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Science Olympiad

Old 03-25-2013, 01:48 PM
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joeyjoejoe
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Default Science Olympiad



Hello, I am new to this forum and know very little about RC planes. I have a question that I think many of you here may be able to answer. My son has just competed in his School's Science Olympiad and he did well. We are looking forward to next year where he will be performing in the same event and I have exhausted my knowledge on the subject so I wanted to pick the brains of the experts. I am confused by all of the new terms: KV? Turns? Pitch? Gearboxes? Its just too much to handle. Here are the abbreviated requirements for his event:
1 Build a magnetically levitate car that travels down a 3" wide track (two 4ft pieces of angled Aluminum attached to a piece of wood). We have this part taken care of. Basically put magnets on the bottom of a 3"X8" piece of balsa and also on the track so that the car levitates. Its the next one that confuses....
2. Attach up to 2 motors (35mm diam max) and props. to drive the device down the 4ft track as quickly as possible. The props don't have a size limit but the max size of the car limits the prop size to <7.5".
3. No voltage can be over 9v. NO IC MOTORS. Only brushed motors.
4. VERY important: The primary thing we are measured on is the "Vehicle score" which is defined as the weight of the car in grams divided by the time to travel down the track in seconds. Consequently, a very heavy car is a must. Our car weighed 1.5kg but we saw cars that weighed nearly 4 kgs at the recent competition! It requires a lot of "push" to get these cars moving. Our arrangement could blow a full can of Coke off of the table if placed directly behind our car (i.e in front of the pusher props).



We used this motor Electrifly T-400 and a multiplex "pusher" prop. Each motor was powered by two 14500 lithium batteries in series (they only needed to run a few runs).



My gut tells me that to increase our speed, we will need a bigget motor. I have seen some t 540 motors with 35mm diameters but I'm not sure if I need (nor sure how to choose) a motor/prop with high or low RPMs, prop with a large pitch etcetera. Most competitors chose pusher props. I attached a picture of our car.
Lastly, fear not for our safety if you want to make a recommendation. I (the coach) lost a few tablespoons of blood standing too close to those whizzing props early on in the project and the student's attitude towards the car changed drastically. We wear gloves and have a strict protocol that is followed when running the car. Thanks in advance for your help!

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Old 03-25-2013, 05:10 PM
  #2  
jester_s1
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Default RE: Science Olympiad

I can offer a few things. First, the design of your car is blocking a significant portion of the front prop's thrust. If the rules allow you to build it wider so the two can be side by side I think that would help a bit by itself. As for the electric stuff, you want the lowest pitch with the biggest diameter you can use. That gives you more static thrust (actual push when the vehicle is not moving) therefore will accelerate the car better. You said you are limited to 9v, but how are you measuring that? A 2 cell Li-ion is going to be 7.2v at roughly mid discharge, while a 3 cell is going to be 10.8. The batteries' voltage will sag under load and so that 3 cell may very well be at 9v during the actual run, but the rules may not allow you to measure it that way since the resting voltage will be a lot higher. Are you using a regulator? Assuming that 9v rule is being enforced, you want to pick the motor that will turn the props you choose at the highest RPM without going over their watt limit (watts are volts x amps). The manufacturer will have data on that, so that part is pretty simple. Given that you are making short runs, you can get away with staying right at the limit or going a little over.

Pusher props are not necessary as on most motors you can mount the prop facing the right direction and reverse the motor by switching the wires. Obviously check with the manufacturer on that to be sure though.

As for your questions, Kv is how many rpm a given motor will turn per volt. So if you pick a 1000Kv motor and put 9v on it, you'll get 9000 rpm if the motor is running at 100% efficiency (that's assuming you are actually putting 9v on it under load). It doesn't matter what load is on the motor. It will draw whatever amps it has to to turn 9k rpm assuming it is being fed with 9v from the battery. The Kv is determined by, among other things, the turns in the motor winding. Car guys talk about turns more than plane guys do because they often used to tweak their motors by rewinding them, but Kv is a more useful term and really all you need to know. Gearboxes are sometimes used because many motors are more power efficient if they have more turns and therefore turn more RPM (higher Kv rating) but in some applications that makes the watts go to high. A gearbox reduces the load when using a big prop letting you keep the efficiency of the higher Kv but slow everything down so nothing goes poof.
Old 03-26-2013, 02:43 AM
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Default RE: Science Olympiad

For the OP -

To add to a bit of Jester's explanation above, consider this.

The prop is like the transmission in a car. Compare this to a standard or manual transmission. The lower gears, first for instance, the car has a lower top speed, but it has "more power" and gets there much faster than in a higher gear, where a higher gear has "less power". So, you pick a prop that has a large diameter but a low pitch... say 10-4 which has a 10 inch diameter and a 4 inch pitch. The pitch is how far, in inches, in ideal conditions, the prop will move in one RPM. So, if it sustains a constant 10,000 RPM, it will move 10,000 inches per minute. You can do the math. It has more pulling power, in other words, than a 10-6 which would have more top end speed.

As for amperage or current draw, be careful and choose a speed controller that can handle the amperage that the motor will draw when turning to the max RPM. You could easily burn it out by "pushing" it to hard. Buy a watt meter - see the link.

http://www3.towerhobbies.com/cgi-bin...&I=LXLMV0&P=ML

This will give you the capability of measuring the amount of power the motor draws. Power = Watts. Watts = Voltage X Current.

You said "only brushed motors"? To bad because there are some real efficient brushless motors out there.

For batteries, look at L.I.F.E. battery packs.

http://www3.towerhobbies.com/cgi-bin...&I=LXYAS9&P=ML

There are several alternatives. Choose what will do the best work for you.

Good luck.

CGr.
Old 03-26-2013, 03:32 AM
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Steve Steinbring
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Default RE: Science Olympiad


ORIGINAL: CGRetired
The pitch is how far, in inches, in ideal conditions, the prop will move in one RPM. So, if it sustains a constant 10,000 RPM, it will move 10,000 inches per minute. You can do the math.

Not to be a lint picker CG but ...... 10,000 X 4 = 40,000
Old 03-26-2013, 05:42 AM
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jester_s1
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Default RE: Science Olympiad

It's going to be tough for the contest directors to make it fair and protect the kids from cheating if the setups are what it sounds like. I'm sure they chose brushed motors because of cost and to eliminate having to install a speed controller. But it's not clear how they determine if the batteries are legal. The only true 9v battery is an alkaline, as NiCd's go to 9.6, LiFe's 9.9 and other lithiums higher than that. They would do better to simply decide on a specific battery and motor the kids have to use and make it all about the car design. A NiCd car pack with a motor that makes good use of it would keep costs down and would give the kids a level playing field. If the school was inclined the invest in the chargers, going to a 3 cell lipo would give them more power to play with.
Old 03-26-2013, 01:05 PM
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joeyjoejoe
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Default RE: Science Olympiad

Thanks for the replies!

Some of this info contradicts what the local Hobbytown USA guy told me and my son just yesterday so I will ask a few more questions.
Thrust is the name of the game here. The winning cars all jumped off of the start line despite being verrry heavy. The usable track is actually less than a meter long so there isn't a lot of time to gather speed. If I am understanding you all, I need as big a prop as is possible (which is around 7") with a low pitch (3, 4, 5???) and drive it with very high RPM(15000RPM? not sure what high means).Would a 3-blade model be better than a 2 blade? Our current prop is a two blade prop that is a5.5X4.5 model. Not sure if that is considered a low pitch or not but I do know that each motor pulled 3.5amps under load so I'm betting this is considered a low pitch. All voltages are measured with the motors off prior to the run. We used two 900mah 3.6v 14500 batteries in series (which actually provided 8v when charged) for each motor. I do not know whatour motorwill provide in terms of RPM.

So, without knowing what RPMs you recommend, suppose I chose a 1000 KV motor and ran it off of my8 volt supply. If I am understanding things, this motor under no load would spina t 8000RPM's right?

In regards to the placement of the motors, yes the front prop is only slightly above but directly in front of the rear prop but I can't adjust them to the left or the right since only the propeller can extend outside the track border. We also experienced problems raising the front motor and prop since this raised the COG significantly causing a wobbly, unstable ride. The motor mounts are hollow to limit the problems associated with a placementlike this.

In summary my questions:
7" prop:Agree or disagree
2 blade or 3 blade
What pitch do you recommend
how many RPM should the motor spin?

I actually have more questions about the motor but my head is full right now.....

Thanks again for the help!
Old 03-26-2013, 07:52 PM
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jester_s1
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Default RE: Science Olympiad

FWIW, I think this is a really poorly designed contest. Power systems are going to be all over the place, and with such a short track differences between vehicles aren't going to make much difference. It's all about which kid's dad can get the most oomph out of his motor for that first 1/4 second without flipping the car sideways. Nevertheless, I'll offer what I can to help him win.

Given the rules as your are giving them to us, the biggest 3 blade prop that will fit on the car with the lowest pitch they make it in is the way to go. Prop style may also be a consideration. There are wide blade props that are designed to optimize static thrust. They are made for "3D" flying where planes spend a lot of time hanging relatively motionless in the air more like helicopters. I've never shopped for a 3 blade 3D type prop so I couldn't even tell you if they are made though.

To answer your questions as you've asked them: 7" prop is fine if that's the biggest prop you can fit on the car. If you are able to go bigger, then do so. Go with a 3 blade if you can since that moves more air, making the prop pull like it is bigger than it really is, although the vast majority of appropriate props for your project will be 2 blade. Don't go over 4 pitch unless you can't get one that low in the diameter you need. The lowest I've ever seen was a 3, and 3.75 is sometimes used for props designed to do 3D flying. For RPM, read the following paragraph.

To properly design this power system, start with the prop. Let's say you wind up with an APC 9x4.5 prop just for argument's sake. Since there are no amp limits and no battery capacity limits, your only RPM limit is on how many watts the motor can take before it burns up. So in choosing a motor, you start with its watt limit, so let's say that's 200 watts. Your battery is at 8v resting so it will probably give you 6.5-7v under load (I'm going to suggest you consider going to a large capacity 2 cell lipo for this. It will be 8.4v hot off the charger and won't sag as much under load). 200 /7 = 28 amps you can safely draw. Then you look at the prop specs the manufacturer gives you to find out how many amps the various different Kv ratings will draw with the prop you've chosen. So say you have a choice between a 1300kv motor and a 1500kv motor, with the 1300 drawing 24 amps with that prop and the 1500 drawing 30 amps. Go with the 1500kv motor because as I said above you can fudge a little on max watts if you're not going to run the motor wide open without adequate cooling for a long period of time. The last thing to buy then is the battery. The slower you discharge any battery the more it can maintain its voltage, so go with the largest capacity that you can. If the battery counts as part of the vehicle weight that's a major plus. A good rule of thumb for high performance setups is to not go above 10 times the battery's capacity in your max discharge unless you spend money on the really good lipo batteries. So in this example, you'd need a minimum of 2800 mah, with absolutely no penalty for going higher other than the cost.

So to summarize, choose your prop, then your motor, then your battery. While we're on the topic, use good thick wire and the highest amp rated connectors you can get. Many of us like the Deans ultra plug for connections like this. If the rules allow it, use a Deans plug as your switch rather than installing a separate one. Few switches can handle the amps your going to be pulling without dropping considerable voltage.
Old 03-27-2013, 02:23 PM
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Default RE: Science Olympiad

ORIGINAL: Steve Steinbring


ORIGINAL: CGRetired
The pitch is how far, in inches, in ideal conditions, the prop will move in one RPM. So, if it sustains a constant 10,000 RPM, it will move 10,000 inches per minute. You can do the math.

Not to be a lint picker CG but ...... 10,000 X 4 = 40,000
Picky picky... Heck, I can't even claim fat finger on that one, can I. How about "mind elsewhere... " [X(]

CGr.
Old 03-29-2013, 04:27 PM
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joeyjoejoe
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Odd but all of this questioning was really geared toware me NOT spending all the money that the hobby requires yet, now that I"ve learned so much, I think I'm going to order a few props, a motor or two and a watt meter and do some experimenting. I'm sure I'll be able to quit whenever I want. My wife is already saying "its rocketry all over again!"......

The instructions for selection of the various components really helped. The selection of the motor is the only problem. I can't find any brushed motor that uses KV or turns in its description but that shouldn't be a problem since most havea "maximum Wattage" rating. I know a bit about electronics but I'm glad you mentioned the voltage sag. I would've missed it and the extra current it allowed.
I'll let you know how it goes.

Old 03-31-2013, 05:46 AM
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jester_s1
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Default RE: Science Olympiad

You can save some money by using a Harbor Freight $5 multi meter. If you need to measure more than 10 amps you'll need to make a shunt for it (do a search on RC groups for a procedure) but that's not too hard.
Motors should have a recommendation for prop sizes, so if you have that info that should get you the right one.
Old 09-24-2013, 06:17 PM
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joeyjoejoe
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Sorry for taking so long to reply! New job (position actually) added with
sending TWO kids to college this semester has pretty much tied up all of my time
(and ALL of my $$) as of late. Anyhow, the rules have changed a LOT for this
years competition but my questions are still pretty much the same. I have not
used my watt-meter as much as I would've liked but I did order several props to
attach to the T-400 motors that I already have and have a few questions. When I
first ordered a motor/prop combo, I chose an electrifly T400 (brushed, as
required by the rules) motor and a Multiplex 5X4 pusher prop only because that
was what was available on the website I was viewing at the time. That combo
(times two since two motor prop combos were allowed last year - only one this
year) won gold for us in 2013. This year we need a heavier car with only one
motor/prop so minimizing friction and maximizing thrust is of utmost importance.
I ordered four additionall APC props and am very dissapointed in the results. If
you look at the attached image, you will see that the APC props I ordered
should've been more aggressive than the Multiplex prop (on the far right) yet
the Multiplex prop provides SIGNIFICANTLY more thrust with roughly the same
current requirements (approx. 5-6 amps). The prop size limitation is now around
5.5 inches. I noticed that the Multiplex prop is wider than the APC props. Is
that the problem? Any specific, more aggressive prop recommendations in the 5.5
inch range??
Old 09-25-2013, 11:14 PM
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tykryuryukrytwen
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In regards to the placement of the motors
Old 09-26-2013, 04:47 AM
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I'm glad it went well for you. Yes, wider props generally will have more static thrust than narrow ones. The narrower props are usually more efficient at faster speeds at the expense of not accelerating quite as hard.

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