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Old 06-25-2019, 01:48 PM
redhawk redhawk is offline
Join Date: May 2008
Location: Boulder, CO
Posts: 198
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...

Last edited by redhawk : 06-25-2019 at 03:47 PM. Reason: terminology
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Old 06-25-2019, 02:33 PM
rv7charlie rv7charlie is offline
Join Date: May 2006
Location: Pocahontas MS
Posts: 3,886

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, 02:43 PM
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smokyray smokyray is offline
Join Date: Jan 2005
Location: TX32
Posts: 1,906
Default Crack me up...

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.
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:

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:


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

Last edited by smokyray : 06-25-2019 at 02:52 PM.
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Old 06-25-2019, 03:34 PM
redhawk redhawk is offline
Join Date: May 2008
Location: Boulder, CO
Posts: 198
Default Weldment cracks

Thanks Smokey for those links... very helpful!
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Old 06-25-2019, 05:06 PM
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Flyhud Flyhud is offline
Join Date: Jul 2018
Location: Burleson, TX
Posts: 92

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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Old 07-01-2019, 01:44 PM
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gmcjetpilot gmcjetpilot is offline
Join Date: Jan 2005
Posts: 4,299

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 01:58 PM.
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Old 07-01-2019, 04:00 PM
rv7charlie rv7charlie is offline
Join Date: May 2006
Location: Pocahontas MS
Posts: 3,886

Well, we could believe you, or the guys at the factory....

Seriously, I asked.

Last edited by rv7charlie : 07-01-2019 at 04:03 PM.
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