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servos , one or two?

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Old 08-01-2007, 06:17 PM
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rmh
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Default servos , one or two?

Here is a little teaser -
let's say you need 300 inch ounces of torque for a rudder
your choice is one 300 in ounce rated or two 150 inch ounce rated. same money eithe way--
pretty straightforward.
you have a battery which is capable and a rx which feeds both servos -if you choose to use two.
two servos won't fight as you can use a floating bellcrank setup .
So - given the fact that linkages are no issue and in this case weight is no issue and the servos all work as advertised -
which setup has the best chance of delivering 300 inch ounces, using standard wires and plugs as supplied.

Then what if you had another setup which needed 100 inch ounces and you could use either two 50 in ounce or one 100 in ounce (same type hypothisis as above)
would you make the same choice (2 or 1) again?
why.
what is different?
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Old 08-01-2007, 06:31 PM
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Default RE: servos , one or two?

I'm going to venture a guess here Dick, and opt for the TWO servo arrangement. You have less voltage drop in TWO sets of wires & connectors than you would in just a single connection...provided everything is in fact the same.

I stand waiting to be crushed....

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Old 08-01-2007, 06:47 PM
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Default RE: servos , one or two?

ok- in both situations?
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Old 08-02-2007, 11:48 PM
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Default RE: servos , one or two?

Sorry Dick...I got dragged clear on across the country...now I can get back to this.

Hmmm. I think in the 100oz/in arrangement, I'd opt for the single servo. While the parallel wiring/connector advantages would likely still exist to some extent, the simplicity of the single servo arrangement would outweigh in my mind any other advantages.

That, and 100oz/in + servos are pretty easy to come by.

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Old 08-03-2007, 08:41 AM
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Default RE: servos , one or two?

Yep - my point was - you have to consider the peak loads of a 300 inch ounce servo.
IF-- you restrict power to these devices - it drops the voltage -which will cause heating in them.
-I se postings of "failures in the big servos which likely are due to the power delivery to them.
Our present radio systems are in a bit of a mess -
the wiring and connectors and in general, the usual batteries etc., are simply not capable of handling some servo arrangements
tie this to the stampede to 2.4 rxs (which being itty bitty computers) and you have a made to order failure of the rx the servos and --the models.
also the proliferation of elaborate power distribution devices -which are quite lovely to look at and use - may actually -accidentally make the situation worse if powered by improperly sized and powered regulators and batteries .
One guy commented he used 11 v batteries -regulated to 5 volts - for maximum safety
Sounds good - unless you know how linear regulators function .
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Old 08-06-2007, 07:27 PM
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Default RE: servos , one or two?

If you truly need 300 oz in (thats 18 lbs on a 1" arm) a servo rated for that thrust will only provide it for a few seconds. The servo will need more than 3A. If the batteries and the wireing can supply it the servo will overheat and will lose its rating quickly. It might even burn out. You need to use two servos.
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Old 08-07-2007, 07:34 AM
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Default RE: servos , one or two?

Interesting discussion. The H9 P-51 150 has two retract servos each rated at 260 oz/in. They operate at the same time and they take 2+ seconds to rotate 90 deg. Seems like most folks are putting them on a separate battery. Based on this discussion its probably a requirement if one is using a computer reciever. Cycling up and down repeatedly probably isn't a good idea either.

thanks,
Brian
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Old 08-07-2007, 07:59 AM
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Default RE: servos , one or two?

So what about a 140 size 72’’ wing plane wt. 12lbs with a servo on each aileron and each side of the elevator? Large plane but a Standard servo on each control service and a good larger servo on the rudder?
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Old 08-07-2007, 10:09 AM
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Default RE: servos , one or two?

The retract 180 degree servos have a large power multiplying gear train and really don't use as much power as the 100 deree gear trains
As for really good servos for the med sized models -I have yet to see a servo a good as the JRSport 126 rated at 140 in ounces of torque
Having used these on 33% models for elevators as well as using the 8411 and 4721 etc., I have to say that the excellent gear train on the 126 plus th overall operation makes it my favorite
before you poo poo it try it - I use a set of the 125's for almost a year in my Funtana 90 /26 Evolution powered ,with ZERO gear slop ever occuring .
it is neither digital or coreless -just a simple 3 pole motor but with my DX7 radio the elevators follow one trim click at a time .
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Old 08-07-2007, 11:09 AM
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Default RE: servos , one or two?

I think it best to not design a control system to the maximum spec. As Dirtybird said the 300 oz-in servos can't put out that much torque for long before they either overheat and drop off, or burn out. Since the connector is the limiting factor, I would say going with two 150 oz-in servos would give you more energetic rudder deflection later in the flight.

Also regulating 11v down to 5v with a linear regulator can eliminate more than HALF of the capacity of the pack (P_waste = (11v - 5v) * current draw), so power wasted (P_waste) is the current times the voltage difference. Most people using those systems forget that! Essentially he is generating 20% more heat than power into the radio. Or, only 45% of the battery power even makes it to the radio. Further, don't linear regulators increase resistance as the temperature increases?

This is why the A123 batteries seem to be the logical choice in many applications. They are low capacity, but also generate little internal heat (wasted energy), and can deliver the full 2600 mAh without regulators. Just putting the reg into the system will require a certain percent greater capacity, and increasing internal impedence of other battery types require even greater capacity, etc. An off the top of my head, rough calculation would say that a 2600 mAh A132 receiver pack at 6.2v would be roughly equal to a 3700 mAh Li-Po regulated to 6.2v (depending on battery impedence, etc).

Sorry, this is getting off topic.
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Old 08-07-2007, 11:14 AM
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Default RE: servos , one or two?


ORIGINAL: aghost

Interesting discussion. The H9 P-51 150 has two retract servos each rated at 260 oz/in. They operate at the same time and they take 2+ seconds to rotate 90 deg.
Brian
Brian, it would be very interesting to measure the current draw of your retracts while they are cycling. The retracts are "rated" at 260 oz-in, but they probably don't use all of this torque and may not draw much current.

Since we are severely "current limited" in our setups, how long will it be before radio manufacturers upgrade their connectors to a system capable of 10A, or even 5A? I could switch everything over to the old Deans plugs, but that is just too much work!
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Old 08-07-2007, 11:43 AM
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Default RE: servos , one or two?

The 123 cells are 2300 mah but usuable is 2000 mah
the voltage under load in our application as a rx pack is as good or better than a 4300 ma sub C 6 volt pack - I load compared em side by side--I was looking at voltage depression under working loads -not stalled out loads - jut max furious back n forth full 150% thro loads on all sticks .
servo ratings are BULLSHI-T except for comparing then and hoping the comparisons were all done in the same manner --that is -on a regulated voltage power supply capable of staying at 6 volts under any load
also servo speed ratings are BULL- they are not rated under a load .
on a model with little surface loads the very fast rated servo MAY outperform th torque rated one but don't bet the farm on that one-- under load the "fast " servo may be slower in response to the load.
The 180 gear train retracts don't pull much power as they run the motors very fast and the output very slowly just like the flashlight cell motors and gear trains in th old Erector Set kits
Note:
for you kiddies -an Erector Set had nothing to do with erectile dysfunction. It was a toy for kids who were actually interested in learning how mechanical things go together and operate.
A lost cause ------
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Old 08-07-2007, 11:52 AM
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Default RE: servos , one or two?


ORIGINAL: BTerry


ORIGINAL: aghost

Interesting discussion. The H9 P-51 150 has two retract servos each rated at 260 oz/in. They operate at the same time and they take 2+ seconds to rotate 90 deg.
Brian
Brian, it would be very interesting to measure the current draw of your retracts while they are cycling. The retracts are "rated" at 260 oz-in, but they probably don't use all of this torque and may not draw much current.

Since we are severely "current limited" in our setups, how long will it be before radio manufacturers upgrade their connectors to a system capable of 10A, or even 5A? I could switch everything over to the old Deans plugs, but that is just too much work!
I think the thing to do would be go to 9V or even 12V. That would mean we would be dealing with much less amperage for the same ratings.
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Old 08-07-2007, 11:54 AM
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Default RE: servos , one or two?

double post
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Old 08-07-2007, 12:33 PM
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Default RE: servos , one or two?

Meccano
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Old 08-07-2007, 12:45 PM
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Default RE: servos , one or two?

I loved my Erector set....

wish I could find them for my son
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