My tractor Dreams

[greywuuf]

I have pretty much finalized my plans for a "pitbull" style tractor, composed mostly of carbon Fiber. I have done a lot of research into the methods and am ready to start constructing a plug to take a mold from. I need to bounce some idea's off of some people familiar with composite construction, dealing mainly with mounting my mast. I am planning on using some struts with heim style ball ends and I need to discuss pro's and cons of several idea's I have. I need to mount the Heim ends to a thick composite "bulkhead" and I would like to discuss methods of doing so ( embedded fasteners, doubler plates, through drilling, through drilling with embedded tubes etc ) I hope to have my Cad drawings finalized soon and would like to be able to post more details in the near future but for now any one willing to just talk about stuff will be cheerfully listened too.

Thanks
Dan

[aerodynamicdon]

I've monkeyed around with foam and fiberglass, mostly making a terrible mess, so I will say with authority that I know nothing about it. You just missed a class from Aircraft Designs, Inc. (Hollman Sportster)-manufacturer on the PRA site, about composite construction. One is planned soon on gyro design. Go dig his site.

[greywuuf]

Thanks, That is an interesting site. Does anyone know how well respected the gyro design info is ? I am considering the class in April. I admit to not having purchased any of the books as I have not heard if the info is good/current/trustworthy. anyone have any opinions?

[rtfm]

Hi,
I have made most of the possible mistakes so far as far as composite construction is concerned, so allow me to ramble on a bit...

First, making the foam plug is non-trivial. I tried seven times before I got it right. Fortunately I had an inexhaustible supply of polystyrene foam with which to experiment. The main problem is getting the sides symmetrical. Unless you have a sure-fire way to do this, be very careful - you could end up wasting a lot of time and/or money. My final solution was to build the mold rather lke boat-builders do - by bending many thin strips of wood over a few known bulkhead shapes. Once reasonably certain that the shape was right, I covered the pine strips with expandafoam, and surformed it to the wooden strips. Worked really well.

Second, getting the glassfibre to stick to the mold is also non-trivial. The only way you can be sure of a good bond (and therefore a good conformity to thew foam plug shape) is to vacuum bag the bugger. Wet out the glass first between two layers of plastic drop cloth, remove the top layer and place the glass wet-side down on the foam plug. Then remove the 2nd plastic sheet. Now, with the glass neatly in place, slip a great big plastic bag (prepared in advance)over the whole thing, and attach a vacuum pump to it. Keep the suction on for 8hrs or so (overnight). Next morning, voila, a perfectly bonded, beautifully formed glassfibre shell.

You can then use this first layer as the basis for your actual mold.

Finally, although you want to make the actual fuselage out of CF, it would be extremely wise to make sure that the inside layer of cloth (ie the layer facing the inside of the cabin) is pure kevlar. CF, if it breaks, produces deadly needles which will puncture your body like rapiers, and cut you to shreds. Kevlar will resist almost all attempts to pierce it, and contain any CF splinters which might seek to turn you into as sieve.

Good luck.

Regards,
Duncan

[greywuuf]

Thanks Duncan,
I am pretty well versed in plug construction, having made a few for snowmobiles over the years. it was easier to build a new hood/cowling than it was to cough up the shipping to have one sent to Alaska.

Vacuum bagging/infusion molding is to be the order of the day. Thank you for the tip about Carbon "Spears" I allready have several area's where kevlar will be used, and I will be sure to include some around the cockpit. I will be using kevlar at all of the "lips" or flanges where a panel will be bolted or Dzus fastened. I was pretty sure the floor area as well, just for abrasion resistance.

My biggest concern right now is fastening a Heim jointed rod end, to a foam cored bulkhead. Any thoughts on this ?

Thanks
Dan

[rtfm]

Hi,
I have the same problem. Generally speaking, the only real way to do this is to substitute something really tough (eg: solid glassfibre piece) for the foam core at those places. For example, my main gear, front gear, wing attach points, and mast attach points all have solid glassfibre inserts in place of the foam core in the rest of the fuse.

Regards,
Duncan

[greywuuf]

Marine plywood ? Can you mix and match core material, like selectively making area's (hard points) out of wood. I would do Aluminum but I hear there can be nightmarish bonding issues with Aluminum.

[rtfm]

Hi,
Yes, plywood's fine. Bonds well with GF/CF. Tough stuff. But remember to use aero-quality plywood to avoid voids. I decided to fabricate solid GF inserts, because they are extremely tough, stable and have no issues related to differential expansion etc. A bit heavy, though.

One of the tricks which was once common was simply to build the core out of foam. Then drill a hole through one surface only. Fix a bent wire into a power drill, insert the wire into the drilled hole and spin her up. The wire would shred a circle of foam within the core, allowing you to pour resin in, forming an internal hard point. Depends on how solid you want the hard point to be. You wouldn't try mounting main gear like this, but it might well be sufficient to mount a door.

Regards,
Duncan

[greywuuf]

Duncan....
how are you making the solid GF inserts? hand layup and then mating it with the foam core to be glassed in ? how thick are your hard points going to be?

I have found building small thick layups to be a Big PITA, as you have no real stiffness to the thing until it sets up some plus you kinda have to be carefull of exotherm when you are going thick. Not a problem if you do multiple lay ups ... but that is as big a pain ( 20 tiny little pieces ) as a big layup and somehow it is not nearly as satisfying as completing another layer on the Fuse, if you know what I mean?

Man, there are some ballistic panels on this helicopter out here that I would LOVE to snag for just this kind of thing ;-)

[rtfm]

Hi,
I'm working through this issue at the moment. So far, the results have been mixed. My design calls for a central load-bearing frame, to which is bonded a light fuse shell. All attachments (ie rotor, tail, legs, wings) are fastened to this load-bearing frame with cheek plates. My problem is that the central airframe is 2" thick. Which means that my hard-points need to be 2" thick, and that's my main issue. I'm now also investigating Airex C70:200 - a 200kg/cu metre density rigid structural foam. This stuff is nearly as hard as solid wood...

I'll be experimenting over the next few weeks, so I'll let you know how I get on.

I've based the structural design on a cute little Russian tractor design. I think the simplicity of the internal airframe is genius. I've also included a drawing of my design, which you can see uses the same basic principles.

Regards,
Duncan

[Cobra Doc]

Marine plywood ? Can you mix and match core material, like selectively making area's (hard points) out of wood. I would do Aluminum but I hear there can be nightmarish bonding issues with Aluminum.

The neat thing about composites is they are just that. Use whatever you need to use. You can't attach anything structural to foam-core. You can attach through it with metal sleeves and large surface plates on both sides of the contact point. Even if I was molding in a fiberglass block I would still use aluminum or steel plates on both sides just to spread out the forces. Same as working with aluminum or kevlar honey-comb structures. Plan ahead and distribute the force over a large area.

[greywuuf]

Thanks Doc,
The more I think about it, the more i think I am going to go with a doubler, only question now is how to rivet a sheet of aluminum on each side of a 1/2" foam core bulkhead?