materials selection

[nhoj62]

hi there
I have built a contra props gearbox and i used 3mm ply for the main bulkheads.
And i was wondering is there a better material i could use for this application aluminium for instance?, or a composite of some sort?.
cheers for any help.
nhoj62.

[davidfee]

I should think that aluminum would work great for this application. It should also be a little more durable than the plywood in the event of an un-scheduled application of terra-braking. It will bend rather than break. Carbon plate would work well also... and it would look really cool. :cool:

-David

[nhoj62]

hi david
thanks for the reply, aluminium i can get but,...

carbon plate???:confused:

where does one get somthing like that, or do u have to make it?

later
nhoj62

[davidfee]

You can buy carbon plate from a place like this: http://www.cstsales.com/carbon-plate.htm

...or you can make it yourself. Either way, carbon is pretty expensive. Realistically speaking, I'd say aluminum is the best way to go... if you ever want to re-do your gearbox.

[cwatters]

I made my own by laminating up thin ply with carbon cloth both sides. Pressed it between two sheets of perspex to get flat surface. End result here...


http://users.pandora.be/colin.watters/images/nieuport/belt_drive.jpg

[Viper Pilot]

cwatters,

Very fine job. You should go into business making custom plates.
I have a couple of planes I'm working on right now taht could use your handiwork.
Thanks for the tip!! I'm gunna give it a try tonight.
Did you use the type of CF that irons on??

VP

[cwatters]

Not the stuff you iron on. I think that is only unidirectional? I used some scraps of woven carbon cloth. Can't recall the weight but you probably need enough layers to make it 2.5 mm thick.

Here is some stuff I wrote for someone else who asked about the belt drive...

I forgot to mention that you need to WAX the old bearing before winding tows around it (see below) or you won't get it out.

Regards,

Colin

--------------------------------------------------------------------------

The pulleys, belts and bearings were from Radio Spares (an electronics distributor that also does industrial/mechanical items) See..

http://rswww.com/ and hit the 'mechanical' link.

I think Farnell also stock similar parts...

http://www.farnell.com/uk/index.htm

The main problem with both of these is that they normally only sell to businesses.

The pulleys are supplied with just a pilot hole in the middle and are quite heavy in the larger sizes. They need to be drilled out, lightened and balanced. Drilling out the center hole needs a good quality pillar drill or the tension in the belt will change as the pulley revolves not to mention the vibration this will cause.

The main shaft is a 10mm carbon rod.

The bearing support plates were made by laminating up carbon cloth between two sheets of Perspex. Once set they were cut to size and an old bearing was temporarily fixed to the plate using four bolts spaced around the circumference. This does not need to be done very accurately because carbon tows are then wound around the bearing and screws to form a cavity molded
around the bearing itself. Once set remove the screws and push the old bearing out. Clean up the area and fit nice clean new bearing.

The bearings support plates are separated from each other by carbon rod spacers braced by more carbon tows in a sort of cats cradle arrangement. The main shaft itself was used to keep everything aligned while the resin sets. Very messy to do but seems to work well in the end.

I cheated with the prop driver - I used a part from an old Kruse belt drive fixed to the carbon shaft.

http://www.kruse-modellbau.de/index.html

It's quite tricky working out how long the belt needs to be and how far apart the pulley's can/will be for a given length of belt (whole numbers of teeth). So I was very glad to find this web page!...

http://www.sdp-si.com/Cd/default.htm

Overall it's only worth building your own for the challenge!

[nhoj62]

hi colin
very interesting technique u got there!.
exactly how many bits of carbon an ply did u put into your plates?
was it just one piece of ply with carbon either side or more?
cheers
later
nhoj62

[cwatters]

> exactly how many bits of carbon an ply did u put into your
> plates? was it just one piece of ply with carbon either side or
> more?

Not sure the answer helps... the ply was about 1.2mm and the cloth somewhere between 150 and 300g per meter. I used two layers both sides. Best to make up a small test piece first and see how stiff it feels.

The bearing mounting trick worked very well. The cats cradle bracing also works but I would probably do differently next time as it's so messy.

[cwatters]

> exactly how many bits of carbon an ply did u put into your
> plates? was it just one piece of ply with carbon either side or
> more?

Not sure the answer helps... the ply was about 1.2mm and the cloth somewhere between 150 and 300g per meter. I used two layers both sides. Best to make up a small test piece first and see how stiff it feels.

[nhoj62]

hi colin thanks for the answer
it is very help full
now i will have to get onto fibretechuk an see wot they got that will do the job, and get some resin.
later john.