Aileron flutter

[t4flyer]

Might seem a silly question bur are you 100% sure its aileron flutter? Just asking because the strut fairing has caused similar effects on a number of Jabiru's in SA. I had violent shaking from the wings which would occur at about 100 knots, to make it worse it did not occur all the time but once it started the only way to stop it was reduce steed to +-85 then go back up to 95+. I also thought it was aileron flutter, really worried me until I found out that it was the strut that was fluttering and the cure was very simple. (and I think now standard on our factory aircraft, well it was standard on my new J170.)

 

Cheers.

 

 

[ianboag]

Not a silly question at all. This is a far-from-new aircraft (belonged to the Adelaide Soaring Club). The problem has not been noted up to now but in my ownership I'd be surprised if it spent 5% of its time over about 95 kts. Having sat there and watched it, I presently go with the aileron flutter notion. I can echo your "not all the time" comments, although my pilot tells me that before we lifted the ailerons it was all the time ....

 

Sometime after Christmas I will get to change the cables and we'll see where that leads ... hopefully to a full resolution of the issue.

 

 

[biggles]

Might seem a silly question bur are you 100% sure its aileron flutter? Just asking because the strut fairing has caused similar effects on a number of Jabiru's in SA. I had violent shaking from the wings which would occur at about 100 knots, to make it worse it did not occur all the time but once it started the only way to stop it was reduce steed to +-85 then go back up to 95+. I also thought it was aileron flutter, really worried me until I found out that it was the strut that was fluttering and the cure was very simple. (and I think now standard on our factory aircraft, well it was standard on my new J170.)Cheers.

What was the " simple cure "

 

Bob

 

 

[facthunter]

Stay............. On the ground. Your revelations scare me. Buzzing strut. BLOW FLYING. Nev

 

 

[t4flyer]

I'm no aerodynamics expert, so I will tell it like I understand it!

 

The strut fairings are shaped equal top & bottom and because of this under some circumstances the airflow over the top and bottom can try to create upward lift on the strut at the same time as it creates downward lift. (sounds like rubbish I know, I didn't believe it myself, but bear with me) under certain circumstances this pulling in both directions can cause the strut to oscillate. On my previous Jabi I would be flying along merrily at 90-95 knots and (sometimes) suddenly the whole wing felt like it was starting to shake itself to hell and gone, scared the crap out of me, slow right down and its gone! I asked my AMO about it he told me he had seen this before and explained about the airflow over the strut fairings and told me the "cure" was to rivet a strip along the bottom trailing edge of strut fairing to disturb the air flow under the strut. I am by nature a skeptic and that sounded to simple to me so I contacted Jabiru SA and they confirmed the problem had been identified and that they do indeed fit a strip of aluminium to the strut to break the airflow. Still not convinced I cut 2 strips of wood 4mm thick x 6 mm wide x not sure of the length now, (but it doesn't have to be the whole length) I carefully duck taped the strips to the trailing edge's of the strut and went flying. As I said I'm no expert but the strips worked 100%, never had the "shakes" again, ever. They stayed duck taped on for almost a year until a trip to the factory where I had them replace my wooden strips with the correct aluminium strips. My new J170 came with the strips factory fitted, (as I think it does on all new Jabiru's from our factory)

 

Cheers.

 

 

[t4flyer]

When my strut issue first reared its ugly head, I asked around on our local Jabiru forum and one of the guys sent me these pictures of the factory fitted strips, so I could see where to stick my wooden temporary strips.

 

 

 

 

 

[djpacro]

I'm no aerodynamics expert, so I will tell it like I understand it!The strut fairings are shaped equal top & bottom and because of this under some circumstances the airflow over the top and bottom can try to create upward lift on the strut at the same time as it creates downward lift. (sounds like rubbish I know, I didn't believe it myself, but bear with me) ...

Von Karman vortex street.Pitts aileron interconnect tubes (between ailerons on lower and upper wings) can suffer from this with a somewhat similar solution.

 

 

[facthunter]

Harmonic I guess. It is fairly long. Anything that flex's can do it. Most struts in simple aircraft have a jury strut but I thought that was mainly for negative "G", in thinner section structures. I always take flutter seriously. Nev

 

 

[t4flyer]

When the strut flutter started on my Jabi it varied from mild shaking (still very noticeable) to quite severe (scared the hell out of me) I realised that a sudden gust would set it off, or maybe a turn into the wind or leveling out after a climb, but slowing down (at least 10-15 knots) would always stop the shaking. What initially gave it away as strut flutter and not aileron flutter was that on one early morning flight it stared up quite bad and with the early morning sun on the strut I could clearly see the strut shaking.

 

Anyway, I just thought for the price of 2 x strips of hardware shop extruded aluminum and a bit of Duck tape you would be able to eliminate strut flutter off the list and be 100% sure it was aileron flutter.

 

Cheers,

 

 

[facthunter]

Agree. ( It's duct tape) BTW. Nev

 

 

[skeptic36]

Bloody iPads, I just read the excellent post from t4flyer and I saw where someone gave it a creative and I thought, who would say that? Had a look and it was me lucky there is an undo button:thumb up:

 

 

[ianboag]

When the strut flutter started on my Jabi it varied from mild shaking (still very noticeable) to quite severe (scared the hell out of me) I realised that a sudden gust would set it off, or maybe a turn into the wind or leveling out after a climb, but slowing down (at least 10-15 knots) would always stop the shaking. What initially gave it away as strut flutter and not aileron flutter was that on one early morning flight it stared up quite bad and with the early morning sun on the strut I could clearly see the strut shaking. Anyway, I just thought for the price of 2 x strips of hardware shop extruded aluminum and a bit of Duck tape you would be able to eliminate strut flutter off the list and be 100% sure it was aileron flutter.

 

Cheers,

One will have to give it a whirl I guess. In the GA world this would have been an SB if not an AD ......

 

 

[dazza 38]

One will have to give it a whirl I guess. In the GA world this would have been an SB if not an AD ......

It wouldn't hurt to give it a try. As it has been proven not to do anything detrimental to the handling of the aircraft

 

 

[t4flyer]

Nev, I was wondering who would fall into that trap. Do you want to win a few beers..... It works for me every time and if we were in a bar you would owe me one right now See below, I won the last bet to the original "Duct tape" was in fact Duck Tape and is still known as Duck Tape in many markets.....

 

Actually he was quite correct... I lost a bet over that some years ago

 

 

 

The original was an adhesive applied to a Duck cotton material .... both are used inter changeable these days, and the brand we buy here in USA is DUCK Tape.

 

 

 

[Guest Andys@coffs]

If the aircraft was on the Australian RAA register then if 19 registered I'd agree wholeheartedly and suggest the owner needs to then do some caveat emptoring testflying......24 registered....not so much!

 

If this is a known issue and there is a known fix then get the manufacturer to issue instructions for retrofit that you can then rely on.

 

There are many things that an individual not in the know could do to screw this up (pun intended) royally....

 

For example ...cant find extruded aluminium locally but did find some stainless steel and I connected it to the strut with Gal self taping screws.....I hangar next to the ocean and in 18 months the strut mechanical integrity has been wholly compromised by dissimilar metal corrosion....flying along and the strut gave way......

 

Unlikely to be so extreme, but raw uncoated extruded aluminium probably shouldn't be added to the strut without corrosion treatment and I'd want to be pretty satisfied that from an engineering viewpoint adding 1-10 holes in the strut doesn't somehow impact on the integrity of the strut. Given that most of us don't have access to, nor know how to interpret, a finite element modelling package that can be used to assess mechanical structures natural resonance frequencies I'd be keen for an engineer to give it the once over to be sure Ive fixed what I intended and haven't introduced something worse....

 

In Australia if its 24 registered LSA you cannot modify unless the manufacturer gives you written authorisation to do so. I would personally, having received written authorisation have a copy of that sent to RAAus to be included in the tech file

 

Andy

 

 

[t4flyer]

Andy, the strut is not touched. The strip is attached to the strut fairing. If you want to try it you can easily attach the strips with Duck/Duct tape as I did. (I used white tape and nobody even noticed it) I have no idea how your Reg's work so was not intending to encourage anyone to break any laws. Hell's bell's, I was just sharing some practical experience.

 

Dazza 38, "In my experience" I could not feel any adverse effect on the aircraft handling. Actually no effect at all other than the strut shaking completely disappeared. As I said plenty Jabi's here have these strips fitted.

 

Cheers,

 

 

[Keenaviator]

Jabiru struts are extruded in a faired shape. They are not a round tube fitted with a fairing therefore any holes drilled into the strut or addons must comply with Jabiru's engineer recommendations and approved practices.

 

Regards, Laurie

 

 

[t4flyer]

Ok, I was only trying to give the benefit of my personal experience to a fellow Jabiru pilot in need, not start an argument! So anyway I looked on your AUS Jabiru site, a bit of digging and I found this.

 

http://www.jabiru.net.au/Manuals/Airframe%20Construction/Sections/Pre-Paint_Wings_Fit%20vibration%20damper%20to%20strut_J1702400.pdf

 

What do you know.... instructions on how to fit a strut vibration damper, technical drawings and all, and all in lovely living colour. There you go. According to Jabiru AUS all that's needed is..

 

Materials required:

 

Vibration damping strips (approx 975 x 11 x 3mm, pack of 2) ►

 

5-minute Araldite and flock

 

TAPK 3-3 1/16” CSK pop rivets (10 of, 5 per strut)

 

 

AND of course whatever legal issues your Regs require you to do to be able to fit the bloody things.

 

 

 

Cheers,

 

 

[planesmaker]

Andy, the strut is not touched. The strip is attached to the strut fairing. If you want to try it you can easily attach the strips with Duck/Duct tape as I did. (I used white tape and nobody even noticed it) I have no idea how your Reg's work so was not intending to encourage anyone to break any laws. Hell's bell's, I was just sharing some practical experience.Dazza 38, "In my experience" I could not feel any adverse effect on the aircraft handling. Actually no effect at all other than the strut shaking completely disappeared. As I said plenty Jabi's here have these strips fitted.

 

Cheers,

Hey t4 thanks for sharing with us it's what the forums are all about. Tom

 

 

[Guest Andys@coffs]

Andy, the strut is not touched. The strip is attached to the strut fairing. If you want to try it you can easily attach the strips with Duck/Duct tape as I did. (I used white tape and nobody even noticed it) I have no idea how your Reg's work so was not intending to encourage anyone to break any laws. Hell's bell's, I was just sharing some practical experience.Dazza 38, "In my experience" I could not feel any adverse effect on the aircraft handling. Actually no effect at all other than the strut shaking completely disappeared. As I said plenty Jabi's here have these strips fitted.

 

Cheers,

t4flyer

 

no issue with what you have provided, my warning only to those within the Australian regulatory regime and those who aren't aviation maintenance savy. No matter who provides info it is best if we all understand the issue and the fix and the consequences. The fact that J have provided full instructions means that anyone doing the fix themselves (rego type allowing) or arranging for a L2 or better to do the work is now fully covered so thanks for providing that extra bit of info.

 

If I came across as argumentative then I apologise it was not my intent, rather just to remind people of the limitations and requirements that their rego type within AU may well apply.

 

Thanks again

 

Andy

 

 

[Oscar]

This sounds to me to be a serious issue and should not be taken lightly. As far as I know, the lift strut end fittings are the most highly-loaded component in a Jabiru, and being aluminium, must be susceptible to fatigue by the nature of the material. While obviously they are sufficiently strong in normal conditions (Jabs don't lose wings), my inexpert engineering understanding is that heavy vibration in a highly-loaded alloy component is a pretty sure-fire way of introducing fatigue.

 

If the strut is fluttering then surely the attachment bolts are being in effect slammed sideways in their holes. It may be that the elasticity of the 'glass lift-strut pick-ups on the fuselage and wing main-spar plus perhaps some bending of the lift-strut attachment bolt itself (which as we know is not tightened down so the end-fitting clamps to the strut pick-ups) dampens the effect of this sudden twisting load-reversal sufficiently that it does not load the actual bolt holes in the lift-strut fittings, but I'd have thought this type of loading would be working the stainless-steel inserts in the lift-strut pick-ups in their holes in the 'glass 'plank' at the very least.

 

 

[Dafydd Llewellyn]

Couple of points: What is being described here is not "flutter" in the normal sense of the word; it is extremely rare to come across a true flutter mode that is not almost explosively destructive; flutter is by its normal definition, a coupling between two separate motions (e.g., but not limited to, wing bending and aileron motion about its hinges) that can interact in such a way as to extract energy from the airflow to build up the ampliude of the motion. This is resisted by whatever inherent structural damping may be present. Generally, if the energy is extracted from the airflow faster than it can be dissipated by the structural damping, the motion "runs away" - and when this happens, what the pilot experiences is not "flutter" but FLUT! and a shower of pieces of aeroplane. DJP is almost certainly correct in ascribing it to a Von Karman vortex street; and the Jabiru fix tends to confirms that.

 

It's therefore a nuisance-value vibration - but if allowed to persist, it could - at least potentially, in the long term - reduce the fatigue life of the lift strut.

 

However most jabs up to & including the 160 do not have mass-balanced ailerons; nor for that matter does the Blanik, and I venture to guess, quite a few recreational aircraft types. For such aircraft, the stiffness of the control linkages and freedom from slop anywhere in the system is important; and the speed limits should be rigorously observed. Any aircraft with non-mass-balanced control surfaces will flutter if you fly it fast enough, and when it does, the effect would be like a bomb going off. The friction of the Jabiru control cables is probably a major part of the damping that stops the system from fluttering.

 

 

[t4flyer]

Dafydd, I have very limited (theoretical or practical) knowledge of flutter, but the little I do know agrees with your excellent explanation. My understanding is that aileron flutter would very quickly lead to in-flight breakup or at the very least parts of your aircraft no longer being present on your aircraft!

 

Having experienced the strut vibration issue first hand and knowing how it can shake/vibrate the whole plane and now knowing there is a very simple solution, I thought it best to throw it into the pot as it may help others avoid a very worrying situation. (and maybe the root cause of Ian's problem) Incidentally, my AMO who first told me about the strut vibration issue said he had never experienced it in any of his Jabiru's himself. On a later trip to his workshop he showed me his J430 which now had the vibration strips fitted, apparently on a long cross country trip he experienced strut vibration, why it started up? but it gave him a fright and the strips were installed as soon as he got home! never happened again. There are plenty of older Jabi's flying around here without the vibration strips, but for some time now all factory new Jabi's are fitted with the vibration strips. Personally, I would fit them to any Jabiru I owned if only as a precautionary measure.

 

Cheers,

 

 

[Oscar]

[quote= DJP is almost certainly correct in ascribing it to a Von Karman vortex street; and the Jabiru fix tends to confirms that.

 

It's therefore a nuisance-value vibration - but if allowed to persist, it could - at least potentially, in the long term - reduce the fatigue life of the lift strut.

 

Would it be unreasonable to suggest that simple vg's applied to the lift-strut upper (or lower) surface would change the airflow so that the Von Karman effect would be negated? It seems to me that what is required is a change in a 'balanced' airflow condition that produces a harmonic interaction: change the characteristics of the airflow on one side and surely the harmonic condition would be dissipated? Thus no intrusion of the structural integrity of the strut - just Araldite the buggers on.

 

 

[Dafydd Llewellyn]

 

Yes, VG strips may well have that effect. The vortex street mechanism is essentially (to my understanding) a consequence of having separated airflow over the back portion of the strut - and the vortex street causes it to re-attach momentarily one one side, then the other side etc which sets up a minute cyclic twisting of the strut, which perpetuates the cycle. If the flow were fully attached - which is, after all, the purpose of vortex strips - this cycle cannot begin.