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RVators article about heating the skins

papalima

Member
In the mid 90th there was an article in RVators about heating the skins before riveting.

Does anyone know which RVator and how I can get it?

Thanks!
 
Possibly 2/93?

There is an article dated 2/93 titled "Heat Shrinking Wing Skins" discussing heating before riveting to give a tighter fit. It says if you heat a skin 80 degrees over a 48" span the free edges move 0.050".

Vans part number CD RVATOR 27 YEARS will get you years of RVAtor on CD.
 
When I built my RV4 I riveted my wings with them sitting on saw horses out in the sun. They were plenty warm. When I built new ones years later I opted for my air conditioned basement.

Could discern no difference in skin ?tightness?

YMMV.
 
Unless you match-drilled them with heated skins, and kept the frame cool, i'd be surprised if you could have any effect on skin tightness. Once dimpled, the dimples will key each hole and prevent any significant shifting... That's kind of the point of a dimple.
 
Unless you match-drilled them with heated skins, and kept the frame cool, i'd be surprised if you could have any effect on skin tightness. Once dimpled, the dimples will key each hole and prevent any significant shifting... That's kind of the point of a dimple.

I can't imagine the self centering function of the prepunched holes isn't going to negate whatever skin growth comes from heating. My opinion is the ribs are going to flex that 0.00whatever so the prepunched holes match the skins when you're drilling to size.
 
https://books.google.com/books?id=4...&q=rvator "Heat Shrinking Wing Skins"&f=false

Put me in the camp that this is a silly thing to do.

ETA lists

CTE, linear 68°F 23.2 µm/m-°C 12.9 µin/in-°F AA; Typical; Average over 68-212°F range.
CTE, linear 250°C 24.7 µm/m-°C 13.7 µin/in-°F Average over the range 20-300ºC

for Al 2024-T4. The rather miniscule effect a perfectly managed (very) hot skin instantaneously attached to a cool substructure is left as an exercise for the reader.
 
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Skepticism aside, the influence of heating skins during installation is real. That is why it was featured in the RVator at one point.

Understanding this requires thinking about the physics involved.
The dimensional difference between a skin being loose or tight in any give rib bay is probably only a couple thousandths of an inch.
A couple thou can be tolerated between ribs without even noticing it when the skin is clecoed on (remember, when rivet holes get dimple countersunk, the hole enlarges slightly).

There is no difference between punched skins or traditionally drilled in place skins. in both cases the holes were put into all of the parts with the intent that they match each other.

The degree of difference noted will always be directly proportional to the high to low temp delta that the airplane operating in.
Example-
If the skins are riveted on when the shop temp is 40 F, when the airplane is parked out in the sun on a 100 F day, there will be a noticeable amount of looseness/oil canning when compared to my first RV that got the top skins riveted on with them sitting outside in the Arizona sun on an 85 F day (the temp delta is small).
Conversely, the wing skins on my airplane were like a Jamaican steel drum when the weather was cold.

What makes this work is for the heating to occur in a way that heats the skin more than the interior structure. If this occurs, the skin will always be tighter in any given temp condition than if it wasn't done. Think of it this way.... the interior sub structure is always in the shade under the wing skins. The substructure material will always be cooler than the top skins in any situation where there is any level of solar influence, and when there is not any, the skins will still be slightly tighter than they would have been otherwise.

Having said all of that, I would suggest the going to the trouble is not worth it for the majority of builders. There are many other things you can do more easily to assure a high quality finish (proper rib fluting and flange angle adjusting, proper quality dimpling, good riveting technique with a 2X gun, etc.). In fact I would suggest that if you are not sure you have all of these other details covered the best they can be, then heating skins is a total waste of time.
I would also suggest that you can cause yourself problems if you decide to do this and you do it incorrectly. I would not recommend attempting this in a cold shop and only heating a portion of the skin with a localized heat source.
 
Skepticism aside, the influence of heating skins during installation is real. That is why it was featured in the RVator at one point.

Understanding this requires thinking about the physics involved.
The dimensional difference between a skin being loose or tight in any give rib bay is probably only a couple thousandths of an inch.
A couple thou can be tolerated between ribs without even noticing it when the skin is clecoed on (remember, when rivet holes get dimple countersunk, the hole enlarges slightly).

There is no difference between punched skins or traditionally drilled in place skins. in both cases the holes were put into all of the parts with the intent that they match each other.

The degree of difference noted will always be directly proportional to the high to low temp delta that the airplane operating in.
Example-
If the skins are riveted on when the shop temp is 40 F, when the airplane is parked out in the sun on a 100 F day, there will be a noticeable amount of looseness/oil canning when compared to my first RV that got the top skins riveted on with them sitting outside in the Arizona sun on an 85 F day (the temp delta is small).
Conversely, the wing skins on my airplane were like a Jamaican steel drum when the weather was cold.

What makes this work is for the heating to occur in a way that heats the skin more than the interior structure. If this occurs, the skin will always be tighter in any given temp condition than if it wasn't done. Think of it this way.... the interior sub structure is always in the shade under the wing skins. The substructure material will always be cooler than the top skins in any situation where there is any level of solar influence, and when there is not any, the skins will still be slightly tighter than they would have been otherwise.

Having said all of that, I would suggest the going to the trouble is not worth it for the majority of builders. There are many other things you can do more easily to assure a high quality finish (proper rib fluting and flange angle adjusting, proper quality dimpling, good riveting technique with a 2X gun, etc.). In fact I would suggest that if you are not sure you have all of these other details covered the best they can be, then heating skins is a total waste of time.
I would also suggest that you can cause yourself problems if you decide to do this and you do it incorrectly. I would not recommend attempting this in a cold shop and only heating a portion of the skin with a localized heat source.

Nobody is arguing the physics or the principles behind...just that unless done with a pretty extreme difference in temperatures between skin and structure (and that temp difference is *maintained* throughout the riveting process) that the effect is swamped by all the factors (flex in the substructure, size of the rivet holes, fluting, flange angle, etc., as you noted).
 
There is an article dated 2/93 titled "Heat Shrinking Wing Skins" discussing heating before riveting to give a tighter fit. It says if you heat a skin 80 degrees over a 48" span the free edges move 0.050".

Vans part number CD RVATOR 27 YEARS will get you years of RVAtor on CD.

Many thanks, I will purchase this.
 
I would also suggest that you can cause yourself problems if you decide to do this and you do it incorrectly. I would not recommend attempting this in a cold shop and only heating a portion of the skin with a localized heat source.

Thanks Scott, can you further explain the possible risk; cracks? Slowly developing or?
 
Nobody is arguing the physics or the principles behind...just that unless done with a pretty extreme difference in temperatures between skin and structure (and that temp difference is *maintained* throughout the riveting process) that the effect is swamped by all the factors (flex in the substructure, size of the rivet holes, fluting, flange angle, etc., as you noted).

Have you ever layed your hand on an unpainted wing skin in the middle of a hot summer day? It is too hot to hold your hand on it.
If you remove an inspection cover and place your hand on the spar or a rib it will be similar to the ambiant air temp. This difference is more than enough to have a noticeable effect.
 
Thanks Scott, can you further explain the possible risk; cracks? Slowly developing or?

The caution is that you must avoid heating the skin unevenly across its surface. Doing so can change the shape and cause fit problems. I think this is discussed somewhat in the article.
 
There seem to be a lot of opinions here. I thought I would post what I did. Mine is a quickbuild so I did not build the whole wing. It was winter when I was putting my skin on. I could not get the clecos in as they were about a hole off. I pointed a shop heater at the skin from a distance that would warm it up but not so hot that I could not put my hand on it. It was cooler than a sheet of metal out in the sun. Like magic the holes moved into place and I clecoed it on good. No more problems! Any doubters can look at an enclosed cargo trailer like they haul race cars in, in the summer some of them look like a shirt that was slept in and some look good. Guess which ones were built in the summer.
 
Have you ever layed your hand on an unpainted wing skin in the middle of a hot summer day? It is too hot to hold your hand on it.
If you remove an inspection cover and place your hand on the spar or a rib it will be similar to the ambiant air temp. This difference is more than enough to have a noticeable effect.

Let's say it's a hot day...90 F. Let's assume the TOP (and only the top) gets heated by the sun to a nice toasty 140 F (I think that's reasonable, but you can use your own numbers if you like). That's 50 F between the internal structure and the skin. 2024-T4 CTE is right around 13 * 10^-6 in/in/deg, or .000013". That gives about 3/100" over a 4' span. That's only about 1/3 of a -3 rivet hole, *undimpled*.

Is there *some* effect of temperature on the overall structure? Of course, that's just simple physics. But I think the claims made of "tight as a drum" and all that are stretching things a bit...the tolerances in *parts* can easily add up to more than .03" in 4' during construction.

And don't forget the coefficient of thermal *conductivity*...those hot wing skins are conducting that heat somewhere, namely *into* the underlying structure, which will itself undergo some degree of expansion.

I'm curious how one would assemble a wing with a skin that's too hot to touch, by the way...
 
When I did a test fit of my top forward fuselage skin (-7 quick build) in my shop at 60?F it was very difficult to cleco to the longerons I needed straps to pull it into place. After sitting in the sun at 80?F for a few hours it lined up perfectly with no effort.
 
It's in "27 Years of the RVator." I read it before starting my RV-3B wings and decided against it. The article wasn't exactly saying it was a good thing.

The best RV wings I've seen are on an RV-3B and the skins are glued on and then, after the glue cured, riveted. The underlying structure was built very, very well. The glue fills very small gaps that can still show up in the finished product. I didn't do that, either, on mine and wish I had. Caveat, there's a whole process to follow if you're going to do this. It's not exactly trivial.

It takes a great deal of time, too.

I've seen unglued RV-3 wings that were almost as good. Randy Lervold's is a good example. Take your time, make certain that everything fits before riveting and that the dimples are good ones.

Dave
 
Let's say it's a hot day...90 F. Let's assume the TOP (and only the top) gets heated by the sun to a nice toasty 140 F (I think that's reasonable, but you can use your own numbers if you like). That's 50 F between the internal structure and the skin. 2024-T4 CTE is right around 13 * 10^-6 in/in/deg, or .000013". That gives about 3/100" over a 4' span. That's only about 1/3 of a -3 rivet hole, *undimpled*.

Is there *some* effect of temperature on the overall structure? Of course, that's just simple physics. But I think the claims made of "tight as a drum" and all that are stretching things a bit...the tolerances in *parts* can easily add up to more than .03" in 4' during construction.

And don't forget the coefficient of thermal *conductivity*...those hot wing skins are conducting that heat somewhere, namely *into* the underlying structure, which will itself undergo some degree of expansion.

I'm curious how one would assemble a wing with a skin that's too hot to touch, by the way...
You seem bent on arguing something that you are just theorizing about. I have done it. I learned long ago that actual results trump my logical reasoning ever time.....
 
Did you measure those results?

I have built 20+ RV's. I think that has given me enough experience to make a judgement on measuring the results, so my answer is yes.

BTW, I am not sure what others have said, but my tight as a drum comment was what it was in cooler temps, when the skins had been installed in hot temps.
 
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Scott, I think the concern is that nobody has explained the mechanism by which this works. The math has been done, and .030" in 4' appears to be nearly the biggest expansion you'd be likely to see. Is that all it takes? How do you ensure this expansion, when holes were match drilled, and dimpled, in a cold shop before any of this happens?

I'm not saying you haven't built RV's with tight skins. But i'm wondering if the reason is your experience in building 20 RV's, and the precision and care with which you're doing it, not the temperature at which it's done. Otherwise, how is it that there are RV's out there flying around with perfectly smooth (and hence tight?) skins that didn't rivet outside in the sun?

Remember: Correlation does not equal causation... :)
 
Scott, I think the concern is that nobody has explained the mechanism by which this works. The math has been done, and .030" in 4' appears to be nearly the biggest expansion you'd be likely to see. Is that all it takes? How do you ensure this expansion, when holes were match drilled, and dimpled, in a cold shop before any of this happens?

I'm not saying you haven't built RV's with tight skins. But i'm wondering if the reason is your experience in building 20 RV's, and the precision and care with which you're doing it, not the temperature at which it's done. Otherwise, how is it that there are RV's out there flying around with perfectly smooth (and hence tight?) skins that didn't rivet outside in the sun?

Remember: Correlation does not equal causation... :)

And lack of belief does not make it untrue..... ;););)

I guess you missed my other post where I said the difference between tight and loose between two ribs is probably only a few thousandths of an inch.

This is beginning to drift out into the weeds as people hyper analyze what has been published and what has been said. Heating skins is not something to do to get tight skins in general. It is done to have the skins stay tight through a wide temp range. It is primarily going to be beneficial to someone that has riveting on wing skins during the winter in a cold shop.

I have already said that it is all about temp delta. If you rivet on skins in the hot sun in Arizona (I have done that), the skins wont be any tighter when out in the very hot sun, than skins on another airplane will be, on a 60 degree cloudy day if they were riveted on in a 60 degree shop.

The difference will be that if both airplanes are parked side by side on a 90 degree hot sunny day, the airplane that had the skins riveted on in the 60 degree shop, will appear looser than the ones on the other airplane.

Now if you look at both airplanes on a cold winter day, the skins on the airplanes riveted on in the hot sun will be super tight compared to the other airplane.

BTW, the mechanism by which it works is quite simple. A little reading on the coefficient of expansion of 2024-T3 aluminum will tell the story. (Hint.... it is huge when talking about skins as big as we put on RV wings)
 
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And lack of belief does not make it untrue..... ;););)

I guess you missed my other post where I said the difference between tight and loose between two ribs is probably only a few thousandths of an inch.

This is beginning to drift out into the weeds as people hyper analyze what has been published and what has been said. Heating skins is not something to do to get tight skins in general. It is done to have the skins stay tight through a wide temp range. It is primarily going to be beneficial to someone that has riveting on wing skins during the winter in a cold shop.

I have already said that it is all about temp delta. If you rivet on skins in the hot sun in Arizona (I have done that), the skins wont be any tighter when out in the very hot sun, than skins on another airplane will be, on a 60 degree cloudy day if they were riveted on in a 60 degree shop.

The difference will be that if both airplanes are parked side by side on a 90 degree hot sunny day, the airplane that had the skins riveted on in the 60 degree shop, will appear looser than the ones on the other airplane.

Now if you look at both airplanes on a cold winter day, the skins on the airplanes riveted on in the hot sun will be super tight compared to the other airplane.

BTW, the mechanism by which it works is quite simple. A little reading on the coefficient of expansion of 2024-T3 aluminum will tell the story. (Hint.... it is huge when talking about skins as big as we put on RV wings)

I am curious how one can heat the skin completely and evenly while NOT heating the ribs & spars below deck? Raise an OH door and the sun will quickly warm a skin but there are a lot of rivets to buck and I am not sure how one would get them all neatly hammered before the heat transferred to the ribs and spar.

But I am willing to learn as I have seen black RV top wing skins that look like melted cheese drooping when the sun comes out from behind a cloud while sitting on the ground.

IMHO white is the only color that has any business on aircraft wings intended to stay airworthy for decades of use as an aircraft.
 
There seem to be a lot of opinions here. I thought I would post what I did. Mine is a quickbuild so I did not build the whole wing. It was winter when I was putting my skin on. I could not get the clecos in as they were about a hole off. I pointed a shop heater at the skin from a distance that would warm it up but not so hot that I could not put my hand on it. It was cooler than a sheet of metal out in the sun. Like magic the holes moved into place and I clecoed it on good. No more problems! Any doubters can look at an enclosed cargo trailer like they haul race cars in, in the summer some of them look like a shirt that was slept in and some look good. Guess which ones were built in the summer.

-3 rivet holes are 3/32", or .09375". With a CTE of .000013 in/in/deg, and assuming a 4' long skin, we we can work this backwards, and you get a temperature change required of 150 degrees F.
 
I started my RV4 in 93 and after reading the RVator I used heat lamps to warm the top wing skins while riveting. I found it hard to get a consistent temperature with heat lamps. Heating the whole shop was easier.
After paint, all black, the skins appeared to be ok on hot days in the Sun. I can not say that was due to the heating process.
I did not heat skins on my other planes and did not notice a lot of difference, however, none of those planes were painted black.
I really liked the black plane, it was extremely visible in the air, and I received many complements on the paint.
Looking around at planes on the ramps at air shows there is good workmanship and some not so good. Better workmanship, on the whole, leaves tighter skins. Although some airplanes can have great top skins but the fuselage skins show oil canning. I believe this has more to do with the structure rather then technique.
Wing skins on Harmon Rockets are tighter then skins on an RV4. The rocket wings are shorter with the same number of ribs and in many cases a heavier top skin. Thus they are “ tighter” then a RV4.
 
I think I have provided enough info for those that are truly interested and not just looking for their daily debate subject, so the only thing I will add is if you doubt the temp difference between a skin and sub structure, do your own test.

Remove an inspection cover and check the temp difference between the wing spar and the interior of the skin.
 
-3 rivet holes are 3/32", or .09375". With a CTE of .000013 in/in/deg, and assuming a 4' long skin, we we can work this backwards, and you get a temperature change required of 150 degrees F.

I think a previous poster mentioned trouble inserting clecos in match-punched skins/ribs at cold temps. Even a quarter of a 3/32 hole would obviously make a difference. That would be 150/4 = 38 degrees.

Seems your math proves rather than disproves the benefit of riveting on skins in the hot sun.

Finn
 
I think a previous poster mentioned trouble inserting clecos in match-punched skins/ribs at cold temps. Even a quarter of a 3/32 hole would obviously make a difference. That would be 150/4 = 38 degrees.

Seems your math proves rather than disproves the benefit of riveting on skins in the hot sun.

Finn

You have trouble flexing a rib 3/128" to get a cleco in? Seriously?
 
There would be a revolt if builders had to use differential heating to make prepunched skins fit.

I agree. But one could reasonably expect builders to work in a reasonably warm shop.

In any case, all this is builder's choice. Like the difference in making things fit to a few mills versus to 1/32 or "good enough". Certainly not mandatory.

My main objection is RV7A Flyer's insistence on it not making any difference at all.

Finn
 
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There would be a revolt if builders had to use differential heating to make prepunched skins fit.

Kyle,
Of course no one typically has to do this This thread originally started as a discussion for do this as an optional part of the skinning process). But it also seems to be ignored that others have already posted needing to do it when they were building in a very cold shop, to get the holes in the skins to fully align with the substructure. If the parts are being assembled at a temp that is a lot different than it was when they were punched, it can make a difference.

Boeing invests huge amounts of money in climate controlled production environments. Particularly in areas where they are machining very large components (such as wing spars), because temp change can have such a large influence when working with aluminum.
 
Spars... Aluminum.
Skins... Aluminum.

I'd hazard a guess that people have all the parts together in their shop and are building that way. Whether the shop is heated or cold, shouldn't matter unless the different aluminum grades used in the skins and spars have CTE's that are wildly different.

While heating the skins may not heat the spars at the same time in the short term, when both have been sitting there in the shop for a while they're probably at the same temp. So we go back to the previous comment about the number of owners who would be revolting if holes didn't line up at assembly time.

Boeing invests time making sure everything is at a known temperature, so parts machined on different days will line up with each other. I've never heard of them differentially heating skins and spars to make things tight after assembly.
 
This thread totally cracks me up....
Posts drift one way and another, twisting what has been said into different meanings and outright calling people liars when they say what they have personally seen and experienced, even when the person posting has never done anything associated with the conversation :rolleyes:

As I have said before..... just because you don't understand doesn't mean something isn't true!
 
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