Brushless Motor Thoughts

[npowell28]

I'm not sure if i've just had a penny dropping moment so i thought i'd share my thoughts.

Am i right in saying that if i fit the same plane with different motors i will get different results with regards to the speed.

e.g. i have a small 36 inch low wing plane. It is fitted with a 1700kv motor running on 3s with a 8' x 6' prop. It really zips along with this configuration and is a bit too fast for a low wing trainer. If i keep everything the same and just change the motor to say a 1200kv motor, will i be able to reduce the speed?

TA

[JetPlaneFlyer]

npowell28,

You don't need to change the motor to fly slower. Fortunately modern radios are 'proportional' (not 'on-off' like the radios of 50 years ago).. All you need to do is move the throttle stick back a little and hey presto.. the motor runs slower. Learning to control the throttle is all part of learning to fly.

If for some unfathomable reason you don't feel able to use the throttle then a lower kv motor should reduce reduce the power, but you could also give much the same effect more cheaply by going to a slightly smaller prop with a little less pitch.

If on the other hand you are finding that when you slow the plane up it stalls and falls out of the sky then that's not a motor issue, that would infer that you are overweight.

[Sir Crash-A-Lot]

The answer to your question is yes, the planes top end speed will be slower. Just make sure you still have enough power to fly.

Mike

[CHELLIE]

All great answers Try a 8x4 or 8x5 prop to slow your plane down a little, that should give you a little more flying time too, because you are not drawing as many amps, and thats a lot cheaper than buying another motor Take care and have fun, Chellie

[AEAJR]

EVERYTHING YOU WANTED TO KNOW ABOUT ELECTRIC POWERED FLIGHT
http://www.wattflyer.com/forums/showthread.php?t=31368

This e-book here on Wattflyer may be helpful.

[kyleservicetech]

Originally Posted by npowell28

I'm not sure if i've just had a penny dropping moment so i thought i'd share my thoughts.

Am i right in saying that if i fit the same plane with different motors i will get different results with regards to the speed.

e.g. i have a small 36 inch low wing plane. It is fitted with a 1700kv motor running on 3s with a 8' x 6' prop. It really zips along with this configuration and is a bit too fast for a low wing trainer. If i keep everything the same and just change the motor to say a 1200kv motor, will i be able to reduce the speed?

TA

Yup, just change the propeller size, and use the same motor and battery. Why does this work??? Because these motors are kind of a constant RPM unit, where the motor RPM stays fairly close to the same value with different props. For example, take the prop off and re-check the RPM.

Not so with a gas or glow engine. Go to a much smaller prop on these units, and the engine might turn over so fast it can damage itself. On the other hand, put a way to big of a prop on a gas or low engine, it will lug down, and maybe overheat. Do that with an electric motor, and it will release the magic smoke. That's why a wattmeter to measure your motors current/volts/watts is so critical to have when playing with different propellers on the same motor and battery.

[NFA Fabrication]

What transmitter do you have? If you have a computer radio, have you considered just lowering the upper throttle endpoint?

[AEAJR]

Originally Posted by NFA Fabrication

What transmitter do you have? If you have a computer radio, have you considered just lowering the upper throttle endpoint?

I don't believe that works. The ESC will read high and low when it comes up. I don't think endpoints will matter.

[JetPlaneFlyer]

Originally Posted by AEAJR

I don't believe that works. The ESC will read high and low when it comes up. I don't think endpoints will matter.

It should work on most ESC's where you have to manually calibrate the ESC when you first set it up. Just calibrate the ESC first then reduce the Tx endpoint.

Only problem would be if you ever powered up with the throttle at WOT the ESC would re-calibrate it's endpoint and you would have full throttle again.

Like i said in my first post though... I just don't see the point If you want less power then just use less throttle. That's why throttle is on a proportional controlled stick rather than an on-off switch. It can be useful to have that extra power available to get you out of sticky situations.

[hayofstacks]

Just prop it down some. Try a 7X4. Mesure how many amps you pull with a watt meter. If you watch your throttle, the amps go crazy near wot, and even 1/2 or 3/4 will draw susbstationally (sp?) less. Putting a smaller prop on it will reduce how many amps your prop will draw total, using less power, and possibly being quite a bit more efficient.

Another thing to consider, a lower kv motor will want a larger prop to get you the same overall effect. So even if it uses less total power, you may be pull more amps at say 1/2 throttle where you might do most of your flying. And you'll need room for a larger prop.

[AEAJR]

Originally Posted by JetPlaneFlyer

It should work on most ESC's where you have to manually calibrate the ESC when you first set it up. Just calibrate the ESC first then reduce the Tx endpoint.

Only problem would be if you ever powered up with the throttle at WOT the ESC would re-calibrate it's endpoint and you would have full throttle again.

Like i said in my first post though... I just don't see the point If you want less power then just use less throttle. That's why throttle is on a proportional controlled stick rather than an on-off switch. It can be useful to have that extra power available to get you out of sticky situations.

I agree completely. Given the option I would prop every motor to near its maximum wattage considering the level that the battery can support based on how I plan to use it. Short high power bursts, like a glider launch, is a different situation from 3D hovering. You may prop and battery differently based on those two use cases.

[SuchIsLife]

Going down from a 1700kv to a 1200kv will not only reduce rpm (thereby reducing speed), but will allow you to use a bigger prop for the same amp draw (reducing speed but increasing power/thrust). It's like changing down a gear in your car.

Another option is to try going from 3s to 2s, will reduce rpm to 2/3 and allow a bigger prop for the same amp draw.

[AEAJR]

If you want to get a feeling for how voltage, prop and kV impact things, play with WebOcalc. It is a free tool.
http://flbeagle.rchomepage.com/softw.../webocalc.html

Put in whatever you like about the aircraft and flight mission, for this excercise it does not matter.

Put in the weight of your airplane as the thrust target, it is just a starting point.

Set the maximum prop diamater to the largest that would fit on your plane for your application. If it is hand launched, set it to 15 inches. If it has to clear the ground on landing set it to a number that works. What this does is limit the prop choices to nothing bigger than that diamater.

Now put in the info about your motor kV, battery voltage and maximum amps. See what the prop outcomes are.

There are really only three things that matter in webocalc. Target speed is just a starting point to bias the outcomes toward speed or thrust. Use 60 mph to start. After that you can play with the number.

Then play with kV, maximum amperage and battery voltage and watch what happens to the prop choices, thurst and speed.

In this case Amp draw would be set to your motor's maximum or your battery's maximum. Now play with 2 cell, 3 cell 4 cell.

As you play with speed, amps, cells and kV you start to understand the relationship between these numbers. Look at the prop suggestions, thrust and speed of each.

Pay particular attention to the white and yellow numbers. They are explained on the right side after the calculation is done. For outrunners you typically want to adjust things to get a 1.0 for the gearbox ratio. This would indicate direct drive. Adjust kV, amps and volts to get to that number.

If you are looking for a new set-up you can use this to find goals.

If you want to estimate the impact of a prop change this can help.

If you want to see what will happen if you go to 4 cells from three, you can model it here. Note that the amps you set will limit to that number so if you go from 3 cells to 4 and don't change the max amps it will go to smaller props to limit the amp draw, so if you want to see what will happen, up the max amps to beyond the limit of your motor. The system will try to prop up to that amp draw.


It is a fun way to learn about these things. It is not nearly as comprehensive as motocalc or some of the others but it is a GREAT learning tool and I use it for quick estimates all the time. I have found it to be pretty accurate in its results. Certainly within 10% which, for my purposes it close enough.



It is easy, it is fun and it is FREE!