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Old 03-05-2011, 01:35 AM   #1
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Default Twin Engine Low Voltage Cut Off?

Installing e power into a Top Flight DC3. I know I need 2 ESC's but I'm wondering about the low voltage cutoff for each engine. It seems possible that the two ESC's might cut the power off at different times resulting in asymetric thrust (which I don't want).

How do I handle this situation with two engines and assure that the low power cutoff works at the exact same time for both engines??

I'm using EFlight 40 Amp Pro ESC's.
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Old 03-05-2011, 01:39 AM   #2
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To my knowledge the ESC makers don't provide a mechanism to accomplish this. This would be a nice feature but probably not used very often and so not worth investing in the engineering and manufacturing modifications.

Best bet is don't fly all the way to LVC. Know how long your power will last and land first.
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Old 03-05-2011, 01:42 AM   #3
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Originally Posted by bsrchas View Post
Installing e power into a Top Flight DC3. I know I need 2 ESC's but I'm wondering about the low voltage cutoff for each engine. It seems possible that the two ESC's might cut the power off at different times resulting in asymetric thrust (which I don't want).

How do I handle this situation with two engines and assure that the low power cutoff works at the exact same time for both engines??

I'm using EFlight 40 Amp Pro ESC's.
That's a problem.

IMHO, best way to handle this is to never fly more than about 60-70% of the battery capacity during any particular flight. First, this should reduce chances of "Single Motor" operation, second, its easier on your Lipo batteries.


Don't know, maybe someone has designed something that will monitor the battery voltage, and when the battery goes below safe values, cut power to both motors.

I could design something like this using a little microcontroller, but am to busy right now, working on another big microcontroller project.
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Old 03-05-2011, 02:03 AM   #4
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Are you using 2 batteries for this? If so you might want to run them in parallel it is more wiring to run, but it will give you piece of mind not having to think about a ESC hitting LVC leaving you with one motor spinning.
GlacierGirl suggested that I do it on a P-38 I am working on. Two batteries with one battery on each side then I ran wire to both batteries to the 2 ESC If I ever hit LVC at least both motor will shut down and not one.
As it was mentioned by the others don't fly that long do the first flight for 5 minutes and check the voltage to gauge how long you can fly your next flight.
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Old 03-05-2011, 02:17 AM   #5
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I have flown many many electric twins. I have never had an issue with one motor quitting before the other. I have also had good luck with them spooling up at the same time. This is also important (maybe even more important).

Here are my rules for twins:
  • Use one battery for both motors. You can use two packs in parallel if needed.
  • Disable BOTH of the ESC's BEC's and use a stand-alone BEC or separate battery pack
  • Never fly to LVC (that is good advice for any plane)
  • Use good quality ESCs of the same brand and rating
  • Use good quality motors of the same brand and Kv
I have flown to LVC before in a twin and have never had an issue with asymmetric thrust. Both motors quit very close to the same time. This is likely due to good, quality ESC's.

Mike
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Old 03-05-2011, 02:17 AM   #6
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Originally Posted by gramps2161 View Post
Are you using 2 batteries for this? If so you might want to run them in parallel it is more wiring to run, but it will give you piece of mind not having to think about a ESC hitting LVC leaving you with one motor spinning.
GlacierGirl suggested that I do it on a P-38 I am working on. Two batteries with one battery on each side then I ran wire to both batteries to the 2 ESC If I ever hit LVC at least both motor will shut down and not one.
As it was mentioned by the others don't fly that long do the first flight for 5 minutes and check the voltage to gauge how long you can fly your next flight.
Response from rcers, #5: That's good to know. Someday, I might be tempted to actually build another twin.

Don't know if the two ESC shut down LVC voltages would be exactly the same on two different ESC's. Might be a good idea to deliberately run the two ESC's to LVC on the ground to verify that they will simultaneously shut down.

If you can do it, parallel connection of the batteries would be a good idea.
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Old 03-05-2011, 07:13 AM   #7
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Originally Posted by gramps2161 View Post
Are you using 2 batteries for this? If so you might want to run them in parallel it is more wiring to run, but it will give you piece of mind not having to think about a ESC hitting LVC leaving you with one motor spinning.
GlacierGirl suggested that I do it on a P-38 I am working on. Two batteries with one battery on each side then I ran wire to both batteries to the 2 ESC If I ever hit LVC at least both motor will shut down and not one.
As it was mentioned by the others don't fly that long do the first flight for 5 minutes and check the voltage to gauge how long you can fly your next flight.
Thats still not going to insure that both esc's will hit LVC at the same time.

Even using higher quality esc's like Castle - which allow you to set a custom LVC at 0.1 volt intervals - its still not at all likely that both esc's will cut off at exactly the same time.

I have played with this for hours trying to sync two esc's and its a waste of time

If your using a cheaper esc's - forget it. You would have to be very lucky to get two different ones that actually agree on a cutoff voltage.

What happens is if one esc decides to cut off even a millisecond sooner than the other one, as soon as that load drops off, the battery voltage rises and now the second esc is seeing a higher voltage and will not cut off.


The only way to have two motors on two esc's cut off at the same time is to have the luck of a mega-jackpot lottery winner

I think I need a signature.
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Old 03-05-2011, 07:47 AM   #8
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I'm building my first twin , a 64inch span scratch build foam Wellington. I was concerned about differential thrust in either motor and then I saw this thread on RC Groups:-


http://www.rcgroups.com/forums/showthread.php?t=1124466

It seems that with a GWS V- tail mixer you can control the speed of either motor and cut the power to one if needed.
This would seem to be well worth a try so I've got a V-tail mixer on order from HC.
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Old 03-05-2011, 12:26 PM   #9
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Originally Posted by Larry3215 View Post
Thats still not going to insure that both esc's will hit LVC at the same time.



The only way to have two motors on two esc's cut off at the same time is to have the luck of a mega-jackpot lottery winner

Thanks for the lesson Larry. I thought if they where set to the same with the batteries in parallel this would help to take care of that problem. Nothing is a absolute except gravity.

Still it does give some added insurance to his question running them in parallel instead of 2 separate packs running independently. Then if the motors are not matched one could draw more amps and definitely shut down before the other. That would be less than a millisecond as you mentioned. So he still would be better off with 2 motors and the batteries in parallel.
I did this with 2 other twins, but one thing I never do is fly to LVC I did that once with a single motor I forgot to set my timer and got into the flight a little to much. Powered off and then throttled back up had enough power to get the plane down safely.
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Old 03-05-2011, 01:42 PM   #10
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Originally Posted by baz49exe View Post
It seems that with a GWS V- tail mixer you can control the speed of either motor and cut the power to one if needed.
This would seem to be well worth a try so I've got a V-tail mixer on order from HC.
Differential thrust ROCKS on twins. I have used it on several that lacked rudder(s) and it is quite effective. I use mixing and each ESC has its own channel rather than a V-tail mixer however.

Here is the appropriate bit of info for the OP. It actually takes quite a bit of differential thrust to make much difference! In some cases one motor is at zero power and the other is at 65-75% before it yaws well!

It differs based on plane, design of the twin, but some like the twin tail OV-10 Bronco takes a vast amount of differential.

At any rate - time your fllights don't use LVC and forget about asymmetric thrust.

Mike
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Old 03-05-2011, 03:31 PM   #11
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Have to agree with everything that's been posted. Like said best advice is not to get to that point, where you have esc's shutting down.

And heck yeah use your tx capabilities to mix the differential steering. Simple two mix set up. I have a quasi P-38, built from 3 wiffle ball bats.
No rudders. I use differential to steer it on the ground, and if I want I can use it in the air to make sharper turns without a lot of banking. Works like a charm. Left the mixes on and tied to the rudder stick.

When I die, I want to go like my Grandfather did, in his sleep...... Not screaming like the passengers in his plane.
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Old 03-05-2011, 04:41 PM   #12
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Wow! Great board. I just joined because electrifying my DC3 is my first attempt at eflight. I've always been a gas guy.

Lots of hours went in to the DC3 so I didn't want to stuff it if I accidentally flew to LVC.

I never did fly the DC3 yet because I had trouble on bench runs trying to exactly sync two 52 four strokes (they're for sale now on ebay) and was concerned about the outcome if one of engines died in flight.

I guess my concern over the LVC was a carryover from concerns about the gas engines.

Thanks to all for your input!
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Old 03-05-2011, 05:35 PM   #13
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Your concern is a valid one. but its like the joke where the guy says to his Doc "It hurts when I do this..." and the Doc says - "Then dont do that"

Dont fly to LVC

I think I need a signature.
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Old 03-05-2011, 08:18 PM   #14
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Originally Posted by bsrchas View Post
Wow! Great board. I just joined because electrifying my DC3 is my first attempt at eflight. I've always been a gas guy.

Lots of hours went in to the DC3 so I didn't want to stuff it if I accidentally flew to LVC.

I never did fly the DC3 yet because I had trouble on bench runs trying to exactly sync two 52 four strokes (they're for sale now on ebay) and was concerned about the outcome if one of engines died in flight.

I guess my concern over the LVC was a carryover from concerns about the gas engines.

Thanks to all for your input!
One of my club members has a lot of experience with twin engine scale models. He's indicated that if you ever have an engine failure on one of these heavy models, it goes into a spin, and will crash. Period.

He's thinking of electrifying an F7F model with 900 square inch wing, and some 33 pounds. Thats over 80 ounces per square foot wing loading!

I'm trying to disuade him from doing it.
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Old 03-05-2011, 10:38 PM   #15
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Originally Posted by kyleservicetech View Post
One of my club members has a lot of experience with twin engine scale models. He's indicated that if you ever have an engine failure on one of these heavy models, it goes into a spin, and will crash. Period.

He's thinking of electrifying an F7F model with 900 square inch wing, and some 33 pounds. Thats over 80 ounces per square foot wing loading!

I'm trying to disuade him from doing it.
80 oz/sq ft??? Talk about a lead sled! Ive seen a few flown at almost that hi a loading. Takeoffs and landings at mach 1

Your other comment about spins reminded me of another good tip for flying twins.

If you do loose one motor for any reason - and it wont come back with an esc re-set - the safest thing to do is go to zero throttle, push the nose down to keep the speed up, and dead stick it in.

I have to warn you, its going to be very hard to resist the temptation to use throttle even though it may be the worst thing you could do

I think I need a signature.
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Old 03-05-2011, 10:43 PM   #16
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Originally Posted by Larry3215 View Post
80 oz/sq ft??? Talk about a lead sled! Ive seen a few flown at almost that hi a loading. Takeoffs and landings at mach 1
Yeah, per motocalc, its going to be difficult to obtain a flying speed much over the stalling speed, using a pair of Hacker A60-16M motors and 10S Lipos.

It's going to cost him several thousand dollars in power equipment what with batteries, ESC's and motors. I'm afraid when its done, it won't fly well.

He does NOT want to go to 30 cc gasoline engines because of vibration and a lot of other issues. My Hacker A60-16M motor puts out more power than a good 30 cc gasser on 12S2P A123's. Might be able to save a pound or three by going to Lipos. But its still a LOT of money for something that might not be a good flyer.

I've had my hands on the cowl for this model. You can fit 24 A123 cells in each cowl! But those danged wings are skinny, with little lifting area.
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Old 03-05-2011, 10:48 PM   #17
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When scale planes get into the low 40's is when they stop being fun for me. 80's is insane. There are better building techniques that avoid such things. Like foam! I don't know what he is doing to get such high numbers but that would sure take the fun out of it for me!

I am lucky to have Keith Sparks as a good friend and flying buddy. He builds most of his masterpieces from foam now. When glassed and finished, nobody knows they are foam, in fact when he opens a hatch people are shocked.

The best thing is how light they turn out. He is generally 1/3 to 1/2 the weight of a balsa/ply model.

Here is are just a few examples of his fantastic airplanes - all of these are foam.

Have you buddy read this thread and save him some grief! http://www.rcgroups.com/forums/showthread.php?t=1287190

Mike


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Old 03-06-2011, 12:49 AM   #18
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Originally Posted by Larry3215 View Post
80 oz/sq ft??? Talk about a lead sled! Ive seen a few flown at almost that hi a loading. Takeoffs and landings at mach 1

Here is a youtube video of this model after it was electrified. Take a look:

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