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Thread: Not mass balanced & cable failure

  1. #11

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    If you are the same guys asking about flutter over in the RV world, you have probably gotten the impression that this is somewhat of a black art. Flight test programs spend a lot of time proving that the airplane is flutter free in the expected operating envelope of the aircraft, and a little more. The challenge is that creating a mathematical model that accurately describes all of the mechanical characteristics of a wing structure is time consuming and expensive. So the engineers are conservative with their design safety margins.

    I will note that when a control surface is in trail and its actuating linkage is disconnected, it has much less leverage to put force into the structure that it is connected to. That is, and aileron is connectect to the wing by the hinges and moves about the hinge line. The mechanical mechanism by way it puts force into the wing structure includes the control linkages, the elasticity of those control linkages, and the points in the wing that anchor them. So in an airplane that uses control cables, those cables must sized and be tensioned do that they do not function as springs that help the aileron resonate against the airflow. The distance between the aileron hinge and the attachment of the cable or control arm form a lever arm that translates the forces into the control linkage and thence into the wing structure. The stiffness of each of those items contribute to the movement, spring response, and possible resonant frequencies of the whole assembly.

    There is a lot more to the topic than can be covered in a web post.

    Hope this makes sense,

    Wes
    N78PS

  2. #12

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    Quote Originally Posted by WLIU View Post
    You should find that if you remove an aileron from the aircraft that do not have aileron balance weights listed as a discrete part, that the aileron assembly is at least 80% balanced.
    I do not know what to say, I would not be so sure about that, I hope you had a reliable information source for that, if they are so, half of my uncertainties would be eliminated. What I found is that mass-balanced might be not even required for some aircrafts, however, that's for normal operation where different parts have an important role on system stiffnes and here comes my issue with disconnected controls.

    Quote Originally Posted by WLIU View Post
    My memory is faulty but I recall that 100% balance is not required to avoid flutter in low performance aircraft.
    I think you are right, reading a lot about flutter and damping techniques, I also found this.

    Quote Originally Posted by WLIU View Post
    Control surfaces are light enough that they will only droop a little against the air flow.
    Actually they will deflect upwards as you slow down (due to high alpha).


    Quote Originally Posted by WLIU View Post
    At this moment I do not recall ever hearing of an aileron control system failure.
    Certainly there were. I don't have right now some links, but sure there are occurences reports on the internet.

  3. #13

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    Without your doing a lot more digging you will not get a definitive answer to your question. You will need to look at data for a lot of airplanes.

    I will suggest that your suggestion that "they will deflect upwards as you slow down (due to high alpha)" needs a better visualization of airflow vs gravity. Both of our statements are correct.

    We do not read about very many crashes any more due to aircraft failures, the vast majority are pilot failures.

    So if your gap in this knowledge area is preventing you from flying, I suggest that you should move on and go fly. If this is an academic exercise, you can work it into a Phd thesis if you have the time and interest. Along the way you might get good at Finite Element Analysis and vibration analysis.

    I am at the stage where I like to fly more than do math.

    Best of luck,

    Wes
    N78PS

  4. #14

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    Quote Originally Posted by WLIU View Post
    If you are the same guys asking about flutter over in the RV world, you have probably gotten the impression that this is somewhat of a black art.
    I'm sorry, RV world? It says nothing to me.

    Quote Originally Posted by WLIU View Post
    I will note that when a control surface is in trail and its actuating linkage is disconnected, it has much less leverage to put force into the structure that it is connected to. That is, and aileron is connectect to the wing by the hinges and moves about the hinge line. The mechanical mechanism by way it puts force into the wing structure includes the control linkages, the elasticity of those control linkages, and the points in the wing that anchor them. So in an airplane that uses control cables, those cables must sized and be tensioned do that they do not function as springs that help the aileron resonate against the airflow. The distance between the aileron hinge and the attachment of the cable or control arm form a lever arm that translates the forces into the control linkage and thence into the wing structure. The stiffness of each of those items contribute to the movement, spring response, and possible resonant frequencies of the whole assembly.
    Basically, you say that sloppy cables might are more prone to flutter than broken ones, right? I think that was the point, I hope I didn't misunderstood.
    Last edited by Steve Jeff; 01-04-2013 at 02:38 PM.

  5. #15

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    Quote Originally Posted by WLIU View Post

    We do not read about very many crashes any more due to aircraft failures, the vast majority are pilot failures.

    So if your gap in this knowledge area is preventing you from flying, I suggest that you should move on and go fly. If this is an academic exercise, you can work it into a Phd thesis if you have the time and interest.
    As I said in my first post, I'm a LSA, PPL pilot, heading towards my CPL. Flight control malfunction are not usually adressed during training, so I think it's not such a bad thing to figure out a bit about some things before you might encounter them in the air, of course if you ever meet them, fortunately statistics show that chances are small.

    P.S. Math was never my passion

    Edit:
    Quote Originally Posted by WLIU View Post
    Without your doing a lot more digging you will not get a definitive answer to your question. You will need to look at data for a lot of airplanes.
    I agree, I would do it, but don't know exactly how. I already dug over the internet and have not found much, maybe because I did not know how to.
    Last edited by Steve Jeff; 01-04-2013 at 02:46 PM.

  6. #16
    steveinindy's Avatar
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    So, how do you figure out those accidents where a disconnected surface triggered flutter?
    I can answer this one at least to a certain degree. Normally in that situation, you're going to find the surface detached from the control cables. Now this normally happens in a lot of crashes due to overload. When that happens, you get a characteristic "broom straw" look. In a case where something that failed prior to impact, you will often have a different look to the control cables. That would be the most obvious finding that would point to a disconnected surface. However, it is rather rare for a control cable to separate in a well-designed and maintained aircraft. I have seen it once in wreckage that I examined and that was in a fatal ultralight crash.

    P.S. Math was never my passion
    I'm in the same boat....aerodynamics when it comes to the more advanced math makes blood want to shoot out of my nose. LOL
    Unfortunately in science what you believe is irrelevant.

    "I'm an old-fashioned Southern Gentleman. Which means I can be a cast-iron son-of-a-***** when I want to be."- Robert A. Heinlein.



  7. #17

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    Quote Originally Posted by Steve Jeff View Post
    So, how do you figure out those accidents where a disconnected surface triggered flutter?
    Did/does a disconnected surface "trigger" flutter? If the surface goes into flutter upon disconnection wouldn't it make sense you were already operating in a flutter condition-which is probably what caused the control failure? I.E. Galloping Ghost.


    It is known that at LSA speeds flutter might be less of an issue than at higher speeds, but what you said sounds like we are immune to flutter at LSA speeds and that's really not true. I would really appreciate to detail your statements a bit.
    Not immune, but if we operate within our allowable speeds flutter usually is not a problem. It's when we excede these speeds that problems develop. Can I get my ailerons to flutter by ripping the cables loose during flight? I'd say no if I'm under Vne. Would they move/flop around? Absolutely. But flutter is far more violent than "movement". I think his original post is refering to the potential of normal flight loads causing control cable failure and then subsequent flutter. I assert that if the suface were to flutter after a cable failure it was probably fluttering to begin with.

  8. #18

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    Since the original poster is a LSA student pilot, I will offer the advice that one skill that a pilot must learn is prioritizing tasks and the addressing of issues. If the airplane is on fire, you don't worry that the landing gear will not extend.

    When you are looking at learning to be an aviator, I will suggest that you focus on the skills required to earn your certificate and the problems you will most likely encounter in your first 100 hrs of flying. Worrying about having a control system break should go way down on the list. You will be a LOT more likely to make a mistake managing your fuel or navigating cross-country. I know LOTS of pilots who have had learning experiences underestimating their fuel supply and becoming surrounded by lostness on a flight away from their home base.

    There are no questions about control system malfunctions on the FAA written exams but there are lots of questions about weight and balance, weather, and regulations.

    If you put the energy into learning about weather and the fuel system of the airplane that you fly, that you are putting into this topic, getting certified won't be a problem.

    Best of luck,

    Wes
    N78PS

  9. #19
    David Pavlich's Avatar
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    I remember several years ago, a local traffic reporter was giving a report when he suddenly stopped and told the morning talk show host that he was setting up for an emergency landing on a levee. This is New Orleans, by the way. Well, it turned out this verteran pilot with thousands of hours in the left seat ran out of fuel. He made a safe landing and with the help of the levee police, made a successful takeoff. Mistakes don't care if you're a student or a grisled veteran.

    David

  10. #20

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    Quote Originally Posted by pacerpilot View Post
    Can I get my ailerons to flutter by ripping the cables loose during flight? I'd say no if I'm under Vne.
    I would say yes, even if you are well below Vne. Also, there were cases when the trim tab became disconnected due to cable failure and it fluttered violently.

    Quote Originally Posted by pacerpilot View Post
    But flutter is far more violent than "movement". I think his original post is refering to the potential of normal flight loads causing control cable failure and then subsequent flutter. I assert that if the suface were to flutter after a cable failure it was probably fluttering to begin with.
    Right. And not necessary due to normal loads, but also due to undetected fatigue and other causes. Since there were few cases when even the airframe developed undetected fatigue, I wouldn't be so sure about cables which are less analyzed for this problem.

    Quote Originally Posted by WLIU View Post
    Since the original poster is a LSA student pilot, I will offer the advice that one skill that a pilot must learn is prioritizing tasks and the addressing of issues.
    Actually I'm not a LSA student, read my first post, however, if you want to call me so, no problem.

    Quote Originally Posted by WLIU View Post
    If the airplane is on fire, you don't worry that the landing gear will not extend.
    Right. I like to keep your idea with a more appropriate analogy. If you start descent and your mixture lever locks toward lean position, I guess you wonder your engine will fail. And to be honest, fortunately, an off-airport landing is more likely to be survivable than an inflight breakup...
    Last edited by Steve Jeff; 01-05-2013 at 02:32 AM.

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