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  #1  
Old 12-28-2019, 09:37 PM
sblack sblack is offline
 
Join Date: Dec 2009
Location: Montreal
Posts: 1,401
Default Struggling with tank skins

I am building the ranks for my 4. I have the ribs clecoed to the rear baffle, that assembly is on the wing and I have the top of the skin match drilled. I have been trying to bend the skin over the leading edge with cargo straps and I need so much force on the straps that it is crushing the inboard rib but still not lying down on the ribs on the bottom surface near the leading edge. I have 4 cargo straps.I bent a radius into the skin as per the manual. With no straps and the skin clecoed on the upper surface of the ribs the skin sticks out a bit below horizontal. But to get it to lay down with straps alone is impossible. I tried slipping some 2x4s under the straps and it helps, but the skin wonít lay down tight just aft of the leading edge on the lower surface, like the first 1 or 2 holes. I know that if I match drill everything else with that gap I wonít have a good airfoil shape. The gap is about 1/8Ē. The straps are tight like a guitar string and things are bending and distorting. I know something is not right.

This is a 30 yr old slowbuild kit, as I assume all the 4s are. I know 1000s have been built successfully so I am not sure what I am doing wrong. The wing skin is from flat stock. The only radius in the LE was put in by me. I donít know if skins from the factory have the radius pre-bent. I am pretty sure that the bending, or lack there of, is the problem but I am afraid to push down more, creat a crease and scrap the skin. Any advice greatly appreciated. Any pics on a build log of how to strap down the rank skins, or pics of how pre bent tank skins should look prior to installation also appreciated. I really want to finish the rank before the end of Christmas vacation.
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  #2  
Old 12-28-2019, 10:11 PM
PaulvS PaulvS is offline
 
Join Date: Aug 2019
Location: Western Australia
Posts: 94
Default Check ribs?

Agree that something is not quite right, it should not take that much pressure on the cargo straps. Perhaps it is due to the shape of the nose ribs, I recall needing to spend some time tweaking the flanges and fluting carefully to get parts to fit. My 6A is similar vintage and wings are the same as -4 I think, but I can't remember if the tank skin was already pre-curved by Vans. I used the plastic strapping that Vans supplied to hold on the skin, from memory it was assembled in a cradle off of the wing.
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  #3  
Old 12-28-2019, 10:19 PM
FinnFlyer FinnFlyer is offline
 
Join Date: Aug 2018
Location: Bell, FL
Posts: 232
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My old, old RV-4 project came with factory pre-bent LE and tank skins.

Looking at the edges of the skins, there were sharpie markings apparently to be used in the bending brake. If I remember correctly at least 4 or 5 such marks.

https://drive.google.com/open?id=1ho...D4EpwYc3-uyf23

Picture of LE skin, but may give an idea of the number of "bends" the factory used.

Sorry can't be of more help.

Finn

Added: I did my drilling on the spar with spacer blocks between tank baffle and spar, main skins already drilled and clecko'd, and was able to tighten the 3 or 4 cargo straps against 2x4 blocks on the rear spar. I did the LE skin before the tank.
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Last edited by FinnFlyer : 12-28-2019 at 10:27 PM.
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  #4  
Old 12-29-2019, 12:13 AM
sblack sblack is offline
 
Join Date: Dec 2009
Location: Montreal
Posts: 1,401
Default

Quote:
Originally Posted by FinnFlyer View Post
My old, old RV-4 project came with factory pre-bent LE and tank skins.

Looking at the edges of the skins, there were sharpie markings apparently to be used in the bending brake. If I remember correctly at least 4 or 5 such marks.

https://drive.google.com/open?id=1ho...D4EpwYc3-uyf23

Picture of LE skin, but may give an idea of the number of "bends" the factory used.

Sorry can't be of more help.

Finn

Added: I did my drilling on the spar with spacer blocks between tank baffle and spar, main skins already drilled and clecko'd, and was able to tighten the 3 or 4 cargo straps against 2x4 blocks on the rear spar. I did the LE skin before the tank.
Ok that’s interesting. I am doing it the same way, with spacer blocks. It must be the LE bending. I am not sure how to bend it more without creasing it. Thanks for the info.
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Old 12-29-2019, 12:53 AM
Kyle Boatright Kyle Boatright is offline
 
Join Date: Aug 2005
Location: Atlanta, GA
Posts: 3,965
Default

To me it sounds like your skins are too far forward on the top, moving the sharp radius of the skins beyond the sharp radius of the ribs.
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  #6  
Old 12-29-2019, 04:21 AM
MS1095 MS1095 is offline
 
Join Date: Jun 2016
Location: West Chester, PA
Posts: 77
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Are you using the proper size spacers behind the tank baffle? I cut several blocks out of MDF and had to experiment with different sizes to get the skin just right.
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  #7  
Old 12-29-2019, 04:32 AM
DeeCee 57's Avatar
DeeCee 57 DeeCee 57 is offline
 
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Location: ZŁrich W, Switzerland, Europe, Earth, Milky Way, known Universe...
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Default

Quote:
My old, old RV-4 project came with factory pre-bent LE and tank skins.
Same here.
Try to prebend the skin, the closer the spacing the better, right and left skin at the same time. A deluxe way would be to fabricate tank rib form blocks outta wood and form the skins over them using the straps.
And check the the thickness of the skin, just wanna be sure it is of the correct 0.32 and 2024-T3 gage...
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  #8  
Old 12-29-2019, 05:51 AM
PaulvS PaulvS is offline
 
Join Date: Aug 2019
Location: Western Australia
Posts: 94
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Quote:
Originally Posted by sblack View Post
Ok thatís interesting. I am doing it the same way, with spacer blocks. It must be the LE bending. I am not sure how to bend it more without creasing it. Thanks for the info.
Thinking about this a bit more, it probably is the LE bending. The factory bent those in my kit, like the other poster said, by making a number of smaller bends close together, presumably on a bending brake. Can you get access to a bending brake? It will probably be a slow and iterative process to make successive bends so as to match the LE rib radius.
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  #9  
Old 12-29-2019, 06:24 AM
RussellT RussellT is offline
 
Join Date: May 2016
Location: Albany Western Australia
Posts: 61
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This pic shows the amount of leading edge pre bend from the factory with my RV 3B wing kit. It played quite nicely without undue force. If (as you say) you are starting with flat stock then a substantial pre form of the leading edge will make it whole lot easier. Hopefully your predrilled top holes with still line up.

Russell
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  #10  
Old 12-29-2019, 11:30 AM
Christopher Murphy Christopher Murphy is offline
 
Join Date: Feb 2007
Location: colorado
Posts: 850
Default The trick

Should have drilled the bottom skins first.


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