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Question on tank vent pressure at high speed

Kahuna

Kahuna

Moderatoring
I have had this very annoying problem of getting air bubbles under the paint on tank rivets. Here is what I know
1. Air bubbles under paint on rivets
2. Size is about the size of eraser heads
3. About 15 rivets on each tank
4. Both Tanks
5. Only on the top side
6. Only on the tanks
7. No fuel in the bubble
8. Bubbles are permanent and will cause me to repaint the tanks.
9. I get more bubbles the more I fly. The first 10 hours I had none. After 100hours I had ~7 per side. 200hours I have ~15 per side.

Here is what I think and you guys tell me if Im whacked.
I have the tank vents venting under the fuse with the #4 line pointing fwd. Like in the picture below. Is it possible that at 200kts I am seriously pressurizing the tank to the point that it is blowing past the proseal and giving me air bubbles?

Does someone know the math on what pressure might be in the tank at say sea level 200kts on a #4 line pointed into the wind?

What do you think?
vent.jpg
 
not much pressure

I can't offer as to why this is happening to you, but I calculate the pressure in the tank to be about 0.9 psi @ 200 kts indicated.
 
I have the exact same problem

After about 25 hours on new paint, except mine is isolated to the left tank for now. About 15 - 20 rivet heads with air bubbles pushing the paint up just enough to notice. Why/what to do? I hope someone knows about this, I thought it was something with the paint!
 
Lift maybe?

Hi Michael,
If there's no fuel in the bubbles, they must just have lifted off the rivets for some other reason. You know, that's about where the greatest amount of lift is generated, isn't it? Could it be that the paint is being sucked up because of a not quite so clean area before paint? :cool:
 
How about solvent evaporating out of the proseal at the rivet site? The fact that it's only on the top and more of a problem the more you fly could simply be due to sun exposure.
 
pierre smith said:
Hi Michael,
If there's no fuel in the bubbles, they must just have lifted off the rivets for some other reason. You know, that's about where the greatest amount of lift is generated, isn't it? Could it be that the paint is being sucked up because of a not quite so clean area before paint? :cool:
Click to expand...

I dont think so cause in every case, the bubble starts at the corner of the rivet and works out away from the rivet. In no case id the entire rivet area getting a bubble.

Also Im VERY surprised its only 1psi smoking along. Can someone confirm the math? Any while we are at it. WHat is the math?

THanks
 
rivet bubbles - me too

Randy,

I am developing the same problem. And it is also only on rivets that are on the top of the tank. First thought was that maybe there was slightly insufficient proseal applied at the factory(quickbuild). Since I did the paint(degreaser, 3M scuffing pad, acetone, etch prime, sealer primer, basecoat, clearcoat) the suspicion that the painter did a poor job does exist. But bubbles only on tank rivets???

On the bright side, I did a terrible paint job so these bubbles are a pretty minor annoyance. :eek:
 
I thought the calculation should be PSI = mph^2 / 57608. That would give 0.694348007 PSI. Might not be correct though, it has been a while.
:rolleyes:
 
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I'm guessing the psi calcs are correct, as Vans has repeatedly told me that during testing of tanks one should only use about 1 psi (or maybe 2, I can't recall).
 
Also Im VERY surprised its only 1psi smoking along. Can someone confirm the math? Any while we are at it. WHat is the math?
Click to expand...
I bought a differential pressure transducer and the manufacturer suggested the 2psi unit (as opposed to 5 or 10) for the speeds we're talking about. So yeah, 1 psi or less sounds right to me. Not sure on the math (yet).
 
I think people have hit on the reason. The paint has failed to adhere to the rivet heads for some reason. I doubt seriously the fuel tank vents have anything to do with it.
 
Mike:

Have the bubbles pentrated the paint? If not, you could take a pen and puncture one of them, then set up a water manometer on your vent line and do a soapy water test to confirm that air is seaping up around the rivets. No air means that there's obviously something else going on.

Also, if you wanted you could simply disconnect your vent line and connect some sort of pressure gauge (i.e. altimeter) to your vent fitting and fly off the other tank to see what sort of pressure you're getting.
 
The Math

Velocity = 200kts = 337.6 ft/s

Density = 0.00238 slug/ft^3 (sea level standard)

Ram Pressure = 1/2 x density x Velocity^2

= 1/2(0.00238)337.6^2 = 135.6 lb/ft^2 = 0.94 psi

Looks like Eric is right on.
 
Thanks for checking my math

keen9a said:
Velocity = 200kts = 337.6 ft/s

Density = 0.00238 slug/ft^3 (sea level standard)

Ram Pressure = 1/2 x density x Velocity^2

= 1/2(0.00238)337.6^2 = 135.6 lb/ft^2 = 0.94 psi

Looks like Eric is right on.
Click to expand...
You mean that someone would doubt my calculations?? ;) Seriously though, I'm glad that someone kept me in check. Since I'm an engineer who loves the SI (basically scientific metric) system, I did it this way:

Dynamic Pressure = (1/2)(Density)(Velocity)^2

Density (@ standard conditions) = 1.2 kg/m^3
Velocity = 200 kts = 102.9 m/s

Dynamic Pressure = (1/2)(1.2kg/m^3)(102.9 m/s)^2
= 6353 Pa
= 0.92 psi

The reason that we have slightly different answers is because I used a slightly different standard atmospheric density than you, but who really cares.

As has already been suggested, the vacuum caused by the lift of the wing will make the differential pressure more than the 0.92 psi (no, I don't know how much more). I still have no idea if the differential pressure is really causing this problem or something else.
 
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Yuck

ericwolf said:
Since I'm an engineer who loves the SI (basically scientific metric) system
Click to expand...
What's being an engineer have to do with loving the metric system. I'm an engineer who hates it :D . Thank God I don't work in one of those industries that has turned itself over to French measurements!
 
Mike, I suspect outgassing of some prep/construction step compound (proseal, primer, filler???? ) residue and probably exasperated by sun heating and airflow (shape).

Deene.
 
Kahuna said:
I dont think so cause in every case, the bubble starts at the corner of the rivet and works out away from the rivet. In no case id the entire rivet area getting a bubble.

Also Im VERY surprised its only 1psi smoking along. Can someone confirm the math? Any while we are at it. WHat is the math?

THanks
Click to expand...


The math:

P = 0.5 * air density * velocity ^2

Using indicated air speed, plug in sea level density of 0.002377 slug/ft^3
velocity in ft/sec

at 200 kts

P = 0.5 * 0.002377 slug/ft^3 * (337 ft/s)^2 = 134.98 lb/ft^2 = 0.93 lb/in^2
 
keen9a said:
What's being an engineer have to do with loving the metric system. I'm an engineer who hates it :D . Thank God I don't work in one of those industries that has turned itself over to French measurements!
Click to expand...
Oh man, let's not start another meaningless debate, we have enough of those. Besides, I'd be the only one on the correct side. :p If you are all into units never working out and messing with slugs, more power to ya. :D
 
JohnR said:
I thought the calculation should be PSI = mph^2 / 57608. That would give 0.694348007 PSI.
Click to expand...

Oops I used 200 mph instead of kts. 200 knots = 230 mph so with my formula it would come up to .91828. Looks like we all agree.
:D

I know, big deal, but I'm getting to the point that remembering something is amazing. :eek:
 
Pain(t)less

First, I take the position that the paint blistering is due to chemical outgassing at elevated temperatures. For a repair, a pinhole and hot air gun might make it lay back down.

So,
I will likely NOT paint my airplane for the first year in the hopes that if it is a proseal issue, it will be done outgassing.
I will probably take my tanks off the wing cradles and start setting them in the sun on convenient days.

On a different subject:

I move next week. I am wringing my hands over it. :( The movers are confident. I will be there helping load and secure the incomplete airframe. I'll be setting up the new shop right after Labor Day. Can't wait to live in a house again (I am already in the new town living in a hotel for the last 2 months). I've gone stir crazy about both not working on the plane and not having my MC's to twist throttle (It's therapeutic, y'know). :D
 
hmmmm

i put my money on a bonding issue...if in doubt pop one with a needle pressurise the tank to a 5 foot column of water(2.4 psi i believe double check the book on that) and apply soap bubbles..if it leaks air it leaks fuel. there is no way that many rivets are leaking any way. good luck let us know
 
Waiting might not help

I flew my RV for 15 months before painting. It had roughly 400 hrs on the Hobbs with several trips to the desert SW to bake in the sun. Blisters didn't begin to appear until several months after painting.

rzbill said:
First, I take the position that the paint blistering is due to chemical outgassing at elevated temperatures. For a repair, a pinhole and hot air gun might make it lay back down.

So,
I will likely NOT paint my airplane for the first year in the hopes that if it is a proseal issue, it will be done outgassing.
I will probably take my tanks off the wing cradles and start setting them in the sun on convenient days.

On a different subject:

I move next week. I am wringing my hands over it. :( The movers are confident. I will be there helping load and secure the incomplete airframe. I'll be setting up the new shop right after Labor Day. Can't wait to live in a house again (I am already in the new town living in a hotel for the last 2 months). I've gone stir crazy about both not working on the plane and not having my MC's to twist throttle (It's therapeutic, y'know). :D
Click to expand...
 
I did not know you were having this problem too Rusty. I would get back to your painter to see what he thinks he can do. I have the same vent tube on my 6 and have no such problem.

Jarhead
 
keen9a said:
What's being an engineer have to do with loving the metric system. I'm an engineer who hates it :D . Thank God I don't work in one of those industries that has turned itself over to French measurements!
Click to expand...

Heh. I had a prof who called metric "commie units".
 
KAHUNA,

About 2 years ago, same thing started with my tank rivets.

This being with quick built tanks, I contacted Van's to see if we could determine what was going on. The symptoms are the same except the total blister count now is now 20-30, top and bottom, I've stopped counting. They just keep coming.

Here's the general dialogue with Van's. (about 2 years ago)
1. They never heard of or saw it before. (I sent images)
2. Van's says no, it is not caused by poorly mixed pro seal that never cured. (my first conclusion) I've come to agree, pro seal probably is not the cause.
3. Poor paint prep The guy who painted the airplane went ballistic. He paints for a living and has NEVER had it happen. The entire machine was prepped the same - how come it was happening only at the tanks?
4. It could be caused by a form of corrosion (filliform) (sp) between the rivet and skin. Why is unknown.

The dialogue with Vans has ended, the cause simply is not known.

The evidence strongly suggests IT IS related to the fuel tank.

I had not considered your vent ram air theory. The problem with it, how come it is not forcing fuel out at the same time? But we can not rule it out. I, too, have the stainless 90 degree ram devices.

To perhaps corner the cause, has anyone had the blisters with Vans standart fuel vent intakes? Also, are your tanks quick built or home built?

If all the blisters are with quick built tanks, there is something going on in the manufacturing process, like maybe the stuff they use to clean the external rivet heads is causing corrosion which is causing the blisters.

We need to have a resolution because it is messing up a lot of nice paint jobs. I thought I was the Lone Ranger with this one and clearly I am not.

dd
RV-7A
N707DD



 
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redbeardmark said:
I flew my RV for 15 months before painting. It had roughly 400 hrs on the Hobbs with several trips to the desert SW to bake in the sun. Blisters didn't begin to appear until several months after painting.
Click to expand...
Perhaps there might be a common thread related to the surface prep solvents being used prior to painting. Perhaps they are being absorbed (adsorbed?) slightly by the proseal around the rivets and then are releasing it later, disbonding the paint. Maybe solvents in the paint system itself are the culprit.
Lots of variables but it seems the prosealed rivets are a common linker.

-mike
 
Rats...

redbeardmark said:
I flew my RV for 15 months before painting. It had roughly 400 hrs on the Hobbs with several trips to the desert SW to bake in the sun. Blisters didn't begin to appear until several months after painting.
Click to expand...

Sheesh,
Always throwin' a wrench in the works, Redbeard! :D

Seriously though, I think this hints at the corrosion mentioned above. I would have thought paint prep issues would have "popped up" quicker. :rolleyes:
 
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mlw450802 said:
Perhaps there might be a common thread related to the surface prep solvents being used prior to painting. Perhaps they are being absorbed (adsorbed?) slightly by the proseal around the rivets and then are releasing it later, disbonding the paint. Maybe solvents in the paint system itself are the culprit.
Lots of variables but it seems the prosealed rivets are a common linker.

-mike
Click to expand...

Mike, I think you might be on to something here. I know some people have used proseal to fill around rivets before painting on other areas of the airplane. Has anyone that has done that had a problem with paint lifting on areas other than the tanks? If so maybe it ca nbe attributed ot the proseal/rivet combo and not just the tanks.

It would be interesting to know if there was a common prep used also.
 
To ad some data.
1. I built my own tanks
2. Did not ad proseal to the top
3. Prepped like the rest of the plane which I painted myself so I know is a professional job:)
4. tanks sat for a year before prime and paint with caps off
5. I rarely run full tanks since I carry 60gal. So the tops of my tanks would generally have air in them.
6. Pressure tests for 24hrs on 3psi with no leaks prior to flying.

If I tought I could just proseal the rivets from the top and repaint I would do that. But Im afraid with no answer Ill go through all that work and again have the bubbles.

This one is a stumper and a good thread. I thought I was the only plane on the planet with this problem. Misery loves company,
Best,
 
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Kahuna said:
To ad some data.

This one is a stumper and a good thread. I thought I was the only plane on the planet with this problem. Misery loves company,
Best,
Click to expand...
MY 1987 RV-4 had the same problem with blisters forming in the paint on the fuel tanks top and bottom. I ended up pulling the tanks off, cutting access panels in each bay, and re-prosealing each rib and rivet.
 
JohnR said:
Mike, I think you might be on to something here. I know some people have used proseal to fill around rivets before painting on other areas of the airplane. Has anyone that has done that had a problem with paint lifting on areas other than the tanks? If so maybe it ca nbe attributed ot the proseal/rivet combo and not just the tanks.

It would be interesting to know if there was a common prep used also.
Click to expand...

John, that common prep could be MEK. Perhapts there is a pro seal out gassing reaction with MEK. I know my machine was cleaned with it.

dd
 
Agent 86 reporting for duty...

"Sorry about that, chief." :D
Corrosion is certainly a possibility.
Also, someone mentioned prep chemicals. I used acetone to wipe the surface before painting... but I did that on all surfaces and only got bubbles on the tank rivets.
Uh, there's one more thing, I did use Marvel Mystery Oil in my fuel a couple of times. Is this a possible commonality?

rzbill said:
Sheesh,
Always throwin' a wrench in the works, Redbeard! :D

Seriously though, I think this hints at the corrosion mentioned above. I would have thought paint prep issues would have "popped up" quicker. :rolleyes:
Click to expand...
 
wyoflyer

just noticed the exact same problem today on my rv-6. Tank vents are the straight tube cut at an angle. Pro-sealed ribs on top. Dupont DB-48 epoxy primer & base-coat, clear-coat paint. Thought maybe I would try a thin hypo needle and some CA glue????

Dave Mader
 
semi smoking rivets?

Just wondering if it may be a possibility that because of the proseal and rivets in the tanks, maybe there is some motion secondary to expansion/contraction of the tanks or due to vibration. The proseal still could be preventing leaks, but because the proseal is there, acting like a rubber motor mount, may be some motion of the rivets is loosening the paint?
Just a thought.
 
self etch

mike did you self etch..or etch, i know on the cars i have painted if you self etch over filler you get a very similar reaction..the seal may absorb the acid temporarirly then give it up later
 
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Matrix the Data?

This is an interesting problem with a lot of interesting data from different sources. It would seem to be iof general interest to get to the bottom of it. When I went out to the airport today I looked at my tank rivet locations and I saw no lifted paint on them. In my old job I would have thought "process" (including the materials) or "contamimation" and called in M&P Engineering and Contamination Control Engineering specialists to investigate in support of the project but we are on our own here (ain't it great?). The development of a matrix in Excel managed by a central controller with inputs provided by builders/owners would be nice but this is what I see so far:

Kahuna - RV-8, red paint on tanks, three color scheme, 15 rivets per side so far affected, linear progression from 10 to 200 hours on both tanks, only on top of tanks, only at rivet locations, no fuel in bubbles, no mention of flight before paint, the bubbles start at the rivet edge and extend outward over the skin, never is a full rivet head involved, built own tanks, did not ad proseal to the top, self painted, tanks sat for a year before prime and paint with caps off, rarely run full tanks since carries 60gal, tops of my tanks generally have air in them, pressure tests for 24hrs on 3psi with no leaks prior to flying, Georgia

Helicopter - RV-7A, Color unknown, number of colors in scheme unknown, 20 rivets after 25 hours on one tank only, now has 175+ flight hours and the problem is not reported to be progressive, no mention of isolation to top of affected tank, the bubble height is described as slight, no mention of surface other than rivet heads affected, Texas

Redbeardmark - RV-8A, Color unknown, number of colors in scheme unknown, only at rivet locations on top, "tank" is used in the description so may be isolated to one side, did the entire preparation and paint process (degreaser, 3M scuffing pad, acetone, etch prime, sealer primer, basecoat, clearcoat), described as minor annoyance, no mention of surface other than rivet heads affected, I flew my RV for 15 months before painting, it had roughly 400 hrs on the Hobbs with several trips to the desert SW to bake in the sun. Blisters didn't begin to appear until several months after painting, wiped down with acetone prior to paint, South Dakota

David-aviator - RV-7A, Color unknown, number of colors in scheme unknown, Quickbuilt tanks, Van's claims ignorance, started 2 years ago, painted by a professional, same vent design as Kahuna, bubbles on bottom as well as top of tanks, plane wiped down with MEK prior to paint, Mississippi

rvator51 - 1987 RV-4 "had the same problem with blisters forming in the paint on the fuel tanks top and bottom. I ended up pulling the tanks off, cutting access panels in each bay, and re-prosealing each rib and rivet", Color unknown, number of colors in scheme unknown, tank builder not mentioned, painter unknown, problem on top and bottom, described as blisters instead of bubbles (may be the same thing - or not), no mention if the fix was effective, Arizona

wyoflyer - RV-6, reported discovering the same problem (assumed bubles in paint isolated to rivets on tanks, no mention that the problem is isolated to the top surface, color unknown, number of colors in scheme unknown, tank builder not mentioned, painter unknown, tank vents are the straight tube cut at an angle, pro-sealed ribs on top, Dupont DB-48 epoxy primer & base-coat, clear-coat paint, Wyoming

Bob Axsom
 
Bob,
Thanks for summerizing the reports.
I have taken the liberty of sending a copy of the summation to Van's support. The dialogue with the factory has been reopened.
At this point, I have done nothing about repainting because it may happen again. We need to know what is causing it.
David Domeier
 
My first vote was for some type of solvent outgassing from the proseal around the rivet or something that soaked into it, but after reading more accounts I'm not that sure. However, I would like to know from the people who make proseal whether exposure to heat causes the stuff to give off gasses long after curing.

I do have a second theory, although it's gonna sound pretty far-fetched. Is it possible that the proseal is insulating the rivet from the surrounding structure and causing the rivet to retain heat? Then the temp differential is stressing the paint bond right at the edge of the rivet head. I said it was a long shot, but maybe?
 
szicree said:
My first vote was for some type of solvent outgassing from the proseal around the rivet or something that soaked into it, but after reading more accounts I'm not that sure. However, I would like to know from the people who make proseal whether exposure to heat causes the stuff to give off gasses long after curing.

I do have a second theory, although it's gonna sound pretty far-fetched. Is it possible that the proseal is insulating the rivet from the surrounding structure and causing the rivet to retain heat? Then the temp differential is stressing the paint bond right at the edge of the rivet head. I said it was a long shot, but maybe?
Click to expand...

Steve,

Rick Galati knows as much about pro seal as anyone on the forum. He has worked with the stuff for years with a major airplane manufacturer. I will try to get him on this thread for his thoughts on whether or not pro seal is a factor. He is aware of my problem and does not think a poorly mixed batch of pro seal will cause out gasssing. But I have not heard his view on MEK or acetone and pro seal, and a possible conflict of chemicals. Whatever he uses to clean pro seal works, so we may be able to illiminate that prep item.

I will contact Rick.

dd
 
More data

I just got a chance to check back on this thread. Wow! I can't believe all of us with the same problem.

My plane came out of the paint shop 1 week before Oshkosh. Everything was fine. It was sanded, fiberglass filled over the rivet heads, sanded again, alodined, washed, primed and painted. The tanks are both all white. Everything was fine after the trip back. Then after about 25 hours post-paint, I noticed, on the left tank only, on the top of the tank only, 10 - 15 rivets where the paint had blistered. It is just noticable and in most cases is only partially the shape of a rivet head. It does not seem to have air under it as you can't "push" the blister around. I have the standard per plans vent inlets (AN fitting cut 45 degrees facing forward). I will post photos this weekend.

It would be nice to get a resolution to this as it is affecting many brand new paint jobs!
 
David-aviator said:
...........I have not heard his view on MEK or acetone and pro seal, and a possible conflict of chemicals. Whatever he uses to clean pro seal works, so we may be able to illiminate that prep item..........dd
Click to expand...
Hey Guys,

The fact that so many builders have reported this common and vexing problem on this forum alone has to suggest that many more RV's are out there that suffer the very same and apparently (according to Van) unreported condition. Although on the production line we routinely used tricloroethane as the cleaning solvent of choice until it was eventually outlawed by the EPA, we sometimes used MEK and that chemical is what I used almost exclusively in cleaning the fuel tanks prior to applying proseal. Since my plane (including the fuel tanks) is mostly polished surfaces, it goes without saying the blistering many of you are experiencing is not a phenomena observed on Darla. I really cannot offer any knowledgeable comments on the use of alternative cleaning solvents. I'm out of my depth on this one.
 
The only firm thing we can say about the problem is, it is happening to rivets set in proseal. Beyond that, we know nothing for sure.

Perhaps if we find another common thread after proseal, we will find the answer.

dd
 
There's more than just the proseal. I suspect the tanks are subject to more dramatic temp swings since they are periodically filled with cold fuel after sitting in the sun. Just trying to touch all the bases. How 'bout we get Paul Dye to run this by some of those nerds over at NASA? They've certainly had more than their share of vexing problems like this.
 
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