Page 52 - 15 Dangerously Mad Projects for the Evil Genius

Chapter 3 ■ Ping-Pong Ball Minigun 33

as it leaves the nozzle from the general roar of the fast as possible. By reducing the effective area of
leaf blower. the end of the nozzle by fitting the barrels over just

From this trace, we can see that our ball took half the area, we can increase the velocity of the
0.6 – 0.08 or 0.52 seconds to travel 5.5 meters, air traveling through the barrels.
indicating an average velocity of 10.6m/s, or 24 Since the leaf blower is open at both ends, if we
miles per hour. Being very light and having a large increase the resistance to flow too much—say, by
surface area, the ping-pong ball will lose speed just having one barrel and blocking up the rest of
quickly, so the muzzle velocity is likely more than the tube—then the air may simply bounce off the
this figure. obstruction and come out the back of the blower,
An alternative to measuring the velocity with or create a vortex that merely circulates the air and
hence the balls.
sound would be to use video. Use a similar
arrangement to the sound, but set up a video Trial and error was used to determine the best
camera at right angles to the flight of the balls. If number of barrels to maximize the speed of the
you know the number of frames per second used balls and the fire rate. For the author’s leaf blower,
by the video camera, then by estimating the this turned out to be three barrels, which
distance the ball has moved between frames, you conveniently fit across the width of the blower.
can work out the velocity.

The only problem with this approach is that you
Summary
need a lot of room so you can put the video
camera far enough away from, and to the side of, This is one of the most enjoyable and easily
the flight of the balls. The author did not have a lot completed projects in the book. As you would
of success with this approach; the balls just moved expect, applying a few kW of power to a bunch of
too fast. But you may have more room for such ping-pong balls is always going to produce some
experiments and a faster camera. exciting results.
In the next chapter, we will embark on another

Theory weapon-type project, but this time it will be a
model weapon, based on a laser and some servo
A leaf blower is a low-pressure air pump. It is motors.
designed to blow out as much air as possible, as