Substituting wing fabric with plywood

HomeBuiltAirplanes.com

Help Support HomeBuiltAirplanes.com:

F

fly scared!

OK, I know, why changing a true and tested configuration for an unsupported one ?
BUT, I never covered a wing with cloth, while I'm more confident to achieve a neat surface with plywood.

So, where should I start to check if the mod is possible ?
In my case (P.130UL, a shoulder braced wing) the plans call for small 1.6 mm birch strips above the ribs.
They have little structural motivation, they mostly provide a larger area to glue the Dacron cloth (1500 Kg).
Extending the band to completely cover the wing would then be geometrically straightforth.

a) would the plywood covering be too stiff for the wing (spar) to take loads properly (even if designed mainly for compression) ?
b) if so, would applying indipendent, small (rib to rib ?) panels aproximate (structurally) a fabric covering ?
c) what is the equivalent thickness for birch to get the same resistance (traction) than Dacron ?
d) how large a weight penalty may I expect ?

Needless to say, ingenious Mr. Pottier leaved us many years ago, so no way to seek advice from the designer here.
 
Last edited by a moderator:

TFF

Well-Known Member
Joined
Apr 28, 2010
Messages
14,648
Location
Memphis, TN
A 4ftX8ft sheet will be about 5 lbs. If you sheet it you still have to cover the ply. Either light weight dacron or fiberglass is normally used. I would guess if you tripled the covering weight would be close. Structure is its own can of worms. Covering with fabric is pretty easy.
 

djschwartz

Well-Known Member
Joined
Jun 21, 2008
Messages
982
Location
Portland, Oregon
Simply putting plywood on in place of fabric is not a good idea. There are many design changes that must be made for this different type of structure. One example: fabric is stretched tight over the structure so it is always in tension even as the wing flexes. Plywood cannot be tightened in this way and as the wing flexes some of the skin will be in comression. Thin plywood has little or no compression strength and it will buckle under compression. To avoid this spanwise stringers are used to provide support and stiffness. The number of stringers required depends on the skin thickness. The thinner the skin, the closer the stringers must be.

Another change is that there is no longer a need for compression members and diagonal bracing when the skin is plywood instead of fabric. While the fabric is stretched tight over the structure, it remains far more elastic than plywood; so, diagonal wires or other similar structures are required to keep the wing from "racking", a term for flexing fore and aft. The fabric also tries to pull the spars together, as do wires if they are used for the diagonal stiffness.

There are other subtle changes as well. These are just a couple of the bigger ones.
 

Topaz

Super Moderator
Staff member
Log Member
Joined
Jul 29, 2005
Messages
14,310
Location
Orange County, California
OK, I know, why changing a true and tested configuration for an unsupported one ?
BUT, I never covered a wing with cloth, while I'm more confident to achieve a neat surface with plywood...
Your discomfort with fabric covering can be overcome at the cost of taking a class about it. Lots of classes going on around the country to show you how to do it.

As has already been pointed out, this is not as simple as merely substituting one material for the other. You'll essentially have to redesign the wing from scratch. Compared to that, taking a class on fabric covering is cheap and easy.
 

Dan Thomas

Well-Known Member
Joined
Sep 17, 2008
Messages
5,901
Plywood doesn't weather well, even when painted with high-grade finishes. The sun will cause it to start checking along the grain, water gets in, and it's all over. Fabric is far more durable, much lighter, and cheaper, too.

Dan
 
F

fly scared!

Simply putting plywood on in place of fabric is not a good idea. There are many design changes that must be made for this different type of structure. One example: fabric is stretched tight over the structure so it is always in tension even as the wing flexes. Plywood cannot be tightened in this way and as the wing flexes some of the skin will be in comression. Thin plywood has little or no compression strength and it will buckle under compression. To avoid this spanwise stringers are used to provide support and stiffness. The number of stringers required depends on the skin thickness. The thinner the skin, the closer the stringers must be.
I'm aware of this concern, but I don't want to use plywood for structural purposes (more stringers to provide extra support and stiffness et c.).
Unless fabric tension is actually required to take loads other than aerodynamic ones (which I strongly doubt), plywood would substitute fabric only as a non-working skin.
Could buckling be avoided with small indipendent panels ?
Most of the wing surface (intrados, aft extrados) would flex mostly spanwise, so maybe in these zones fabric could be substitued by spanwise strips.
The leading edge however would flex in a compound curve so maybe smaller longitudinal panels would be required.

Another change is that there is no longer a need for compression members and diagonal bracing when the skin is plywood instead of fabric. While the fabric is stretched tight over the structure, it remains far more elastic than plywood; so, diagonal wires or other similar structures are required to keep the wing from "racking", a term for flexing fore and aft. The fabric also tries to pull the spars together, as do wires if they are used for the diagonal stiffness.
In my case the wing has only a drag brace, I guess all the stiffness is provided by the leading edge box. (btw I never heard of buckling problems with plywood l.e.)
Still, the prototype wing was modified by the designer to increase stiffness and improve handling in stalls (the wing has a small negative sweep angle), so I don't think some extra stiffness would do any harm (provided spar loading is unaffected of course)
 

Topaz

Super Moderator
Staff member
Log Member
Joined
Jul 29, 2005
Messages
14,310
Location
Orange County, California
I'm aware of this concern, but I don't want to use plywood for structural purposes (more stringers to provide extra support and stiffness et c.).
Unless fabric tension is actually required to take loads other than aerodynamic ones (which I strongly doubt), plywood would substitute fabric only as a non-working skin.
...
It doesn't work that way. Unless the plywood panels are "free floating" (not connected to each other), they're going to absorb load, being the farthest element from the neutral axis of the wing. The results will be unpredictable without a complete structural analysis of the wing with the modification. And not to mention that the wing will be considerably heavier than the fabric-covered version, affecting weight and balance and reducing the useful load.

If you attempt to create free-floating skin panels out of ply, you're in for a pretty major engineering challenge. Seriously, it's just a lot easier to become skilled in fabric covering of aircraft than go through this exercise. I understand that you're not comfortable with fabric right now, but that's fixable and your proposed alternative is just going to absorb useful load and require extensive structural analysis. Why go through all that when you can spend a weekend and a couple hundred dollars and learn to do fabric like the designer intended?
 
F

fly scared!

Your discomfort with fabric covering can be overcome at the cost of taking a class about it. Lots of classes going on around the country to show you how to do it.

As has already been pointed out, this is not as simple as merely substituting one material for the other. You'll essentially have to redesign the wing from scratch. Compared to that, taking a class on fabric covering is cheap and easy.
Well, I'm sure you are right... for US ! Nothing like that exists in Italy. I've done some workshops with CAP (the local homebuilding association) but they were mostly a knowledge sharing opportunity, not "classes".

As for redesigning the wing, this is exactly the matter of the topic: how would you modify the wing for this substitution ? I'm not saying it would be sane, easy, or worth of, beware.

(I know I'll end up using fabric, but I would like to discuss the topic, so I would kindly ask to avoid saying "why don't you simply use fabric ?" thanks)
 

Topaz

Super Moderator
Staff member
Log Member
Joined
Jul 29, 2005
Messages
14,310
Location
Orange County, California
...I'm not saying it would be sane, easy, or worth of, beware....
Then why bother? You know you're going to use fabric anyway, so what's the point of this exercise? How would I modify the wing for this substitution? I wouldn't. I'd either redesign the wing from scratch or I'd stick with the designer's plans. Period. Full stop.
 
F

fly scared!

It doesn't work that way. Unless the plywood panels are "free floating" (not connected to each other), they're going to absorb load, being the farthest element from the neutral axis of the wing
This is exactly what I'm proposing, free floating panels. I believe the distance from the neutral axis is critical for cantilevered wing, braced wing will flex far less.

If you attempt to create free-floating skin panels out of ply, you're in for a pretty major engineering challenge.
Well, I believe this is the right place to discuss any major engineering challenge, isn't ? :)
 
F

fly scared!

Then why bother? You know you're going to use fabric anyway, so what's the point of this exercise? How would I modify the wing for this substitution? I wouldn't. I'd either redesign the wing from scratch or I'd stick with the designer's plans. Period. Full stop.
OK, full stop.
For you.
Nobody forced you to enter this topic if you're not in interested in discussing it.
I'm interested in it, is this a problem ?
 

Topaz

Super Moderator
Staff member
Log Member
Joined
Jul 29, 2005
Messages
14,310
Location
Orange County, California
OK, full stop.
For you.
Nobody forced you to enter this topic if you're not in interested in discussing it.
I'm interested in it, is this a problem ?
Quite right. It's your time to waste. Everyone here has told you this is a bad idea with only a poor outcome. I understand that that's not the answer you wanted to hear, but it's what you're getting. It's a problem when you brought it up and then don't want to listen to the answers you're getting.
 
F

fly scared!

Quite right. It's your time to waste. Everyone here has told you this is a bad idea with only a poor outcome. I understand that that's not the answer you wanted to hear, but it's what you're getting. It's a problem when you brought it up and then don't want to listen to the answers you're getting.
I WANT to listen to the answers but what is YOUR answer ? I haven't read one. You're only saying it's not worth of.
Let's dig WHY it is not.
You're not interested ? I understand it, I'm not interested in many of the topics here, so what's the problem ?
 

Topaz

Super Moderator
Staff member
Log Member
Joined
Jul 29, 2005
Messages
14,310
Location
Orange County, California
I WANT to listen to the answers but what is YOUR answer ? I haven't read one. You're only saying it's not worth of.
Let dig WHY it is not.
Fair enough. Here's why it's not worth it:

1) Substituting plywood for fabric will add significant weight to the aircraft. That weight comes out of the useful load, meaning you aren't able to carry as much people, baggage, or fuel. You don't get to add it to the MTOW.

2) Unless you free-float the plywood panels, they WILL take up loads from the existing wing structure. Unpredictably. You'll need to run a full analysis of the new structure to understand the new structure that you've got. That exercise is considerably more time-consuming than simply learning to cover an airplane in fabric.

3) Free-floating the panels adds yet MORE weight (the flexible mounts required, as well as covering between the joints to preserve a smooth airfoil). Again, this comes from the useful load of the aircraft.

4) If you butt-join the panels with some small gap to allow for flexure, whatever you use to bridge the gap will bunch up in compression, ruining your airfoil. The only other alternative is to shingle the panels on, which also destroys the airfoil.

5) You'll further need to make sure that air cannot get under the free-floating panels under ANY flight condition, including stall, where air over the wing could actually be going *backwards* over the upper surface. Any air gets under the edges, you're most likely to start shedding skin panels.

6) You're only embarking on this path because, as you stated, you've never done fabric covering before and are uncomfortable with it. The solution to that is TRAINING, not some quixotic exercise in material substitution. If you don't have classes available there, delve through YouTube and then make some test panels to train yourself. Or get something like the Stitts system and they'll show you how to do it through DVDs and manuals. The short and practical path here is to get rid of your discomfort, not redesign the wheel.

7) Lastly, if through some strange twist of fate you're actually unable to learn how to cover an airplane in fabric, hire the task out. There are professionals that do this in your country, too. The time cost of redesigning this wing for plywood is probably less than the monetary cost of hiring someone else to do the job, and the latter is going to be a lot faster.

Fabric covering has been used on aircraft for nearly a century, by tens of thousands of people building airplanes. If they can do it, you can do it.
 
F

fly scared!

Plywood doesn't weather well, even when painted with high-grade finishes. The sun will cause it to start checking along the grain, water gets in, and it's all over. Fabric is far more durable, much lighter, and cheaper, too.

Dan
Do you think that would apply also to a plywood skin covered with some glass ? I've had bad experiences with boats built this way, but I guess a clean hangar is a very different environment...
 

4trade

Well-Known Member
Joined
Nov 1, 2010
Messages
527
Location
Lahti/Finland
Do you think that would apply also to a plywood skin covered with some glass ? I've had bad experiences with boats built this way, but I guess a clean hangar is a very different environment...
Plywood skin must cover with class or fabric. Plywood itself cannot protect for just paint. Small amount or moisture will come on a surface of plywood skin and it start to become "hairy", small wood fibers will separate of plywood surface and ruin your wing smooth surface.

Fabric with good layers of protective paint (include sun resistant layers and final color) will be 250- 300 grams/ sq meter. It is significantly lighter than any plywood skin, because your plywood need that glass and all of those sun resistant/ final paint anyhow. That plywood covering for protection weight approx same than regular fabric job overall. Your plywood wing surface will be (weight of a plywood) heavier than just fabric.
 
F

fly scared!

Fair enough. Here's why it's not worth it:
Thank you. So:

1) weight penalty
2) I'm actually proposing free panels
3) weight penalty
4) gap compression
5) the skin must be sealed
6) OT
7) OT

I understand extra weight will detract from MTOW; at least, that weight would be placed in the best possible position, and would not increase loads on the fuselage structure.
The problems with panels gaps would require some investigation about the suitable composite fabric; maybe a flexible, kevlar-like tape ?
As for wing sealing, none would fly a wing with open gaps, I hope !
 
F

fly scared!

Plywood skin must cover with class or fabric. Plywood itself cannot protect for just paint. Small amount or moisture will come on a surface of plywood skin and it start to become "hairy", small wood fibers will separate of plywood surface and ruin your wing smooth surface.

Fabric with good layers of protective paint (include sun resistant layers and final color) will be 250- 300 grams/ sq meter. It is significantly lighter than any plywood skin, because your plywood need that glass and all of those sun resistant/ final paint anyhow. That plywood covering for protection weight approx same than regular fabric job overall. Your plywood wing surface will be (weight of a plywood) heavier than just fabric.
I agree. That would imply that the thinner the plywood the better, since the the glass would be both lighter and stronger than plywood or fabric.
However, since the aim would be to exploit the plywood stiffness to get a straight surface, a minimal thickness would be required to support the glass.
On the other hand, you could place a very thin plywood layer in very large panel dimensions, and exploit the camber of the wing for stiffness while covering with glass.
Then you could cut the small panels and seal the gaps with some more elastic tape.
All in all, very complicated...
 

4trade

Well-Known Member
Joined
Nov 1, 2010
Messages
527
Location
Lahti/Finland
If you really want to cover you wing without fabric, and keep it light, you may consider to laminate thin glass layer on a straight surface, bend it over your ribs and bond that skin on your ribs. It is heavier than fabric, but probably slightly lighter than plywood/ glass combo. It is critical to have excellent, strong bond between skin/ ribs and it is time consuming work compared just basic fabric work.

There is lot of issues for mixing different materials like that, and you should take those issues seriously if you take that road. I recommend traditional fabric cowering way....it is well proof for decades and safe.
 
F

fly scared!

If you really want to cover you wing without fabric, and keep it light, you may consider to laminate thin glass layer on a straight surface, bend it over your ribs and bond that skin on your ribs. It is heavier than fabric, but probably slightly lighter than plywood/ glass combo. It is critical to have excellent, strong bond between skin/ ribs and it is time consuming work compared just basic fabric work.

.
That would be the glass equivalent of plywood covering . I wonder if a thin skin would keep straight during bending like the plywood does.
You could use some temporary stringer inside, but removing them could prove difficult.
Maybe you could tack the stringer during lamination and then place the stringer face outside.
Then you should cut/sand them off.
 
Top