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  #11  
Old 02-12-2008, 08:23 PM
mkraus01 mkraus01 is offline
 
Join Date: Aug 2005
Location: Grass Lake, MI
Posts: 128
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This is a very common issue. My friend had it on this -7 after about 100 hours. We fabbed a new one with 0.093 aluminum.... So far so good, but it has only been another 100 hours...
-Mike
RV-4 Flying
RV-10 FWF and wiring
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  #12  
Old 02-13-2008, 01:17 AM
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sprucemoose sprucemoose is offline
 
Join Date: Jan 2005
Location: MKE
Posts: 1,519
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I discovered very similar cracking on mine at the first annual (150 or so hours.) I made a new plate out of .090 al sheet, and I replaced the washers with large area washers to spread the load out more. Looking at your picture, the crack seems to follow the outline of the washers on the outer portion of the plate.

I think making it out of 4130 is overkill and will add weight. And powdercoat is not a great idea as it will hide cracks. Make it out of aluminum, don't prime it at all and replace it every five years, and you'll be fine.
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RV-6, RLU-1 built & flying
Tech Counselor, Flight Advisor & President, EAA Chapter 18
Milwaukee
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  #13  
Old 02-13-2008, 01:47 AM
Steve Sampson Steve Sampson is offline
 
Join Date: Aug 2005
Location: N. Yorkshire, England
Posts: 1,050
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Does this happen because the cowl/air box junction is too tight, causing the cowl to be stressing the airbox when the engine shakes at shut down?
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G-IKON Build log here , or Index to blog here.
RV4 #4478 - Flying since 16th June '08. First flight video here.
Circuits at my 1000' strip.
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  #14  
Old 02-13-2008, 05:58 AM
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pierre smith pierre smith is offline
 
Join Date: Jan 2005
Location: Louisville, Ga
Posts: 7,840
Default Could be.....

......engine shake, Steve.

A friend placed putty on either side of the airbox at the closest area to the cowl. He then started the engine with the top cowl off and shut it down, watching it shake. He then looked at the putty and was happy with the clearance.

Regards,
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Pierre Smith
RV-10, 510 TT
RV6A (Sojourner) 180 HP, Catto 3 Bl (502Hrs), gone...and already missed
Air Tractor AT 502B PT 6-15 Sold
Air Tractor 402 PT-6-20 Sold
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Louisville, Ga

It's never skill or craftsmanship that completes airplanes, it's the will to do so,
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Dues gladly paid!
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  #15  
Old 02-13-2008, 07:53 AM
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smokyray smokyray is offline
 
Join Date: Jan 2005
Location: TX32
Posts: 1,891
Default Never liked the FAB...

I'm pretty sure the failure wouldn't have killed you, probably not even a little bit scared. Hey, what doesn't kill you makes you stronger!
I flew my RV4 for 1500 hours with no FAB at all. I used a home-made ram air system with a K&N filter on top.
The problem with the FAB is it sticks out into space with no support, the vibration inflight causes the frame to crack as yours did. The easy fix is to fabricate a support bracket that attaches to the forward bolt hole on top up to the lower engine area. I fixed my buddies RV4 FAB this way with no further issues. Works well, lasts a long time...

Rob Ray
RV4ormerly
F16 sadly formerly
HR2 currently
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  #16  
Old 02-13-2008, 05:25 PM
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SMO SMO is offline
 
Join Date: Oct 2007
Location: Salmon Arm, BC
Posts: 933
Default I'm convinced

Thanks for the input, I have changed my mind on using steel and will use a little thicker aluminum. I will also add a support as suggested by smokyray, and will inspect it every time the cowl comes off. The suggestion regarding getting the prop dynamically balanced seems very worthwhile as this is not the only part that could be helped by reduced vibration.
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1987 RV-4 Sold
2003 Super Decathlon - Sold
F1 EVO Rocket, first flight May 31/14
First in line for the Sonex JSX-2T kit
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  #17  
Old 02-15-2008, 08:39 PM
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dougweil dougweil is offline
 
Join Date: Jun 2005
Location: Hudson, WI
Posts: 244
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A very common situation on RVs. When mine cracked I made up a new mount using 6061T-6 which is MUCH less likely to crack.
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Doug Weiler

Hudson, WI
president, Twin Cities RV Builders Group
RV-4, sold
RV-7 completed and flying N722DW, 840 hours
Based Lake Elmo, MN (21D)
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