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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, 11:37
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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, 12:15
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Re: Team 665 Fan/s and More

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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

Quote:
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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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, 22:32
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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 03-02-2009, 00:25
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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!
You're off by about one order of magnitude.

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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Unread 03-02-2009, 01:40
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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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Unread 03-02-2009, 13:08
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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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Unread 03-02-2009, 18:07
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Re: Team 665 Fan/s and More

I am not an aeronautical engineer, but I do sit next to one. I also attended the AE interweb school of hard knocks when I decided that work well or not, propellers are going on the 2091 machine. Here’s what I found.

Your standard RC airplane propeller is designed to run at high RPMs AND at high forward velocity. Of course they work stalled, lest a plane wouldn’t take off, but their peak efficiencies are when they are moving. I don’t like the idea of gearing up a motor either, and the 5krpm type props were too big for what I was going for. They make propellers designed to run on motors (not engines) with typically higher, low-RPM torque requirements and lower total RPM sweet spots. I was poking around looking at those when I found the so called “slow fly” propellers. These are designed to fly at low RPM, and in stalled conditions. APC propellers makes FRP slow fly propellers in left and right handedness -- so you can have a counter rotating arrangement -- in lots of pitch and diameter combinations, surprisingly (to me) cheaply. The ones I settled on were 12x3.8 through dragan fly innovations. They are marketed for dual rotor RC helicopters in the 300W motor range, which looked perfect for cim motors. The slow fly propellers maximum RPM is about 65,000/D[=]in, so for 12” props it is ~ 5500, which I should never see in a loaded cim. This was one more piece of evidence that this was the right propeller/motor combination. A couple of suspect online propeller calculators indicated that each propeller would be in the neighborhood of half the motive force of what the wheels could punch out, which is testing enough for me. I’m bring’n the Heavy Metal (well, FRP) Noise to New Orleans (world capital of airboats), whether they work or not; 2024-T3, solid rivet construction enclosures by the way, before anyone jumps all over me.

Any real AEs see anything wrong with that?

Travis
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