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Thread: Flutter


  1. #26
    lopflyers's Avatar
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    RE: Flutter

    Nice videos SpeedsterDEN, thx.
    Keep your wings level
    Club Saito Member #693

  2. #27
    speedracerntrixie's Avatar
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    RE: Flutter

    I don't recall the OP stating what size airplane, servo brand and model number or a picture of the setup showing us mechanical advantage. While most of what I read regarding the cause of flutter is correct, the most common cause on an airplane like the Sbach is improper setup. Things to check is whether the servo has the power for the job, the servo arm should be at least 1/4" shorter then the control horn and there should be no slop in the linkage. Being that the first few flights on a fresh RX pack went well clues me in a a little . With slightly higher voltage the setup had enough power to not flutter. Look to make sure the servo has enough power and you have good mechanical advantage. Pictures would be a big help.

  3. #28
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    RE: Flutter

    Yes I did mention is a 73" WS30cc  Sbach.
    Servoes are Hitec, I will post model # and pics of the set up later.
    I just bought a new LiFe 2100 mAh 6.6 v to change the existing one.
    Keep your wings level
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  4. #29
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    RE: Flutter

    I did skim back over the thread and noticed it was a 30cc airplane. What model # are those Hitecs? How long of a servo arm are you using? Are you using ball links? Are you using the stock servo arms?The LIFE batteries you are using will top out at close to 7V so the additional power gained from that may have been what allowed your setup to work until the battery dropped down to nominal voltage. Do not do any mods to the airframe just yet such as counterbalances or cutting on the ailerons. Things like that are not what is causing your issue. I have built many 35% to 45% airplanes that had the ailerons all the way to the tip. Some of those airplanes had over 200 sq. in. of aileron per side. The more information you can give me the faster we can hone in on the actual cause.

  5. #30
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    RE: Flutter

    Here is a short clip of flutter on a twin turbine RC Jet. The aircraft was a total loss.

    http://youtu.be/Unwq7IpNLIQ
    The dreamers of the day are dangerous men, for they may act their dreams with open eyes, to make it possible.

  6. #31
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    RE: Flutter

    They are JR DS 8231 Digital, the arm is 1" the rod is 2".
    The rod is metal, ball bearing at the end. Here are two pics. Tomorrow ill take out the receiver battery and measure the volts. I'm changing to a new one anyhow.
    Keep your wings level
    Club Saito Member #693

  7. #32
    lopflyers's Avatar
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    RE: Flutter

    [img]webkit-fake-url://AC731F0D-F5A5-41B0-8601-DAB3FE0E96FE/imagejpeg[/img]
    Keep your wings level
    Club Saito Member #693

  8. #33
    lopflyers's Avatar
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    RE: Flutter

    [img]webkit-fake-url://AF0A1C49-10E6-49E3-8299-7DDB20679497/imagejpeg[/img]
    Keep your wings level
    Club Saito Member #693

  9. #34
    lopflyers's Avatar
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    RE: Flutter

    [img]webkit-fake-url://7B201181-8FF1-472E-A325-86FA347C9200/imagejpeg[/img]
    Keep your wings level
    Club Saito Member #693

  10. #35
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    RE: Flutter

    The pictures didn't come through. The servo torque seems sufficient provided the mechanical advantage is there. Some would advise against nylon gears but I have run them in a 70cc airplane without issues. I would not do so today though. A 1 inch servo arm would be a good length but what is the arm made of? Ball bearing at the end of the rod? I am going to assume you mean ball link. If so ball links are not a good idea on plastic arms. Under load it will allow the arm to twist and that can be your cause of flutter. So far you have not mentioned how long your control horn is. That would be the part of the control system that is secured to the aileron. You will want to measure from the hinge to the point where the pushrod connects. With a 1" servo arm you would want that dimension to be at least 1 1/4", 1 1/2 would be even better.

    bottom line is that to avoid flutter is to have servos that you think are a good 20% more power then you need, have good mechanical advantage, use ball links only on metal arms, tight hinge lines and make sure the servos get enough voltage to make their max torque. The latter means all wiring should be 20 ga and minimize connections whenever possible.

  11. #36
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    RE: Flutter

    The servo arm is plastic. Control horn is 1 1/4, I'm not surprised the pics didnt load. I'm still seeing them from my end.
    Good advise, thx. I'm planning to fly her wed with the new battery. I think that is where the problem is.
    Out of laziness I always charged this batt with one port instead of balance- charge. 
    I'll let you guys know if it was the battery.
    That's why I love RCU, you get tons of advise from the gurus.
    Keep your wings level
    Club Saito Member #693

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    RE: Flutter

    Hope this link works great article by Mike

    http://www.rcuniverse.com/magazine/a...rticle_id=1289

  13. #38
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    RE: Flutter


    ORIGINAL: mikes767

    Hope this link works great article by Mike

    http://www.rcuniverse.com/magazine/a...rticle_id=1289
    +1 I remember helping out with that article
    ANY hinged flight surface WILL flutter... the trick is determining at what speed ... (preferably above or below the normal flight envelope )
    Jim Buzzeo AMA 74894
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    RE: Flutter

    you must be Mikes son ,He helped me out many times [8D]

  15. #40
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    RE: Flutter


    ORIGINAL: mikes767

    you must be Mikes son ,He helped me out many times [8D]
    hehe... well, you're close at least. I'm Mikey's 'little' brother. His son is 20 years younger than me
    yup, I miss the old bugger too!
    Jim Buzzeo AMA 74894
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  16. #41
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    RE: Flutter

    It is a combination of aerodynamic and mechanical instability. The quickest fix for our models is to make sure your linkages are tight, servos adequate, and surfaces strong and properly hinged.

    Action and Reaction:

    Flutter happens when some force (usually an aerodynamic force) impacts part of the control surface system - it could be a change in attitude, implementation of aileron/elevator/rudder, a wind gust, or possibly even something like vortex shedding (spiraling air that breaks off opposing edges of a surface creating an oscillating instability). Whatever force is present is the action.

    Every action has a reaction. When the above action is beyond the limits of the surface/servo/linkage, something will bend, break, or give. If it doesn't break, it will snap back like a rubber band (servo regains control when force reduces, push rod flexes and reflexes back, etc). It snaps back with a force that is about equal to the initial force, however the aero force is probably gone or moved. Now you have a large reactive force with nothing to balance it out, and the process continues until it finds some kind of balance. Hopefully it oscillates out, but often something breaks.

    Flutter has a lot to do with the bending and resonant properties of the materials of the control surface and linkage, and even the wing and airplane frame. So the cause of flutter is usually inadequate structure (including loose parts) for whatever situation the airplane is in. The aerodynamic force sets off the flutter, but is not really the cause. Airplanes should be able to fly within their operating envelope and not experience flutter.
    Joe Marri
    Enjoying all things aviation.

  17. #42
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    RE: Flutter

    I just took the LiFe battery out, it was puffed. It took charge, 30 mins at 1A, up to 7.1 volts.
    I'm not trusting it. Just slided a new freshly charged one. I will test fly her tomorrow and let you guys know if flutter is back!,!!
    Keep your wings level
    Club Saito Member #693

  18. #43

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    RE: Flutter


    ORIGINAL: hogflyer

    Other items to look at are the slop in the control system
    Binding in the control system .... or slop.
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  19. #44
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    RE: Flutter


    ORIGINAL: lopflyers

    I just took the LiFe battery out, it was puffed. It took charge, 30 mins at 1A, up to 7.1 volts.
    I'm not trusting it. Just slided a new freshly charged one. I will test fly her tomorrow and let you guys know if flutter is back!,!!
    You were lucky surviving the first case of flutter, I would not suggest flying the airplane again until you are sure it is fixed. I am 35 years into R/C, 15 of those flying IMAC. I have NEVER seen a battery cause flutter. I have seen poor linkage setup cause flutter more times then I can count. Your previous posts are leading me to beleive that you are running ball links on plastic servo arms. On any size gasser that is a bad idea. IMO at this point you should take your aileron servos apart and inspect the gears very carefully. If all is well with the gears then replace the ball link at the servo with a metal clevis and have it no more then 3/4" out on the arm. This would be the minimum I would suggest, best case senario you would replace the servos with 150+ oz metal geared servos and metal arms.


  20. #45

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    RE: Flutter

    ORIGINAL: Charlie P.

    One of my attempts at a counter-weight.* It worked - but caught on grass and was generally a nuisance.

    Ultimate solution was ball& socket comtrol linkages and a re-prop to a lower pitch/higher torque prop.

    [img][/img]

    [img][/img]

    [img][/img]
    Unless that weigth was some ultra dense material I'd doubt the CG of the moving system was anywhere near the hingeline or just in front of it.

    Mass balancing of a surface does two things (among many):
    (1) changes the frequency at which it flutters and as a result the airspeed at which flutter occurs
    Some motorcycle use lead weights in the handle bars if vibrations build up at cruising speeds to shift the natural frequency a bit so that they build up at speeds the bike will be expected to pass through quickly.


    (2) moves the CG of the moving system to the hingeline or very slighty ahead so that aeroelastic movements in the wing/stab/fin can't excite flutter in the moveable surface.
    Grab a bit of timber at one end and move it up and down and you'll need a strong wrist to holt it level as it tries to tilt up and down, grab it at the GC and move it up and down and it will stay close to level with much less effort.


  21. #46
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    RE: Flutter

    Generally when people talk about 'balanced control surfaces' it is not about mass balance, it is about aerodynamic force balance. A large, balanced stabilizer has some portion of the surface in front of the hinge line, such that when elevator is applied (say down elevator), the smaller portion that goes (up) in front of the hinge line is exposed to the flow path and takes some of the load off the actuator/servo.

    This is about rudders in marine vessels, but the point is the same:
    http://en.wikipedia.org/wiki/Balanced_rudder
    Joe Marri
    Enjoying all things aviation.

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    RE: Flutter

    Years ago, early 70's..many RC'rs flew a popular design called the UGLY STICK...The early STICKS had ailerons all the way out to both tips of their wing...and a few modelers that had a penchant to go Mach 2 with their hair on fire, they spent bigger bucks and installed one the new Schnerle ported fire breathing engines on their STICKS and shortly thereafter began experiencing FLUTTER..... Some of the RC club "wise guys" cautioned everyone to keep their aileron/elevator hinge lines tight to reduce FLUTTER.. That fix didn't entirely solve the problem and the Mach 2 guys still suffered from gross aileron/elevator vibrations on their STICKS.... Since I was a member of the local EAA chapter and had a modicum of aerodynamic learning & experience (engineering degrees sticking out of my ears and other orifices) I told them the FLUTTER problem was caused by TIP VORTICES or TIP RECIRCULATION and to cure the problem.. INSET or shorten the length of their ailerons to inside of the wingtip(s)...Several followed my advice and inset their ailerons and wouldn't ya know it.... VOILA'... The FLUTTER disappeared... But I still cautioned them, to assist in eliminating aedrodynamic FLUTTER...one must strive for a zero surface airleron/elevator gap to help cure the problem...then reduce the DOWNWARD movement of each aileron and increase the upward movement...referred to as AILERON THROW DIFFERENTIAL, Almost all RC modelers now own a computer radio which have the added ability to 'dial in' differential aileron deflection (ATV)..just takes a few moments to accomplish...

  23. #48
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    RE: Flutter

    Well, good thing it is windy and raining today sine I was ready to fly her.
    I'll certainly look at replacing those servoes.
    Thank you guys, there is a 1000 yrs of wisdom here
    Keep your wings level
    Club Saito Member #693

  24. #49
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    RE: Flutter

    I just took the LiFe battery out, it was puffed.

    I never knew they would do that. Have always heard they were tolerant of abuse. 

    Charlie, John B., Rodney et al, great info. The point behind Charlie's 'unaerodynamic' mass balance pic is to get as much weight ahead of the hinge line. We've also gotten rid of flutter on ailerons by tapering the last 10-20% of the aileron trailing edge. 
    I might not be very good, but I'm fun to watch!

  25. #50

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    RE: Flutter

    LiFe's are very sensitive to being overcharged or fully discharged. They are more physically tough than Lipos, but that's it. That said, I have a couple of puffed ones in planes I fly now. They both cycle fine and maintain voltage fine, so I've kept using them. I saw something a while back that said early Lipo puffing was caused by water vapor that got trapped inside the cell casing and didn't affect the battery's performance. I have to wonder if that's the same situation with LiFe's as well.
    No kid, I said break ground and fly into the wind!


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