Does the motor wattage and kv really matter when selecting a motor instead of thrust? Instead of buying a motor that is similar in volts/kv/current to the ones in a power pack, shouldn’t buying a motor that can provide a similar amount of thrust and have a similar weight regardless of kv or voltage be more of a consideration?
Electric motors
- Thread starter BoredGuy
- Start date
You are correct, the motor thrust is the critical factor.
There is a direct correlation between watts and thrust. The Kv is an expression of at what speed that thrust will occur. Take 2 motors with the same watt ratting, one 1000 Kv, one 2200 Kv. The lower Kv motor will always have more thrust on a test stand. The higher Kv motor will always produce a higher top speed in a plane.
Two motors with the same watt rating and Kv, they will have similar thrust. If you are a super competitive quad racer, then you will be able to detect the difference. For us mere mortals, we will not notice any difference.
Bricks
Master member
And the size and style of plane plays a roll. You have a big fat fuselage with a high KV motor swinging a 6X3 prop more then likely is not going to work, as a 1200 KV motor swinging 10X6 prop will work great. You cannot just use watts when comparing for the same plame..
Dear Merv,
thanks for the xplanation.
Is there a formula or a calculation table/tool to find the right motor for a plane?
I've two examples, a Great Plane F4U Corsair 40 with 6,60lbs and an Old Timer Junior 60 with 3,30lbs. Which motors to buy?
Thanks to the team
António
Thank you very much
Upon doing some more research (random googling), I have found another alternative: http://www.drivecalc.de/
It looks like it’s free and the database is supposedly more accurate than eCalc’s.
It looks like it’s free and the database is supposedly more accurate than eCalc’s.
danskis
Master member
This is could be wrong so take it with a grain of salt - its just something to chew on. High kv motors have become really popular because of quads and they list really high power for small motors - with small props. So what kind of planes are these good for? I'm thinking their good for pushers like the Arrow or slot jets like the F22 which have limited room for a large prop. They could also be good for a puller plane with two small motors mounted on the front. And as @Bricks said if you're making a conventional plane with a fat fuselage a low kv motor might be just as good. You also have to think about how long you're battery is going to last. My 2207 2750kv motors suck the life out of a 700mah battery in about 2 - 2.5 minutes
No formulas that I’m aware of. Here are some guidelines, they are not exact but are intended to get you close. It depends on how you want to fly and the total flying weight of the plane, including motor and battery.
• 50 to 70 watts per pound is the minimum level of power, good for park flyers and lightly loaded slow flyers.
•70 to 90 watts per pound is perfect for trainers and slow-flying aircraft.
•90 to 110 watts per pound is good for fast-flying scale models and some sport aerobatic aircraft.
•110 to 130 watts per pound is what you want for advanced aerobatics and high-speed aircraft.
•130 to 150 watts per pound is needed for lightly loaded 3D models and ducted fans.
•150 to 210+ watts per pound gives unlimited performance for any 3D model.
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Thank you very much for your complete explanation.
I've just bought the eCalc and it's indeed a great tool but too complex. I've just tried some calculations for my two planes and it follows some comments I saw in the forums.
I'll try now drivecalc.
The main problem I face while trying to choose a motor is the complexity of the nomenclature used by the suppliers/distributors.
There is a learning curve, we all had to lean at some point. Some vendors do give more information than others.
I’m not go to do the research for you, but free to ask what you don’t know & learn the process.
