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.
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.
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".
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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).
Thanks Scott, can you further explain the possible risk; cracks? Slowly developing or?
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.
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.....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?
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)
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.
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.
I am curious how one can heat the skin completely and evenly while NOT heating the ribs & spars below deck?
-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
You have trouble flexing a rib 3/128" to get a cleco in? Seriously?
You just have to be right, don't you?
There's also the holes along the main spar web which are not flexible.
Finn
There would be a revolt if builders had to use differential heating to make prepunched skins fit.
There would be a revolt if builders had to use differential heating to make prepunched skins fit.