Extreme Speed Prop Planes Discuss the need for speed with fast prop planes (Screamin Demon, Diamond Dust, Shrikes or any REAL sound breakin'''' plane)

Questions regarding Flutter

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Old 01-26-2008, 08:58 PM
  #1  
Isaac F
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Default Questions regarding Flutter

Guys, I have a airplane similar to a Kaos with a West 50 and Weston Pipe and on the firts run this thing was very fast but there was a flutter...... The noise was like RRRRRRRRRRRRRRRRR.

This is the first time I have a airplane with fluter so I dont have experience with this.

1)How do I know were I have flutter? Ailerons, Elevator or Rudder???

Reading here says to cover with tape the aileron gap on the bottom part of the wing. Do this solve the problem 100% or do I have to doit both part top and bottom?

What about the Elevator and rudder?

Just curious, if I dont do nothing and I keep flyint the airplane like this, what exactly will happend? What I mean, I know I can crash, but why the airplane will crash? Do this flutter will damage the hinges, the servo link conectors, the servo arm or do the same servo?

THX

Isaac

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Old 01-26-2008, 09:22 PM
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cncswiss1
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Default RE: Questions regarding Flutter

flutter can destroy stuff in seconds, and usually the plane is in a crater soon after that. servos strip gears, flight surfaces rip clean off etc..

"most" of the time it's the ailerons, they are the biggest...

likeyou said, look for slop in any of the linkage, tape the gaps, stiffen things up, and try to slow down when you hear the noise, get a spotter and some binoculars to watch the plane real close in flight to see what's up
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Old 01-26-2008, 09:25 PM
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aussiesteve
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Default RE: Questions regarding Flutter

First off - My advice is Do Not fly the plane until you solve the flutter. If you really have to - keep it slow and away from the speed where the flutter occurs.

I have seen flutter tear a plane apart in mid air in a couple of seconds. (U-Can-Do60 using the standard control gear that came with it - being flown fast). the control surface basically vibrates at such a frequency and force that it simply shakes the plane apart. The pilot has no control over the plane while that occurs and it has the possibility to put people in danger.

It is difficult to find exactly where the flutter is without observing the flutter. I wouldn't recommend making it flutter on purpose. Just look closely at all the control surfaces. For any of the conditions below.

There are a lot of things that will contribute to flutter.
A wide hinge gap - that can contibute - taping them will help but if they are too wide anyway, it is better to fix that problem first - then tape the gap.
Slop in the control linkages - Clevises, ball joints, z-bends through oversized holes in servo arms will all contribute to this - make suere there is no excessive slop in the connection between the control surface and the servo motor (That includes the servo gear train).
Weak Servos - are they adequate for the job?
Flying a plane with large control surfaces or throws too fast. That will exacerbate any of the issues noted above.
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Old 01-26-2008, 10:18 PM
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combatpigg
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Default RE: Questions regarding Flutter

......use the pivot points on the servo arm and control horn that gives the servo the most leverage while still allowing enough travel for high and low speed flight.

Trying to control a 2 foot long aileron with a hidden torque rod that only extends into the inboard end of the flap about an inch is asking for flutter, the surest way to control a full span aileron or elevon is at mid span.

A good checkout of all the controls against moderate hand pressure will sometimes reveal a problem.

I believe that all control surfaces should be sanded even with the part of the plane they are hinged to.......that means after hinging, the control surface should not stand proud of the part it is hinged to anywhere along it's span.
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Old 01-26-2008, 10:30 PM
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Isaac F
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Default RE: Questions regarding Flutter

......use the pivot points on the servo arm and control horn that gives the servo the most leverage while still allowing enough travel for high and low speed flight.
Dont undertand what you mean. Can you explain better

Trying to control a 2 foot long aileron with a hidden torque rod that only extends into the inboard end of the flap about an inch is asking for flutter, the surest way to control a full span aileron or elevon is at mid span.
Ok, this airplane use one servo in the middle of the wing with hidden torque rod. So is better to install one servo in the middle of each wing in order that one servo contro one aileron. I can do this because I just today bough some monokote to cover the wing.

Let me know,

THX

Isaac

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Old 01-26-2008, 11:27 PM
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combatpigg
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Default RE: Questions regarding Flutter

Isaac, if you use the inner hole on the servo arm and the top hole on a tall control horn, the flap won't move very much, but this is how to get maximum leverage and the finest control resolution. Torque rod systems can be made to work well, but the faster the plane and the bigger the flap, the beefier the torque rod needs to be. Centering a servo in each wing panel and using stiff balsa for the flaps and a carbon tube push rod will give the strongest set up.
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Old 01-27-2008, 09:28 AM
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Ed Smith
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Default RE: Questions regarding Flutter

Taping the hinge line will probably not solve the flutter problem. It is not the hinge line that is fluttering. It is very likely the outboard trailing edge tip of the aileron, elevator or rudder. The picture shows the cure for aileron flutter. A small piece of tape is stuck across the corner of the aileron (In this case). The control surface still has to move so the tape does not restrict the outboard tip completely. Try one control surface at a time until the culprit is found.

Flutter/vibration can be caused by many things unrelated to control surfaces.

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Old 01-27-2008, 09:40 AM
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Default RE: Questions regarding Flutter

I had a Chipmuck that had flutter because the holes that the torque rods when into elongated. I split the aileron open for about 1 inch then layed some 2oz glass cloth going under and over the torque rod end wetted with epoxy. This helped, but the torque rods where too small so now I'll fix it with aileron servos driving from the center of each aileron.
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Old 01-27-2008, 10:09 AM
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Default RE: Questions regarding Flutter

I had an aileron flutter problem with my Cermark Bobcat...

The ailerons originally went all the way out to the tips, but after I cut about 3" off and glued 'em solid to the T.E. of the wing, it hasn't fluttered since. (knock on wood )
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Old 01-27-2008, 10:43 PM
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Flyboy Dave
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Default RE: Questions regarding Flutter

....yep, ailerons are usually the culprit....but I've had elevators (2) flutter, and rudders
(2) flutter....and (believe it or not) a nose wheel that fluttered.

I did what proptop did on the ailerons on my big F-20....shortened them. I whacked off
(on the airplane) about 1/3 of the ailerons, and it cured the aileron flutter.

FBD.
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Old 01-27-2008, 10:53 PM
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soarrich
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Default RE: Questions regarding Flutter


ORIGINAL: Flyboy Dave

I whacked off
(on the airplane) about 1/3 of the ailerons, and it cured the aileron flutter.

FBD.
Now that was funny!
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Old 01-28-2008, 12:19 AM
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Default RE: Questions regarding Flutter

FBD ! [X(] LOL
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Old 04-16-2008, 08:10 AM
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Brainiax
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Default RE: Questions regarding Flutter

Hi folks, I recently lost a nice plane due to flutter, this red one: [link=http://www.rcuniverse.com/forum/m_6860430/mpage_2/tm.htm]140 Extra 330L[/link] , the yellow one is new and almost the same. I want to do everything I can to prevent any chance of flutter on the ailerons. Is the info avove all ther is or ca I do other things to remedy this? Thanks in advance.
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Old 04-16-2008, 08:40 AM
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soarrich
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Default RE: Questions regarding Flutter

The plane looks well made. I would say check your linkages to the ailerons, you should have zero slop. Two things I would look into, only because you said you had one before and it got flutter:

First make sure that the servo tray that the aileron servo screws into is not flexing, also make sure it''s glued in well if it breaks loose you could get flutter and destroy the plane so quickly you just thought it was the flutter that caused the problem, when really it was a broken out aileron servo causing/allowing the aileron to flutter.

Second I''d really crank down on the servo mounting screws to take out as much of the servo mounting grommet flex as possible. Knowing I had flutter last time on the ailerons, I might even remove the grommets entirely, put some Goop on the mount and screw the servos in.
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Old 04-16-2008, 08:54 AM
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Default RE: Questions regarding Flutter

Make sure you have no slop
Make sure your hinge line is tight seal if needed
Make sure theres no slop in the gear train of the servo.
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Old 04-16-2008, 09:49 AM
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Default RE: Questions regarding Flutter

The leading edge of the aileron should not be thinner then the trailing edge of the wing. heavy ailerons can cause flutter. Keep light but stiff. Plus all of the above.
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Old 04-16-2008, 11:26 AM
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Default RE: Questions regarding Flutter

Had a lot of experiance diagnosing flutter in Dynamic Soaring planes that see airspeeds in excess of 300mph. What combatpigg is talking about is essential regarding maximizing servo torque. You want the tallest possible control horns on the shorters possible servo arm (closest hole to the center). With the servo's at max throw it should give you the high rate you need. IF this is a speed plane there is no need for big throws, just the minimum to have sufficient control authority at landing speeds. Also make sure the hole on the control horn is right over the hinge line and not infront or behind it. I make my own control horns using 1/8" G-10 fiberboard available from Mcmaster Carr for cheap. The next issue is the stiffness of the control surface. If the control surface is long and lacks rigidity you will get flutter on the section that is furthest from the control horn. You can tell by arming the radio so the servos hold position and then flexing that part of the surfaces furthest from the control horn to see how much movement you get. IF you have a ton of flex you can either stiffen up the surface by embeding carbon/metal rod into, cut it down to reduce the size, or tape the tip solid and depend on flex of the aileron for control movement. You can also reduce flutter by mass balancing the control surface (what real airplanes do) which involves putting weight ahead of the hinge line. If you have a more major problem such as the entire wing fluttering you have a more serious issue that would require a major design change of the model.
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Old 04-16-2008, 12:04 PM
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Default RE: Questions regarding Flutter

Is the speed on the dynamic soaring planes really ground speed or airspeed?
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Old 04-16-2008, 01:52 PM
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Default RE: Questions regarding Flutter

We measure ground speed via radar but at certain points in the circuit the air speed is much higher than the ground speed. On a big speed day when there is strong wind it is not uncommon to have wind at a steady 55 gusting to 70+. In those conditions you can easily be cruising in the 200''s ground speed but when the airplane hits the sheer layer and see''s that 70mph headwind the airspeed over the plane is now way into the 300''s. The current record is pretty much based on the limit of high end radar guns and being lucky enough to get a signature return which at this point is 313mph. However I''m sure many circuits have been much faster but beyond the range and resolution of a radar gun. When you consider that these types of speeds require a 60-70mph wind you are looking at airspeeds in portions of the circuit exceeding 400mph in the most extreme cases. Flutter city if the aircraft is not set up properly. All the purpose built DS ships have the absolute minimum size control surfaces with the strongest servos you can cram into the area availble.
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Old 04-16-2008, 07:58 PM
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Default RE: Questions regarding Flutter

All of the above and;
On clevises or Z bends, if there is a tiny bit of clearance, I like to add a very small amount of CA to it and hit it with kicker than flex it. Periodically I check the linkage for ANY slop and do it again if needed.

The best of servos with time get some backlash. A one time only cheap fix is to carefully open the gear case, mark the gears with a felt tip pen, Than remove all the gears (except the motor’s gear) Than reinstall all the gears 180 degrees (except the output shaft) from the way you found it. If there is a next time you’ll need a new gear set.
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