Air Trajectory B/C

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Re: Air Trajectory B/C

Post by bernard » January 24th, 2015, 4:30 pm

swheeldon16 wrote:I had a quick question. I live in rural Alaska, and am currently having a hard time obtaining supplies necessary for this event. I wanted to see if using a bike pump to build up pressure into an air chamber would be legal in the context of this events rules and was wondering if anyone else had tried this before in the past. Thoughts?
If you plan on using a bike pump, keep in mind that the handle of the bike pump (any parts that decrease in potential energy and supply launch force) must be able to be separated from the device and included in the 3.5 kg mass. This is something I didn't consider when I was going to use a bike pump.
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Re: Air Trajectory B/C

Post by XYZ_SMS » January 25th, 2015, 4:40 am

Is using a Laser as part of your tool kit for setting up the device before launch is within the rules?

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Re: Air Trajectory B/C

Post by Xystus » January 25th, 2015, 5:56 am

XYZ_SMS wrote:Is using a Laser as part of your tool kit for setting up the device before launch is within the rules?
bernard wrote:
Xystus wrote:Do you guys think laser pointers are legal to use for aiming as long as they don't pose any threat to the eyes?
They aren't explicitly prohibited by the rules, so as long as you follow General Rule #1 and the Science Olympiad policy for lasers I don't think you would have any problems.

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Re: Air Trajectory B/C

Post by peteb » January 25th, 2015, 8:19 am

A bike pump will hardly work, because you will need a larger amount of air moving faster than the that type of cylinder will provide to project the ball the needed distance.

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Re: Air Trajectory B/C

Post by chalker » January 25th, 2015, 10:20 am

peteb wrote:A bike pump will hardly work, because you will need a larger amount of air moving faster than the that type of cylinder will provide to project the ball the needed distance.
Not true at all. I've personally seen a device that consisted of nothing more than a bike pump, some weights and a small piece of PVC to hold the ball in. Was it a national caliber device? No, but it was able to launch the ball sufficiently to reach most target distances.

Air pressure (or any fluid pressure) can be a very strange thing to conceptualize sometimes. It's not always correct to assume you need a LOT of volume to generate a lot of pressure. Small hand pumps that bicyclists take with them for road repairs can easily generate over 100 psi.

A standard golf ball has a diameter of 1.68 inches, which means it has a cross section area of 3.14 * .84 * .84 = 2.21 square inches. That's also about the same cross section as a typical bike hand pump piston. Hence, with the right design, the majority of the force of the mass dropping will be transferred to the face of the ball (e.g. 3.5 kg or 5.0 kg depending on the division you are in). Let's work with the 3.5 kg number for 'worst case' (and ignore friction and other factors for simplicity).

Force = mass * acceleration -> 3.5 kg * 9.8 m/s^2 = 34.3 kgm/s^2 = force due to the mass dropping which is applied to the face of the golf ball. A golf ball weighs ~46 g, so doing the reverse calculation, Acceleration = Force / Mass -> 34.3 kgm/s^2 / 0.046 = 745 m/s^2 is the initial acceleration. Now air resistance will have a huge impact immediately on the velocity of the ball, as well as the duration of time the air pressure is applied to the face of the ball to accelerate it, but regardless, there is easily plenty of force for the ball to fly a few meters.

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Re: Air Trajectory B/C

Post by retired1 » January 25th, 2015, 4:59 pm

The calculation ignores the fact that the pump handle will be over 2' high on a standard tire pump. It also ignores the fact that resistance will build up rapidly further slowing the force applied to the ball. Air flow thru the small hose will be restricted vs a larger diameter. Most pumps that I have seen will pump about 40 psi, some a bit higher. It will take a large force on the handle to get into that range, a lot more than 5Kg dropping a foot to 18" before meeting significant resistance.
I tried a much larger pump and with 1/2" pipe it failed to get a ping pong ball more than 5m.
The bottle does better and a bellows does a lot better.

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Re: Air Trajectory B/C

Post by Toms_42 » January 25th, 2015, 9:23 pm

Has anyone had much luck with the racquetballs and practice golf balls? I've almost entirely seen people using ping pong balls, but they have a really poor flight pattern and tend to curve in the air. Practice golf balls don't fit well in any of the pvc tubes I've tested them with, anyone have luck finding a good tube for them?

Sorry if this has been asked, the thread is really long and I haven't looked through all of it.
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Re: Air Trajectory B/C

Post by bernard » January 25th, 2015, 9:28 pm

Toms_42 wrote:Has anyone had much luck with the racquetballs and practice golf balls? I've almost entirely seen people using ping pong balls, but they have a really poor flight pattern and tend to curve in the air. Practice golf balls don't fit well in any of the pvc tubes I've tested them with, anyone have luck finding a good tube for them?

Sorry if this has been asked, the thread is really long and I haven't looked through all of it.
How do the practice golf balls you have seen compare with the PVC you have tested? Regulation ping-pong balls are 40 mm, and golf balls must be no less than 42.7 mm. I know for sure a 1.5" ID PVC pipe is large enough for a ping-pong ball (slightly larger). Might also work for golf balls.
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Re: Air Trajectory B/C

Post by Toms_42 » January 25th, 2015, 9:36 pm

bernard wrote:
Toms_42 wrote:Has anyone had much luck with the racquetballs and practice golf balls? I've almost entirely seen people using ping pong balls, but they have a really poor flight pattern and tend to curve in the air. Practice golf balls don't fit well in any of the pvc tubes I've tested them with, anyone have luck finding a good tube for them?

Sorry if this has been asked, the thread is really long and I haven't looked through all of it.
How do the practice golf balls you have seen compare with the PVC you have tested? Regulation ping-pong balls are 40 mm, and golf balls must be no less than 42.7 mm. I know for sure a 1.5" ID PVC pipe is large enough for a ping-pong ball (slightly larger). Might also work for golf balls.
I tried that, the practice golf balls are just too big to fit. You can kind of jam them in, but unless you want to bore out the inside of the tube, 1.5" pvc is too small.
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Re: Air Trajectory B/C

Post by peteb » January 26th, 2015, 4:07 am

Toms_42 wrote:Has anyone had much luck with the racquetballs and practice golf balls? I've almost entirely seen people using ping pong balls, but they have a really poor flight pattern and tend to curve in the air. Practice golf balls don't fit well in any of the pvc tubes I've tested them with, anyone have luck finding a good tube for them?.
I searched all over and found a piece of custom pipe from a factory installation that fit a racquetball perfectly. Nothing else worked. Another team told me they had success with a ring inside the pipe to make a perfect fit for the ball. they were getting enough distance with a ping pong ball that way. Haven't tried that method myself.

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