Author Topic: Bolt-on wing bending strength (Read 2206 times)

Keith Renecle · « **on:** March 04, 2011, 12:00:10 AM »

Hi All,

Just reading the thread below on geodetic wing stiffness brought this to mind, and it's something that I can't seem to get figured. I've built quite a few different take-apart wing systems, and some were even "break-apart" systems! Most of mine were based on the top and bottom bolt system like the Yatsenko's use. Basically you bolt the main spar together, but I would like to know if this resists bending as well as a wing that is built in one piece. In other words, is the sheeting on either side of the main spar helping to resist bending?

A sheeted foam wing relies on the surface skin for strength, but I suppose that the critical area is at the upper and lower surfaces around where a main spar would be anyway.

I always built my wing halves with a thin ply root rib, plus another one of the fuselage. I see that the Yatsenko wings have a balsa root rib, but of course the surface of the wing is composite, as is the body. I could build one of my systems with just balsa root ribs and a soft balsa fuz sheeting to see if there are any deformations after a hard flight, but I thought that the engineering types out there would know this stuff anyway. Please help! Thanks.

Keith R

Bill Little · « **Reply #1 on:** March 04, 2011, 10:42:29 AM »

hi Keith,

Not going to be much help in that all I know on the subject is that you really need to be careful in the spar/wing construction when making a two piece/removable wing. It no longer has the unified "spar system" to carry the load across the center span.

Bill

Tim Wescott · « **Reply #2 on:** March 04, 2011, 12:53:06 PM »

The sheeting is going to resist bending, the same way it does in a foam wing. If it's tied together in the center it'll also resist breaking. If you want to get full benefit from a "bolt together" spar, then you need to bring all of the forces from the sheet into the spar attach point. Assuming that the spars are bolted together tightly (I don't know how the Yatsenko wing is designed) then you won't get more bending, but unless you make sure the spar can carry the full load you may get a break.

Instead of building a wing with a balsa center rib, then flying (and possibly crashing), why don't you build a mock-up wing center section? Or build a whole wing in a hurry -- let it be ugly, warped and unfinished, but make sure that anything that carries load is the same as what you really want. Then prop up the center section to simulate the load of a fuselage, and weigh it down with sandbags until it breaks. Weigh the sandbags, and you know how much weight your real wing can carry. Moreover, as it starts to buckle you'll see which part is failing, so you can either stop the test and beef up that part, or you can make the next test article with a beefed-up part.

phil c · « **Reply #3 on:** March 04, 2011, 02:27:05 PM »

You could do what they did on the FW-190. The top and bottom wings were bolted to the fuselage with about 15 bolts, top and bottom, along the chord. They went in at about a 45 deg. angle and were covered by the fuselage fillets. Use Tim's prototype idea, but I would guess 5 screws top and bottom on each panel would be enough. The carry-through structure inside the fuselage could simply be half inch wide strips of PC board, stiffened with some really light balsa blocks so they don't wiggle.

If you messed around with some socket head, flat-head screws, they could be made flush with the skin and I bet the appearance judges wouldn't even notice them.

Keith Renecle · « **Reply #4 on:** March 04, 2011, 11:27:01 PM »

Thanks guys. Hi Phil............I'm not looking for hard work! Sticking screws all around the perimeter would be a nightmare. I reckon that the bending load is taken by the main spar, so if you have a foam wing, you would need to add some sort of hard spar under the surface that at least holds the joiner device.

I've added a few pics of Lauri Malila's Shark to show how the wings go on. The rear bolt basically holds the trailing edge from moving away from the fuz, and I don't think it will add any strength to stop bending.

My own system works by joining the spars to some aluminium bits and if you look at my 3-D pics, you can see how the center section works. You can basically make most of it with hand tools. I have noticed some bending in hard maneuvers, so I guess that it must be the spars and the actual structure of the wing. I was just curious to know if attaching it more securely to the fuselage would make any difference. The spar joiners do not budge at all, and I think (?) that the main spar is where the bending takes place. So if this is secure, then the bending is taking place further along the spar. Interesting stuff!!

Keith R

phil c · « **Reply #5 on:** March 07, 2011, 09:14:47 AM »

That is a perfectly good way to do things. Putting the screws vertically puts a shear load on them, which is good. The fits would, I think, have to be very good though. If the joint works just a few hundredths of a mm the wing tip can move 6 mm or more.

An I-beam spar capped with 1/8 x 1/2 in. balsa and joined through the fuselage with 1/8 x3/8 hardwood is plenty strong.
A friend of mine built a wing like this and made some aluminum "L" joiners about 2 in. long glued to the outer faces of the spars. The were tapered and had a slot machined in so a 4/40 screw(~3 mm) could be slipped in and screwed into a piece of 1/4 in hex aluminum glued into the fuselage. He also added locating screws at the leading edge and the trailing edge so the angle of attack could be adjusted if needed for trimming. It was plenty strong.

Chris Wilson · « **Reply #6 on:** March 07, 2011, 03:35:12 PM »

Quote from: Keith Renecle on March 04, 2011, 12:00:10 AM

Hi All,

Just reading the thread below on geodetic wing stiffness brought this to mind, and it's something that I can't seem to get figured. I've built quite a few different take-apart wing systems, and some were even "break-apart" systems! Most of mine were based on the top and bottom bolt system like the Yatsenko's use. Basically you bolt the main spar together, but I would like to know if this resists bending as well as a wing that is built in one piece. In other words, is the sheeting on either side of the main spar helping to resist bending?

A sheeted foam wing relies on the surface skin for strength, but I suppose that the critical area is at the upper and lower surfaces around where a main spar would be anyway.

I always built my wing halves with a thin ply root rib, plus another one of the fuselage. I see that the Yatsenko wings have a balsa root rib, but of course the surface of the wing is composite, as is the body. I could build one of my systems with just balsa root ribs and a soft balsa fuz sheeting to see if there are any deformations after a hard flight, but I thought that the engineering types out there would know this stuff anyway. Please help! Thanks.

Keith R

Hi Keith,
It would seem to me that comparing a take apart system with centralized aluminum supports to a one piece hardwood/balsa system is a bit like comparing apples to oranges, and anyway continuous center sheeting is going to help either system to be stiffer regardless.

With the Yatsenko system all the forces are channeled into the hard mounting points and they form a large enough triangular base to resist flight loads, so I doubt that the inner rib of the wing half will do much more than act as a separator and sound dampener despite what it is made of.

If the choice of inner rib material did matter greatly then I would venture to say that you actually have a problem with the localized strength of your wing joining system instead.
(I note that the take apart system you have built is bolted together and not a solid one piece, if it were solid then it resist bending all the better.)

Cheers.

ash · « **Reply #7 on:** March 07, 2011, 04:49:51 PM »

Keith, what are the materials in your picture that are in between the main spar joiners? The spacers and bellcrank mounting bits.

Also, what is the thickness of the aluminium parts?

I have an almost identical design to implement, but mine uses (apparently) thicker spar joiners to better resist bending and shear in the centre section. I've see one that uses extruded aluminium angle in the centre for the same reason.

Rather than stiffness, I'd be concerned about fatigue life of the spar joiners between the bolt hole and the first lightening hole.

Keith Renecle · « **Reply #8 on:** March 07, 2011, 10:24:36 PM »

Thanks guys for the comments. My aluminium parts are 3 mm thick and the width varies according to the size of the model. My Electron stunter center section is 12 mm wide. Chris, I actually glue the whole thing together with epoxy. The lighter coloured parts are hard balsa, and the wood between the threaded round bar is hardwood like beech that I use for engine bearers. I take your point, however about the center assembly being rigid, like the Yatsenko system. The aluminium is also the harder type. I forget what the hardness number is at the moment. I think that you have answered my question on the issue of the root ribs. Thanks for that.

On my take-apart profile model, I glue the main assembly into the inner wing, bellcrank and all. The outer wing is then bolted on and this holds the fuselage in place, so there I reckon that my hard ply ribs on the wings and fuz are justified. So in this instance, the wing holds the fuz in place and not the other way around.

I use hardwood 12 x 3 mm spars under the leading edge sheeting. I bind them to the joiners with thread, and epoxy them in place. The joiners are made out of of 6 mm thick aluminium and machined as per the drawings. "Machining" of my original joiners meant hacksaw and file. I have a "sort-of" milling machine now using my lathe that does this a little better.

It seems that the bending that is visible on some of my models is coming from the wing half bending itself and not from the joining system. I need to re-think the structure of the actual wings. Thanks for the feeback so far.

Keith R

ash · « **Reply #9 on:** March 08, 2011, 03:52:18 AM »

OK, 3mm is the same as mine. I use 3x10 strip but with less in the way of lightening holes than your picture suggests, so probably about the same result.

I think a lot of people make things harder than it needs to be with bolt-on wings but your method looks just right to me.

Chris Wilson · « **Reply #10 on:** March 08, 2011, 03:34:38 PM »

Keith, have you seen our mate Herb Hanna's home brewed wing joining system?

Linked here -

http://www.clstunt.com/htdocs/dc/dcboard.php?az=show_topic&forum=103&topic_id=307979&mesg_id=307979&page=371

Dallas Hanna · « **Reply #11 on:** March 08, 2011, 06:29:39 PM »

Hi Keith. Thought I'd add a bit here. I've been using the Yatsenko style spar for a while now. The 51 size model has been flying continually for a few years now in some rather windy conditions at times with no problems whatsoever.

I built a bigger and of course heavier 800 sq in model for for the big R & B 75 engine from Belko and Ian Russel. I went a bit small on the T-section in the wing component and it had a bit of a problem strength wise with the 4.5 secs/lap and T-bird turning rates

so I dug those bits out and replaced them with heavier section units. It now has quite a bit of air time on it with a more sedate 5.2 sec/lap with no sign of stress problems.

As for flexing, there isn't any more if any than a conventional wing structure. I was more more worried about the flex of the standard I-beam wing in my electric USA-1. Now that does flex!!!

HH

Keith Renecle · « **Reply #12 on:** March 09, 2011, 12:05:10 AM »

Thanks Chris and Herb, I did see that thread, and marvelled at the neat work of Herb on those parts. As I said, I don't have a proper milling machine. My system is easy to make however, and from what I can now gather from all of the nice feedback, my bending is coming from the spar or "virtual spar" if I use the foam wing option, and not from the root rib structure. That Grondal Nobler is great!

Most of my designs over the last 10 years or so, had wings based on the Tom Morris Millenium wing concept with the riblets and I-beam spar much like a true I-beamer. The leading edge up to the spar is sheeted, so it is something like a D-box section. I still had some flexing however and on my last attempt, I built up the spar with cris-cross supports in between the ribs. This was a lot more rigid. The trick is to get the overall model weight down and this is not such a problem. I've seen the I-beamers flexing many times, but all those ribs just look so good!

Keith R

jose modesto · « **Reply #13 on:** March 16, 2011, 06:24:05 AM »

Photos of Dallas Hanna Yatzenko take apart system. Sina is building a plug in wing Take apart system Impact Master
Photo #1,2 system fitted in place
Photo #3 Systen glued in place

Jose Modesto

Derek Barry · « **Reply #14 on:** March 19, 2011, 05:54:08 AM »

Jose, these pictures still scare me. I think that Sina should wrap that Kevlar (i am assuming) all the way around the spar. I used copper wire (a lot of it) to wrap mine, it also was wrapped all the way around the spar. I made little notches to allow for the wire. I will try to find pictures of it.

Derek Barry · « **Reply #15 on:** March 19, 2011, 06:00:57 AM »

Here are some pictures of the Dreadnought wing.

Derek Barry · « **Reply #16 on:** March 19, 2011, 06:03:17 AM »

a few more.

Jose, the last pic is the repair I made in Hungary. I have not had any problems since.

Lauri Malila · « **Reply #17 on:** March 22, 2011, 08:01:57 AM »

Hi.

I think the most important wrapping is the one that just goes around the aluminium part in wing. Gluing aluminium directly to wood spar is a little unsafe, and the wrapping (Kevlar) between these 2 materials strenghten the glue joint a lot. After that you can add some Kevlar and wrap the Al part and spars together but it is not absolutely necessary. Actually, if there is no gaps in the assembly, cyano glue is strong enough.
For gluing aluminium, I know there are products and methods for etching the metal or anodizing it to improve glue adhesion but I have never really bothered with those. What I have found sufficient is:

-Good de-greasing with acetone.
-Surfaces must be roughened with sandpaper. The problem is, that when done dry, aluminium surface very quickly oxydises, which weakens the glue joint. To prevent this, I first apply glue (slow curing epoxy) and then sand with #80 paper. The epoxy acts like cutting fluid and it prevents the contact between oxygen and aluminium. It can get messy but so what.
-Whenever possible, the epoxy should be cured in raised temperature. That would be about 50-70 degrees C. When the other part in the joint is wood, the ultimate strenght of epoxy is not the most important thing. The important thing is, that in raised temperature, epoxy's viscosity goes down and it penetrates deeper into wood.
When gluing two differend materials together, it is better to turn heating off before the glue hardens completely. Especially with long and narrow parts (I'd say over 50mm long). There, the problem comes from differend thermal expansion of differend materials. If the joint is let to harden completely in raised temperature, there is a risk of warping or at least a shear tension inside the joint. L

Chris Wilson · « **Reply #18 on:** March 22, 2011, 05:43:59 PM »

Interestingly there is never a mention of the leading edge ever getting a hard point mount on any two wing system that I have seen so far.

Its not necessary I assume but should it be adventured into?

Lauri Malila · « **Reply #19 on:** March 23, 2011, 10:06:17 AM »

There is no need, to carry the loads you only need to have 2 attachment points as far from wing center line as possible. Naturally, these are at the thickest point of the wing. The t.e. mount is mainly for alignment. Nothing at wing cl is going to stop bending. L

ash · « **Reply #20 on:** March 23, 2011, 03:45:53 PM »

The TE attachment point is also big factor in two other ways. It resists the wings swinging forward (clapping hands kind of motion) on landing (intended or otherwise) and it significantly increases stiffness in the pitching axis. All that pitching moment provided by the wing with flaps deflected has to be transferred to the fuselage somehow. The two main points will do it, but the three with a wider spacing do it much better.

Howard Rush · « **Reply #21 on:** March 23, 2011, 11:49:29 PM »

Quote from: Chris Wilson on March 22, 2011, 05:43:59 PM

Interestingly there is never a mention of the leading edge ever getting a hard point mount on any two wing system that I have seen so far.

Its not necessary I assume but should it be adventured into?

I'm no stress guy, but I'll bet it wouldn't hurt. In an inside loop, for example, the LE is lifting up, and the engine is pushing the fuselage front down. Tying them together might save a little weight.

Keith Renecle · « **Reply #22 on:** March 24, 2011, 09:49:30 AM »

I normally put a 1/4" dowel through the front part of the wing near the leading edge. The root ribs on each wing half are 1/16" ply and this just locates the wing quite well. I then clamp the wing at the rear section as you can see from the photos below. My older models had a small bolt on section near the TE, but my last few models just had an aluminium rod through the fuselage with a simple clamp on the inside of the wing root ribs. I then just squeeze it by turning in the M3 bolt. A 440 bolt would most likely work better with a coarser thread. The pics are from my take-apart profile stunter.

Keith R