Replies to This Discussion

Permalink Reply by Paul D Moller on July 26, 2011 at 6:32am

Ok cool. That's what I wanted to hear.. Looking forward to your video! 

Thanks.

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3D RoboticsPermalink Reply by Chris Anderson on July 26, 2011 at 4:52pm

Yes, the ArduCopter IMU is more than up to the camera stabilization job. The limiter is the quality of th pan/tilt mechanics and servos. Regular RC servos aren't smooth enough for really pro smoothing.

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DeveloperPermalink Reply by Olivier ADLER on July 26, 2011 at 7:23am

 

Propeller size (inertia is proportiaonal to the square of diameter) and motor torque is an important factor i think, even more than ESC update speed as soon as the frame is in the heavy lift size.

 

Processor power is important as well for best results, because PID closed loops are simple and efficient, but not optimized for best dynamics.

 

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Permalink Reply by Denny Rowland on July 26, 2011 at 10:55am

Prop efficiency is something that starts to become very noticeable with heavy lift quads. My latest design is 35% more efficient than it's predecessor. No matter how many times you do the calculation a quad comes out on top in comparison to a hexa or an Okto. The others are absolute non starters in that race. Having the props below the arms is a big improvement as aerodynamic drag is a square law. Mounting the props close to the backplate reduces overhang and thus vibration also. Lift efficiency is almost a function of how much diameter you can absorb the available power into. The thrust velocity should be matched to be just above the airspeed that you will fly at. So if you fly at 100 mph then you need a small toothpick prop with a high pitch angle. but in our case we need a slow moving high volume of air movement i.e. large dia small pitch angle and wide blade profile. a 14x4.7 came out on top when testing on my motor dyno. It was also the top performer on Holgers tests. Better stabilization from a high degree of efficiency does not always follow. A little less dia. can smooth out the response if the pid settings are not great.    

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DeveloperPermalink Reply by Adam Rivera on July 26, 2011 at 11:12am

I am considering mounting my props on the underside of my octo frame. Main reason I went for the octo configuration was for redundancy. When carrying expensive equipment I just appreciate that extra peace of mind. I am really curious to see how the inverted prop mount will affect my efficiency. That translates into longer battery life correct?

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DeveloperPermalink Reply by Adam Rivera on July 26, 2011 at 11:41am

Could you explain how having the props below the arms offers increased efficiency?

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Permalink Reply by Denny Rowland on July 26, 2011 at 12:19pm

The low pressure air that flows into the top side of the prop. swept area comes in from all directions especially from the sides. It does not really notice the laminar flow arm. The thrust side of the prop. converges that flow inwards and therefore at greater dynamic pressure. As that flow is a square law i.e. double the velocity and get four times the drag then you are effectively using your own thrust to push the model downwards due to the less than ideal profile of a square arm. I cant remember exactly how much additional lift that gave but it multiply it by 4. 

 

There is one other factor that starts to play a role and that is due to the center of aerodynamic pressure created by the lifting propellers. If the center of mass is significantly lower, then the motors will need to work a lot harder to stabilize the model. This is because the low center of mass wants to swing about like a pendulum when there is a lateral movement. I have two models here one with a high thrust line and one with a low thrust line. guess which one gives 35% more duration.

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DeveloperPermalink Reply by Adam Rivera on July 26, 2011 at 12:23pm

Thanks for sharing. That is incredibly interesting! I love physics! I think that switching my props to the bottom side would really be beneficial in my case because I carry heavy equipment under and having the props closer to the CG would be more stable. I never considered that. Thanks again!

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Permalink Reply by Dave Keck on July 26, 2011 at 1:00pm

Is there any reason one could not mount two opposing motors pointing downward and the other two upward?  Then all four props could be the same without the need for pusher props.

Dave

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Permalink Reply by Don Cramer on July 26, 2011 at 1:04pm

Yes, props are designed to spin a certain direction, you simply can not reverse the direction the prop spins....

Permalink Reply by swordfishBob on July 26, 2011 at 2:13pm

It's also getting a bit off-topic but Denny's post is very informative.
Take a spinning motor with a prop, and turn it over. You've changed whether it's CW or CCW as viewed from top, and simultaneously reversed the airflow. To get the airflow back, reverse the motor, and.. you're back where you started just with the motor pointing down. So no saving on the props.
Accepting that you'd need 2 types of prop, I think the asymetric tuning would be awkward, and you'd be tied to + config, but at least you'd know which dimension is better tuned!
It would get the centre of thrust closer to the centre of mass though.

Permalink Reply by Rui Manuel Cravo Marques on July 26, 2011 at 2:57pm

as Don said, you cannot simply flip a prop and invert it's rotation. The props are not simetric, they are build to rotate in one way. If you want to use 2 motors counter rotating each other, you'll still need CW and CCW props on both, with the bottom one flipped regarding to the motor (pushing air down).

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