THE BATTERY CLINIC - 2012/07

Many have chosen Eneloop cells
for transmitter packs, primarily
because of their improved
charge retention compared to standard
NiMh cells. I decided it was time to find
out how much charge would be left after
a month on a transmitter pack that had
been used for more than two years.
First, I charged the pack and then
discharged it. I recharged it and let it set
for 30 days, then discharged it again.
From the included graph you can see
it lost roughly 1% of charge per day. If
you don’t charge your pack for a month,
you will still have approximately five
hours of flight time left based on the 250
mAh load the transmitter puts on the
battery. This a significant improvement
compared to the 700 mAh packs we
have been using.
Free RC Advisor Calculator
Carlos Reyes has launched a new
version of his website, RCadvisor.com,
with two major changes.
His model airplane calculator is now
free. He is posting approximately one
high-quality article per day. With more
than 10,000 registered calculator users
already, he expects the calculator to be
more popular than ever. The feedback on
the articles has also been positive, and he
hopes you like the changes.
If you have not visited his website,
you are missing out on some excellent
modeling material. Here is a portion of
his article series, Quick and Easy Model
Airplane Sizing, which can be found on
his website:
How big of an electric motor do you need
to power a 6 foot (1.8m) Piper Cub model?
Finding the answer is easy!
Six Foot Cub
Let’s say we decide to design and build
a Piper Cub because the world cannot
have enough Cubs. We want to make
the wingspan exactly 6 feet (1.8 m) because that is the size of our
building table. How big of an electric motor do we need to power it?
Whoa! Did your mind just go blank? You say you have no idea? It’s
not as hard as it looks. Let us take it one simple step at a time until
we get the answer.
Full Size Cub
Look up the specifi cations for the Piper
Cub. For the sake of argument, let’s say
that it weighs 1,000 pounds (454 kg). The
wingspan is 35.25 feet (10.75 m) and
the wing area is 178.5 feet squared (16.6
m²).
The formula for the aspect ratio of a
wing is (wing span²)/(wing area). That
doesn’t look too hard, does it? The units
are not important as long as they are
consistent.
Plug in the numbers from the
specifi cations and you should come up with
the value 7. I know that it’s really a little
bit less than that. Let’s just round it out.
For the sake of this discussion, the aspect
ratio of a Piper Cub is 7.
Given the wingspan and aspect ratio,
let’s compute the wing area of a Piper Cub
as a check on the numbers. The wing area
of a wing is (wing span²)/(aspect ratio).
Plugging in the numbers, you should get a
value close to 178 feet squared (16.5 m²).
Again, we are still talking about the fullsize
Piper Cub.
Similarly, we can plug in the numbers
we have to compute the wing area of our
6 foot Cub. I get
5.1 feet squared
(0.47 m²).
Airplane Type Constant
There’s a very simple formula that relates
the weight, wingspan, and wing area of
an airplane. The value that you get is an
indicator of how that airplane will handle in
the air. The formula is (weight)/(wingspan *
wing area). The units are not important as
long as they are consistent. In this example,
we will use ounces as the unit of weight and
feet (or feet²) as the units of length and area.
Let us plug in the numbers for a full-size
Piper Cub to fi nd its airplane type constant. I
get a value of about 2.5. So if we come up with
a 6-foot model of the Piper Cub that has an
airplane type constant of 2.5, it will behave in
the air similarly to the full-size Cub.
Note that I said similarly, not exactly.
The point of this article is to quickly come up
with some numbers that can be used as the
starting point of a detailed airplane design.
The airplane type constant formula may
look very simple, but it is extremely handy.
In fact, this is the formula from my fi rst book
that I use the most.
To read more, visit the RCadvisor.com
website at the address listed in “Sources.”
Ed King
Ed has provided the specifi cs on
his beautiful Constellation, which
was pictured in this column in the
March 2012 issue. The model was built
from Don Smith plans, with many
construction changes. The full-scale
Connie he modeled is located at a
museum in Kansas City, Missouri.
Wingspan: 11 foot, 8 inches
Fuselage length: 10 foot, 6 inches
Weight: 55 pounds
Finish: Fiberglass/epoxy, automotive acrylic
enamel, automotive clear urethane
Radio system: JR 12X
Total servos: 20
Battery: Dual 6-volt airborne battery capacity,
5400 mAh total
Motors: AXI 4130-20
ESCs: JETI 77 Opto
Total motor power: 6000 watts (30 volts
x 50 amps) x 4
Total motor battery: (LiPo) capacity; 22,000 mAh
Total battery packs: 10
Includes: Fowler fl aps, operating beacons,
navigation lights, landing lights, fuselage
lights for checking de-icing boots,
retractable landing gear, doors opening
and closing
Construction time: 21/2 years
RC Hall of Fame
The Radio Control Hall of Fame
website is worth visiting if you are
interested in RC modeling history. This
site covers nearly all manufacturers of
RC equipment. It is a small organization
attempting to do a large job. Anyone
who can do it better is welcome to do so,
or to join in the cause.
The focus of the Radio Control Hall
of Fame and Museum is on the Radio
Control itself—not the wonderful
airplanes, cars, and other vehicles that
employ it (and are well covered by other
museums and sites). See “Sources” for
website information.
Audible Alarm Module
Gregg Stockdale sent this email:
“Red, I’ve stopped using my timer on
my transmitter in exchange for using a
2-in-1 LiPo Battery LED Low Voltage
Meter Tester 1S-8S Buzzer Alarm.
“This little beauty connects to the
balancing plug on your battery pack. It
measures the charge in each cell, and
when any one cell hits the low (you can
set the limit) voltage, it buzzes quite
loudly. It’s easy to know when you should
think about landing rather than just
running out of juice.
“I set mine for 3.5 volts. That way,
when the buzzer goes off, I have plenty of
time left to reduce throttle and wait my
turn at landing (we have a very busy fi eld
and there are almost always fi ve planes
up at the same time). This also saves the
issue of consumption not matching the
timer. If you fl y hard, you’ll run out of
juice prior to the timer going off. If you
are not pushing it, you’ll land with too
much juice left unused. Also, if you have a
cell that’s not up to par, you’ll be warned
about that as well. Good stuff to know!
“I’ve had the device imbedded deep
inside a foam plane and I’m still able to
hear the buzz. At around $10 (eBay), it’s
a lot cheaper than a telemetry system
and very light. I can’t keep mine in stock
because I keep giving them to new fl iers
... it’s a real ‘plane-saver.’”
Don’t Forget the Females
I messed up. I thought it would be cool
to get my great-grandson an AMA Youth
Membership. Then it hit me; I forgot
someone.
I exited the doghouse by adding
great-granddaughter, Jenna. If you are
taking advantage of the free AMA Youth
membership program, don’t forget the
females.
That’s all for this month. I enjoy your
communications. No email connection?
Then drop me a note at The Battery
Clinic, 12219 NW 9th Ln., Newberry FL
32669.