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INDUCING flutter on a trainer

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INDUCING flutter on a trainer

Old 01-15-2003, 03:26 AM
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Unstable
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Default INDUCING flutter on a trainer

here is something you probably thought you would never hear.

I want to cause a avistar trainer to expirience a flutter condition on the rudder.

I am writing an article for RCU mag that is a intro to flutter and prevention (nothing too advanced, targeting the everyday flyer) and want good video *with sound* for the section on recongnizing and reacting to flutter.

so I bought a trainer and am going to mod it to flutter and record the results.

the problem is although i know what flutter is, how it happens, and what to do to stop it, im not sure on how to cause it reliably.

this is a stock avistar arf with a os .46fx on the nose.
I was thinking the rudder because I feel that is the one surface I feel I can "loose" and still be able to land.

also any suggestions, online links, and vids/pics that you guys have would be much appreciated.

Thanks.
Old 01-15-2003, 04:23 AM
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Default INDUCING flutter on a trainer

Yer right.... this IS a new one....

I'd say off the top of my head that you're looking to make it more flexible and sloppy. The easy way would be to insert a spring override device into the rudder pushrod. You know the type? two pushrods coupled with bits of tubing and springs on either side so there's a soft end travel? That'll give you a soft pliable neutral.

With Murphy riding shotgun you'll probably NEVER get it to flutter.... Or after all this the ailerons will still flutter and eject and you'll have to land it with the soft, floppy rudder.....


..... OR you could cheat and use a pocket of covering material stuck to the wing or whatever and only the front and sides are stuck and the rear is open. Put a little bagginess in it and in a dive it should set up a fluttering reed sound. May have to play with it's position to find just the right place.
Old 01-15-2003, 05:28 AM
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Default INDUCING flutter on a trainer

Yip. This is a strange, but intriguing request.

Put lots of slop on the pushrod-linkage-horn. Put the rudder throw to maximum i.e. place the push rod linkage as far in on the horn i.e. the hole closest to the rudder hinge.

Also, put only two hinges close to the center of the rudder so that there is lots of slop where the horn is (at the bottom).

And, cut some lightening holes in the rudder sheet (if it's not built up) and this will make the rudder twist easily.

Make sure the rudder is exactly at neutral so that there is no aerodynamic forces on it in normal flight. Any turbulence (prop wash, etc.) will setup a flutter in no time.

All of the above will make for a sure-fire accident waiting to happen.

Then all you need to do is fly the plane fast. A long shallow dive from altitude should do it.

Let us know how it goes. I think that if you do the above you might just have your hands full right after takeoff.

-Q.
Old 01-15-2003, 06:27 AM
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PigMan Buggerus
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Default Avistar Flutter

I think you are really going to have to try to make it flutter. I have an Avistar, and I know it's overkill, but I put a tuned pipe on my OS .46 FX. I am turning somewhere close to 14000 rpms and a guy at the field said I was doing close to 70-75 MPH. Of course he was eyeballing it, but he flys jets so I gues he has some idea of speed, anyway, I have never experienced flutter.

Good Luck,
Allan
Old 01-15-2003, 06:41 AM
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Default INDUCING flutter on a trainer

The more I think about this one the more I think the rudder isn't going to cooperate. I may be wrong but I think it's just too short to really set up a good flutter. But with all the "fine workmanship" that's being suggested perhaps it'll cooperate.
Old 01-15-2003, 06:52 AM
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William Robison
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Default INDUCING flutter on a trainer

Unstable:

Getting the rudder to flutter might be easy. Make the vertical fin very flexible. You can have all the slop and gap in the world without inducing flutter, but a weak vertical stab will almost guarantee it. A surface with a high aspect ratio, an aileron, will flutter much more readily than a low aspect ratio, rudder and elevator. By increasing the flexibility of the vertical surfaces you will increase the possibility of flutter, but still not guarantee it.

Just my suggestion.

I hope Gordon will pay you enough to replace the engine and radio gear, if not the airplane and labor time. Haw. Give him a pizza gift certificate. Deluxe supreme. Plenty of tomato sauce and no anchovies.

Flutter, rudder, flutter away!
. I wonder if Gordon will pay?

Bill.
Old 01-15-2003, 01:49 PM
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Unstable
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Default INDUCING flutter on a trainer

thanks for the suggestions.

I'm thinking maybe making a "extention" off the rudder out of thinner balsa might be an idea. I already planned on dropping the control horn into the closest hole (after opening the hole WAY too much) then swapping out the good servo for a junk one so i dont lose a good servo to this "project".

BMatthews, I think you are right about Mr Murphy, but I will see if he wouldnt mind taking a back seat just this once (yeah, right)
Old 01-15-2003, 02:11 PM
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Dsegal
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Default INDUCING flutter on a trainer

> and what to do to stop it <

The fastest way to do that is to pull up sharply which is the quickest way to reduce airspeed.

Dave Segal
Old 01-15-2003, 05:09 PM
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Ollie
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Default INDUCING flutter on a trainer

Another measure that the other guys haven't mentioned is to add mass to the trailing edge of the rudder. Tape on a length of round solder wire with a little masking tape. If it is used with a sloppy servo linkage it will flutter. If you enlarge the inside hole in the rudder control horn to a slot about 1/8 inch long you will have plenty of slop. The weight can be easily increased or decreased to vary the frequency of the flutter.
Old 01-15-2003, 05:14 PM
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rwh
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Default INDUCING flutter on a trainer

Heck, you can borrow my Morris Su-Du-Khoi!

When the rudder pull-pull cables are a bit loose, the rudder flutters away! You can see it and hear it. Level flight, Saito 0.56 on the nose. Doesn't particularly affect the flight path.

So.... copy the rudder of the Su-Du-Khoi and you've got it made.
Old 01-15-2003, 05:24 PM
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Ben Lanterman
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Default INDUCING flutter on a trainer

Bruce's idea of the springs is a very good one. Somewhere I think I remember that it does require the spring to get a system that has a natural frequency that can be driven by the aerodynamics. But keep the vertical tail and rudder strong and rigid (most models are over built so flutter is probably due to linkages and hinges ) since you would like to be able to duplicate the flutter on multiple flights (don't want to rebuild them after each flight) to be sure you have good video. Vary the stiffness of the springs until you have a set of springs that will allow flutter at a medium flight speed. Then it will be very repeatable and allow getting the video at a low airspeed and altitude. You can approach show center at a high speed level flight condition, cut throttle and have flutter as it coasts through the flutter speed at a low motor noise condition. (well it sounds good anyway, never tried it personally)

Since at least half of the flutter in actual flight is caused by sloppy flexible linkages it will also give you a feel for the amount of slop and flex that is required to allow flutter to build.
Old 01-15-2003, 06:12 PM
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Default INDUCING flutter on a trainer

build a correx combat plane, theres one here that will flutter away on any pass with no damage and no loss of surface or control... i could if hes out get some AVI with sound. pics arnt to sharp tho
Old 01-15-2003, 07:03 PM
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Unstable
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Default INDUCING flutter on a trainer

Originally posted by rwh
Heck, you can borrow my Morris Su-Du-Khoi!

When the rudder pull-pull cables are a bit loose, the rudder flutters away! You can see it and hear it. Level flight, Saito 0.56 on the nose. Doesn't particularly affect the flight path.

So.... copy the rudder of the Su-Du-Khoi and you've got it made.

sure just send it over and I'll "borrow it"

seriously what do you guys think about this design
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Old 01-15-2003, 11:48 PM
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Default INDUCING flutter on a trainer

Gees ... it seems like some of us have enough trouble keeping planes in one piece without inventing ways to make them crash ...

After you are done with the flutter video, maybe you'll make a video that demonstrates overloading the wings. I've been told that my flying is putting my trainer at risk of overpulling the wings off. A good educational video might be just what I need to ease up on the up elevator...
Old 01-16-2003, 01:49 PM
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rwh
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Default INDUCING flutter on a trainer

I once bought a partially built trainer at an auction. It had all the control surfaces hinged, but no control rods.

I drove up to the field to show the plane to the guys after the auction, took it out of the truck and held it out in the 15 mph breeze. All the surfaces starting fluttering away! They thought it was wicked funny!

One thing about that plane, it had a significant gap at the hinge line, maybe 1/8 inch. I cut out the old hinges, finished the plane and we still use it today as a club trainer.

Maybe you could try flying your plane with the rudder control rod disconnected?
Old 01-16-2003, 11:20 PM
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Default INDUCING flutter on a trainer

I was just reading about gyros.

Apparently, if you put a gyro in your rudder circuit and turn the gain up to high, you may induce flutter, particularly at higher speeds. I'm not sure what is meant by higher speeds (and don't know if a trainer can get there), but if you have a gyro on hand, you might give this a try.
Old 01-16-2003, 11:38 PM
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JimTrainor
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Default INDUCING flutter on a trainer

However, the high gain gyro instability is not likely flutter. What'd you'd probably be seeing is the feedback controller in the gyro becoming unstable.

The gyro holds a position by comparing where it wants to go (straight ahead) with where its sensor tells it it is actually going. The gyro's controller tries to drive that error to zero. The controller will multiply the error by a gain and use that to generate the servo command. If the gain is too high, the whole system will oscillate, or go completely unstable.
Old 01-21-2003, 02:07 PM
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Default INDUCING flutter on a trainer

I put the "extension" on the rudder.(no weights yet) the dimensions are... (holds out his hands)ummm about this big by that big. (here is evidence of my extreme care and calculations while building)

I secured it with two peices of econokote to the exsisting rudder.

I messed with the control horn giving it about a 1/4 each way of free play.

all I need now is some warmer weather so me and marc can go out and film this travesty in aviation engineering (anything I'm involved in is bound to be a disaster)
Old 04-30-2003, 04:11 PM
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Default INDUCING flutter on a trainer

Originally posted by rwh
Maybe you could try flying your plane with the rudder control rod disconnected?
well I FINALY got out to test this plane out and my extension didnt produce enough flutter to be noticable on tape.

so I diconnected the rudder all together and took her up.....

after the 3rd unplanned roll I was able to level it out and get it down minus the vert stab.

I should be submitting the article soon so you will be able to see the first part of that incident. (Marc took away the camera after the second roll because he didnt know what was happening.. but you do get to see my "landing" (I killed the engine and put it down kinda level were it wanted to go as I didnt want to fight it.)
Old 04-30-2003, 06:06 PM
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Tall Paul
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Default INDUCING flutter on a trainer

I had a foamie sloper.. ANABAT.. taped everything... after a few flights with combat collisions etc. the aileron hinges (tape full length top and bottom) got so loose I could induce flutter on command. And get out of it!
You might try a vertical/rudder with the areas equal above and below the centerline.. your fluttering rudder probably overloaded the vertical because of all the load being above the vertical attachment point on the tail end.
Old 05-01-2003, 12:22 PM
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Default INDUCING flutter on a trainer

wow! I cant wait to see that video. I really appreciate you making that video, as I have never made a control surface flutter and would like to know what to do and how to diagnose it.
Old 05-04-2003, 07:09 PM
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Default INDUCING flutter on a trainer

Unstable,

Quite an interesting topic. This reminds me of the lengthy discussion that Carmudgeon Paul, myself, and a few other RCOnliners had regarding the factors that influence the onset of flutter on model aircraft. We were looking at a very elementary wind tunnel test of various trailing edge shapes and how that effects flutter. For that particular experiment, I don't recall any solid conclusions aside from flutter being an incredibly complex phenomenon that is not easily modeled or reproduced.

Beware in your discussion not to mix up flutter and control surface buzz. I don't claim to be an expert or even competent in either field, but I do know that they are very different from each other.

Control surface 'buzz' is a limit cycle oscillation. Meaning that if the initial conditions that cause this behavior remain, the buzz will continue indefinitely at the same amplitude. A lot of flutter discussion really falls under this category, where the model emits a very noticeable buzzing sound but it does not have a big effect on the performance or flight path.

Flutter, on the other hand, is an unstable phenomenon. By it's nature, flutter will propogate in time and lead to the destruction or damage of the airframe. This is one of the reasons why flutter is so hard to study, test models keep destroying themselves This is, of course, assuming you can ever get them to flutter.

I had talked to one of the people at NASA Langley about this, and he brought up the notion that most of what's referred to as "flutter" is really far from it. Another aspect of flutter is that it involves an interplay between aeroelastic and aerodynamic forces. Meaning that a simple rigid-body movement of a control surface isn't flutter at all.

Finally... one bit of anecdotal evidence on flutter... I have a Senior Telemaster that only developed a flutter problem after I recovered the airframe. I don't know what it was that changed in the large equation of life, but I'm assuming that the change to fabric covering from Monokote made sufficient difference in the structural properties of the wing to cause the flutter. One the few occasions I observed it, the entire wing would bend and twist at a frequency of approx 5 - 8 Hz, enough so that you could see the deformation and hear the "whoomp- whoomp - whoomp" from the ground.

Best of luck with your article!
Old 05-04-2003, 07:14 PM
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Default INDUCING flutter on a trainer

when i rebuild my Dust Unlimited i thought by having the servo arms on the underside of the wing ( still internal ) and the horns on top i would have a neat exit hole in the film, well i did, but i also got bad flutter, the whole plane shook and the boom flexed, it slowed down the plane very quickly, but didnt cause damage, i swapped the horns to the upper posistion and the problem was sloved
Old 05-12-2003, 08:56 PM
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Default INDUCING flutter on a trainer

Tack a couple of streamers to it. 8)
Old 05-16-2003, 02:26 PM
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Default INDUCING flutter on a trainer

joojoo,

alot of good points in there.

the article (which is mostly written, I am just waiting to get my laptop back from repair to finish it) is made to be an introduction to the very basic theory. I dont go into the diffrence between buzz and true flutter as it would just confuse most people, as well as give me a headache trying to put it in words

I am far from an expert on aerodynamics but I feel I know enough to give general guidelines on what to look out for and what to do.

I know that when I submit this article that some people are going to start nitpicking it apart but hopefully I get the main point of the article across.

there is a quote I found that applies to this.

Some fear flutter because they do not understand it. And some fear it because they do.
Theodore von Karman, aerodynamicist.

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