8 replies to this topic

[nitromethane]

Hi

Currently I'm in a process of designing and building a Y6 copter.

While looking for motors, I started wandering how efficient are "one on top of the other" motor configurations.

Let's say one motor outputs 2kg of thrust, so in prfect world on a Y6 that would mean 4kg of thrust per arm, but there's no such thing as prfect world, right ?

Question: What kind of thrust should I expect ?

[jes1111]

Depends on a lot of factors - can really only be determined by experimentation. The reason the bottom prop may produce less thrust than expected is to do with 1) the speed of the airflow from the top prop and 2) the spiral/vortex of that air. A partial remedy for the first problem is to fit a higher pitch blade on the bottom - this will allow the bottom prop a better "bite" on the fast-moving airflow. Rule-of-thumb is about 20% more pitch, e.g. 14x4 + 14x5. Cause number 2 is a bit of a unknown and will vary with prop design, pitch, rpm, arm shape/size, motor spacing, etc. It's unlikely that you'll ever get precisely equal thrust from top and bottom but 1) IMO the "inefficiency" is less than many people might quote and 2) the controller will cope just fine.

I have no mathematics to back up the claim, but I reckon the losses can be "balanced out" by the potentially lower weight of a Y6/X8 (versus an equivalent flat hex/octo) and correct choice of props.

[CrispyCargo]

agreed, it does depend on a lot of factors, but what makes sense to me is if you have full power going to each motor, then they should be producing very similar numbers even though the lower motor might have an easier job making that number.

I used to have a Y6 and loved it! I was using RCTimer 2830 750kV motors with APC 10x4.7 props on 4S and the thing was a monster! It flew very well. I'm actually going to have another crack at it and build it again. For me it was by far the best copter for descending into it's own prop-wash (and this was before flashed ESCs).

I'd say give it a go. You don't need to go too crazy with props just yet, just build it and fly it! Get some stick time in and THEN tweak the heck out of it. (my opinion btw, not gospel)

Good luck and have fun.

[Jake Bullit]

Well I can lift a bear with mine:



Torxpower 2216, 4s ,and graupner e prop 11X5.

[nitromethane]

Thanks guys, good info to know

I didn't realy know what exsactly to expect, but as I understand there's no dramatic diference, so there should be no problem.
I'll probably try higher pitch prop on the lower motors.

BTW: this is what I'm working on:
 2012-06-09 18.52.04.jpg   83.17K   34 downloads

[Jake Bullit]

Keep us up to date

[nitromethane]

I'll do a build log

[CrispyCargo]

BTW: this is what I'm working on:
 2012-06-09 18.52.04.jpg   83.17K   34 downloads

Looks like it can take a beating! Good work.

[comster]

I remember kevinv033 used a custom throttle curve where the lower motors always spun faster than the upper ones by a certain ratio. I think he was happy with the results, but I don't know how much he experimented or tuned the ratio.