Problem, 4 motors but different RPM's

[aap]

I am building a cargo plane with four motors. I am using 4 Emax 2822 outrunners going to 4 esc's, and then splitters to ch. 3 on the RX.

The problem is two motors on one wing are running at a different RPM than the other two. I don't have a tach, but it is obvious as the plane turns immediately from the greater thrust from one pair of motors. The only thing I can think of is the esc's. I have 2 Red Brick, and 2 Hobby wing esc's. Could that be the cause?? Also the motors don't start at the same time all the time. See Video

[gramps2161]

Possible that the different brand ESC have the timing set differently causing the problem with higher rpm difference. Do you have the 2 redbrick ESC on one side and the hobby wing on the other side? Myself never did a 4 motor plane yet just twins, but I used matching motors and ESC to avoid problems.

[dahawk]

Originally Posted by aap

I am building a cargo plane with four motors. I am using 4 Emax 2822 outrunners going to 4 esc's, and then splitters to ch. 3 on the RX.

The problem is two motors on one wing are running at a different RPM than the other two. I don't have a tach, but it is obvious as the plane turns immediately from the greater thrust from one pair of motors. The only thing I can think of is the esc's. I have 2 Red Brick, and 2 Hobby wing esc's. Could that be the cause?? Also the motors don't start at the same time all the time. See Video

Cool plane. Looks like a winner ! Is that from the "Twins" ?

Try resetting the ESC's? Perhaps the default settings are slightly different. Are these props counter rotating? or are all 4 going in the same direction? One battery running all four? If more than one battery, is the C rating the same? Mmmm


-Hawk

[aap]

Yes, I think I have a matched pair of esc's running 2 motors on the same wing.

[rcers]

Some ESC's have very different throttle calibration. Some are very linear some are on/off switches. I suspect that is it.

Split the ESC's so the two in use for the left side control the inboard motors and you should be fine.

[aap]

Originally Posted by dahawk

Cool plane. Looks like a winner ! Is that from the "Twins" ?

Try resetting the ESC's? Perhaps the default settings are slightly different. Are these props counter rotating? or are all 4 going in the same direction? One battery running all four? If more than one battery, is the C rating the same? Mmmm


-Hawk

I don't know about he "twins" It's just a seat of the pants design. It maiden perfect with 2 motors.

Not counter rotating, 2 batts with same C rating. I should check the Esc timing.

[aap]

Originally Posted by rcers

Some ESC's have very different throttle calibration. Some are very linear some are on/off switches. I suspect that is it.

Split the ESC's so the two in use for the left side control the inboard motors and you should be fine.

I will give that a try. Thanks.

[solentlife]

When I did multi's with Glow IC ... it was copmmon practice to link the inner motors together ... and similarly link the outers together. That way we could have most thrust from the inner motors and a touch less on the outers to avoid yaw / torque effects ........ It was also common to have inners on the throttle stick and outers on a separate slider ch. ....

We would open up inners on take-off .... then bring in outers ... take-off. Outers would basically stay at throttle setting and we would vary the inners. That way we always had flying speed. For landing we would reduce outers and then use inners as the control to land ... killing the outers once down and then taxying with inners.

Old habits as they say ........ I'm going to be building a Lanc later and I think I will follow the old ways there ...

[JetPlaneFlyer]

Originally Posted by rcers

Some ESC's have very different throttle calibration. Some are very linear some are on/off switches. I suspect that is it.

Split the ESC's so the two in use for the left side control the inboard motors and you should be fine.

What he said.. Also you will have to calibrate the ESC's to set the on/off end points, the ESC instructions should cover it.

Steve

[Larry3215]

You really need to use all the same brand and model of esc. Like the others said, there are differences in throttle response, internal resistance, timing etc. But if your using the cheap esc's that may not guarantee them all being the same. QC sucks with those.

Its not only the esc's that are different. Your two batteries are different and will likely hold different voltages under load - even if they are the same brand, age and C rating. No tweo batteries ever respond exactly the same.

The RPM of a motor is determined by voltage times kV. Any difference in voltage will cause a difference in RPM.

Connect the packs together with a Y harness so all the esc's always see the exact same voltage. That will help.

Finally, no two motors ever have the exact same kV. This is especially true with cheaper motors. I have seen as much as 30%+ difference between two motors that were labeled exactly the same and bought at the same time. The cheaper motors have very poor quality control. Even top rated motors are never exactly the same. Its quite common to see 5% or more variations in kV.

If you dont want to buy all new stuff - use a tach and test each motor to find its peak RPM with each esc. Try each motor with each different esc. Then pick the two combinations with the closest RPM values and use tham on the outboard motor locations. Use the next best pair in the inboard spots - bbut if the left outboard motor runs faster than the right, put the fastest inboard motor on the right side. That will help balance out any remaining differences in thrust.

[aap]

Thanks Larry,

I reset the throttle limits on the esc's and it seems to have balanced things out pretty well. I will do as you have suggested and relocate the motors based on their RPM differences. Thanks again!

[dahawk]

aap,

Can you post a video when you get er airborne? I like the plane !

Subscribed

-Hawk

[aap]

Originally Posted by dahawk

aap,

Can you post a video when you get er airborne? I like the plane !

Subscribed

-Hawk

Here is the maiden flight with two motors.

[Larry3215]

Originally Posted by aap

Thanks Larry,

I reset the throttle limits on the esc's and it seems to have balanced things out pretty well. I will do as you have suggested and relocate the motors based on their RPM differences. Thanks again!

You need to be careful doing that. ESC's will run hotter at partial throttle than they do at full throttle. You want to try not to limit them more than about 10% or so.

Good luck!

[dahawk]

Originally Posted by aap

Here is the maiden flight with two motors.

Well puff me a Lipo ! Thanks for posting. Nice flying machine. Great flight time. Would like the plans for that one. I'm starting a scratchbuild build on a twin edf airliner. I call it: "The Money Pit"

Need to find a thread on wiring twins. Have two 3500kv 66mm eflight edf's each with a 30 amp esc's. Will also have electric servoless retracts, 4 9g servos for the ailerons and elevator. Do I need an external UBEC? Would like to run off one Lipo if possible. Not familiar with the wiring scheme for this.

Didn't mean to hijack your subject matter here.

-Hawk

[Larry3215]

Wiring twins is easy.

2 motors, 2 esc's wired as normal. Connect the signal leads to a Y harness, then to Rx throttle channel.

Do the same for the battery packs. Y harness the power leads from the esc's to a single pack. Size the pack to twice the size needed for one motor.

If you use well matched, quality motors and esc's, you shouldn't have too much of an rpm difference issue.

If you do, you can run each esc into a different channel on the rx, then use a mix with a curve to get them running close to each other through the entire range. Put the motor with the lower rpm on the throttle channel and the faster turning one on the mixed channel. Then use the mix to slow down the faster one.

[rcers]

Originally Posted by Larry3215

If you do, you can run each esc into a different channel on the rx, then use a mix with a curve to get them running close to each other through the entire range. Put the motor with the lower rpm on the throttle channel and the faster turning one on the mixed channel. Then use the mix to slow down the faster one.

I spend little if any time matching RPM differences. It is really not necessary.

I figured this out when I realized how much differential thrust was needed when playing with mixing throttle on two different channels. I figured out it took 30% mix to start getting yaw - AMAZING.

This is after years of matching glow engines - not all that important! I think of all that time with a tach and adjusting needle valves. Hmmmm.

It is amazing how much it takes to really start affecting yaw. Try it!

Mike

[Larry3215]

Mike, its the principle of the thing

[rcers]

Originally Posted by Larry3215

Mike, its the principle of the thing

I get it!!!

When I think how many hours I used to fuss with glow engines to match the sync....man....



I remember that first time with setting up the "yaw" mix with differential thrust. I started with 5% thinking, o yea that will do it... Nothing!

Then when I setup the OV10 Bronco - it was like 50% before it would really start to Yaw at all.

LOL.

Mike

[Larry3215]

A lot depends on the model - how far out from center line are the motors, how much yaw stability the model has etc.

It also depends on if your controlling both motors or just one. Ive tried it where only one motor slowed down while the other stayed the same. Thats the simplest mixing option.

But if you have one motor speed up while the other slows at the same time... you need lots of free mixes or use Vtail mixers I guess would work too.

I built a Depron Mig something, twin pusher that would do the most insane flat spins with the mix full on! Fun stuff.

having the rpms close also gives you that cool reverb action