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#1
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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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#2
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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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#3
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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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#4
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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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#5
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Re: Team 665 Fan/s and More
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#6
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Re: Team 665 Fan/s and More
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For the first of the 2 I'd have to say that the main argument there is merely about suction such as a vacuum cleaner would provide. How ever if you have it so that the fan draws the air from a large area around it then it shouldn't create a vacuum and should dispel that particular argument. As for the second with it breaking rule <R06> from what i see of the rule it isn't breaking it. Quote:
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#7
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Re: Team 665 Fan/s and More
I think i know which thread you were talking about. The one i found was:Team 2526 - Propeller Propulsion Prototype. As well as that post on the Q&A. i guess that you are right about not being able to use it to provide a downward force.
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#8
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Re: Team 665 Fan/s and More
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#9
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Re: Team 665 Fan/s and More
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#10
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Re: Team 665 Fan/s and More
I do not have a link handy to the rules right now, but i thought somewhere in the rules you could not use a fan of any type to deflect balls that people are shooting at your trailer. So are these aimed sideways or are they aimed front and back?
Like i said i will come up with a quote when i get a miniuite to do so. |
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#11
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Re: Team 665 Fan/s and More
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I am looking forward to seeing these fan-bots in action. |
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#12
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Re: Team 665 Fan/s and More
What rule states this? Sorry i'm just trying to get clarification.
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#13
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Re: Team 665 Fan/s and More
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#14
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Re: Team 665 Fan/s and More
Thank you very much it settled an argument. I appreciate your fast response.
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#15
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tere must be a hopper but where>?
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