Mass balance and cable failure

[SteveJeff]

I found that many certified LSA aircrafts, unlike most standard certified FAR 23 light aircrafts, don't have mass-balanced surfaces. I'm interested in how they are designed to show freedom from flutter, especially control surface flutter and if these aircrafts can handle a disconnected freefloating surface emergency (due to cable failure, for example).

 

Thank you very much!

 

 

[djpacro]

Just an interim response to part of that post.

 

Many small FAR 23 airplanes have no mass balance for the rudder and elevator. And no consideration of a tab disconnect.

 

Aileron flutter is more of a concern for low speeds.

 

FAA Report #45 may be of use - can't remember the title but something about flutter prevention for small airplanes.

 

More later.

 

Edited to add the reference to Rept 45 in FAR 23.629 at http://www.astech-engineering.com/systems/avionics/aircraft/faapart23d.html

 

And further edited with a reference to another thread here http://www.recreationalflying.com/threads/a-little-bit-of-a-history-of-flutter-in-aerobatic-aeroplanes.9661/

 

 

[Head in the clouds]

....Aileron flutter is more of a concern for low speeds.....

How so DJ?

 

 

[Kyle Communications]

Flutter is usually at high speed.....well it certainly is a factor in my model turbine aircraft doing 400 kph the servos and hinging and some mass balancing is required to stop the aircraft disintergrating in flight

 

 

[SteveJeff]

Just an interim response to part of that post.Many small FAR 23 airplanes have no mass balance for the rudder and elevator. And no consideration of a tab disconnect.

 

Aileron flutter is more of a concern for low speeds.

 

FAA Report #45 may be of use - can't remember the title but something about flutter prevention for small airplanes.

 

More later.

 

Edited to add the reference to Rept 45 in FAR 23.629 at http://www.astech-engineering.com/systems/avionics/aircraft/faapart23d.html

 

And further edited with a reference to another thread here http://www.recreationalflying.com/threads/a-little-bit-of-a-history-of-flutter-in-aerobatic-aeroplanes.9661/

No consideration of a tab disconnect, are you sure? If so, they don't comply FAR 23.629:

 

(f) Freedom from flutter, control reversal and divergence up to VD/MD must be shown as follows:

 

(1) For aeroplanes that meet the criteria of sub-paragraphs (d) (1) to (d) (3) of this paragraph, after the

 

failure, malfunction, or disconnection of any single element in any tab control system.

 

Aileron flutter is more of a concern for low speeds.

 

I'm sorry, it is mistyped or fact? I thought high sppeeds.

 

Read the topic and it reinforces my concerns. Are these airplanes at risk for an inflight breakup due to a control cable failure?

 

Report 45 seems to have a lot to do with mass-balance. I don't think that using it exempts mass-balanced surfaces, on the contrary, it seems that according to it requires that and using it and being mass-balanced makes flutter less of an issue.

 

I don't believe that FAR requires flutter concerns regarding a small trim tab disconnected scenario and won't do that for a primary control surface, but my question is more from a technical point of view, how do they show freedom from flutter if not using mass-balanced surfaces as others have.

 

And regarding LSAs aircrafts, the problem is even more complex considering ASTM standards, unlike FAR, are no very specific on this issue.

 

 

[djpacro]

To clarify my comments a bit more:

 

• there are not too many aeroplanes certified to the current and recent versions of FAR 23.629 so you will see many types certified to old versions of FAR 23 which were not required to consider the disconnect. I can think of two small GA airplanes certified to the newer 23.629 - one uses a spring trim system to eliminate elevator trim tab and the other has a dual drive to the elevator tab.
   
   
  
• aileron flutter in particular can have a mode at fairly low speeds so my comment was to suggest that designers of low speed aeroplanes may easily avoid flutter at the tail end (and likely not require mass balance) however should be more wary of aileron flutter - another example indicated in this report http://researchbank.rmit.edu.au/eserv/rmit:6164/Norrison.pdf (you only need to read the summary page).
   
   
  

 

 

 

 

[SteveJeff]

The new FA 23.629 is not really so new. I read a document from 1991 and it had those requirements and I didn't have time to search more, but I guess they are much older than that, so I can figure out that they are at least 30 years old. It's true that the mean age from first flight of GA aircrafts may be more (and usually it is) than 30, but in this case I can figure out that most aircrafts have not undergone any changes since they were first certified? Basically all FAR safety improvements changes made over the years does not apply to them? That hurts if it's true.

 

I thought most of FAR 23 aircraft (and all of them must, considering FAR) have two cables for actuating the trim tab (if I remember correctly, Cessna 172 has it), but I may be wrong. Also, I saw some have a spring which (in a cable failure situation) will make the trim tab to stay fixed in position in case of single cable system.

 

 

[djpacro]

Two cables? Need to be dual cables in parallel which I have never seen. I.e. everything has to be duplicated to cater for failure of a cable or fitting etc.

 

Cable with a spring? Nope, springs do not make a tab fixed.

 

This requirement was introduced in FAR 23 many years ago along with many other onerous requirements. That is why the cost of certification is so high. If a type was certified many years ago and has a safe history in service there is no reason to require redesign.

 

I recall a push some years back for simplified FAR 23 certification for slow, small airplanes and there was a draft advisory circular including treatment of the tab failure criteria with appropriate consideration for simple, robust systems with remote likelihood of such a failure.

 

 

[djpacro]

That advisory circular is AC 23-15 http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/1ab39b4ed563b08985256a35006d56af/976d95d659b6c11d862569b2006a7694/$FILE/ATTVE21P/AC23-15.pdf

 

"For small low performance airplanes, designing the actuating structures to a factor of safety equaling four and providing redundant fastener safety means to minimize loss of single fastener joint integrity would be an acceptable method of showing compliance with this request."

 

Interestingly, for anyone interested in the debate on stall speed limitations for recreational aeroplanes/minimal certification requirements, take a look at that AC in the section on emergency landings on page 23.

 

 

[SteveJeff]

Two cables? Need to be dual cables in parallel which I have never seen. I.e. everything has to be duplicated to cater for failure of a cable or fitting etc.Cable with a spring? Nope, springs do not make a tab fixed.

Not redundant, I thought that usually being a two cable system and the trim tab is linked in two points to these cables, so if one breaks the other prevents the tab to free-rotate.

And about the spring. Have a look at this photo here , zoom on the trim tab and at the bottom of the trim tab you'll see a spring. The cable is atached on the top of trim tab. If the cable fails, I assume the trim tab wouldn't freefloat having that spring there and thus it prevents flutter, or am I missing smth?

 

 

[djpacro]

1. I just see one wire (cable) going to each side of the trim tab.
    
    
   
2. Anyway, if there was a spring on one side:
   
   • 
      
      
     springs do not stop movement - there is still a degree of freedom
      
      
     
   • what if the cable doesn't break but the spring does?
      
      
     

 

 

 

 

 

 

 

 

Regardless, all that is required per AC 23-15 is to design it so that a failure of the tab drive mechanism is remote (well, on small slow aeroplane anyway).

 

 

[SteveJeff]

1. I just see one wire (cable) going to each side of the trim tab.
    
    
   
2. Anyway, if there was a spring on one side:
   
   • 
      
      
     springs do not stop movement - there is still a degree of freedom
      
      
     
   • what if the cable doesn't break but the spring does?
      
      
     

 

If the spring breaks, I figure out the trim tab remains linked to cable, so it's not freeflaoting or just a little. Even that would be critical to flutter?

 

 

[djpacro]

Perhaps I leave you with this magazine article to read http://www.flightglobal.com/pdfarchive/view/1952/1952%20-%200107.html

 

I'll stick with the simple things.

 

 

[SteveJeff]

Ok, I read it, quite useful piece, I figure out that it's a very complex subject and I knew that in some particular condition you can encounter flutter even in a normal system without any failure. What I wonder is regarding my first question, about disconnected unbalanced surface.

 

Basically, the question was, are these airplanes with unbalanced surfaces more at risk than the balanced ones in a disconnected situation, considering the airplane didn't have flutter occurences in normal operation and it was also tested for flutter during certification?

 

Also, I would really appreciate to detail a little bit this, if it's not a mistype:

 

Aileron flutter is more of a concern for low speeds.

[SteveJeff]

Also, have a look here at the C172 flight control systems. Page 210-211. There are two cables which run from trim wheel to trim tab pushrod. Only if the pushrod would detach from the tab surface you'll have a freefloating tab which is extremely unlikely cause we can consider it as strong as the tab itself. This kind of fail-safe I was talking about when I referred FAR 23 requirements.

 

 

[Head in the clouds]

Also, have a look here at the C172 flight control systems. Page 210-211. There are two cables which run from trim wheel to trim tab pushrod. Only if the pushrod would detach from the tab surface you'll have a freefloating tab which is extremely unlikely cause we can consider it as strong as the tab itself. This kind of fail-safe I was talking about when I referred FAR 23 requirements.

My C172 only had one cable, it started at the bellcrank in the tail (the bellcrank which the pushrod attaches to) then it came forward, was wrapped around the trimwheel drum several times and then went back to the bellcrank, so it was a single cable with pull/pull effect which might look like two cables in the diagram. In the middle of the cable wrappings on the drum it had a ferrule swaged to the cable which located in an indent in the drum. If the cable broke on one side you might still have control on the other side or it might unwrap itself and the ferrule fall out of the indent so you wouldn't have any control. It's unlikely that the tab would flutter with the pushrod still connected to it.

 

 

[facthunter]

My view is that even looseness of un balanced control linkages can be a problem, let alone a disconnect. Wasn't this sort of problem to do with the zenith?

 

Regarding the slotted flapperons on the Nomad, I would see that as a special case. It's questionable whether that system was ever "safe". The particular aircraft never had a good name, and had problems in other areas of structure.

 

On a slightly different tack ,... Has anyone ever experimented with friction or hydraulic damping?... Nev

 

 

[Head in the clouds]

On a slightly different tack ,... Has anyone ever experimented with friction or hydraulic damping?... Nev

Yes, they have, it was the subject of a lengthy and involved discussion on another forum. The upshot was that it was tried on quite a number of gliders and some fast powered aircraft and was only partially successful, and then only if the friction or damper was located right at the control surface itself. Neither of those methods actually nulled the flutter response and so 'rattle' was not prevented and it could be damaging to the hinges if allowed to continue for lengthy periods. The only real solution for flutter is to employ methods which move the response frequency out of the flight envelope i.e. lighter or heavier surfaces, changing the CG of the surface (mass balance) change the aerodynamics of the surface (camber), change the trailing edge condition (sharp T/E flutters more readily, thick T/E is more flutter resistant, flat plate on the T/E is very resistant and doesn't have to extend along much of the T/E). On wings with a swept L/E a cusp or dogtooth like the C22 for instance, creates a vortex which resists flutter.

 

 

[Garry Morgan]

Once you get over about 108 kts you should mass balance, many of the imports go to so much trouble so save weight, the last thing they will do is add lead to the aircraft. many of there wing top skin show buckling in level flight, showing the design and low strength and stiffness,which will have a short lift span, some of the import hand books are stating a 10 year life, I was looking at buying a LSA glider but its 10 yrs is up and only 30hrs TT ?????????

 

 

[Head in the clouds]

Once you get over about 108 kts you should mass balance....

'About 108kts' is a very precise number, how have you determined it as the speed for the likely onset of flutter?

 

 

[facthunter]

Perhaps , put another way .A/C that fly below 100 kts,( OTA) might have less of a problem with flutter, than faster aircraft. As I see it flutter speeds are a dark art. The onset may be provoked by a particular situation one of which is speed.. Does the word aeroelasticity help the comcept? Nev

 

 

[tdfsks]

The answer to the original question is no, most LSA's without mass balance cannot be guaranteed to be flutter free with slack control cables. The basic problem is that the flutter requirements in ASTM F2245 are poorly written. Perhaps they were written by those with a vested interest in an easy and cheap path to compliance (remember this is a concensus standard written by a committee including manufacturers) or it might simply be that those who wrote the requirements simply didn't understand flutter. Probably a combination of both. Most LSA's are certificated for flutter via flight test. The evidence is that few of these flight tests are conducted correctly with adequate instrumentation and proper means of exciting the vibration modes of the structure. Also I fail to see how they can do a proper flight test without first doing analysis and a ground vibration test to understand the critical modes that need to be checked during flight test.

 

In the case of a convention light aircraft with a Part 23 certification basis, FAR 23.629 (and in particular the associated AC's) are quite clear that you do need to show freedom from flutter for the case where the control surface natural frequencies tend to zero ... i.e. a failed or slack cable. That is why virtually every Part 23 aircraft has mass balance .. either 100% balanced or some form of product of inertia balancing. Note: that 100% mass balance is usually not necessary.

 

The inflight breakup of at least 8 CH-601XL's was due to flutter caused by this exact problem. No aileron mass balance. The bellcrank on which the aileron bellcrank was mounted bent due to the cable tension and let the cables go slack. The aileron / wing then fluttered .... failing the wing spar in the root area. One of the NTSB recommendations was that the ASTM F2245 committee review the flutter requirements .... who knows if they took took any action or even understood the issue.

 

 

[Head in the clouds]

The inflight breakup of at least 8 CH-601XL's was due to flutter caused by this exact problem. No aileron mass balance. The bellcrank on which the aileron bellcrank was mounted bent due to the cable tension and let the cables go slack. The aileron / wing then fluttered .... failing the wing spar in the root area. One of the NTSB recommendations was that the ASTM F2245 committee review the flutter requirements .... who knows if they took took any action or even understood the issue.

Whilst I agree that flutter seems like the most likely explanation I think you're going way out on a fragile limb by categorically stating that that is what caused any, let alone many, of the 601XL structural failures. I've been following the various investigations and recommendations and reluctant factory modification actions quite closely, and never seen anything officially documented which can absolutely point to any of the failures' cause. I may well have missed something though, so do you have any links or leads to a definitive finding? I also haven't seen anything documented about the bent bellcrank, and that also would appear to be a likely candidate, are you able to point me at where that came from?

 

In case you're wondering I do have a keen interest in the subject since I narrowly escaped a very frightening situation where one of my own designs fluttered violently some 25yrs ago. All my recent projects have incorporated all anti-flutter methodology and I've not had a similar problem since but I still like to keep abreast of any investigation findings.

 

 

[tdfsks]

Let me just correct an typing error in my previous post:

 

"The bellcrank on which the aileron bellcrank ....."

 

Should have read:

 

"The rib on which the aileron bellcrank ......"