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  #16   Spotlight this post!  
Unread 02-02-2009, 10:01
Greg Peshek Greg Peshek is offline
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Re: Team 665 Fan/s and More

Quote:
Originally Posted by martin417 View Post
I have a suggestion for measuring prop speed. The device you are using in the video looks for changes in light intensity. The room you are in has florescent lights. Florescent lights flicker 120 times per second. This flickering is picked up by the photo-tach and you will not get a good reading, if you get one at all. (try aiming the tach at the overhead lights, you should get a reading of around 3,600 RPM if you are set for a 2-bladed prop). You should make your readings outside, or turn off the overheads and use an incandescent light source like a flashlight, aimed from the far side of the prop (so the beam passes through the prop) at the tach.

Good luck at your regional!
The device in the video is actually a wind speed meter. It has a little (muffin-like) fan in the center that spins and then it turns that into a wind speed. So we're not actually clocking the RPMs coming out of it, just the physical speed of the wind Thanks for the advice in the rest of the post, we'll make sure to look into it.
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Unread 02-02-2009, 11:03
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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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Unread 02-02-2009, 11:30
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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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Unread 02-02-2009, 11:37
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Re: Team 665 Fan/s and More

Quote:
Originally Posted by Tom Line View Post
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!
How much power would be consumed, in order to generate that? I don't think that airspeed is anywhere close to uniform over the propeller disc.

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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Unread 02-02-2009, 11:43
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Re: Team 665 Fan/s and More

Quote:
Originally Posted by JamesByrne View Post
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?
You can't use fan systems to de-score balls that are already in the trailer, but you are free to intercept incoming balls if you should so please.

I am looking forward to seeing these fan-bots in action.
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Unread 02-02-2009, 12:15
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Re: Team 665 Fan/s and More

Quote:
Originally Posted by Tristan Lall View Post
How much power would be consumed, in order to generate that? I don't think that airspeed is anywhere close to uniform over the propeller disc.

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.
Like I said - 320 pounds if uniform. Assume it's not, so I halved the value to 150. Even if you want to cut it 80%, they're going to get similar drive from that fan as they do from the wheels, so they'll accelerate twice as fast as a non-ducted bot.

They better not have the air-intake on the bottom of the robot though.....
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Unread 02-02-2009, 12:25
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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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Unread 02-02-2009, 12:45
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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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Unread 02-02-2009, 13:11
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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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Unread 02-02-2009, 13:56
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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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Unread 02-02-2009, 14:01
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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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Unread 02-02-2009, 14:25
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Re: Team 665 Fan/s and More

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Originally Posted by zrop View Post
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
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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  #28   Spotlight this post!  
Unread 02-02-2009, 22:32
Greg Peshek Greg Peshek is offline
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Re: Team 665 Fan/s and More

Quote:
Originally Posted by Tom Line View Post
Like I said - 320 pounds if uniform. Assume it's not, so I halved the value to 150. Even if you want to cut it 80%, they're going to get similar drive from that fan as they do from the wheels, so they'll accelerate twice as fast as a non-ducted bot.

They better not have the air-intake on the bottom of the robot though.....
During the video I posted, you can see our mentor measuring the wind speed at different locations around the blade. It is roughly 26 mph at the far tips of the blade and 28 mph toward the middle of the blades. Don't worry, the air-intake on the bottom was our original plan until we pulled up our test flooring on the third day. Then we saw the update and that was categorically disqualified.

Quote:
At some point you have to start wondering what all the backwash from your props will do to the other robots on the field, including your own partners. We'll find out in Orlando I suppose
There's a big volume of air being pushed out, if you watch the video closely, you can see the papers blowing 20' away from the prop, but we are fairly sure by the time the backwash diffuses after being pushed out the fan there shouldn't be any giant surfaces for the air to catch on with the other robots. Let's just hope no one puts sails on their robot.

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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Unread 02-02-2009, 22:36
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Re: Team 665 Fan/s and More

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Originally Posted by zrop View Post
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
Keep it in your own thread please.
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Unread 02-02-2009, 23:02
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Re: Team 665 Fan/s and More

Quote:
Originally Posted by Bongle View Post
You can't use fan systems to de-score balls that are already in the trailer, but you are free to intercept incoming balls if you should so please.

I am looking forward to seeing these fan-bots in action.
What rule states this? Sorry i'm just trying to get clarification.
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