New fabrication method

[rtfm]

Hi,
I am building a tractor gyro with aluminum airframe and fibreglass fuselage. Bear with me, if you will, as I trace the evolution of a new fabrication method I am currently exploring.

I had initially intended to build the airframe from 2" square aluminum (as is common practice in the gyro world). For a number of reasons, I abandoned this idea, and began thinking in terms of welded 4130 chrome moly tubing (eg: Jukka Tervamaki). Then Skyguynca put me on to using 1" aluminum tubing, riveted together with 2024-T3 gussets. Far easier to build. I thought I had made the final decision.

The idea was to build the airframe, build the fibreglass fuselage, and then marry the two. Not too much of a problem, but it would entail making fittings of some sort.

Then I discovered Jim Marske's web site, (http://www.continuo.com/marske/ARTICLES/Carbon%20rods/carbon.htm) where he extolls the virtues of carbon rods. At first I thought - hey, why not create the triangulated airframe from carbon rods? But on more mature reflection I don't think this is optimal. Here's my latest idea...

I hadn't been thinking laterally enough. It is just so natural to think along the usual lines, travelling again the familiar paths. The idea struck me that I had it arse about face. The first thing to do was to build the fuselage shell, not the airframe. Once this is finished, simply laminate carbon rods directly onto the inside of the fuse. The airframe thus becomes integral with the fuse, and not separate from it. No fittings, no lost internal space taken up by the airframe tubing. The fuse acts as a great big shear web, transferring the loads between the carbon rods. Add some strategically placed bulkheads, some tortional bracing, and you have a terrifically light yet rigid airframe. And it'll be considerably cheaper to make as well, since carbon rods are comparatively cheap compared with 6061-T6 tubing.

What do you think? Would this work?

Since with a mid-mounted tractor gyro almost all the weight and stress is centred on the mast, it would (in addition) make sense to build the mast, engine mount, seats and main gear as a unit, and fit this into the fuse. Easy enough, and it relieves the rest of the fuselage from the major stresses.

Regards,
Duncan

[Jeremy Harris]

You don't really need the CF rods I suspect, as if you build the structure as a straightforward epoxy glass monocoque then I would think that you could size the laminates to ensure that the stresses were all within limits in the skin. If you use a foam core, or better still foam sandwich construction, like yachts, then you'll get the stiffness you need to keep the skins working properly WRT even load distribution (i.e. avoiding buckling and consequent crippling failure). Take a look at some of the foam/glass (Rutan method) aircraft construction techniques for ideas. Here is a link to a chap that's building a light hang glider buggy as an epoxy glass monocoque: http://curedcomposites.netfirms.com/hangbuggy.html Hopefully this might give you some ideas.

One area that will need very careful design and construction is the mast, as the loads are high and concentrated. I'd be inclined to compromise and use a well-proven alloy structure here, bolted to multiple, robust, shear webs with some fastener/bonding failure redundancy.

Mixing glass and CF in the same matrix is frought with challenges, as the CF is much, much stiffer and far less resilient than the glass. The result is that load sharing is difficult to predict and internal bond failure can occur if a "stretchy" glass element is laid up next to a stiff CF element and exposed to a tensile load. Careful design can avoid this pitfall, but it isn't as easy as it looks at first!

Jeremy
SALISBURY
UK

[rtfm]

Jeremy,
Thank you for your suggestion, and for the link to the hang glider pod project. I have bookmarked the web site, since it is exactly what I need.

I will be using 6061-T6 for the mast, engine mount, seat fixtures and main gear attachments. Plenty of good old-fashioned shear webs, AN hardware and over-design.

If I decide on the monocoque design (not sure yet...), I might use C-Fibre instead of fibreglass. Both lighter and more rigid. In the overall context of the entire cost of the aircraft, the extra cost of the C-Fibre is minimal. I have ordered Rutan's manual on fibreglass sandwich construction technique, so this will provide some much needed info. Also, the builders manual for the Quickie series of aircraft is available on the web, and provides some excellent technical info on composite construction.

Thanks for the help.

Regards,
Duncan

[rtfm]

Hi,
Actually, thinking about this... does anyone have the web addresses of builders busy with composite manufacture? I've Googled this, but without too much success. I've also ordered Burt Rutan's manual, as well as a number of other composite how-to's. Real live examples would be great, however.

Cheers,
Duncan