Quote:
|
Originally Posted by Rick TYler
We rejected a catapult early on because of the inherent
limitations of rate of fire. Spinning wheels or belts have such an obvious
advantage in fire rate that we didn't seriously consider a catapult for long,
although some students did some interesting math on one.
So, this is just FIRSTnoodling, and not a serious design proposal. I'm also
not going to do the math, so forgive me for that, please. 
I agree that one of the problems with catapults is the waste of energy
involved. A lot of energy is used accelerating the arm, which is then usually
wasted at the end of the stroke. [...] |
Hey Rick! Email me privately and tell me how your build is going
(and what you did with the data I sent you... )
Yea even if you COULD do one, it's obviously not going to win anything
(but the crowd) this year, so don't take my comments too seriously.
But as you can see, if someone asks
me to "noodle", I'll take a
real crack at it...
Now if a catapult
was a serious payload widget contender, you'd
still have to power the thing, so my comments about pneumatics
vs motors (et al) stand.
Another one of "Keith's Rules":
"Stall can be managed. You must first look at the 'total energy
per round' and the 'energy transfer rate' requirements of
each device, before attempting to allocate actuators to
widgets on your machine. Right off the bat, those two criteria
may point you to the best actuator for the job."
I'm hoping at least
that part of this discussion may be useful to
the next student trying to the classic "motor vs spring vs
cylinder" choice.
But yea, though it wouldn't
win, I still feel a catapult, a "Cesta Wheel"
or some other "Rube Goldberg" throwing widget would offer a
LOT
of "ohh, ahh" crowd entertainment value! (Especially if it whips
the ball wildly off course!)
Quote:
|
Originally Posted by Rick TYler
How does this calculus change when the arm weighs essentially nothing?
You could build a very stiff launching arm three feet long of hand-laid
carbon fiber and fiberglass on a foam form that would weigh about about
six ounces.[...].
|
Well, please realize that a catapult arm is
not simply a stick,
whose strength you're trying to maximize, while at the same time
minimizing weight. It's a
distributed spring. Just like a bow, the
arm itself acts as an energy storage device. The material local
to the pivot rotates
much faster than the load itself. In essence,
the arm "<sets>" itself (there's that banned CDF "c" word again!)
by transferring potential energy from the spring into its length.
It then re-releases that energy, converting it into Kinetic Energy
in the load mass as it accelerates. IOW, the
compliance of the arm
is a
huge part of the total system.
- Keith