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Old Mar 07, 2011, 03:21 PM
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Question
Difference between an inrunner and outrunner motor ?

What is the difference in performance between two similar powerd inrunner and outrunner motors ? For a .36 size 3D plane what would be better ?
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Old Mar 07, 2011, 05:00 PM
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Very little difference in performance between an outrunner and a suitably geared inrunner. Often it depends on which is easier to mount.

Electric motors are generally more efficient at high rpm (40,000rpm+)... props are most efficient at low rpm...so a geared inrunner does the trick.

For a .36 size plane you are looking at 720W or so.
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Old Mar 08, 2011, 09:01 AM
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Ok cool thanx for your comment. I have been looking at mostly outrunner motors but if an inrunner will do the job just as well I gues I can take a look at those too.
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Old Mar 08, 2011, 06:49 PM
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Well, I personally wouldn't say that a brushless inrunner will do the job just as well as a brushless outrunner. The inrunner uses a gearbox (as has been mentioned) but remember that a gearbox adds friction thus robbing a little power, and adds weight to the airplane. It is also a source of potential mechanical problems due to extra moving parts, as well as extra lube and maintenance.

If I had a choice as to which to use, I would always pick the outrunner. Direct drive to the prop (all outrunners that I have, the prop bolts directly onto the output shaft of the motor) so you bypass the extra friction of the gearbox which is simply slowing down the higher RPM inrunner, and converting the RPMs to torque, which the slower turning outrunner already has. Fewer parts, easier install, less maintenance, and quieter.

If the airplane you've picked can take either, I would go with the outrunner, but if the airplane specifically calls for an inrunner with a gearbox, you might want to follow the directions unless you have modeling experience and could do the conversion yourself.

My two
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Old Mar 08, 2011, 07:55 PM
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In general, many people will argue that inrunners are better (me included) however, when it come to sport flying and bang for the buck (dollar that is!) it is very hard to due better than an outrunner. There are many manufactures that offer decent performing outrunners for very low prices! Just run some simulations on the various motor calculation software programs available and you should be fine.

You may also think about getting some extra tools like a rpm meter, amp or watts up meter, battery checker aka cell checker / power left - meter, so on and so forth!

Nothing like being able to check the preformance of your system prior to getting into the air!

Michael
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Old Mar 08, 2011, 08:50 PM
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Quote:
Originally Posted by elecfryer View Post
In general, many people will argue that inrunners are better (me included) ...
Michael
Curious as to your reasons for thinking that inrunners are better than outrunners. I gave my reasons for thinking that outrunners are better in my previous post, and I'm wondering what your thinking is.
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Old Mar 08, 2011, 09:25 PM
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This is my logic:

Electric motors are generally more efficient at high rpm (40,000rpm+)... props are most efficient at low rpm.

If you want to turn a large prop, a low Kv outrunner has the inherent disadvantage of lots of windings and high resistance... a geared inrunner is the way to go.

Especially for high-powered units (720W in this case) ... weight is somewhat less of an issue...and a geared inrunner WILL win the efficiency battle.
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Old Mar 09, 2011, 12:34 AM
The 6 P principle works for me
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Quote:
Originally Posted by SandHog View Post
Curious as to your reasons for thinking that inrunners are better than outrunners. I gave my reasons for thinking that outrunners are better in my previous post, and I'm wondering what your thinking is.

I apoligize for any confusion however, how does one define better? Certainly you listed many reasons why you think out is better.

1: are all your reasons all valid?
2: you gave one side of a debate, is the debate now over?
3. you listed generalities and no specifics. Here is a specific, I can list an inrunner with a gearbox that with the same esc and battery turns the same prop at a higher rpm than an outrunner and yet, the inrunner with a gearbox weighs less than the outrunner. If true then you may agree that the inrunner with a gearbox is a better motor however, did I mention that the inrunner with a gearbox costs over twice what the outrunner does?
4. I did NOT state that all in's are better than all out's!
5. To repeat, for many and perhaps most applications outrunners will do the job and cost less than inrunners.
6. If you want to disagree, please talk to all the people that use Nue inrunners in their limited run time hi performance models instead of out runners.
7. Sometimes it is not what is better but what works better for a specific application. As we are discussing mechanical applications, there will always be pluses and minuses.

FYI, I build Delta's that go over 100mph and some use in's and some use out's. It comes down to hoped for performance and dollars willing to be spent as they all work!

P.S. can you speak "kt" aka torque constant? Interesting reading, IMO!

Michael (if your not fryin, your not tryin!)
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Old Mar 09, 2011, 07:06 AM
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Hi

The difference between an outrunner and a geared inrunner assuming equal Kv and power is w.r.t efficiency marginal.
The innrunner has due to the more higher Kv less R. The efficiency is slightly better compared to a similar outrunner.
The gearbox needed for the inrunner has an efficiency of about 96% (a good gearbox). So the advantage of a geared inrunner
is only slightly better.

The difference betwenn inrunner/outrunner is more a difference between motor sizing and price. The diameter
of a outrunner is larger, but length is shorter compared with a similar geared inrunner.

I'm flying gliders with narrow fuselages. In that case an outrunner normaly will not fit. So I'm using inrunner with
either a maxon 4.4:1 gearbox or Kontronik inrunner with gearbox. The next important fact is the Kv of the drive.
Flying powered gliders needs an other setup as motor planes. Gliders usually needs larger props with less pitch compared to
motor modells. Even with outrunners it is hard to find one with low Kv suitable for my gliders. So for powered gliders with narrow fuselage
an inrunner with gearbox is the better choice.

Sure the price for a good gearbox is high, so I use for my
motor modells normally an outrunner, if motor diameter is not an issue.

Best regards
Micha
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Old Mar 09, 2011, 09:09 AM
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Micha and I are of a like mind! Horses for courses.
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Old Mar 09, 2011, 10:35 AM
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I think I am probably the only guy with a geared outrunner in a narrow fuse sailplane. I had the motor and it just needed a home.

I just like to be diffierent.

Now who wants to argue that geared outrunners are better than geared inruners are better than outrunners, dogs are better than cats, or that blue is better than red?

The best motor out there is the one I am flying at the moment.
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Old Mar 09, 2011, 07:17 PM
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Quote:
Originally Posted by Dr Kiwi View Post
This is my logic:

Electric motors are generally more efficient at high rpm (40,000rpm+)... props are most efficient at low rpm.

If you want to turn a large prop, a low Kv outrunner has the inherent disadvantage of lots of windings and high resistance... a geared inrunner is the way to go.

Especially for high-powered units (720W in this case) ... weight is somewhat less of an issue...and a geared inrunner WILL win the efficiency battle.
Dr; as Elecfryer pointed out to me (correctly) you cannot speak in generalities. Stating that "Electric motors are generally more efficient at high rpm (40,000rpm+)" is a glaring generality. What is the Kv of the motor(s) in question?

40,000 RPM / 11.1V (3s) = 3603Kv motor (inrunner speeds)
40,000 RPM / 14.8V (4s) = 2702Kv motor (inrunner speeds)

Inrunners -in general- are built for high RPM and are thus more efficient at higher speeds

Outrunners -in general- are built for more torque and are thus more efficient at low speeds.

Semi truck diesel VS Import Tuner Car engine: which is more "efficient"??

To answer that, you need to ask what each is going to be used for. If racing, I'll take the tuner engine; if hauling 48,000 pounds plus the weight of the truck itself = 80,000 pounds total, I'll take the semi truck diesel. Now I'm not saying that outrunners are more efficient in terms of power consumption, but elecfryer is right: you need to whole picture.

And has been pointed out, price is a major concern as well.

I would have to say that both sides are right, and both sides are wrong. I'm not made of money, so I have to stick with outrunners. If I inherit a gold mine, I will take a serious look at inrunners with high-efficiency gearboxes.
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Old Mar 09, 2011, 07:36 PM
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Quote:
Originally Posted by SandHog View Post
I'm not made of money, so I have to stick with outrunners. If I inherit a gold mine, I will take a serious look at inrunners with high-efficiency gearboxes.
Heck, I'm not made of money either; which is exactly why I prefer an inrunner with a few different cheap GWS gearboxes of varying ratios.

For me, optimizing the weight of the plane is a major concern.... so if I can use a lightweight inrunner with the proper gearing to produce the same thrust as a MUCH heavier outrunner, using fewer amps from my battery, then I'm ahead of the game.
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Old Mar 09, 2011, 10:16 PM
The 6 P principle works for me
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Ventura CA
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ah.....having many options allows us many choices and dare I say it......have our cake and eat it tooooo!!!

I love this hobby!

either the above or paraphrase Sinatra and say, I did it my way?

Michael
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Old Mar 10, 2011, 11:58 AM
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Quote:
Originally Posted by crypto666 View Post
I think I am probably the only guy with a geared outrunner in a narrow fuse sailplane. I had the motor and it just needed a home.

I just like to be diffierent.

Now who wants to argue that geared outrunners are better than geared inruners are better than outrunners, dogs are better than cats, or that blue is better than red?

The best motor out there is the one I am flying at the moment.
Hi crypto666

You are not alone. I'm using a AXI 2217D outrunner with a multiplex gearbox 3.3:1 in a 2.5m wingspan glider. I have bought this
Axi together with the AXI PG3 gearbox. But the AXI gearbox PG3 has had only a lifetime of about 10 minutes runtime.
Inspection shows very very small gears. Not able to deliver the proposed 350W.

The setup of the drive depends on several parameters. There is no contradiction to use an inrunner/outrunner with a gearbox. Best fit is to have a drive chain
with best efficiency w.r.t to plane weight, wing aera, motor efficiency and flight speed. So different setups are possible.

It is always a trade off between money, availability, size constraints and efficiency.

Best regards
Micha
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