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Old May 24, 2015, 02:22 AM
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Weight of plane versus motor power

Hi guys I did this thread to know proportional calculation weights of planes vesus the motor power needed.
If there are people who want to share.
Thanks
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Old May 24, 2015, 02:44 AM
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Search for "watts per pound" or just look at "Similar Threads" at the bottom of the page.

You'll get plenty of different suggestions. It will always depend on how you want the plane to fly...motor gliders need a lot less power than 3D aerobats and EDF jets need even more.

Steve
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Old May 24, 2015, 05:37 AM
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Originally Posted by slipstick View Post
Search for "watts per pound" or just look at "Similar Threads" at the bottom of the page.

You'll get plenty of different suggestions. It will always depend on how you want the plane to fly...motor gliders need a lot less power than 3D aerobats and EDF jets need even more.

Steve
Thanks Steve I will search.
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Old May 24, 2015, 05:44 AM
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sorry double post
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Old May 29, 2015, 09:28 AM
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Hi !

I found this very useful document on a forum (may be here).
It works very well.
You just have to "link" your plane's AUW and the power of the brushless you plan to use to read the behaviour you can expect of your plane.
That graphic also allows you to determinate the power of the brusless you will need to match a determinated behaviour regarding the AUW (or estimated AUW ) of the plane you want to build.

I hope it can help



Best regards
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Old May 29, 2015, 10:21 AM
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If you must use the 'watts / pound' method, also consider wing loading and how you want the plane to fly.

A 3Lb large electric glider is a different beast to a 3lb 3D model, etc.

Likewise, any 'static thrust' testing also needs the prop 'pitch speed', and again how you want the plane to fly.

Basically you need to consider the whole package including what you want from the flight characteristics.

An easy trap, -- you want to go faster, so need a bigger motor, which will probably need a bigger battery, all which makes the model heavier.
You can easily end up with a fast model, but flies and lands like a pig.
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Old Jun 02, 2015, 06:28 AM
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Originally Posted by Dreamcatcher View Post
Hi !

I found this very useful document on a forum (may be here).
It works very well.
You just have to "link" your plane's AUW and the power of the brushless you plan to use to read the behaviour you can expect of your plane.
That graphic also allows you to determinate the power of the brusless you will need to match a determinated behaviour regarding the AUW (or estimated AUW ) of the plane you want to build.

I hope it can help

Best regards

Hi Dreamcatcher can you please elaborate more on this chart cause I could not understand it?
Thanks
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Old Jun 02, 2015, 07:31 AM
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To add to my post #6

Don't forget the propeller. The thing that does all the work.

Select the right sort and size of prop to fly the model how you hope, then select a motor and battery voltage to turn it fast enough.

Alternately, have a look in the Power Systems 'Sticky' thread -- e-flight calculators (compilation)

Motocalc is one of the best.
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Old Jun 02, 2015, 10:26 AM
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Quote:
Originally Posted by guruji View Post
Hi Dreamcatcher can you please elaborate more on this chart cause I could not understand it?
Thanks
You must know any two of the three items. Then draw a straight line between them that also extends to the third item.
So if you know the value of the outer two items and draw a line between them, it will give you what the center item should be as it crosses that item.
If you know the AUW (all up weight) of the plane, and the Watts of the motor, the chart will tell you the flight characteristics of the plane. Or if you know the AUW and how you want the plane to fly, it will tell you what motor watts you need for the plane to do that.
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Old Jun 04, 2015, 12:35 PM
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Originally Posted by emckay7067 View Post
You must know any two of the three items. Then draw a straight line between them that also extends to the third item.
So if you know the value of the outer two items and draw a line between them, it will give you what the center item should be as it crosses that item.
If you know the AUW (all up weight) of the plane, and the Watts of the motor, the chart will tell you the flight characteristics of the plane. Or if you know the AUW and how you want the plane to fly, it will tell you what motor watts you need for the plane to do that.
Ok emckay now I understood it thanks.
Why is it that wingspan not included in this chart cause it makes alot of difference.
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Old Jun 04, 2015, 04:59 PM
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Wing SPAN makes almost no difference. Wing AREA makes a bit more difference but all of these things are just aimed at getting a power system that works reasonably well....NOT the best possible system. You'll have to put a lot more work in to find that.

Steve
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Old Jun 07, 2015, 02:21 AM
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Wing SPAN makes almost no difference. Wing AREA makes a bit more difference but all of these things are just aimed at getting a power system that works reasonably well....NOT the best possible system. You'll have to put a lot more work in to find that.

Steve
Hi Slipstick regarding my motor can you please tell me it's wattage cause I don't know exactly.
It's an A2208 kv800 brushless motor.
Thanks.
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Old Jun 07, 2015, 03:32 AM
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I don't know that motor (who makes it ?) but most 2208 motors are good for around 80-100W continuous.

Steve
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Old Jun 07, 2015, 04:43 AM
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I don't know that motor (who makes it ?) but most 2208 motors are good for around 80-100W continuous.

Steve
Specs:
A2208-KV800 Brushless Motor
Type: A2208.
Rotational speed: 800 KV.
No load current / 10V: 0.8A .
Number of battery (Ni-MH): 6-10 Ni-MH/ 2-3 Li-Po.
Current capacity: 9A / 60s.
Max efficiency: 80%.
Motor for ESC: 18A - 30A.
Motor for Propeller: 7045, 8045, 9045.
Motor for small fixed-wing, helicopter, Multi-axis aircraft.
RC Pull weight: 250g - 600g.
Note: RC Pull weight Different, Optional ESC and Propeller Different.
Size: 27.8 x 36 mm.
Weight: 39g.
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Old Jun 07, 2015, 07:09 AM
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Watts = Volts x Amps

3s = approx 12v, Current capacity: 9A / 60s.

so 12v x 9A = 108 watts max theoretically, though you may squeeze a bit more out with good cooling..

Watts don't actually fly a plane, you could get the same watts with a flywheel, but it wouldn't fly.

As you already have the motor, select the type and size of prop that hopefully will fly the model, based on whether you use a 2s or 3s Lipo and and a max amps draw of 9A.

It's the prop, (the load on the motor), that will determine just how many amps are drawn. Multiply that by the Lipo voltage and there is the theoretical watts.
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