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  #11  
Old 11-06-2017, 11:06 AM
Vac Vac is offline
 
Join Date: Jul 2009
Location: Niceville, Florida
Posts: 306
Arrow 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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Mike Vaccaro
RV-4 2112
Niceville, Florida

Last edited by Vac : 11-06-2017 at 11:19 AM.
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  #12  
Old 11-06-2017, 02:03 PM
sblack sblack is offline
 
Join Date: Dec 2009
Location: Montreal
Posts: 1,195
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What are the RV8 style gussets suggested by Vans?
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Scott Black
RV 4, with an engine...and other stuff
VAF dues 2016
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  #13  
Old 11-07-2017, 09:24 AM
SJordan SJordan is offline
 
Join Date: Sep 2016
Location: Scottsdale, AZ
Posts: 120
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Quote:
Originally Posted by sblack View Post
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.
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  #14  
Old 11-07-2017, 10:15 AM
Sylvester Sylvester is offline
 
Join Date: Mar 2017
Location: Uk
Posts: 5
Default 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?
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  #15  
Old 11-07-2017, 11:06 AM
SJordan SJordan is offline
 
Join Date: Sep 2016
Location: Scottsdale, AZ
Posts: 120
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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.
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  #16  
Old 11-07-2017, 11:39 AM
rv7charlie rv7charlie is offline
 
Join Date: May 2006
Location: Pocahontas MS
Posts: 2,380
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Quote:
Originally Posted by SJordan View Post
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.
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  #17  
Old 11-07-2017, 12:50 PM
SJordan SJordan is offline
 
Join Date: Sep 2016
Location: Scottsdale, AZ
Posts: 120
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Quote:
Originally Posted by rv7charlie View Post
The only image in post #4 is a ransom note from Photobucket.
I have the Photobucket add in for Chrome and it works great, it was mentioned in another VAF post.

Here is the pic for post #4.

[IMG][/IMG]
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  #18  
Old 11-15-2017, 02:36 PM
Sylvester Sylvester is offline
 
Join Date: Mar 2017
Location: Uk
Posts: 5
Default Firewall wrinkle

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!!
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  #19  
Old 11-15-2017, 03:00 PM
sblack sblack is offline
 
Join Date: Dec 2009
Location: Montreal
Posts: 1,195
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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.
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RV 4, with an engine...and other stuff
VAF dues 2016
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  #20  
Old 12-05-2017, 01:08 PM
TIPSYNIPPER TIPSYNIPPER is offline
 
Join Date: Jan 2014
Location: Ardleigh, Essex, UK
Posts: 43
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I have just noticed this wrinkle in my firewall, not exactly sure when it happened, but two possible occasions come to mind.
Gutted

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

Last edited by TIPSYNIPPER : 12-06-2017 at 05:06 AM. Reason: text change
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