Not mass balanced & cable failure

[Steve Jeff]

Hi everyone here!

I'm a LSA and PPL piIot, working on my CPL, and I have some questions about the aeroelastic effects and flutter phenomenon. I would like to talk a little about aerodynamic flutter onset speed and flight control malfunction. It is known that freeplay, worn-out control rods or slop in flight control cables might induce flutter.

What I'm interested in is how critical flutter speed is affected by those problem. I'm wondering especially about a cable control failure where the surface would be disconnected and freefloating. There are small light sport aircrafts and even some FAR 23 standard certified aircrafts which don't have mass-balanced surfaces, especially ailerons which I guess would be more prone to flutter. How critical flutter speed lowers in a situation like that (and how prone to violently flutter are these ailerons in an emergency disconnected sitation)? Is there a linear drop in flutter speed? Can it reach even lower speeds in the normal operating envelope e.g. lower than Vno or Va? Normally, assuming no malfunction, flutter speed is at least 10% above Vne or Vdf, which is dive test speed used during flight testing. How do you think things might change?

The same about the others control surfaces e.g. a broken trim tab linkage.

I really appreciate your help. Thank you very much!

[pacerpilot]

The information your looking for has been compiled in great detail by NASA. The affect of worn parts on flutter is a function of how the component can withstand the flutter, not inducing it because of looseness. As for flutter at low speeds, I've never heard of any occuring. Also, don't confuse flutter with "flop". After you view some of the videos you'll understand more about it and see that we're not typically flying within a range that is anywhere near a problem.

[Steve Jeff]

The information your looking for has been compiled in great detail by NASA. The affect of worn parts on flutter is a function of how the component can withstand the flutter, not inducing it because of looseness.
As for flutter at low speeds, I've never heard of any occuring. Also, don't confuse flutter with "flop". After you view some of the videos you'll understand more about it and see that we're not typically flying within a range that is anywhere near a problem.
So, how do you figure out those accidents where a disconnected surface triggered flutter?



Also, don't confuse flutter with "flop". After you view some of the videos you'll understand more about it and see that we're not typically flying within a range that is anywhere near a problem.
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.

[WLIU]

Are you most interested in theory or practice?

In practice, the first step in dealing with a control system problem is to slow down. Get the airspeed down to landing approach speeds seems to work best. That reduces the forces on the control surfaces and system components. Lower airspeed also generally, but not always, means lower energy and lower frequency inputs to any component prone to flutter.

Typically only one thing breaks. So your next task is figuring out how to fly home with what you have left.

Example - With one elevator cable corroded or shot away (know of both instances), you can use trim to put the remaining cable in some tension so that you can carefully use the stick or wheel normally.

Aileron and rudder control failures are challenging as trim on those are much less common. But the good news is that one of these controls can compensate for a problem with the other if you can quickly learn to fly the airplane a little crooked. That said, the landing may be a little messy.

So rule #1 is slow down. Then move the control stick to see what works.

One interesting exercise is to try flying around without using one control. For instance, try not using the elevator control but instead flying the airplane down final using the elevator trim and the throttle. Plan a long, stable, final. Make small changes in power and trim.

In the aerobatics and skydiving world we get to explore some of these challenges. One pilot that I know was flying an Unlimited category aerobatic competition program at practice altitude when he broke his control stick off. Recovered the airplane using trim and carefully landed OK. A skydiver driver that I know had a parachute deploy into his tail, jamming the rudder and elevator. Got everyone out of the airplane, used the ailerons to point the airplane at a big open field, and used his parachute.

Best of luck,

Wes
N78PS

[Steve Jeff]

I'm interested in the practical outcome of a cable failure. I know those things, practiced them all, simulating them as close as possible, what I wonder regarding a cable failure is flutter, especially on an aileron which is not mass-balanced. That's why I'm asking. When you encounter flutter, the recovery seems easy: retard throttle and slow down as fast as you can, also you might try to load the surface, but I wouldn't think you have time to do that on such a destructive flutter onset the aircraft might be pieces in seconds.

[WLIU]

Many airplanes have rudders and elevators that are not mass balanced. My observation and limited experience is that because those surfaces are short and stiff they are very unlikely to flutter in a typical light plane, whether or not all of the cables are connected.

Ailerons are a different story. All of the ailerons that I have seen are mass balanced. Wings are longer and more potentially elastic structures. I think that I recall that Kitfox tried not having mass balance and ran into problems. So I will suggest that in the event that an aileron cable fails, the probability of running into a flutter issue is small if you identify the problem and slow down. And if the aileron does act like it wants to flutter, the appropriate action is to slow down.

With all of that said, we are talking about extremely rare events for your typical general aviation operation. I would not over-think this. You are more likely to be struck by lightning assuming that you maintain your equipment to expected aviation standards and do a real pre-flight inspection of the aircraft.

Hope this info helps. I will suggest that you should walk down the flightline of your airport, and walk into the maintenance shops, and look at how the different makes and models of airplanes are put together. Books are great but actually looking and touching fills in the blanks that books pass over. Each machine that you look at is the physical implementation of an engineer's solutions to the problems that you are thinking about. And each different make and model shows the thinking of different engineers or teams of engineers. You will see some great ideas, some OK ideas, and some bad ideas. All educational.

Best of luck,

Wes
N78PS

[Steve Jeff]

All of the ailerons that I have seen are mass balanced
Many LSA are not. Also, FAR 23, American Champion series (Scout, Citabria etc.) as an example.

[WLIU]

The balance weight is hidden in the leading edge of the aileron.

Wes
N78PS

[Steve Jeff]

Right. There are different designs on different aircrafts, some are outside you can see them, some are inside at the leading edge, and some are nowhere, cause they are not mass-balanced and thereto I referred.

[WLIU]

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. My memory is faulty but I recall that 100% balance is not required to avoid flutter in low performance aircraft. I am not where I can dig out the reference right now.

The point though is that if an aileron cable fails, the wing does not fall apart immediately. Control surfaces are light enough that they will only droop a little against the air flow. And you get to learn how to use the remaining controls to fly the airplane.

At this moment I do not recall ever hearing of an aileron control system failure. I have heard of them being connected backwards. Some of those pilots figured it out and some crashed on takeoff.

Fly safe,

Wes
N78PS

[WLIU]

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

[Steve Jeff]

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.



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.


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).




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.

[WLIU]

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

[Steve Jeff]

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? :confused: It says nothing to me.


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.

[Steve Jeff]

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.
:DAs 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:rollseyes:

Edit:

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.:|

[steveinindy]

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

[pacerpilot]

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.

[WLIU]

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

[David Pavlich]

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

[Steve Jeff]

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.



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.


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.



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...

[David Pavlich]

Steve...reading the above post, it almost sounds like you're trying to talk yourself out of flying. Undetected fatigue, cable failure, etc. These kinds of thing have been concerns since man became mechanised. Every time you get into your car, there's a chance that you could have a flat tire at high speed while going around a turn and end up in the woods.

All we can do is inspect and follow the maintenance recommendations set down by the FAA, Piper, Beach, Ford, GM and so on. Or, we can wake up and decide to stay in the house and lower the risk factor to near zero.

David

[steveinindy]

Or, we can wake up and decide to stay in the house and lower the risk factor to near zero.

Actually the odds of being hurt in your own home are pretty high. If that's not enough, take a look at the risk of complications (blood clots, etc) that come with not even bothering getting out of bed! ;)

[David Pavlich]

Actually the odds of being hurt in your own home are pretty high. If that's not enough, take a look at the risk of complications (blood clots, etc) that come with not even bothering getting out of bed! ;)

Like I said, near 0.

David

[Steve Jeff]

Actually the odds of being hurt in your own home are pretty high. If that's not enough, take a look at the risk of complications (blood clots, etc) that come with not even bothering getting out of bed! ;)

Btw, you can also have blood clots from too much time in the left seat:D

Guys, there is no need to tell me how safe is to fly, even if I wanted to leave aviation, I couldn't. It's in my blood, I think you know the feeling:rollseyes:

This wanted to be more like a technical discussion, not one which takes care about my worries.

As I said, I think it's worth it to know in advance what to expect in an emergency and how to deal with it, even if it's one of the rare. I prepare to be a lifetime in aviation, I think it worths to know even some advanced things which are not usually adressed during normal training.

[Bob Dingley]

Flutter was a hot topic among fellow builders a few years ago. There was a string of in flight break-ups. I won't name names. We had our "flutter deniers" but NTSB proved flutter in the end. My friend and co-worker knew a retired USAF Aerodynamacist in our area. We took my dwgs for a show and tell. Got great advice. He finished with "If the CG of the control surface is ahead of the hinge, flutter is impossible."

If you want to see real counter weights, go find a KitFox and check out the flaperons. KitFox is the son of an Avid and so are EuroFox, Aerotrek and Appolo. Excelent safety records. I know a couple of guys with kitfoxs.

A couple of ultralights had fatal flutter break-ups in the past. One was a US plagerized French design that deleted the aileron flutter protection. The French had no problems, only the US version. If you look at pictures of Ercoupes taken in the 40s & 50s, you will notice aileron balance weights. The FAA allowed their removal (don't know when) A wing came apart about 5 years ago and killed two. The resulting AD still lets you remove them. NTSB said flutter.

I was building a Thorp T-18 in the late 60s. Mid 69, one came apart in the air and killed two. November 69, a solo pilot in CA had what seemed to be an explosion at 160-165 KTS. He got it on the ground. Miracle! The (snail mail) newsletter pictures showed a pretty much destroyed stabilator. The rivet seam on the top R. longeron was sheared fwd to the cockpit. Point of interest: John Thorp designed the first PA-28 stabilator and it also went on the T-18. FAA & Thorp found accumulated slop in two bearings of the cockpit to stabilator control rod linkage. Enough to cause flutter. I got dwg revisions dated Feb,70 in the mail.

I learned that flutter is related to TAS more than IAS/CAS. I was looking at the POH for a glider and and found charts for TAS to IAS and IAS to TAS. Kind of like a helo where Vne is reduced as you climb.

If you want a free resource on flutter related data, I have a Yahoo group for you. The "files" area is a gold mine. Used to be mucho old NACA reports related to the subject.

http://groups.yahoo.com/group/ZBAG/

There was one report from the 40s-50s where they put full size wings/tails in a wind tunnel and tested to destruction. Yeah, I know, real scientist don't do that to wind tunnels. Read the report. They recorded freqs in the 600-800 CPM remaining constant with changing AS, as I recall. Only changing with the size/ mass of the panel being tested. My take is: Don't let anybody tell you that flutter is a silent killer. In fact, if you dig into the many NTSB reports herein you will find more than one eyewitness reports that state that they looked up only when they heard what they thought was the pilot jazzing the engine. Could this be the sound of flutter?This was followed by what they called an explosion. See how many you can count that were in the traffic pattern. Most of these NTSB reports have the annexes that are hard to get.

Bob

[steveinindy]

Btw, you can also have blood clots from too much time in the left seat

Only if you're not using your legs to control the rudder. Moving your leg muscles helps to minimize stasis (pooling) of the blood in the venous beds of the legs which contributes to blood clots. Staying hydrated also helps. Using oxygen at lower than normally considered altitudes might also help since hypoxia (even of a low-grade nature) is a pretty well established trigger for the clotting cascade.

If you can't tell I am a physiology geek. ;)

[Steve Jeff]

Bob, great info, I really appreciate!


If you can't tell I am a physiology geek. ;)
You really are:D

[David Pavlich]

Maybe you've seen this already. Very timely:

https://www2.gotomeeting.com/register/980209042

David

[Frank Giger]

I sure am glad my little Nieuport has push rods for aileron and elevator control.

And that it won't go faster than 60 mph.

:)

[Bill Berson]

Trim tab flutter is probably more likely ( I think)because of small size and lack of balance. A small slop in twin tab has more effect. The modern FAR 23 requires "irreversible " control system for trim tab.
But old regs allowed Aeroncas and others with reversible cables. Cubs don't have tabs.

[Steve Jeff]

Maybe you've seen this already. Very timely:

https://www2.gotomeeting.com/register/980209042

David

It can be watched here (http://www.eaavideo.org/video.aspx?v=2085824205001) . It was quite interesting. I think the idea, which I also knew, was that such a problem is pretty manageable, however, looking into reports I wouldn't say so. Yeah, I know, one would say that looking in statistics you'll also find that stall/spin is unpreventable/unrecoverable considering the high number of occurences.

LATER EDIT:


The modern FAR 23 requires "irreversible " control system for trim tab.
But old regs allowed Aeroncas and others with reversible cables. Cubs don't have tabs.
Actually the CAR 3 dating from 1949 has that requirement. I think it was a requirement since the first CAR 3 regulations, if I recall corectly dating from 1945. However, I found Aeronca aircrafts are even before that.:P So, no requirement for them.

It's quite interesting to see that regulations dating from WW2 are very well developed regarding a high factor of safety and in the same type, LSAs certified under years 200x regulations don't have so strict rules. No argue, take it as an amateur observation.:|