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Electric Motor Effciency (Density / Watt / Losses)

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Patrick

Tue Sept 24 2013, 02:22AM

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

Steve Conner wrote ...

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

yes this is right, but with high inertia acting on the far half of the prop, there needs to be more material for a stronger tip to resist those forces... the inertia doesnt speak to energy, but the consequence of a lot of fiberglass and CF toward the circumference of a 11" prop at 9,000 rpm, certainly does imply battery and motor are using more energy than the opposing case, using a narrow low force tip.

[i]

BigBad wrote ...

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.

i thought about this, but he passed away in 2009... there is a company and some surviving freinds of Pual lipps, so i may ask them for some details, particularly the significance of Reynolds number, and static thrust for micro UAV hovering purposes.

Sulaiman wrote ...

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 ?

well Sulaiman, this idea should be investigated, ill have to add this to the list of possibilities. But Common fans for human purposes dont need energy/aerodynamic effciency (i dont mean the motor's effciency) since they plug in to the wall, so that might be a problem.

Steve Conner

Tue Sept 24 2013, 08:42AM

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

No, a lot of material towards the circumference just means more inertia, and as I said previously, inertia doesn't consume energy, it just stores it.

Patrick

Tue Sept 24 2013, 09:46AM

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

Steve Conner wrote ...

No, a lot of material towards the circumference just means more inertia, and as I said previously, inertia doesn't consume energy, it just stores it.

let me ponder that which you have said.

Ash Small

Tue Sept 24 2013, 10:06AM

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

Steve is correct here. Also, as I mentioned earlier, most of the losses are at the interface of the accelerated air, and the static air around it, that's why slow props are generally more efficient than small, fast ones. The Lipps design has the accelerated air moving slower at the circumferance than nearer the centre. 'Conventional' props are designed to have constant velocity throughout the whole accelerated air mass, that's why the profile changes from the centre to the tip in the way that it does.

The Lipps design will be more efficient because there is less turbulence at the interface between accelerated and static air, because the air at the tips is moving slower relative to the air towards the centre.

It is more efficient because it has less losses.

Dr. Slack

Tue Sept 24 2013, 11:23AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

A propeller is a rotating wing, and then some. Many things that will optimise a wing will also optimise a propeller, and then there are a few other considerations due to the rotation.

The dominant thing to optimise is lift and drag, or rather their ratio. Under the wing is high pressure, above is low pressure. The airflow from high to low pressure around the tip wastes energy, it's called tip drag or induced drag. The many mitigations against it are tip wings or winglets as used in commercial airliners, very long wing as in gliders, or optimum elliptical shaped wings as the WW2 Spitfire. If the wing is a propeller, then mounting it in a duct with a small clearance will reduce the spill of air round the tip. Obviously winglets and ducts increase the surface area wetted by the airflow, so increase the surface (viscosity) drag, resulting in a tradeoff between induced and surface drag.

If you have different constraints, you end up with your optimum looking quite different. Compare Lipps' and a C130 propellor to a more conventional design. Given the fact that the outside of a prop travels faster than the inside, it's possible that Lipps' design is heading in the direction of a more elliptical distribution of lift along the length of the blade, Spitfire style, to minimise overall drag. Eschewing maximum lift in the outer half of the blade radius results in a larger prop for the same thrust, think of the under-used tip as an additional winglet that lies in the plane of the blade. A C130 on the other hand just wants maximum thrust given the aircraft envelope and the blade tips not going supersonic, so they are cut off square, hang the induced drag and fuel consumption. Most props of course fall between these two extremes.

Patrick

Fri Sept 27 2013, 07:35PM

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

sorry all for going dark for so long...

ive made some decisions and have been fabricating parts. your contributions have been useful, and ill get this new machine up and flying. Then ill make some comparisons and improvements after I validate this new tricopter prototype.

So keep the comments coming. And ill post more pics and explanations in about 10-12 hours.

got to go to work...

BigBad

Sat Sept 28 2013, 02:12AM

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

Steve Conner wrote ...

No, a lot of material towards the circumference just means more inertia, and as I said previously, inertia doesn't consume energy, it just stores it.

The inertia itself doesn't consume energy.

But the tip is heavier if/because it has more wing there, and the wing does consume energy in drag. Also, if you have too much lift at the tip you get worse tip vortexing; whereas the idea of the elliptic wing is you reduce the tip lift somewhat and this reduces the vortexing, which reduces losses, and then you move the lift inboard to get the same overall lift (but not too much because then you'd scrub it back off on the fuselage.) In this case though there's very little fuselage.

It would probably make a few percent difference or something; normal propellers are IRC ~77+% and you could push it to ~80+%, maybe.

Andy

Sat Sept 28 2013, 02:33AM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
Location:
Posts: 874

Designing wind turbine blades , the benz limit

describes the jet turbine inlet fans, with multiable(inf) blades that are skinny and don't weight anything. What others have said with the wing tips tapped to remove votrex will have the highest efficiency but need more torque from the motor.

Patrick

Wed Oct 02 2013, 12:39AM

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

sorry i dropped off there, have had internet connection problems, and work...

the betz rule is profound ! i knew there must have been something formally known along this line, but didnt know what it was called.

ive got more components made, and will soon be ready for non-flight testing...let me post some pics.

tricopter

Patrick

Wed Oct 09 2013, 06:08PM

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

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?.

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