of many factors - from a pure configuration point of view I cannot believe the RV8 exhibits negative dynamic yaw stability - maybe weak but not negative. Its it actually hard to even design for that mode in conventionally configured fixed wing airplanes...function of the area behind the CG vs forward of the CG provided you remain in the CG envelope.

Now however, if your spiral mode is divergent (roll tends to increase from a fixed AOB when stick free) and is combined with weak dynamic yaw stability then the dutch roll mode can become divergent. There is a term called the spiral determinant which is the difference between the product of the yaw control derivative and Dihedral derivative and product of the Roll Control derivative with the yaw stability derivative. If this term is positive = damped dutch roll, negative = divergent dutch roll. Zero = dutch roll continues (or something like that...its been awhile)

Also, it would be important to characterize the frequency of your rudder doublets as every mode also as a natural frequency...if the Fn is high enough to not be in the range of normal pilot inputs (which are of low frequency) then its generally of no consequence. 1-2 Hz rudder inputs are a good guess as to the Fn of the airplane.

Also, another way to help determine the more offending mode in the dutch roll is to input aileron doublets of slightly higher freq (but keep the amplitude low) with fixed rudder at neutral. See if the dutch roll is excited by this input.

A minor point: adding friction intentionally to the system is generally not going to be a good thing. While it may help with apparent stability by simulating a fixed pedal condition - you will have altered the mechanical characteristics of the flight control system. Specifically adding friction increases the breakout force (which is just that force required to start the pedal moving from a static position). Why is this important? Because higher break-out forces lead to over controlling in high gain tasks such as yaw control during gusty crosswind landings. The pilot will want to make many small inputs but the friction forces the pilot to push harder just to get the pedal to move, this increased force causes a slight overshoot (since static friction will be higher than dynamic friction), requiring a compensatory input in the opposite direction..and so on - this results in PIO (pilot induced oscillations) which is essentially a rudder doublet and therefore excites the dutch-roll mode...and we are back to the reason we added friction in the first place..make sense?

whew that hurt my head :)
 
I'm wondering if there is something different than other RV-8s?
Like perhaps there is a difference in the gear fairings or wheel pants? A different scoop on the cowl?
The reason I ask is because adding surface area ahead of the CG can cause directional instability. :confused:
 
Ken,
Nice post. Good questions and observations. I will try to answer them below.
John

You've described the character of the vertical axis (yaw) but can you tell us more about the longitudinal axis (roll)? As a previous post points out, this is most likely a dutch-roll mode and I suspect it is as not many configurations produce pure yaw moments. Would you say the yaw divergence grows faster than the roll divergence?

The yaw divergence is definitely more pronounced. There is a roll component but it is not dominant.

Also, you need to test the spiral stability - tendency for the airplane to recover, maintain, or increase AOB during a stick free turn.

Have not done this. I guess we'll have to go out and fly. Darn. Will report back.

As stated Dutch Roll is a complex combination of many factors - from a pure configuration point of view I cannot believe the RV8 exhibits negative dynamic yaw stability - maybe weak but not negative. Its it actually hard to even design for that mode in conventionally configured fixed wing airplanes...function of the area behind the CG vs forward of the CG provided you remain in the CG envelope.

This is what has me puzzled. There is nothing unique about this 8. Stock wheel pants, cowl, etc. The only thing unique is it is sporting a tail wheel fairing. Hmmm, extra mass swinging around? The tail wheel is not directly linked to the rudder. It has chains. The fairing is actually fairly long.

Now however, if your spiral mode is divergent (roll tends to increase from a fixed AOB when stick free) and is combined with weak dynamic yaw stability then the dutch roll mode can become divergent. There is a term called the spiral determinant which is the difference between the product of the yaw control derivative and Dihedral derivative and product of the Roll Control derivative with the yaw stability derivative. If this term is positive = damped dutch roll, negative = divergent dutch roll. Zero = dutch roll continues (or something like that...its been awhile)

Like I said. Standard Van's. I have not noticed any divergence in AOB but if you're not looking for it you tend to compensate without thinking. We'll test next time out and report.

Also, it would be important to characterize the frequency of your rudder doublets as every mode also as a natural frequency...if the Fn is high enough to not be in the range of normal pilot inputs (which are of low frequency) then its generally of no consequence. 1-2 Hz rudder inputs are a good guess as to the Fn of the airplane.

Your guess is about right. I'd say my inputs we're in the 1-2 Hz range with pedal deflections of about 2". This produced results exactly like perturbations caused by turbulence.

Also, another way to help determine the more offending mode in the dutch roll is to input aileron doublets of slightly higher freq (but keep the amplitude low) with fixed rudder at neutral. See if the dutch roll is excited by this input.

This is another good test! Will do and report back.

A minor point: adding friction intentionally to the system is generally not going to be a good thing. While it may help with apparent stability by simulating a fixed pedal condition - you will have altered the mechanical characteristics of the flight control system. Specifically adding friction increases the breakout force (which is just that force required to start the pedal moving from a static position). Why is this important? Because higher break-out forces lead to over controlling in high gain tasks such as yaw control during gusty crosswind landings. The pilot will want to make many small inputs but the friction forces the pilot to push harder just to get the pedal to move, this increased force causes a slight overshoot (since static friction will be higher than dynamic friction), requiring a compensatory input in the opposite direction..and so on - this results in PIO (pilot induced oscillations) which is essentially a rudder doublet and therefore excites the dutch-roll mode...and we are back to the reason we added friction in the first place..make sense?

No, adding friction was never a consideration. Friction in a control system is not a good thing. It was brought up by another poster as a possible cause of the yaw instability.

So to summarize your inputs and those of another A.E. who posted: What we are experiencing is probably a divergent "dutch roll" mode with the dominant divergence in the vertical axis. However, further testing needs to be done to see if the roll mode is also divergent. In addition some testing on AOB stability is in order including inducing some aileron doublets to evaluate roll stability.

whew that hurt my head :)[/QUOTE]
 
Yaw test; rudder cable pinch mechanism

A few thoughts have popped up within this thread that entertain the idea of this RV8 being unstable in yaw. One simple test to check for static yaw stability is to smoothly displace the rudder, and see it pedal forces build up commensurate with deflection. If pedal force at some point actually drops off with increasing deflection, that's a problem.


One way to hold the rudder in position during cruise: I've seen a neat little cam-pinch device that can grab the rudder cable and prevent the rudder from moving. A friend of mine had one in his T-18 at OSH and it was pretty neat. He uses it instead of a trim tab. Just get where you want in cruise, press the cable lock lever and you stay trimmed that way until you release it. Its also designed to be easy to overcome by the pilot and I think will self-disengage if the pedal is pressed firmly enough.
 
(snip) I flew with a friend in his -8 to OSH last week. We're from Oregon. The airplane flew beautifully. However, I did notice a disconcerting lack of yaw stability. H(snip)
Anyway, have others encountered this? What's the thinking?

My experience is that the -8s have better yaw stability than -4s and -6s, I haven't flown a -7.

Many airplanes I've flown, especially 33/35 Bonanzas, have a lot more tail wag in turbulence than my -8, or others that i've ridden in or flown.

Is it possible that you're seeing some rudder "snatch?" How does the trailing edge radius of the rudder compare to plans, or to other RVs?.

It's my understanding that an overly generous radius on the trailing edge can cause the rudder to "hunt" back and forth somewhat.
 
Bill,
Thanks for the additional test. We'll try it. I have flown hamerheads with this airplane which require full rudder deflection. I noticed no bad behavior.
Do you have a picture or sketch of the locking device. I'm uncomfortable with any device that locks the control system unless it can be overcome such that it is then free. Having a rudder with high drag in the landing regime would be bad juju.
JB
 
James,
That's been my experience as well. I've flown a 6 with aft CG all the way to OSH and back and never experienced anything like this.
Hmm, trailing edge radius. We'll check and compare to plans.
JB
 
James,
That's been my experience as well. I've flown a 6 with aft CG all the way to OSH and back and never experienced anything like this.
Hmm, trailing edge radius. We'll check and compare to plans.
JB

Since I fly both the -8 and the -6 regularly, I'd have to say that I find the back end of the CG envelope to feel quite different in the two different models. The -6 at the aft limit is pretty easy to handle, while the -8....well, in my airplane, I try to observe a more conservative limit than that published by Van's. It is much more more unstable when I reach th limit than the -6. Now, like most folks, I tend to think Pitch when discussing stability, so I haven't done an extensive yaw stability test - I am thinking this might be fun to do after reading all this! ("Fun" equating with getting the parachute re-packed and getting suited up for testing, of course....)

Great thread BTW - htis is interesting stuff!

Paul
 
Rudder cable clamp / Possible rudder issues

Here's a pic of the rudder cable clamping device:

IMG_1339.JPG"


Its on a friend's T-18 and he gave me a detailed explanation of how it worked, but not how its built (although it looks dirt simple).

The clamp is used in lieu of a rudder trim tab, and is capable of holding trim force on the cable. It is not a hard lock on the cable. This has one primary advantage: it works at all speeds and power settings, unlike a fixed tab which is only precisely set for one specific condition.

He did say the clamp was easy to overcome with pilot input. He's test landed the Thorp with the clamp engaged and says it was no big deal. I, for one, wouldn't want to forget to release it prior to landing. (put it on your checklist)

Still, this is a neat simple way to trim the rudder at any point you wish. Some day, I think I'll fab one of these up for my 8 and test it out.

*******************************

More thoughts:

Spiral mode on these airplanes will be divergent, but very controllable. Dutch roll, although present, is typically not a problem on any of the RV's, so this case is very unusual.

There's just gotta be something different; something not right about this 8. I think the problem lies with the rudder itself, and the previous idea to take a hard look at the TE radius is spot-on. But, possibly even more important than the TE radius is the shape of the rudder skin: it needs to be dead-flat right up to where the TE bends around to the other side. The internal stiffeners help to keep flat skin shape up to where they end, but there are a couple inches of skin unsupported right near the TE. This area needs to be FLAT. If not, I believe there might be some hinge moment building up in the direction of the yaw angle, which would encourage divergence.
 
Could someone do a yaw stability test for us?

I for one would be VERY interested in someone doing a static yaw stability test on the RV8.

For static yaw stability, the test is to hold the rudder displaced and measure (guess at) pedal force. Do the test over a range of displacement values (small, med, large to keep it simple)

More information on doing a test for static directional stability can be found in AC 23-8B: "To check static directional stability with the airplane in the desired configuration and stabilized on the trim speed, the airplane is slowly yawed in both directions keeping the wings level with ailerons. When the rudder is released, the airplane should tend to return to straight flight. See paragraph 63.a. for discussion of ground adjustable tabs."

The test should also be done across a speed range, to sample the rolling moment couple due to dihedral effect. Also, sample middle of the envelope CG and for sure get a test at aft CG. Power on/off will also influence both static and dynamic yaw stability (more power = less stable)

I'm still a couple months away from having my 8 flying, but will test this for sure when I get her in the air.

Paul, maybe you could do some testing next time you fly??

Kevin Horton, any input on this?
 
Last edited:
Trailing edge radius

I think Jim Freeman is onto something here. He is the only one that came up with this in the thread and it was the first thing I thought of when I read the first post.

When I first flew my 4 I had a heavy wing and corrected it with the squeeze the aileron method. In the discussion I had with someone at Vans during the time, (It may have been Van himself), they mentioned that if the aircraft wasn't stable in yaw to try squeezing the trailing edge of the rudder "very carefully". You don't want to over do it apparently.

Anyone else remember this?

Joe Hine
RV4 C-FYTQ
 
Just a thought. I don't want to get you guys off on a tangent, but what about rudder hinge alignment? If the rod end bearings aren't in near perfect vertical alignment, could they be causing an "over centering" action that leads to a divergent control condition? Something else to check?

I apologize if this is way out in left field.
 
Caveman has a point. If the rudder hinge pins are not aligned the rudder will 'pop over center' to one side, then the other. It will be spring loaded to not stay centered. If any adjustment was made to the horizontal stab angle of attack (after initial construction), the vertical stab front spar attach is altered up or down. Then the rear spar is pulled and the hinges will be out of alignment.
Also, the faint possibility exists that the rudder counterweight fairing or rounded leading edge of the rudder (both are fwd of the hinge line) are mis-shaped and contribute to the problem.
 
only in turbulence

If you have a problem with blunt trailing edge causing hunting, or hinge misalignment causing an over-center action, these things will exist regardless of smooth or turbulent air. And they won't cause an oscillation, they will bias the rudder to one side and stay there until you push on the other pedal and make it 'snap over' to the other neutral point.

But, if you are getting a swinging oscillation in turbulence, that is Dutch Roll.

Normally, the Dutch Roll mode is most evident when turbulence gets it started. Of course you can start it yourself with a rudder doublet.
For short winged, not too much dihedral airplanes like RV's, the Dutch Roll motion is more 'snakey', that is, yaw dominated.
It is also possible to design airplanes that have a Dutch Roll mode that is more twisty, that is, roll dominated. This is really unpleasant.

Another poster mentioned the roll divergence mode - or what some pilots call the over-banking tendancy. Most airplanes are either weakly stable or unstable in roll divergence, but the time scale of the mode is so slow that pilots just automatcially level the wings. I'm not sure you can automatically say that having an unstable roll-divergence mode will give you a divergent Dutch Roll mode - although they are both indicators of insufficient dihedral. For example, my sailplane with the winglets removed has an unstable roll divergence mode, but still very well damped Dutch Roll. It has a big fin.

My impression is that the Dutch Roll on the RV-7 is a bit worse than the -8 (at least with the older, pre-RV-9 rudder), but the back-seat person in an -8 will definitely find it unpleasant if you just let it swing back and forth.

Putting both feet firmly on the rudder pedals will damp it out quickly.
Thats all thats needed.
 
Time to test:

Thanks all for your inputs on this problem. At this point our test card is pretty full. I think one of the first things we'll test is if this airplane shows the same tendency when solo and no baggage. Then we'll load it back up and fly some of the tests recommended. Not sure when I'll get this done since this is not my airplane and I'm in the final frenzy of finishing up my 8 (top skin and windshield is all that's left then off to the hangar for engine start). I promise to post when I get the tests flown.
One final thought: Since I have not heard otherwise, I'm assuming that this condition is unique to this aircraft? No one else (at least so far) has posted having a similar problem.
 
I'd say it's an unusual problem, maybe unique to your aircraft. RV8 fliers just don't report problems like yaw instability, and yours is the first I've heard of. But we don't really know until enough RV8's have been tested, yours included.

I'll look forward to reading about your findings.
 
I'd say it's an unusual problem, maybe unique to your aircraft. RV8 fliers just don't report problems like yaw instability, and yours is the first I've heard of. But we don't really know until enough RV8's have been tested, yours included.

I did a very quick and dirty check of yaw stability this evening on a local evening flight, and I found nothing unusual - of course, I had an empty back seat, and was at about 25% CG.
 
I had a yaw back and forth in my 8 in smooth air at high cruise, which for me is above 175kts. Pinching the trailing edge of the rudder and it went away.

Displacing the rudder now and it returns to smooth operation.
Best
 
Good to know that pinching the trailing edge helped. Reduces the Coanda effect? Or does it increase it. I can't remember.
 
Back
Top