[Canberrauav] catapult design

Jack Pittar jpittar at bigpond.net.au
Thu Nov 8 23:14:03 EST 2012

Good ideas from Chris and Matt. This catapult is taking on a life of its own
I spent too much time thinking about it today. I can only theorise so far -
then I need some numbers to close in on what is practical.

The maximum weight of the aircraft is likely to be about 5Kg.
Stall speed is about 10 m/s, so maneuvering speed is about 15 m/s.
>From my school maths, this means the average force required is:

34 Kg for a 1.7 metre rail (the plane takes up the last 300mm of the rail)
29 Kg for 2m
21 Kg for 2.7m
19 Kg for 3m, or 14 Kg if we opt for 13m/s launch speed.

A longer slide than this means folding or joining lengths of slide. Some
effort will be required to make the shuttle holding the aircraft negotiate
the join in the slide.

16 Kg for 3.7m
14 Kg for 4m, or 10 Kg if we opt for 13m/s launch speed.

The force required is proportional to the square of the launch velocity and
inversely proprtional to the length of the slide.

Is 19/14 Kg manageable? That is alot of tension. Someone check my figures

I measured 3m from the dashboard of my car to the rear door.
I just did an experiment with some 8mm shock cord - it takes 4.8 Kg to
double its length, and it will not stretch much farther than that.
Another hither to useless fact in my notebook for a previously explored
project says rectangular section plastic downpipe comes in 3 metre lengths.
100mm x 50mm or 100mm x 65. Plastic pipe is light and easy to work with, and
is much more car friendly than a metal structure.
The yachting fraternity sell suitable multiple pulley sheaves, and sheaves
suitable for mounting on the side of a mast so an internal halyard can be
used to hoist the sail.


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