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Dutch Roll - 5/5/2008 10:40:14 PM   
billf



Posts: 257
Joined: 2/1/2002
From: Hudson, WI, USA
Status: offline 		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
       Post #: 1

RE: Dutch Roll - 5/5/2008 10:58:29 PM   
da Rock



Posts: 6132
Joined: 10/11/2005
From: western, NC, USA
Status: offline 		http://en.wikipedia.org/wiki/Dutch_roll

(in reply to billf)
       Post #: 2

RE: Dutch Roll - 5/5/2008 10:59:28 PM   
gboulton



Posts: 1862
Joined: 5/28/2005
From: La Vergne, TN, USA
Status: offline 		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.

_____________________________

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(in reply to billf)
       Post #: 3

RE: Dutch Roll - 5/5/2008 11:05:28 PM   
da Rock



Posts: 6132
Joined: 10/11/2005
From: western, NC, USA
Status: offline
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.

(in reply to billf)
       Post #: 4

RE: Dutch Roll - 5/6/2008 8:22:21 AM   
pimmnz


 

Posts: 447
Joined: 11/9/2005
From: Auckland, NEW ZEALAND
Status: offline 		S'funny, I thought dutch roll was caused by too much fin area...
Evan, WB #12.

(in reply to da Rock)
       Post #: 5

RE: Dutch Roll - 5/6/2008 11:28:28 AM   
da Rock



Posts: 6132
Joined: 10/11/2005
From: western, NC, USA
Status: offline
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.

(in reply to pimmnz)
       Post #: 6

RE: Dutch Roll - 5/6/2008 11:57:50 AM   
B.L.E.


 

Posts: 1271
Joined: 8/27/2005
From: Austin, TX, USA
Status: offline
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.

(in reply to pimmnz)
       Post #: 7

RE: Dutch Roll - 5/6/2008 2:58:23 PM   
HalH


 

Posts: 613
Joined: 11/25/2002
From: Madison, WI,
Status: offline 		I would like to suggest that all read the paragraph at http://thedutchroll.com/HomePage.html

(in reply to B.L.E.)
       Post #: 8

RE: Dutch Roll - 5/7/2008 1:57:51 AM   
Shoe



Posts: 170
Joined: 9/28/2003
From: Ridgecrest, CA, USA
Status: offline
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.

(in reply to da Rock)
       Post #: 9

RE: Dutch Roll - 5/7/2008 2:28:42 AM   
da Rock



Posts: 6132
Joined: 10/11/2005
From: western, NC, USA
Status: offline
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.

(in reply to Shoe)
       Post #: 10

RE: Dutch Roll - 5/7/2008 4:54:54 AM   
Shoe



Posts: 170
Joined: 9/28/2003
From: Ridgecrest, CA, USA
Status: offline 		Must be a poor definition you are referring to

(in reply to da Rock)
       Post #: 11

RE: Dutch Roll - 5/7/2008 1:13:08 PM   
Strat2003


 

Posts: 605
Joined: 3/9/2003
From: Mt. Pleasant, OH, AFGHANISTAN
Status: offline 		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.

(in reply to Shoe)
       Post #: 12

RE: Dutch Roll - 5/7/2008 2:55:03 PM   
da Rock



Posts: 6132
Joined: 10/11/2005
From: western, NC, USA
Status: offline 		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.












< Message edited by da Rock -- 5/7/2008 4:44:02 PM >

(in reply to Strat2003)
       Post #: 13

RE: Dutch Roll - 5/7/2008 4:04:25 PM   
billf



Posts: 257
Joined: 2/1/2002
From: Hudson, WI, USA
Status: offline 		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

(in reply to da Rock)
       Post #: 14

RE: Dutch Roll - 5/7/2008 4:22:03 PM   
CrateCruncher



Posts: 532
Joined: 10/14/2005
From: Austin, TX, USA
Status: offline 		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.

(in reply to billf)
       Post #: 15

RE: Dutch Roll - 5/7/2008 4:39:33 PM   
da Rock



Posts: 6132
Joined: 10/11/2005
From: western, NC, USA
Status: offline
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.

(in reply to billf)