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Old 03-03-2011, 09:55 PM   #1
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Default Analog or Digital on large electric planes?

What's everyone using? I'm getting ready to build a 78" Tiger Moth + a couple of other large models and was curious.

http://www.hobby-lobby.com/supertigermoth.htm
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Old 03-03-2011, 10:00 PM   #2
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Depends on the type of plane. When I need strong light small servos I go digital. They hold great and are monster power.

In a scale airplane - much less concern and analog is fine.

Budget comes into play too.



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Old 03-03-2011, 10:02 PM   #3
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My next build will be the 78" Tiger Moth from Hobby-Lobby.

http://www.hobby-lobby.com/supertigermoth.htm
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Old 03-03-2011, 10:16 PM   #4
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Zero reason for digital servos on that plane IMHO.
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Old 03-03-2011, 10:46 PM   #5
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I've been using these cheapies from Hobby-King:

Hextronik 5010

On the smaller stuff I've been using them on they do OK but they don't always center well and from what I've read have very poor resolution. I was thinking that with a huge rudder like on that Tiger Moth, I may want to go with something better (but still cheap lol).

I can get these for $15 locally.

Hitec HS-5485HB
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Old 03-03-2011, 10:57 PM   #6
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Yea I am a Hitec fan. Very much about not risking a 1k investment on a cheap, crap China cloned servo. I am NOT a fan of them as you might be able to tell.

Saving $20-40 on a 1k investment plane (plane, motor, servos, batts, rx, time, blood, sweat, tears) just makes no sense to me.

Again in all candor I am NOT impressed with Chinese servos. That beautiful Moth deserves a few bucks in Hitec servos.

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Old 03-06-2011, 09:49 PM   #7
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Originally Posted by rcers View Post
Zero reason for digital servos on that plane IMHO.
What type of planes do you use digital servos on? What's your criteria for deciding whether to use analog or digital on any given plane?
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Old 03-06-2011, 11:57 PM   #8
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Originally Posted by bicyclemonkey View Post
What type of planes do you use digital servos on? What's your criteria for deciding whether to use analog or digital on any given plane?
Read post #2 above.

I use digital servos in planes that require what digital servos excel at. Holding power, torque, speed and centering.

Good analog servos are certainly able to do those things as well, I don't mean to indicate they can't, but digital servos are even better.

So digitals in aerobatic and 3D type planes. Heli guys use them all the time as well for those same reasons.

Many scale models just don't need digital servos - especially the intended Tiger Moth the OP was talking about. It is not terribly demanding on servos so cheaper analog are just fine.

Price is another factor! I use them when I can afford too! So on my expensive high dollar planes tends to send me toward digital as well.

Mike
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Old 03-07-2011, 12:57 AM   #9
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So for warbirds, which is mostly what I fly, analog will be fine? I'm talking large scale, not park flyers here.

My next build after the Moth will be an 82" Texan...analog?

So:

Sport/3d planes...digital?

Scale planes...analog?
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Old 03-07-2011, 01:11 AM   #10
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The Moth is a gentle calm flyer. I just don't feel the need for Digital servos for that.

For a 82" Texan you might want to consider digitals. Ask yourself what you are unhappy with on your analog servos? Do you need more power for the Texan? Do you need more speed? If so you can look at digital servos.

So look at how much torque, speed, holding and if you need programmable servos. Then look at servos that meet your specifications. Then you can decide. Budget comes into play as well. For an 82" Texan if you need 5-8 digital servos that can run you $400-$800.

For 90% of my planes I choose analog as I can find good analog servos that do the job just fine.

Mike
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Old 03-07-2011, 01:50 AM   #11
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Originally Posted by bicyclemonkey View Post
What's everyone using? I'm getting ready to build a 78" Tiger Moth + a couple of other large models and was curious.

http://www.hobby-lobby.com/supertigermoth.htm
I've got a 17 pound Extra 330 by Carl Goldbury (who???) with a 2400 watt power system. Servos are Hitec HS 645MG's, four on the ailerons, two on the elevator, and a HS 5625MG on the rudder.

Not been flown yet, waiting for the snow to go away on our flying field.

SERVO TORQUE CALCULATOR
http://www.mnbigbirds.com/Servo%20Torque%20Caculator.htm
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Old 03-07-2011, 03:49 AM   #12
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If you can afford it - always go digital.

Even if you cant, its still worth it

Id use digitals even on the Moth. Why fly sloppy just because your flying slow?

Digital servos work better, center better, hold better, are more precise. They give you a better "feel" for the plane.

Once I started using digitals, I swore I wouldnt go back on anything larger than a foamy.

Even then, many of my foamies use digitals

The new Hyperion digitals are excellant servos and the pricing isnt much more than that of a decent analog servo.

http://www.allerc.com/servos-hyperion-c-40_259.html

I wont ever use a cheap Chinese servo in any plane.



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Old 03-07-2011, 02:03 PM   #13
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I have used the Hyperion digital servos and have been impressed, will all things except one. They were power hungry. That is common and one of the traits of digital you must account for. My 5 amp BEC could not give them the power they needed (4 servos) and my system locked out. It happened 50 feet off the ground in a landing approach. It did not end happily. Perhaps that soured me just a bit too.

I have used analog servos switched to digital on sport planes and can't tell much if any difference, except in my wallet.



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Old 03-07-2011, 04:36 PM   #14
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So I guess a good rule of thumb for large electric planes would be that if you use digital servos, you should use a RX battery instead of a BEC/flight battery?
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Old 03-07-2011, 04:37 PM   #15
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Originally Posted by rcers View Post
I have used the Hyperion digital servos and have been impressed, will all things except one. They were power hungry. That is common and one of the traits of digital you must account for. My 5 amp BEC could not give them the power they needed (4 servos) and my system locked out. It happened 50 feet off the ground in a landing approach. It did not end happily. Perhaps that soured me just a bit too.

Mike
Agreed:
I've got a CC 10 amp uBEC in my giant scale Extra 330, with 6 Hitec HS645MG servos, plus one of their digital servos for the rudder.

Moving all seven servos at once caused the 10 Amp uBEC to drop its voltage, causing the Spektrum AR7000 receiver to reboot. Therefore, take a look:
http://www.wattflyer.com/forums/showthread.php?t=58310 (I've added the extra diode mentioned in the text to the schematic)
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Old 03-07-2011, 04:49 PM   #16
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Originally Posted by bicyclemonkey View Post
So I guess a good rule of thumb for large electric planes would be that if you use digital servos, you should use a RX battery instead of a BEC/flight battery?
Or, get the real high power Castle Creations uBEC. IMHO, a giant scale model should really have dual receiver power systems, just in case.

And, don't use those 2500 Milliampere Hour "AA" size receiver batteries. They can't put out the 10 amps or so peak current drain required by some of those giant scale models with all of their servos.

In my case, I've put together an A123 2300 Milliampere battery pack as a backup to the Castle Creations 10 Amp uBEC. Works well, and those 7 servos in my giant scale model are not going to pull down that A123 pack. Expecially when that same pack can easily put out 30 - 40 amperes, running an electric motor.
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Old 03-08-2011, 12:42 AM   #17
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I agree - digital servos draw more power than analog servos.

But even with analog servos, when you have several large ones on a model the internal BEC just isnt going to cut it.

For large models, you need to have a bullet proof power supply for the rx and servos - no matter what equipment your using.

For my money, a hi quality 10 amp BEC is the absolute minimum and as noted above - it wont be enough in many cases.

On my big balsa models where I have tons of money invested, I use 2ea 2S lipo's driving 2ea 10 amp regulators that feed the rx through a failsafe switch thats designed to draw power from either source if one fails.

Thats not a cheap setup. The regulators are about $40 ea and the failsafe switch is another $60 plus the lipo's, wire and connectors.

My next step down is to use a pair of 2S A123 packs instead of the lipos and regulators. You have to be sure your servos can handle 7 volts peak though.

On my giant foamies I usually just use a 2S lipo and regulator with a normal switch or no switch or in some cases just a 10 amp UBEC.

In any case - rx power is a very poor place to cheep out on a giant scale model.

Even if all the stuff comes from Hobby City, you still have a substantial investment.

More importantly though - that larger model can do a lot more damage than a small foamy if you loose control and hit someones house, car or worst case - another person.

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Old 03-08-2011, 12:51 AM   #18
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Originally Posted by Larry3215 View Post
But even with analog servos, when you have several large ones on a model the internal BEC just isnt going to cut it.

For large models, you need to have a bullet proof power supply for the rx and servos - no matter what equipment you are using.

For my money, a hi quality 10 amp BEC is the absolute minimum and as noted above - it wont be enough in many cases.

My next step down is to use a pair of 2S A123 packs instead of the lipos and regulators. You have to be sure your servos can handle 7 volts peak though.

IMHO, anyone using the ESC's built in LINEAR voltage regulator on anything larger than a small model is really asking for problems. When these linear regulators are run over their rating, they get hot, and shut down to protect them selves. And you crash. By the time you get to what's left of your model, that regulator has cooled off, and will start working again.
To really make that regulator work well, it needs a finned heat sink, of a physical size larger than the ESC itself.

Or pick up one of those BEC's with the switching power supply uBEC's such as the Castle Creations ICE series.

I just happened to have done some testing on the 2S 2300 Milliampere Hour A123 cells, versus a brand new 1500 Mah "Sub C" five cell Nickel Hydride receiver battery pack. That Nih pack actually ran closer to 2000 Mah under test.

Interesting, their characteristics are virtually identical. Take a look at the attached JPG of a discharge test of the A123 pack, at room temperature and at freezing, plus that Sub C Nickel Hydride pack. (FYI, I also tested that A123 pack pulling 15 Amps at 31 degrees F. Its output voltage dropped to 5.6 Volts DC after one minute at 15 Amperes.)

Conclusion, if your receiver and servos can handle a 5 cell Nickel Hydride pack, it can also handle a 2S A123 pack.


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