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Electric Motor Effciency (Density / Watt / Losses)

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Ash Small

Wed Oct 09 2013, 06:13PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Patrick wrote ...

ive been thinking of Steve Connors point on inertia.

let me ask, will it take more energy to accelerate/decelerate a heavy prop Vs light prop? of the same type and of the same shape in the same time interval?.

I think, while it WILL take more energy to accelerate a heavier (presumably larger) prop, a larger prop will provide more lift at lower RPM.

Patrick

Wed Oct 09 2013, 09:32PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Ash Small wrote ...

I think, while it WILL take more energy to accelerate a heavier (presumably larger) prop, a larger prop will provide more lift at lower RPM.

but i mean a heavy prop vs light prop all else being exactly the same, a multirotor must constantly modulate the prop RPM (and quickly), a normal heli or fixed wing does not.

BigBad

Thu Oct 10 2013, 01:18AM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

Often it can be worthwhile wacking it all in one spreadsheet, like one design per row or section of the spreadsheet and filling in plausible equations for things like weights, powers, batteries etc. And just doing a comparison like that.

Steve Conner

Thu Oct 10 2013, 07:28AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Yes, of course it will. If you plan to be changing the RPM of the prop quickly and often, then you'll want one with a low moment of inertia.

That's not quite the same thing as "light". Moment of inertia is proportional to mass times the square of the "radius of gyration". The same mass out at the blade tips will give much more inertia than if it were in at the blade roots.

On the other hand, putting more material out there might let you make the same thrust at a lower RPM.

Patrick

Fri Oct 11 2013, 01:12AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Steve Conner wrote ...

Yes, of course it will. If you plan to be changing the RPM of the prop quickly and often, then you'll want one with a low moment of inertia.

That's not quite the same thing as "light". Moment of inertia is proportional to mass times the square of the "radius of gyration". The same mass out at the blade tips will give much more inertia than if it were in at the blade roots.

On the other hand, putting more material out there might let you make the same thrust at a lower RPM.

i suspect this trade off must be formally studied, along with low Reynolds number props, for us multirotorists to know where to draw the line.

5 days till first flight of the most advanced drone I've ever built....

EDIT, after much thought:

Steve Conner wrote ...

Yes, of course it will. If you plan to be changing the RPM of the prop quickly and often, then you'll want one with a low moment of inertia.

This describes the life of a multirotor fan (and I mean ESC, Prop, Motor, duct) perfectly... contrary to a fixed wing prop(s) or heli main rotor disc.

Patrick

Sat Oct 12 2013, 05:48PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

i have the whole tail tilter assembled and working. ill be attaching the forward fans today and then the airframe will be complete. Tomarrow, ill attach all electronics.

pics

tail fan, and tilter.

The main body, and what it will look like when fully ready.

Tilter mechanics.

Patrick

Wed Oct 16 2013, 03:42PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

not quite finished, but ive had some technical issues. should be ready to fly in 2 more days...

BigBad

Wed Oct 16 2013, 09:04PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

Just had a thought.

Isn't the performance-optimum design for this kind of aircraft going to be a large main rotor giving all the lift, maybe a tail rotor to stop it spinning and something to control the attitude; like small puffer jets or small rotors?

Dividing up the lift into multiple rotors is highly likely to be a net loss.

In other words, more like a standard helicopter. They are presumably designed for efficiency, after all.

Chris Cristini

Wed Oct 16 2013, 09:19PM

Registered Member #1749 Joined: Fri Oct 10 2008, 02:04AM
Location: Claremont New Hampshire
Posts: 497

I don't see how a helicopter is any more efficient with the given size one large motor VS three motors 1/3rd of the size also the idea behind a multirotor is being more agile it is a little more complicated but well worth it. Also his tricopter could survive a crash well before a helicopter.

BigBad

Wed Oct 16 2013, 11:24PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

Technically it doesn't have to be one big rotor, but you want the biggest total rotor area you can because that reduces the disc loading.

In other words, the larger the rotor area, the slower the air is being thrown downwards to keep the vehicle up, and slow air improves the energy efficiency because that's wasted kinetic energy that comes out of the battery.

One big rotor that just touches the bounding box that you're designing for gives you very large rotor area.

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