Super strong Built up wing???

[soholingo]

Gentlemen,

I would like to build a built up wing that can withstand the same forces as a molded wing. is this possible?

I am going to sheet the first half of the wing to the main spar, which is wooden.

How would I further strengthen it? I would like to add a CF spar but I have no idea of how to do this...

Also any commercial hotliners with a built up wing? What techniques were used?

Lastly it will be a TWO piece wing....

Thanks,

j

[tim hooper]

J,

I've recently built a little S400 Minibat. I modified the wing to accept a couple of cf rods (spaced by balsa webbing) as a spar.

I can't believe it's as strong as a fully moulded job, but for the pennies it cost, it really is very rigid indeed.

tim

[tim hooper]

As you can see, the spar lies flush with the 1/16" wing sheeting.

Any help?

tim

[soholingo]

Thanks Tim. The real challenge is the fact that it will be a two piece wing. How would I go about integrating the CF spar AND two pieces....

And is it possible to make a built up wing strong enough to with stand high speed dives?

thanks...

j

[vintage1]

Can't answer on specifics, but there is no reason why a built up wing should not be as strong as anything else. Might be heavier tho, if you build it up out of titanium :)

Look, I have NO practical excperinec, but I do have a degree in engineering, and we did structural up to the second year, before I dived off into electrons.

A wing - structurally - is a cantilever type structure. It needs to be light, stiff, and strong. Light for obvious reasons, stiff because we don't want flutter, although some bending under High G is acceptable, and strong so it don't snap in 15g corners :)

Light means take out all unstressed material. If it ain;t stressed, its not adding anything except weight!

Stiff neans you need to make the classic I beam type structure - that is, in teh case of a wing, you need strong spars top and bottom and webs in between that take the compressive loads that would otherwise cause it to buckle. A foam and skin wing is simp0ly one where the tops of the I beam become the whole wing skin and the foam stops it all buckling - sometimes.

The very best structure I have ever seen for stiffness and strength to weight is honeycomb structure - where you build up a hexagonal cell structure of stiff ribs, and sheet it with a very strong tensile skin.

I would suspect that this is the way to go, and with a fully built up wing, you can vary the spacing to make it less strong towards the wingtips, where the bending moments are far less, and save weight.

So, here is a purely theoretical and I-never-tried-it-but-it-ought-to-work idea.

First of all, decide what material to use to skin it. It may be only film, in wich case you will need to add stiffness with some sheet or spars.

Then make up a structure with straight ribs, and ribs running at 60 degrees to it, to form the honeycomb. Use the grain in the straight ribs running horizontall, to get good strength in the fore and aft plane, but cut the cross ribs out of vertical grain wood. This will give very good resistance to buckling. You will end up with a structure that is very stiff in all planes except warping, but that is what teh stressed surface will fix.

You may then decide to fully sheet it, in which case my gut feeling is that no spars are actually necessary - just like a foam wing. If you must put a CF spar in, i'd make it the LE itself, and the TE because the thinnest part of the section is where he structure tends to bend more.

If you want to be really classy, you could sheet the center section with something thicker and stronger, or glass it. You might only partially sheet the outer section and use the covering film itself to provide adequate strength there..

What you will have discovered tho, is just how much work it is gaining an ounce on a glassed up foam wing :)

However, all this stuff is what is used in aerospace and formula 1 car design - where light stiff and strong are the magic words in structural engineering. Eggshells are natures answer to a similat problem. What you need is wing shaped eggshell with internal bracing to stop it buckling...

There is nothing new in structural engineering except new materials and fabrication techniques that allow even more fancy ways to engineer structures that fail uniformly at the rated loads!

Honey comb is THE way to go, BUT its a pig to build! and it makes installation of internal kit a right royal pian in the butt, so its not used where the cost is prohibitive. I came across it in the context of some cutting edge design work being undertaken by a company that was using RF power to set the glue to bond the skins. I picked up a sheet of composite - looked like paper epoxy comb, with CF epoxy skin - that was about 1/2" thick, and weighed about as much as solid light balsa, and I could not bend it let alone snap it.

I did a quick google search on 'honeycomb stressed skin structure' and came up with lots of stuiff that may be really useful in this context.

Take a look if you are really serious..

[soholingo]

vintage....

A LOT of good stuff in that post. I will have to read several more times to assimilate everything...

However, my thought was this....

Sheet the leading half of the wing. Place a secondary removeable spar (or two) made of CF into the wing. Then glass the trailing half of the wing. Lastly cover everything in oracover light...

Would this suffice?

j

[Sparky Paul]

This is the wing I'm currently building for the Baker marathon..
It will be light, stiff and strong.
I've never had one like this fail.
The requirement for two pieces adds a little complication at the root.
A steel rod in brass tubing, a maple rectangular joiner.. fitting between the spars with ply webs on each side of the spar is normal, with an anti-rotation fitting about 75% chord to keep the thing aligned.
For a wing that will outlast a Buick, look at Mark Drela's wing construction method at the Charles River site...
http://www.charlesriverrc.org/articles/allegrolite2m/markdrela_allegrolite2m.htm

[vintage1]

That looks pretty close to a honeycomb already Sparky!

I lthink that is about as good as it gets for a simple structure.

I have a bit of a concern for a wing with *just* film covering at high speeds...I can envisage turbulence ripping the covering off...

I wish I could be more help soholingo. As I said, all I was trying to do was point you in a direction of a possible technique.

I'll do a web search and see what stuff I can come up with.

Hmm. Not much more really.

This is a high speed glider right? 170mph+?

So its a very thin wing section.

I can see why they glass em up already.

You want maxium tensile strength in the skins at minimum weight - that's carbon fibre or glass or kevlar with epoxy to stiffen it a bit. Maybe very thin plywood would do nearly as well. 1/32" or so perhaps.

Then a compression structure between the skins to stop it folding. Foam isn't ideal there really. Bit too compressible.

I'd still go for lots of vertical grained ribs at different angles to create a hexagonal cell structure. Sparky's design tends towards that with his ribs and spar webs all capped. In addition you could use thicker wood around the center section, and maybe a layer of glass there as well. Or double up the ply thickness over the center third of the span with another layer of ply.

So, the essence of the design is roughly this.

(i) No open areas of covering - could be unsafe at high speed
(ii) high tensile strength skin - best would be carbon fibre composite, second best glass fiber, then probably plywood. Make it a bit thicker in the wing center.
(iii) a way of stopping it all buckling. Best would be a full honeycomb of ribs with vertical grain, graded to be closer and/or thicker near the wing root. Second best is high density foam with holes punched in to save weight, third best is low density foam, and probably a vertical grain spar or two to increase compression strength around the center section.

Or you could use HD foam in the center section and LD foam on the outer panels.

How are these wings made normally? glass over foam? seems about right really :(

[BMatthews]

First off you're on the right track but I'd extend that upper surface sheeting back to about the 50 or 60% chord mark. This is more for aerodynamic reasons than strenght but the torsional stiffness would be helped a little too. You can add carbon to wings like this. First, use spruce for the spars, at least for center panels, and cap them on the lower surface of the lower spar and the upper of the upper with .007 x 1/2 carbon premolded strips glued on with thin laminating epoxy. Use two layers for the first third of each panel and one for full span.

Another hint is to bring the spars out to the very outside surface of the wing. We often bury the spars under the sheeting and this is fine for looking nice but seriously degrades the load carrying of the spar system. The lower surface will be fine and the upper can be coated with a thin layer of balsa or lightweight wall filler and sanded to match the airfoil profile. That extra 1/8 inch offers BIG returns to keeping the wing strong. On a 1 inch wing that's 16% increase on the load carrying (.125/.875= .16).

Vintage, you have lots of great ideas there. Just a couple of things though. Your egg crate wing with full sheeting would still be prone to collapse just because the balsa just doesn't cut it for compressive or tesile strength. But if you added a carboned spar like above then you'd have a wing that would take a helluva stress and not flutter thanks to the geodesic rib layout and full skins. With proper light wood selection I suspect you could make such a wing to be competitive in weight to a bagged sloper wing but it would be slightly heavier than a bagged thermal wing but would stand the same or more punishment.

A buddy of mine built a 100 inch electric sailplane a few years back with full skins and regular rib layout. It wasn't that much heavier than the open frame, only 4 extra sheets of 1/16. But it flew great.

The big drawback to built up like this is that it's harder to faithfully replicate some of the zoomy airfoils used these days. The foam core vacumn bagged design allows for smoother and more accurate airfoils provided the foam is cut well using accurate templates.

So what's this Uber Wing going to be mounted on?

[vintage1]

Totally agree with you - if you look carefully I reckoned that plywood was a better bet than balsa in this app for the skins.

In terms of crushability of the ribs - its a straight exercise in crushability versus weight. I haven't got my book on wood properties any more, so I cxan't say whether e.g. thing ply ribs are better than thick balsa ribs etc.etc.

Also 100% take your point on wing sections and accuracy. My impression of hotliners - is that weight is not as critical as it is on some other model types.
So glass skins would in the end seem to be the correct approach :)

[Sparky Paul]

I have a CR "Renegade" sloper 60 inch wing made with CF skins over foam, vacuum bagged. It's probably the strongest wing I have. It's also seriously heavy!
Does fly well though, especially at speed.
The dynamic soaring guys are always talking about exploding wings, maybe they've got some skinny on skinning for strength.
This is an 18 foot CF foam and glass wing the Embry-Riddle Daytona guys brought to Aero Design West 2002... it crashed on its only flight.. It survived a static test load after the event.. But another 200 pounds did it in.

[planeman]

You guys are doing it all wrong!

For a really strong wing you first of all need to build a wing-shaped male mold and then pull a female mold from it. You then take some 1/2" steel rebar and locate it lengthwise along the span and within 1/8" from the surface if the wing. Eight or ten rebars should do. Using ready-mix concrete from your local supplier, pour it into the mould and wait about six days.

I haven't had one break yet. However, I haven't flown one yet.

:D

Planeman

[BMatthews]

Originally posted by planeman
You guys are doing it all wrong!

For a really strong wing you first of all need to build a wing-shaped male mold and then pull a female mold from it. You then take some 1/2" steel rebar and locate it lengthwise along the span and within 1/8" from the surface if the wing. Eight or ten rebars should do. Using ready-mix concrete from your local supplier, pour it into the mould and wait about six days.

I haven't had one break yet. However, I haven't flown one yet.

:D

Planeman

LOL. Test glide it underwater... :D

Vintage, I missed the bit about the plywood. That would work for sure. Good point too about options. We are all stuck on balsa but if veneer or 1/64 ply is good enough enough for sheeting foam wings then it should be good enough for sheeting built ups also. Not to mention using fiberglass layed up on a glass plate and then formed over the built up structure while still "green" and flexible. These thinner options would let us use finer designed airfoils as well. But I'd want to assemble them in a cut foam bed to support the lower skin for accuracy.

[stumax]

Take a look at a modern F1B and F1C wing (that's free flight rubber and power). They both have very high aspect ratios (F1C approaching 20:1!) and very stiff wings. F1B's use a carbon moulded D-box LE back to 25-30% with carbon spar caps/shear web then balsa ribs capped with carbon strips, and a carbon TE, with the open section covered in some exotic stuff called tissue. The F1C guys vacuum stick (with epoxy) high tensile 1 thou alum foil (not cookin foil!) to the outside of their wing skins then wrap it around the wing structure. I did this on an F5D model, only on the inside of a moulded wing and it really adds torsional stiffness. Their wing structure usually consists of balsa ribs (sometimes capped in carbon) carbon spar caps with a carbon shear web with fibres running at 45 degrees (you can do this with ply @ 45 degrees too). With the wing being in 2 pieces you'll need to have a long joiner of preferrably rectangular cross section with the joiner tubes longer still, and supported by ply or carbon reinforced ribs - check what the F3B and F3J guys use. The joiner should be inline with the spar otherwise nasty stress concentrations will arise. If it was me, I'd just cut a blue foam core and glue a pre laminated (1 layer of 2 oz + 1 layer of 6oz glass layed up on a waxed sheet of window glass) glass skin to the offcuts (sandwich the foam between 2 bits of 1" MDF before cutting) to make a quick mould and just mould the bloody thing. Don't need vacuum bags or any fancy stuff, just use 1/2" foam rubber around the "wing" part of the foam core (wrapped in plastic) and squash it between the mould havles with a few bricks. Do this both to glue the mould skins on and when laying up the wing skins. Use 1.5 oz glass (@45 deg to the span) either side of 1/16" balsa for the skins, make the spar as above with the carbons caps etc, glue the spar to the upper skin (leave the skins in the mould) then to the lower skin (with thickened epoxy) with spacers between the MDF to maintain thickness, then release it from the mould the next day and glue a spruce LE on the front and shape it nice. Instant moulded wing with no hassles and no ribs to cut! I "invented" this method for quick prototyping wings for F3D pylon over ten years ago and it was so good that we never bothered to make proper moulds until recently. Hope this helps! Good luck, Stu.

[Joel K. Scholz]

It's easy to build a strong lightweight wing using carbon fiber tubes. I use one in the leadfing edge and one at more or less the cg. I prefer skyshark wrapped graphite. a 32.5 inch tube weights around 11 grams and balsa glues to it like you wouldn't believe. If you need a long wing or two piece one, simply ferrule the rods via an internal carbon ferrule. Want to buy some, go to skyshark.com. Here is a picture of a 2 piece wing. you can see the carbon on the wingtip..They are almost indestructable. I have a pile of wings left over from long demolished fuselages.

[vintage1]

I have found this all fascinating, especially because I am trying to combine theory - as far as I can remember it - with practical problems of building light strong and stiff wings. However speed is not *my* aim - mostly light and slow...

I do think tho, that some of the presented solutions would be 'interesting' at hotliner speeds.

Has anybody ever used an open structure on a hotliner?

[Sparky Paul]

It probably wouldn't work as well...in the fuselage. Composites let the skin take the stress. Built-ups intrude in the internal area so the volume available for the equipment is less in an equivalent airplane.
As there'd probably be a reason to glass part of the fuselage, it's more sensible to make the whole thing glass/CF...
The only CF on this latest version of the endurance plane is at the wingtips, to stiffen the sheet structure..

[vintage1]

Is that your 'mule'?

[Rudeboy]

A very important aspect in building wings is experience...
Don't be afraid to experiment.
There's no better way to test your structures then taking them to their braking point... Now I know it isn't fun to watch all those hours of work being destroyed, but it does give you insight in what some structures can or can't take.
If you experiment (and fail...) enough you get a "feeling" on the matter...
I've done my share of experimenting and you'd be surprised of the loads exotic (read complicated, difficult and time-robbing to build...) balsa structures can take.
And yes, eventually you end up with some sort of honeycomb structure...

[Sparky Paul]

Originally posted by vintage1
Is that your 'mule'?
Vint, the mule is the pusher.. I'm using it to see what performance battery/motor/props deliver, and how that performance agrees with the calc programs.
So far the results aren't all that bad, maybe 10-15 percent difference between the numbers and the hands-on.
Here's what the interim final configuration looks like, less motor.
Weighs 58 oz right now, empty, with 2 servos installed
Can't decide on a Hacker or a Mega.
Anticipated AUW is 8 to 8-1/2 pounds. Wingloading will be near 16 for reasonable performance, although motocalc predicts only a 32 mph cruise.

[vintage1]

Looks good. I'd use motocalc to work out what best efficiency is with teh motor propped up and geared down.

hacker are obviously more efficient at full power, but who knows what works best at low throttle settings? Might be that the controllers with less advance are better in that area.

I'd definitely out some dayglo panels on that for visibility :-)

[Sparky Paul]

Vint, flew it today. Couldn't wait for me to make up my mind as to the "real" motor, so I installed a Turbo 700 9.6v, Graupner 12.5x6.5 prop direct (only thing I have that will fit the shaft on the motor), 10;x3000 Nimh. 5 lb 15 oz AUW.
Takeoff was short and didn't need any corrections,but I did a couple just to check out the 4->1 mixing, which worked.
Easy flier.
Good rate of climb.
Easily seen at distance.
There's yellow dayglow panels on the top.
Seems to have a good speed range, flew well with reduced power, but tucks with speed build up.
I'll be moving the c.g. forward.. currently at 33%.
Almost tempted to loop it.. but that's not in the flight plan. :D

[Troy]

Sparky,
I'm glad to see you are entering the Baker Marathon. I have done the gas portion three times. My best run was third place for 180+ miles. I actually only did it on 1/4 gallon of gas because we had trouble getting off the ground after my teammate busted our special wheels the day before. If we had been able to get off with all of the fuel, I would have held the record for a few years. I was thinking of doing it again this year. I used an OS .20 Surpass 4-stroke, a 2 meter RG15 "glider wing" mated to a combo F5B/flying gas tank fuselage. People thought we were nuts but we kicked the non-believer's butts. Next time I am going gas/ignition and a slightly larger motor for reliability. I can't bring myself to do diesel because they are so hard to keep running with the altitude changes on the course. As it was, we had an onboard fuel mixture adjustment. Your planes look really nice. I hope to make it out there this year and root you on!!
-Troy Peterson

[Sparky Paul]

I have an OS 20 also. I've flown it in the '97 plane.
To get the whole experience, I think I'd enter it in the Sportsman class on Sunday. (The after-event raffle is one of the world's best!)
I'm wondering if some of the "electric only" props might be happy on a low-revving 4-stroke.

[vintage1]

Originally posted by Sparky Paul
Vint, flew it today. Couldn't wait for me to make up my mind as to the "real" motor, so I installed a Turbo 700 9.6v, Graupner 12.5x6.5 prop direct (only thing I have that will fit the shaft on the motor), 10;x3000 Nimh. 5 lb 15 oz AUW.
Takeoff was short and didn't need any corrections,but I did a couple just to check out the 4->1 mixing, which worked.
Easy flier.
Good rate of climb.
Easily seen at distance.
There's yellow dayglow panels on the top.
Seems to have a good speed range, flew well with reduced power, but tucks with speed build up.
I'll be moving the c.g. forward.. currently at 33%.
Almost tempted to loop it.. but that's not in the flight plan. :D

It looks gorgeous. Flying? Hah. 20 knots below freezing here - Yuk.

I guess you will be tuning up the props and boxes soon.

I am well curious to see how really big props work out.

[vintage1]

Originally posted by Sparky Paul

I'm wondering if some of the "electric only" props might be happy on a low-revving 4-stroke.

I would say not. For two reasons - you need weight for the flywheel effect, and the torque comes in sharp bumps, so you need more strength. Wood CF or titanium wold seem to be the way to go.

Titanium for sheer one upmanship :)

There's got to be someone on the west coast who can sneak a billet onto a CNC mill over a weekend L:)

[BMatthews]

Sparky and Vintage, a few thoughts about the props.

First off if the prop used was a larger than normal choice for the engine so that the rpms are held down to well within the range of the prop I can't see any problem.

As for the second bit about shock impulses and flywheel effects a heavy spinner and backplate would take care of both these problems. It'll have to be a full sized spinner with a heavy backplate I think. Something like a 2 inch spinner with a 1/4 aluminium backplate to act as a flywheel. The flywheel would help the engine run smoother and would absorb a lot of the power impulse shocks.

And it's not like we are talking about using them on a daily basis around lots of other unsuspecting people in crowded pit areas. With some consideration for the safety aspects there's no reason why you couldn't go for this option in this special case.

Sparky, I say do the homework and go for it. Start and takeoff using lots of safety room and knowledgable helpers that keep their body parts out of the danger zone and/or wear a face shield until it's well tested and even then until after takeoff. And use an electric starter to avoid undue stress on the lighter blades.

You'll obviously want to do lots of ground testing first but I think if it holds up you've got some real possibilities as long as you don't exceed the prop's max rpm range. You'd be starting at idle and then stepping back behind the engine before "flooring it" anyway so I think you can get away with at least testing some of these without being Guillotined.

If you go for the APC electric series props you have a unique way of inspecting for stress damage. The resin they use seems to show up stress overloads as white'ish cobweb like areas where damaged. I've seen this on their regualr props after they get dorked one too many times.

I've been using some highly thinned wood props on 1/2A Texaco models for years without problems. These are thinned Zingers with the airfoil cut down to less than 1/16 thick at the blades mid point. But being 8 x 4 and 8 x 5 they engine can't turn them at more than 6 to7 thousand rpm. By strictly legal considerations I shouldn't be doing this either but with a little forethought it's no more dangerous than many other practices we do in this hobby.

[Sparky Paul]

Originally posted by vintage1
It looks gorgeous. Flying? Hah. 20 knots below freezing here - Yuk.

I guess you will be tuning up the props and boxes soon.

I am well curious to see how really big props work out.
.
It's low 40's here. Almost cold enough to wear long pants and a second shirt! :D
.
Apparently the Turbo 700 failed the amp test.
12.5x6.5 was way too much prop for direct.
I let a lot of smoke out this morning trying to takeoff with a fwd c.g. and some downthrust. :mad:
.
Next sacrificial motor will be an ASTRO 15G.

[Sparky Paul]

On tbe OS 26 powered model I used one of those distinctly odd-shaped Top Flite 10x4 prop. Looks like a poorly whittled prop, but the engine liked it.
Wood Zingers or composite Masters are good candidates.

[stumax]

It's low 40's here. Almost cold enough to wear long pants and a second shirt!

It's low 40's here too! Centigrade, that is! :eek:

[soholingo]

WOW!!!

Great discusion about the wings. I am new to building so I think the best (read easiest) route is to do the following


[list=1]
Make the LE a CF rod
Mke the shear webs IBeam for the entire length of the wing.
Add that second spar that Sparky uses (for honeycomb effect)
Sheet the Dbox area of the wing with some sort of thin balsa
[/list=1]

This is going to add some weight but the rest of the plane will be stock. The plane??? Skimmer 400

j

[soholingo]

Ok.... changed my mind again...

I think this set up will offer the best design as far as strength and low weight.
[list=1]
CF rod for LE
cover the wooden sparrs with CF...
I-Beam shear webs
[/list=1]
I like this... Simple and easy.... I wasn't looking forward to sheeting the wing...

j