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Old Dec 04, 2012, 04:03 PM
Inherent Tinkerer
Wichita, Kansas, USA
Joined Jul 2003
1,428 Posts
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Regulating pneumatic launcher

I have a pneumatic catapult launcher that I made but am curious if anyone knows of a way to make the launch "softer". When I trigger the launch the valve dumps all of the pressure at once making for a pretty harsh acceleration on the plane. I don't know if it's possible to have a dual stage or regulated release of the pressure to slow the initial acceleration or if that's even feasible. Maybe a short section of bungee between the plane and the launcher? Any thoughts?

Happy flying,

Jimmy
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Old Dec 04, 2012, 04:56 PM
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Palo Alto, California, United States
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Quote:
Originally Posted by jtprouty View Post
I have a pneumatic catapult launcher that I made but am curious if anyone knows of a way to make the launch "softer". When I trigger the launch the valve dumps all of the pressure at once making for a pretty harsh acceleration on the plane. I don't know if it's possible to have a dual stage or regulated release of the pressure to slow the initial acceleration or if that's even feasible. Maybe a short section of bungee between the plane and the launcher? Any thoughts?

Happy flying,

Jimmy
Can you add a second valve in series that is adjustable before launch so that it is partially open? that should slow the pressure rise and decrease the peak pressure in the launcher.

Steve
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Old Dec 04, 2012, 07:13 PM
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Near Austin
Joined Dec 2001
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Jimmy,

Don't know the configuration or length of your piston chamber, but we built one that had a set of holes in the piston chamber that would vent incrementally as the piston moved up the cylinder. This decreased the attenuated "punch" the vehicle received resulting in a smoother impulse curve.

Gene
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Old Dec 04, 2012, 08:11 PM
Registered User
Banff Alberta Canada
Joined Nov 2004
569 Posts
I started a project last year that involved using a pneumatic launcher to launch my Skywalker. I haven't finished it yet but it involved using a sabot being slid down the
barrel of a pneumatic cannon. The sabot has a surgical tubing connecting it to my
plane to give a sling shot affect. The sabot also has a rocket parachute attached
to the aft to limit the projecting of the sabot after the launch.
The following thread I've attached uses the same technique that I plan to use.


http://www.rcgroups.com/forums/showthread.php?t=526832
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Old Dec 04, 2012, 09:49 PM
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Near Austin
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hehehe.... yep, that would be the PALS system based on the plans I used to sell here.

http://home1.gte.net/texhills/page13.html

We used an aluminum rod encased in a swim noodle with a lightweight "piston" with a leather "ring" on the pressure end. Drilling holes in the "barrel" reduced initial acceleration but still allowed the aircraft to get up to flying speed.
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Old Dec 05, 2012, 11:21 AM
Inherent Tinkerer
Wichita, Kansas, USA
Joined Jul 2003
1,428 Posts
Thanks, Guys. That's pretty much the same setup that I'm currently using (I originally designed mine as an RPG and mortar for launching Nerf mini pocket Vortex footballs for our airsoft games.) I have an air nozzle that we use to trigger it but will switch to an electric valve for this project. Going to be launching 5 - 10 lb planes with it.

Gene - having problems visualizing the holes you're speaking of. Are they drilled in the barrel?

Jimmy
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Old Dec 05, 2012, 07:28 PM
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Quote:
Originally Posted by jtprouty View Post
Gene - having problems visualizing the holes you're speaking of. Are they drilled in the barrel?
Yes. Depending upon where you want to slow the acceleration curve, you would start drilling holes of varying diameters. Heck.... .I'll just draw a picture. Right back...

OK. First, until you get it right I would suggest that you put a threaded coupling in so you can change out of barrels easily. You might need to try a few if you put too many holes in one, etc. etc......

So, If you want to reduce the acceleration impulse, you'll start with holes closer to the valve (red). The number and size of holes will be varied by how much you want to flatten the impulse. If you want to keep the initial acceleration properties, but slow the piston toward the end of the stroke, then move the holes towards the muzzle end of the barrel (blue). The holes just act as an incremental pressure dump as they are exposed to the incoming pressure when the piston goes by. The spacing of the holes will also change the characteristics of the curve as well.

I don't know whose autopilot you are using these days, but anything with a logging accelerometer in it should give you pretty good idea of what's happening to the vehicle in the first few milliseconds of acceleration.

Let us know how you fare with this little project!

Gene
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Last edited by CenTexFlyer; Dec 05, 2012 at 07:49 PM. Reason: picture is worth a thousand words
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Old Dec 06, 2012, 08:41 AM
Inherent Tinkerer
Wichita, Kansas, USA
Joined Jul 2003
1,428 Posts
Thanks, Gene. Makes sense now. I already have a threaded barrel so that should work out well. Been playing with pneumatic launchers of various types for years now:

Airsoft Mortar (0 min 50 sec)


Jimmy
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Old Dec 06, 2012, 08:15 PM
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...... ah... the bad ol' days of the AquaNet hairspray spud gun.......
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Old Dec 06, 2012, 10:42 PM
Inherent Tinkerer
Wichita, Kansas, USA
Joined Jul 2003
1,428 Posts
Oh, I remember those days! Sent more than one flaming tennis ball soaring towards the heavens.... The mortar in the video is actually CO2 powered. I'm going to convert it to trigger electronically rather than manually. Did you have to modify the sprinkler valve at all?

Jimmy
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Old Dec 07, 2012, 07:24 PM
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Near Austin
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Suprisingly, no! The sprinkler valve will hold against the pressure just fine and you can cycle it easily. All you need to do is use a momentary contact switch to trigger the valve in line with a 12V source. I will say that when you launch, you will need to hold the button down through the launch cycle instead of just "tapping" the button because the valve is fast enough that it will "short stroke" on you.
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