Wing Oscillation

Australian designed and manufactured Jabiru Aircraft/

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checkride
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Wing Oscillation

Unread post by checkride » Thu Feb 27, 2014 4:33 pm

The wings on my Jabiru J400 oscillates at certain speeds.

When I am lightly loaded the speed will be about 113kt indicated,
when I am heavy load the speed will be anything from 104kts to 109kts.

Any Ideas what would cause this.
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Re: Wing Oscillation

Unread post by RogerRoger » Thu Feb 27, 2014 4:42 pm

its probably the wing strut that picks up a harmonic resonance...some Jabi owners attached a little strip to the bottom of the strut cover (the oval tube running up to the wing) to brake the symmetric airflow. my 50c
Alternatively have your wing attach bolts checked by an AP or better, the factory. DO NOT..tighten any strut, wing or aileron bolts yourself unless you know exactly what the book says!
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Re: Wing Oscillation

Unread post by checkride » Thu Feb 27, 2014 4:52 pm

It will be going to the Factory for servo installation so will get them to check it. It has only started happening since the plane came back from
the factory where they did a respray and new fairing and cowl.
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Re: Wing Oscillation

Unread post by T4flyer » Thu Feb 27, 2014 8:22 pm

Hi Checkride, 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 (J120) I would be flying along merrily at 90 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 Leon at Legair 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)

If (like me) you are still skeptical have a look at the AUS Jabiru site link below, .... instructions on how to fit a "strut vibration damper", technical drawings and all, and all in lovely living colour.

http://www.jabiru.net.au/Manuals/Airfra ... 702400.pdf

Regardless, I would get the factory to fit them while you are there, even if only as a precaution.

Cheers, Don.
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Re: Wing Oscillation

Unread post by checkride » Thu Feb 27, 2014 8:36 pm

Thanks Don

I thought for a minute I was the only one having this problem.

I will do that mods.

Thanks again

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Re: Wing Oscillation

Unread post by BigglesSA » Fri Feb 28, 2014 7:12 am

Hi Checkride
I agree with Don but while at the factory just ask them to check the wing and strut mounting point. Know of a recent problem where the wing mounting points on one wing where modified (drilled) incorrectly by an AMO causing wing vibrations at just at certain speeds. Then some years ago the bush on the top strut mounting on Marius's plane was discovered to be a little loose during one of his thorough pre-flights. This did not caused vibration but needed to be corrected.
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Re: Wing Oscillation

Unread post by gavinwolff » Fri Feb 28, 2014 10:00 am

When I still had a share in a Jabi, I did the mod with the aluminum strip to the strut and it worked 100%
Quick easy job, needing ally strip, drill, pop rivets and spraypaint.
Just need to ensure 100% accurate placement so both sides are exactly equal.
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Re: Wing Oscillation

Unread post by T4flyer » Fri Feb 28, 2014 10:43 am

My suggestion, Buy a length of 6mm extruded aluminum from Builders Warehouse or the such like. (I know they sell it at the Builder Warehouse Fourways branch) while you are there purchase a roll of white Duct tape. Cut the aluminum strip to length, position it and stick it "firmly" in place with the white Duct tape. Then go fly your Jabi and see if the vibration has gone. Trust me, my wooden strips stayed "duct taped on" like this for about a year :oops:

But this will only cure vibration caused by "normal" strut oscillation. Biggles is 100% right, check all the obvious mechanical issues such as wing mounting points and excessive aileron cable play (I read on the Aus Jabi forum that excessive aileron cable play also causes "vibration/shaking at certain speeds, cured by installing new cables, apparently a swine of a job!)

Cheers, Don.
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Re: Wing Oscillation

Unread post by Wingsoverafrica » Fri Feb 28, 2014 5:28 pm

One has to assume that all is good with the wing mounting as Jabi George removed the wings for the spray job or not........... :) :) . Well try this mod on the weekend.
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Re: Wing Oscillation

Unread post by Len alford » Sat Mar 01, 2014 12:34 pm

Hi ,
J 400 J160 sp/120 do not need turbulators on the wingstruts.
This particular 400 does it have the mass balanced rudder? An easy way to check is if the rudder is symmetrical or l shaped.
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Re: Wing Oscillation

Unread post by Wingsoverafrica » Sat Mar 01, 2014 6:39 pm

Len alford wrote:Hi ,
J 400 J160 sp/120 do not need turbulators on the wingstruts.
This particular 400 does it have the mass balanced rudder? An easy way to check is if the rudder is symmetrical or l shaped.
Len
Hi Len, if this is the case what could be causing this oscillation surely not the rudder? I'm not aware that you can feel anything through the control surfaces
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Re: Wing Oscillation

Unread post by Len alford » Sun Mar 02, 2014 10:21 am

Wingsoverafrica
This is why I do not post on avcom!After the years of building Jabiru I do believe I have the knowledge and experience to compently answer any question raised on the aitcraft.I am not here to listen or comment on other opinions and guesswork.
You free to contact the factory and ask for myself.( which should be your first recourse)
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Re: Wing Oscillation

Unread post by Wingsoverafrica » Sun Mar 02, 2014 10:59 am

Len alford wrote:Wingsoverafrica
This is why I do not post on avcom!After the years of building Jabiru I do believe I have the knowledge and experience to compently answer any question raised on the aitcraft.I am not here to listen or comment on other opinions and guesswork.
You free to contact the factory and ask for myself.( which should be your first recourse)
Len

Thanks Len, you know this particular aircraft CYR.......you guys did the refurbish.
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Re: Wing Oscillation

Unread post by checkride » Sun Mar 02, 2014 3:19 pm

Hi Len

Here is a tail picture not to sure which tail I have
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Re: Wing Oscillation

Unread post by checkride » Fri Jan 15, 2016 9:39 am

Ok here is a new one . I have no idea what has happened but sunday I will find out and report.
Have a big issue.

I recently replaced both aileron cables about 14 hrs ago same time as the new engine was installed. I did the installation and followed the manual for the rigging and setting of deflections. What transpired today was very poor climb out from Morning star airfield. My son was on his way to Robertson but had to divert to Worcester airfield. The noticed a oscillation from both wings at the wing tip. They reduced speed to around 85 knots and landed safely at Worcester. On further investigation, please see attached video they observed excessive movement/ play on both ailerons. With holding the one side /left this video was done apparently the play is there on the left side but not as bad but still not right. If both ailerons are physically held in place there is very little movement on the stick in the cockpit. If the stick is held in position excessive play again on both sides is noticed. Attached are other pictures the guys took. Everything is in place nothing is loose. Pulled back the black rubber boots on the cable ends as requested by Ricky and no cracks were found.

1. Are these cable made up here or do they come from Australia?
2. All hard points have been checked , all OK. Swaged ends are OK as well.
3. Im thinking there might be an issue with the inner lining maybe the wrong spec causing this place
4.Does anyone else have this issue. have you checked.
5.They noticed that both ailerons were up equally in flight , this is what caused the bad climb out performance, so wing is in reflex right.
6.On the ground stick straight both ailerons are equal in line with the flaps no reFlex. This has to be due to the amount of play on these cables.
7. MY PLAN IS TO REPLACE ONE OF THE CABLES WITH THE OLD ONE THAT WAS REMOVED AND COMPARE. ALSO TO COMPARE THE NEW CABLE I ORDERED IN CASE WITH THE OLD CABLE AND SEE IF THERE ARE ANY DIFFERENCES.

On another note. Flew at 85kts to Robertson as they are closer than Cape Town .The AMO looked at the plane and indicated that this is definitely not right but is unsure of the issue. Plane is now grounded and stuck in Robertson in a Hanger.
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