Static/mass balanced or not

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David36

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Primary control surfaces require 1/8" minimum, 7 x 19 galvanized cable. It has a breaking strength of 2000 pounds and a rated working load of 400 pounds. When fittings are attached, it is tested to 60 percent of breaking strength, 1200 pounds, for a minimum of three minutes, far beyond what it will experience in the airplane. 1200 pounds on the cable once installed would crush pulleys and buckle their brackets and and tear out control surface bellcranks and probably do extensive damage to the rest of the airframe structure.

Dan
Unlike FAR 23, there are no requirements for cable specifications on LSA aircrafts, so they there is no warranty they are really so strong as you said.

Worrying about properly designed systems and their cables will lead to worrying about every other little thing in the airplane and you will never fly it.
LATER EDIT: I can't recall any other mechanical failure which can lead to inflight breakup, of course, excepting itself structural failure and I won't make it an issue, there are a lot of founded reasons for that: they are strong enough structurally, there are specific strength standards regarding this issue, also, I can't recall any structural failure crashes where the causal factors were others than pushing the envelope, initial type flight tests, poorly structural designed types with obvious and numerous crashes due to that, or very high carelessness maintenance. Unfortunately, can't say the same for cable failures... if it would be so, I would not wonder about it.

And even if there are any structural failures, they are very isolated cases. The airframe structure undergoes more thorough inspections to detect early any structural failure and generally it's given much more attention to structural fatigue and other problems and that makes the structure less prone to failure, because any problem it is earlier detected. It's normal and obvious that the structure is more closely analyzed than cables to detect these problems.

LATER EDIT: And maybe that's all we can do from a practical standpoint to detect fatigue and other structural failure issues, it can't be 100% virtually safe, but regarding this thread issue, if not mass-balanced surfaces choice would be likely to flutter, it seems it's just unnecessary risk taking for an issue with catastrophic consequences. You got my point?
 
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Dana

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As I said, I just can't rely on this. I’m really not satisfied relying on cables since there are old pilots who had even more than one occurence regarding control cable malfunction over years.
What kind of malfunction? Jammed controls, or cable breakage? I have had brake cable failures, trim tab cable jam, but never a failure of primary flight controls. I did find some frayed strands on a rudder cable during an inspection, but it never actually failed.

For example? Like what? Please give some examples to figure out better what do you mean.
Pilot error, engine failure, assembly error of any critical component, load shifting...

-Dana

The speed of light is greater than the speed of sound. That's why some people seem very bright until you hear them speak.
 

David36

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What kind of malfunction? Jammed controls, or cable breakage?

-Dana
All kinds of malfunction, but when I said I referred to cable failures.

LATER EDIT:

Pilot error, engine failure, assembly error of any critical component, load shifting...

-Dana
I agree, it is obvious that pilot error and engine failure are some of the most common crash causes. We were basically talking about mechanical issues and I think an engine failure wouldn't lead to an inflight breakup.
Assembly error of any critical component? Maintenance error, but what are flight tests for after high maintenance tasks? Load shifting? You mean unsecured loads in cabin? That's gross pilot error due to pure carelessness.

No one is immune to pilot error and I do not want to get more deep into details about the reasons for engine failure leading to fatalities, I think you know what I mean. The airplane flies very good with the engine stopped just as the glider was doing when I took my first flying lessons. The L/D ratio is lower, that's what you have to pay attention for. All you need is some free field on ground if you are too low. If you don't have that, you are in big trouble. However, you know these aspects, don't want to detail them anymore, I figure out there is a gross difference between loosing a wing vs loosing engine power. For the first one, I simply wouldn't fly if I would know the plane is prone to such a failure.
 
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Head in the clouds

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Hi David,

I still think you sound like an investigator looking into some incident/accident or whatever, not that it matters but your line of questioning seems rather too subject specific, to me...

I'm also wondering if you've had a quick change of name and domicile. All of a sudden you seem to be SteveJeff and apparently living in Romania and asking the same question on another forum...? Not that that matters either.

The thing is, I think you're still missing the point -

First you may not have differentiated between LSA aircraft and homebuilt aircraft, and experimental aircraft and experimental ultralight aircraft. There are some differences around the world, so I will only refer to the Australian rules/varieties. LSAs cannot be built at home from kits or plans and they do have a great deal more testing required of them than any homebuilt/experimental aircraft. They do need to have been fully flutter tested and they either have mass balances to take the speed of flutter onset higher than the Vne or the Vne is set according to the flutter onset speed. I think Vne is set at 10% less than flutter onset speed in that case.

The reason that LSAs do have to complete a full test regime, including sandbagging, stability tests etc is that they can be used for commercial purposes i.e. flight training. LSAs in Australia can be identified by their registration prefix which is 25-xxxx.

All kit, plan or scratch built aircraft in the same weight limit (or a bit less for single seaters) are experimental ultralight aircraft and have registration 19-xxxx.

All kit, plan or scratch built aircraft that do not fit those weight limits are homebuilt GA aircraft and must be proven/tested designs or be shown to comply with the appropriate legislation and have Australian ICAO registration VH-xxx.

So, kit built CH-601s for example are not LSA aircraft they are ultralight experimental aircraft and placarded to that effect i.e. 'fly in me at your own risk etc'. The legislation has been put in place to prevent litigation in event of mishap, since they are experimental and/or may not be built to any particular standard.



I have had a fully developed case of aileron flutter on one of my own designed aircraft, you can read about it here - https://www.homebuiltairplanes.com/forums/aircraft-design-aerodynamics-new-technology/12360-rudder-design-3.html#post129339 If you also read my posts nearby on the same thread, including my erroneous one about stepping on both pedals to stop flutter of the rudder, and also read several of Aircar's comments before and after mine you will discover something else that you have missed. Flutter is not a function of the control cables being connected or not, in fact it's quite possible that the ailerons would flutter with the cables connected whereas they might not if they were disconnected because the spring-rate of the cables affects the resonant rate of the whole system, so for example, if the ailerons themselves were heavy, and connected, they might want to flutter at speeds below flying speed and then be above their whole system flutter speed by take-off speed.

Also, you seem to assume that if the cables became disconnected and the ailerons weren't balanced (and aerodynamic balance is another matter not yet touched on, and quite as effective as mass, in changing the flutter speed/freq) then they would flutter and the wing fall off. But in fact you need to be flying at or close above the flutter speed for that to happen. And if that was by chance the case then simply slow down until they stop fluttering. There can be considerable hysteresis so you might need to slow right down to the stall but it will stop at the stall due to the detached flow removing the excitement from the upper side. Once it has stopped you can fly at any speed below the flutter speed and flutter begins sporadically (see the video I posted earlier) so you can find the bottom end of the flutter speed quite easily. So - flutter is not necessarily catastrophic as long as you recognize it early and act appropriately.

Another thing, if the ailerons were going to flutter you would discover it while the cables were connected because they would shake the stick as they reached their flutter speed and also would flutter at another speed when they reached the speed at which they were in harmony with the springiness (and there is always springiness) in the system.

Additionally, during a flutter incident it can be stopped almost instantly simply by applying a load to the surface(s). In the case of the rudder I was wrong, pressing both pedals might not stop it, but pressing one of them and flying a little slipped while slowing down, would do so. Similar with the ailerons, slip the plane while slowing and they will stop fluttering immediately. If the aileron cable system had failed then just apply rudder one way and then the other, while slowing down, in any case a firm pull-up to slow down should stop the flutter due to changing airflow over the wings. Deploy a little flap to stop flaps fluttering if they had system play as AR pointed out.

So, in addition to the extreme unlikelihood of inflight primary control system failure (although I do know one man whose elevator cable broke and he did not even have a trim control but got it down OK using throttle for pitch control) if you learn your emergency procedures, and practice whichever of them are possible to practice then there are other matters that are far more likely to bring you undone than the one particularly remote possibility you seem to be focusing on.

From my own brush with flutter I discovered one major issue which would contribute to, or cause, catastrophic structural failure resulting from flutter. Flutter is very noisy and frightening and quite violent, you can't even see properly because your eyes are being vibrated. If you haven't pre-thought what to do in event of flutter it is very unlikely that you would do the correct action which is to slow down of course. The first thought when hell breaks loose is to get the machine on the ground and so the tendency, even though you would probably reduce power, is to lower the nose to get down when in fact you must raise the nose lots so as to slow down quickly. In that sense spins and flutter are similar in the manner of their danger i.e. the correct thing to do is not intuitive. Unfortunately you cannot practice flutter so you need to think about it, imagine it and drill the correct response into yourself. As for spins - go and practice them.
 

David36

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Hi David,

I still think you sound like an investigator looking into some incident/accident or whatever, not that it matters but your line of questioning seems rather too subject specific, to me...
Yes, investigating the consequences might have a control cable failure in some of the airplanes I use to fly. That's all. I've no specific interest in Zodiac 601 crashes or others, if that's the point.

I'm also wondering if you've had a quick change of name and domicile. All of a sudden you seem to be SteveJeff and apparently living in Romania and asking the same question on another forum...? Not that that matters either.
You made me laugh. Anyway, even if it doesn't matter, I must say that I'm living in neither the UK nor in Romania, however my homeland is UK. At the moment, I'm living in Republic of Moldova and I think it was more like an autodetected location, maybe due to my ISP having datacenters in Romania or a random choosen country from the list when I registered, I don't know, I don't pay too much attention at this things and I won't start to make an investigation on this absurd issue. Also, it would be absurd to ask me also why my IP is also changing and thus my location being automatically changed from time to time, having a dynamic IP. Also, the same for nickname, being an internet forum, there is no problem using a nickname other than my real name, anyway, my name is David, not SteveJeff, but I would like to call me so.:gig: But as you said, I think this has nothing to do with this topic issue. Sorry being offtopic, but I felt it was necessary to clarify it.

All kit, plan or scratch built aircraft that do not fit those weight limits are homebuilt GA aircraft and must be proven/tested designs or be shown to comply with the appropriate legislation and have Australian ICAO registration VH-xxx.
Just curiosity, what is the appropiate legislation for it? I figure out they shouldn't comply FAR 23 standards, right?

So, kit built CH-601s for example are not LSA aircraft they are ultralight experimental aircraft and placarded to that effect i.e. 'fly in me at your own risk etc'.
Were all those 601 crashes involved kit builts, not factory-built models? Do they differ in from a design point of view or they are the same just under different regulation? Again, just pure curiosity, if you don't want to talk anymore about this, please don't answer it.

Also, you seem to assume that if the cables became disconnected and the ailerons weren't balanced (and aerodynamic balance is another matter not yet touched on, and quite as effective as mass, in changing the flutter speed/freq) then they would flutter and the wing fall off.
Not necessary, but I found a lot of sources saying that is more likely to flutter being so.

But in fact you need to be flying at or close above the flutter speed for that to happen.
That's simply not true, not all accidents involved pushing the envelope. If so, there would be simply pilot error, not a flutter issue.

And if that was by chance the case then simply slow down until they stop fluttering. There can be considerable hysteresis so you might need to slow right down to the stall but it will stop at the stall due to the detached flow removing the excitement from the upper side. Once it has stopped you can fly at any speed below the flutter speed and flutter begins sporadically (see the video I posted earlier) so you can find the bottom end of the flutter speed quite easily. So - flutter is not necessarily catastrophic as long as you recognize it early and act appropriately.
Already read all those techniques, if it were that easy I guarantee I wouldn't bother about it. You described how violent it is. "Flutter is very noisy and frightening and quite violent, you can't even see properly because your eyes are being vibrated." That says it all. These techniques may help somehow, useful to know them, but it's really hard to think fighting the airplane breaking inflight.

Some of you have insisted more on my worries than on my technical question. Why not call a spade a spade and speak plainly, yes it is likely to flutter, no it is not or we can’t predict that, we know cables fail very rare, however it would be better to have mass-balanced surfaces to prevent flutter in a free-floating situation considering that most writings on the subject advise this. Note: Strictly technical, it was just an assumption on the surface behaviour in that situation, because as I said, my understanding is somehow confusing and that’s why I asked about it.

EDIT: Excuse me to say, but it's really inappropriate and awkward what you did on the other forum before having enough information.
 
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Head in the clouds

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Just curiosity, what is the appropiate legislation for it? I figure out they shouldn't comply FAR 23 standards, right?
For the benefit of other readers this referred to what were/are the required standards for approval of homebuilt GA aircraft in Oz. The answer is a can of worms unfortunately and that is what held up aircraft development in the private sector in Australia for around 50 years since Oz doesn't have its own standard anyone wanting to get a type approval had to get it approved under any acceptable overseas standard, be it FAR, BCAR, JAR etc etc.



Were all those 601 crashes involved kit builts, not factory-built models? Do they differ in from a design point of view or they are the same just under different regulation? Again, just pure curiosity, if you don't want to talk anymore about this, please don't answer it.
I don't know the answer here because I thought all 601s were kit built, did the factory also sell them factory-built? We don't always see all models of planes here because of the freight difficulties with Australia being so far from everywhere else. I've never seen a flying school here operating any CH types and there probably would be if there were any registered in 25 category.



I said - But in fact you need to be flying at or close above the flutter speed for that to happen.

You answered - That's simply not true, not all accidents involved pushing the envelope. If so, there would be simply pilot error, not a flutter issue.
For other reader benefit - I was referring to whether flutter would automatically happen at the moment the control cables disconnected... This is where you still don't get it David. If you were flying at a speed above or below the flutter excitation speed they would not flutter - watch that video I posted, you will see the car accelerating and as the speed increases it eventually reaches a speed where the aileron flutters a bit and then more and then sufficiently to break the wing. So the only way the aileron would flutter would be if you were exactly at, or a little above the flutter speed. If you were more than a little above the flutter speed it would not flutter and generally it will not flutter as you slow down unless you slow down slowly if you follow me.

So the point is, it doesn't have to do with "pushing the envelope" or "pilot error", we're not talking about exceeding Vne here, we're talking about exceeding the flutter speed. And whilst the flutter speed should be above the Vne we're exploring the area of your fears here i.e. that the disconnected flutter speed might be below the Vne.


Already read all those techniques, if it were that easy I guarantee I wouldn't bother about it. You described how violent it is. "Flutter is very noisy and frightening and quite violent, you can't even see properly because your eyes are being vibrated." That says it all. These techniques may help somehow, useful to know them, but it's really hard to think fighting the airplane breaking inflight.
Here you are responding to my explanation of the ways to stop aileron flutter when the flutter has already started. David, consider other adventure sports - snow skiing, scuba diving, skydiving, mountain climbing. There is no way to practice for an avalanche right? But you learn techniques for survival, you traverse the slope, you take cover, whatever, I don't ski much so I don't know all the procedures. There are many things that can go wrong underwater but you can't practice some of them so you learn what the symptoms will be and you learn what to do if you recognise those symptoms. You don't practice main-chute failure but if it happens you have already learnt what to do and have gone over and over it in your mind so when/if it happens you check to be sure it cannot be recovered, then you cutaway, check again that it is gone, and deploy your reserve. You don't practice falling off a mountain, or a piton failure, or a descendeur failure but you have already learnt the procedure to avoid it becoming a catastrophe. And you don't practice flutter but you already know that if it happens just pull back on the stick a bit, reduce power - it's not that hard, as long as you know what to do.

Maybe flying isn't for you if you must obsess over something because perhaps when anything at all goes wrong you will assume it is flutter?

Some of you have insisted more on my worries than on my technical question. Why not call a spade a spade and speak plainly, yes it is likely to flutter, no it is not or we can’t predict that, we know cables fail very rare, however it would be better to have mass-balanced surfaces to prevent flutter in a free-floating situation considering that most writings on the subject advise this. Note: Strictly technical, it was just an assumption on the surface behaviour in that situation, because as I said, my understanding is somehow confusing and that’s why I asked about it.
The fact is that it is not likely to flutter, if it was going to flutter it would have been trying to flutter while it was connected and you would have felt that through the stick. The connected controls will not stop it fluttering slightly, and I did already tell you that, so a spade was a spade, and very plainly spoken as well... ;)

And - unless an amateur designer is very well versed in the calculations and/or has access to full-size testing facilities which is unlikely then he/she would, of course, be wise to apply 100% mass balancing to ailerons at the least and should consider aerodynamic balancing where practical. The plane I am building will have 100% mass balanced ailerons fitted to a tip horn which will also act as partial aerodynamic balance. This is partly because I will be using push-pull cables and they do have some lost motion in their action. My tailfeathers will also be balanced both ways but that is because I do not have the facilities to test or sufficient knowledge to calculate that the plane's envelope is outside of the flutter zones.

EDIT: Excuse me to say, but it's really inappropriate and awkward what you did on the other forum before having enough information.
No, it was not inappropriate at all. As you did here you came to the forum with the appearance of a complete novice and junior pilot asking an innocent question and giving the appearance of someone with no knowledge of the subject at all whereas you had already gained much from information supplied here. Not only that, within a post or two you were able to quote lengthy and technically involved passages from various Regulations. You have been asked several times what your underlying reason for these questions is and you have avoided saying anything very much at all, so how can I be expected to 'have enough information' if you won't supply it?

I didn't do anything other than alert folks there to the fact that you know more than you are letting on and that I believe you have a hidden agenda, I've already said that to you here. It is a requirement of use of the (that) forum, that members report any such issues, to be considered by the moderators. You have correctly objected to my post and so it is not visible to any other members until the moderators consider what I said, and your objection to it.

If you were a bit more open about your real purpose I think more folks would be a lot more helpful, but since you have been given the answers to questions you asked, several times, some of us imagine there is more to your inquiry than meets the eye.

One last thing - from your copious quoting of the various Regulations and airworthiness Standards it would appear that you might think that if the plane is built according to those requirements then it won't flutter? Wrong - there are still maintenance and other matters that could have you end up in a situation with flutter. As Percy (I think it was) mentioned earlier, the weights on C172 types are well known, not only for coming loose, but also for falling off inflight. You see lead and aluminum just don't like each other when a bit of damp is around and the aly rivets that secure the weights corrode in the middle of their length. So each end of the rivets look fine on visual inspection but there comes the time when the middle separates and the weights just fall off. So, then fly at the right speed and you now have to deal with flutter. It's better to know what to do than to believe you're in a plane that can't flutter...
 

David36

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I don't know the answer here because I thought all 601s were kit built, did the factory also sell them factory-built? We don't always see all models of planes here because of the freight difficulties with Australia being so far from everywhere else. I've never seen a flying school here operating any CH types and there probably would be if there were any registered in 25 category.
Even it doesn't make the main subject, I found some of those crashed were factory-built, some were kits.

For other reader benefit - I was referring to whether flutter would automatically happen at the moment the control cables disconnected... This is where you still don't get it David. If you were flying at a speed above or below the flutter excitation speed they would not flutter - watch that video I posted, you will see the car accelerating and as the speed increases it eventually reaches a speed where the aileron flutters a bit and then more and then sufficiently to break the wing. So the only way the aileron would flutter would be if you were exactly at, or a little above the flutter speed. If you were more than a little above the flutter speed it would not flutter and generally it will not flutter as you slow down unless you slow down slowly if you follow me.

So the point is, it doesn't have to do with "pushing the envelope" or "pilot error", we're not talking about exceeding Vne here, we're talking about exceeding the flutter speed. And whilst the flutter speed should be above the Vne we're exploring the area of your fears here i.e. that the disconnected flutter speed might be below the Vne.
Got it, this post was entirely close of what I was expecting since my first question.

Considering we were talking generally about flutter in this paragraph, I have some questions, I hope to make a better idea of how flutter really works.

I thought that, for example, the aileron become disconnected, let's assume it would go up, that up aileron will cause a down force on the wing, and it will bend down, overshoot, then spring back up, the air will be forcing the aileron back down, but it will overshoot too, down, and so push the wing up. If this happens just as the wing was already springing up, it will push the wing higher, leaving the aileron behind and the whole problem repeats, with the wing bending more and more with each oscillation. Consider this happens very quick and in seconds you'll be wingless and hopeless.:grin:

Now, from your post, I understand that some airspeed values can dump these oscillations, but I know that the flutter condition is wing bending frequency and the aileron flapping frequency must be the same. The question is when they coincide? Anytime but they are damped in speeds below flutter speed?
In normal operation when the flutter speed is above Vne, if you are flying above that I figure out that the low capability of airspeed to dump oscillations is what triggers flutter, but I can't explain when, why and how the frequencies coincide there, if they do that then.

I know it is a very complex theoretical subject, but maybe in a simple summary, understood by all, I would be able to understand the general idea of how it happens. If it's too complex to explain it like that, I'd settle for that instead of having a wrong idea about what actually is an intricate subject like this.

The fact is that it is not likely to flutter, if it was going to flutter it would have been trying to flutter while it was connected and you would have felt that through the stick.
If so, that doesn't explain why a slopped cable might trigger flutter, even in flaps which are not so prone to flutter.

One last thing - from your copious quoting of the various Regulations and airworthiness Standards it would appear that you might think that if the plane is built according to those requirements then it won't flutter? Wrong - there are still maintenance and other matters that could have you end up in a situation with flutter. As Percy (I think it was) mentioned earlier, the weights on C172 types are well known, not only for coming loose, but also for falling off inflight. You see lead and aluminum just don't like each other when a bit of damp is around and the aly rivets that secure the weights corrode in the middle of their length. So each end of the rivets look fine on visual inspection but there comes the time when the middle separates and the weights just fall off. So, then fly at the right speed and you now have to deal with flutter. It's better to know what to do than to believe you're in a plane that can't flutter...
I know that in some particular cases even a mass-balanced surface might flutter because there are situations when the balance may be affected by some extremely abnormal factors. I read a crash when some water froze in the aileron and that unbalanced it, so it fluttered violently leading to inflight breakup. I don't worry about this, because it's a very particular case, if so, I would really be paranoid as someone here said in the first posts, that is a very particular case, maybe even the only one occurence having that causal factor.

LATER EDIT: Also, how it differs when we talk about wing or aileron flutter and other surface flutter, like rudder. Are they the same? It seems that in aileron flutter, a significant factor is the fact the wing bends due to aerodynamic forces being changed by aileron deflection.

Also, about recovery technique. Basically, you are becoming somehow a test pilot in that situation. I found the recovery techique in some flight test tips articles.

Forgot to ask, would it be appropiate a high G pull up (of course, not above aircraft G limit) to stop flutter or that may aggravate the situation. Basically, it would add wing or airframe load and make the situation worse or the main purpose is to reduce speeds as fast as you can, and that load from G would be unsignificant to situation?

Also, from your statements, if you are above flutter speed, you must jump somehow over it when you reduce the speed, I figure out that from your statement: If you were more than a little above the flutter speed it would not flutter and generally it will not flutter as you slow down unless you slow down slowly if you follow me. However, it's possible I misunderstood the phrase.

I wrote above about that frequency coupling, I know that the flutter condition is wing bending frequency and the aileron flapping frequency must be the same, what I wonder is, usually this frequency coupling is between two particular components (e.g. wing and aileron) or between one part (e.g. aileron) and the hole airframe?

And how flutter speed changes? Is there a way to predict the range in which it can drop below Vne considering my fears that it might show below Vne due to a disconnected aileron? Is there a pattern of how flutter speed changes or it maybe random anywhere in the speed range from low speeds even below stall to high speeds above Vne? Basically, is there a linear or gradual drop regarding this flutter speed? It is more likely to drop in the high speed range, let's say between Vne and Vno, that is at the top of yellow or green arc or it has no rule?

The flutter point is only flying that flutter speed or at and above it? If the oscillations grow with it, it is possible to fly above it?

I hope I was not too hard to follow. Thank you very much!
 
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Dana

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...I know that the flutter condition is wing bending frequency and the aileron flapping frequency must be the same...

Also, how it differs when we talk about wing or aileron flutter and other surface flutter, like rudder. Are they the same? It seems that in aileron flutter, a significant factor is the fact the wing bends due to aerodynamic forces being changed by aileron deflection.

...I know that the flutter condition is wing bending frequency and the aileron flapping frequency must be the same, what I wonder is, usually this frequency coupling is between two particular components (e.g. wing and aileron) or between one part (e.g. aileron) and the hole airframe?
I think you misunderstand. Certainly if the flutter frequency is the same as the wing's resonant frequency you could have a very destructive situation, but that's unlikely. You could have destructive flutter even with an infinitely rigid wing... it might not make the wing fail, but it could rip the aileron hinges out.

Flutter of other surfaces is equally likely, and is the same phenomenon, though elevator and rudder flutter may also be initiated or aggravated by the pulsating airflow from the propeller.

-Dana

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Head in the clouds

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I thought that, for example, the aileron become disconnected, let's assume it would go up, that up aileron will cause a down force on the wing, and it will bend down, overshoot, then spring back up, the air will be forcing the aileron back down, but it will overshoot too, down, and so push the wing up. If this happens just as the wing was already springing up, it will push the wing higher, leaving the aileron behind and the whole problem repeats, with the wing bending more and more with each oscillation. Consider this happens very quick and in seconds you'll be wingless and hopeless.:grin:

Now, from your post, I understand that some airspeed values can dump these oscillations, but I know that the flutter condition is wing bending frequency and the aileron flapping frequency must be the same. The question is when they coincide? Anytime but they are damped in speeds below flutter speed?
In normal operation when the flutter speed is above Vne, if you are flying above that I figure out that the low capability of airspeed to dump oscillations is what triggers flutter, but I can't explain when, why and how the frequencies coincide there, if they do that then.

I know it is a very complex theoretical subject, but maybe in a simple summary, understood by all, I would be able to understand the general idea of how it happens. If it's too complex to explain it like that, I'd settle for that instead of having a wrong idea about what actually is an intricate subject like this.



If so, that doesn't explain why a slopped cable might trigger flutter, even in flaps which are not so prone to flutter.



I know that in some particular cases even a mass-balanced surface might flutter because there are situations when the balance may be affected by some extremely abnormal factors. I read a crash when some water froze in the aileron and that unbalanced it, so it fluttered violently leading to inflight breakup. I don't worry about this, because it's a very particular case, if so, I would really be paranoid as someone here said in the first posts, that is a very particular case, maybe even the only one occurence having that causal factor.

LATER EDIT: Also, how it differs when we talk about wing or aileron flutter and other surface flutter, like rudder. Are they the same? It seems that in aileron flutter, a significant factor is the fact the wing bends due to aerodynamic forces being changed by aileron deflection.

Also, about recovery technique. Basically, you are becoming somehow a test pilot in that situation. I found the recovery techique in some flight test tips articles.

Forgot to ask, would it be appropiate a high G pull up (of course, not above aircraft G limit) to stop flutter or that may aggravate the situation. Basically, it would add wing or airframe load and make the situation worse or the main purpose is to reduce speeds as fast as you can, and that load from G would be unsignificant to situation?

Also, from your statements, if you are above flutter speed, you must jump somehow over it when you reduce the speed, I figure out that from your statement: If you were more than a little above the flutter speed it would not flutter and generally it will not flutter as you slow down unless you slow down slowly if you follow me. However, it's possible I misunderstood the phrase.

I wrote above about that frequency coupling, I know that the flutter condition is wing bending frequency and the aileron flapping frequency must be the same, what I wonder is, usually this frequency coupling is between two particular components (e.g. wing and aileron) or between one part (e.g. aileron) and the hole airframe?

And how flutter speed changes? Is there a way to predict the range in which it can drop below Vne considering my fears that it might show below Vne due to a disconnected aileron? Is there a pattern of how flutter speed changes or it maybe random anywhere in the speed range from low speeds even below stall to high speeds above Vne? Basically, is there a linear or gradual drop regarding this flutter speed? It is more likely to drop in the high speed range, let's say between Vne and Vno, that is at the top of yellow or green arc or it has no rule?

The flutter point is only flying that flutter speed or at and above it? If the oscillations grow with it, it is possible to fly above it?

I hope I was not too hard to follow. Thank you very much!
OK, now you're digging deeply into the subject and whilst I have studied it to some degree my knowledge is not sufficient to give you definitive answers. There is a considerable amount of information available on the net, here is a link to something very useful and which covers a lot of the different modes of flutter - http://www.cs.wright.edu/~jslater/SDTCOutreachWebsite/aerodynamic_flutter_banner.pdf

Another suggestion I have is that you visit or correspond with the lecturers in a university that offers studies in Aeronautics. They would be able to suggest the best texts for you to study.

And further research can be conducted on the net and in technical libraries, the keywords you need are 'Aeroelastic effects'.

Things for you to consider are - in the case of your aileron disconnect - ailerons are often controlled via a return loop system and the disconnect may, or may not, break the loop. If the loop is broken then both ailerons will deflect upwards and that will affect the speed of the flutter onset for two reasons. First they want to stay in that deflected state and second they have nothing to react against i.e. there is no springiness in the 'system' because there is no system.

In event that the disconnect leaves the return loop closed i.e. both ailerons are connected together but not to the control stick - they will not deflect upwards of course but will trail evenly provided that there is no slip or skid. Their flutter frequency/critical airspeed would typically be lower than the broken loop example because they can react against each other. If they flutter in that condition their frequency is determined by their system frequency and is not dependent on any wing bending frequency. In that state the hinges could fail as Dana mentioned but no wing spar damage need necessarily occur. There are two modes of wing bending that can harmonize with the aileron frequency (note that the frequencies of ailerons and wing bending do not have to be the same, in fact usually will not be, the aileron frequency will usually be a higher harmonic of the wing. The wing bending can be in torsion or vertical flapping. Usually the torsional bending (twisting) of the wing is the more problematic, more usually the case and more destructive. Also note that if the aileron and wing bending and twisting frequencies are well separated in their harmonic frequencies then the aileron flutter would not excite the wing and so it would not cause spar or torsional failure modes even though the possible hinge failure might be enough to spoil the day.

As far as managing the flutter to a successful outcome, yes you might have to slow down through the flutter zone if you happened to be already faster than it, the deeper/longer the control surface and the heavier it is the lower will be the flutter frequency, many unbalanced rudders for example would have a CG well behind the hingeline and might want to flutter at half the stall speed but they don't get to flutter because the take-off is accelerating and the landing is decelerating and if the fin is stiff there is nothing for the rudder to excite/react with since the stiff fin would want to oscillate at a different frequency from the rudder...

If you speed up to reach the flutter frequency then simply slow down by raising the nose promptly and reducing power. If you did, by some remote chance, have a disconnect which happened while you were flying exactly at, or a little above, flutter speed then just do the same, promptly raise the nose and slow down. And if it was a disconnect which broke the loop and the ailerons reflexed then you would have a condition where the lift of about half the wing would be almost completely dumped a bit like a glider applying full spoiler so you would be descending at a very steep glideslope, maybe as little as 3:1 but you could still keep the nose up and the speed down because you would have no fear of the outer panels of the wing stalling, just look for a landing place quickly...
 

David36

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Ok, thank you so much for your detailed technical post, even if I didn't understand it at all (I wasn't expecting to, because don't have all that knowledge) I will keep reading on it and searching for useful articles on the Internet.

What surprised me is how did you figure out that both ailerons will come up due to a disconnected loop failure. I searched for some flight controls diagrams, I couldn't explain myself how they gonna deflect both upwards. Also, when thinking about this, have you considered the mass balance of the surface or it has nothing to do in that situation and it will be the same and you generally said what would happen, even being mass-balanced or not?
 

Dana

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What surprised me is how did you figure out that both ailerons will come up due to a disconnected loop failure. I searched for some flight controls diagrams, I couldn't explain myself how they gonna deflect both upwards. Also, when thinking about this, have you considered the mass balance of the surface or it has nothing to do in that situation and it will be the same and you generally said what would happen, even being mass-balanced or not?
I don't get it either. Absent any input, any disconnected control surface will trail back in the slipstream... it would have to be very heavy (and unbalanced) to do otherwise, and nobody would built a surface that heavy.

With a typical looped system, what happens would depend on where the break occurred, and the design of the system. You might be able to pull both ailerons down but not up, or pull both up, or pull one up and the other down but not vice versa, or you might lose all control if the chain comes off the control wheel sprocket (if the design has one, like my T-Craft did). Yes, any of those things could affect the flutter modes.

-Dana

There is always a law against doing anything interesting.
 

Head in the clouds

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I don't get it either. Absent any input, any disconnected control surface will trail back in the slipstream... it would have to be very heavy (and unbalanced) to do otherwise, and nobody would built a surface that heavy.

With a typical looped system, what happens would depend on where the break occurred, and the design of the system. You might be able to pull both ailerons down but not up, or pull both up, or pull one up and the other down but not vice versa.....
......What surprised me is how did you figure out that both ailerons will come up due to a disconnected loop failure. I searched for some flight controls diagrams, I couldn't explain myself how they gonna deflect both upwards. Also, when thinking about this, have you considered the mass balance of the surface or it has nothing to do in that situation and it will be the same and you generally said what would happen, even being mass-balanced or not?
I think I'm getting flutter fatigue and my hinges are coming off...bugger.gif


I should have been more specific - with a complete disconnect where the ailerons are not connected together or to the control stick i.e. each aileron can move quite independently of anything else... and let's assume no slip or skid, then they will both move upwards but I didn't mean vertically upwards, the amount of upward deflection is dependent on the alpha (angle of attack). I've seen this demonstrated in a wind tunnel but for real-life examples - if you've ever had manually operated flaps that have one or two reflex positions then you will know that as you retract them they want to pull the operating lever past the zero position to the reflexed position. I had a friend who used to build motorgliders and he had flaps with reflex positions and installed strong springs to assist with finding the zero position detente. Another example, many flapped aircraft have some play in the flap actuation i.e. you can rattle the flaps on the ground (and even with that play I've not heard of them fluttering). Anyway if you watch the flaps while taxiing they will rattle away but primarily stay slightly drooped. When a certain speed is reached on the take-off run the flaps will move up to the zero position, they stay there because they are trying to move up a bit further but have hit their hard stop.

So - at crusie speed and alpha the disconnected flaps would reflex a bit, perhaps around 5 degrees up, and if you have flown something which has flaps which can reflex you will know how much that reduces the lift - a lot. Which is why I said that if that happened to both ailerons you will be coming down rapidly even without high speed. If the ailerons were not mass balanced then the reflex might be a bit less due to the weight of the ailerons holding them down a little, if they were balanced the deflection would be a little more.

As the aircraft slows down the ailerons will reflex more and more due to the change of slipstream angle even though the pressure field is moving forward while above the stall. If the stall is reached I would think the ailerons would then deflect very highly upwards, most likely to their hard stops. So the slower the aircraft gets i.e. the higher the alpha, the more the lift will be reduced by the reflex, this is exactly the effect experienced in the normal operations of tailless aircraft where the elevators have the unfortunate effect of reducing the CL (co-efficient of lift) at increasing alpha during the round-out for landing at a time when a high CL is most wanted.

Regarding the aileron control system David, if you don't already have a good understanding of all of the different types of aileron control systems then it's no wonder you're paranoid about their possible failures, you must be imagining all sorts of things. Take a simple one - cable from stick to a bellcrank in one wing. Another cable from the other side of that bellcrank to one side of a bellcrank in the other wing. Another cable from the other side of that second bellcrank back to the stick. That is a simple closed loop. Then each of the bellcranks have a pushrod to their associated aileron. Assuming the cables attached to the stick pull the ailerons down (some aircraft may be rigged so that the cables from the stick pull them up, in which case this example doesn't apply but you can work out all the possible different combinations...), if the long cable between the two bellcranks breaks then the ailerons will stay centered because the cables from the stick are preventing them from reflexing (in the air that is, on the ground they would droop due to their own weight if not mass balanced), but the ailerons would still work almost normally since the stick will still pull the appropriate aileron down and the other will reflex up somewhat, because that is where it wants to go as previously discussed, so you might not even notice you had a failure.

If the cables from the stick pulled the ailerons up and the long return cable broke then both ailerons would reflex a bit as discussed earlier but you might still have some roll control because you could still pull one aileron up further than its 'trail, slightly reflexed' position.

What I have described is a simple closed loop, sometimes the closed loop is independent of the connection to the stick and a pushrod from the stick moves a bellcrank which moves the closed loop, if the pushrod from the stick disconnected then you would have no control of the ailerons at all but they would not reflex because the loop is still intact.

If one or other of the aileron pushrods disconnected then that aileron would reflex slightly but you would still have full control of the other aileron.

If one bellcrank broke away from its mounting.... you see there are many ways to have a failure but they hardly ever happen, yes mass balanced is better than not but may not be necessary if the flutter speed is higher than Vne and there are other factors too (some surfaces have a flat plate on their trailing edge, trim tabs usually, which prevents them from fluttering at any speed). And the answer is still the same - if you get flutter just slow down.
 

David36

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To be honest, I'm not convinced that the ailerons might deflect upwards as you slow down, that would mean that the change in slipstream you mentioned has a greater effect than the decreased pressure (due to low airspeed). If so, as an analogy, the flaps would be harder to deflect at low speeds and easier at high speeds and that's not true, I don't remember that ever happened. (even my last flight was 2 months ago due to winter season :grin:) I always thought that the flaps are harder to deflect at higher speeds due to increased pressure and that's the way I feel they do. (Of course, not necessary to specify, as you said, we are talking mechanical actuated flaps, usually through a lever which gives you a lot of pressure feel when you move them at different speeds.) However, it is possible that I don't have enough knowledge to figure out your assumption, but that's the way I thought it is.

What I figure out it's normal that reflex flaps tend to deflect upwards when you move them. Their reflex position is usually 0 degrees (that is, neutral lever positon is not really 0 degrees on flap surface, actually reflex lever position is 0 degrees on surface and neutral lever position is few + degrees, like +3 and even not being so, read further my explanation) or maximum minus few degrees (e.g. -3) and it's normal to tend upwards those few degrees considering the pressure created due to speed. That's why the lever tends to reach the stop position coresponding the reflex flaps position and they usually do that and the effect is greater at higher speeds, not lower.

What I think it would happen is that at high speeds the free aileron(s) eventually will rise slightly, but not too much because its pressure due to speed acts on both sides of the surface. Only if, as Dana said, the surface would be very heavy (and unbalanced) to do otherwise, and nobody would built a surface that heavy. Even not talking about aerodynamic effect and considering other control system failure mode, I don't figure out how a hardover might occur to ailerons or other surface and, in our case, the ailerons could deflect on their own, the only case that comes to my mind would be to deflect them normally using the stick and they jam or stuck there. The same regarding the other surfaces. Let me know if you don't agree.

Btw, I flew at least two airplanes with reflex flaps and they perform very well selecting the reflex position at cruise giving you some extra knots, they kill lift, but also kill drag and that's magic cause it seems to work in your favor. It's great to see how your airplane speeds up thanks them.


P.S. HAPPY NEW YEAR to Australia, I think you're already in 2013 there, and HAPPY NEW YEAR to everyone here, I really appreciate your help!:)
 
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Norman

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A control surface will tend to float toward the low pressure side if there is not a mechanical restraint. Rudders and elevators trail straight behind their airfoils because those airfoils are usually symmetrical. Ailerons are usualy part of cambered airfoils. If the aileron has camber the camber will act like a trim tab and bias the aileron to move away from the neutral position (tabs just add aft camber to a small part of the span). This is why a lot of times designers will fill in the cusp of aft cambered airfoils although doing so completely negates the reason for choosing that airfoil in the first place. Many commonly used airfoils have a slight curvature in the last 25% of the chord so their ailerons do tend to float up if the balance cable separates, as Head in the clouds described, if the aileron construction preserves that camber. Even if the ailerons are built with a symmetrical triangular cross section the pressure field of the wing will suck them up a bit, especially if the gaps are sealed. Aerodynamic balances can reduce these effects to zero and produce an aileron that trails straight back but probably not at all angles of attack.

The ailerons of reflexed wings tend to float down because the control surface is cambered in the opposite direction
 

David36

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And what about the flaps analogy I wrote above? I figure out flaps are usually the same, cambered surface and even not being so, why do they behave differently in the same conditions?
 

Dan Thomas

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And what about the flaps analogy I wrote above? I figure out flaps are usually the same, cambered surface and even not being so, why do they behave differently in the same conditions?
They don't move upward. They experience pressure only beneath them, normally, and won't tend to flutter downward.

A flap actuation failure will just cause an uncommanded roll. Depending on the power of the flaps, it might or might not be uncontrollable with aileron. Tha one cable failure I mentioned earlier was a flap cable in a 185 on floats, and it broke right after takeoff from a river. The flap popped up and the sudden roll dug a float in and flipped the airplane. The cables are small 3/32" affairs and were likely stainless for float ops, and stainless frays much more easily than galvanized and needs watching. Proper inspections prevent such stuff. Cessna typically demands cable inspections every 200 hours but most private and many commercial airplanes don't get it, believe me, based on what I've found. Further, a fraying cable usually telegraphs its distress and only a dope of a pilot will overlook it. Cables fray over pulleys and fairleads and will make scratching sounds and will also be felt in the controls during preflight.

Dan
 

David36

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I'm sorry, but it seems no sense. That two statements oppose and should not. If ailerons deflect upwards as you slow down, the same thing should happen regarding flaps, but it doesn't.

I didn't reply cause I expected an answer from Head in the clouds, I hope he will explain his assumptions to clarify how things are and remove confusion (mine).
 
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Head in the clouds

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I'm sorry, but it seems no sense. That two statements oppose and should not. If ailerons deflect upwards as you slow down, the same thing should happen regarding flaps, but it doesn't.

I didn't reply cause I expected an answer from Head in the clouds, I hope he will explain his assumptions to clarify how things are and remove confusion (mine).
I didn't reply further because I don't like to contradict other forum commentators, so you must make your own mind up.

I did mention that I have seen the disconnect effect for ailerons in a wind tunnel, it was the wind tunnel at RMIT in Melbourne Australia, the ailerons quite clearly deflected upwards when disconnected (and they weren't even mass balanced so their weight would be opposing the upward deflection) and they deflected upward by an increasing amount according to increasing alpha. I can't find the photos now although I used to have them. I am sure you could find similar wind tunnel tests on YouTube, I have seen lots of them (not all about ailerons but worth a search).

Also, Norman is probably the most knowledgeable forumist on aerodynamics, and he agrees.

Consider that all things move from higher pressure to lower pressure, so where is the higher pressure and where is the lower pressure? Based on that the ailerons would move upwards on any plane I know of.

You can google a condition known as 'control snatch'. It refers mainly to flying wings but is relevant to any wing I would think. As the stick is pulled back and the alpha increases in the flare the stick force becomes less and less and eventually might even be a negative force i.e. the stick wants to move back on its own, because the elevons are being 'sucked upwards' by the increasing low pressure field on the top surface. In the flying wing the elevons are similar surfaces to the flaps and ailerons on a tailed aircraft.

Lastly regarding flaps, and the same applies to the ailerons - with manual flaps do you have to pull on the flap lever to deploy the flaps, or does the lever want to spring up when you press the release button? You have to pull on it right? And if the flap lever is up (flaps are deployed) and you press the button without holding the lever what happens? The flaps slam closed right? So is the aerodynamic force holding the flaps up or down? If the flaps did not have a stop at the zero position would they stop there anyway or would they continue up into a reflexed position? Yes they would, you know that because there is pressure on the flap lever holding it in the retracted position, if the flaps wanted to stay at the zero position you would not have to pull on the lever much at all to begin the deployment.

Dan's example of flap failure shows exactly that. He described a flap failure where the flaps were already deployed and then one flap connection failed, so that flap retracted, leaving the other flap deployed and so causing the aircraft to roll.
 

autoreply

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I'm sorry, but it seems no sense. That two statements oppose and should not. If ailerons deflect upwards as you slow down, the same thing should happen regarding flaps, but it doesn't.
Completely true. It does happen in fact. If you remove the stops. Those stops are sized for loads at VA (usually) and are very large, think of a thousand pound-ish for a typical Cessna. If you remove those (which given their strength isn't going to happen otherwise) your flaps will go up and might flutter. You can also see this in reality in any flap or flaperon sailplane where the flaps go to negative positions. To pull them into neutral positive does require force, while -7, -5 degrees (up) goes a lot easier.
 

David36

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Lastly regarding flaps, and the same applies to the ailerons - with manual flaps do you have to pull on the flap lever to deploy the flaps, or does the lever want to spring up when you press the release button? You have to pull on it right? And if the flap lever is up (flaps are deployed) and you press the button without holding the lever what happens? The flaps slam closed right? So is the aerodynamic force holding the flaps up or down? If the flaps did not have a stop at the zero position would they stop there anyway or would they continue up into a reflexed position? Yes they would, you know that because there is pressure on the flap lever holding it in the retracted position, if the flaps wanted to stay at the zero position you would not have to pull on the lever much at all to begin the deployment.

You can also see this in reality in any flap or flaperon sailplane where the flaps go to negative positions. To pull them into neutral positive does require force, while -7, -5 degrees (up) goes a lot easier.
Flew some aricrafts with negative positions. How did you find that effect changes in terms of speed? I always thought and found that at HIGHER speeds they require MORE force to pull them into neutral positive, while -7, -5 degrees (up) goes a EASIER, not vice versa, that was my point.

Also, why is there a Vfe speed? Why you are more likely to break them at higher speeds?

I respect your opinions, not argue, the idea is just to understand how things work. Thank you!
 
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