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My plane experiences snaking

Old 08-06-2010, 10:11 AM
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Paul_BB
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Default My plane experiences snaking



My airplane, a super chipmunk clone, experiences snaking as soom as the slightest breeze picks up.
What can I do about it ?

Another one of my planes, a sort of Ugly Stick, flies as if on rails, even if a good breeze is blowing.
So what makesan airplane snake ?

-Paul

Old 08-06-2010, 10:44 AM
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ARUP
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Default RE: My plane experiences snaking

'Snaking' ?????
Old 08-06-2010, 11:39 AM
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Default RE: My plane experiences snaking

Excuse me about that, snaking is the term used in France. Funny, its an english word, I though it came from the States.

The plane "snakes" when it wiggles around its yaw axis, as a snake.
It's a yaw movement, from left to right and right to left.

-Paul
Old 08-06-2010, 11:44 AM
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Default RE: My plane experiences snaking

It seems that your plane needs more vertical fin area.
Also locating the CG a little more forward could help.

Read this reference:

http://resources.yesican-science.ca/...light1g06.html

You can estimate the centroid of the lateral area by copying the shape in cardboard, scaled down.
Double the areas of the lateral projection of the wing and stab and paste them in place.
Then, find the CG of that piece of carboard.
Old 08-06-2010, 11:50 AM
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HighPlains
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Default RE: My plane experiences snaking

Try a smaller diameter prop and see if the problem goes away.
Old 08-06-2010, 12:49 PM
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Default RE: My plane experiences snaking

@ Lnewqban: thanks for the reference. I'll take a close look at it.
But the geometry of my airplane is identical to the Goldberg Super Chipmunk and thecenter of gravityis at the maximum fore position recommended by Goldberg ?

@HighPlains: I don"t see the relation between the prop diameter and the yaw wiggle ?

Anyway thanks for helping me.

-Paul
Old 08-06-2010, 04:07 PM
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Default RE: My plane experiences snaking

On conventional (tractor) designs, the prop disk area is ahead of the CG, and because of this destabilizing in yaw to the airpalne. As pointed out by Lnewqban the solution can be adding more area to the vertical stabilizer. On the other hand, you might also achieve the same effect with a smaller diameter prop, and that is a simple experiment.

Is this an electric powered model? Was it originally designed for wet power? Is it stable when the power is off and it is gliding?
Old 08-06-2010, 08:46 PM
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Default RE: My plane experiences snaking

ORIGINAL: Paul_BB

@ Lnewqban: thanks for the reference. I'll take a close look at it.
But the geometry of my airplane is identical to the Goldberg Super Chipmunk and the center of gravity is at the maximum fore position recommended by Goldberg ?
Keep in mind that any oscillation of the model in flight happens only around the center of mass or CG, and that it is always caused by a torque (force acting at certain distance from the pivot point or CG).
The amplitude and/or intensity of that oscillation can increase or decrease with time (the force becomes smaller or bigger respectively).

Allow me to direct you to another website, that shows very interesting animations about control and stability, and from which I have copied the following paragraph:

“When the aircraft slips the fin will turn the aircraft back into the relative wind. In most aircraft this tendency is nearly dead beat. This means that the aircraft does not overshoot and yaw back and forth much, it just turns quickly into the relative wind and stops there. However, if the aircraft has a lot of angular moment it will overshoot and slip in the opposite direction. The fin will then yaw it back and so on. The nose may wind up yawing back and forth several times before it settles down to non-slipping flight.”

http://www.geistware.com/rcmodeling/..._stability.htm

If your plane reacts to disturbance in a different way than others, either the geometry or balance or both are not identical.
Many times, small differences cause unexpected reactions, such as a big and heavy propeller or weight in the tail for proper balance. Both create bigger angular moment.
Old 08-06-2010, 11:01 PM
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Default RE: My plane experiences snaking

Hi Paul,

I have a Goldberg Chipmunk and it does the same thing. I believe it is due to the enlarged rudder. As I am sure you know, the Super Chipmunk's rudder is quite a bit bigger than the standard Chipmunk.

I consider it part of the price you have to pay for the advantage of having higher yaw authority, or at least having a (sort of) scale model of an aeroplane that had the rudder enlarged to give more yaw authority.

According to my Principles of Flight (Aerodynamics) training, snaking occurs when there is strong static yaw stability (eg large fin and rudder) and/or low dynamic yaw stability (eg heavy wing and fuselage, full tip tanks etc). In other words when a yaw disturbance occurs there is a strong restoring force (static stability) but a lowish damping of the resulting phugoid (reduced dynamic stability). The airfoil section of the fin/rudder comes into it too (the Goldberg is flat plate of course). Something to do with the slope of the Cl/AOA curve.

I have flown the Airsail Chipmunk, which is a similar size and weight model, but more scaleish and with the original size rudder and a 'proper' airfoiled section. It doesn't snake.

If it really bothers you it would be pretty easy to try a smaller prop and/or larger rudder as suggested above. If that doesn't help, with a bit more effort you could either cut your rudder down or make a new one to the original (non super) size and see what happens. I would be interested in your results.

Good luck,

Dave H

Sorry Lnewqban, your post wasn't there when I wrote this. I just said the same thing that you did, only not as eloquently.
Old 08-07-2010, 07:31 AM
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Default RE: My plane experiences snaking

Hey Dave, nothing to be sorry about; your post is great and helps Paul![sm=thumbs_up.gif]
Old 08-07-2010, 02:35 PM
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Default RE: My plane experiences snaking

So the guys are basically suggesting that your Chipmunk wings and perhaps the fore and aft ends of the fuselage are enough heavier that the vertical tail area does not have the power required to dampen out the snaking. I'm going to take a guess and suggest that your Stik is much lighter for the overall size (ie: lower wing loading) and so the fin is able to dampen out any snaking like oscillations before they are large enough to see. There's no doubt at all that both planes snake a little when they encounter the turbulence found at lower altitudes on a windy day but the one is able to damp them out more quickly so they are not noticable. If I'm right on this then there's not much you can do about it since to reduce the weight at the tips of the Chipmunk isn't practical at all. And adding a bigger fine and rudder for more vertical tail area to give it more ability to damp out the oscillations brings other bad side effects that you will not enjoy. So you may well be stuck with the situation. Or if you really like the shape of the design in the air then the only option may be to build another one but this time concentrate on keeping the wing light at the tips and the fuselage lighter in the nose and tail.
Old 08-07-2010, 03:14 PM
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Default RE: My plane experiences snaking

Hi BMatthews,

I agree with you.

Dave, I love too much my Chipmunk to butcher it ! So I will let you test your idea with your own Chipmunk.
Everybody at the fieldis captivated by this good looking (and cheap) airplane (designed in the seventies ! ).
So I will live with the snaking.

Right, the Chipmunk wings are much heavier than the Stick ones. I beleive the Izz moment of inertia about the yaw axisalong with the short fuselage of the Chipmunk is the responsible for the poor damping of the snaking (dutch roll). The rudder area is already big. I may increase damping if I increase the rudder area but I could deteriorate the spiral stability. This is what iswritten in my "Flight Stability and Automatic Control" book.
And sure, as Dave said, the flat airfoil and the square bottom of the fuselage may help somehow to trigger the oscillations.

I also discovered while surfingthe netthat there could exist a coupling between the mechanical link of the rudder control surface and the lateral oscillations:
http://history.nasa.gov/monograph12/ch4.htm

-Paul
Old 08-07-2010, 04:06 PM
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Default RE: My plane experiences snaking

Even the full size guys experience this, the early jets, bereft of a prop at the front, were particularly susceptible. The German answer was to rivet a couple of ally angles to the rudder trailing edge. The theory being that, aerodynamically speaking, it thickened up the trailing edge of the rudder. Worked too. The British (Meteor) answer was to offset the trim tabs either side of neutral, achieving the same end. You could try the same wheeze, a couple of bits of 1/8" square tacked to the rudder trailing edge, and just see if it helps. If you make them a couple of inches long, and slowly trim them as experience is gained, it might save lots of work.
Evan, WB #12.
Old 08-07-2010, 11:43 PM
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Default RE: My plane experiences snaking

Paul, that spiral stability issue is eactly what I was thinking of when I wrote about "bad side effects".

Like I suggested the best solution is to build the same size but lighter. Nothing like adding another project plane to the build list, eh?
Old 08-08-2010, 03:41 AM
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Default RE: My plane experiences snaking

Another very easy and simple way to test for a fix is to use card stock and masking tape. Fold some card stock and tape it to the rudder to increase the vertical fin area however much you feel should work. Fly and decided. The same technique can be used to make the rudder larger if you feel that's a better solution. You get greater rudder authority along with better stability.

It's easy to undo if it isn't the solution. It's easy to do again larger if it helps but isn't quite enough.
Old 08-08-2010, 06:15 AM
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Default RE: My plane experiences snaking

@BMatthews: I used to live in Calgary when I was young and I remember these lonnnng winters overthere. Aperfect time to build. But here in France the wintersare short, 3 monthes maximum. But your idea is interesting because Iwas thinkingabout buying the real CG Super Chipmumk kit. In that case I will try to lighten the wings.

What I was hunting for is rather a coupling between the mechanical link (servo-pushrods-control area) and the naturally poorly damped lateral mode, as descibed in the document:
http://history.nasa.gov/monograph12/ch4.htm

Mymechanical link is not that stiff and Iwas wondering ifsomebody had seen some better (with respect to the snaking behavior)when stiffening the link.


@da Rock: I should have though about that, great idea.


-Paul
Old 08-08-2010, 09:06 AM
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Default RE: My plane experiences snaking


ORIGINAL: Paul_BB

Dave, I love too much my Chipmunk to butcher it ! So I will let you test your idea with your own Chipmunk. [img][/img]
Everybody at the field is captivated by this good looking (and cheap) airplane (designed in the seventies ! [img][/img]).
So I will live with the snaking.

-Paul
Hi Paul,

Thought you might say that, but was hoping you might to save me butchering mine. Like you say they are great models

That NASA article is very interesting. I hadn't thought of the rudder floating with yaw contributing to snaking, but I think that is probably more of a stick free stability phenomena. Our models are all stick fixed of course because the control surfaces are (more or less) rigidly 'fixed' to a servo.

I wouldn't worry too much about spiral instability, as the article says that is a long period instability which means you won,t have a problem controlling the model. So go ahead and try temporarily enlarging the fin or rudder (within reason) it might not improve things but it shouldn't be uncontrollable.

I still think reducing the size of the rudder would solve the problem though. This would change the balance between static and dynamic yaw stability. Or as you say just live with it.

Cheers Dave
Old 08-08-2010, 09:08 AM
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Default RE: My plane experiences snaking

G,day Evan,

How about them All Blacks?

The Strikemaster (RNZAF jet trainer, not the model) had trailing edge thickening on the rudder and elevator also. It was to reduce the dead band for small control movements though, rather than anything to do with stability.

Dave
Old 08-08-2010, 12:40 PM
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Default RE: My plane experiences snaking

G'day Gerry! What, did some team win something? I didn't know about the Strikemaster fix, but this particular malady isn't a matter of stability, really, more to do with an airflow problem, 'rough air sinuosity'. I believe that it was attributed to an airflow breakaway from the fin, one side to 'tother, when the aircraft was upset slightly in yaw with a bit of turbulence. You will note that the writer says it's fine in the calm. I have exactly the same problem, if it is a problem, with my Super Kaos. Sometimes it's even called the 'Kwik Fly wiggle'. Seems to be a well known phenomenon, with all sorts of fixes. Lovely bit of weather we are having...
Evan.
Old 08-08-2010, 01:01 PM
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Default RE: My plane experiences snaking

my escapade experiences a similar flight characteristic.It seems to ride the bumpy air like a surf board rides waves.

if anything I think its due  to too much vertical stab area. My stik did not exhibit a sikilar chracteristic but my trainer does.

The Extra seems to demonstrate it as well but not as sever possibly die to a lower tail moment.

Was watching a fFuntana yesterday as well as a Pawnee, Both showed similar tailwagging in the bumpy air.

I fix it by flying higher...
Old 08-08-2010, 08:50 PM
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Default RE: My plane experiences snaking

I had a large scale aerobatic model with an aerodynamic mass balance tab on the rudder. It snaked a lot till I cut it off and fixed it to the fin. This made the rudder smaller and the fin larger. Complete cure. Didn't change the look of the model either.
The linkage should be solid of course as should the servo mounting. If the rudder can flex then that could be it.
Also make sure there is no large air gap at the hinge point. Seal it up if you can.
You could try taping some tabs on the rudder just to see if that helps, then you can remove or change them easily.
Old 08-09-2010, 12:25 PM
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Default RE: My plane experiences snaking

That NACA article about how the rudder flexes on the hinges may have some merit in your case. Took me a while of reading to get down to that part.... For anyone that wants to read it the bit on "snaking" is about 3/4's of the way down the page in the earlier link.

If hinge friction and slop in the system are the culprit as described in the NACA report then the "fix" would be to install flexing hinges such as the usual mylar strip hinges to replace any pinned style hinges. That would eliminate any hinge friction and replace it with a light self centering spring action that is typical of those hinges. Beyond that ensure that your pushrods have tight but not TOO tight pin joints with little or no slop at the servo arm and control horn.
Old 08-09-2010, 02:03 PM
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Default RE: My plane experiences snaking


I wouldn't worry too much about spiral instability, as the article says that is a long period instability which means you won,t have a problem controlling the model.
The spiral mode is slow, therefore controlable as you said it,but when I bank the plane and release the stick, Idon't likewhen itbanks further, towards the spiral.

I still think reducing the size of the rudder would solve the problem though. This would change the balance between static and dynamic yaw stability. Or as you say just live with it.
When you increase the vertical fin area you increase both the static stability (the force)and the dynamic stability (the damping).
In my book it is written: "Unfortunately,increasing the yaw dampingis not easy to achieve simply by geometric design changes."
Dang ! You don't need to butcher your Chipmunk neither.

It seems that this Dutch roll mode is poorly damped on most of the airplanes, and this is why the electronic yaw damper is common nowadays.

Regarding the NACA article, the motion is fixed stick: it is the compliance of the mechanical linkalongwith thetendency of the rudder to lean windwardfor small sideslip angles that induces the snaking.
Old 08-09-2010, 05:49 PM
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Default RE: My plane experiences snaking

Make sure the engine sits at zero-zero, or just a degree or two of right thrust. The worst thing you can do is have just a tiny amount of left thrust.
The "exercise" of trying a smaller diameter prop will be very worthwhile to prove if your balance of vertical areas is marginal.
Old 08-10-2010, 08:18 AM
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Default RE: My plane experiences snaking

'Lovely bit of weather we are having'

Yep sure is, thought I'd pop over to Brisbane for day or two, huge improvement, rain is vertical and temperature above freezing. Much better.

If your Kaos 'wiggles' I wonder if my Ultra sport (next project) will?

BTW Gerry is the (ex) cat, I'm Dave.

Cheers Dave

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