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Electric Motor Effciency (Density / Watt / Losses)

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Steve Conner

Thu Sept 19 2013, 10:00AM

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

Patrick, I think you are starting to understand what real engineering is all about.

Any fool (even a computer) can blindly optimise one variable, but a good engineer weighs up all the variables to make a system that's best suited to its intended purpose. Often there are too many variables to optimise globally, so you have to resort to intuition, experiments, and so on.

You might want to read some of the literature on human-powered flight. You have a very heavy and low-powered engine so the craft needs to be extremely efficient to get off the ground. The recent Sikorsky prize winning helicopter used four rotors that were very big and slow turning indeed.

Patrick

Thu Sept 19 2013, 09:20PM

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

Yeah Steve, I realized optimizing multi variables was a real chore when I studied the NASA helios back in 1995 I think at 15? But anyway, its also a problem with scale of the prop, props of less than 14" are in the low Reynolds number region, so the prop shape cant just be a 10 foot turbo fan prop scaled down to 8 inches...

Ive just built a circular FG and CF straight duct, so ill post pics when I get back from class...

EIDT: On previous occasions ive tried to optimize many variable systems, energy conversion like from sofc fuel cells and the whole transformer design is also a case too...

pics

Patrick

Sat Sept 21 2013, 07:34AM

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

ive found a radical propeller design, its shocking in its difference from the traditional shapes...

look here :

the elipse propeller, the tips are very different, so thin and narrow, with that low mass out on a far radius, less power would be used to turn the prop. hes also getting more thrust out of the root-spinner part of the prop, normally the inner part of the prop is considered to contribute little or no thrust. The AR Drone 1.0 also has the inner pitch/camber steeper on the inner radius.

BigBad

Sun Sept 22 2013, 03:07PM

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

Those kinds of tricks- that bring the thrust inboard will work best if the central motor is quite compact and streamline- I would expect it to work relatively well for electric propulsion.

On the other the Reynolds* number is very different, so it's not guaranteed.

*-fixed

Patrick

Sun Sept 22 2013, 08:43PM

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

(BigBad, did you mean Reynolds number?)

Yeah, its just the opposite of what Im used to seeing. usually 80% of the thrust comes from the outer half radius, what Paul Lipps has done is make the thrust high near the root, and tend towards extinction and the tip. This means strength and mass does'nt have to be far out on the rotating circumference. I just think about the Helios wing drone... it had a larger contribution of the inner radius too.

Paul Lipps was clearly on to a unique implementation for a prop, but i think he draws some negative conclusions on conventional props, not really justified. Big aerospace corps and government agencies spend a whole lot of time effort and money on CFD to predict the flow and forces for modern props. Lipps used some advanced math and reasoning, but not fluid flow, to arrive at his improved prop. So, there maybe hidden problems or special cases at take off or landing, near static thrust where a 400,000lb cargo plane could'nt use his design...

And im not suggesting that big gov/corps get things right becuase there so dam high and mighty, there are cases where individuals have made advances that the establishment missed or dismissed as useless.

BigBad

Sun Sept 22 2013, 11:11PM

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

What he seems to be doing is optimising the prop to the aircraft. The standard prop seems pretty good, but you can get better performance/efficiency in many cases.

If you can get a suitable prop fabricated I would expect this would work well for you, an elliptical wing design is likely to work well with the smooth aerodynamics of an electric motor compared to a relatively messy aircraft aerodynamics; I'm pretty sure you don't really want the tips giving you most of the lift.

Patrick

Sun Sept 22 2013, 11:51PM

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

BigBad wrote ...

I'm pretty sure you don't really want the tips giving you most of the lift.

yeah I think so too, the slow-fly APC props are specifically meant to be used with electric only, (low RPM compared to Nitro insane rpm) so their thin optimized prop is easy to drive but thin and flexible. You can here them all the sudden go too high in rpm and they make a whap-whap-whap sound when the tips curl up from high thrust force. So avoiding tip loading might give a stronger stiffer prop with out the high inertia of a long heavy tip...

Steve Conner

Mon Sept 23 2013, 02:57PM

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

Remember that inertia doesn't actually waste power. It just stores energy, making the prop slower to accelerate and decelerate.

BigBad

Mon Sept 23 2013, 04:07PM

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

Yup.

If you could work out what shape you need, maybe you could 3D print the right aerofoil. If you ask the guy really nicely, maybe he'd run his program for you.

Sulaiman

Mon Sept 23 2013, 08:16PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Prop. designs for commercial aeroplanes are for very high speed air
prop. designs for commercial helicopters large volumes of fast air

Would the fan shape and speed of a 'stand fan'

,or.&bvm=bv.52434380,d.bGE,pv.xjs.s.en_US.RJfod4sw qLE.O&biw=1024&bih=664&dpr=1&wrapid=tlif1379967034 14011&um=1&ie=UTF- 8&hl=en&tbm=isch&source=og&sa=N&tab=wi&ei=G6BAUo7g N82UhQfZmIDoDw
be suitable for a quad-copter ?

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