[NSRCA-discussion] Digital and analog servos on sport plane

Fri Mar 27 10:45:02 AKDT 2020

I normally just watch the conversations, but fell compelled to reply to this one.

When the 2.4 radios first came out, I was an early adopter.  I bought the DX7 "package" that came with radio, servos, receiver, NiCd battery, and switch.  I built two airplanes that winter with that technology.

When I started flying that spring, I was very careful about monitoring the battery using an "old school" expanded volt meter after each flight.  I lost both airplanes a week apart under suspicious circumstances.  During the third flight of each airplane, the receiver apparently browned out during the down side of a loop.  I had just regained control of the airplane before it pancaked into the ground at full speed.  3.5 seconds is a long time when the airplane is pointed straight down at the ground at full throttle.

The day after the second crash I found an article that discussed how much current a digital servo could draw.  I had 4 of them in that airplane.  So I set up a simulated environment on my kitchen table with the four servos, the two batteries from my crashed airplane, and a volt watch plugged into the receiver.  Fresh off the charger everything was fine.  Using the battery from the last crash (without charging) it still read OK on the volt meter, but went into Red zone on the volt when the sticks were stirred.  VERY expensive lesson learned - a "normal size" Nickle battery could not supply the necessary current required by digital servos.  Within two weeks of my crashes warnings started appearing on all the on-line websites warning about current draw with digital servos and advising against Nickel battery technology with their use.

I started buying LiFe batteries to use for flight packs, initially using Black&Decker VPX power tool packs with an adapted harness and heavy duty (with larger gauge wires) switches.  End of problems and the additional advantage that you didn't need to charge the night before you wanted to go fly.  When the airplanes came out of the truck they went right onto the charger.  And the batteries held 95% charge for months.  What was not to like about this chemistry?

In today's world, the price of a quality LiFe packs are the same as a Nickle pack or less.  They're about the same size and weight.  And if you're flying electric and have any kind of modern advanced charger, you are already the setup to handle this chemistry.  If you're not familiar, investigate the Turnigy Nano-tech line of LiFe batteries as a starting point.  The connectors are a JR/Hitec style plug so they are pretty universal.  Don't forget the Heavy Duty switch - I've been using the Mpi MAXX PRODUCTS line because they offered a version that had Deans connectors, but also have a JR/Hitec version.

With a single digital servo you might avoid the problems I experienced.  However, unless you're using servos that will not tolerate 6.0 volts (and you already said you're using a 5 cell pack pack) everything should plug right in and work.  New technologies are always a bit mysterious and intimidating.  This one has been a real positive for me.

I started using the VPS packs which lasted for 5 - 7 years.  When Hobbico got into the business of LiFe packs, I jumped on those instead because they were already to go (no wiring harness build needed).  They worked well with most lasting 4+ years.  Sadly that production has ended, but others picked up the slack.  I'm using the Nano-tech and NOS Hobbico LiFe batteries on all my planes now.  I always cycle the pack 3 - 5 times before putting it in service.  If it doesn't measure up, it gets relegated to desk-top duty.  Sadly, the new production stuff doesn't have quite the quality materials that the old stuff did.  I buy 3 to 5 at a time so I always have a replacement and cycle test my current fleet every year and a specific airplane when necessary if the replaced milliamps seems suspicious after charging.

Your mileage may vary.  Hope this was helpful.
Robert

________________________________
From: NSRCA-discussion <nsrca-discussion-bounces at lists.nsrca.org> on behalf of Phil Spelt via NSRCA-discussion <nsrca-discussion at lists.nsrca.org>
Sent: Friday, March 27, 2020 7:51 AM
To: John Pavlick <jpavlick at idseng.com>; General pattern discussion <nsrca-discussion at lists.nsrca.org>
Subject: Re: [NSRCA-discussion] Digital and analog servos on sport plane

Right you are, John.  I forgot that little detail.  I always use 6-volt Rx packs, anyway, so as long as I monitor pack capacity during a long flying day, current draw is not a problem.

Phil Spelt, AMA 1294, Scientific Leader Member
SPA L-18, Board Member, NSRCA 2032, KCRC Emeritus
Knoxville, TN  37931   (865) 604-0541c

On March 27, 2020 at 8:51 AM John Pavlick via NSRCA-discussion <nsrca-discussion at lists.nsrca.org> wrote:

Mike,

Make sure that your airborne batteries or the BEC in your speed controller are capable of delivering sufficient current. Digital servos can draw a lot more current than analog servos!

John Pavlick

Cell: 203-417-4971

[idslogo2]

Integrated Development Services

From: NSRCA-discussion <nsrca-discussion-bounces at lists.nsrca.org> On Behalf Of Phil Spelt via NSRCA-discussion
Sent: Thursday, March 26, 2020 6:36 PM
To: Michael Cohen <precisionaero at hotmail.com>; General pattern discussion <nsrca-discussion at lists.nsrca.org>
Subject: Re: [NSRCA-discussion] Digital and analog servos on sport plane

Hey, Mike,

I don't know about Airtronics, but the Spektrum radios I fly (DX-9 and iX12) are capable of two frame rates -- 22 msc and 11 msc.  In order to use the faster frame rate, you need all digital servos, as the analog can't follow the faster frame rate.  If  your Tx works the current analog servos, I don't think you will find any electronic issues, as the digital servos are equally capable of following the slower frame rate.  The pulse widths and voltage changes to create the pulses are well-standardized, these days.  Whether you would experience any "flying feeling" differences between functions with and without digital servos, I don't know.

What functions are you ging to use digital servos for, and why?  Just curious...

Phil Spelt, AMA 1294, Scientific Leader Member

SPA L-18, Board Member, NSRCA 2032, KCRC Emeritus
Knoxville, TN  37931   (865) 604-0541c

On March 26, 2020 at 6:23 PM Michael Cohen via NSRCA-discussion <nsrca-discussion at lists.nsrca.org<mailto:nsrca-discussion at lists.nsrca.org>> wrote:

Hope everyone out there is COVID-19 free and using this time to build or fly, with social separation, if the weather is nice enough.

I have a sport plane where I want to put a digital servo in with the inexpensive analog ones the plane came with.  Any know issues with such a setup?  This will be with an Airtronics radio, if that makes a difference.

Be safe!

Mike Cohen

Sent from Mail<https://go.microsoft.com/fwlink/?LinkId=550986> for Windows 10

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