Motors facing up or down??

Discussions about quadcopters/multi-rotors
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Solek
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Motors facing up or down??

Post by Solek » Thu Feb 07, 2013 6:02 pm

Hi guys, I have heared that if you face the motors downwards so they push the quad up it is more efficient...
what do you think is that right??



arnaud
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Re: Motors facing up or down??

Post by arnaud » Sun Feb 10, 2013 11:10 am

We have been thinking about it. I am not sure the efficiency would be changed but the center of gravity would be higher which would make the copter more stable (some would say less fun ;)).
We also though to test motors facing up and down. Not too sure what would happen: best case we get twice as much lift, but thinking about it the top propeller will certainly affect efficiency of the bottom one (or vice-versa...). Anyway that would be fun to test.

tobias
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Re: Motors facing up or down??

Post by tobias » Sat Feb 16, 2013 11:59 am

That would be really interesting to test and shouldn't be that hard. Just turn the motors upside down and make sure the props spin in the right direction. I think they need to be revered. A firmware change could also fix that. Making some landing gear would be good as well.

I'm a bit skeptical that you would gain that much efficiency but I'm not an aerodynamics specialist. I guess the airflow underneath the propeller is more directional and would benefit from having a "clearer" path.

absoloodle37
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Re: Motors facing up or down??

Post by absoloodle37 » Mon Jun 10, 2013 1:24 am

Sorry, I don't think there's an advantage. Thrust is the same no matter which way the motors spin or the props are facing. I thought about doubling up the motors (see pics)

Image
Image

Heres' the problem with the double motor idea: There's no net thrust gain because the power output going through each motor terminal is still the same, regardless of how many motors are attached to it. (two motors using the same power = each spins with half as much torque/speed). You could change the software/firmware to increase the power output (e.g. changing max thrust from 80 to 100) but the extra 20 'units' of power would be split by both motors (10 for each), which would be the same lift as one motor on 20 'units' of extra power. Also, 4 extra motors means extra weight, so again, the idea is less efficient than just increasing the output power to the existing 4 motors.

Bottom line: As payload weight increases (larger battery, video camera, etc), you'll need more available throttle to keep the CF flying--until you've hit the stop on your control stick and the CF can't gain any altitude--it just hovers. That's the maximum upward force the motors/propellers can generate from the output power they're being fed through the terminals. That maximum force is determined by the amount of torque the motors/propellers can generate per second (i.e. prop power), with the electrical power they've been given. A specific payload weight requires a specific amount of prop power to keep it hovering. As weight/required prop power increases, the motors/propellers have to spin faster to keep the CF aloft and require more electrical power. Eventually, the maximum power output is reached (max throttle), yielding a maximum RPM. The propeller blades that come with the crazyflie may not be the most efficient for this specific combination of RPM and power.

Another idea: Slightly bigger propellers. There's enough clearance between the blades of each motor to put on larger propellers. This would increase lift, but add more required torque to the motors so the max RPM is decreased. Most propellers are more efficient at a lower RPM. The lower RPM means the air that the trailing blade is entering is less disturbed. So depending on how much extra torque the motors can take at a given RPM, this may be a great way to get more lift in order to carry a larger battery/payload. Usually, the most efficient propellers are two bladed. Because the diameter of our propellers is so small, multiple blade propellers disturb the air that the trailing blade is entering. Therefore, 3 and 4 blade propellers are less efficient.

Does anyone know where to find slightly larger propellers?

-dingo

tobias
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Re: Motors facing up or down??

Post by tobias » Tue Jun 11, 2013 1:54 pm

Grrrr, I always have to little time writing my posts so it won't be that long...

Yes, you can double the power this way but you must put the motors in parallel. If you put them in series you will get the effect you are talking about (the same with loudspeakers). Each motor is about 2.3ohm and when you put them is parallel you will get half the resistance which will double the possible power, good luck :D, it's going to be a very powerfull Crazyflie.

Putting bigger props might not be the way to go as the props are already in the upper zone. Having more efficient props would be the way to go but we have been searching all over for other suitable props but without any luck.

absoloodle37
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Re: Motors facing up or down??

Post by absoloodle37 » Wed Jun 12, 2013 2:04 am

Thanks Tobias,
Of course, you are right. I thought the motors would be connected in series for some reason. Thanks for keeping up with this thread/idea. Even short posts are helpful!

Ryan

psatya
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Re: Motors facing up or down??

Post by psatya » Wed Oct 09, 2013 2:12 am

I just got the crazyflie kit. Started assembly after watching the videos and WIKI. Got into a problem: the first motor went upside down :)
Stopped assembly. Should I take take the motor down or assemble all the motors upside down? This thread is experimenting with
the upside down set up, so I thought I would post my query here.
If I need to take the motor down and re do it, then what is the best way to go about doing it?
Thanks for your help.

absoloodle37
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Re: Motors facing up or down??

Post by absoloodle37 » Wed Oct 09, 2013 11:03 pm

Motors facing up or down doesn't change performance. The position of the motor is irrelevant to the aerodynamic effects of the prop. In real airplanes, most aircraft have props that are pulling instead of pushing, but that's because the plane balances better when the engine is in front. The only real difference is that with the motors upside down, the center of gravity is higher, making the CF slightly less stable. This means the motors and speed control have to work harder to keep the CF steady during maneuvers, which means slightly less flying time. Thus, I don't recommend mounting the motors upside down, based on the above assumptions.

If you don't agree, try testing both configurations and post your results for the rest of us!

tobias
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Re: Motors facing up or down??

Post by tobias » Thu Oct 10, 2013 1:05 pm

See this tread for some advice on desoldering.

orcinus
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Re: Motors facing up or down??

Post by orcinus » Thu Jan 23, 2014 11:26 pm

The matter of CoG is not as simple as you think, absoloodle.

It's not so much about the CoG as about the position of CoG in relation to the center of thrust.
CoT is parallel to the rotor planes of the propellers. Contrary to what you might expect (and somewhat counter-intuitively), having the CoG below the CoT makes the system more unstable and more prone to oscillatory behaviour.

Having the CoG above the CoT causes divergent instability - the kind that the Crazyflie's FW can deal with.

Ideally, the rotor discs should coincide with the CoG, i.e. CoG and CoT should match.
But having CoG above CoT is preferred over having CoG below the CoT.

Here's an interesting paper lifted of a similar-themed DIY Drones thread:
http://eprints.qut.edu.au/33767/1/33767.pdf


PS: as far as efficiency of the propellers is concerned, the position is irrelevant - it doesn't matter whether you're blowing air over the quadcopter's arms or sucking the air over them.

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