JollyPopper

My Feedback: (6)

I am still new at electrics and am trying to force as much knowledge into my thick head as I can. One vendor is offering the same size motor in three different Kv ratings, 950, 1100 and 1450. I realize that is telling me how many RPMs the motor is capable of, but what does that mean in terms of performance? Will the slower turning 950Kv motor have more torque and swing a bigger prop? And what does that mean in choosing a motor for an airplane? How would I know which one to choose for a given airplane? BTW, all three of these motors are the same brand and the same price so the decision would strictly be what those motors would do on a given airplane.

Walt Thyng

My Feedback: (9)

I'm not an EE, but I do know that Kv is related to torque, which in turn is related to the nearly undefinable term "power." A lower Kv motor will swing a larger prop (obviously more slowly) and a lower Kv will spin a smaller prop faster. so, if you're flying a big draggy WWI type bipe, you want the slower turning motor. If your flying a racer then the faster. The mid-range Kv might be a more universal use motor.

It would be helpful if you down-loaded "Drive- Calc", a free online performance predictor/estimator program. It takes a bit of learning to get the hang of it, but it's worth it. All you'd have to do is change the Kv in a given set-up to see the difference it makes.
Walt

guver

My Feedback: (325)

Ultimately you can achieve similar performance on all three motors provided there are available props and all three had the same voltage. So max prop diameter may dictate using one of a higher kv at times , but selecting the biggest prop (and thus a lower kv) in most cases will be good thing. Beyond that if the voltage is variable then using higher voltage will dictate a lower kv for similar performance and is usually a good thing too as it lowers the current.

MJD

My Feedback: (1)

Power is clearly defined.. it is the rate at which you can do work.

RPM x torque = power

Power flies airplanes, torque does not.

Half the rpm and double the torque at the shaft = same horsepower, same potential top speed, same potential climb rate etc. How efficiently you can use the horsepower in your particular application is where the mystery of prop choice comes into play.. and that is the complex part.

More efficient motors deliver more horsepower output for the same horsepower input, and vice versa. More efficient props, same deal. The remainder goes into heating the atmosphere. In many applications a larger prop, properly selected, is more efficient. Not all the time however.

hugger-4641

My Feedback: (6)

Most of the above comments are functionally correct. But the Kv rating is more related to rpm. The Kv and number of poles determine how many rpm the motor will theoretically turn per volt supplied. So yes, all other things being equal, if you are using a lower voltage on the same prop, you will need a higher Kv to get similar performance, and vice versa. But as guver mentioned, it is usually better to use a higher voltage supply and a lower Kv rated motor to keep the current draw lower. The battery, esc, and the motor will run cooler and last longer that way.

Here's an analogy that might help: Let's say you have a V-8 car engine that produces 400 ft/lbs of torque at 3000rpm. Theoretically, you could get that same 400 ft/lbs of torque from a 4 cylinder engine, but you would need a much higher rpm to do it. So, economically, if you actually needed 400 ft/lbs of torque, you would be better to use a V-8, but if you actually only need 100 ft/lbs of torque to do the job you need, you would probably be better to shed the weight of a V-8 and use the 4 cylinder. Hope that helps!

chuckk2

Backing up a bit - - -
Ground clearance places an upper limit on prop length.
Longer props generally reduce the maximum safe RPM for a specific design propeller.
Next, what battery voltage and current might be appropriate. 3 or 4 cell (common choices) maybe 30-80A for various motor sizes
Using 10,000 RPM for convenience, 12,000 could be appropriate as well for most electric propellers ~ 14,000 is usually close to safe limits
(Above this RPM range, consider a propeller rated for glow engine use.)
950kv = 10.52 v (3 to 4 cell might be appropriate)
1100kv = 9.1 v (3 cell)
1450kv = 6.9 v (3 cell max, 2 cell maybe)

How heavy is the model?
100W per pound for gentle flyers, 125-150 for just a bit above. 150+ or so for moderate aerobatics, 200+ for 3D
If the KV rating is low a larger prop or a higher battery voltage might be required. A 6 cell battery is often a practical max, due to ESC voltage limits.
(8 cell or higher escapes cost!)
So, how do you put this together?

5- 6 Lb model example
Ground clearance limit 12-13" propeller length
Motor max watts 800
Battery 4 cell (use ~ 16v for a fully charged battery, this allows for some drop in wire, connectors, FETs, etc.)
KV 770 x 16 = 12,320 RPM (Use ~9,000 to 10,000RPM as a practical minimum, 14,000 as a max, due to Prop RPMsafe limits for electric props in the 12" - 13" length range.
A 3 blade prop might be considered when ground clearance is an issue.
60-80 A ESC might be appropriate for an 800 W rated motor at 16V 800w /5lb = 160 w /lb But, when all is said and done this might be a bit high, 700W might be closer (140w/lb)
Next, use E-Calc or a similar program to see what prop sizes and pitch might be appropriate.
From motor specs, a 12x6 to a 13x8 might be ball park. E-Calc allows you to play with the variables. 13x6.5 is a commonly available APCelectric prop
Flight time and weight might be used to give a battery capacity number. Desired ~ 5 minutes flight time at "normal" throttle (About 1/2 throttle)
This might work out to be 3,300mah to 4,000 mah (fairly common and available battery capacities)
Actually, I have three models that use E-Flite 32 motors, and the numbers I ball parked are a result of mfrs and E-calc results.
One model using a Power 32 motor, a 4 cell 4000mah, and a 12x8x3 will show a peak static wattage close to 1000Watts with a fully charged Lipo.

An E-Flite 25 or 15 also usually works out to 3 to 4 cell batteries, and 2,200 to 3,300 mah capacity for appropriate models.
I try to size the props for these motors so that I can use either a 3 or 4 cell battery without changing props.

The difference between a "led sled" and a "floaty" model can be power and weight. Too much motor and battery vs the alternative.
I have a Stinson that can almost go from one to the other by changing from a 3 to a 4 cell battery of the same mah capacity.

Don't forget Servos and BECs. Digital servos can draw peak currents higher than 1 Amp. I recently measured a commonly used servo
that was acting strangely, and discovered that it was drawing 1-1/2A when moving in one direction. The inital clues were a large shift in center, and slow movement in one direction.
This would likely have resulted in a crash if a commonly used 2A BEC or a 4 cell NMIH battery was providing receiver and servo power.
Thus, a 6A BEC can be just adequate for models using 6 digital servos, assuming that the servos will not all draw peak current.
at the same time.