gerryndennis

'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.'

Hi Paul,

Yep, another reason for yaw dampers, on multi engine aircraft anyway, is to allow for lower take off speeds. Many (most?) multi engine aircraft are fitted with a larger fin/rudder than 'normal' stability requirements would dictate. This allows the aircraft to remain controllable at a lower speed following an engine failure. This means that we can either safely take off from a shorter runway, or lift more weight from a given runway. The downside is that the aircraft is now snakes in the dutch roll mode, especially at high altitude.
a problem completely solved by the yaw damper.

Yaw dampers are good. Flight Attendants don't like yaw excursions while they are working. Grumpy bun runners means no coffee for me.

Dave

pimmnz

Sorry Dave, haven't learnt to read to the bottom of the page...First aircraft I worked on that had yaw dampers fitted as standard kit was a 737-200. Belonged to a company called, er, some time ago, NAC! Unfortunately I am stuck behind a computer these days, and do all my engineering with a keyboard, and all the nice weather is outside the window. Weekends, of course, is reserved for all the other weather we didn't get all week. Leaves a bit of time for buiding, but.
Evan.

Paul_BB

Hi Dave,

I understand now why you want to decrease the area of thetail fin.
I've read somewhere that the damping worsens with the Mach number and the altitude.

I found a MIT document about lateral stability. It says on page 9to damp the Dutch roll mode with a largetail fin.
It is an interesting documentbecause itexplains in a clear waythe origin of the lateral stability derivativesLp Lr Np Nr.

But the document does'nt analyse the respective influences of the airspeedand thealtitude on the static and dynamic lateral stabilities.

-Paul

gerryndennis

Paul,

The Mach/True airspeed versus Indicated airspeed issue obviousely has nothing to do with our models, but is another example of how stability changes when the relationship between static and dynamic stability changes. Static stability stays the same, but as altitude and therefore TAS increases dynamic stability or damping reduces.

I'm pretty much at my limit of knowledge here, and would have to draw a diagram to explain it. I'll see if I can find anything with Google or Wiki, but that stuff isn't really my forte.

Look for roll damping as well. It varies with Mach/Tas for the same reason.

Dave

gerryndennis

oops

BMatthews

.... "Grumpy bun runners"? ? ? ? LOL!

I'M TELLING THEM THAT YOU CALLED THEM THAT! ! ! ! NO MORE COFFEE FOR YOU! ! ! !

ndb8fxe

My Feedback: (1)

Okay so you really want to change this charateristic of your plane WITHOUT changing it's looks. Ever think of using a gyro on the rudder to act as a yaw damper? I think it could work.

Paul_BB

Hi Foxman,

Didyour aerodynamic balance tab looked like the Chipmunk one I drew in the attached file ?

-Paul

Foxman

Hi Paul, yes the aerodynamic balance tab did look exactly like the one you have drawn.
I cut it off and glued it to the fin. It looked similar to what it had done before except there was more fin area and less rudder.
I can't remember what the model name was. It was similar to an Extra but it was about 1980 and the name escapes me.
I did wonder if the problem was helped by area being removed that was high up above the centre line. Maybe right rudder was rolling it left a little bit and the lower rudder helped.
I also had a Club 20 pylon racer with a large V tail that wobbled during turns.
I cut the area down and that cured that. Never a definite answer is there?
Jim

ARUP

I remember a model airplane called 'Total Zero' and it was built very symmetrically (other than the gear) in order to make it as 'neutral' a flying airplane as possible. The goal(?) was to eliminate adverse yaw with aileron, Dutch roll and etc. (the only thing it wasn't: a twin engined aircraft w counter rotating props!) The F1 raceplane, Kelly 1D (?) has its fin/ rudder equally distributed on the thrust line to enhance its flying qualities. I reckon the Chipmunk will have this 'snaking' tendency because it is the nature of its design.

Lifer

My Feedback: (1)

Had the same problem with a Hangar 9 .60 sized P-47. Installed a $18 gyro from Hobby King and it disappeared. First time using a gyro, too!

Foxman

Hi Paul, I just had a thought, is the rudder mass balanced?
If it isn't then try getting enough weight into the leading edge of the tab till it is actually nose heavy. If you lay the fuselage on it's side the front of the rudder should be heavier and it will go down.
This will naturally dampen out the the snaking, it may not cure it but it's an easy fix to try.
If the rudder is tail heavy then as the body snakes to the left the weight of the rudder tries to carry on making the effect worse but if it is nose heavy it will do the opposite which could cancel out the snaking effect.
Mail me if that isn't 100% clear, lol
It's confusing to me and I wrote it, ha ha
If you do add a lot of weight don't forget to check the cg of the entire model just in case you have moved it back.
Not sure if the cg of the overall airframe would affect the snaking, the model should very slightly drop the nose when flown inverted but only a little bit.
A customer of mine a long time ago had a large Mig 29, this had an elevator snaking effect. He found out it was because he had mounted the servos on the fibreglass body skin and they could flex. You could bend the skin by moving the stab.
Some twin finned model jets like the F15 can have snaking and can benefit by putting the rudders outwards a little bit which gives them "toe in".
I can't explain why this is but it seems to work.
Some aircraft have a plate fixed on the rudder trailing edge. Someone mentioned this earlier in the thread but that is an easy experiment to try although I think it is to stop rudder flutter or rudder buzzing rather than snaking.
Jim

Paul_BB

Hi Jim,

Thanks for your suggestion.

But I think that if you add weight to the rudder, on the TE or on the LE, the problem will worsen because it is the moment of inertia that counts, remember the rudder rotates about its hinges.
So in any case the rudder will feel "heavy", in a rotational point of view.

I discovered that I have too muchdeflection on my rudder. I fly with only 50% (dual rates). I found also that I have 1 inch of slack.
So I will start by that: reduce the throw and get rid of the slack/compliance, like your customer did on hisMig 29.
I will test this mod weather permitting.

-Paul

Foxman

Hi Paul, if there is a lot of slack then that could be it.
I understand what you say about adding weight to a moving surface but if it is "mass balanced" it can only improve things.
As an example if you lose a wing from a model then as the wing comes down it rotates. If some weight is added to the leading edge it doesn't.
By adding weight to the leading edge of the rudder you are counteracting this tendency.
As a test take the linkage off the rudder and quickly move the body from left to right. The weight of the rudder makes it carry on when you stop the movement but if it's "mass balanced" it doesn't want to carry on.
If it is mass balanced to the point of being nose heavy then the leading edge of the rudder will want to carry on moving which has a damping effect instead of exagerating the problem.
You could always just tape some weight on and try it, if it doesn't work you can take it off easily.
Jim

Paul_BB

Hi Jim,

Okay now I understand what you mean. I could give it a try if my first mod does'nt help.
But did'nt you say that you solve your snaking problem by removing this counterbalancemass on yourplane?

-Paul

Foxman

Hi Paul, yes I cured my problem by removing the aerodynamic tab and glueing it to the fin, but I have been wondering if I had mass balanced it with weight then I might have achieved the same result.
It was just that you could tape some weight on and try it with out having to cut anything.
I never tried it at the time so I don't know if that would have worked or not.
I hope you cure it. Let me know if you do and how you did it.
I wish I could remember the name of that model. Spinks Akromaster, maybe?? Not sure. Is that an aeroplane even, lol
Jim

AA5BY

Balance tabs are restricted in their percentage to avoid the issue being discussed but there are two other issues that factor. 1. The lever arm of the balance tab. The longer fore-n-aft the balance tab is the greater its lever and thus likely the percentage rule more conservative. 2. Any unusual air flow. For example if the airflow favors that upper fin area and the balance tab area over the broader area of the rudder then again the balance tab area should be conservative.

If the balance tab is over sized or leveraged, it will produce over controlling that will take advantage of the slightest bit of play in control linkages to produce hard impacting rudder shifts. In fact there is some danger here because if an unsecured metal clevis is in the system, the oscillations will greatly accelerate thread wear and within a few flights failure could occur. So, if the plan is to fly it this way, keep a check that clevises are locked by a back up nut.

I know you don't want to do surgery... but it seems to me that it is warranted. I'd not do away with the balance tab by cutting it off and adding it to the stab (major surgery), but I'd cut a little off the top to reduce the tab percentage (minor surgery).

Paul_BB

Jim,

If I add 1 oz at the rudder, that means much more in the nose to keep the plane balanced. So this I will not do because whatI hate above all is to add weight to an airplane.
I will try the plane with a stiffer mechanical link and will get rid of the slack.
If this does not cure the snaking then I will live with it. I will not fly in turbulent air.
Thanks for your ideas, its always interesting to discuss the issues, even if we don't solve them.

-Paul

vasek

My Feedback: (4)

I'm with Lifer on this one. A cheap Gyro will eliminate any tail wagging 4 sure

Lifer

My Feedback: (1)

vasek,

I think the gyro weighed about 6 or 7 grams.

Incredible!

Foxman

Hi Paul, I guess the other guys are favouring a gyro. I suppose for the cost and the weight it is a good idea but If it was mine I would like to find the cause anyway.
Sloppy and flexible servo mount/linkages are the first thing to change.
The more solid the better.
How about 2 servos ? 1 either side? You could use small lightweight ones or is that too much hassle?
Jim

Paul_BB

Lifer,

did you mean this kind of gyro ?
http://www.hobbyking.com/hobbyking/s...o_Gyro_2.3gram

Lifer

My Feedback: (1)

PaulBB,

That's not it. It was made by Turnigy and was about the size of a sugar cube. I'll see if I can get the model number. Costs about 17 or so dollars, plus shipping.

Lifer

My Feedback: (1)

PaulBB,

Found it! TG-380.....$18 plus shipping. Ridiculously easy to install and use.

Paul_BB

Thanks,

-Paul