Check out my small steel frame wings, advice on coating? - RC Groups
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Oct 30, 2012, 06:47 PM
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Check out my small steel frame wings, advice on coating?

I am getting these wings cut this week. The wings are made from .04 steel, snapped together and either brazed or spot welded to make a frame. The wings are about 14 inches long and 8ft total.

I was thinking of coating with canvas but would covering with lexan and vacuum bagging do a good job here? Any advice guys?

The drawing shows the peices I had 3D cut and the frame they will make when snapped together.
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Oct 30, 2012, 09:00 PM
B for Bruce
BMatthews's Avatar
An interesting project. VERY interesting in fact.

You are missing out on the sort of spar caps needed to achieve a proper spar system. The flat .04 webs of the "spar" members will simply collapse under compression during lift. What you need is one or two "L" angles to form the caps for a "[" or true "I" beam style of spar. Also the webs of the upright main spar section should have had diagonals or at least deeper horizontal webs to best support the angle caps which form the top and bottom portions of the spars.

So are these intended to actually fly? Steel is an unusual material for a flying model. And the coverings you are suggesting are even less suitable for model flying.
Oct 31, 2012, 02:25 AM
Registered User
yeah this is made to fly! I know the frame does not follow aerospace standards in engineering but it will be strong enough to hold itself together. Lexan will be heated and rolled over it. Maybe double layers.
Oct 31, 2012, 04:22 AM
B for Bruce
BMatthews's Avatar
Between the steel and a thick enough layer of lexan to substitute for proper spar caps the sort of structure you are suggesting is going to be overly heavy for the amount of strength required. Simply put no one uses a molded skin of lexan plastic because of the simple fact that by the time you use a thick enough skin to do the job it's going to be overly heavy. Especially since other materials, such as glass fiber and epoxy, have a higher strength to weight ratio and could do more with less in your application.

But even so the skin of the wing is not a good substitute for a proper spar. Or if you do want to use a "stressed skin" structure then you need multiple light but stiff vertical stringers in the framework to better support the skin to allow it to act as the outer elements of a "distributed I beam" like structure.

There is a reason why we have some "classic" structural styles. It's because they work well at providing support with the least amount of material. You ignore such standards at your own peril.

The structure you show would certainly be able to hold itself together on the ground. But what about in a steep turn or during a dive recovery where the G loads will double or possibly triple the apparent weight of the model? It's this ultimate stess value that you need to anticipate and design the structure to withstand. AND you need to do so in a way that you have a design which has a wing loading which is proven to be decently flyable for the size range you have chosen.

Your idea for the frame is an interesting one. But I would suggest you reconsider your choice of materials and add on the L sections needed to form a proper I beam style spar.

I actually think that an interlocking steel framework such as you are suggesting coud be made to work. But .040, otherwise close to what I see is 19 Ga, is rather heavy. A thinner gauge steel formed with bends of the sort similar to how aluminium ribs are made for regular planes could be made far thinner and still stiff enough to carry the chord wise loads courtesy of the flange bends that aid in producing three dimensional stiffness from thin and otherwise uselessly flexible metal sheet. This is the reason why the "accepted standards" ARE the accepted standards. It's not someone's whim. It's simply that forming the ribs and other parts with bends and flanges and other shapes that pull the metal away from a simple 2 dimensional cutout is what gives the component the ability to achieve stiffness and load bearing capabilities out of otherwise overly thin and light material.
Last edited by BMatthews; Oct 31, 2012 at 04:29 AM.
Oct 31, 2012, 03:53 PM
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machinate's Avatar
I'm afraid I have to agree with BMatthews. Thin, flat metal has appalling structural characteristics, and I can't think of a single situation in the real world where it's used structurally. It's always used either in a beam shape (I-beam or tube), or it's stamped into a shape with depth (kitchen sinks, car bodies). Your design will be very heavy compared to a typical wood model of the same size, and still won't have anything close to the same level of strength.

Here's a comparison for you: Last year, I crashed one of my smaller wood planes quite badly, and decided not to fix it. As I was loading the wreckage into my car, I had the idea to stand on the wing. It was maybe a 40-inch span, with the model weighing less than 3 lbs ready to fly. I set the wing flat on the concrete and stood on it, and it supported my weight. It wasn't between blocks or anything super impressive, it was just lying flat on the concrete, but I had to jump on it to noticeably crush the spar area.

In the same situation, your design appears to be nothing more than a very open cross of 19ga steel. I strongly syspect it would buckle if I stood on it, and it's meant to carry a model many times larger than mine!

The most important thing to add to your design, no matter what the building material, is a strip on top and bottom of the spar, perpendicular to the main spar, to convert it to an I-beam. Without this, it will easily buckle forward or backward.

Even with this addition, though, it's going to be extremely heavy. It's rare in models, but airplanes can obviously be made of metal; almost every full-size plane is. But those are all made of aluminum. Usually not even the spars are steel, because it's far too heavy for its strength. The only exception is with tubular steel frames, which are built like a race car chassis but with much thinner steel. However, these require very careful engineering, and a much more complex design.

Finally, Lexan is a lot heavier than you really need to cover your wings (although if you're sticking with the current steel design, you'll need all the help you can get). It's also pretty heavy for its strength, especially when you're planning to use so much of it.

On the other hand, now you've got me thinking I'd like to build a model with conventional techniques, but using thin acrylic in place of balsa. That'd look neat.
Oct 31, 2012, 04:46 PM
B for Bruce
BMatthews's Avatar
Here's a picture of what machinate and I are talking about;

See how the otherwise thin and far too flexible metal is formed to create L's and U shaped stiffening. Without these things the ribs made from this thin a metal would simply collapse under any sort of load. But with this shaping for stiffness the ribs are both strong and light.

Machinate, if you could set up and vacu-form ribs of this style from pop bottle plastic, whatever that is, I suspect that it would be quite adequitely stiff. Then a thin sheet of that same plastic over the framing and you'd have an all clear model. Wonder Woman's invisible jet comes to mind....
Nov 01, 2012, 01:20 AM
Registered User
I am thinking about after brazing, cleaning up and putting this plastic on all edges of the metal, then securing and stretching surplus heatshrink fabric to a wing shape.
Do you think the material will be tight enough to not dip between the spars?
Nov 01, 2012, 03:46 AM
B for Bruce
BMatthews's Avatar
I have no idea what you expect to gain by adding that edging to the structure. In any event adding the edging is not in any way a suitable substitute for the flanging and stiffening seen in that aluminium rib photo. And it most certainly will not add the spar function mentioned already that this structure desparately needs.

But hey, if you want to try your own thing and as long as you are not sitting inside when it folds up more power to ya.
Nov 04, 2012, 02:12 AM
Registered User
Yeah this is for a large RC plane proof of concept, so even if the wing is overly heavy and fragile it doesnt really matter.
Nov 04, 2012, 02:37 AM
Wake up, feel pulse, be happy!
Piece's Avatar
Any particular reason you decided to go with steel in the first place?

You could achieve a much higher strength-to-weight ratio and probably greater absolute strength if you built a "proper" structure from wood with more traditional covering methods.
Nov 04, 2012, 05:06 AM
Registered User
Lasercut steel sheet is very cheap, it requires magnitudes less elbow grease than making foams and using composite or doing a wooden rib design. Its good for someone that wants to make some wings without buying a ton of materials, tools, etc
Nov 04, 2012, 05:08 AM
Registered User
So I think I will line all the edges with rubber and then cover the frame with heavy polyfiber, sew the spars, heatshrink and then add paint/hardeners. Do you guys thing polyfiber would dip between my spars which are 8" apart if properly stretched taught?
Nov 04, 2012, 08:53 PM
B for Bruce
BMatthews's Avatar
Originally Posted by nkalex
Lasercut steel sheet is very cheap, it requires magnitudes less elbow grease than making foams and using composite or doing a wooden rib design. Its good for someone that wants to make some wings without buying a ton of materials, tools, etc
Simply said I would suggest that it is not. If you had done the same wing from 1/8 luan door skin ribs with some wood spars the structure would be both lighter AND stronger in all the ways that are needed for a wing. AND you could have produced the wing with few tools and a few hours of hand work instead of the cost of having laser cut steel done by a shop.

You are also wildly missing the points on wing structure that have been posted for you to consider. No offense but I would say you simply do not understand about the basic structural needs for a wing structure or how the classic I beam works to achieve the most strength from the least cross sectional area.

There are very valid reasons why "classic structures" become classic. The primary one is that the "classics" survive for long and trouble free lifetimes instead of suffering from catastrophic failure.
Last edited by BMatthews; Nov 04, 2012 at 09:03 PM.
Nov 04, 2012, 10:22 PM
Wake up, feel pulse, be happy!
Piece's Avatar
Its good for someone that wants to make some wings without buying a ton of materials, tools, etc
Mmmkay, then. Acid test: Why doesn't ANYONE else do it your way, anywhere, ever, with success? Surely there are plenty of other people who'd like to make wings cheap-n-easy. I've found that this one question is by far the most reliable method of checking simple ideas for validity.

On the other hand, I don't feel like you're too keen on listening to our advice, so go ahead and build your steel wings. Let us know how they turn out
Nov 05, 2012, 12:09 PM
You know nothing....
Stuart A's Avatar
Magnetic building board?

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