RE: What do you think about JETLEGEND L39?  
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RE: What do you think about JETLEGEND L39? - 5/7/2008 6:34:39 PM   
jeff sewell


 

Posts: 321
Joined: 7/26/2005
From: hull, UNITED KINGDOM
Status: online 		Hi guys,

Just reading the thread and must say i am very surpised at the loss of an L39 due to this failure. It surprises me a lot as JL use this method on all their jets and i have never seen this fault before on any of the models i have sold. I work as a test control engineer at BAE Systems Structural Test Facility which tests components and full aircraft (military) on both static and fatigue tests, so i decided to approach one of the senior stress and structures guys about this issue. He said that a tube in itself was not the correct method really as compression and tension occurs on both the outer and inner walls of the tube. A tube really handles torsional forces much better than a bending force. However when i asked him about the introduction of further tubes inside the main one, he said that they increase the overall strength of the component by a very high factor as long as the tubes are bonded to each other. After showing him the images he concluded that the tube was probably slightly damaged at the entry point to the receiver which would then lead to a catastrophic failure of the spar. This could be mitigated by gently removing the sharp edge to the receiver which i would advise everyone to do. In full size testing the general rule is the first time is fluke, the second is coincidence and the third is science in other words repeatability is the only way that anyone can legitimately say there is a fault. As there have not been a number of cases, reported or otherwise, of this occurring to other L39's or other JL models then we have to assume either, 1. Manufacturing defect, 2. Post manufacture damage to the spar, or 3. Airframe taken beyond maximum load. I would be inclined to think either point 1 or 2 as a major failure to the airframe structure is more likelythan breaking the spar due to high G.
I would ensure that the tubes in the wing are bonded to each other by running some thin cyano carefully around the spar ends. I will pass on my finding to Yang also and suggest the introduction of a solid spar. I will perform the mods as i have outlined and mentioned previously in this thread, although i personally won't bother adding any extra rods to the spars.

Jeff



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       Post #: 751

RE: What do you think about JETLEGEND L39? - 5/7/2008 6:48:37 PM   
dbarrym



Posts: 1201
Joined: 10/21/2003
From: Carlsbad, CA, USA
Status: offline 		Jeff -

I agree 100% with your assesment, and I suspect that there was damage (nicks/cuts in the tube) or possibly a manufacturing defect in the bonding of the tubes. I am not an ME, but I have built two full scale A/C and have a fair amount of metal and composite fabrication experience. I always go over the fittings and hardware of any new kit or plane looking for failure prone spots such as these.

When I put together my L39, I did my best to de-burr the edges of the aluminum wing spar receivers in the fuse using a hand-held rotary deburring tool. It's tough to get access through the wing root as the tool is by necessity of larger diameter than the wing rod. It would sure help if you asked Yang to have these parts cleaned up before assembly - AND had some added CF stiffening to the fuse bottom for handling purposes.

I haven't seen any signs of wear or damage to my tubes, but then I only have 5 flights on mine so far, and I keep my flight G-loads down to "scale" levels for flight realism anyway. I am still planning on bonding in some aluminum rod, if for no other reason than peace of mind (and some extra added protection should the tubes crack at some point).

Barry

(in reply to jeff sewell)
       Post #: 752

RE: What do you think about JETLEGEND L39? - 5/7/2008 6:51:59 PM   
aerialcameraman


 

Posts: 58
Joined: 1/19/2007
From: Limassol, CYPRUS
Status: offline 		Jeff

It sounds like we are on the same track, I think that the solid bar insert will suffice to make the model safe but the tube idea still sucks.

Any chance that you can get the factory to send me a new clear canopy that was badly scratched when I received my kit? It seems that all communication with Mr Yang is only done through a third party?

Denny Rowland

(in reply to jeff sewell)
       Post #: 753

RE: What do you think about JETLEGEND L39? - 5/7/2008 7:19:18 PM   
Don Perry



Posts: 407
Joined: 4/17/2002
From: Oklahoama City, OK, USA
Status: offline 		I have around 50 flights on mine and just inspected the tubes.

No visible cracks or scuffing on any of the rods.

Everything ok so far.



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       Post #: 754

RE: What do you think about JETLEGEND L39? - 5/7/2008 7:41:31 PM   
jeff sewell


 

Posts: 321
Joined: 7/26/2005
From: hull, UNITED KINGDOM
Status: online 		Hi Denny,

I will ask Yang for that canopy when i send him info on this subject [)]

Anyone who owns one of these models or is thinking of buying one should consider the slight mods, if only for peace of mind. However please don't let this issue put you off considering owning what is a very good jet.

Cheers

Jeff


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RE: What do you think about JETLEGEND L39? - 5/7/2008 8:39:26 PM   
avicom


 

Posts: 996
Joined: 7/3/2003
From: Rio Grande, PUERTO RICO (USA)
Status: offline 		I migth be wrong, but this plane does not come with the bridge former on top of the engine bay like the F-18 kit from others manufacturers. When the wing spars are short like in this plane this rienforment help prevent the structural disformation to the fuse cause by the g-force. This bridge is not necessary when you have a wing tube passing all the way inside the fuselage to both ends to insert the wings. After seeing this I'm thinking to make that bridge to reinforce Dr.'s plane plus inserting some alum rods inside the wing carbon tubes. My 2 cents.

Alex

(in reply to jeff sewell)
       Post #: 756

RE: What do you think about JETLEGEND L39? - 5/7/2008 10:16:01 PM   
Don Perry



Posts: 407
Joined: 4/17/2002
From: Oklahoama City, OK, USA
Status: offline 		Well for peace of mind I just Hysol'ed 4 10.9 inch long al aly 6061-T6 rods in all of my wing tube spars the od dia for mine was 5/16.

each of the 4 al aly rods was 1.3 oz in weight.

My carbon fiber spars appeared to already have a slightly smaller cf tube in them already. Now they are solid!

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RE: What do you think about JETLEGEND L39? - 5/7/2008 11:36:02 PM   
ash 26


 

Posts: 88
Joined: 4/3/2006
From: Belmont,Co.Offaly, IRELAND
Status: offline 		Hi Cris
I am very sorry to hear about the faith of the Spider jet L39 as we all have been inspired by it on the great forum Nicolas did on it .
I did look at my setup and it looks the same as every one's what I was going to do to mine is hone the edges as Jeff proposed and first I was going to insert a pice of aly bar but I think a hardwood dowl would do the very same job
I was just going to maiden mine on Monday when I read the bad news so I will put it off till all the modifications are done
all the best

Andreas

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       Post #: 758

RE: What do you think about JETLEGEND L39? - 5/8/2008 5:54:59 AM   
aerialcameraman


 

Posts: 58
Joined: 1/19/2007
From: Limassol, CYPRUS
Status: offline 		A somewhat irrelevant but interesting point is that the full size L39 has a one piece wing that is held on by four bolts only. Thats because they put them in the right places!!

Denny

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RE: What do you think about JETLEGEND L39? - 5/8/2008 4:57:03 PM   
jeff sewell


 

Posts: 321
Joined: 7/26/2005
From: hull, UNITED KINGDOM
Status: online 		Hi Guys,

I've just found out why a tube is used for the spar systems on our JL jets; there are two reasons:

If you look at the stresses induced on a tube using a shear force diagram you would see that the most stress obviously occurs on the wall of the tube in both compression and tension forces. There is a neutral point in the centre of the tube which has little or no forces induced into it when a bending force is applied; effectively you can take that section away hence the use of tube. The introduction of more layers increases the strength of the component, as in the use of more tubes, but the neutral point still exists and is essentially redundant.
The other reason involves the construction of the most common carbon fibre rods; nearly all the fibres are laminated in more or less the same direction which means that the rod can fail dramatically in the event of surface damage. Using a number of separate tubes means that the direction of fibres in each tube is different, therefore spreading the load over a wider area.

Just thought you all may be interested...

Jeff


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RE: What do you think about JETLEGEND L39? - 5/9/2008 7:16:33 AM   
aerialcameraman


 

Posts: 58
Joined: 1/19/2007
From: Limassol, CYPRUS
Status: offline 		Jeff

What you say is true about the neutral point in the centre of the tube but it also applies to the section of tube wall thickness that is not at the top or bottom so very little of the cross section is actually effective. the I beam has been proved to be the best structural load bearer, an example of which is the common RSJ which is still the number one load carrying component used throughout most construction industries.
The other main problem with tubes is that the more load that is applied, the more the tube tries to distort towards an oval section as the top tries to close towards the bottom. this scenario creates longditudinal stresses that try to separate the carbon substrate. Therefore the inclusion of a central bonded plug serves to keep the carbon rod round and add to the overall strength . It can nevertheless only be described as a quick fix solution to a basic design fault.

The rear wing mounting is fine as the loads are different at that point, but the main aerodynamic loads are focussed at the Center of Aerodynamic Pressure (centre of lift) which occurs close to the front rod which is a small diameter in comparison to the wing thickness, the loads are focussed from the wings skin (in compression and tension) to that point thus concentrating the stress. A much larger diameter tube would have improved the situation by reducing the concentration.

Denny

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       Post #: 761

RE: What do you think about JETLEGEND L39? - 5/9/2008 12:06:43 PM   
cairoman



Posts: 738
Joined: 12/6/2001
From: Nicosia, CYPRUS
Status: offline 		This is what I have received from JL this morning. Now Chinese copiers are doubting and questioning the building abilities of European Kit Manufacturers (Spiderjets), as this model was fully built and initially flown by the Master himself (Nicolas, Spiderjets), as you all know, since the beginning of this thread.

As of today, I am moving all my business elsewhere, as this kit maker (JL) only wants to take my money!!!!

{qote}Dear Mr. Christos,

I looked through all the photos and integrated all your statements, I don't think we should be responsible for this accident.

Denny's flutter in L39 rudder can't prove it's the reason to cause the crash, there are so many factors leading to rudder flutter. And there are so many factors leading to crash too. It can't always be attributed to R&D as long as there is a problem. I am not 100% sure R&D has no problem, but it's not proved R&D definitely has problem. Proper installation of model is also very important. Good installation job is essential premise for model show it's performance.

Finally, I would appreciate you purchased our model. Thanks!

Best regards,
Yang {un-quote}

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RE: What do you think about JETLEGEND L39? - 5/9/2008 1:10:37 PM   
SpiderJets



Posts: 850
Joined: 12/26/2001
From: .., BELGIUM
Status: offline 		I definitely think he did not understand what both of you are talking about.

He speaks about rudder flutter on someone elses plane, you talk about a broken carbon wing tube on your plane.



Also, I put my big question marks behind his comments/insinuations your model was not properly built... Oops, that is a tough one for me ! hehehe !
Speaking about an example of manufacturers' total ignorance of customer respect or service , this is indeed one where a manufacturer says in an indirect way to complaints or remarks to customers just to F...-OFF. It's a pitty.

...AND...

I find it very strange that a manufacturer makes such comments on 'proper installations' if he even fails to put the most simple manual in all his kits and leaves EVERYTHING up to the creativity (good or bad) of eventual buyers... There was absolutely NOTHING in the kit, not even a notification of the CG !


I do understand the disappointment of Christos. He simply was not taken seriously.


Just my 2 cents.

Nicolas.





< Message edited by SpiderJets -- 5/9/2008 1:20:08 PM >

(in reply to cairoman)