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Dutch roll tendency

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birddog486

Well Known Member
Has anyone found a way to reduce the amount of dutch roll? I've searched the forum but haven't found a lot of info on correcting whatever's causing it.

My RV-7 was rigged per the plans and has flown hands off with the ball centered since first flight (about 60hrs) but, it has quite a bit of dutch roll tendency with and without turbulence.

I flew the first 20hrs. without wheel pants and fairings and was hoping adding them would help but that wasn't the case.

All control surfaces have been checked and adjusted to have almost a perfect shape using a straight edge back to the trailing edge radius. There's also no measurable twist in the wings or control surfaces so I'm stumped.

I've tried using a slight yaw, feet off the pedals and both feet hard on the pedals with nothing solving the issue and possibly making it worse using hard pressure on both pedals.

I'm flying solo with a W&B ARM of about 81.5" except for a couple flights with ballast at gross weight so I'm not sure if W&B has an effect.

Travis
 
Not necessarily an RV thing.

birddog486 said:
Has anyone found a way to reduce the amount of dutch roll? I've searched the forum but haven't found a lot of info on correcting whatever's causing it.
My RV-7 was rigged per the plans and has flown hands off with the ball centered since first flight (about 60hrs) but, it has quite a bit of dutch roll tendency with and without turbulence.
I flew the first 20hrs. without wheel pants and fairings and was hoping adding them would help but that wasn't the case.
All control surfaces have been checked and adjusted to have almost a perfect shape using a straight edge back to the trailing edge radius. There's also no measurable twist in the wings or control surfaces so I'm stumped.
I've tried using a slight yaw, feet off the pedals and both feet hard on the pedals with nothing solving the issue and possibly making it worse using hard pressure on both pedals.
I'm flying solo with a W&B ARM of about 81.5" except for a couple flights with ballast at gross weight so I'm not sure if W&B has an effect.
Travis
Click to expand...

It's pretty common on short coupled airplanes.
 
Mel said:
It's pretty common on short coupled airplanes.
Click to expand...

Especially short coupled and lower static and dynamically stable airplanes.

In simple terms... RV?s would have much less of the light and responsive handling we all love if they were rock solid like on rails when not being maneuvered.
 
As Scott says, there is stability at one end of the scale and maneuverability at the other end. Which do you prefer. In my opinion (Which is worth just what you pay for it). The Short-Wing RV series of aircraft are just about at the perfect point.
They are as maneuverable as practical without being unstable.
 
birddog486 said:
Has anyone found a way to reduce the amount of dutch roll? I've searched the forum but haven't found a lot of info on correcting whatever's causing it.

My RV-7 was rigged per the plans and has flown hands off with the ball centered since first flight (about 60hrs) but, it has quite a bit of dutch roll tendency with and without turbulence.

I flew the first 20hrs. without wheel pants and fairings and was hoping adding them would help but that wasn't the case.

All control surfaces have been checked and adjusted to have almost a perfect shape using a straight edge back to the trailing edge radius. There's also no measurable twist in the wings or control surfaces so I'm stumped.

I've tried using a slight yaw, feet off the pedals and both feet hard on the pedals with nothing solving the issue and possibly making it worse using hard pressure on both pedals.

I'm flying solo with a W&B ARM of about 81.5" except for a couple flights with ballast at gross weight so I'm not sure if W&B has an effect.

Travis
Click to expand...

The two ways to change the design to reduce the dutch roll mode have other consequences for handling. The easier of the two is to make the fin and/or rudder bigger. In the case of the RV-7, you have the option to put the RV-9 rudder on. This will help some, as long as you keep your feet firmly on the rudder pedals. The bigger rudder wont help much if it is swinging back and forth.

The other way to change it is to reduce the dihedral, which makes the airplane less laterally stable and less enjoyable to fly. Also not practical to do on an existing design. Just mentioning it for background.
 
Dutch Roll

I'm a newer RV pilot but I have lots of time in a variety of aircraft and on my first flight I noticed that familiar little tail wag of an early Bonanza from years ago. Not quite as much as the famous Beech but it was there.
Certainly never stopped me from buying it because the control response was so well balanced plus I just accepted it like I did on the Bonanza and hardly feel it anymore..I really like how the RV flies, near perfect in my opinion.:D
 
Keeping rudder fixed ( like Steve already mentioned) helps a noticeable amount in turbulence.
If you leave your feet flat on the floor you are essentially reducing the vertical stab area by the rudder size in square inches.
 
I have noticed the motion you describe in several of the RV models I have flown and I just reverted to my Cessna twin days of "apply pressure to both rudder pedals" technique. It always seemed to stop in about two oscillations. I am surprised that you are not seeing an improvement in motion when pressing on both pedals.
 
rvbuilder2002 said:
Keeping rudder fixed ( like Steve already mentioned) helps a noticeable amount in turbulence.
Click to expand...

Or actually USE the rudder to make small yaw corrections as they happen. Really kills those oscillations in turbulence and gives you something to do while you're droning along XC.
 
I would probably just let the airframe exhibit the slight Dutch roll before I tried to constantly correct it via rudder inputs. I think some pilots me included could end up making things worse (PIO)and placing unneeded stress on the rudder. I never really found it to be a issue in the RV6 and it?s non existent in the Rocket.
 
luddite42 said:
Or actually USE the rudder to make small yaw corrections as they happen. Really kills those oscillations in turbulence and gives you something to do while you're droning along XC.
Click to expand...

I've tried that and after a short time trying to time random swings in turbulence I decide the benefit isn't worth the effort....instead of concentrating on killing Dutch Roll I just keep my feet on the pedals and soon forget it's happening.
 
The -6, -8, and -10 were pussycats

in turbulence compared to my V-tail Bonanza --- always thought it was trying to swap ends, with me!

Ron
 
Most of the above post sound like what a yaw damper does. I have not flew with one in an RV, though I put the support bracket in my tail cone on my current build. Some airplanes I have flown in the damper is to slow to respond (not quite pio but makes it almost worse) others function really well and eliminate Dutch roll or reduce felt yaw oscillations from turbulence disturbances.
 
I wanted to circle back to this to point out that compared to many airplanes RV's are not heavy in tail wag or dutch roll in turbulence.
It does get mentioned here in the forums occasionally but if you do a search you will not find a lot of discussion about it.

They will exhibit it more than some airplanes people are used to flying (particularly if they were high wing because low wing airplanes with higher degrees of dihedral will tend to do it more) because of some of the reasons already mentioned.

Another rather significant one is C.G. position. The further aft the C.G. is, the lower the static and dynamic yaw stability will be.

So it is not something that indicates a problem with the design (all the RV's have proven to have more than adequate yaw stability). It is just something inherent to the design.
 
Yup

Agree with Scott. We have an RV-6 and while I?ve noticed a small bit of wag in turbulence, I?ve never noticed the wag coupling into roll so I don?t think I?d call it dutch roll. It feels perfectly fine and wag-free in smooth air. I?ve wondered if another square foot or so of vertical fin would dampen the yaw and I?ve had fantasies of taping a dorsal on just to see... but then I remember the yaw is barely a trifle in the airplane I like better than any other airplane I?ve ever flown, and other effects might be, well, unpleasant. In flying free-flight models I learned that spiral stability can be made worse if the vertical is too small or too big. Actually, the Piper Tomahawk was worse, with a slow (~50 hz) wag in smooth air. Other airplanes wag in rough air, too. Usually I can damp it out with my feet. It?s really just a quirk, not a problem. The 7AC Champ with the small vertical is noticeably livelier handling than the later Champ variants with the bigger fin, at the loss of a bit of directional stability. My 2 cents: Van got it just about exactly right. That said, I?ve noticed that the vertical is bigger on the RV-7 and wondered if this was to address that small bit of wag in the similar RV-6?
 
Lancair solved that problem by using differential ailerons. IOW, the "up" aileron travels further than the "down" aileron on the other wing. Rudder is hardly needed in a turn. Lancair accomplished that with the bell crank out in the wing which pushes/pulls the aileron up and down.
 
snopercod said:
Lancair solved that problem by using differential ailerons. IOW, the "up" aileron travels further than the "down" aileron on the other wing. Rudder is hardly needed in a turn. Lancair accomplished that with the bell crank out in the wing which pushes/pulls the aileron up and down.
Click to expand...

All RV's (starting with the RV-3) have had differential ailerons accomplished by the exact same method you described.

This is to counter adverse yaw, which is different than steady flight yaw inducing dutch roll.
 
Thanks for 'splaing. My Lancair exhibits tail wag (or wiggle) in turbulence. I even added a ventral fin and larger rudder but it still does it.
 
The ventral fin probably helps at high angles of sideslip and maybe in spins, but won't do much in normal flight.

The larger rudder will help if you force it not to float. For this, it either needs to actively prevent yaw excursions of at a minimum, or stay relatively fixed. But if your feet are relaxed and passive, it probably won't do much. If you're a current taildragger pilot, your feet will know what to do. It can be a relatively tiring bit of work.

It's been a long, long time since Aero 101 but this is what I seem to remember. It could be wrong, it's been so long.

Dave
 
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