[Canberrauav] X8 catapult

Matthew Ridley dr.matt.ridley at gmail.com
Thu Nov 8 08:57:45 EST 2012

I gave this some thought last night. Some other options to provide the
necessary force in 2-3m might be:

- Jack mentioned the "compound bow" approach. How about a leaf spring
from a old car, and form a sort of giant cross bow ? mount the track
perpendicular to this. Pulleys and cams could be added to make the
force more constant and increase the travel to 2m. This would be cheap
to build.

- Pneumatic ram spreading a block and tackle ? Should be easy enough
to get 100-200 psi into a small reservoir and drive a ram about the
size of a bicycle pump at 50-70kg over 50cm. If the reservoir is large
enough, the force will be more constant than a bungee. With losses on
a 1:4 block and tackle I'm guessing maybe 10-15kg of force over 2m ?

- I was also thinking of a flywheel/cam arrangement that can be spun
up to provide the energy. Where the excess energy goes is a bit of an
issue... compressed air can hiss out a hole, bungee cords can flap
about without too much harm.

- Could we spread a block and tackle with a car starter motor driving
a worm screw ?


On 8 November 2012 07:54, Andrew Tridgell <tridge at samba.org> wrote:
> Hi Chris,
>> Ok, I'd imagined more of a "high start" style, like a glider (maximise
>> range). Thats why i thought your hook was to far forwards, the gradual
>> acceleration was to flat. If you just want to slingshot it where its
>> pointing that's different.
> yep, I was always hoping we'd end up with a catapult with a fairly flat
> trajectory (say 20 degrees or so). I think that will provide us with a
> launcher that can work in a small area, with it getting on the motor and
> under pilot control as soon after takeoff as possible.
>> Are you sure the attitude solution will be OK with massive
>> acceleration? If the horizontal accelerometer saturates, "down" swings
>> forward, "forwards" meets ground...
> It should handle it, although I may end up adding a special case like
> Curtis does and disable the accel based correction for the first couple
> of seconds after launch, operating on gyros only.
> The accels are currently set to +/- 8g full scale. We hit around 1.8g on
> takeoff on Sunday with Stans bungee. Using your bungee we got to around
> 1.4g.
> On the first launch we got to around 13m/s in 1 second before starting
> the motor. On the second launch I started the motor at close to the 2
> second mark, at an airspeed of 10m/s.
> I'm guessing that with a catapult the acceleration would be much higher,
> perhaps 3g. If that is the case, and we have a 2m ramp, then we would
> get to 10.8m/s at the end of the ramp, after 0.37 seconds.
> If we get 4g, then with a 2m ramp we'd get to 12.5m/s after 0.32
> seconds.
> That assumes a constant force while on the ramp, which of course it
> won't be. So we'd actually need a bit more force at the start.
> If we measure the force on the bungee with a luggage scale, then 4g
> accel on a 4kg model will show 16kg "weight" on the luggage scale.
> I think if we use shock cord (I have 10m of 8mm shock cord), then if we
> have it loop back once on a 2m ramp, and have it stretched so that it is
> still under some tension when it has released all the way forward then I
> think it should work out OK. I tested this by stretching my shock cord
> from a pole with my luggage scales and it looks reasonable. We could
> measure the force at different stages of release by pulling againt a
> pole to see how much the force drops as it gets released.
> Cheers, Tridge
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