"Skinning the Bat" Open Bays- Part II

As suggested by the moderator, a good idea, a second thread on the
following: (this is cut and paste from the first Open Bay post)

"Larry and gents,
What's your thoughts on open bay -v- sheeted wings in the weight department?
Yes the open bay will be lighter, but depending on the area that's left open
is it enough to make a major difference in performance?

If total airframe weight is targeted to be "no more then X oz.'s" Is it not
possible to sheet the wing completely, and save the weight else where?

Any other thoughts on this kind of thinking?

What are your thoughts on Open Bay -v- Built up Sheeted -v- Foam/Sheeted
construction, as they relate to weight? The Sheeted wing seems to offer
better (read: easier by degree) finishing technique, ..or not? what do you
all think?

Tony"

Mr. Stuka Sr. replies below (also a C-n-P).

"My comments: A light foam wing will end up an ounce or two heavier than a
light built up wing. A fully sheeted wing is always easier to finish IF you
don't have ripples which can happen on a fully sheeted-over-ribs type of
wing. However, there are "fixes".

In theory, a partially sheeted foam wing would save weight (as per Tom
Dixon's suggestion.) However, instead of using 1/16 inch wood on a partially
sheeted wing, why not use 1/20 or even 1/32 wood on a fully sheeted wing?
You will maintain the strength and actually save weight on a fully sheeted
1/32 wing over a partially sheeted 1/16.

On most of our wings we use the same construction troughout. They are thus
overbuilt at the tips and weakest in the center where the maximum load is.
We need to change our building techniques to increase the structure in the
center and decrease it at the tips. There are several ways of doing this
including tapering our spars, using carbon fiber, tapering the leading and
trailing edge sheeting.

And, yes, you could make the wing a little heavier and try to shave the
weight elsewhere, but why not save the weight elsewhere and then make the
wing lighter, too? Keep the weight down while maintaining adequate flying
structure, and you will always come out in the flying end.

Leonard Neumann"

Also: from Rocket City Jim:

If you want a foam wing plane that is light enough to perform
extremely well, then I suggest that you E-Mail Randy Smith or
Bill Rich and find out what they did to build 55 to 57 ounce
Foam wing 675 SQ In. Intrepid X/L's. They look like they didn't
skimp any on their finishing process either. Mine weighs 62 ounces and does pretty well.
RCJ

Thanks Leonard,
Tony

Tony,

One other thought that I have seen mentioned by Igor Burger of Slovokia is that he believe that a carefully crafted foam wing that is as light as possible and perfectly true in it's airfoil shape is superior aerodynamically to a built up wing structure.

Yes, if you hollow the daylights out of it (literally and figuratively). No sag between (nonexistent) ribs. And it's straight.

But how many people can produce a stunt foamie that, when ready to fly, is truly competitive from a weight standpoint?

Granted, there are a few.

Igor also uses drafting paper to cover foam wings. Anyone tried that?

If so, what's the weight difference over balsa sheet?

What might the Foam WIng weight penalty be 15%? 20%?

I wouldnt have any issue with thinking that the wing aerodynamics could be 15-20% more efficient in L/D when it is proper (as designed) over the entire span.

THe only place a ribbed wing is correct is at the ribs. Sheeted back to the spar really helps, but it would be an iteresting tunnel series--bet its been done already for full size!

FWIW

Assuming we have a weight penalty of 2 ounces with the foam wing and a 50 ounce airplane, we then have a 4% weight penalty. Will there be even a 4% efficiency gain on the wing? I don't know. It is interesting that there are an abundance of different airfoils out there, and it is all in the hands of the flyer.

Is Paul's airfoil better than Ted's? How about Windy's? The Nobler was basically a Barnstormer airfoil with moveable flaps. Not nearly as efficient as most modern airfoils. But successful, nontheless. And was Gieseke "scientific" when he increased the airfoil by a pencil width all around?

Maybe a foam wing is more efficient, but when we get into the verticals and the overheads airfoils are only used for turning and weight is still weight.

Still, a foam wing nearly won last year's Nats. And Baron's plane, when he won a few years earlier, was extremely heavy. It is still in the handle of the flyer.

I think if a man were to actually check, the split between foam and built up wings is still about 50/50, even at the Nats. Now the top-top guys might all be using the lost foam geo wings, but I think you will find that there might be a connection between the guy who makes them and the flyer. I think a lost foam wing goes for $200 or more? I am not sure.

Another good alternative would be laser cut wings. Good fits make for a better wing, even in a traditional design. There are a lot of laser-cut wings in Dallas, and I have seen at least one super light example because of the little glue required for construction. Read this EASY TO BUILD.

I still think foam is better for the regular guy, if the airfoil is the correct airfoil. Foam is an almost assurance of a warp free wing. The Dixon method even gets rid of the seams (my biggest problem). If the two halves are prepped descent at the joint, assembly is pretty hard to screw up.

The truth of the matter is that most guys do not build super light. Most build medium, and some heavy. I build about medium these days, but I am getting a little lighter as I go. I do not know if I will ever build super light, as I have not noticed that the designs I am using require super light wing loading. Here’s the point. The airfoil does make a difference, period. When the wing loading is high, the airfoil makes ALL THE DIFFERENCE. When the wing loading is low, I doubt there is a major difference.

Most participants in PA have higher wing loadings, this is my belief. These same participants can be competitive. I think that a lot of the top guys have higher wing loadings than they would admit to in public. A lot of lead sleds have won the Nats; another fact. A lot of foam has won the Nats. Hunt won the Worlds with a foamy, and now condemns the foam as inferior to the built up wing.

Let’s all be a little honest and admit we will never build and finish like Werwage. Ted Fancher admitted he could not build 750 sq. inch airplanes at 52-55 oz. and still manages to win (a lot). With today’s horsepower, why bother. The numbers we are using are based in the Fox 35 days. I have seen some big, fat, airplanes fly AWESOME (when they were designed for it). Fat wings open the envelope just like pipes, heavy-duty controls, horsepower, good props, etc. Why not use them? Skinny wings are for Classic. Even Werwage has gone fatter, and likes it!

Al Rabe was right 30 years ago. Most of the good designs are based on his airfoils.

Now, back to foam. The bulk of the weight realized with foam is in the glue and finish. Take all of the components required for a foam wing and put them on a scale before assembly. The foam cores with lite-ply spars should be about 1 oz. Using 6# 1/16” balsa sheeting on 600 sq. inches of wing would be 2 oz. Front and rear spars in 6# ¼” balsa would be 1 oz. The bellcrank and pushrod probably weighs 1.5-2 oz (somebody check this). The inboard tip should weigh about ½ oz. The outer tip should not count! The outer tip is tip weight and anything under 1.5 oz. should be generally acceptable for most 650 sq. inch modern designs.

All told that is 4.5 oz (before glue) without the outer tip and tip weight box and hardware. These are fixed values and are present on built up wings also. With hardware we should be at 6.5 oz without glue. Add 2 oz. for flaps and that is 8.5 oz, not counting tip weight. I would take that! My last foamy (Dixon style) was 14 oz with 2 oz flaps (which included a solid outboard tip) so we will subtract 1.5 oz for tip weight box and outboard tip and say 12.5 oz. That still says 4 oz of glue and fiberglass (for the center section)!

Here is my method for my next wing. I plan to use 1/32” 6# wood, which will shave 1 oz. I would like to try the Polyurethane glue to attach the skins, or I may go back to 3M-77. Epoxy has never worked well for me. I plan to cover the wing with Minwax Polycrylic. I will use no paper or dope. Polycrylic is water based and will not melt the foam through the thin balsa and leaves a surface very similar to CA soaked wood. It is super hard. 2 coats, sanding between, auto primer and paint. Done.

Bill Wilson, our local paint guru, discovered Minwax Polycrylic. I am currently trying it for the first time, as I was reluctant to believe anything was a good as dope and paper. I must say, I am very excited. This stuff looks like milk, has no smell and is water based. It dries in 2 hours and leaves bare balsa ROCK HARD, like it is soaked in thin CA. Bill claims the wood is as strong as wood covered in dope and paper. He has performed tests to prove it. Also, since it is water based, it swells the wood nicely to reveal all the scratches, which can then be knocked flat with the sandpaper.

Guess I just felt talkative today.

Bradley,

What kind of color paint would you use over the Minwax polycrylic and autoprimer? Auto Enamel?

I use basecoat/clearcoat these days and do not plan on going back. The only substitutions I make are lacquer spray cans that can be purchased at AutoZone and Pep Boys. Minwax Polycrylic should be inert and should accept any kind of primer. I use automotive gray primer made by Evercoat.

One thing, DO NOT wet sand the primer over the Minwax Polycrylic. The Polycrylic is water based and will cut too easily with water. Dry sanding of the primer will result only in cutting the primer and not the Polycrylic. As a general rule, water based should not be wet sanded.

The next thing I want to try is to apply paper with the Minwax Polycrylic.

I finished the new fuselage with the Polycrylic and no paper. I would say this is a very viable technique, but did not significantly reduce the work required for a good finish. I would say that sanding time would be equal (2 coats of Polycrylic-two coats of primer) to using paper and dope. There is a huge strength and stink advantage over dope, and the paper is not required (for those who do not like paper). All in all, I really like the stuff.

I also like Jap tissue for the ultimate finishing surface.

Jap tissue really requires no primer to fill. My Oriental, finished in automotive clear, has no filler or primer. Brodak clear and Jap tissue was the base. I plan on doing some test to see if Polycrylic can be used to apply paper to wood surfaces. It is water based, so why shouldn’t it work? The beauty is that since Polycrylic is water based, it does not attack foam, so thinner sheeting can be used. The cores could be papered right after being sheeted, before assembly.

Also, there is a new ultralight polyester tissue in Tower Hobbies. It is 1 OZ per square yard and is pre finished with clear epoxy. Has anyone tried it?

>But how many people can produce
>a stunt foamie that, when
>ready to fly, is truly
>competitive from a weight standpoint?

Almost anyone. Weight, particularly a few ounces, is just not much of an issue. I think that they fly more consistently, which is a lot more important than 2 ounces.

My foam airplane, at 61 oz., is right in the ballpark with other built-ups, and lighter than the last 16 NATS winners.

What I was trying to say was: produce a foam wing at the same, or less weight, when compared to the conventional method. Competitive strictly from a wt. perspective, not a purely competitive (as in PA contest) one.