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Re: Team 665 Fan/s and More
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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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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 ;) |
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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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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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 |
Re: Team 665 Fan/s and More
I don't think you will be anywhere near 5 lbf per CIM with a slow fly prop, although I'd love to be wrong about this.
For reference, A Draganflyer X6 Helicopter has 3 sets of 16inch/15inch contra-rotating blades, i.e. 6 blades operating at 2000 RPM at hover with a maximum gross weight of 3.3 lbs (and max power for its 6 motors of 450 watts total. So if you can beat this by 50% with a single smaller prop they will probably have a job waiting for you ;) The mechanical power you can get from the CIM operating from 40 - 50 amps is about 125 - 185W. But you can get that only with a torque load of 100 - 125 oz-in. You need a prop that will give you that torque load at 3800 to 3300 RPM respectively. Slow fly props do have higher loads than normal props but I suspect that a 12 x 3.8 is still pretty far from the sweet spot on a CIM. A normal clark airfoil type prop is only going to give you about 1.2 - 1.5 lbf at 5000 RPM. BTW, a traditional 15 x 3.8 prop at 2000 RPM would produce about .61 lbf. So 6 of them is within 10% of the max gross of the X6. Drop the prop to 12 inches and you've got a quarter pounder. I've seen test data showing an APC 10x3.8 slow fly turning at 6850 RPM producing 1.54 lbf static thrust which is pretty good agreement with 1.69 lbf for a normal prop. Note that this is already above the max recommended speed. Slow fly props are not as sturdy as conventional props. You won't get the CIM anywhere near 6850. I think you'll see 1 - 1.5 lbf. I think the major difference in the slow fly is that you will have better performance at slower speed but nowhere near the improvement of dropping the pitch under 2. Once the robot gets moving, the effective pitch drops and the slow fly should increase faster to its max thrust point. The normal prop will never get to that point. The lower pitch prop is starting out well ahead of the slow fly and will also improve. It should be interesting to see what end's up working out the best and what can survive the abnormal precession forces resulting from robot collisions. This is unexplored territory. |
Re: Team 665 Fan/s and More
Based on some preliminary, very unscientific testing* with bare props, directly mounted to Fisher Price motors, in pusher configuration, we were getting ~2.0 - 2.5 Lb.s of thrust from APC slow fly 10 x 3.75 props. They were running at about 9,000 RPM. If you look at this paper (yes it is old, but the science is still good) you will see that it is possible to gain about 50% to 75% in thrust, as well as improved efficiency by building a shroud. Since you have to protect the props in something for safety anyway, why not make it an asset? Here is a picture of the fiberglass shroud we made for the prop:
Link: On another note, due to the internal resistance of the battery and other components, if we ran two of these at the same time, the voltage dropped enough that the RPMs fell off to about 8,000, and the thrust went down to less than 2 lb. per prop. We are building these modules, but still don't know whether or not we will add them to the finished bot. We still have to do testing to see whether the benefits outweigh the hassle and if they fit within the weight budget. *Unscientific test: stand on digital scale holding motor/prop in hand and record displayed weight. Point the prop straight down and engage motor (use caution here, very dangerous, do not attempt this at home! these are |
Re: Team 665 Fan/s and More
So, if a 12x3.8sf propeller is not the best candidate, what is a better one? Let’s make the design criteria D<=12” and a cim with 1:1 transmission. I had a hard time finding pitches below 3 in the 12” class, especially ones that didn’t run at the more like 15Krpm nitro speeds. Would an APC 12x10E thin electric be more my speed maybe? If there isn't a good option in the 12" range, I could maybe squeze into a size 14.
I have a working drive train, so the propellers are just gravy at this point. Even a modest 1/4lb thrust is justification enough for installation in my opinion. Thanks, Travis |
Re: Team 665 Fan/s and More
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Re: Team 665 Fan/s and More
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We are using direct drive FP motors and our thruster is still optional. We were originally planning to machine a custom prop but are now planning on off-the-shelf adjustable pitch props with a custom machined 3 blade hub to get us a 1 - 2 pitch in a pusher configuration. As another poster pointed out, careful attention to the shroud will help. I suspect we may see both geared up CIM's and Direct Drive FP's with total thrusts approaching 10 lbf (at the start of the match). |
Re: Team 665 Fan/s and More
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Be aware that 9,000 RPM is about 138% of the manufacturers safety limit for a 10 inch slow fly prop. The battery issues (and wiring) are very important. While the internal resistance of the battery is spec'd at 10 milliohms it behaves more like 5 to 10 times that at higher currents and low frequencies. It's best to design around 11 volts at the motors to account for all of the drops. |
Re: Team 665 Fan/s and More
i got the rule that states you can't de-score game pieces already in the trailer.
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Re: Team 665 Fan/s and More
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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. |
Re: Team 665 Fan/s and More
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-Greg |
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