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Unread 02-02-2009, 11:37
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Re: Team 665 Fan/s and More

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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 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 06-02-2009, 19:05
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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!
I'd have to say your estimate is quite generous. We have run a fan test on a 28 inch prop with it directly connected to a CIM motor (1:1). Xoar 28x10 prop gives us approximately 8lb static thrust, whilst a 28x12 gives approx 7lb of static thrust.
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Unread 07-02-2009, 12:56
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Re: Team 665 Fan/s and More

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sorry, but Q&A got to that one pretty quickly when it was brought up. http://forums.usfirst.org/showthread.php?t=11025

Yeah, it's intended to deflect incoming moon rocks. But I can almost guarantee you that at least one inspector per event will not allow the up-angle. (Most likely the lead inspector, but could be another one.) It's going to be really hard to build a deflection system that works using air jets, because it has to be horizontal. That said, anyone who does a fully legal one deserves at least a Xerox Creativity Award.
well i looked at the thread you gave me but id rather have to dissagree with it. There are 2 main arguments in there against it. The first being the vaccume will damage the floor and the second being it goes against <R06>.

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:
<R06> ROBOTs must use ROVER WHEELS (as supplied in the 2009 Kit Of Parts and/or their equivalent as provided by the supplying vendor) to provide traction between the ROBOT and the ARENA. Any number of ROVER WHEELS may be used. The ROVER WHEELS must be used in a “normal” orientation (i.e. with the tread of the wheel in contact with the ground, with the axis of rotation parallel to the ground and penetrating the wheel hub). No other forms of traction devices (wheels, tracks, legs, or other devices intended to provide traction) are permitted. The surface tread of the ROVER WHEELS may not be modified except through normal wear-and-tear. Specifically, the addition of cleats, studs, carved treads, alterations to the wheel profile, high-traction surface treatments, adhesive coatings, abrasive materials, and/or other attachments are prohibited. The intent of this rule is that the ROVER WHEELS be used in as close to their “out of the box” condition
The fan is not a traction device because it doesn't provide traction. It merely causes an increase of the force caused by frame of the robot (and the components of the robot) in pushing/pulling downward on the wheels. The robot already naturally does this through gravity but the fan just increases it further and so increases the wheels natural friction without any form of modification to the wheels.
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Unread 07-02-2009, 17:16
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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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Unread 02-02-2009, 01:37
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Re: Team 665 Fan/s and More

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We tried the fan at a 1:6 ratio, we tripped the breaker before we could even get to a decent speed. We're looking at 1:1 as a very good ratio right now. We're looking at the possibility of two non-rotating fans for directional control. 2 weeks is plenty of time....

-Greg
I'd suggest trying a 1:2 ratio. Because at peak power output you're gonna get about 2.6k rpm out of the CIM, and if you double that, for that kind of prop, you should be at a very good thrust. Also try to minimize your pitch, otherwise you'll be stalling the whole time. XD
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Unread 02-02-2009, 09:57
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Re: Team 665 Fan/s and More

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It puts out about 28 mph windspeed,
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
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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, 11:43
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Re: Team 665 Fan/s and More

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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, 23:02
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Re: Team 665 Fan/s and More

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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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Unread 02-02-2009, 23:13
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Re: Team 665 Fan/s and More

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Originally Posted by JamesByrne View Post
What rule states this? Sorry i'm just trying to get clarification.
Q&A in http://forums.usfirst.org/showthread.php?t=10943
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Unread 02-02-2009, 23:15
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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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Unread 07-02-2009, 15:30
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Smile Re: Team 665 Fan/s and More

tere must be a hopper but where>?
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