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Old Jul 11, 2016, 01:44 AM
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Help!
Inrunner vs Outrunner Motors

Hi guys, I need some help regarding inrunner and outrunner motors.
We are building a drone similar to the germany E-Volocopter which can carry a human. But right now we are just building a prototype first which is about 40 Kg for the whole prototype.
We have been searching for various motors for weeks, we have came across an outstanding performance outrunner motor: U11 Tiger series motor which can provide the thrust we need. I understand that there have been many threads regarding the 2 different types of motor but I haven't came across one which talks about the thrust.
I have realised that higher end or better thrust providing outrunner motors are relatively expensive and inrunner motor tends to be cheaper but they don't really provide the chart for thrust etc.
My question is in terms of thrust, which one (inrunner or outrunner) is better?
The U11 Motor cost about 300 USD and provides around 7Kg of thrust using 29 inch prop.
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Old Jul 11, 2016, 02:57 AM
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Generally speaking, outrunners provide more torque than similar-sized inrunners. They do that because their magnets are mounted on the rotating outer casing, so are further away from the shaft and, hence, have greater "leverage" (so more torque).

That said, by checking specs carefully, as you're obviously doing, you might find an inrunner that's better for your application.
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Old Jul 11, 2016, 03:16 AM
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Hi abenn, thank you for your reply.
I understand generally outrunner have lower efficiency and higher torque. Inrunner on the other hand has higher rpm and better efficiency, however does higher rpm have higher thrust? Because most inrunner provides only Kv and Rpm in their data and not thrust.
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Old Jul 11, 2016, 03:32 AM
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Hi,
what rules are the longer leverarm , the bigger posible mass of magnets that can be installed ,
possible higher pole count that reduce iron mass . You have to look for the torque to size ratio if you want to turn the propeller without a gear . If you look around you will find the answer or do you know a helicopter / multicopter with inrunner without a gear ?

Happy Amps Christian
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Old Jul 11, 2016, 04:02 AM
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Hi lucas,
its my first time with inrunner, all the while since my senior's time we have been using outrunner for our multicopter. Thus I am not quite well versed on inrunner.
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Old Jul 11, 2016, 07:33 AM
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Guys, do you have any idea how to determine the thrust provided for inrunner motors?
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Old Jul 11, 2016, 07:48 AM
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Quote:
Originally Posted by 1405255 View Post
Hi guys, I need some help regarding inrunner and outrunner motors.
We are building a drone similar to the germany E-Volocopter which can carry a human. But right now we are just building a prototype first which is about 40 Kg for the whole prototype.
We have been searching for various motors for weeks, we have came across an outstanding performance outrunner motor: U11 Tiger series motor which can provide the thrust we need. I understand that there have been many threads regarding the 2 different types of motor but I haven't came across one which talks about the thrust.
I have realised that higher end or better thrust providing outrunner motors are relatively expensive and inrunner motor tends to be cheaper but they don't really provide the chart for thrust etc.
My question is in terms of thrust, which one (inrunner or outrunner) is better?
The U11 Motor cost about 300 USD and provides around 7Kg of thrust using 29 inch prop.
Motors do not provide any thrust. Propellers do provide thrust. I think you want to focus on the props. And as you do that I think your focus should be on the input power (in Watts) at a given (and practical) RPM and the resulting thrust you get.

The thrust from a propeller, at a given RPM, will be about the same with any motor that can get it to that same RPM. And the input power will be about the same too unless the battery voltage and the Kv of the motor are badly mismatched to the prop.

To determine the Kv you want to know the max RPM you want to attain and the battery voltage. For a prop to attain 5000 RPM on 16.4V (4S LiPO) you would want to have a Kv of about 305 or so because 5000 / 16.4 = 305. To allow for losses in the power system and falling battery voltage I would raise that to 325 or maybe even 350.

Typical quality outrunners will typically be running at about 75% of the no load RPM (no load RPM = Kv x V using the charged and rested pack voltage) at their continuous rating and a bout 90% of the no load RPM at full throttle.

The continuous rating is the throttle setting that gives you a safe (within max limits) and stabilized (no longer rising) temperature in the windings. The full throttle rating will normally be time limited and have a rising temperature that could eventually damage the motor to the point of failure.

We have a rule of thumb that typical quality outrunners will handle an input power of 3W per gram of motor weight at their continuous rating and outrunners do a little better, maybe 4W or 4.5W per gram. This helps us in choosing a motor (by weight) that will be likely to meet the demands of the prop we want to use.

As far as how much power your prop needs you will have to rely on performance data from the prop makers or from static testing. Choosing a prop is not a trivial thing. It is really the starting point for designing a power system. And after you have one and put it in flight, you may find that you missed the mark on the prop you thought you needed and have to use a bigger or smaller prop. And that can change the validity of the chosen motor weight and Kv. But you will usually have some room for experimenting up and down a little on the prop's diameter and pitch. And you may even be able to experiment with the battery voltage too as that will change the no load RPM and bring the prop size and pitch back into consideration for further changes.

By coincidence I just posted a reply about using and the usefulness of some published prop performance data in another thread here. That dealt primary with the widely used props from the APC Propeller company. That post may be of interest to you also:

Making Use of the APC Prop Performance Data - http://www.rcgroups.com/forums/showp...2&postcount=78

You can enter that thread from that post (click on the yellow link in the top right) and it may be of further interest as it alludes to the performance characteristics of the "pancake style" (i.e., motors with stators that are higher in diameter and thinner) motors.

The pancake style motors are believed have improved torque characteristics over outrunners of typical diameter and length because of the increase in leverage that results from the larger diameter. And I think that also contributes to efficiencey. And further, the pancake style seems to be a good thing for you project as the pancake motors would have a lower and more compact mounting and probably contribute less to drage in forward flight.

Jack
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Old Jul 11, 2016, 04:07 PM
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Quote:
Originally Posted by 1405255 View Post
... I understand that there have been many threads regarding the 2 different types of motor but I haven't came across one which talks about the thrust. ...
.... Guys, do you have any idea how to determine the thrust provided for inrunner motors? ...
Because motors don't produce thrust, props produce thrust. (edit: jack beat me to it)

Figure out what prop you want turning at what rpm. That will give you power and torque. Once you know those numbers, you can select the motor.
Inrunners are just outrunners turned inside out (fifteen years ago I would have written that the other way round), that's all there is to it.
If both types have the same Kv and the same max.power, max.current, they 'are' the same from user point of view.

Vriendelijke groeten Ron
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Old Jul 11, 2016, 07:10 PM
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Thank you all.
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Old Jul 11, 2016, 07:44 PM
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Yes Jack, thank you. I have been told of 200Watts per Kg and all these but it doesn't really match. 200W per Kg is too high and your 4.5W per g is too low. I understand that there are props and other factors involved. Would it be better if I have given you guys the specs of the inrunner and outrunner motor?

Outrunner: U11 Tiger Motor (The grey boxed chart image below)
We are looking at 12S 29*9.5 CF Prop which provides around 10 Kg of thrust at 100% throttle and 1531Watts.

Inrunner: Electrifly GPMG 5290 (The white boxed chart image below)
There is a chart for these inrunner motors but no thrust is provided. We are looking at GPMG 5290, the second last motor.

Do you think that this inrunner can provide at least 5Kg of thrust? with the prop size mentioned of 10 inch.
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Old Jul 11, 2016, 09:58 PM
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Quote:
Originally Posted by 1405255 View Post
Yes Jack, thank you. I have been told of 200Watts per Kg and all these but it doesn't really match. 200W per Kg is too high and your 4.5W per g is too low. I understand that there are props and other factors involved. Would it be better if I have given you guys the specs of the inrunner and outrunner motor?

Outrunner: U11 Tiger Motor (The grey boxed chart image below)
We are looking at 12S 29*9.5 CF Prop which provides around 10 Kg of thrust at 100% throttle and 1531Watts.

Inrunner: Electrifly GPMG 5290 (The white boxed chart image below)
There is a chart for these inrunner motors but no thrust is provided. We are looking at GPMG 5290, the second last motor.

Do you think that this inrunner can provide at least 5Kg of thrust? with the prop size mentioned of 10 inch.
To answer the question about the inrunner first the anwer would be no, I do not think it will work. I'll explain that.

I see this in the specs for the GPMG5250:

2300 Kv
243g

10X7E 13X8E
14.8-18.5V
continuous rating 60A 1110W
peak rating 100A 1850W

One of the first things that catches my eye is the 14.8-18.5V voltage spec. That must be for NiMH cells or something that is not used much any more. Either that or they are the 3.7V nominal voltages for 4S and 5S LiPO packs. So the specs are either very old or idiotically phrased theoretical values.

No one launches a charged and rested LiPO pack at less than 4.1V and at 3.7V under load the pack's capacity is probably fully depleted and it is time to land.

But to continue the answer, if you started with a charged and rested 4S LiPO pack at 16.4V (4.1V per cell) the no load RPM with the 2300 Kv inrunner would be:

2300 x 16.4 = 37,770 RPM

And there are not any 10" props that you can run at that high an RPM nor any that would even work at that high an RPM. On a multirotor (with a very low advance speed) they would be badly stalled and I would be surprised if they did not fail.

As an example the APC 10 x 7 Thin Electric has a max RPM recommendation of 145,000 / 10 or 14,500 RPM. Some of the CF 10" prop will handle more RPM than the APC props but again, they would be badly stalled at 37,770 RPM.

So again, I think those specs are very unbelievable and maybe even dangerously so.

But the inrunner clearly will not work with that high a Kv rating.

As far as the input power ratings, the 1110W puts the 243g inrunner at 4.6W per gram continuous and 1850W puts it at 7.6W per gram peak. Those values are probably credible enough and the motor would survive that I think. .

If you look at the performance data for an APC 10 x 7 Thin Electric prop at 15,000 RPM it has an input power of 3.754 HP or 2799W. And that is way over the motor's1850W max rating. It would be a death sentence for the motor at 11.5W per gram on a 243g motor.

APC show performance data for RPM readings that are higher than their suggested maximum RPM limit of 145,000 / diameter in inches. They show up to 24,000 for the 10 x 7 and that is a theoretical calculation. But it shows an input power of almost 8000W for that prop at 24,000 RPM. The reality is that no one uses those APC props at their limits simply because they are both inefficient and grossly power hungry at their limits.

The U11 motor, with it's 90 Kv and has a better chance of working for you. How many motor will your drone have? And what weight (with battery and pilot) do you hope to launch at?

Jack
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Old Jul 11, 2016, 10:15 PM
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Getting 5kg of thrust from a 10x6 prop will require about 17700 rpm and absorb about 1900W at the prop. Motor input power will of course be higher dependent on motor efficiency. That power is above the realistic limits of the motor, and at
the low voltage required to get ~18k rpm on a 2300 rpm/V motor, the current will be prohibitively high.

For your high static thrust low pitch speed need, large diameter props at low rpm will have much better power specific thrust. That will lead you to a low effective Kv, most likely an outrunner such as you've found. Geared inrunners might be feasible but would likely need a lower voltage higher current setup to turn the more optimum props.

Re motors not having published thrust data - you can get ballpark estimations by using eCalc

Look at the Neu motors website for possibly the widest range of high power handling inrunners.

But remember, it's not the motor format (in or out) or shape (long skinny, short fat) that determines torque. In the end it's Kv and ability to handle current. Format and shape contribute to Kv, but torque is purely a function of the inverse of Kv and current.
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Old Jul 12, 2016, 02:03 AM
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Our Drone is 40Kg for the whole structure.
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Old Jul 12, 2016, 02:14 AM
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We would need the motor to be capable of 40Kg of thrust. We would only need 6 of the U11 motor for the 40Kg drone.
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Old Jul 12, 2016, 07:22 AM
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To get a multicopter with a 40 Kg AUW off the ground and flying safely and nicely in light winds you need about 1.2 to 1.4 times that weight in thrust. So you want about 48Kg to 64Kg of thrust from six motors. And that would be 8Kg to 10.7kg per prop.

If you look at the testing data the first two props will do that on the 12S voltage. The 27 x 8.8 prop is better than the 26 x 5.5 and it also runs the motor a little hotter at 58.5C.

Heat is what kills motors and T-Motors says that the temperature shown there is taken after 10 minutes at full throttle. 58.5C/137.3F is just above our "too hot to keep a finger on it" temperature but it should not damage the motor depending on where that reading is taken. I would not want the outside of the magnet housing to get anywhere near that temperature because it would threaten the magnets with permanent damage. The housing should never get to the too hot to touch 54C/130F range. But the windings can safely get to more than that with decent quality wire. Good magnet wire is OK at as much as 155C to 200C even for continuous use.

So if the windings get too hot to touch in flight but the housing stay cooler, as they typically will, the U-11 motors should work for you with those first two props.

As you fly you will want to run the motors as lower throttle as you end the flight and after you land to make sure that the winding are cooling down a little. That only takes a short time, 10-15 seconds at lower throttle, to safely cool things down. If you stop the motors with heat soaked windings that are up near 80C that can put the magnets at risk.

Jack
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