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Old Nov 28, 2012, 11:23 PM
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In praise of analog components

I screwed up this weekend and allowed my Rx battery to get dangerously low in flight. It ought not to have happened, and I'll be more careful, and I deserved to lose the airplane.

That all said-- had I been using 2.4 and all digital servos, I *would* have browned out and lost the airplane. As is, I'm using 72MHz PCM and a mix of digital and analog servos. The digital flaps went inoperative, the digital tail became intermittant (I could sometimes get very sloppy pitch control when I wasn't doing *anything* else.)

The analog ailerons, however, stayed operative, albeit with a veeeeerrrrrryyyyy sslloooooooooow response. Frightening, but controllable, and I was able to limp home in one piece. *phew*

Saved from my own err by the old technology! Adama would be proud.
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Old Nov 29, 2012, 05:51 AM
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as I have gotten into more complex gliders, all digitals, 2.4 AND 5 cell pack I have noticed the packs don't last as long. No surprise. This is especially true if I am doing a lot of launches as I think that is probably the highest drain time.

My 2M rud/ele/spoiler Spirit flew all day on a 600 mah pack.
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Old Nov 29, 2012, 06:34 AM
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And the answer lies behind door #3, modern battery technology!!! Today's LiFe and regulated LiPo batterys offer so much higher energy density, capacity per oz, temperature stability, etc., that there's no good excuse for "not enough battery".

I fly LiFe's in everything now. An 850 mah 2S in my 2M planes and 1450 or 2100 mah 2S in everything bigger. By taking each system and flying normally for 1 hour without turning off (generally 4-5 flights to include launching) then recharging with a modern charger that reports back current returned to the battery, I get my average current draw/hour. Then multiply the battery capacity by 80%, divide by the average draw and you get the number of flyable hours on the battery with a good safety margin. Typical draws per hour range from 225 mah for the 2M Sprite (6 digitals) to 270 mah for the Supra/Maxa types (6 digitals).

Long live the digital era!

JT
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Old Nov 29, 2012, 07:03 AM
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Long live the digital era!

JT
+1!

Digi servos should also not be lumped into one category when it comes to their draw on power: they differ quite a lot. Futaba s3150's I find are particularly economical.

Chris
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Old Nov 29, 2012, 07:33 AM
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Originally Posted by jtlsf5 View Post
And the answer lies behind door #3, modern battery technology!!! Today's LiFe and regulated LiPo batterys offer so much higher energy density, capacity per oz, temperature stability, etc., that there's no good excuse for "not enough battery".

I fly LiFe's in everything now. An 850 mah 2S in my 2M planes and 1450 or 2100 mah 2S in everything bigger. By taking each system and flying normally for 1 hour without turning off (generally 4-5 flights to include launching) then recharging with a modern charger that reports back current returned to the battery, I get my average current draw/hour. Then multiply the battery capacity by 80%, divide by the average draw and you get the number of flyable hours on the battery with a good safety margin. Typical draws per hour range from 225 mah for the 2M Sprite (6 digitals) to 270 mah for the Supra/Maxa types (6 digitals).

Long live the digital era!

JT
Thanks JT. Good info.

I used to use about 250 mah/h as my estimate when I was using 72 MHz and analogs servos on a full house Legend or Thermal Dancer. Today my full house contest planes are all digital, 2.4 GHz, 5 cell NiMh and I launch a lot harder. I have been using about 350 mah/h as my guestimate during contests or about 3 hours per 1500 mah pack allowing for safety margin. That will easily do 8 contest rounds, but I still tend to charge up over lunch.


One thing that often catches people by surprise is that 2.4 GHz receivers typically use more power than the 72 MHz receivers they replace. For example:

Receiver R148DF
(FM Dual conversion)
Receiving frequency: 50 or 72 MHz bands
Power requirement: 4.8 - 6.0V Ni-Cd battery
Current drain: 14 mA
Size: 1” x 2.2” x .9” (25.4 x 55.8 x 22.9 mm)
Weight: 1.1 oz (31.18 g)
Channels: 8


Futaba R617FS 2.4GHz FASST 7-Channel Receiver
SPECS: Size: 1.6 x 1.1 x .35" (40 x 27 x 9mm)
Power Requirement: 4.8 - 6V
Current Drain: 80mA (at no signal), 74 mA(w/ signal no servos on 4.8)
Weight: 0.34oz (9.8g) with case, 0.25oz (7g) without case
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Old Nov 29, 2012, 08:05 AM
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Side note: Anyone able to find the current draw of Spektrum 7 channel receiver with remote? In documenting my last post it occured to me that the Spektrum design, with remote receivers, may draw well over 100 mah, but I can't find any specs on the Horizon or Spektrum sites. What would be the draw of a 9 channel with 3 remotes? Could be significant.

Would be useful in guiding new pilots as to battery consumption as a lot of them are flying spektrum.
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Old Nov 29, 2012, 09:31 AM
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And the answer lies behind door #3, modern battery technology!!! Today's LiFe and regulated LiPo batterys offer so much higher energy density, capacity per oz, temperature stability, etc., that there's no good excuse for "not enough battery".
True. In my case I checked the on-board meter, found it green, and neglected to recharge because I thought I was safe.

Even with the green light on the meter I had less than 3 minutes of time left.

The trick is-- the on-board meter doesn't check the voltage under load, so you need to push on a servo or something and make sure it *stays* green.

Or find another way to monitor the voltage.

Or, perhaps best, recharge every time.
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Old Nov 29, 2012, 09:39 AM
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Digi servos should also not be lumped into one category when it comes to their draw on power: they differ quite a lot. Futaba s3150's I find are particularly economical.
No argument. But I believe they all do quit working altogether when the voltage drops below a certain level. (I know all 3 brands have do) My older analogs have a gradual deterioration all the way down to zero.


The takeaway is merely the same thing you find everywhere-- that the higher performance equipment tends to have more catastrophic failure modes, (in this case quitting altogether at a voltage level where the low-tech stuff simply degrades) so when you use it, you need to be more vigilant to ensure that failures don't happen in the first place.

Fortunately, as jtlsf5 points out, that extra vigilance isn't terribly difficult to come by.
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Old Nov 29, 2012, 10:21 AM
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I put zero stock in those cute little on-board LED things. I throw away any that come with planes I acquire, and strongly urge any of my flying buds to do the same. All they do is suck up current and provide no realistic measure of usable capacity. Kind of like starting a trip across 200 miles of open desert as long as the car starts.

Nothing beats good battery maintenance practices, high quality components, and good slop/bind free linkages.

JT
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Old Nov 29, 2012, 12:42 PM
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I put zero stock in those cute little on-board LED things
+1

Your writeup above of charge, timed flights, charge noting juice put back in, 80% for total usable flight time is exactly what I have been doing.
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Old Nov 29, 2012, 02:21 PM
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Originally Posted by aeajr View Post
Side note: Anyone able to find the current draw of Spektrum 7 channel receiver with remote? In documenting my last post it occured to me that the Spektrum design, with remote receivers, may draw well over 100 mah, but I can't find any specs on the Horizon or Spektrum sites. What would be the draw of a 9 channel with 3 remotes? Could be significant.

Would be useful in guiding new pilots as to battery consumption as a lot of them are flying spektrum.
Current draw listed on the instructions that came with one of my JR R921X is 70 ma. This is a 1 satellite 2.4 RX.

My Spectrum 7600 instructions do not list a current draw.

JT
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Old Nov 29, 2012, 05:59 PM
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Thanks!
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Old Nov 29, 2012, 10:28 PM
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Originally Posted by jtlsf5 View Post
I put zero stock in those cute little on-board LED things. I throw away any that come with planes I acquire, and strongly urge any of my flying buds to do the same. All they do is suck up current and provide no realistic measure of usable capacity. Kind of like starting a trip across 200 miles of open desert as long as the car starts.

Nothing beats good battery maintenance practices, high quality components, and good slop/bind free linkages.

JT
Any gadget that tries to assess a battery's level by measuring voltage alone (e,g, on-board LED things) is not very useful. I have had a great experience with an on-board audio low-voltage alarm. Like most flyers, I wiggle the sticks just before I launch. If I hear a beep, I don't launch. Wiggling the sticks puts a load on the battery and gives me a warning before the battery is too low.
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Old Nov 30, 2012, 02:22 AM
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Even AA nimh are up over 2 amp hours now. I don't know if they are as reliable as the old nicads, but that's a lot of capacity. Ever since I got a pulsed charger (similar to a Sirius), I've done pretty well with batteries. Except for the time a wire in the charger came loose!
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Old Dec 02, 2012, 01:18 AM
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