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  #31  
Old 04-10-2008, 12:46 AM
Steve Sampson Steve Sampson is offline
 
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Location: N. Yorkshire, England
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Dave- I feel for you. Clearly he was not an expert.

I would be interested to know which generation of weldemnts you had. The current generation, delivered I think since '94, can be seen at item 3 here.

There are two earlier parts which VANS shipped and then beefed up to try to stop this happening. I wonder if any of the pictures we have seen are with the current post '94 parts?
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  #32  
Old 04-10-2008, 02:29 PM
WSBuilder WSBuilder is offline
 
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Default Hash Marks?

Dave,
What are the four lines on the MLG receptacle? Are those scratches, gouges or jjust markings?
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  #33  
Old 04-10-2008, 04:00 PM
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777Dave 777Dave is offline
 
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Default weldments

Most people seem to feel the weldments are of the heavier type and possibly were replaced. They seem to be ok but I am going to make sure. I have had about 10 different builders/experts have a look at the airplane and everyone feel s it is serviceable but I would like to repair the damage. I intend to fly it this summer and plan to fix it in the fall...
I believe the marks on the engine mount are scratches. The mount looks ok.

Dave
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  #34  
Old 04-10-2008, 08:17 PM
kleecker kleecker is offline
 
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Location: Elkton, MD
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Quote:
Originally Posted by 777Dave View Post
I am dealing with the same issue, pics enclosed. In my case the previous owner claimed no knowledge of the damage but is willing to pay to have it fixed. I know its a big job but feel for resale it would be wise. Any opinions? Estimates of time required?
Whay annoys me is that I paid to have the airplane inspected by an RV4 expert and it went unnoticed.

Thanks..
I was quoted $5K by someone who owns an RV-4 which I looked at. Now this was to replace all the 4 weldments as well.
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  #35  
Old 04-11-2008, 02:27 AM
SvingenB SvingenB is offline
 
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I don't see how the weldments will fix this unless they are considerably larger and will make the fuselage stiffer. Does anyone have any pictures of the two versions to see the difference?
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  #36  
Old 04-11-2008, 09:08 AM
Steve Sampson Steve Sampson is offline
 
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Bjonar, I think it works like this. VANS have twice increased the thickness and design of the weldement. Most recently in '94.

The early weldements were just cracking up so not doing their job of keeping everything else in their relative position, thus releasing unintended forces into the rest of the structure. The structure while it is naturally flexible since it is a tube, was then flexing even more than intended. And as you summarised somewhere below, the firewall which in the area of the right angle bend, but in fact was never designed to resist the compressive force then buckles, or worse, cracks.

So I think VANS intent was to provide a part that would not crack. What I dont know is if all the buckled firewalls are on aircraft that started out with the flimsier part. No one ever says.

There is in fact (I think) another part that was changed. the triangular fillets on the engine frame, where it bolts to the weldements were not there originallly. Again they stiffen the whole structure.

Having said all that, I think in fact the firewall is still being stressed by heavy landings with the new parts, so I am sure if you give it enough bad treatement you will still get firewall damage.

I had a conversation with a Czech who has asked the Technical Aero Uni in Prague to run a stress analysis on the -4 U/C. The point they came up with allegidly was the design clearly was not ideal. I think the issue is because in a heavy landing, because of the aft sweep of the gear legs there is a very strong twisting force. The design of the frame is triangulated to try to resist this, but it is quite a light structure with short resisting arms.

The -8 I think in contrast has an u/c which is almost perpendicular to the landing forces. (Possibly bolted to the spar?) The -7 and -9(a) do have swept u/c but it attaches to heavy steel parts which bolt very directly onto the spar, so little load transmits through the fuse'. The -4 is unique in that the loads are taken into the frame, with the engine trying to bend it in one direction, and the entire fuse' in the other.

I think Smokey's comments are about right. Nice smooth low energy landings. I have done 1086 landings in my Supercub to date. I had better do some more and you work on your score. The cub you are involved in looks fun!
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  #37  
Old 04-11-2008, 10:17 AM
szicree szicree is offline
 
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I was just looking at my firewall and it occurs to me that placing something small and heavy (like a battery) right on top of the firewall shelf could produce an additional tendency to crumple the lower firewall in the event of a hard landing. Also, I'm wondering if the addition of wooden gearleg stiffeners might make the situation worse.
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  #38  
Old 04-11-2008, 01:21 PM
Steve Sampson Steve Sampson is offline
 
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Steve, first off you wont do hard landings!

Did you put yours on the shelf? I think Smokey's is/was that way. I have put mine above, but not on the shelf. I put a piece of angle across which ties in to one of the vertical angles on the inside of the firewall, and the rudder pedal brackets. Both of them, with AN3. I can find pictures if you are interested.

In all honesty I would think it makes little difference. The shelf is 'hung' on the firewall and the angle cross member above, so little extra strain I would have thought.

The way that I visualise the movement is all about the bottom corner weldements moving up wrt everything else.

How is it going? You must be nearly ready to fly? I am 6 or 8 weeks off I think.

PS I dont think VANS are so enthusiastic about the wood stiffners these days. I didnt have them on the -9a and never missed them and I even flew that a few times on concrete. Why not leave them off unless you need them. Its all more work.
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G-IKON Build log here , or Index to blog here.
RV4 #4478 - Flying since 16th June '08. First flight video here.
Circuits at my 1000' strip.

Last edited by Steve Sampson : 04-11-2008 at 01:23 PM. Reason: To add PS
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  #39  
Old 04-11-2008, 02:39 PM
szicree szicree is offline
 
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Steve,

I actually put the battery between my feet, but I still can't quite get my head around how the firewall gets so bent. I know everyone is focusing on the corner weldments, but I can't understand how the firewall gets wrinkled without also deforming the firewall framework, particularly the vertical members.

As far as my completion date, I really have no idea. All my plane needs at this point is for me to get the cowling and spinner in paintable shape and then to paint the whole project. That being said, work has to come first so things are going slowly. I'm hoping to be done perhaps by the end of summer.
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paid =VAF= dues through June 2013
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  #40  
Old 04-11-2008, 05:11 PM
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jsharkey jsharkey is offline
 
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Just a thought - Is there some pitch (Y) axis rotation going on at the lower mounts during a hard landing that would only cause elastic (recoverable) flexing of a flat 6/7/9 firewall but which causes plastic (permanent) buckling of the footwell folds on a 4? It might even happen on the rebound.

There is after all a huge pry-bar/torque arm in the form of the landing gear leg pivoting roughly about that point.

Simple compression of the firewall to cause buckling seems unlikely since it implies that the engine mounting frame compresses too.

Beefing up or cross-bracing the vertical tube of the engine frame might help to reduce the rotation and avoid the buckling.

Sounds like a neat problem for a college student with access to non-linear, dynamic FEA - and a fast computer.

Jim Sharkey
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