# Shrink Wrap Wings !

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##### Well-Known Member
My plane with me in it is under 500lbs. I has 26ft*5ft=130 sq ft wing area or 18,720 sq in.

I need strength of at least 500lbs / 18,720 sq in = .027 lbs/sq in. or 4lbs/sq ft.

74lbs/sq in for oracover /5 strength ratio = 14.8 lbs/sq in for shrink wrap. I should be good by a wide margin.

do the calculickulations make sense to you?

#### proppastie

##### Well-Known Member
Log Member
My plane with me in it is under 500lbs. I has 26ft*5ft=130 sq ft wing area or 18,720 sq in.

I need strength of at least 500lbs / 18,720 sq in = .027 lbs/sq in. or 4lbs/sq ft.

74lbs/sq in for oracover /5 strength ratio = 14.8 lbs/sq in for shrink wrap. I should be good by a wide margin.

do the calculickulations make sense to you?
need the thickness of shrink wrap the 74 lb/in width of test sample..... to get psi need thickness. You have the right idea though.

##### Well-Known Member
the thickness of the shrink wrap I used is .006 in...the test sample was .009 in .....how does that come into play?

I think the width would be less than an inch

##### Well-Known Member
PSI is pounds per square inch

#### proppastie

##### Well-Known Member
Log Member
First off I am not an experienced aircraft stress specialists. I had not planned to examine this issue in my design as I planned to use existing known methods. The question of "is this strong enough" is a not normally answered here. There are many aspects of stress analysis and aerodynamics/wing loading that are beyond my experience. That being said and hopping that someone that knows how to do it is watching I will throw out some thoughts ....be very careful here, sometimes you get what you pay for.

PSI ...pounds per square inch...lb/sq in.... of fiber strength is different than one square inch of wing loading. The air load on a square inch of wing is different than the tensile load on one square inch of fiber cross section under tension.

fabric tensile strength of one square inch of fabric is the perimeter of the inch square times the thickness of the fabric. = 1 in sq = 4 in x thickness (.006) = .024 sq in of shrink wrap cross section area in tension. .024 x 3000 = 72 lbs that square inch should fail at.

You have calculated that at 500 lb you have .027 lb/sq in wing load.......you have forgotten your other factors such as "normal, utility or aerobatic" plus the safety factor of 1.5.

I do not know what other factors go into an aircraft covering design...I do not know how the fabric test results (Maule punch test) relate to PSI or wing loading. I can compare or use FAA minimum standards, but it all takes time and research.

I hope this helps, and that someone else will chime in an fill in the blanks.

Edit: What is not listed is the yield point of the materials. The Shrink Wrap elongates up to 800 % before it fails, so at some point your skin will flap too much to fly even if it does not fail. The Oratex has a listing of up to 20% "Breaking Extension". If it is the same specification then perhaps you could still fly and perhaps not. Usually with stress we try to stay in the lineal point of the graph, and the material should not permanently deform up to limit load (normal,utility,aerobatic). Which means it will return to its former size. This could be a key specification and might be why the fabrics seemed to be so over-designed. Again I would guess this is a question for someone that died in the 30's but maybe there is an expert here.

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#### Dana

##### Super Moderator
Staff member
My plane with me in it is under 500lbs. I has 26ft*5ft=130 sq ft wing area or 18,720 sq in.

I need strength of at least 500lbs / 18,720 sq in = .027 lbs/sq in. or 4lbs/sq ft.

74lbs/sq in for oracover /5 strength ratio = 14.8 lbs/sq in for shrink wrap. I should be good by a wide margin.

do the calculickulations make sense to you?
No, what you're calculating is wing loading. Fabric strength is generally measured in lbs / lineal inch, i.e. the breaking strength of a strip of fabric 1" wide. For standard aircraft covering fabric, the limit is 46 lbs/inch, which is 70% of the strength of new fabric. Using the 3000 psi strength in the link you posted, that would be 18#/inch, which is a lot weaker than aircraft fabric. Then there is the elongation or "stretchiness" as proppastie mentioned... it might not actually fail, but it may stretch and balloon out under air loads, altering (possibly dangerously) the airfoil. For some very back of the envelope calculations, Dacron has 15-20% elongation at break (at it's 46#/in breaking strength) whereas the shrink wrap has a 650% elongation at its much lower 18#/in breaking strength. This means that polyester aircraft fabric is 83 times less stretchy than .006 shrink wrap. Put another way, if standard aircraft fabric balloons out, say, 1/8" in flight (a made up but not unreasonable guestimate), then the shrink wrap will balloon out ten inches under the same load. Not very aerodynamic...

Dana

#### proppastie

##### Well-Known Member
Log Member
Is standard aircraft fabric or Oratex 600 over designed for a 2 lb/sq ft or 4 lb/sq ft loading. Are there characteristics of fabric covered aircraft that require the strengths they have. For example 1 sq ft has 48 lineal inch of fabric in tension. That works out at even 46 lb/in to 2208 lbs. but the wing loading is only 4 lb. What gives? Am I missing something here?

#### bmcj

##### Well-Known Member
HBA Supporter
Is standard aircraft fabric or Oratex 600 over designed for a 2 lb/sq ft or 4 lb/sq ft loading. Are there characteristics of fabric covered aircraft that require the strengths they have. For example 1 sq ft has 48 lineal inch of fabric in tension. That works out at even 46 lb/in to 2208 lbs. but the wing loading is only 4 lb. What gives? Am I missing something here?
You misunderstand. The perimeter of the piece means nothing. When he says 46 lbs per lineal inch, he means that a long strip (single layer) 1" wide will hold 46 lbs. In other words, you can attach one end of that strip from the ceiling, and hang 45 lbs from the other end without it failing. If your strip is 2 inches wide, it should support 2 lbs before failing. It is a TENSION rating.

#### Dana

##### Super Moderator
Staff member
^^^Right.

But picture this: If you attach both ends of the fabric to the ceiling and the weight in the middle hanging down, then you can support 92# with that one inch strip, since there are in effect two strips. But as you move the upper ends apart, the weight it can support decreases (basic trigonometry). If you attach the strip horizontally between the walls, the tension in the fabric goes to infinity even with little weight (though in practice it'll give and sag a little so it's not quite horizontal, so the stresses in the fabric, while not infinite, will still be quite high). That's the same thing happening from the air loads perpendicular to a tightly stretched wing fabric. And that's not taking into account the initial tension from shrinking the fabric, which can also be very high.

Dana

#### proppastie

##### Well-Known Member
Log Member
So given that the sag/angle is variable according to say age, tautness of install is our only design guidance the manufacturers recommendations. In other words at 46 lb/in strength at what wing loading will the 1 sq ft of fabric fail. Say I wanted to design a fabric system .....what might be the test data/geometry assumptions I use in my calculations. How do you calculate strength required for a fabric system?

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#### Dana

##### Super Moderator
Staff member
I suppose you could cover one size of a test box the same size as one wing bay, pressurize it to the same pressure as your wing loading (at max g's), and see what happens. It won't be a realistic test since the pressure distribution isn't constant along an airfoil, but it could be a useful sanity check.

Dana

##### Well-Known Member
Based on common sense, and flying Ultracote on models. I say this will be plenty strong. Trust me this is NOT going to stretch 800%. I can pound my fist on the shrink wrap and get no stretching.

Fabrics tend to tear along the thread lines, plastics, have none. Once heated is stays tight. 4lbs/sq ft is not near as much as me beating it with my fist.

Dacron is used on much heavier planes than an ultralight.

If I wanted to I could use 12 mill shrink wrap with twice the strength. It's not needed.

Also, I used heat shrink tape over the shrink wrap to reinforce it.

Not to worry, it will be over kill on strength for what I need. For aerobatics use thicker shrink wrap and more shrink tape if you want.

One thing we have not talked about is how light it is compared to painted dacron.

Just my opinion.

#### don january

##### Well-Known Member
HBA Supporter
Log Member
What would be the result of a wing covered with a fabric and then sealing to where you can add a vacum to pull the fabric down over the wing? You want you wing air tight if possible and with a vacum force pulling the fabric down among your rib's and spar would you not be ahead of the game?

#### Topaz

##### Super Moderator
Staff member
Log Member
What would be the result of a wing covered with a fabric and then sealing to where you can add a vacum to pull the fabric down over the wing? You want you wing air tight if possible and with a vacum force pulling the fabric down among your rib's and spar would you not be ahead of the game? View attachment 43710
Is the current system of making tight fabric not adequate? In what way would this vacuum system be an improvement over what we already have?

#### don january

##### Well-Known Member
HBA Supporter
Log Member
I thought maybe it would bypass rib stitching and the force from airflow would already be interduced to the wing, and the best thing would be a person would know that moisture would not be entering the wing if the vacum held without leak's. The only negitive I see would be if the skin's were'nt tight after the vacum was interdused,then your back to resewing a smaller set of cover's.

#### Hot Wings

##### Grumpy Cynic
HBA Supporter
Log Member
In what way would this vacuum system be an improvement over what we already have?
If the fabric were actually porous it might be a novel way to control the boundary layer? It's not my idea. Read it somewhere a couple of decades ago........

#### Topaz

##### Super Moderator
Staff member
Log Member
If the fabric were actually porous it might be a novel way to control the boundary layer? It's not my idea. Read it somewhere a couple of decades ago........
Do you think a fabric covered airplane would have low-enough drag otherwise to benefit from active boundary layer control, and all the weight and complexity associated with such a system?