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#1
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Re: Team 665 Fan/s and More
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#2
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Re: Team 665 Fan/s and More
Back of the napkin tells me that if you're putting out a uniform 28 mph across a 32 inch fan, you'll get around 340 lbf out of it. Anyone want to check my numbers? I guessed on some of my constants - been a while since I cared about air density
If my numbers are right, conservatively you may get 150 pounds of push out of it. Not too shabby! |
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#3
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Re: Team 665 Fan/s and More
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Anecdotally, I've dealt with a propeller-based system: a 100 lb model aircraft with a pair of 1 250 W (mechanical output) motors and Ø20 in two-bladed propellers with 12 in pitch. In long-duration maximum-power static thrust tests on the ground (there was resistance at the wheels, but not much), it had between 30 lb and 40 lb of thrust. The blades were spinning at over 6 000 rev/min, and the motors were drawing over 40 A each at 40 V. (The aircraft had a conservative flight duration of around 8 minutes at maximum power. When cruising, it needed far less power than that to sustain estimated airspeeds of 100 km/h.) Incidentally, it's the experience on that project (everyone stayed safe), that fuels my healthy distaste for propeller systems in crowded spaces. |
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#4
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Re: Team 665 Fan/s and More
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They better not have the air-intake on the bottom of the robot though..... |
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#5
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Re: Team 665 Fan/s and More
I'm a big fan of 665...tee hee. Actually I think this is going to be really fun to watch
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#6
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Re: Team 665 Fan/s and More
I like how no one is really that concerned about safety here when the guy has his hand REALLY close to the prop, yet when our team posts anything about prop testing, we are continually nominated for the Darwin award XDD
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#7
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Re: Team 665 Fan/s and More
his hand is close to the blade but not in the plane of rotation like you guys were. Also there protection around the fan is far superior to the one you have currently proposed. I think people are becoming tired of your blatant disregard for safety. It's ok if you want to hurt yourself, but by not shielding the fans properly you will be putting hundreds of people at risk.
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#8
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Re: Team 665 Fan/s and More
I think the safety officails will have fun with this team
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#9
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Re: Team 665 Fan/s and More
I like it, great concept, can't wait to see it in action!
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#10
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Re: Team 665 Fan/s and More
It has a lot to do with the way your post came off. It has been talked about intesely else where. You should stop considering yourself a "victim" but rather extremely assisted. We all want you to pass inspection and these are the steps necessary to make that happen.
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#11
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Re: Team 665 Fan/s and More
Keep it in your own thread please.
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#12
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Re: Team 665 Fan/s and More
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Quote:
For anyone interested, we measured the amp draw at different power outputs.. it's about 20 amps on startup, 8-10 amps on 50%-75% speed, and 4 amps at 100% throttle. As far as the safety aspect, the metal for the cowling is only 1/32", so it was fairly easy to bend and make into a circle. Although we are confident that if the blade is to come off and shatter, the cowling would stop or drastically reduce the velocity of any dangerous debris. Since there really isn't any scale, the mesh we have protects against putting fingers in. One girl on our team can fit her pinkie finger in the mesh, but even then it wouldn't be long enough to get to the blade. So we figure a 10 year old won't be able to hurt themselves even if they tried. Thanks for all of the comments and input guys. -Greg |
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#13
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Re: Team 665 Fan/s and More
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Spinning a 22 inch prop at 4000 RPM is going to get you a little over 11 lbf of static thrust. Forget about windspeed. Your goal is not to move air. It doesn't matter how fast you move air. The static thrust from your propeller (i.e. what moves the robot) is a function of prop diameter, number of blades, and RPM. The propeller pitch will determine how much torque (and therefor mechanical power) is required. Nothing else matters much. The most mechanical power you can get out of a single CIM is about 300W. While a pair looks like it could get you 600W, you will have serious voltage drops so 500W is a more realistic maximum mechanical power for 1 to 2 second intervals with 60 amps on each motor. If you want to operate sustained, you have to limit your input current to not much more than 40 amps each. At the max power point 60% of your input power is going to go into motor heat. Your continuous mechanical power is unlikely be more than 440W for the pair. You'll loose at least another 5% in your gear train. You could theoretically reach about 21 lbs of thrust with a 22 inch prop spinning at 5500 RPM with a pair of CIMS geared about 1:2. But to do this you will need a prop pitch under 2. A normal prop pitch of 4 is just going to go click - click - click because it's torque load at 5500 RPM requires more than 800W and you don't have that kind of power. You'll top out at 4000 RPM and about 11 lpf static thrust. BTW We are using a pair of FP motors each driving a 12 inch 3 blade adjustable prop. Our CIM's are dedicated to other uses and call me old fashioned but I just don't like the idea of gearing up a motor. (Although it appears unavoidable if you want to use CIM's). The "best practice" way to determine your static thrust is with an engine test stand setup that measures the actual force exerted at the prop shaft. R/C airplane folks do it all the time. Google is your friend. The above numbers are for open air at sea level. The safety shroud will normally have a bit of a positive effect and your safety cage will have a negative effect. Any negative pressure created by inadequate air flow behind the prop will have a (possibly severe) negative effect as well. And if you plan to compete at the Colorado regional, you'll loose about 20% to Denver's density altitude ![]() |
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#14
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Re: Team 665 Fan/s and More
I fly large (25 lb) radio control airplanes with 52 cc gas engine that swing 20 x 10 props at 7000 RPM static. The engines idle at just below 2000RPM. Your not going to get much thrust using one of these props at the 2600 RPM that a CIM turns.
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#15
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Re: Team 665 Fan/s and More
You biggest problem is going to be finding a large prop like that, that has a pitch of 2 or less. At this point, you should either try to custom order one, or get a nice piece of wood.. and carve your own =)
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