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Permalink Reply by Troy Reabe on April 27, 2010 at 11:42pm

First; great idea and I like the idea of printing out a wing The few things that I see as a problem I know the the FEA in solid works is giving you data that says that this should work but what base plastic are you using and what happens when you cure your wing in the oven under vacuum? The next area that I have had problems with myself with foam wings it to do a good job of covering the wing and not having marks from bagging on the surface of the wing. If you are using a straight cross section, or just a tappered wing you can use a mylar sheet with holes punched in it and with it streached out right you will get the surface you want. You can try different thicknesses to see what works best for you.
Also did you look at a 4 axis hot wire? you would be able to cut most wing shapes untill you start to go eliptical or something like that. I know at Madison we were able to have a few test cores cut for us and the place that did it normaly cut foam for chairs.

Small minor note looking at your model you would like to bond you spar to your skin. this will add some weard forces to your spar but if you are buying your spar it will most likely be stronger than your skin and haveing a good way to directly transfer the force from the skin to the spar would help. Or you could glue your spar in but that costs weight.

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Permalink Reply by Gerry Lichter on May 28, 2010 at 5:14pm

I think it would be cool if you could print it between centers on a lathe. CNC convert you lathe, add an extrude to the tool post, and belt drive the lathe dog.

Maybe a spool gun for aluminum welding could be modified to lay individual carbon fibers?

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Permalink Reply by Nick on June 5, 2010 at 11:27am

Just a thought: it looks as though your design is enclosed in a skin. Depending on the printing process you use, that could be a problem. If you use SLS or SLA, you're going to have powder/resin stuck in the cells when the product is finished. If you use fused deposition modeling, you won't have that issue. As well, if you print these off at sea level and fly them at higher altitudes, it is possible that the pressure inside the cells coupled with the small honeycomb wall thickness could drastically reduce its structural strength and make it brittle, especially if the wing is exposed to heavy temperature changes. If the cells are able to vent into each other and the interior is printed without the skin, these problems can be eliminated, though you will run into the issue with sagging that was mentioned before.

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Permalink Reply by Oliver on June 6, 2010 at 9:12am

Hi Nick,

Thanks for your interest. You would be right to worry about trapped volumes as the the pressure changes at altitude.....Im not sure how the vacuum bagged foam cored wings vent, but I imagine there is not a complete seal between the skin and the core allowing the trapped air to escape. I am currently working on a way of making the geometry build without any support material on the RP machine so there is no trapped material, the other issue I have come up against is the deflection of the material when it is put under vacuum for processing the carbon fibre skin. Making an FEA model of the wing shows a maximum deflection of 0.3mm at 1 atmosphere. I think I may have come up with an "out of the box" solution to overcome this. If I was making an autosport part or a bicycle frame I dont think these problems would be such an issue, I would just increase the thickness of various parts until the deflection under vacuum disappeared, but because I am trying to get to a target weight similar to a foam core these problems are harder to overcome.

Regards


Oliver

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Permalink Reply by Sam Steeb on June 27, 2012 at 6:39pm

For the honeycomb areas, it would be possible to use laser cut parts to come to very near the same design. Then you could choose from sheet foams like depron, styrene sheet, woods, balsa, lite ply... Could keep costs down and allow enough weight savings to allow other design changes, like sheeting under the skin.

It was mentioned above as well, what material does your RP use? Can it handle the curing temps and pressures?

Have you done any FEA with the core 45° off current?

When using prepreg is a high vaccum needed during cure?(I'm not a composites guy)

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Permalink Reply by Andrew Rabbitt on July 1, 2012 at 5:53am

It looks like a nice idea until you analyse it closely. The honeycomb adds a lot of weight and stiffness for construction loads, but little benefit, in my opinion, for flying loads.

I have spent some time analysing construction methods for the wings of my HDwing glider concepts and so far nothing can beat a fully balsa skinned, balsa structure wing for weight and stiffness, unless you're trying for some impossibly ultra thin aerofoil. Even a CF skin on its own made unmanufacturably thin is still heavier than a fully structured built-up balsa-skinned wing in my application.

Larger structures may close the gap, but I'll bet balsa will always take some beatng

Adding die-pultruded wet carbon filament spar caps can add a a lot of CF-style stiffness for very low weight (and cost)

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Permalink Reply by Oliver on July 2, 2012 at 1:53am

Hi Andrew,

Thanks for your reply,

you are right about what you say, the honeycomb structure is no good for wing designs as I have shown it...You are also right that its hard to beat Balsa as a lightweight material......but thats not quite what I was trying to do....I wanted to design a small UAV <10kg you can print off, something that needs no modelmaking skills and something that is strong enough to be catapault launched or crashed a few times without falling apart...I am looking at combining printed parts with CF coated printed parts....I have some single skinned CF bits that are simple to make but really strong and tough...Lets not forget that the plated RP parts I have made are over 10 x stiffer than the plastic substrate...surely they could be used for spars etc?

 

Regards

 

 

 

 

Oliver

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Permalink Reply by Andrew Rabbitt on July 3, 2012 at 3:01am

I see what you are trying to do Oliver and it is indeed pause for thought, but going for broke with printing the entire thing, I am struggling to see how you can avoid winding up with an airframe with next-to-no payload.

On the other hand, a series of printed ribs with more three-dimensional detail than can be achieved by simple laser-cut balsa might be a good compromise solution, especially if you can clip/slide them on to a couple of (for example) laser-cut balsa or tubular CF spars.  This structure could then be relatively easily skinned in  balsa or EPP/EPO sheeting.

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Permalink Reply by Oliver on July 5, 2012 at 2:21am

Hi Andrew,

 

You are right......as with everything the design is a compromise....if you print parts from the standard ABS material it's hard to make them stiff and strong enough....

This link shows somebody doing what I have wanted to do for ages but not got round to doing...make an entire airframe...http://www.stratasys.com/Resources/Case-Studies/Aerospace-FDM-Techn...

As you can see they had a few issues with strength, my idea is to use the printed parts and then selectively  metalise them to add stiffness and strength The key is to optimise the design with FEA to understand the loads and only add material where it's needed ...the wings are much more of a challenge....I think the best option is to print a thin skin hollow aerofoil and then fill it with pourable foam, from the work I have done already I think a single skin of CF is all you need.

This new approach to construction also works better when you tailor the airframe design to exploit the strengths of the process. Blended body/wing designs that allow maximum payload space work well with this process, high wing monoplanes perhaps less so.....

 I think this approach will give you a durable airframe that will survive multiple hard landings something balsa and foam UAV's just cannot do (in my experience...!)

 

Print, Build, Fly the future of UAV Construction......

 

 

Ollie

 

 

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Permalink Reply by Andrew Rabbitt on July 7, 2012 at 2:41pm

If you're talking about pourable foam mouldings, consider using the printed part as a disposable mould - print from PLA or some form of disolvable material.  Pour the foam interior and then strip off the printed material - a la shell moulded casting.

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