Frequently Asked Questions - 2009/12

The step on the float is slightly aft of the model’s CG (marked on the
fuselage side). The Cruiser has a slightly positive attitude with
respect to the floats when they are level.
Twin float installation and options
Bob Aberle | baberle@? Frequently Asked Questions optonline.net
December 2009 75
Also included in this column:
• Digital voltmeter with a load
• Hitec’s brilliant Aurora RCsystem
feature
• Measuring motor
p aramet er s on mi c ro RC
airc raft
Bob spaced the twin floats at 7 inches center to center.
Crossbracing wires were unnecessary for this spacing.
The author installed 16-inch-long pink-foam floats from
FoamFly.com on his Live Wire Cruiser. Mounting hardware is
included in the floats’ purchase price.
PLEASE WRITE IN with your questions; that is the only way we
can keep this column format going. When referring to published Qs
and As (for follow-ups), provide the number as a reference.
Contact information for resources mentioned within the text are
placed in a group, separate from the text, at the end of this column
under “Sources.”
Q425: “I remember seeing a while back in your column a reference
to a twin float installation on park flyer and indoor size aircraft. I
can’t find that article.
“Can you point me to it and add any additional information that
might help me install a set of floats on my model?”
A425: The question-and-answer the reader is referring to is 353,
which was published in the July 2008 MA (page 85). I showed a pair
of FoamFly.com twin pink-foam floats being attached to one of my
small indoor RC designs. I provided only the company’s contact
information and hadn’t attached the floats to my aircraft.
This past summer, I revisited an old Hal deBolt design called the
Live Wire Cruiser. I reduced it to 150 square inches, and the
resulting model weighed only 7.7 ounces with electric power.
I learned that when it came to locating the floats on that aircraft,
I needed a reference that could guide me on such things as float
spacing, the location of the float steps to the CG, and the incidence
angle between the airplane and the floats. I also needed to know
whether or not I needed crossbracing wires on my tiny aircraft and if
an articulated water rudder was necessary.
In the process of researching twin float installations on model
airplanes, I found a wonderful paper that Robert Thibodeau had
written. It was edited and compiled by Leon H. Raesly and
originally published in the BARCS News in July 1998.
12sig3.QXD_00MSTRPG.QXD 10/23/09 10:07 AM Page 75
In the “Sources”
listing, I’ve
included the Web
site where you can
obtain this
document, which is
titled “Float
Selection and Fine
Tuning Your Float
Plane Set-Up.” This
address is long and
complicated. If you
can’t open it when
you transcribe the
address to your
computer, drop me
an e-mail. I can
send you the link to
the site.
Installing a pair
of 16-inch pinkfoam
floats added
only 1.1 ounces to
my Live Wire
Cruiser, bringing the
total flying weight
to 8.8 ounces. Crossbracing wires were
unnecessary for this size aircraft, and no
water rudder was employed.
Remember that to maneuver the aircraft
while it’s in the water, you must keep its
speed up (a technique called “plowing”).
You also need more power than you would
for land flying.
The final motor-power input was 39
watts. With the weight of my model, that
resulted in a power loading of 81 watts per
pound, and the Live Wire Cruiser flew
great. A construction article for this aircraft,
with full-size plans, was published in the
December 2009 Fly RC magazine.
Q426: “I read recently in your column that
we should all be checking our RC receiver
batteries before flying. You stated that the
proper way to do this was to use a voltmeter
that applies a load to the batteries.
“The one meter that you recommended
gave three choices of load current, but it
cost quite a bit. Can you recommend a more
economical substitute?”
A426: The meter I recommended several times in my column costs
approximately $70. But I did like the fact that it gave you a choice
of 250, 500, or 1000 mA load currents with the flip of a switch.
However, for a long time I have successfully used the simple
Hobbico Digital Voltmeter MkIII, which is available from Tower
Hobbies. It sells for $19.99 (as of August 2009).
This meter applies a single load of 250 mA, which is certainly
good enough for most applications—except for very high-capacity
battery packs. You can turn off the load and use this unit as a
regular voltmeter for checking purposes. It will provide digital
readings from zero to 19.99 volts. The meter is battery powered but
has a timer that shuts it off after a 10-minute period, to save the
battery.
The purpose of this meter, or any similar to it, is to measure your
receiver-battery voltage before flying. Suggested minimum voltage
readings are included with the unit. When that minimum is reached,
the LCD screen will flash “Recharge.”
A word of caution about using a voltmeter with a load is to let
the load remain on for roughly 10 seconds (count to 10) before
taking your reading. A good-capacity battery should drop off only a
few tenths of a volt, if at all.
76 MODEL AVIATION
Hobbico’s Digital Voltmeter MkIII applies
a 250 mA load to test receiver batteries.
Apply the load for at least 10 seconds.
Bob connected a Li-Poly battery cell to one side of a Super Whattmeter. The other side
goes to his Micro Kerswap’s motor system. A four-cell receiver battery pack permits
readings of less than 4.5 volts.
Checking a 7mm pager motor’s power parameters, the
Whattmeter measures 0.80 amp of motor current. Power input is
3 watts. Voltage while the motor is running 3.77, and below that is
power taken from the battery.
Hitec’s Aurora RC system has a built-in lowvoltage
sensor that broadcasts a warning alarm
back to the transmitter when the receiver
battery gets low.
12sig3.QXD_00MSTRPG.QXD 10/23/09 10:07 AM Page 76
But a battery with poor capacity might
indicate a rapid voltage falloff. If that is
noted, you should not fly; test the battery
for its capacity with a device similar to the
West Mountain Radio CBA II or CBA III.
I am always concerned about the RC
receiver voltage dropping off to the point
where the receiver quits altogether or, in
the case of a spread spectrum receiver, it
temporarily shuts down and must complete
a rebooting process. That is why I keep
preaching to test your receiver batteries (if
using a separate battery) before every
flight.
However, I do have some interesting
news for you. I received a new Hitec RCD
USA Aurora digital spread spectrum RC
system that includes a seven-channelfunction
Optima spread spectrum receiver.
I quickly learned about a new feature of
this system; the receiver is actually a
“transceiver.” That means it can accept a
signal broadcast by the transmitter. It can
also send a signal back down to the
transmitter.
Because of the send/receive capability,
Hitec engineers built a low-voltage sensor
into the receiver circuit. If the voltage drops
below something near 4.4, a warning signal
is sent back to your transmitter. As soon as
the transmitter receives that signal, a
warning is sounded that alerts the pilot that
he or she should land the aircraft.
The receiver sensor can accept voltage
sources from four- or five-cell battery
packs, Li-Poly battery packs, and even a
BEC as used in electric power. With this
kind of capability, all types of RC pilots
can be protected against receiver low
voltage, whether they use electric power,
fueled power, or fly a sailplane with no
power.
I think that Hitec should be
congratulated for developing this feature.
I’m hopeful that other RC manufacturers
will join in and produce similar receiverpower
warning systems.
Q427: “Like many people I have enjoyed
flying my ParkZone Cessna, but after a
while wanted to remove the power and RC
system and move on to other aircraft. I never
did get a handle on parameters like motor
current, power input and so on.
“This info would help me more if I
decided to go to a higher power motor,
which might, in turn, require a higher
capacity battery cell. How would I go about
measuring these needed parameters?”
A427: As shown in an accompanying photo,
I did what you are planning to do. I designed
a reduced-size Kerswap Old-Timer replica to
have 50 square inches of wing area and a
total weight of 35 grams. The design is
featured in the September 2009 Micro RC
World Online Magazine.
The Cessna brushed motor ran close to
275 mA current and roughly 1 watt of input
power. That was okay for a 16-gram Cessna,
but not for a 35-gram Kerswap. I switched to
a 7mm red geared pager motor from BSD
Micro RC and, as you are, I was curious
about the motor current and power input for
the propeller I selected.
Motor current, power input, and battery
voltage can be read on an AstroFlight digital
meter. I use the 101N Super Whattmeter,
which comes without connectors.
I installed a pair of Anderson Powerpole
Connectors on the input and output lead
wires. To those connectors I can plug in
essentially any custom-made adapter cable
to cover all current-measuring needs.
The nice part of the AstroFlight meter is
that it will read voltage down to zero when
you attach a standard four-cell Ni-Cd or
NiMH battery pack. This is important,
because most micro-size motors operate on
78 MODEL AVIATION
only a single cell at roughly a nominal 3.7
volts.
The only other thing to note is that
special connectors are required to allow you
to attach the single Li-Poly battery cell on
the one side and then plug into your aircraft
motor system on the other side.
The best way to obtain the correct
microconnectors for your needs is to contact
Bob Selman at BSD in Missouri. He can set
you up with exactly what you need for your
application.
As I found out, my 7mm pager motor had
a much higher current draw at 800 mA and
at 3.77 volts; the power input was close to 3
watts. The meter reads to the nearest watt,
rounded off. If you need more accuracy,
multiply the 800 mA (or 0.800 amp) by the
3.77 volts, and you get exactly 3.02 watts.
The ParkZone Cessna’s built-in ESC (in
the Brick) could handle the extra current
with no problem. But there is a limit to how
far you can go; if you’re uncertain, contact
the Horizon Hobby technical-service people.
That’s basically it. You can use the same
AstroFlight meter for all of your motorcurrent
readings, as long as you select the
proper connectors to mate with your
particular batteries and motors. MA
Sources:
FoamFly.com
www.foamfly.com
Float Selection and Fine Tuning Your Float
Plane Set-Up:
www.smilesandwags.com/Floatsite/FLOAT
%20SELECTION%20AND%20FINE%20T
UNING.html
Hobbico
(217) 398-8970
www.hobbico.com
Hitec RCD Aurora
(858) 748-6948
www.hitecrcd.com
ParkZone
(800) 338-4639
www.parkzone.com
Micro Kerswap
www.cloud9rc.com
BSD Micro RC
(417) 358-9521
www.bsdmicrorc.com
AstroFlight
(310) 821-6242
www.astroflight.com