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Author Topic: Geared motors  (Read 1039 times)
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SP250
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« on: January 01, 2019, 05:41:33 AM »

Asking for some help in propellor sizing.

If I know the size of prop a motor will turn as direct drive and the number of cells etc.
Is there a ratio of increasing dia/pitch prop for a gear ratio of 2:1 then 3:1 etc?
Or do I just go by the area of the increased dia of prop using pi. r squared? 
I.E. for a 2:1 ratio I can double the area of prop disc - so a direct drive 8" prop can go up to an 11.3" dia with the 2:1 gearbox??

I have an inrunner motor used direct drive with a small prop in a small heavy hotliner glider. 
I can put a 5.25:1 gearbox on it for use in a larger thermal soarer for competition.

If someone could kindly point me in the right direction please.
Thanks

John M
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Russ Lister
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« Reply #1 on: January 01, 2019, 06:35:10 AM »

Your question has made me realise just how rusty I have become with model electric power!
I have used MotoCalc extensively in the past ... if there are no other replies, I will run a couple of test cases for you.
Pitch speed against stall speed and finding the optimum loading of the motor complicate any scenario, it's very much a sliding scale with electrics!
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Konrad
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« Reply #2 on: January 01, 2019, 09:17:11 AM »

As I understand the question you are asking for how a prop absorbs power.
 
The short answer is no. The relationship of rpm to diameter is not linear.

Per the Abbott equation;  Static Power = K x Dia.(4th) x Pitch x RPM(cubed).

"K" is a constant that addresses issues like blade shape and air density etc..
If you are keeping things the same other than the prop's  basic dimensions you can ignore it for now.

This is from a chart I found in Bob Boucher's "Electric Motor Handbook" (AstroFlight Inc)
Assuming that your direct drive is 100%.
A gear drive of 2:1 will drop the rpm 50%, the diameter can go up 152%, thrust goes up 132%, and speed drops 72%
A gear drive of 3:1 will drop the rpm 33%, the diameter can go up 192%, thrust goes up 155%, and speed drops 64.4%
A gear drive of 5:1 will drop the rpm 20%, the diameter can go up 263%, thrust goes up 190%, and speed drops 52.5%

I hope this helps,
Konrad

I found this HIP discussion.
http://www.hippocketaeronautics.com/hpa_forum/index.php?topic=14638.0
« Last Edit: January 01, 2019, 09:37:24 AM by Konrad » Logged

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USch
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« Reply #3 on: January 01, 2019, 12:36:33 PM »

A gear drive of 2:1 will drop the rpm 50%, the diameter can go up 152%, thrust goes up 132%, and speed drops 72%
A gear drive of 3:1 will drop the rpm 33%, the diameter can go up 192%, thrust goes up 155%, and speed drops 64.4%
A gear drive of 5:1 will drop the rpm 20%, the diameter can go up 263%, thrust goes up 190%, and speed drops 52.5%

Konrad, I dont want to teach you the English language, but I believe the sentence should read:

A gear drive of 2:1 will drop the rpm to 50%, the diameter can go up 152%, thrust goes up 132%, and speed drops 72%
A gear drive of 3:1 will drop the rpm to 33%, the diameter can go up 192%, thrust goes up 155%, and speed drops 64.4%
A gear drive of 5:1 will drop the rpm to 20%, the diameter can go up 263%, thrust goes up 190%, and speed drops 52.5%

Otherwise it could be read as ...drop the rpm by xx%...

Best wishes for a Happy New Year to everybody,

Urs

Urs
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Konrad
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« Reply #4 on: January 01, 2019, 12:42:17 PM »

Point taken.

So if the 100% rpm is 100, it would be down to 33 rpm with a 3:1 gear box.

All the best,
Konrad
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frash
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« Reply #5 on: January 01, 2019, 01:16:52 PM »

Thanks, everyone. I anticipate that this will be useful to me.

Fred Rash
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Indoorflyer
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« Reply #6 on: January 01, 2019, 01:55:51 PM »

What is the term "speed" referring to? Flying "speed"?
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Konrad
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« Reply #7 on: January 01, 2019, 02:07:05 PM »

Pitch speed. This was taken from Bob Boucher's Electric Motor Handbook on "Understanding Propellers" Chapter 4.
Flying speed deals a lot with drag and trust at flying speed. So to keep things simple we are only talking about the prop. Matching the prop to the airframe is another topic.

Bob Boucher (AstroFlight, Inc) is known as the father of electric flight as he holds the USA patent #3957230 along with his brother Roland (Leisure Flight, Inc).
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« Reply #8 on: January 01, 2019, 02:31:17 PM »

Thanks for that; was about to chime in, wondering why airframe effects were not being factored in!
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SP250
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« Reply #9 on: January 01, 2019, 03:34:05 PM »

Thanks all

So in summary,
If my motor does 2200Kv / volt then on a 3cell LiPo (nominal volts 11.1) will result in 24,420 revs.
With a 5.25:1 reduction gearbox, the output shaft will be doing 24,420 / 5.25 = 4651.43 revs
Which is a reduction to 19.04% of the un-geared motor shaft speed.
By your info Konrad (Astroflight), I can fit a prop at 263% bigger.
Original direct drive was 6" dia, so geared I can fit a 15.8" dia prop (6" x 263% = 15.8")
Being as the thermal soarer will fly a lot slower than the hotliner (52.5% according to Astroflight) I can adjust the prop pitch to a lower number as well and save amps or run with a smaller capacity battery.
Then check it all out on a wattmeter for safety's sake.

Thanks again John M
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Konrad
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« Reply #10 on: January 01, 2019, 04:19:22 PM »

Work it back into the static power equation. If using thin carbon blades a "K" of 1.18 seems to work out ok. Units are Watts, Diameter and Pitch are in feet* and RPM is in thousands.

Keep an eye on the pitch and flight speed. If one is trying to climb as high as possible on a fixed battery charge with a light wing loaded glider, like an old timer, then gearing is often advantageous. If one is flying in a limited motor run, then aerodynamic efficiency of the aircraft may not be as important as flight speed.

It is a common misconception that gearing improves motor efficiency. This is false. What gearing does is allow a motor to turn a larger prop. Gearing only improves propeller efficiency if the direct drive propeller's pitch speed is much higher than the actual flight speed.

*Sorry Boucher worked in the cumbersome imperial units.

All the best,
Konrad
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Konrad
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« Reply #11 on: January 01, 2019, 06:44:19 PM »

For my sport ships, I like to start with a pitch speed that is about 2.5 to 3 times that of the stall speed.
This is what I use to base line (guesstimate) the stall speed.
This is an approximation of the stall speed, by Keith Shaw:

Stall speed = 3.7 x (square root of wing loading)

speed in mph
wing loading in oz/sqft

Again sorry about the units.

All the best,
Konrad
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SP250
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« Reply #12 on: January 01, 2019, 08:24:07 PM »

Thanks Konrad - I can work in either imp or metric units - the advantage of being 60+ and brought up with the old and having to learn the new.

Cheers John
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Konrad
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« Reply #13 on: January 02, 2019, 03:47:23 AM »

Thanks Konrad - I can work in either imp or metric units - the advantage of being 60+ and brought up with the old and having to learn the new.

Cheers John

You and I are much alike in this respect. But having worked with both for decades as an engineer and machinist, metric is a far superior system. That is why I apologize for using imp units. Other than the naming convention, the imp system is cumbersome to work with.
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TimWescott
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« Reply #14 on: January 02, 2019, 06:47:01 PM »

Thanks Konrad - I can work in either imp or metric units - the advantage of being 60+ and brought up with the old and having to learn the new.

Cheers John

You and I are much alike in this respect. But having worked with both for decades as an engineer and machinist, metric is a far superior system. That is why I apologize for using imp units. Other than the naming convention, the imp system is cumbersome to work with.

Aw, c'mon, man -- there's certainly a rich choice of divisors!  All you can divide by with metric is 10.  With Imperial units you can divide by 2, 4, 8, etc., 12, 11, and more!
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« Reply #15 on: January 02, 2019, 08:16:00 PM »

It is a common misconception that gearing improves motor efficiency. This is false. What gearing does is allow a motor to turn a larger prop. Gearing only improves propeller efficiency if the direct drive propeller's pitch speed is much higher than the actual flight speed.

Hmmm, not sure ...... is there something online that references this?

Curious about this, I just ran a set of trials and Motocalc doesn't bear this out. Checking with similar pitch speed on the same model with the same battery, motor, and ESC, a Speed 400 running direct drive shows both a lower motor output shaft efficiency plus a lower prop efficiency (due to a shorter prop) than a 3 to 1 geared example.

The total drive efficiency is markedly lower, thrust at any speed lower, and battery life shorter. This doesn't depend on the model/pitch speed relationship since the two props were chosen to keep pitch speed similar.
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Konrad
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« Reply #16 on: January 03, 2019, 02:44:53 AM »

Not sure what you have an issue with.

The MOTOR'S efficiency does not change with the use of a gear box.  The motor's efficiency stays the same for any given power input regardless of the load source. It has no idea if it is driving a gear box, a prop or an EDF fan. So If you load a motor to to draw 10 amps at 11.1 volts  and drive a prop directly. Then do the same thing, load the motor to draw 10 amp at 11.1 volts and drive a prop through a gear box. Both set ups will have the same MOTOR efficiency (heat loss).
(So will the battery and ESC and wiring losses be the same)

Curious about this, I just ran a set of trials and Motocalc doesn't bear this out. Checking with similar pitch speed on the same model with the same battery, motor, and ESC, a Speed 400 running direct drive shows both a lower motor output shaft efficiency plus a lower prop efficiency (due to a shorter prop) than a 3 to 1 geared example.

For the motor, this will true if the current is higher in the direct drive set up. I don't know what the drag profile of Motocalc's airplane is but is seems fine as it is showing that a geared set up usually will fly better with most sport models.

The total drive efficiency is markedly lower, thrust at any speed lower, and battery life shorter (with the direct drive system). This doesn't depend on the model/pitch speed relationship since the two props were chosen to keep pitch speed similar.
I don't know what physics engine Motocalc uses so I can't comment on that.

NACA has a lot of papers on how to size a prop to the airframe. My quote is a paraphrase from Boucher's handbook. It is generally understood that the closer the pitch speed is to the actual airframe speed the more efficient the prop is at driving the airframe through the air. Yes there is a lot more to this like diameter to pitch ratios, tip losses  etc..

I like that you used the Mabuchi motor as an example. It has a higher sloped efficiency curve. Most electric motors have a rather flat efficiency curve making these studies rather difficult.

All the best,
Konrad
« Last Edit: January 03, 2019, 02:58:20 AM by Konrad » Logged

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Konrad
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« Reply #17 on: January 03, 2019, 02:48:27 AM »


Aw, c'mon, man -- there's certainly a rich choice of divisors!  All you can divide by with metric is 10.  With Imperial units you can divide by 2, 4, 8, etc., 12, 11, and more!
LOL,
Sounds like a Microsoft sales pitch. "That's not a bug it's a feature !"

All the best,
Konrad
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« Reply #18 on: January 03, 2019, 09:35:01 AM »

Konrad, I don't have an "issue" with what you wrote. More akin to curiosity about the behavior of power combinations, and understanding what you meant in light of different results with Motocalc.

Sure, I agree, if you measure the same voltage and current draw in a specific direct drive and a geared setup, then the motor's output will be the same in both cases. And the motor's efficiency will be the same, obviously.

But that's not a real world system. It would require only one specific set of props to achieve, and the geared motor's prop would be sub-optimal for any particular model. Meaning that the overall drive efficiency was intentionally hampered to provide a single data point where motor efficiency was equivalent. That would not be a reasonable use for a geared motor.

On the other hand if you chose props with the same pitch speed, for both drives, (the condition I gave in my earlier post), you could either reduce the current draw on the geared drive's motor while maintaining the same thrust, or increase the thrust at the same current draw, depending on the length of the prop chosen. Both of those would yield a more efficient drive system, and depending on the motor's efficiency curve and the current drawn, may also produce a higher motor efficiency.



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« Reply #19 on: January 03, 2019, 09:58:17 AM »

Motor efficiency is calculated by comparing power input to mechanical ouput is it not? Heat has little to do with measuring output in a direct sense, but is part of the equation if you are trying to maximize efficiency in a system. Limited motor RC car racing will teach you a lot in regards to heat vs performance in an electric motor. In foam tire racing, if you don't measure the diameter of your tires and calculate gearing/rollout before each run, you will get your arse handed to you by the guys who make sure their gearing is optimal.
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vtdiy
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« Reply #20 on: January 03, 2019, 10:11:58 AM »

I think the real confusion is caused by our language, English.

The difference is in two concepts of a motor's "efficiency" in a drive system.

A motor's efficiency curve does not change when you connect it to a gearbox in a particular application.
But a gearbox may allow a motor to run at the highest point in its efficiency curve for a particular application.

In the first case you can say that "a gearbox doesn't change a motor's efficiency."
In the second case you can say that "a gearbox makes a motor run more efficiently."

Which is correct?

Both are.
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Konrad
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« Reply #21 on: January 03, 2019, 10:45:57 AM »

Well that is how I match my power systems, be them direct drive like in an old F5D or the new geared F5D racers. That is I want the motor to give me the most power that I can use. The use of a larger prop disk allows the plane to accelerate faster out of the turn. Straight line speed is about the same with vastly different pitches. But the lap speed is faster with the gearbox as a result of the larger prop disk.

Now in the F5E tasks I'd sacrifice motor efficiency for the loss in weight using a smaller geared motor. I actually get a better climb rate from the loss of weight and the slight improvement in prop efficiency with the smaller geared motor, even though I'm turning more of my battery's energy into heat rather than thrust. This is looking at the aircraft system as a whole in the real world.

If you get more power out of the geared system for the same input, it is because the prop is more efficient, not that the motor is more efficient. That is all I'm saying.

I need to make a caveat, I and Boucher are assuming one is setting up the motors to be run to the right of the efficiency curve peak, (after the efficiency peak). Setting up a motor on the ground to run before one reaches peak efficiency is thought to be using too large a motor for the application, as the prop load drops sometimes over 20% at speed. This puts the current on the wrong side of the efficiency curve (the motor's efficiency gets worse as the current goes down).  You want to place the current so that the motor's efficiency increases as the motor unloads in the air.

Again I don't know the physics engine Motocalc is using. I do know that most motor/prop simulator do give a warning about sizing the motor before peak efficiency is reached.

All the best,
Konrad
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Konrad
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« Reply #22 on: January 03, 2019, 10:52:25 AM »

I think the real confusion is caused by our language, English.
...
In the second case you can say that "a gearbox makes a motor run more efficiently."

Which is correct?

Both are.

I'm sure language is the problem!
Nope, the gearbox does not make the motor run more efficiently. It may allow the power system to run more efficiently. And in our case that would be in the dynamics of the prop where we find the added efficiency.

All the best,
Konrad
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Konrad
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« Reply #23 on: January 03, 2019, 11:02:32 AM »

Motor efficiency is calculated by comparing power input to mechanical output is it not? Heat has little to do with measuring output in a direct sense, but is part of the equation if you are trying to maximize efficiency in a system. Limited motor RC car racing will teach you a lot in regards to heat vs performance in an electric motor. In foam tire racing, if you don't measure the diameter of your tires and calculate gearing/rollout before each run, you will get your arse handed to you by the guys who make sure their gearing is optimal.

And Maxwell's laws of thermodynamics says that the difference between input and output is heat. That is what the motor efficiency curve is showing. In the case of the foam tire, the tire adds load to the system as the diameter grows. This lowers the effective gear ratio putting more load on the motor. And if you are on the bleeding edge of the motor's thermal limits you will burn up the motor.
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« Reply #24 on: January 03, 2019, 12:11:49 PM »

I think the real confusion is caused by our language, English.
...
In the second case you can say that "a gearbox makes a motor run more efficiently."

Which is correct?

Both are.

I'm sure language is the problem!
Nope, the gearbox does not make the motor run more efficiently. It may allow the power system to run more efficiently. And in our case that would be in the dynamics of the prop where we find the added efficiency.

All the best,
Konrad

Naturally, you are using the first meaning of running the motor more efficiently. That is, altering the fundamental characteristics of the motor. A gearbox does no do that.

The other meaning of running the motor efficiently, in common usage, is that gearing allows the motor to be run at it's most efficient when driving a particular application to which it is not well matched via direct drive.

There is no real physical problem, just a semantic problem.

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