I learned this many years ago when covering the two halves of a trainer wing. I had not planned very well and had cut the pieces for the tops of the wings oriented two different ways. I didn't notice the difference when I was covering, but when I had finished both sides I was shocked at how much difference it made. To make sure this is really what caused it, I purposely repeated the same thing on another wing. It did exactly the same thing! The amount of sag between the rib bays on one side was definitely different on the other side. You guys that are doubting this probably have not cut the material for your sufaces with it oriented 90 degrees off before, because when covering a wing you usually lay the material out oriented the same way as the wing before cutting it. I think that's what happened to me. I had covered several wings before that and never noticed a problem, but after making the mistake one time (and once more for the experiment) I have been very careful to avoid it since then. By the way, it seems that the softer, lower heat coverings exhibit more of this behavior than high temp coverings such as monokote. I decided this after thinking about my own experience and talking to friends who had, at one time or another, had the same problem.

Hmmm, the doubters astound me. I will post pictures tonight with a ruler laid across the tops of the ribs so you can see the difference in sag between the top and bottom of the wing.

John

Heh ! Heh! There are many "Thomases" around here. Blessed are those that do not see but believe...........

Guess my method of covering is a little different . I first seal all around the edges, pulling it tight & then shrinking it between the ribs & last ironing it to ribs. All the old tissues had a very pronounced grain to them & would sag between ribs if installed with the grain wrong. You won`t have that trouble with a wing with turbulator spars or a "D" tube leading edge. I also use a heat gun to do most of my shrinking , it won`t stick to the ribs
until you use an iron on them.
>>>>>>>>>>>>>>>>>big max 1935 >>>>>>>>>>>>>>>>>>

That's exactly how I cover all pieces. Start at the center of one end and tack it down securely. Move to the opposite end and tack the center pulling the covering as tight as you can without popping it loose or breaking the model. From there, I pull the width tight and tack the center of each side and alternate from one side to the other back and forth until I work to the corners. At this point, I have pulled the covering so tight and evenly that it appears to already be shrunk. Next, break out the heat gun and shrink the whole panel. I never use an iron over the center of a piece, only a heat gun.

John

Not true! I just finished covering a wing, and since the wing sides were only 24", I decided to cut the pieces across the roll, mostly because it was a lot easier. The Monokote went on very easily, and tightened right up with the heat gun. To me, it felt exactly like the three other wings I've built in which I cut the Monokote the other way on the roll.

I'm not really doubting whether there is some subtle difference. I'm just saying in my experience, I couldn't tell the difference, and if I must say so myself, it's really a beautiful wing. I'd post a picture, but all you'd see is a nicely covered yellow wing with no wrinkles, sags, or anything.

I also can not figure out in my mind why it would make a difference even if it were true. Suppose Monokote only shrunk in one direction. Period. Zero shrink in one direction, a lot in the other. I still don't see why it would be better to place it one way rather than the other. The only difference would be if you got any wrinkles, they would go one way rather than the other. Why would one be better than the other?

OK, picture your newest wing. It's built up with open bays right? If you cover it with the roll direction inline with the chord, the covering will dip more between the ribs than if the roll direction were inline with the span. There are no wrinikles or difficulty in getting the covering the shrink, it's all about how much the covering dips between the ribs. The more dip, the less accurate the airfoil and the less aesthetically pleasing the covering job.

Again, I will post pictures this evening of the top and bottom of a wing with a ruler across the ribs to demonstrate the difference this makes.

John

All right. I get it now. If you are correct that the stuff shrinks significantly more in the roll direction than across the roll, then I would agree that given that the ribs are curved and the leading and trailing edges are not, you would want the shrinky way to go spanwise. Otherwise, the increased shrinky between the leading edge and trailing edge would tend to pull the covering down in the area between the ribs. (Did that help anyone?)

Still. I can't help but note that I've got a beautiful wing here.

One thing that I'm convinced of is that the different colors shrink differently. That makes the science very difficult, because even if you were to build identical wings, you might not be using the same color each time.

Another thing I learned from my experiment is that the stuff shrinks surprisingly little. I would have expected 1/8" or more over 6", but in fact, I've now measured more carefully, and it's a 1/32", if that.

By Jove I think you've got it!!!!!

Here are the two pictures as promised. Note the amount of dip/sag between the ribs. It's very close to 1/64".



and now compare this picture of the same wing with the ruller placed over the same two ribs. The gap is now just slightly over 1/16".



Now before anyone jumps up and says "But they are different colors and different colors behave differently" let me say right now, they don't shrink THAT much differently. We're talking a 1/64" vs. 1/16" dip over a 3" span!

Does the wing look good? Oh yeah baby! Both top and bottom are very nice. Am I going to hide the bottom at the field? Only during landings I'm not afraid to say this is a good covering job. But, there are differences between the top and bottom and the top simply looks better if for no other reason than the ribs are not as pronounced as on the bottom. Will anyone else notice the difference? More than likely not. But, if you are trying to do the best job possible, you will see the difference and know you could have done better.

How's that?

John

As I recall, or as it maybe used to be. Monokote is an extrusion (blown generally) and Orocover (ultracoat) is a casting. This should cause Monokote to tend to be unidirectional and Orocover to be non-directional.
I believe that's why I always found that Orocover would go around compound curves easier than Monokote.

As far as Mono being an extrusion and Oro being a casting I just recall someone telling me that back in "06".
But the two coverings always seemed to act that way to me.

People here seem to be saying that this issue does NOT apply to Ultracote.

Is that the consensus of opinions???

The difference in shrink rate affects all film coverings regardless of brand.

John