chuckk2
Posts: 371
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Joined: 7/24/2007
Last Login: 5/20/2013
From: Warner Robins,
GA, USA
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ORIGINAL: JollyPopper
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.
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 RPM safe 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 APC electric 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.
< Message edited by chuckk2 -- 3/30/2013 3:40 AM >
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KV and performance - 
