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Firewall cracks from tough field ops

redhawk

Well Known Member
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
 
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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.
 
Crack me up...

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/community/showthread.php?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/2006/05/if-you-are-going-to-build-4-there-are.html#links

V/R
Smokey

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

 
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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.
 
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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The firewall isn't structural. My -4 firewall has cracks, with no damage at all to any aluminum or steel around the stainless cracks.
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. 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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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.
 
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/community/showpost.php?p=729280&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/community/showthread.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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Weldment Cracks

I apologize everyone as in my header for this post I meant to say weldment cracks not firewall cracks. The weldments on the earlier RV4s are a weak point in the aircraft and I am looking at buying one that actually has cracked weldments and was looking for owners with experience at replacing these.
 
I apologize everyone as in my header for this post I meant to say weldment cracks not firewall cracks. The weldments on the earlier RV4s are a weak point in the aircraft and I am looking at buying one that actually has cracked weldments and was looking for owners with experience at replacing these.
You are good... me and Charlie got side tracked... My bad....

So if you are not a builder there is time to kearn.:D That thread I linked to was pretty good start. Search these forums for more threads. There are threads going back over 10 years. You might reach out to those members.

It is kind of a big job. I "supervised" my hanger neighbor do this. You are going to take the engine and mount off (someone may have different opinion). You have to support the plane. It is a matter of unbolting and drilling out rivets. Removing the weldment and installing new ones (back drilling to match existing holes). There is a lot of fiddling and possible shimming. Do you have drawings and builders manual? You can get RV-4 drawings and manual on USB drive if you are registered owner from Vans for not too much money. Someone on the fourm might have manual and drawings to loan you.

Many people upgrade their engine mount and gear at this time. However that is more expense and time, with new brake lines, fairings... etc.... However the longer gear helps prop clearance and landing/takeoff angle of attack... Looks better to.

I would put out feelers on these forums (classified) and call Van's for hired hands if it comes to that. Good luck.
 
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RV4 Firewall cracks

Hey guys. My RV 4 has the dreaded firewall and weldment cracks.

Has anyone who has done this repair kept a blog or record of the repair process? I did not build my 4 and my mechanic/friend has no RV repair experience. Instead of re-inventing the wheel, we'd like to learn from others if possible.

Thanks in advance for the help!!
 
Weldment cracks

Mine has the weldment cracks but my firewall is fine. Pls include me in the circulation of any repair logs.
 
Hopefully someone chimes in with a link to a documented repair effort.
My short geared -4 came from Canada and the weldments were replaced some years ago. I’ve built a 7A so I’ve installed those weldments.
Steve, since you’re weldments are cracked you must make the repair. It’s a fair amount of work but but a straight forward project that you and your mechanic friend can handle.
Slats, as stated earlier in this thread, the FW is not structural. Lots of -4s flying with cracked and wrinkled FW’s.
You may want to consider reenforcing the lower weldments to preclude replacement going forward. The weakest aspect of the gen 1 weldments is that they were split at their vertex where they nest into the lower longeron. The newer ones were welded closed.
Adding a doubler angle on the inside that effectively ties the split closed is likely all that needs to be done. Structural screws can be used to replace the rivets.
Others have reinforced the weldments with external gussets.
 
I have replaced firewall weldments on 3 RV 4s over the years .Would talk on the phone about the process. Butch 541 326 7851.
 
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