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low voltage in futaba flight pack?

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Old 08-19-2006, 11:31 PM
  #1  
aileron101
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Default low voltage in futaba flight pack?


I posted this in the manufacturers forum but perhaps some of you may have ideas to share about solving the problem.

Hello

Bear with me if this is a stupid question, but I have spent the last few hours scanning through 30 plus pages of posts trying to find an answer without much luck.

I have a GP Christen eagle that will be guided by my trusty 8uap radio. I am using the 8 ch. pcm rx that came with the radio and with a 6.0v 2700mah nimh battery pack that will run 8 hitec 645mg servos and one std s148 servo on the throttle. The plane also has a slimline showtime pro smoke pump with its own battery that plugs into the rx. I have used this radio before with the hitec servos (only then there were only 5 and it was a 4.8v nicd pack) so I know the servos work well with the rx. I have an mpi power switch with led warning lights and a 6.0v voltage regulator. All leads are 20 ga wire and are generally less than 24 inches. I got the radio programmed and there is no binding of any of the controls. The problem is that when you are banging the sticks around (say when doing snap rolls or lomcevaks) the servos will slow way up then all of a sudden they "catch up" to where they are supposed to be. Sometimes when the sticks are released the servos/control surfaces will pass through center to nearly full travel before centering. At times they bounce back and forth with a little less travel each time before they finally center. If the controls are each moved individually there are no problems. I suspect that the current is dropping too low when all the surfaces are moving at once and this is confirmed when looking at the led warning lights on the switch. The problem is a lot better without the voltage regulator but it is still there sometimes. My question is how do I eliminate this problem? The local hobby shop didn't have an answer and I see nothing in Tower's catalog to help.

Any suggestions would be much appreciated.

Thanks for your time!

~aileron101
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Old 08-20-2006, 10:02 AM
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Default RE: low voltage in futaba flight pack?

Hi,

I don't really have an answer to your problem, but I too am also interested in the answer. I am working on a project that is using (2) HS645MG's, (1) HS635, and (4) HS475 servos. I think the problem is that the HS645MG's have a No-Load operating current of 450mA each at 6V. If you are using (8) of these in your plane you are talking 8*450mA, or 3600mA. This is just the normal current draw when the servo is moving. It does not account for any appreciable load on the flight surface. I imagine the current goes up considerably when any amount of torque is required to move the flight surfaces.
So looking at your 2700mA flight battery, it will drain in 2700mAh/3600mA in about 45mins of operation. (Again this is just with all servos moving under no load conditions.) One thing I am considering is using two flight batteries of equal capacity in parallel. This should help handle the larger current drain which is required with these servos. I think the voltage regulator is a good idea. Just make sure it has enough load current capability to supply these high torque servos. If they are drawing 3.6amps no load, I would at least double that and look for a reg that can supply at least 8Amps.

Here are the specs on the HS645MG servos.



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Old 08-20-2006, 10:08 AM
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Default RE: low voltage in futaba flight pack?

I would say your problem is your pack.
A standard 2700maH pack has too much internal resistance for a large load.
You will do better with a 1800mah SC NiCad pack.
It will be a little heavier but will handle the load.
Contact Dave at Radical RC if you are interested in this pack
www.radicalrc.com
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Old 08-20-2006, 10:23 AM
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Default RE: low voltage in futaba flight pack?

Just a thought here, can you try try connecting the battery directly to thr Rx and eliminate the possible current limitation in the MPI switch/regulator, I question the need for a regulator when using a 6V battery as long as the servos are rated for 6V operation. As I write I see Geistwares post re: the battery, any chance of getting a meter on the battery with the servos moving? Do you have access to a Watt meter, if you can put one in line with the battery, that will tell all Good luck.
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Old 08-20-2006, 10:25 AM
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Default RE: low voltage in futaba flight pack?

I have done this.
Just load the pack.
You will see that above C/2 the packs voltage starts to drop quickly.
ORIGINAL: pilotpete2

Just a thought here, can you try try connecting the battery directly to thr Rx and eliminate the possible current limitation in the MPI switch/regulator, I question the need for a regulator when using a 6V battery as long as the servos are rated for 6V operation. As I write I see Geistwares post re: the battery, any chance of getting a meter on the battery with the servos moving? Do you have access to a Watt meter, if you can put one in line with the battery, that will tell all Good luck.
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Old 08-20-2006, 01:40 PM
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Default RE: low voltage in futaba flight pack?

Try powering only the receiver with a 4 cell pack and power the servos off your other battery. This involves providing a seperate power buss for the servos but it will make sure there are no dropouts or failsafes from the receiver due to power glitches in the buss powering the servos. No need for more than 4 cells on the receiver as even that is lowered (regulated) to a lower voltage inside the receiver itself for power to the RF and filter stages.
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Old 08-20-2006, 10:57 PM
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Default RE: low voltage in futaba flight pack?

Thanks guys for trying to help. If I what Wydah says is correct (and I believe it is), how would using a smaller pack be better? If I understand the example above wouldn't using only an 1800 mah pack only give me 1/2 hour at the max? I did test the battery after using it while programing the tx with my hobbico voltmeter. I realize the load the voltmewter is putting on the pack is not even close to the load I am putting on it, but, after having it on for well over an hour it still showed 6.22v under load (6.3?v with no load) I did also try plugging the battery direct into the rx and the problem was much better, but it was still there. I have never set up a plane with 2 packs and am not sure how to do it. I have been looking at something called a power expander that sounds like it may be what I am looking for but I dont want to shell out another $100 or more if it wont fix the problem. Surely there are some giant scale guys out there who have run into this before.
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