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Old Feb 06, 2011, 06:06 PM
King of Crash
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ok so i'm also a noob only im using one arm now but i need to replace my motor and esc now which is a brushed 370 size greatplanes piece of crap spinning an 11x4.7 piece of pain with an 11.1v 3 cell 15-20c parkzone battery and a 25a deep fried esc with a gross weight of 15 oz. what would i need to go brushless at a budget.
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Old Feb 06, 2011, 06:22 PM
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ok so i'm also a noob only im using one arm now but i need to replace my motor and esc now which is a brushed 370 size greatplanes piece of crap spinning an 11x4.7 piece of pain with an 11.1v 3 cell 15-20c parkzone battery and a 25a deep fried esc with a gross weight of 15 oz. what would i need to go brushless at a budget.
You're running a brushed 370 motor with a 3s LiPo and an 11" prop, with no gearbox???? No wonder the motor got toasted.
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Old Feb 06, 2011, 06:28 PM
King of Crash
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You're running a brushed 370 motor with a 3s LiPo and an 11" prop, with no gearbox???? No wonder the motor got toasted.
Lemme rephrase that bla bla bla bla bla Brushed 370 WITH A 5:1 gearbox, bla
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Old Feb 25, 2011, 01:10 AM
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hai all ....
i need help, how can i determine prop saiz if the vendor not say what suggestion prop can used for certain motor...
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Old Mar 03, 2011, 05:42 PM
King of Crash
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Originally Posted by hanafeearis View Post
hai all ....
i need help, how can i determine prop saiz if the vendor not say what suggestion prop can used for certain motor...

flip a coin
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Old Mar 03, 2011, 05:47 PM
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hai all ....
i need help, how can i determine prop saiz if the vendor not say what suggestion prop can used for certain motor...
See this tutorial on how to do it easily...

http://www.rcgroups.com/forums/showthread.php?t=1136470
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Old Mar 04, 2011, 04:29 AM
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Originally Posted by hanafeearis View Post
hai all ....
i need help, how can i determine prop saiz if the vendor not say what suggestion prop can used for certain motor...
There are a variety of "calculators" on the web. Some are free and some are not. The free ones tend to be simple but not very comprehensive ( read the first couple of pages of this discussion )

One that I use is Webocalc. Not very comprehensive but it is free.
http://flbeagle.rchomepage.com/

If you have a specific plane in mind, put in its numbers. If not, then just approximate something.

Put in the max prop size, (based on ground clearence deisred and a starting pitch speed of 45 mph and any thrust number.

Put in the max amps and kV rating of the motor as well as the the proper battery pack cell count. Some props will be suggested.

Vary the amperage or pitch speed to see various prop suggestions.
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Old Mar 04, 2011, 06:52 AM
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Drivecalc is free and has a motor library. It also has a propcalc for computing effiency.
That's the only calc I use. But, use the calcs as a starting point and then measure live data.

Peter
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Old Mar 07, 2011, 06:46 AM
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After reading a lot of this stuff, I have come to the following "recipe",
It is an attempt to simplify things, by an example.
could you please verify it is correct ?

[1] We begin by knowing (or guessing) the weight and speed required for the model.
[2] From the weight we determine the Watts and from the speed we determine rpm/prop combination.
[3] We try to find a proper motor/prop combination that conforms with the above.
It is difficult to determine the optimal power system for our model, but it is relatively easy to determine a decent one.

For example, for a 16 oz, 1m Cessna, we could assume 120 Watts power and 50 mph speed. Suppose some motor works at 12 Amps at 3 cells, revolving a 9x7 prop at 8000 rpm. This yields power = 12 x 11.1 = 133.2 Watts, and speed = rpm x pitch/1056 = 53 mph. Ok this sounds good enough !

Suppose another motor works at 12 Amps, revolving a 10 x 4.7 prop at 6000 rpm. This yields 133.2 Watts which is nice, but speed = rpm x pitch / 1056 = 6000 x 4.7 / 1056 = 26 mph. Oh gee, this is too slow fly, even at full throttle it will not go faster than 26 mph, so this is not a good power system for our airplane. Even if it manages to take off, it will remain dangerously close to stall speed, and therefore will not provide a fun experience.

Could you please verify that the above reasoning is correct ?
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Old Mar 07, 2011, 07:04 AM
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Quote:
Originally Posted by dtrip View Post
After reading a lot of this stuff, I have come to the following "recipe",
It is an attempt to simplify things, by an example.
could you please verify it is correct ?

[1] We begin by knowing (or guessing) the weight and speed required for the model.
[2] From the weight we determine the Watts and from the speed we determine rpm/prop combination.
[3] We try to find a proper motor/prop combination that conforms with the above.
It is difficult to determine the optimal power system for our model, but it is relatively easy to determine a decent one.

For example, for a 16 oz, 1m Cessna, we could assume 120 Watts power and 50 mph speed. Suppose some motor works at 12 Amps at 3 cells, revolving a 9x7 prop at 8000 rpm. This yields power = 12 x 11.1 = 133.2 Watts, and speed = rpm x pitch/1056 = 53 mph. Ok this sounds good enough !

Suppose another motor works at 12 Amps, revolving a 10 x 4.7 prop at 6000 rpm. This yields 133.2 Watts which is nice, but speed = rpm x pitch / 1056 = 6000 x 4.7 / 1056 = 26 mph. Oh gee, this is too slow fly, even at full throttle it will not go faster than 26 mph, so this is not a good power system for our airplane. Even if it manages to take off, it will remain dangerously close to stall speed, and therefore will not provide a fun experience.

Could you please verify that the above reasoning is correct ?
Hi Dtrip
Weight and speed is important but
** plane style and piloting is also important
If you have a hand launching or launching from grass ..
If you have a trainer or a glider or a racer ..
If you have a solar plane ..
Louis
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Old Mar 07, 2011, 07:31 AM
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Quote:
Originally Posted by dtrip View Post
After reading a lot of this stuff, I have come to the following "recipe",
It is an attempt to simplify things, by an example.
could you please verify it is correct ?

[1] We begin by knowing (or guessing) the weight and speed required for the model.
[2] From the weight we determine the Watts and from the speed we determine rpm/prop combination.
[3] We try to find a proper motor/prop combination that conforms with the above.
It is difficult to determine the optimal power system for our model, but it is relatively easy to determine a decent one.

For example, for a 16 oz, 1m Cessna, we could assume 120 Watts power and 50 mph speed. Suppose some motor works at 12 Amps at 3 cells, revolving a 9x7 prop at 8000 rpm. This yields power = 12 x 11.1 = 133.2 Watts, and speed = rpm x pitch/1056 = 53 mph. Ok this sounds good enough !

Suppose another motor works at 12 Amps, revolving a 10 x 4.7 prop at 6000 rpm. This yields 133.2 Watts which is nice, but speed = rpm x pitch / 1056 = 6000 x 4.7 / 1056 = 26 mph. Oh gee, this is too slow fly, even at full throttle it will not go faster than 26 mph, so this is not a good power system for our airplane. Even if it manages to take off, it will remain dangerously close to stall speed, and therefore will not provide a fun experience.

Could you please verify that the above reasoning is correct ?
That is a pretty good summary. Add Fourdan's comments about type of flying and you can further tune your target motor system.

If you are going to select your own power systems, as opposed to buying planes that have motors already installed, then it is probably a good idea to invest in once of the calc programs that help you "model" a variety of combinations till you find one you really like.
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Old Mar 07, 2011, 07:44 AM
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Originally Posted by Fourdan View Post
Hi Dtrip
Weight and speed is important but
** plane style and piloting is also important
If you have a hand launching or launching from grass ..
If you have a trainer or a glider or a racer ..
If you have a solar plane ..
Louis
Hello !

Pilot and plane style is of course greatly important, but don't you think that it can be expressed by "max speed" ?
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Old Mar 07, 2011, 07:46 AM
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Hello !

Pilot and plane style is of course greatly important, but don't you think that it can be expressed by "max speed" ?
Not really at all.

Take the case of a plane like your example Cessna propped to only go 20 mph top speed.

For one pilot, looking to fly 3D with it, it may be ideal for his style. But for another, looking to fly scale like, it would not be what he was after.
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Old Mar 07, 2011, 07:49 AM
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Originally Posted by aeajr View Post
That is a pretty good summary. Add Fourdan's comments about type of flying and you can further tune your target motor system.

If you are going to select your own power systems, as opposed to buying planes that have motors already installed, then it is probably a good idea to invest in once of the calc programs that help you "model" a variety of combinations till you find one you really like.
Thanks !
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Old Mar 07, 2011, 07:54 AM
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Antony (France)
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Quote:
Originally Posted by dtrip View Post
Hello !

Pilot and plane style is of course greatly important, but don't you think that it can be expressed by "max speed" ?
Hi
In my opinion .. yes but
The two major "parameters" are
a) Static thrust
b) Pitch speed (or speed for prop transparency)
The product is proportional to watts out
If you count on the motor-gear box efficiency (could be 80%) you have all.

So I prefer a) and b) parameters instead of a) OR b) + the product a)b)

For take off from grass you need static thrust
For hovering you need static thrust
For a racer you need pitch speed
...

Louis
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