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Tail Wagging RV-7

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birddog486

Well Known Member
I've been spending a lot of time in the RV-7 this year and I've checked rigging, alignments and twist in all the control surfaces without any luck in finding whats causing this tail wagging.

It only takes a slight amount of turbulence to set it in motion. I can keep both feet pressed hard on the rudder or just let it float and neither will stop it. It doesn't make a difference being fast or slow but the stronger the turbulence the more aggressive the wagging.

Anyone have thoughts on this.

https://youtu.be/fgnwp1I3nfI
 
Maybe be my nosewheel acts as a yaw stabilizer ? 1100+ hours and have never seen that with my 7A. Something isn’t right. Highly doubt it’s a normal trait.
 
What happens with the wheel and leg fairings off ? Maybe one of those parts has an incorrect angle of incidence ? Is there any possibility of setting up cameras to observe rudder/ vertical stabilisor and wheelpants in flight ?
 
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I’ve got decades in a Bonanza, so I know what tail wag is. My RV7 has never experienced wag, either in smooth air or turbulence. Something isn’t right. The landing gear pants or fairings, as mentioned earlier, might be the cause…
 
Hmm. When I took a CFII friend for a ride in the back seat of my RV-8A, after an hour he had had enough of being sloshed side to side in the back seat. I didn't notice any up front, but I was sitting pretty much on the c.g. And in some videos shot in the RV-9A, you can discern a small amount of tail wagging / Dutch roll mode.

Maybe the phenomenon is one of perception -- some people are more sensitive to the phenomenon. It's my impression that tail wag is common in low wing aircraft.

Quantitative data from those who do and those who don't perceive tail wag would clarify the discussion...

The RV-9A is still awaiting an estimate from the shop to clean out and repair any soot damage, so I can't gather any data myself. But I have noticed that with the video camera looking straight ahead over the nose, a little tail wag is noticeable in the video that I don't perceive myself. I'll see if I can find a video that shows that...
 
In the video it looks like the AHRS is very sensitive. If you look outside do you see the same? Where's the AHRS sensor located?
 
to answer a couple questions, the AHARS is in the back of the plane but looking out the front mimics exactly what the ball is doing.

As far as the gear, the wheel pants and leg fairings have no effect on the wagging. In smooth air the ball is centered and it flies hands off straight and level with fairings on or off.
 
the wheel pants and leg fairings have no effect on the wagging. In smooth air the ball is centered and it flies hands off straight and level with fairings on or off.
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They could still have an effect… assuming for instance the legs fairing are symmetrically installed, but with some divergent angle (to neutral), the gear would be aerodynamically pre-loaded, and an upset as encountered in turbulence, set the wagging in motion…
Lightly exciting the rudder in smooth conditions might reveal something.
 
My RV-7 doesn't wag like that, either.

It just seems like your plane is hunting for that equilibrium state. You said you've checked the wheel pants and gear leg fairings for alignment, but is there any chance this may have been done incorrectly? The gear has to off the ground and the aircraft datum leveled to ensure it's all true. Those fiberglass components may be the only thing that can be adjusted, as the tail rigging and wing settings are pretty well set when they're drilled.

The idea of setting up a GoPro where you can watch the gear legs for what they're doing in flight is a good idea as well.
 
Ed_Wischmeyer said:
The RV-9A is still awaiting an estimate from the shop to clean out and repair any soot damage, so I can't gather any data myself. But I have noticed that with the video camera looking straight ahead over the nose, a little tail wag is noticeable in the video that I don't perceive myself. I'll see if I can find a video that shows that...
Click to expand...

My RV-9A doesn't wag even a little bit, neither visually, reflected in the EFIS/instrumentation, nor seat-of-pants. I've experienced it in some larger airplanes ("Bonanza Boogie") but didn't know it occurs in RV's. Are yaw dampers much of a thing in RV's?
 
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Yaw damper

I’ve tried my 14 yaw damper on vs off and in Florida turbulence makes a huge difference and very effective. My wife is even more sensitive to tail wag.
 
birddog486 said:
As far as the gear, the wheel pants and leg fairings have no effect on the wagging. In smooth air the ball is centered and it flies hands off straight and level with fairings on or off.
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FWIW, my -8 can wiggle quite a bit in rough air.

Ok, not wheel fairings. Which rudder to you have, riveted trailing edge or folded trailing edge?
 
What’s really puzzling to me is that the OP experiences this with feet on or off the rudder pedals. It’s a well-known phenomenon that the airplanes without rudder return springs wag their tails with feet off the pedals because you’ve effectively cut your vertical tail area in half when the rudder is free….but for it to happen either way is a stumper….

I recently flew a Kitfox with a three-axis Garmin autopilot, and was amazed at how ell the yaw damper worked - I would never have guessed it would be as effective as it was ( and the Kitfox can be a little squirrelly in its undamped form in bumps) - hate to see a guy spend money on one for an RV when it isn’t an issue in most examples of the marque….

We get tail wag with rudder from in the -8, a little in the -3, and not much in the -6 (which has reasonably stout rudder return springs).


This is a good question for Steve Smith!

Paul
 
Riveted trailing edge on the rudder and its been the last thing I've focused on before coming back here for ideas. Using a straight edge vertically on the trailing edge is almost perfectly straight and from the trailing edge forward its flat.

One thing I notice is a straight edge from the trailing edge of the rudder forward across the verticle stab reveals the trailing edge of the vertical stab is about 1/8" wider than the rudder on both sides.

I'm not sure if that's enough to cause turbulence across the rudder but I think its a possibility.

As far as the leg fairings and wheel pants go, there is no difference in the wagging with them either on or off.

I've flown 750 hrs this year in this plane and half of that has been without the fairing installed. Running at
45% power and 5 gph wasnt slow enough.
 
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750 hrs this year in this plane
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Whoaaa, nice :cool:

Now it seems you have basically checked everything... ok, another idea, is your rudder free moving, no undue resistance when the pedals are moved?
Also, has the VS been mounted with offset?
 
There's zero friction in the rudder or pedals and the tail was built per plan with an offset.
 
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Power setting doesn't effect it, but faster airspeed CAN make the wagging faster.

Moderate turbulence makes it uncomfortable not from the bumps but the side to side motion.
 
Trail?

Is the rudder actually in a straight trail position when the pedals are neutral? Watching the video it appears to nose up and slightly roll left as if the counter weight is not doing its job. As if the rudder was slightly off center to the left (left pedal applied.)

Can you feel movement in the rudder cables when the tail is wagging? Push the pedals forward so there is no slack in the cables and light touch one. Is there slight movement in the cable. How tight are your chains? Are the two chains the same length? How are the springs in your cables, they do wear out and after 750 hours in a year they could be worn out.

A solution could be a small strake mounted on the bottom of the fuselage under the elevator. A lot of aircraft use strakes to reduce tail wag / shuffle. I have no idea as to the size needed, but probably not very large.
 
MacCool said:
My RV-9A doesn't wag even a little bit, neither visually, reflected in the EFIS/instrumentation, nor seat-of-pants. I've experienced it in some larger airplanes ("Bonanza Boogie") but didn't know it occurs in RV's. Are yaw dampers much of a thing in RV's?
Click to expand...

That's what I thought about mine, too, till I saw the video...
 
Aileron issues ??

I would check for slightly heavy left wing.

Check radius on ailerons
Hunting for level.

There is play in rigging somewhere.

Also check servos in wing.

My .05
Boomer
 
D-Dubya said:
My RV-7 doesn't wag like that, either.
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RV-7A here...same thing. Occasionally after getting smacked around by turbulence, but nowhere near that and it damps out quickly. During normal cruise, smooth as silk.

Something is out of whack somewhere on this one...
 
Ed_Wischmeyer said:
That's what I thought about mine, too, till I saw the video...
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I too have seen the video, both from wingtip and tail tie-down, so...pretty confident, but next flight I'll try it making a concerted effort to keep feet off the rudder pedals.
 
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sahrens said:
Is the rudder actually in a straight trail position when the pedals are neutral? Watching the video it appears to nose up and slightly roll left as if the counter weight is not doing its job. As if the rudder was slightly off center to the left (left pedal applied.)

Can you feel movement in the rudder cables when the tail is wagging? Push the pedals forward so there is no slack in the cables and light touch one. Is there slight movement in the cable. How tight are your chains? Are the two chains the same length? How are the springs in your cables, they do wear out and after 750 hours in a year they could be worn out.

A solution could be a small strake mounted on the bottom of the fuselage under the elevator. A lot of aircraft use strakes to reduce tail wag / shuffle. I have no idea as to the size needed, but probably not very large.
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It appears the rudder is straight in trail but I don't have a camera I can mount to confirm in flight.

In the video my feet were hard on both pedals. If I step on one side it stops the wag because of the slip but once centered or just left to trail freely the wag will start (in turbulence)

The tail wheel springs and chains are kept fairly loose, probably 1/2" slack which should have no effect on the control surface. Theres no springs on the rudder pedals or cables themselves.
 
birddog486 said:
It appears the rudder is straight in trail but I don't have a camera I can mount to confirm in flight.

In the video my feet were hard on both pedals. If I step on one side it stops the wag because of the slip but once centered or just left to trail freely the wag will start (in turbulence)

The tail wheel springs and chains are kept fairly loose, probably 1/2" slack which should have no effect on the control surface. Theres no springs on the rudder pedals or cables themselves.
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I meant the springs at the tail wheel chains. If you are on the pedals there is no slack, so if the chains are different lengths you pull the rudder out of alignment. The aerodynamic force moves the rudder back to neutral but the chain spring pulls it slightly back out of alignment setting up the oscillation. It wouldn't take a lot of rudder deviation to get that much nose movement. But with 1/2" of slack in each chain it may not be the cause.

Just a thought
 
Somebody earlier mentioned the “ Bonanza Boogie “ and since I own a V-tail, I know it well. If your model-specific suggestions don’t pan out, I suggest a little research on this phenomenon which as I recall, is an inherent instability caused by the flat-bottom design of the fuselage. RVs are pretty flat on the bottom so maybe there’s a correlation. I must be compensating for it with rudder pressure because I don’t even notice it anymore but I don’t ride in the back seat.:D I put a GoPro on the tailring once and almost got sick watching the video!
 
No smoking gun yet but I do see the signature of turbulent air across the rudder after flying through some dirty water.

If someone can compare there Vertical and rudder I'd appreciate it.

My vertical stab is a consistent 1/8" wider than the leading edge of the rudder when in trail top to bottom and with very light pressure on the rudder pedals in flight I can feel it being pulled side to side.
 

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birddog486 said:
... with very light pressure on the rudder pedals in flight I can feel it being pulled side to side.
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That sounds like a smoking gun to me! It seems a pretty clear indication that aerodynamic forces are pushing the rudder back and forth. No surprise that is going to cause a slight back and forth effect on the yaw. Then again, this is counter to your observation that it still wags when held firm with pressure on the pedals. Unsure if what you are exploring here is the cause, but seems like a decent place to start. Tape on some plastic sheets to cover that gap and test fly.
 
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Check rudder hinge bearing locations (in/out). Could they be out of spec?
 
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lr172 said:
Tape on some plastic sheets to cover that gap and test fly.
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As the rudder is narrower than the VS, that will be difficult with a plastic sheet. But! If you have some old 1" venetian blinds (with the curved slats) you could probably tape one on each side with the curve towards the rudder and it would fill the gap. Only tape to the VS, let it rub on the rudder.
 
A couple of thoughts. One check the rudder bearing distances as it looks from the photos that the rudder might be a little bit spaced from the fin. Second check that the rudder has not been built with a twist. (The trailing edge might be straight but actually the whole rudder can be twisted. I had this with a rudder I once burrowed, as mine was damaged - my perfectly trimmed aircraft now flew out of balance, then when I built another rudder it went back into trim again)

Regards Peter
 
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Snowflake said:
As the rudder is narrower than the VS, that will be difficult with a plastic sheet. But! If you have some old 1" venetian blinds (with the curved slats) you could probably tape one on each side with the curve towards the rudder and it would fill the gap. Only tape to the VS, let it rub on the rudder.
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I personally would be EXTREMELY reluctant to fly with something taped near my flight controls! The thought of a jammed control surface isn't pleasant.......:eek:
 
Mine is a bit smaller

birddog486 said:
If someone can compare there Vertical and rudder I'd appreciate it.

My vertical stab is a consistent 1/8" wider than the leading edge of the rudder when in trail top to bottom and with very light pressure on the rudder pedals in flight I can feel it being pulled side to side.
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I measured mine and it is right at 3/32" so a bit smaller but not that much.

One thought that came to mind is the rudder horn. The bolts that attach the rudder cables can elongate the mounting holes. Could the rudder horn on your aircraft be worn enough to allow the rudder to wander?
 
Gap Seals

Gap seals are very common on aerobatic airplanes. The OP problem is the relationship between the trailing edge of vertical and the rudder. For maximum efficiency the leading edge of a control surface should be approximately 10% thicker then the area ahead of the control. Commonly seen on aerobatic aircraft ailerons.
Why this airplane and not others I don't have an answer.
The question with the mylar strips is whether they will stay in place at higher speeds.
 
No wag...

birddog486 said:
No smoking gun yet but I do see the signature of turbulent air across the rudder after flying through some dirty water.

If someone can compare there Vertical and rudder I'd appreciate it.

My vertical stab is a consistent 1/8" wider than the leading edge of the rudder when in trail top to bottom and with very light pressure on the rudder pedals in flight I can feel it being pulled side to side.
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Here's mine -- RV-7 2015 Vertical Stab & 2015 -8 Rudder. (zoom in)

The vertical stab aft spar web/flanges and skin stack-up are about 3/16" wider than the rudder skin at the hinge line location.
 

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After looking at Brian’s pic above

I would also check the tailwheel and all attachments

Bent wheel, bracket, etc, transferred to rudder ??

Boomer
 
Curious what it does with A/P on? Assuming you have A/P? Any change in severity of oscillations?
In an old V tail Bonanza I used to drive ide always have to fly slightly out of balance to stop its wagging, horrible thing!
 
The wagging behavior suggests something more than a misalignment issue. The vertical stab interacting with the propellor's wash/tip vorticies, maybe.

Fixed (static) alignment devices/protuberances usually just displace the flight control or airframe component a certain amount at a given airspeed (e.g. trim tab) and doesn't demonstrate a positive static + neutral dynamic stability.

OTOH, maybe incorporate a rudder trim wedge/tailstrobe plate and bias the rudder slightly one way or the other and see what effect that has...

-=Guessing=-
 
I appreciate everyones thoughts and input. Keep it up.

This is Serial #74585 and it's built per plans with an offset in the vertical. I've been flying it for almost 4 years (750 hrs so far this year, 990TT) and I'm tired of being sloshed around in it and need to find the fix.

The rudder doesn't appear to have any twist but the gap between it and the vertical does seem excessive although when the rudder is deflected to the stop there's only a very small space between the rudder and vertical skin.

It looks to me that the rolled leading edge of the rudder could be longer to take up some of the space between the rudder and vertical.

You can see in one of the pics theres air flow traveling up the front of the rudder and exiting out the top hinge opening shown by the exhaust traces.

I'm considering moving the rudder closer to the Stab but at most theres only two threads beyond the jam nuts on the joints.

I'll add that the plane has done this since first flight so it's not new, I'm just tired of it.
 

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HS effect

The first post states that all rigging has been checked, assuming that also includes the angle of incidence for the horizontal stabiliser? It is designed to exert a downward force so as to provide positive stability. If the shim blocks are too thick and the angle of incidence is too high then pitch stability will tend toward neutral. An easy check is to sight the elevator in level flight and verify that there is slight down elevator to counter the normal effect of the HS.
 
Thinking out loud here... What about the interaction between the rudder horn and the VS? Is there anything around there that may be non-standard, or somehow "not like the others?"
 
Tail light

It’s hard to tell from the photo but the tail light looks like it has something sticking out the right side? Is this some sort of mini tab?

Looking at my construction manual it suggests that from the centre of the pivot bolt hole to the fwd face of the spar

Top 51/64
Mid 63/64
Bottom 1 3/64

Then says adjust these measurements as necessary to make the rudder swing without binding.

This may mean having to also adjust the top fairing for clearance if the rudder moves forward.
 
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Rudder concavity?

From the third picture it looks like the rudder itself might have a slight concavity to its surfaces? It might just be the lighting, but it might be a very very tiny curve in the surface causing it to behave like a flag (wagging back and forth). Might be worth checking.
 
Rudder

Concave near the trailing edge will increase centering force, convex will diminish the centering force.
I believe the OP problem is the rudder gap/shape of the rudder leading edge.
 
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