I am very new to this whole world, but I have recently become involved with an airplane construction group at the university i attend. We have run into a small problem though. the design we have come up with requires a counter-rotating prop with a 17 inch diameter and so far, we have not been able to find any suppliers who carry a prop like this. do any of you know where one would find such a prop? Thanks in advance for any advice you can give.

No one makes a counter rotating prop setup that I know of. I have been playing around with the idea but you are going to add a lot of weight to a model with the gearbox. You will need a right and left hand prop along with a gear box to drive the props and a motor with enough power to drive two props of the size you want. The gear box would be similar to what is drawn with a shaft right through the the box attached to the gear on input side and gear on output side drives counter rotating prop. The yellow in the drawing is bearing material and the pink is the outer case. Built one in the 80's but would only spin 10" props with a 90 size engine for power. Modern engines are more powerful but still not realy up to spec.

Cheers.

Are you talking about dual props per shaft, counter rotating, or are you talking about a twin engine, with counter rotating props?

Look or ask in the rc helicopters forum...alot of helos use them if you are talking about inline...

Or are you just talking about a pusher prop, thats is turned the other way to be on the rear?

Or are you talking 2 seperate props with one drive, im confused

Dave Platt had an airplane on the cover MA years ago with counter rotating props. Might try asking him.

As I recall, that was a model Fairy Gannet, with two engines geared to drive contra-rotating props. I haven't heard of anything like it since then.

I guess i really didnt specify the plan well enough earlier. The basic idea is to make a pusher/puller plane. It will be electrically powered, so we just have to wire one of the motors with reverse polarity (or just turn it around so it is facing the opposite direction). We wanted the reverse rotation prop so that we could use it as a pusher in addition to the motor in front acting as a puller. This way, we would have double the thrust, with no resultant torque on the airplane.

Aha!! That's a tractor-pusher, not contra-rotating props. There are few technical probs with tractor-pushers.

what you are describing is simple compared to what some of the readers are describing. You just need to buy a reverse pitch prop, also called a "pusher" prop. these are made in various sizes. some of the previous comments thought you wanted two props running off of the same engine on the same axis. I'd search the popular web sites for prop manufacturers:
heres one: www.apcprop.com

they show a pusher prop in 14 and 20 inch diameters. you can cut down a 20 inch prop to smaller diameter if you have to.