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-   -   Dutch Roll (http://www.rcuniverse.com/forum/aerodynamics-76/7459454-dutch-roll.html)

billf 05-05-2008 04:40 PM

Dutch Roll
 
Here's hoping someone can help me with remembering how to correct "dutch roll". Years ago, when flying
polyhedral aircraft such as the Zombie, there was a cure for the dutch roll. It involved making the vertical fin/rudder either
larger in area or smaller and I can't remember which!

I've converted a 44" wing span Zombie to electric with rudder and elevator for R/C, and I notice that it dutch rolls. Would
moving the CG farther forward help without fooling with the vertical fin/rudder dimensions?

Thanks in advance for any suggestions.

Bill F
Hudson, WI

da Rock 05-05-2008 04:58 PM

RE: Dutch Roll
 
http://en.wikipedia.org/wiki/Dutch_roll

gboulton 05-05-2008 04:59 PM

RE: Dutch Roll
 
Given that the "problem" is caused by insufficient lateral area aft of the CG, and the resultant insufficient yaw dampening, I'd wager that increasing the size of the vertical fin would be the answer. That's merely an "educated guess", however.

da Rock 05-05-2008 05:05 PM

RE: Dutch Roll
 

Quote:

ORIGINAL: billf

Here's hoping someone can help me with remembering how to correct "dutch roll". Years ago, when flying
polyhedral aircraft such as the Zombie, there was a cure for the dutch roll. It involved making the vertical fin/rudder either
larger in area or smaller and I can't remember which!

I've converted a 44" wing span Zombie to electric with rudder and elevator for R/C, and I notice that it dutch rolls. Would
moving the CG farther forward help without fooling with the vertical fin/rudder dimensions?

Thanks in advance for any suggestions.

Bill F
Hudson, WI


As mentioned in the article, full scale designers do things like add anhedral to big cargo high wing hulks. And some have used swept wing planforms on low wingers. More fin is what is usually suggested for our models.

pimmnz 05-06-2008 02:22 AM

RE: Dutch Roll
 
S'funny, I thought dutch roll was caused by too much fin area...
Evan, WB #12.

da Rock 05-06-2008 05:28 AM

RE: Dutch Roll
 

Quote:

ORIGINAL: pimmnz

S'funny, I thought dutch roll was caused by too much fin area...
Evan, WB #12.

Actually, it's caused by an imbalance between the yaw stability and the roll stability. And truth is, it's cured by fixing or overpowering what ever is weak about the design that is causing it.

B.L.E. 05-06-2008 05:57 AM

RE: Dutch Roll
 

Quote:

ORIGINAL: pimmnz

S'funny, I thought dutch roll was caused by too much fin area...
Evan, WB #12.
Too much vertical fin causes spiral instability. Not enough causes dutch roll. Or so that is what I understand from reading the free flight articles in magazines.

Too much fin and the plane doesn't want to return to level flight on its own, instead it stays in a bank which gets steeper and steeper as the plane goes into a spiral dive.

Not enough fin and the plane doesn't just return to level flight on its own, it overshoots level flight and then re-overshoots, this is dutch roll.

HalH 05-06-2008 08:58 AM

RE: Dutch Roll
 
I would like to suggest that all read the paragraph at http://thedutchroll.com/HomePage.html

Shoe 05-06-2008 07:57 PM

RE: Dutch Roll
 
Quote:

ORIGINAL: da Rock
Actually, it's caused by an imbalance between the yaw stability and the roll stability. And truth is, it's cured by fixing or overpowering what ever is weak about the design that is causing it.
I'm not sure what you mean by roll stability. If you release a directionally stable airplane at an angle of sideslip, it will yaw to reduce the sideslip angle (or return to the trim sideslip angle if it's not zero). I think that's what you mean by yaw stability. If you put an airplane at an angle of bank with zero sideslip, there's nothing to make it roll back to the original bank angle (with zero sideslip, there is nothing to tell the airplane which direction to roll). If you allow sideslip, the aircraft might roll back toward wings level, but that's due to spiral stability, not "roll stability". I'm just trying to point out that the roll axis is fundamentally different from the pitch and yaw axes. If you apply a coordinated roll input, you will not get a second-order response like you will in pitch and yaw.

da Rock 05-06-2008 08:28 PM

RE: Dutch Roll
 

Quote:

ORIGINAL: Shoe

Quote:

ORIGINAL: da Rock
Actually, it's caused by an imbalance between the yaw stability and the roll stability. And truth is, it's cured by fixing or overpowering what ever is weak about the design that is causing it.
I'm not sure what you mean by roll stability.
What do "I" mean?
That's a direct quote from the definition of dutch roll where it mentions the cause.

Shoe 05-06-2008 10:54 PM

RE: Dutch Roll
 
Must be a poor definition you are referring to

Strat2003 05-07-2008 07:13 AM

RE: Dutch Roll
 
From experience....
In general, Dutch roll comes from too little fin area for the dihedral, or too much dihedral for the fin area.
Spiral instability comes from too much fin area for the dihedral, or too little dihedral for the fin area.

They can be fixed by changing either dihedral or fin area, which ever is more practical. That's usually adjusting the fin area, but I've done it both ways.

da Rock 05-07-2008 08:55 AM

RE: Dutch Roll
 
Roll stability is demonstrated by our RC trainers coming back to level flight when they aren't level and our controls are restored to center. Yaw stability is what restores from yaw when that axis has been disturbed and the disturbance is removed.

Personally, I thought the wikipedia definition of Dutch Roll is decent enough as definitions go. And the reference was actually the link I included in Post #2 of this thread.











billf 05-07-2008 10:04 AM

RE: Dutch Roll
 
Thanks to all who offered suggestions to my original dilemma and confusion. Having read all the posts I am beginning to remember an article a number of years ago in Model Aviation (?) that showed a cartoon of a fellow with a big pair of scissors about to clip off part of the vertical fin on his model. The caption talked about correcting a problem. I think in this case the problem was spiral instability.

So what I plan to try is simply attaching a playing card with a clamp to the vertical fin to see what happens.

Thanks again for the responses.

BillF
Hudson, WI

CrateCruncher 05-07-2008 10:22 AM

RE: Dutch Roll
 
Billf,
I was just going to suggest this! Having tinkered with fin and rudder area myself, I learned that one needs to use something as stiff as the fin itself so the extension doesn't flop around confusing the result. I experimented with several sizes and shapes of sheet balsa covered in plastic film and attached on both sides of the fin with heavy packing tape. Good luck!
Crate.

da Rock 05-07-2008 10:39 AM

RE: Dutch Roll
 

Quote:

ORIGINAL: billf

Thanks to all who offered suggestions to my original dilemma and confusion. Having read all the posts I am beginning to remember an article a number of years ago in Model Aviation (?) that showed a cartoon of a fellow with a big pair of scissors about to clip off part of the vertical fin on his model. The caption talked about correcting a problem. I think in this case the problem was spiral instability.

So what I plan to try is simply attaching a playing card with a clamp to the vertical fin to see what happens.

Thanks again for the responses.

BillF
Hudson, WI

Don't use a clamp.

Fold the card over and tape it. Generations and generations of modelers have done just that for years and years.

da Rock 05-07-2008 10:41 AM

RE: Dutch Roll
 
It's also very easy to shape scrap balsa to fit and tape that. You get a lot better shape that is not apt to deform from the pressure of the airflow.

buzzard bait 05-19-2008 08:33 PM

RE: Dutch Roll
 
In the future, you might want to reduce the dihedral when you do an RC version of an old timer like the Zombie. They don't need so much when built as RCs, and then you probably won't need more fin area.

Also, CG can affect Dutch Roll. I found this on my Goldberg Skylark, which had Dutch Roll when I moved the CG back to the rear limit, but was fine when it was moved forward. I assume it was because moving the CG back increased the side area in front of the CG and reduced it in back. The effect was very dramatic, and it worked after an attempt to add rudder area failed to solve the problem.

Jim

deothoric 01-27-2009 01:19 PM

RE: Dutch Roll
 
Not for nuthin' Boss but the webpage refered to is wrong in almost every statement. Starting with history "Dutch Roll" is the motion ascribed to a ship when it has wind and wave under side quarter stern. Absolutely the worst ships under these conditions were the 15th Century trading cogs the Dutch used, hence, Dutch Roll.

On the more recent statements, the statement seems to refer to "dihedral" as "tip above root" while talking about dynamic conditions. Mixed messages, since the swept wing mentioned will act dynamically as extra dyhedral. I'd go on but rather I suggest not quoting this page as a definition would be better. Thanks.


Quote:

ORIGINAL: da Rock


Quote:

ORIGINAL: Shoe

Quote:

ORIGINAL: da Rock
Actually, it's caused by an imbalance between the yaw stability and the roll stability. And truth is, it's cured by fixing or overpowering what ever is weak about the design that is causing it.
I'm not sure what you mean by roll stability.
What do "I" mean?
That's a direct quote from the definition of dutch roll where it mentions the cause.

rmh 01-27-2009 01:48 PM

RE: Dutch Roll
 
On and on and on we go ( Da Rock got itright.)
This is like the ground effect stuff which became a farce in 3 acts.
some of the " aerodynamic"explanations were horribly off target
Dutch roll - is simply a "balancing/ rebalancing of forces" which is has just enough energy to allow an oscillation to set in

Simply taping a cup to the fuselage at some point FORWARD the CP will typically setup the problem -or at least a rudder lwagging at a fixed frequency
Old Buicks would do a "Dutch Roll which was deadly. On those old "ships of the boulevard" the combo of castor angle/ steering geometry , soft springing etc., would let the steering wheel, if moved sharply -then released , allow the car to roll in response - then the steering would self correct and the resulting lock to lock oscillation, rapidly set in. Shimmey in other cars was also a problem but nothing like the Buick issue.
same problem on ships, same on aircraft , same on control surfaces of aircraft
if the "self correcting force is "just right" a oscillation or flutter -or Dutch Roll -call it what you will , occurs.

Taurus Flyer 01-27-2009 03:30 PM

RE: Dutch Roll
 
1 Attachment(s)
Gents,

I did make a picture for you,

Quote:

ORIGINAL: dick Hanson

On and on and on we go ( Da Rock got itright.)
This is like the ground effect stuff which became a farce in 3 acts.
some of the " aerodynamic"explanations were horribly off target
Dutch roll - is simply a "balancing/ rebalancing of forces" which is has just enough energy to allow an oscillation to set in

Simply taping a cup to the fuselage at some point FORWARD the CP will typically setup the problem -or at least a rudder lwagging at a fixed frequency
Old Buicks would do a "Dutch Roll which was deadly. On those old "ships of the boulevard" the combo of castor angle/ steering geometry , soft springing etc., would let the steering wheel, if moved sharply -then released , allow the car to roll in response - then the steering would self correct and the resulting lock to lock oscillation, rapidly set in. Shimmey in other cars was also a problem but nothing like the Buick issue.
same problem on ships, same on aircraft , same on control surfaces of aircraft
if the "self correcting force is "just right" a oscillation or flutter -or Dutch Roll -call it what you will , occurs.


Dutch Roll, continuous oscillation after horizontal piano impact in the tail, around two axles with phase shift so not to compare with flutter.

Cees (Dutchman)

rmh 01-27-2009 03:43 PM

RE: Dutch Roll
 
For what it's worth.
I used both occillation and flutter to clarify the idea that these cases are really the same.
It's all about motion becoming self perpetuating.
That's also why I mentioned the old Buick.
The car would roll wildly from side to side and the steering wheel would spin left to right. It was "rock n rollat it's finest -sorry I have no pictures or graphs but then -the "Galloping Gertie" bridge which failed and fell in the 1940's(?) also falls into this catagory and the wild twisting and swaying was well documented .
It's all related -

Bax 01-28-2009 11:23 AM

RE: Dutch Roll
 
Quote:

ORIGINAL: dick Hanson

then -the "Galloping Gertie" bridge which failed and fell in the 1940's(?) also falls into this catagory and the wild twisting and swaying was well documented .
It's all related -

Ahh....the Tacoma Narrows Bridge. I remember seeing the film in physics class when I attended university. A classic demonstration of resonance once the threshold was reached. As long as the wind was blowing, the bridge kept twisting. I'd not be surprised if it wasn't on YouTube somewhere.

http://en.wikipedia.org/wiki/Tacoma_Narrows_Bridge


Tall Paul 01-28-2009 01:28 PM

RE: Dutch Roll
 
1 Attachment(s)
The videos from this plane as a twin were pretty awful until I increased the vertical area.
OTOH, on this Evans Slo-Mo-Shun, I had to reduce the vertical to eliminate a high-speed wiggle in yaw.
Dutch Roll in airplanes generally is said to look like this.. From the motion of the -shoulders- of the skater on the ice. :)

rmh 01-28-2009 02:40 PM

RE: Dutch Roll
 
A lot of the rythmic rocking n rolling is due to the weight and/or weight arrangement of the craft.
The problem can often be cured by simply moving SOMETHING or removing weight
Just as in curing a flutter or a noise .


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