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Thread: Can you Stop Castor Wheel Flutter?

  1. #21
    Senior Member TheAbleChef's Avatar
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    Quote Originally Posted by Wheelchair Designer View Post
    Glad to hear you're now flutter free! Be sure to pass the information on to other people who have caster flutter, now that you know all about it and how to prevent it.
    I will most definitely! I can`t believe how easy it was to fix. Even with a bearing that needs to be changed it is still flutter free!
    Never Give Up!

  2. #22
    i gotta say, as an aerospace engineer whose field is flutter, you are not experiencing flutter. it is a bearing problem. flutter is self-feeding and leads to destruction of the structure and cannot be stopped once excited. your problem is much simpler. and and apparently fixed.

    it is a misnomer calling it flutter. lay people do not understand the difference between divergence and flutter and simple vibration issues. i assure you, no wheelchair has a flutter problem.
    Last edited by cass; 04-09-2009 at 03:22 AM.

  3. #23
    Quote Originally Posted by cass View Post
    i gotta say, as an aerospace engineer whose field is flutter, you are not experiencing flutter. it is a bearing problem. flutter is self-feeding and leads to destruction of the structure and cannot be stopped once excited. your problem is much simpler. and and apparently fixed.

    it is a misnomer calling it flutter. lay people do not understand the difference between divergence and flutter and simple vibration issues. i assure you, no wheelchair has a flutter problem.
    Perhaps the better word is "wobble"?

    wise.

  4. #24
    cass,

    I totally disagree.

    However the important distinction here is that I used my knowledge to help someone, and you come along after the fact and try to use yours to incorrectly nit pick my semantics.

    I'm fine if you want to call caster flutter, wobble, shimmy, flitter, skittering, quavering, oscillation, flagellating, flapping, vibration, etc. However flutter is still an appropriate designation even in the sense we aerospace engineers use it.

    From Wikipedia on aeroelastic flutter: Flutter is a self-starting and potentially destructive vibration where... forces on an object couple with a structure's natural mode of vibration to produce rapid periodic motion. Flutter can occur in any object... under the conditions that a positive feedback occurs..

    All of that is true with regard to wheelchairs, and flutter is the most accurate engineering terminology that can be used to describe it. The entry also recommends damping as a cure for flutter, just like I did.

    flutter is also a medical and electronics term and would also be analogous by those definitions.

    Now let me take the time to refute you point by point:
    you are not experiencing flutter. it is a bearing problem.
    It is not a bearing problem it is a damping problem, which is why TheAbleChef was able to damp it out without replacing his bearings.

    flutter... leads to destruction of the structure and cannot be stopped once excited.
    Nonsense! Any aerospace engineer(who knows his field) will tell you that if you slow down below the incipient speed of flutter the flutter is no longer able to sustain itself and stops. I can't tell you how many rivets transonic flutter sheared off of the T-tail on the Citation X prototype I was working on while doing high mach testing and every single time they decelerated the aircraft to stop the flutter.

    lay people do not understand the difference between divergence and flutter and simple vibration issues
    Some self proclaimed aircraft engineers seem to be getting it wrong also.

    no wheelchair has a flutter problem.
    Google "caster flutter". The US Patent Office issues patents for flutter resistant casters. And trust me, as a wheelchair patent holder, Patent lawyers can be real sticklers for correct semantics.

    Here is an excerpt from the engineering study I cited in an earlier post.
    Caster wheel shimmy can be
    experienced in everyday equipment, such as wheelchairs,
    grocery carts, gurneys, teacarts, and the like, and is universally
    recognized . Self-excited vibration is characterized
    by vibration that is produced by the motion of the system (e .g., wheelchair speed) itself. The flutter due to
    the motion of turbine blades or aircraft wings is a good
    example of this instability . In addition, machine tool chatter,
    internal flow-induced vibration of piping, and cross
    flow-induced vibration of wires and structures are treated
    under this topic in modern vibration texts
    Conclusion:
    Caster Flutter is the most correct description. Wobble is defined by webster as:
    to move or proceed with an irregular rocking or staggering motion or unsteadily and clumsily from side to side
    Flutter is neither irregular or unsteady, but is a periodic motion characterized by regular sinusoidal angular acceleration like a pendulum or metronome. Now you will note that I also responded to a thread called "wheel wobble" and never stooped to bitching about peoples semantics even though I tend to be anal about it. It is commonly accepted on the web that if you pick on minor semantics or spelling or grammar issues when the meaning is quite clear then you are just "trolling"(picking a fight just for something to do). I however while I prefer correct terminology am not going to nit pick peoples posts unless they try to get into it with me, and then I'm happy to defend myself. I'm fine if Dr. Wise wants to call it wobble to appease cass's erroneous opinion. I'm smart enough to read between the lines and know we're all talking about the same thing and semantics are less important than whether you're helping someone, or just arguing to be a smug contrarian(even when your wrong and clueless about it). So, I'll keep correctly referring to it as caster flutter, and others can call it what they want, and cass can keep griping about it. Please don't insult me in my field of expertise with your ignorant personal contrivances while I'm helping, as this forum was intended, to provide people with real world applicable knowledge and solutions.

  5. #25
    well, the field of flutter has always been a bit of a black box with aero engineers debating it to this day. so, call it what you want.

    btw, ever been on a flight test airplane that experienced actual flutter and stopped it by slowing down? you said slow down before the flutter speed is attained. that is why a 15% margin is used in the envelope. and yes, i agree, flutter can be experienced in smaller components without the destruction of the entire structure. but if you hit flutter speed on a major airfoil, such as a wing or even an aileron, it is going to rip off. never heard of a wheelchair wheel doing that. so, i dispute the term. flutter is a coupling between aerodynamics and the natural vibration mode of a structure. i don't think a wheelchair wheel is subjected to enough aerodynamic force to truly be termed flutter. but, my opinion.

    i think it more appropriate to compare it to landing gear shimmy. that is not flutter in the generally accepted sense.

    have you seen flutter in an aeroelastic model in the wind tunnel? it happens so fast, well, frankly you won't see it except on the slowed down videotape. one second the model is there, the next it is down the tunnel.

    but, no point in debating. flutter engineers still debate tacoma narrows amongst themselves.
    Last edited by cass; 04-09-2009 at 10:14 PM.

  6. #26
    cass,
    As a pilot I've felt "buzz". I backed off the airspeed of course.

    I understand you're associating flutter with aerodynamic forces, but the term can apply to fishing lures and casters also which have their oscillation powered by other fluids or even contact with solid ground. Flutter can also describe things which happen slowly and visibly. There is no speed limit on the word. I have not seen an engineering definition of the word flutter that says it can only apply to aerodynamically induced flapping. The US Patent Office should not have issued a patent for flutter resistant casters if it is an obvious misnomer. I think your personal definition of flutter is far less inclusive than the ones in the dictionaries and common engineering usage.

  7. #27
    i don't want to argue. it's not my personal definition. i got my pilot's license in 1979 and my aero engineering degree in 81. did wind tunnel, GVT and flutter flight test on B2 and since on Boeing commercial. flutter is defined in engineering terms as aerodynamic coupling with the natural mode of a structure. the aerodynamics is the excitation. in GVT (Ground Vibration Test), we excite the structure by different means (inputting measured vibrations to excite the structure). i've done this by a few different means; one of the least precise was strings attached to the B2 aeroelastic wind tunnel model that i (or another engineer) yanked on when tunnel velocity approached our CAD modeled (finite element) flutter speed.

    you are completely correct in disputing the term flutter as us flutter engineers like to argue it, too. however, i have spent my engineering career calculating flutter speeds on aircraft. yes, it is a definite velocity, depending on the structure. first you build a mathematical finite element model using a 3D program such as NASTRAN with 6 degrees of freedom elements. then the wind tunnel model followed by fine tuning. by GVT, it better match as GVT is performed on a real FOM (first of model) airplane, off its gear by soft supports to eliminate landing gear coupling.

    btw, i do think you are mixing up structural dynamics and flutter.

    i meant no disrespect. but i'm still sticking to what i said
    Last edited by cass; 04-10-2009 at 02:32 AM.

  8. #28
    cass,
    And I'll stick by my proper use of the widely recognized and government approved engineering term "caster flutter". Since you have still provided no contrary definition precluding it.

    Did you get to read the study I referenced? http://www.rehab.research.va.gov/jour/00/37/3/kauzl.htm
    It is helpful.

    I like your Avatar. Is that you and your horse?

    The important thing is to let people know how caster flutter can easily be damped out so nobody should have to live with it. People need to know that it is not caused by some minor error in their caster geometry, but that it is inherent to casters and the system needs to be properly damped so that the incipient flutter speed is going to be higher than any speed they will end up going in their wheelchair.

  9. #29
    well, if you really want to see something cited, you'll find this interesting, i think. this is what i've spent 28 yrs doing, give or take. i tested at Langley, AEDC, Northrop, Boeing and EAFB.

    http://www.cs.wright.edu/~jslater/SD...ter_banner.pdf

    btw, that source you cited calls it shimmy, which i also called it a couple of posts ago.

    if you want to continue this discussion, we could take it to pm because i really doubt most ppl here give a damn what we call it!

    the avatar is Viggo Mortensen in Hidalgo. i'm not a cowboy or male.
    Last edited by cass; 04-11-2009 at 04:59 AM.

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