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Old Feb 18, 2012, 06:13 AM
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how to calculate motor for arf kit

i have a kit of 63inches wingspan.which motor for this kit? i have alpa 40 arf kit.
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Last edited by imran09125; Feb 18, 2012 at 06:21 AM.
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Old Feb 18, 2012, 08:29 AM
Jack
USA, ME, Ellsworth
Joined May 2008
16,338 Posts
If you want you can try what worked for others. Go to the Glow to Electric Conversions and do a search for Alpha 40 or post the question there. You'll find stuff like this thread:

http://www.rcgroups.com/forums/showthread.php?t=1241544

To your question, the Alpha 40 is recommended to have a 36–.46 (2 stroke) engine. There is a rule of thumb that 2000 x displacement in cu. in. = electric power in watts. So you would need a 720 to 920 Watt motor. I would look for that number to be the continuous power rating.

And my experience has been that the rule of thumb produces planes with more power than was in the fuelie version. So I would not hesitate to go with the 720 Watt electric motor.

I think the Alpha will fly at 5.25 lbs and with a 9" or 10" prop, I'm going to guess that an APC Thin Electric 9 x 6 or 10 x 7 would be candidate props. So I look at those props at flybrushless.com:

APC E 9x6 - http://www.flybrushless.com/prop/view/78
APC E 10 x 7 - http://www.flybrushless.com/prop/view/42

I see the thrust with those props would be about 0.5:1 and that should fly the plane. A thrust of 0.33:1 assures flight, 0.5:1 is enough for good power in a sports flyer.

And the data also shows what the efficient and safe RPM levels for those props are. APC TE props are rated for up to 145,000 / Dia. (inches) so you want to limit them to to the 14,000 to 16,000 RPM range at the absolute tops.

As you can see, they are getting the thrust you need at only half to two thirds of the max. So now you know if you can turn the 10 x 7 at 10,000 RPM you should be in good shape to fly.

You need to decide a battery and a Kv for the motor. For a 5 lb or so plane I would go with a 4S battery. That has a charged and rested voltage of 16.4V or so. That is one of the numbers that will help you choose a Kv for the motor.

A motor running with a prop that draws current at its continuous rating will be running at 75% of it's no load (no prop on) RPM. So you know that your 10 x 7 at 10,000 RPM is running a 75% of the no load RPM.

10,000 RPM is 75% of about 13,400 RPM so you need a motor that turns 13,400 when 16V is applied to it (dropping from 16.4 to 16 allows for the load the motor places on the prop with no prop). 13,400 / 16 = 836 and that is the Kv you need for a 4S battery. If you went with 3S or 5S, the Kv would be different.

As a ballpark guess, you want a 720W, 836 Kv motor.

Typical motors produce 3 Watts of power for each gram of motor weight so you also know that a 720W motor will weigh about 240 grams (720W / 3g = 240g).

As a double check on those numbers, Tower Hobbies recommends the Power 25 for that plane:

E-flite Power 25 - http://www.e-fliterc.com/Products/De...odID=EFLM4025A

To take the plane to the upper limits of power the Power 32 would probably be a good choice:

E-flite Power 32 - http://www.e-fliterc.com/Products/De...odID=EFLM4032A

To save money, you can look for motors with weights, Watts, and Kv ratings that are in the same range. Sometimes the most difficult part is trying to figure out what the advertised specs really mean because they list a wattage and range of battery sizes so is it hard to narrow it down as to what the wattage is for one specific battery. But if you look for prop testing data from users or the experiences of other builders it will sort out the details and lead you to the best choices.

Jack
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Last edited by jackerbes; Feb 18, 2012 at 10:53 AM.
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Old Feb 18, 2012, 10:41 AM
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Canada, MB, Winnipeg
Joined Jan 2006
192 Posts
Now that is a very definitive answer. Very thorough.
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Old Feb 18, 2012, 10:52 AM
Jack
USA, ME, Ellsworth
Joined May 2008
16,338 Posts
Once you get more deeply involved there are a number of flight calculators that will do that for you but sometimes it is easier to just bone it out the simpler way.

The complexity of some of the flight calculators and all the answers they demand for inputs can be a little intimidating. You'll learn it all eventually but starting with just learning a little about power system seems simpler.

When I want to use a calculator I usually turn to Webocalc. It is a great one to start with. There are many other and more "powerful" calculators but that almost always equates to more complicated too.

Jack
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