vtach688

My Feedback: (8)

I am new to electrics but have been flying nitro since 1982. My question is this: how do you size up a motor? I see specs such as 2212/12 or park 480 and so on. What does this mean, more importantly for me, how do i cross ref. this information. Is there somewhere that lists the differences? Thanks in advance.. V.

rmenke

My Feedback: (13)

Vatach:

Unfortunately, there is no real consistant point of measurement or compairson between the motor manufacturers IMHO. One close area is the "Watt's" the motor is supposed to produce, watts being a measurement of work. Again, unfortnately many/most figures given by the factory can not be obtained in the field. Leaders in the field like Hacker, and AXI do produce power close to specs given. I recently purchased a 700 watt motor for a 25 size glow airplane, 3D type. It does provide good power and unlimited vertical and around 10 minutes of flight time. Actual watts measured by a watt meeter is 375-400, a little shy of claims. Some manufacturers give a glow equivelent of power to help sizing, some will give you "thrust" which for me is the best way to measure power. If I want to size a 3lb airframe for 3D, I would be looking for a motor with 6 pounds of thrust or 2 to 1 power ratio. Nice thing about thrust measurements is that you can verrify the motors output at home with a simple fish scale.

Regardless of the confusion of motor power among different manufacturers, stay with motors known to be quality until you get your feet on the floor. It is sort of counter productive to have a powerfull motor with very poor efficiency that ends in poor flight time. For now, try to stick with package deals by known quality manufacturers or shops that guarantee their product. This will leave you the time to practice control of your throttle stick. A experienced electric guy may well get twice the flight time you do on the same system. Something you will learn to do with time. You do not need to fly around wide open all the time. After around 3 years playing with the electrics, I enjoy them very much while the glow birds gather dust on the ceiling. ENJOY

vtach688

My Feedback: (8)

thanks rmenke, that helps. so is there a way to tell how much thrust a motor gets, if it not list by the manufacture?

Dr Kiwi

Thrust has to be measured.... and for a certain amount of power into a particular motor it will be dependent on prop diameter/prop pitch.

vtach688

My Feedback: (8)

Ok, that makes sense, but what is the calculation for measuring thrust? One other question is have is this: is the # of watts a better measure that thrust? (to have both would be good too)

vtach688

My Feedback: (8)

ok, i need to read a little better, you said use a "fish scale", perfect. thanks

Dr Kiwi

You can measure thrust directly with a fish scale or a thrust stand. There are programs which calculate the amount of thrust you'd get from a certain prop at a certain rpm (http://www.badcock.net/cgi-bin/powertrain/propconst.cgi). The # of watts in, tells you how much power your motor is consuming, but the more important measure is how much it is putting out (without using a dynamometer, it can be crudely measured by thrust, in combination with pitch speed).

The efficiency of the motor obviously comes into it, as does the prop you are using... for the same watts-in, if you want lots of thrust you can get it with a large diameter prop, but pitch speed will necessarily have to be low... if you want speed, you use a small diameter high pitched prop, but your thrust will be low.

It is a trade-off... no free lunch!

jdetray

A couple of other notes about thrust. If you know the RPM of a prop, which can be measured with a tachometer, you can find the thrust using a [link=http://www.gobrushless.com/shop/index.php?app=ccp0&ns=display&ref=thrustcalc]calculator like this one[/link] from GoBrushless.com.

And besides a fish scale, you can also measure thrust with a dedicated test stand, such as the one below, which is my version of Dr Kiwi's design.

Also, remember that thrust is only one measure of power system performance. For some applications, speed is more important than thrust. Thrust is vital, for example, for 3D flying. But speed might be more important for a fast-flying warbird.

- Jeff

vtach688

My Feedback: (8)

Thanks Dr. Kiwi, great info, and that helps alot & thanks jdetray, that web site for calcs was very helpful. i can use this info to find the right combination for the type of flying i want to do. Thanks a lot.

Fidy$Trainer

My Feedback: (1)

Question: I'm a total noob to this and have a quick question as it relates to engine Kv and speed / thrust: What's the relationship between a low Kv and high Kv motor as it pertains to speed.

In my case I want to hopup a little F4U Corsair Warbird. It's currently has a 960kv motor with a 9.5x7.5" prop. How do you determine the prop size to motor kv? From what I understand the lower the pitch the more thrust it provides the higher the pitch the faster it will fly. How does one figure out the optimum prop size to motor kv? Or does that make sence.

The motor I was considering is a 1250kv outrunner with a 8x7 prop. I'm struggling to understand the relationships between motor size, ESC, and batteries to determine optimum configurations with out burning anything up.

Dr Kiwi

I'll cover this in two places!

You have it correct - for the same power-in, bigger diameter/lower pitch generates more thrust at the cost of lower speed: smaller diameter/higher pitch gets you more speed but lower thrust. A higher Kv motor is better suited to a smaller prop (for speed), than a lower Kv motor.