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  #1  
Old 06-25-2019, 12:48 PM
redhawk redhawk is offline
 
Join Date: May 2008
Location: Boulder, CO
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Default Landing gear weldment cracks from rough field ops

Hello out there. Just want to get some advice from anyone who has had to repair landing gear weldments from this common problem of Weldment cracks from landing on grass strips. I understand there is this reinforcement kit / heavier duty gear weldments from Vans. (WD-402-L&R, WD-403-L&R)
How big a job would this be?
Also thinking of converting to the long gear leg mount and legs while doing this...
Thanks

Last edited by redhawk : 06-25-2019 at 02:47 PM. Reason: terminology
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  #2  
Old 06-25-2019, 01:33 PM
rv7charlie rv7charlie is offline
 
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Is the structure ok, and you just have cracks in the stainless firewall? If so, fill the cracks with intumescent caulk. The firewall isn't structural. My -4 firewall has cracks, with no damage at all to any aluminum or steel around the stainless cracks.
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Old 06-25-2019, 01:43 PM
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smokyray smokyray is offline
 
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Location: TX32
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Default Crack me up...

Quote:
Originally Posted by redhawk View Post
Hello out there. Just want to get some advice from anyone who has had to repair a firewall from this common problem of firewall cracks from landing on grass strips. I heard there may be a reinforcement kit from Vans but did not find it on their website and the parts lady did not know about it.
Thanks
Hey Dick,
Pretty common issue on most Fours, most of the originals being over 40 years old. The firewall isn't structural so stop drilling the SS sheet is acceptable however comma, the aluminum parts and aluminum structure behind should be inspected as well. The lower engine mount fillets as well as the gear tubes should also be closely inspected for cracks. Many years ago my RV4 guru/builder friend Arvil fabricated a set of what would later become known in the RV4 world as "Heavy weldments". He liked them so much he shared the plans with Van and magically they appeared in later RV4 kits.* They reinforce the lower engine mount area and provide much needed structural enhancement in a critical area.

Here is an excellent blast from the past thread on that very subject:
http://www.vansairforce.com/communit...t=28481&page=4

The weldments are triangular in shape and located in the lower corners of the forward fuselage. A close look behind the rudder pedals will confirm if you have them. From Steve Sampson's excellent blog:
http://gikoncnsdr.blogspot.com/2006/...are.html#links

V/R
Smokey

*Arvil said he never heard back from Vans but the new weldments were identical to his.


Last edited by smokyray : 06-25-2019 at 01:52 PM.
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  #4  
Old 06-25-2019, 02:34 PM
redhawk redhawk is offline
 
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Default Weldment cracks

Thanks Smokey for those links... very helpful!
Dick
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  #5  
Old 06-25-2019, 04:06 PM
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Flyhud Flyhud is offline
 
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My -4 had firewall cracks, upon close inspection, the engine mount also had cracks on both sides around the lower clusters. I decided to replace the lower firewall and replace the engine mount (long leg). When I got the lower firewall off I also found cracks on the left center engine mount attach bracket as well as slight deformation of one of the center angles that runs from the firewall to the spar. The aircraft already had the late style heavy weldments, which were in good shape. I added the rv-8 style gussets that tie in between the lower longeron and the vertical angle. The fuselage side skins have to be partially removed to install these gussets. There is no kit, and as far as I know the gusset is not an approved modification from Vans. I used pictures and posts from other members on here to fabricate mine. (Search rv-4 gusset)The gussets should help mitigate flexing in the lower firewall corners and hopefully reduce the likelihood of cracking in the firewall. I have not seen any long term reports of how well this modification has worked. My aircraft finished in ‘94 had about 500 hours at the time I did this , it had spent its entire life operating out of a rough grass strip. Although as previously posted, the firewall is not structural, I would encourage you to inspect the entire area closely for other signs of damage.
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  #6  
Old 07-01-2019, 12:44 PM
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gmcjetpilot gmcjetpilot is offline
 
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The fire wall is not structural? Really. Let's exam that. The Engine mount is ridged weldment. It connects at four points (landing gear loads also go through the engine mount as well).

The firewall has four corner weldments that mate with engine mount. Longerons from the weldments go into the fuselage where engine (gear) mount attaches. The out of plane loads (perpendicular to the fire wall) are reacted by the longerons going into the fuselage.

There are some angles on the firewall to stiffen the firewall from buckling. Despite some gussets there is not a lot of righty without the fire wall. The firewall is picking up some of the load. This is called statically indeterminate structure, where there are more than one load path. To be sure the firewall is picking up in plnae loads or shear.

The fuselage skin attaches to the firewall around the edge. This provides stiffness to the fuselage and allows fuselage loads to transfer INTO the firewall or engine loads to transfer into the fuselage..... If this was a welded fuselage airplane yes the fire wall is just a shield. This is a semi- monocoque airplane (monocogue means "one shel"l and the "semi" means it is a stiffened shell not a pure shell). The firewall is part of the shell and closes out the fuselage to react torsion.

What if you had NO FIREWALL (forget about heat, fire and fumes for a second). What would support the fuselage skin? What would support the engine mount points (besides the few angles) to the fuselage? YES there are longerons, but most of the load they react are out of the firewall's plane.... reacting overturning moments from engine and landing gear. The SHEAR (or vertical load) is also reacted by these longerons but somne is in the firewall... likely for some load cases a good bit of load is going into the SS firewall and into the fuselage skin.

You would not have a structurally sound airplane without the firewall. It is "PRIMARY STRUCTURE". FIREWALL TAKES SHEAR LOADS or in-plane loads. Repair all cracks. It is a big part of the structure. As mentioned if you have a firewall crack you may have other cracks. I am NOT SAYING that a crack stopped drilled in the firewall with all other structure sound is unsafe of can't be done. however you should not dismiss it as trivial. After time those cracks will start up again.
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Last edited by gmcjetpilot : 07-01-2019 at 12:58 PM.
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  #7  
Old 07-01-2019, 03:00 PM
rv7charlie rv7charlie is offline
 
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Well, we could believe you, or the guys at the factory....

Seriously, I asked.

Last edited by rv7charlie : 07-01-2019 at 03:03 PM.
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  #8  
Old 07-02-2019, 05:53 AM
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gmcjetpilot gmcjetpilot is offline
 
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Quote:
Originally Posted by rv7charlie View Post
The firewall isn't structural. My -4 firewall has cracks, with no damage at all to any aluminum or steel around the stainless cracks.
Quote:
Originally Posted by rv7charlie View Post
Well, we could believe you, or the guys at the factory.... Seriously, I asked.
I would take cracks in the firewall seriously. Who did you talk to? What did they say? Do you have this in writing? I just disagree the firewall is "not structural". Only one person at Van's Aircraft is an engineer, designed and did stress analysis on RV-4, Richard VanGrunsven. Be careful what you asked and understand what they actually said or mean.

Really no hard feelings if you believe me. [/b] My personal educated opinion is the firewall provides strength, all I'm saying. I doubt Van's aircraft company or the founder are saying the firewall is not structural. [b]Clearly the "hard points" (weldments, heavy angles they attach to) take most of the load by design, but these structural items attack to the firewall. The SS firewall does pick up has flight/ground loads and therefore structural. Is it the prime structural load path for engine/gear loads? No. So it is structural, but secondary to the main hard points. It does not mean FW is not important or cracks do not affect the strength at all. Saying not structural is misleading to me.

When an RV makes a really hard landing what can (and does) happen to the firewall? It buckles. It is taking load. Cracks reduce FW structural integrity. If you disregard cracks without analysis or data that is your choice.

The cracking engine mount weldments on the RV-4 is well known. Van has published info in RVAtor (decades ago) and the topic has been posted in these forums... I'd go there first. If a crack or cracks are large, the repair is more than a stop drill, Sketch all your FW cracks out in detail, location, length, send it to Van's. Ask them if it's OK in writing. We'd all like to get that data point.

The FW is stainless steel for fire resistance, which makes it stronger than aluminum, and I am assuming (guessing) it has more strength than needed just for taxi, flight and landing loads. This may be one thing the FW has going for it to mitigate the crack issue. There may be a small crack acceptable length, but again show me where Van has that in writing. A specific allowable damage that has been analyzed is different than firewall isn't structural. Believe as you like; fly with firewall cracks, up to you. I'd take all cracks in metallic fuselage structure seriously.

A small crack in firewall will case catastrophic failure if all other structure is 100%? No, unlikely if the crack is small. This is due to failsafe or residual strength of the unaffected semi-monocoqeue structure. It is not without effect on the strength.... Redundancy of the structure is why it's light, strong and resilient, from multi-load-paths. Loss of some strength or stiffness means it has to go elsewhere, but can the undamaged structure still take normal loads? Is the residual strength sufficient for all loads with a margin of safety? Don't know unless you do the analysis. If you choose to fly with cracks, I'd inspect them often if not every flight. My choice would be repair it at some point.

Richard VanG says it is good, OK. Someone on the phone at Van said A-OK. I’d ask some more detailed questions, have them email it to me (no offense to people at Van's customer service but not all of them are experts on everything). I'm conservative when it comes to airworthiness of aircraft structure. Bottom line a crack reduces the strength. Is it critical? I'd like to see that analysis or at least have someone explain why it's OK. However fact this area cracks a lot tells me it is not overbuilt and as less than ideal design details... It also tells me this area is under significant loads, both ultimate and fatigue.

"The term semi-monocoqeue refers to a stressed shell structure that is similar to a true monocoque, but which derives at least some of its strength from conventional reinforcement. Semi-monocoque construction is used for, among other things, aircraft fuselages, car bodies and motorcycle frames." Wiki [The firewall is part of the stressed shell with reinforcement. You cannot do without the shell or reinforcement, they work together as a structural component.]
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Last edited by gmcjetpilot : 07-02-2019 at 07:33 AM.
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  #9  
Old 07-02-2019, 07:18 AM
rv7charlie rv7charlie is offline
 
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To be clear, the closest thing I have to credentials on the subject was a literal stay in a Holiday Inn Express a couple of weeks ago. And when I posed the question to Van's, it was about the -7 I'm building; not a -4. But given the factory's incredibly conservative approach when asked about virtually *any* mod to their airframes, I had pretty high confidence in the firewall (the stainless sheet itself, to again be specific) not being structural. What they told me was that I could cut holes anywhere I wanted in the stainless, as long as I avoided the steel weldments and the aluminum angle braces. *My interpretation* (and I always try to be open to correction) was that the steel bits & aluminum angles form a truss (and likely the motor mount itself contributes) to resist racking, and the stainless sheet is there purely for heat/fire/CO protection. Again, my interpretation of what they told me.
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  #10  
Old 07-02-2019, 07:41 AM
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gmcjetpilot gmcjetpilot is offline
 
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Quote:
Originally Posted by rv7charlie View Post
To be clear, the closest thing I have to credentials on the subject was a literal stay in a Holiday Inn Express a couple of weeks ago. And when I posed the question to Van's, it was about the -7 I'm building; not a -4. But given the factory's incredibly conservative approach when asked about virtually *any* mod to their airframes, I had pretty high confidence in the firewall (the stainless sheet itself, to again be specific) not being structural. What they told me was that I could cut holes anywhere I wanted in the stainless, as long as I avoided the steel weldments and the aluminum angle braces. *My interpretation* (and I always try to be open to correction) was that the steel bits & aluminum angles form a truss (and likely the motor mount itself contributes) to resist racking, and the stainless sheet is there purely for heat/fire/CO protection. Again, my interpretation of what they told me.
You are right. The main bits are the primary load path. The SS Firewall is secondary. So we are talking terms, primary structure vs. secondary structure. Just looking at the structure you can see the "Beef" is not the firewall. The fuselage skin aft of the firewall is also structure of course. It is a shell with reinforcement. It all counts, just some of it counts more. This is the point it all works together... What Van's told you is not totally correct IMHO.

My point firewall is part of the structure and takes some load. Therefore it is structural. A crack in the firewall also can be indication of worse. So inspect and repair as required.

This thread has great pictures... You can see where the main strength is.

http://www.vansairforce.com/communit...0&postcount=23

This is the whole thread. The original installation was a mess so they rebuilt it... they took this structure seriously.
http://www.vansairforce.com/communit...d.php?t=94509&

Is it worth fixing for a small crack in firewall? I would take the engine mount off and get at back and have a good look. People rebuild this area all the time. I have seen it done by a hanger mate for cracked weldments. It is a lot of work. I know Van beefed up the weldments in later versions. Good discussion. Regards.
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Last edited by gmcjetpilot : 07-02-2019 at 07:51 AM.
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