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Firewall wrinkle

Sylvester

Member
Hi,
I've attached some pics of my firewall. The aircraft is a fairly early 4, and had issues here when I first started flying it. 9 years ago I rebuilt it after it suffered a landing accident, using new engine mount, long legs, firewall, side panels and new style corner weldments.
The aircraft operates from a grass strip, and has not had a heavy landing; I'm the only pilot. Within weeks of the rebuild I could hear the firewall flex as I taxied, and just new what was going to happen!
It's now back in the workshop for instrument and radio upgrade, and I'll be doing the firewall as well.
Vans have suggested incorporating the gusset in the lower corners, and I'll be taking great care with alignments.
Does anyone have any other suggestions. I don't really want to do it again!
Thanks
PS. I'm disabled from posting photos. Could do pm's perhaps ? [ed. Upper left corner look for 'Inserting Pics' link. That's how to do it. v/r,dr]
 
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Pictures

Hi All,
Below are some pics of the RV-4 firewall in question.
Cheers,
Tom.
xomy34.jpg

j14mf4.jpg
 
Hmmm

Had those same marks on my -4 way back when...~3000hrs later, it's still going with no signs of distress. If I can recall correctly, this sort of wrinkling is visible on more than a few -4's.

Yes - those showed up after a not-so-smooth landing on grass.

I wouldn't worry about the wrinkles, but I would have a look at the fwd corners to make sure all is well - seems like that area might benefit from some additional interior parts to spread the loads around better. Seems that corner area might be a wee bit too flexible.

I would also add a 1/8" 4130 gussets to the lower attach tubes..
 
Very common. When I bought my plane, I saw that and the guy doing the pre-buy said not to worry. He's done many pre-buys and assured me they are present on almost half the RV4's he's seen. I had some gussets added to mine.
 
I would have expected the vertical loads to be taken by the aluminum angles. Are these wrinkles from lateral loads? Is there a recommended beefup from Vans? I would prefer to do it now before I fly.
 
The loading at that point is complex with the back and outwards sweep of the legs, the twisting moment that the brakes have, torque from the prop and the weight of the motor. I've sat and looked at it for ages and still can't see how it happens!
 
potential fix

The loading at that point is complex with the back and outwards sweep of the legs, the twisting moment that the brakes have, torque from the prop and the weight of the motor. I've sat and looked at it for ages and still can't see how it happens!

The Harmon and F1 use the same sort of setup, but none of those have any firewall wrinkles. I would say that fwd fuselage side skin should be .040, as is done in the larger, heavier examples. Might add some skin stiffeners too, since the HR and F1 fwd side skins are curved, which changes the equation.
 
My early '90s era -4 has firewall wrinkles. But no evidence of skin, attach point weldments, or aluminum structure damage. No detectable distortion to the airframe. The motor mount still aligns when removed/reattached.

Perhaps it's worth pointing out that the firewall is non-structural. If it's wrinkled, it obviously indicates further inspection is needed, but it isn't structural damage in and of itself.
 
Same wrinkles

My 1994 RV-4 has same wrinkle. It has been this way for years and many hours. I didn't build it, but it was this way when I bought it. The A&P/IA maintaining it for almost ten years said if had been that way and unchanged it that time. I've had a couple of other RV builders look and no one seems to think it is an issue. Everything else is straight and solid. The plane has just over 1000 hours TTAFE.
 
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Firewall Wrinkle

I feel your pain! It's fairly common for -4's to suffer wrinkling in the lower corners, usually as the result of a hard landing load or continuous operation off of rough surfaces. The aluminum fuselage structure flexes under load (as it's designed to do), but the hard stainless firewall suffers stress wrinkles or cracks. As long as there is no permanent fuselage primary structure or landing gear leg deformation, the firewall itself is non-structural. If the stainless has any holes or cracks, they should be smoothed and/or stop-drilled, as appropriate and filled with high temp silicone to preserve firewall integrity.

Similar to your situation, I also watched my brand new replacement lower RV-4 firewall crack in the lower corner within a few landings because I didn't "pre-stress" the lower firewall thru-bolts...i.e., before drilling the replacement firewall rivet holes and riveting, use a nut, bolt and fender washers to properly compress the stainless in the corners to mimic the compression imparted by the engine mount. So that would be my primary suggestion in addition to adding the RV-8 style gussets as suggested by Van's!

Likely you are already aware, but be sure to check the engine mount for any cracking as well, since the same stress that induces the firewall wrinkling causes stress at the welds in the gear leg sockets. Engine mount fatigue cracking is not uncommon (mine cracked at about 1100 hours). If your mount is finished in black powder-coat, visual inspection can be difficult, and may require you to remove the powder coat in suspect areas to make a determination. If the engine mount is dark, sometimes the presence of a small amount of grease at the weld can provide a visual cue that the weld may be cracked. If the weld cracks, it's possible for some of the grease to be forced out of the gear socket through the failed weld as the area flexes during taxi, takeoff or landing. Given the option, a painted mount that uses dark primer and light finish color can simplify visual inspection.

Cheers,

Vac
 
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What are the RV8 style gussets suggested by Vans?

I would also like to know this but my guess is they are what is pictured in post #4 on page one. Does not look like they can be added after the side skins are already on. Would love to see any pictures of braces added to a already completed -4.
 
Wrinkle

I think I'm getting to the bottom of this one. The lower weldment bolts to the lower longeron, and is riveted to the side skin. The vertical angles behind the firewall have a gap to the sideskin. On mine, the origin of the crease on the firewall is where the top of the gusset on the weldment is. The additional gusset as suggested by Vans, and mentioned by others, would stop any movement of the weldment transmitting to the vertical angle. This would be confirmed on my aircraft by the evidence of movement on the rivets from the weldment to the sideskin.
I hope this is clear. Let me know if not and I'll try another way. I have a picture of the RV7 plan which shows this, but still can't work out how to post it. I could PM the shot to someone who can perhaps?
 
Dose anyone have a part number for those braces in the picture? Could they be pop rivet to the vertical angle? Bottom rivets would be easy to re rivet but no way to rivet the brace to the vertical angle after side skins are on.
 
I would also like to know this but my guess is they are what is pictured in post #4 on page one. Does not look like they can be added after the side skins are already on. Would love to see any pictures of braces added to a already completed -4.

The only image in post #4 is a ransom note from Photobucket.
 
Having studied the structure fairly closely as it came apart, I think I can see the origin of the problem. The wrinkle starts with movement of the lower weldment (very small) along its longitudinal axis. The start of my deformation is exactly at the top of the diagonal steel gusset on the weldment. I think this is confirmed by slight evidence of rivet movement where it's riveted through the sideskin in the bottom front corner.
Previous threads have said that they don't see problems on aircraft with the extra diagonal gusset. That makes perfect sense with the above in mind.
What makes some aircraft more prone? Uneven grass strips I think, along with the complex loads that the 4 gear and brakes feed in.
That's my take anyway!!
 
Darn, that doesn't look like something that could be added after the build. Oh well, I will just have to make all my landings gentle ones.
 
I have just noticed this wrinkle in my firewall, not exactly sure when it happened, but two possible occasions come to mind.
Gutted :mad:

Has anyone replaced just the bottom 1/3 of the firewall? If I replace the whole firewall it will mean a respray, as the a/c is metallic painted, and blending it in won't be an option.
I am considering replacing the lower 1/3, and making the joint on the firewall.

Any tips/tricks/suggestions very gratefully received.

As the engine mount is so well braced with straight runs bottom lateral and sides vertical, the only way I can think the firewall is being distorted is the upper mounts are being squeezed inwards where there is no horizontal brace....perhaps a brace bar inside the fuselage between the upper mounts might stop the flexing of the firewall?

Neil
 
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Lower firewall

I replaced the lower firewall and it's not that hard to do. If you drill out the rivets along the forward bottom of the fuselage side skin you can then roll it back and add the gusset shown in post #17. After you have removed the engine & mount, it all comes apart pretty fast. If the cowl cheek extensions remain on you will have a hard time riveting the firewall side flanges where they overlap. I think there are two rivets there. Maybe some other type of fastener (?). As I recall, the stainless steel lower firewall only cost about $26. The stock part fit very well and only required slight tweaking to fit with my existing upper f/w. Thin stainless is more forgiving then aluminum. Take everything out of the inside of the fuselage so you can put down some plywood and padding for lying down while working, bucking rivets, etc.

Sounds daunting but its not. In the process of reinstalling the engine I did a better job with all the accessory wires compared the first time around on the initial build ? a plus! Go for it.
 
I have just noticed this wrinkle in my firewall, not exactly sure when it happened, but two possible occasions come to mind.
Gutted :mad:
Has anyone replaced just the bottom 1/3 of the firewall? If I replace the whole firewall it will mean a respray, as the a/c is metallic painted, and blending it in won't be an option.
I am considering replacing the lower 1/3, and making the joint on the firewall.
Any tips/tricks/suggestions very gratefully received.
Neil

Are you doing it for cosmetic reasons? If so, that's your call. But the firewall is non-structural, according to Van's staff. If you can seal the cracks with something like
http://multimedia.3m.com/mws/media/201508O/3m-fire-barrier-2000-silicone-sealants.pdf?&fn=3MFB_Silicon_Sealant_2000+_cg1.p
there should be no functional or safety issues.

Charlie
 
Yes, having spoken with Vans builder help desk they did confirm that is considered non-structural.....but if it is being bent it must be a load path.
I am more convinced having studied photos just now that a brace bar across the top must help stop the flexing of the firewall.
It would be for cosmetic reasons I suppose, but also a/c value when I come to sell it.
Neil
 
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When Steve built the a/c he did not anticipate engine removal with regard to the wiring loom to the engine. Thermocouples I can't cut, but putting a plug in-line would ease this and future engine removals. Can anyone suggest a good plug/socket system for the loom?
Neil
 
RV8 gusset ?

Thanks to all for this heads up, my RV4 side skins are fitted but NOT riveted. So perfect timing to add the gusset in the pic. My vertical aluminum angle has two rivets in the WELDED steel corner fitting. There is no gap to flex the firewall under loads ? That said, does anyone have specs on the RV8 gusset and I will fab and install.
 
Hi Larry. I can't post pics, but pm me and I can email the 7 drawings.
The more I've crawled around mine, it's apparent that the flex is in the vertical 3/4" angles, since the undercarriage loads go up them to the top mount. The side skin doesn't take the load because it's not structurally supported between the two mounts; it's only held there by the 'non structural' firewall. The gusset stiffens it all up. I've seen pics with a gusset top and bottom. The 7 drawing shows top and bottom.
Vic
 
Having trawled the web to find what I can this seems to be one of the best threads currently.
I am considering replacing the damaged section of the firewall, and adding the lower 1/4 gussets, BUT, not riveting them to the skin at the angled side skin longeron, but making a deeper gusset that goes all the way to the bottom stringer and bolting it on.
Cherrymax rivets to fasten it to the vertical....I think there may be room enough between the vertical and the skin to get the CM rivets in before setting them.
This way I will get the same reinforcement, but without disturbing the rivets on the fuselage skin.
Thoughts and comments welcome.
Neil
 
Design and installation almost complete:
0.040 4130 gusset installed from the lower stringer to the vertical angle. Bolted to the lower stringer and solid rivets to the vertical alloy angle on the firewall. No need to disturb any rivets on the skins.
The match drilling of the gusset was achieved by drilling access holes in the fairings (non structural) and using a guide to drill pilot holes so the original holes were not enlarged. The access holes are covered by the u/c leg root fairings so do not require addressing.

https://www.flickr.com/photos/136077758@N05/39801606102/in/dateposted-public/

https://www.flickr.com/photos/136077758@N05/38934771185/in/dateposted-public/

Neil
 
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Hi Guys,

An update on the firewall repair:

I have been in touch with Vans who have been very helpful.

The final repair has included gussets that fit to the lower stringers and to the upper firewall supports made from 0.040" 4130. To facilitate the repair I needed to match drill the gussets at the stringer attachment bolts, and as the fairings on the lower sides of the fuselage are not structural I drilled those to allow me to drill through the original holes using a guide for pilot drilling to ensure I did not open out the original holes. This has meant that I do not need to drill any rivets on the fuselage skins other than those hidden by the u/c fairing.

The replacement firewall section was relatively easy to fabricate.

If you follow the link below it will take you to other photos of the repair scheme.

https://www.flickr.com/photos/136077758@N05/25095731267/in/album-72157692033806604/
 
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