Author Topic: Design question, relating to wing spar construction (Read 3015 times)

John Miller · « **on:** September 23, 2007, 12:09:32 PM »

Some of you know that I'm working on the plans for Walter Umlands rendition of the Sterling Spitfire. It's my desire to change design features wherever possible to improve, and, or, remove excess weight, from the design.

This, of course is internal, and not affecting the external dimensions. We hope to arrive at a lighter. stronger, plane, that when finished will externally be the same as the original.

Excessive weight seems to be one of the problems to address, esp. with the relatively narrow wing, compared to todays designs. So, much of my effort at this time is to reduce, even if it's a small amount, structure weight, while maintaining enough strength to carry the load.

I have been told, and do believe that a stunt ship when crashed hard, should turn to dust. If it doesn't, you've over built it, and it's therefore too heavy.

I have a queston though, and would like your input.

It has to do with spar design. The usual practise, and, what I would normally use with the Spitfire wing, uses a pair of 1/4" sq. medium hard balsa spars, top and bottom, with possibly some vertical shear webbing between them. Literally thousands of planes have successfully been built with this style of spars.

Lately though I've gone to a different style that has worked very good so far, and comes out slightly lighter in weight. They also seem to be, and should be, from my studies, much stronger.

For a weight savings of 25%, I can use 3/8" X 1/8" medium hard balsa oriented with the flat to the outside of the airfoil. According to my studies, this puts more "section modulus' at the extremes, adding considerably to the spar strength.

I can also change from a simple vertical shear web, to a Warren Truss arrangement, which depending on the type and amount of glue used, could save an additional percentage of weight.

My problem is that I fear some builders may not understand the thinking behind this change, and consider it poor engineering at best, or modify and over build, believing that it's a better way to go.

I know that many builders re-engineer their kits, to reflect their own beliefs, style, or, because that's simply the way they have always done something. In most cases that's fine, and perhaps even needed.

Anyway, I'm posting this on both forums to get some feedback from others. Hopefully, I can put together a redesign that will accomplish our design goals, be understood, and not be subject to a lot of re-engineering by the builder.

What do you think?

Dennis Adamisin · « **Reply #1 on:** September 23, 2007, 01:09:56 PM »

John:
the 1/8 x 3/8 flate with shear webs is standard practice in wooden saiplanes. If you are nervous you can substitute spruce for balsa at negligable weight gain. SHEAR WEBS ARE THE KEY for torsion and stiffness in bending! A good trick is to plank about 1/2 way over the spar caps then use the other half to hold the rib cap strips.

BTW my recent Sterling Spitfire kit build I kept the standard 1/8" square spars BUT I angled the planking so it followed the spar, then added shear webs, that closed-up the D-tube.. The wing is very strong in bending and torsion - FAR better than the stock wing.

Circlejerk · « **Reply #2 on:** September 24, 2007, 06:24:24 AM »

Why? If your goal is to reproduce the Sterling Kit then why not provide a faithful reproduction. Allow the individual builder to make changes as he/she sees fit. The biggest problem with the old Sterling kits was wood selection.

Paul Smith · « **Reply #3 on:** September 24, 2007, 08:30:03 AM »

The writer's theory of structure is entirely correct:

The food chain would go.... 1/4 x 1/4, then 1/8 x 1/2, then 1/16 sheet over the whole wing, then no spars, just firbreglass or carbon. The farther from the neutral axis the better.

Bill Little · « **Reply #4 on:** September 24, 2007, 09:41:50 AM »

Quote from: Circlejerk on September 24, 2007, 06:24:24 AM

Why? If your goal is to reproduce the Sterling Kit then why not provide a faithful reproduction. Allow the individual builder to make changes as he/she sees fit. The biggest problem with the old Sterling kits was wood selection.

Good question.

Knowing Walter, he is intent on providing the best possible kit he can, without changing the external dimensions, or appearance. A lot of internal changes (allowed by Classic rules) can really improve the flight characteristics of most of the old Classic kits. Short cuts were often taken in kitting the design from it's original state.

Wood selection WAS a problem in 99.9% of all the Sterling kits! Walter always has impeccable choices in his wood selections. The correct weigh and grain is the norm in his kits.

I have a Sterling Spitfire I built in 1972. Due to no particular reason, it has only had probably a dozen flights in its lifetime! I am intending to strip it down, put it on a diet, and install a "modern" engine. I have seen a few that flew surprisingly well!

Bill Little · « **Reply #5 on:** September 24, 2007, 09:43:57 AM »

Hi John,

I would note on the plans your rationale for the change. It should help out, for sure. Sheer webbing is a necessity, I believe!

Marvin Denny · « **Reply #6 on:** September 25, 2007, 10:54:46 AM »

Us old combat fliers discovered a long time ago that the spars laid flat were stronger than those stood on edge, when coupled with spar webbing.

Now I ask--- what is the purpose of verticle grain spar webbing?? Also, Is it supposed to be between each and every rib the full length of the span?
I'm serious.

Bigiron

Mark Scarborough · « **Reply #7 on:** September 25, 2007, 01:44:48 PM »

The vertical grain shear web prevents the spars from moving towards each other. their function simply prevents the wing from collapsing when it is stressed. As long as the spars remain correctly located with respect to each other the integrity and strenght of the wing is maintained. The vertical grain uses the directional strength inherant in the balsa to align the compresion resistand in line with the anticipated stress. To visualize this, think about how easy it is to dent balsa across the grain.

Leo Mehl · « **Reply #8 on:** September 25, 2007, 06:20:12 PM »

I build my wings with Spruce spars and cross brace sheer pieces. I also sheer web the the trailing edge. This make a very rigid wing and helps keep the wieght down. In The sixties Gene Matheny and I used 1/8" inch square spars on a C tubE configuration that worked very well. But you had to really watch your wing when you painted. It din't take much to warp it. This really made the wing light. I carried this one step further by spreading the rib spacing. I think three inch open bays are acceptible. That is my method. I do think spruce spars are the answer for spar strength without adding wieght.

Steve Helmick · « **Reply #9 on:** September 30, 2007, 09:39:29 PM »

The diagonals (i.e., Warren Truss) are a very poor substitute

for shear webs. The shear webs keep the spars the correct distance apart, and also keep them from sliding spanwise, relative to each other (prevent bending). PLUS, they 'close the box', increasing torsional stiffness tremendously. Diagonals don't do any of that. If you are using diagonals, and getting away with it, it's because your wing is strong enough without the diagonals. You don't see spar diagonals used on any F1A wings, and there just aren't many thinner or more highly stressed wings in aeromodelling.

Steve

Leo Mehl · « **Reply #10 on:** October 02, 2007, 08:02:08 PM »

Quote from: Steve Helmick on September 30, 2007, 09:39:29 PM

The diagonals (i.e., Warren Truss) are a very poor substitute for shear webs. The shear webs keep the spars the correct distance apart, and also keep them from sliding spanwise, relative to each other (prevent bending). PLUS, they 'close the box', increasing torsional stiffness tremendously. Diagonals don't do any of that. If you are using diagonals, and getting away with it, it's because your wing is strong enough without the diagonals. You don't see spar diagonals used on any F1A wings, and there just aren't many thinner or more highly stressed wings in aeromodelling. Steve

I think cross bracing is as strong as sheer webs. Also if you are a good builder you do not have to worry about your rib spacing because you already have them nailed down. I have proof of this Grinder wing only broke in the middle. I also use warren truss for my motor crutch which is as srong as blocking between motor bearers.

Gordon Tarbell · « **Reply #11 on:** October 08, 2007, 07:20:00 PM »

So if I understand this correctly the grain of the 2" by 30"(for example) spar sheet runs top to bottom not tip to root? I didn't know you could by balsa sheets with the grain running anything other than lengthwise. I am getting ready to do a Morfadite/Frankenwing and will need to cut some spar material. The caps are going to be 3/8 by 1/8 spruce laid flat. Pleas school me.

Mark Scarborough · « **Reply #12 on:** October 08, 2007, 07:32:50 PM »

Ok let me jump in here for a sec,,,,the first thing to realize is that the purpose for shear webs is NOT just to strengthen the wing. It is to strengthen the wing in a particular direction. The shear webs basically counter the movement of the tips up and down. by themselves they realy dont contribute anything to crash resistance. We really shouldnt be building to try to resist crashing anyway. (unless your me right Leo?) Shear webs are peices of thin sheet material which has the grain running top to bottom between the spars. Typically it is cut to fit between the ribs and as such does not need to be 30 inchs in length across grain. it only needs to be the same length as the distance between your ribs. warren truss spar reinforcements are effective if used correctly however they cannot be as strong by definition as true shear webs. They could be somewhat lighter and arguably easier to fit. However in my opinion, to effectifly use diagonal bracing to replace shear webs, you need to have an "X" between each rib otherwise the spars are still allowed to move relative to each other and that is what the shear webs are trying to avoid. The grain of the reinforcement material needs to run in the same direction as the stress you are trying to counter. In the case of shear webs, we are trying to eliminate the ability of the spars to move closer together or farther apart which would ultimaetly lead to a failure in the wing. this movement comes about primarily by the tips moving relative to the root of the wing. to make another point,Twisting motion in the wing is controled by the sheeting and or covering used on thewing, this is why an I beam wing needs silk or silkspan instead of monokote. Monokote is STRONGER than silkspan in that it wont fail until a higher load is placed upon it. HOWEVER it is more flexible and will distort before it fails , this defeats the purpose in an ibeam and allows the ribs and spars to move relative to each other and as soon as you allow things to move that way, failure occurs. Hence silkspan or silk is better for this application, it maintains the cofiguration of the componants inside the wing, doesnt let them move, and in doing so prevents failure.In a conventional wing, the leading and trailing edge sheeting combined with the ribs controll the twisting. HOWEVER, warren truss ribs actually do a better job of aligning the strength of the rib with the stress it is trying to counter which is the twisting motion. IMHO.

Gordon Tarbell · « **Reply #13 on:** October 08, 2007, 08:59:08 PM »

The reason I feel the need for a thirty inch long shear web is that I don't have any ribs in the wing and each panel is about thirty inches long . Maybe it would be ok to use long grain wood laminated with carbon veil and the spruce caps that I described.

John Miller · « **Reply #14 on:** October 08, 2007, 09:41:01 PM »

Quote from: gordon tarbell on October 08, 2007, 08:59:08 PM

The reason I feel the need for a thirty inch long shear web is that I don't have any ribs in the wing and each panel is about thirty inches long . Maybe it would be OK to use long grain wood laminated with carbon veil and the spruce caps that I described.

Gordon, A spar web using the grain oriented legnth wise, can be used. I used such a web in my version of Gierkes All American Eagle. The trick of course is to not overbuild while trying to gain strength.

I suspect that you could construct a laminate, using 1/16" balsa. Three layers, with the center layer oriented at right angles to the outside layers, should be plenty strong. Using CF sandwiched between two layers of balsa oriented legnthwise will be a little stronger than without it, but, this may not be the best place to use CF in spar construction.

You would gain the most strength, using CF in spar construction, by laminating .007 CF between two 1/16" X 1/2" balsa. You would then use these laminations as you would the balsa 1/8" X 1/2" balsa spars. If you are using a sheeted leading edge, You could use the 1/8" X 1/2" balsa spars, with a strip of .007 CF at the top of the spar, between the spar and the sheeting. This would put the greatest strength at almost the maximum distance from the center of the bending moment, at the maximum stress points in compression and expansion. The best way to use the strength gained with CF in spar construction, without over building, or adding extra weight. IMHO.

Marks explanation of the function of a shear webbed spar is pretty good, Ya done good Mark.

Mark Scarborough · « **Reply #15 on:** October 08, 2007, 10:07:42 PM »

Guess I didnt catch the part about no ribs there eh John? Thanks for the comments, from you thats high praise!

Leo Mehl · « **Reply #16 on:** October 09, 2007, 08:53:00 AM »

Quote from: Mark Scarborough on October 08, 2007, 07:32:50 PM

Ok let me jump in here for a sec,,,,the first thing to realize is that the purpose for shear webs is NOT just to strengthen the wing. It is to strengthen the wing in a particular direction. The shear webs basically counter the movement of the tips up and down. by themselves they realy dont contribute anything to crash resistance. We really shouldnt be building to try to resist crashing anyway. (unless your me right Leo?) Shear webs are peices of thin sheet material which has the grain running top to bottom between the spars. Typically it is cut to fit between the ribs and as such does not need to be 30 inchs in length across grain. it only needs to be the same length as the distance between your ribs. warren truss spar reinforcements are effective if used correctly however they cannot be as strong by definition as true shear webs. They could be somewhat lighter and arguably easier to fit. However in my opinion, to effectifly use diagonal bracing to replace shear webs, you need to have an "X" between each rib otherwise the spars are still allowed to move relative to each other and that is what the shear webs are trying to avoid. The grain of the reinforcement material needs to run in the same direction as the stress you are trying to counter. In the case of shear webs, we are trying to eliminate the ability of the spars to move closer together or farther apart which would ultimaetly lead to a failure in the wing. this movement comes about primarily by the tips moving relative to the root of the wing. to make another point,Twisting motion in the wing is controled by the sheeting and or covering used on thewing, this is why an I beam wing needs silk or silkspan instead of monokote. Monokote is STRONGER than silkspan in that it wont fail until a higher load is placed upon it. HOWEVER it is more flexible and will distort before it fails , this defeats the purpose in an ibeam and allows the ribs and spars to move relative to each other and as soon as you allow things to move that way, failure occurs. Hence silkspan or silk is better for this application, it maintains the cofiguration of the componants inside the wing, doesnt let them move, and in doing so prevents failure.In a conventional wing, the leading and trailing edge sheeting combined with the ribs controll the twisting. HOWEVER, warren truss ribs actually do a better job of aligning the strength of the rib with the stress it is trying to counter which is the twisting motion. IMHO.

Mark- You need to build something that will just bounce.

Mark Scarborough · « **Reply #17 on:** October 09, 2007, 09:04:59 AM »

THANKS LEO

Leo Mehl · « **Reply #18 on:** October 09, 2007, 09:59:13 AM »

Quote from: Mark Scarborough on October 09, 2007, 09:04:59 AM

THANKS LEO

No, I will not leave!

don Burke · « **Reply #19 on:** October 09, 2007, 06:50:20 PM »

I disagree with some of the statements above. A truss type construction CAN be just as stiff in bending as a shear-webbed configuration IF there is no bending in the spars and diagonals when loaded. Otherwise you wouldn't see any truss type stuctures in use anywhere. A truss type construction can be lighter than a shear web design, but usually not in the sizes we use in models.

The failure mode in our wing designs is usually compression failure of the spars due to bending(spars moving together). Shear webs or diagonals help alleviate this problem.

Lastly, a transparent covered wing looks at lot more 'COOL' when it has diagonals instead of shear webs.

phil c · « **Reply #20 on:** October 10, 2007, 01:51:52 PM »

Has anybody taken the time to build a few test spars and load them until they break? That is the only way to settle this kind of a discussion, since most folks can't remember or never got the math to do the calculations. Kind of like the science fair bridge buidling contest they have in school. Build your spar(only need one half) onto a piece of 2x6 lumber. Hang a gallon jug on the end and start filling it slowly with water, meanwhile measuring how far it bends with each half cup or so.

I did this with spars in a combat foam wing. 1/8 x 3/8 spars set flat at the high point are about 1/3 stiffer than the same spars set on edge. 1/4 in deflection vs. 3/8in. with the spars set on edge.

Leo Mehl · « **Reply #21 on:** October 18, 2007, 10:50:47 AM »

I saw this demonstrated By Jerry Holcomb. He made up a bottom and top spar suspended from the end about 12 inches long with wieghts suspended from the middle. He used string in a cross braced pattern. He cut one string and then two strings When he cut the third string in the next bay it collapsed. The up and down movement that we get from root to tip of the wing is what we are trying to stop. As far as strength is concerned we all know in a crash we are going to loose the wing. If we build it so it Doesn't break it would be way to heavy. After all this is a stunt plane and it needs to be reasonable light. The weakest place in any chrash is the center of the wing. I should know, I have crashed more than my share of planes over the years! I have never had a rib collaps on me from the compression of the spars, this just does not happen.

Leo Mehl · « **Reply #22 on:** December 22, 2007, 05:21:27 PM »

My latest wing. Sheer webs in trailing edge and cross braced on main spars. when i sheeted it I can no longer twist the wing. If I wanted it lighter I would have used 3/16" spruce spars. Or even 1/8".

Crist Rigotti · « **Reply #23 on:** December 22, 2007, 06:47:13 PM »

I use a 3/32 x 1/2 balsa top and bottom spar with 1/16 x 1/4 cross braces.

mike mullis · « **Reply #24 on:** December 22, 2007, 08:01:45 PM »

man i gotta tell ya..some of the work i see on here is truely amazing!!!!!!!....wish i had the knack to do someof the stuff yall folks do...high five to ya all!!!!!!!!

Jim Pollock · « **Reply #25 on:** December 27, 2007, 11:36:22 PM »

Hi there,

Checking in for a post here.

The way Crist built his wing is identical to the way Tom Morris builds his wings.

They are very sturdy.
That is also how my Cavalier was constructed and probably the only reason it survived a flameout crash

on
it's fourth flight so it could be rebuilt

. The wing didn't turn to dust

, mostly because the impact was not quite as hard as it could have been, due to it slowing down flying slightly upwind.

Jim Pollock

Brett Buck · « **Reply #26 on:** January 01, 2008, 12:32:34 AM »

Quote from: phil c on October 10, 2007, 01:51:52 PM

Has anybody taken the time to build a few test spars and load them until they break? That is the only way to settle this kind of a discussion, since most folks can't remember or never got the math to do the calculations. Kind of like the science fair bridge buidling contest they have in school. Build your spar(only need one half) onto a piece of 2x6 lumber. Hang a gallon jug on the end and start filling it slowly with water, meanwhile measuring how far it bends with each half cup or so.

I did this with spars in a combat foam wing. 1/8 x 3/8 spars set flat at the high point are about 1/3 stiffer than the same spars set on edge. 1/4 in deflection vs. 3/8in. with the spars set on edge.

Leo Mehl · « **Reply #27 on:** January 01, 2008, 10:43:29 AM »

Quote from: Brett Buck on January 01, 2008, 12:32:34 AM

Sam thing Jerry Holcomb Did> Only on a smaller scale.

Alan Hahn · « **Reply #28 on:** March 01, 2008, 03:56:53 PM »

Just came back from Geneva Switzerland, sitting in the window seat above the wing of a 747 Jumbo. It was fun watching that wing move up and down with turbulence. Maybe a little flexibility isn't a bad idea---especially in windy weather. Of course you don't want the wing to break, but maybe give a little.

Steve Helmick · « **Reply #29 on:** March 02, 2008, 08:01:51 PM »

Quote from: Alan Hahn on March 01, 2008, 03:56:53 PM

Just came back from Geneva Switzerland, sitting in the window seat above the wing of a 747 Jumbo. It was fun watching that wing move up and down with turbulence. Maybe a little flexibility isn't a bad idea---especially in windy weather. Of course you don't want the wing to break, but maybe give a little.

Uh....NO! Stiffer is better, until you hit the ground, at least. We can get away with what they can't on 747's. I was kinda shaken by seeing the engine pods flapping around in turbulance, someplace around Fiji. I knew the wings would flex, but those engine pylons just looked wrong! I wonder what Brett was going to write? I bet it would be good stuff.

Steve

Mark Scarborough · « **Reply #30 on:** March 02, 2008, 09:43:49 PM »

Steve,
with all due respect,, the B-52 was engineered to allow the wing to flex. if they would have desinged it to be rigid, or stiff, it would have been either to heavy or it would have failed from not being able to absorb flight loads. In our case however, the dynamic that I see as a problem comes to the whole precision aspect, if it flexs, you have variable control response. It would also cause some binding of the hinge line as well. Yes rigid is good, however you can easily overengineer a system and create a stress focal point that will fail.

Bill Little · « **Reply #31 on:** March 03, 2008, 07:45:27 AM »

Mark,

I *believe* Steve was making a comparison with his opening statement. *WE* being C/L builders, not full size aircraft.