[NSRCA-discussion] Digital and analog servos on sport plane
John Pavlick
jpavlick at idseng.com
Fri Mar 27 12:20:29 AKDT 2020
Sorry to hear that you had to learn this the hard way. To be clear it's not
that NiCads themselves are bad or inferior in any way. In fact there are
plenty of NiCad batteries that could easily provide power for multiple
digital servos. The problem is that the NiCads that we typically use in
model airplanes are of the "high-capacity" variety, NOT the "high-current"
variety. Although all NiCads have the same output voltage, they don't all
have the same output current capability. We typically use 700mAh - 1000 mAh
NiCads in 60-size and smaller sport airplanes. Even the 1000 mAh cells could
create the conditions that Robert described. It's because the
"high-capacity" variety usually have higher internal resistance. Anyone who
flies electric Pattern knows what this means: they can't deliver their rated
output voltage at high current levels. The servos want what they want, when
they want it. If the current demand is high and the resistance is high as
well, the voltage will sag.
There are plenty of good batteries available that will be able to provide
enough current to feed the digital servos. I was using dual 900 mAh LiPos
with 2 TechAero regulators in my glow Pattern planes. This setup is light
and it gives you redundancy: if one pack fails for any reason you can still
fly.
Just do your research before you risk your airplane.
John Pavlick
Cell: 203-417-4971
Integrated Development Services
From: NSRCA-discussion <nsrca-discussion-bounces at lists.nsrca.org> On Behalf
Of ROBERT SOX via NSRCA-discussion
Sent: Friday, March 27, 2020 2:45 PM
To: chuenkan at comcast.net; General pattern discussion
<nsrca-discussion at lists.nsrca.org>
Subject: Re: [NSRCA-discussion] Digital and analog servos on sport plane
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
<mailto:nsrca-discussion-bounces at lists.nsrca.org> > on behalf of Phil Spelt
via NSRCA-discussion <nsrca-discussion at lists.nsrca.org
<mailto:nsrca-discussion at lists.nsrca.org> >
Sent: Friday, March 27, 2020 7:51 AM
To: John Pavlick <jpavlick at idseng.com <mailto:jpavlick at idseng.com> >;
General pattern discussion <nsrca-discussion at lists.nsrca.org
<mailto: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 <mailto: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
Integrated Development Services
From: NSRCA-discussion <nsrca-discussion-bounces at lists.nsrca.org
<mailto: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
<mailto:precisionaero at hotmail.com> >; General pattern discussion
<nsrca-discussion at lists.nsrca.org <mailto: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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