Which is the most efficient

[Erfolg]

The other day I was discussing the merits of motor types with a club member.

Removing the ins and outs of the conversation and stating the general issues raised.

I personaly like outrunners and consider them more efficient than the inrunner type. For me the disadvantage is the rather blunt nose when used in a glider. My friends contention is that the inrunner is better when used with a gearbox to swing a big prop.

My own general view was and is, a low Kv outrunner can swing a large prop and does not need a gearbox. From my general experience a gearbox consumes a lot of power. It then becomes a balancing act of the increased efficiency of the large diameter prop against the loss from the gearbox.

I have noted that very high efficencies are claimed for some epicyclic boxes ( I have one and it gets very hot suggesting it is not that efficient), although some gearboxes will consume 50% of the energy.Yet the inrunner could benefit from swinging a very large diameter propeller, which may be greater tham can be swung by an outrunner

I am still undecided, which is the better set up when supplied with the same watt input. Does anybody definatively know. Rather than my own vague postulations?

Erfolg

PS my outrunner set ups all seem to be better than my inrunners, with or without gearboxes(inruuners).

[Tim Mackey]

Rather than my own vague postulations?

Erfolg

My similarly vague postulations agree with yours ! never did like gearboxes, noisy energy sapping PPOfs.

One thing that is a fact re in versus out - inrunners do tend to run cooler like for like against their outrunning counterparts. Inrunners have their windings on the outside in contact with the motor case and therefore conduct heat away better than the outrunner, which has its windings on the stator which is the non revolving innerds of the machine, with only the magnets whirling around in the airflow.

[Ed Kershaw]

I usually go straight to the manufacturer's data for stuff like this.  Have a look at the Mega motors website www.megamotor.cz  They have all the data, and efficiency curves to keep you amused well into the wee small hours.

From their data, inrunners themselves seem quite capable of high eighties, early nineties efficiency. But as you say, the gearbox is the big unknown, although you could estimate this by measuring how hot it gets.  The energy absorbed by the gearbox must turn directly into heat.  Weigh the gearbox and do a quick calc based on the specific heat capacity of the gearbox material versus the increase in temperature and the time it take to get there.  There will be a few unknowns and assumptions, like the heat lost to convection etc, but it should be a reasonable indication.

[Former Member]

[This posting has been removed]

[Steve Hargreaves - Moderator]

Eric I think you're exactly right....rather than which is most efficient the crux of the matter is what you want the motor to do as power is produced in different ways. I came up with an analogy in another post recently which I will repeat here....Timbo has already seen it (& even liked it!!) So apologies for repeating myself here Timbo.....

In my garage sit two vehicles.....a Kawasaki ZX9R motorbike & a VW Passat 2.0 TDI. both engines produce about 140bhp....the bike does it at 12,000 rpm,(a high "kv" or inrunner motor) the car at about 4,000 rpm (a low "kv" Outrunner motor). The bike is a lot faster than the car but it won't tow a caravan (not that I have such a thing I hasten to add!!!) Imagine you swapped the motors over.....each vehicle still has the same power but the diesel bike would be rubbish to ride & I'm not sure the Kawasaki engine would even move the car!!!!

In other words choose the right motor for the sort of aeroplane you want it to fly......

[Erfolg]

Steve

You are probably right.

Inrunners are probably best suited to EDF where high revs on small fans are wanted and very fast flying models where small propellers and coarse pitch are being used (inefficiently). The use of a gearbox is an attempt to extend the range of usefulness, by turning a bigger propeller at slower speeds.

The outrunner seems to generally have the advantage in not needing a gearbox, to swing a larger propeller in the useful range for sports, scale, aerobatic types or even glider. Yet is generally not the obvious choice for EDF even though high KV motors are available

I do think that efficiency does cover this concept, in a loose way. In that typically both inrunners and outrunners claim the circa 90 percent efficiency, yet it is the total package that matters, incl.  propeller etc.

Yet both types can be used to achieve similar performance, I guess just as with full size Radials and Inline aircraft engines, the best solution is not  obvious.

I think the main reason your Passet will not achieve the same speed as your Kawasaki, is that of cross section area and possibly weight. BMW used to make a 4 cylinder inline motor cycle  that had much in common with car type concepts, high torque, lowish revs. Yet in a straight line pretty fast, although not as fast as Quackers and CBRs and Fireblades etc. I think you are re-enforcing the concept that the overall package matters and you can do lots to the output of the motor to optimise the performance for a duty.

For me KISS is best, which pushes me towards outrunners. But as I started, I am less than certain.  

Erfolg

[Mal Quelch]

Erfolg - Hi!

A picture tells a thousand words and this example shows that a multi pole/magnet outrunner is effectively 'electronically' geared and explains why they are able to turn large propellers.

Inrunners have far fewer poles/magnets and this effect does not occur hence the need for an external less efficient gearbox to provide the same effect .      outrunner gearing   Scroll down to the animation - although the whole article is very interesting if you are into this sort of thing!

I know its not strictly about comparable efficiencies but I'm sure you will see the connection and relevance.

There is also a Table available that shows different gear 'ratios' for different pole and magnet counts if you are interested. reductions from 1 to 20 depending on configuration used.  ratios

 Hope my links work ( first time) and that you find them interesting !

 Mal   

[Erfolg]

Mal

I have successfully opened the links, they are both interesting and informative.

Although it does seem you not only need to know the magnet tooth numbers, but also the wind type, as it has a significant effect on the gearing effect. Which obviously knew, but is a surprise to me.

Thanks.

Your information confirms my prejudice, in that I had found the performance of my inrunners disappointing compared to my outrunners  (although  inrunners are more expensive).

It seems unless i desperatley need that needle like nose or am modelling a DF, outrunners are generally going to be a better type.

Erfolg 

[Tim Mackey]

Sevral EDFs are now deploying high Kv outrunners very successfully