What type of props are those? They look like 9"x something. Are they balanced? If not, I can tell you how to balance a prop.
Also, I’m not quite sure that that cage is going to be enough. I’d put another layer on on top or underneath so that the area of each open area is divided into four areas–if the props do explode, I’m pretty sure they could get through the pictured version. Reducing the open area will also reduce the risks.
Wow that totally is a unique and different propulsion system! I cant wait to see that in actual competition!
I would tend to agree with Eric though, I’d add some more protection to that cage. The cage will protect from reaching hands, but if those props were to ever break, (and going 7000 rpm, they very well might) you dont want blades going flying… I’m not a safety Nazi, I just like being safe.
Overall though, Awesome!
The inspectors will be looking for both. One piece of prop shrapnel gets through, and <S01> is probably going to be called. Then you have to remove both.
Let’s say those are 9" blades going at 7000 RPM. That’s C=pid=pi9=28.26" per revolution. That’s about a 2’ circumference (rounded down to emphasize the need for protection here). So, the tips of that prop are traveling at 7000RPM=116.7RPS over a 2’ circle. That’s a tangential speed of 233.3 feet per second–or about 4 FRC fields in one second. If a tip breaks off at that speed, you WILL get a safety violation if it leaves the robot! And, if I were the head ref, I’d make you either remove the props or add protection to the cage. Play it safe and add material now.
Nice work, looks like you put some thought into the design. Good use of model airplane props.
Are you worried that having the props mounted in a biased configuration (not dead-center on the robot) will tend to make the robot turn or move awkwardly? I assume you offset them to make room for a ball-handling system. Will the airflow pass through the ball system or will you reverse the rotation to reverse the direction of the robot?
On a separate note, have you checked the metal hardware on your collector to make sure it’s not going to rip up the balls? Looks iffy to me.
Sadly that hasn’t worked with the tests our team has doen, the porous nature of the ball’s geometry tends to not move it at all. Our tests were all done with a 22" blade with a 3D-10A pitch blade that was moving a lot of air at a 1:1 ratio from a CIM. YMMV.
The propellers are perfectly safe as long as they don’t exceed their maximum rpm, the rate at which we are spinning them is no where near their maximum rpm (the maximum is about 13k rpm). Having them spontaneously exploding is not a huge concern to us, having balls getting in the way of the props is a much bigger concern. (oh and its 12.25 in in diameter btw ;D)
Yeah, that’s why we have a 3.5 multiplier on our props. So the 2.5 k rpms you get on a CIM motor at peak power output you don’t have nearly enough pushing power. Our props are actually push them at about a walking pace.
See, ideally you reach 88% - 94% the speed of sound on the tips of your props in order to get maximum power.
Well, our blade/s pull too much power to gear it up any. We start tripping a 40A breaker at a 1:2 ratio. The size of our prop combined with the really high pitch, and the fact that the blade is cut to give a lot of thrust in one direction… gave us a pretty good acceleration to around the speed you’re describing at 3/4 ratio.
Of course we’re going for different things. You have them for turning and maneuvering plus a good amount of power. And we’re going for sheer power, mainly for getting across in a straight line fast, pinning, and then dumping.
Tell you what, you make it to Atlanta for Championships, we’ll have an Overdrive style race.
You might want to reduce your pitch then. High pitches are made for high speeds, therefore you’re being really ineffective and ideally you’d have about 1-2 pitch prop from max speeds of about 7-15 mph. Otherwise, even with our pitch of 3.75, we hit ideal speeds at about 30 mph.
Let’s run the numbers. That’s 374.15 feet per second if something breaks! If a prop breaks, it’s going 7, yes 7, FRC fields per second.
As for perfectly safe if they don’t exceed maximum RPM, that’s all well and good, but R/C aircraft don’t hit things. That’s the facts of it–if they hit anything, they crash (or just crashed) and the prop is broken anyway. These props are going to be jostled through shock loads when they get hit. This could potentially weaken them. If they’re weak, and they get one hit too many, somebody is going to get hit, hard. I haven’t run the numbers on KE for, say, 1/3 of a prop, but that’s far more than I want to have hitting anybody!
As for who’s going to poke hands in, I can think of a few things–little kids, poles from a tipped trailer, unsuspecting students… I am pretty sure I could get my hand in enough to contact at least one prop.
Bottom line, don’t worry about keeping balls out, worry about keeping the props in. If you do that (better than the cage shown will), the balls will take care of themselves.
Oh, and I just remembered–if you reverse those blades while they’re going full bore, that’s a pretty hefty acceleration force you put them through. This will contribute to propeller weakening. They aren’t designed to run in reverse.
Yes, the cage does make it more safe, but why isn’t it on during testing? In the 2nd video, I think the person bending down gets a little too close; one slip and people will be watching the video for different reasons…