|Mar 24, 2013, 08:20 PM|
Joined Mar 2013
Hello, I am new to this forum and know very little about RC planes. 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 (BTW, its not that I'm not trying to learn but within 1hour of looking for an appropriate motor/prop combo, my head exploded!- 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 am attaching 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!
|Mar 29, 2013, 09:09 AM|
Ill have a stab at it.
So you're working with a maximum voltage, but im not reading anything about amperage limits?
This is what I would do.
Convert straight over to a 2S lipo pack (are these allowed?) A fully charged 2S lipo will measure 8.4V, but will pack much more punch. (Im assuming you're running Li-ions)
Ditch the multiplex props. Convert over to something made by APC or Graupner. Much better propeller designs there. Try and run the biggest diameter props you can. Aim for high thrust. You only have 4ft of track to run with so look for a setup that will provided the highest acceleration. Large diameter, low pitch props for example.
Try and avoid the inline motor design. See if you can make something with the motors side by side so you don't get inefficient flow characteristics by having one propeller blow into the other one.
Also, that definitely doesn't look like a pusher prop. If you look front on from the motors perspective (shaft front) the prop should be rotating in the counter clockwise direction
As far as motors go, I don't know, everyone on here is using brushless motors, brushed motors are rarely ever used anymore.
If you can get a hold of a pair of wattage super cobalt 400 motors, then you should be able to make something pretty fast. That's the only brushed motor Ive ever used in a model and I was very happy with them.
|Mar 29, 2013, 08:33 PM|
Joined Mar 2013
Yes LiPo are allowed and I've got a 2100maH 2s right now but I think I will up the mah for the competition.
I actually just oredered a 7X4 APC slow fly pusher prop . Why pusher? I don't know. Everybody had pusher props in this years competition.
Side by side won't work. The rules state that nothing but the prop can extend outside the limit of the 3" track. I saw several designs where the front prop was very high but, as we discovered when we tried it, those cars had a tendency to tip. You really have no real support floating on a magnetic field and with a COG 5or more inches above the track, the slightest thing will cause the car to topple.
Keen eye regarding the prop. It was advertised as a pusher prop but, yes, I did have to reverse polarity to the motor to get it to push.
I will investigate the super cobalt motors. If I can find some info on the dimensions of the motor (it must be less than 35mm in diameter for example), I'll buy one ASAP.
Thanks again for you reply!
|Mar 31, 2013, 01:01 PM|
If you cant find the cobalt 400 motors. I have one here that I can send to you. Just pay shipping and its yours.
Its certainly a lot better then those electrify motors.
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