104 MODEL AVIATION
Since I’ll cover motor selection when we get back to class, I’ll
touch on what was at the Toledo Show. Everything from the tiniest
to the biggest motor was displayed, and with three flying events
going on during the weekend I got to see a lot of them in action.
Now comes the tough part: how do you choose? There are so
many motors on the market I can no longer keep up with them. It
used to be easy, but I can’t even keep my database updated with the
new offerings, and that is compounded by so many vendors who
have no idea what the motor is intended for.
Unless the manufacturers provide some motor constants and
suggested setups, they can’t tell you for what purpose the motors are
designed. Fortunately most of the vendors do their homework ahead
of time and are forcing the manufacturers to help.
I need to mention one thing that still goes on at the shows. You
cannot, under any circumstance, tell how good a motor is or
anything about its power by holding it in your hand or placing it on
a test stand and running it up to feel the pull and the wind!
You can feel that it pulls, although you shouldn’t be holding it in
your hand! You can tell that it blows a great deal of wind or makes
plenty of noise if it’s on a stand, but that’s all the information you’re
going to get.
Don’t get me wrong. It’s interesting to see some of the dualmotor
contrarotating setups in action and the big Plettenberg is cool
to see run, but the useful information comes from the data sheets;
don’t be blinded by the show.
Outrunners are plentiful on the landscape
and I still maintain that a quality inrunner
is better. Check out Steve Neu’s recent
article in Quiet Flyer about inrunners vs.
outrunners. It’s a good primer on the two
designs.
I use outrunners in some applications and
they certainly have their place, so don’t place
me in the “outrunners are bad” corner.
Everything has its pluses and minuses.
A field trip to the 2007 Toledo Show
Electrics Greg Gimlick | [email protected]
Also included in this column:
• Back to school
Keith Shaw’s scratch-built DFW T28 FLOH uses an Astro geared
020, Castle Creations Phoenix-25 ESC, and two E-moli cells. Yes, it
was a full-scale airplane.
Jim Ryan flew his new Army Chinook at the RC Groups indoor fly. He scratch-built it
from stock parts off the shelf. Look for an upcoming MAN construction article.
RECESS FROM SCHOOL: In the last
column I wrote about some of the basic
things you need to consider as you look at
getting into Electrics. I’m going to continue
that, but first we’re going to step outside the
classroom and go on a brief field trip—to
the 2007 Weak Signals’ Toledo R/C Expo!
This is my favorite big show of the year;
it’s like a giant hobby shop and you live
there for three days. All the new stuff is
there, and you get to touch it and talk to the
manufacturers and vendors. I’ll be
sprinkling in a bit from the show now and
then, but MA Associate Editor Michael
Ramsey was there doing the main coverage
so I won’t go deep into it.
I came up short in looking for a specific
item regarding Electrics that jumped out at
me. What did jump out at me was that we
got what we asked for: much more electric
stuff. Almost every booth had something to
do with electric flight, be it motors,
controllers, propellers, batteries, or kits.
Yes, kits, although it’s clear that the ARF
market is the dominant force.
The other thing that jumped out at me
was that electric-powered helicopters are
everywhere! I’ll provide more about them
in time, but I don’t want to infringe on
Mark Fadely’s RC helicopter column.
Suffice it to say that the prices and
selections are incredible, and the quality of
the offerings is excellent. Wait until you
see the review I’ll be doing on a scale
electric Cobra!
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August 2007 105
Winged Shadow Systems’ How Fast is
inexpensive and light enough to be
installed on something as small as the
Cox warbird (shown).
Terry Nitsch is well known for big jets but is becoming a force in the electric realm. His
new TeeRific micro jets look great and he offers a custom carrying case.
If you’re using a junk gearbox that is set
up improperly, you would be better off
using an outrunner to spin that same-size
propeller. Outrunners are simple and
inexpensive, which isn’t all bad. Buy a
quality motor regardless of design and
you’ll be happy.
Indoor Flying at Toledo Abounds! If you’ve
read anything I’ve written about Toledo
through the years, you know I’ve lamented
the loss of the informal indoor flys-ins we
used to enjoy. I’m happy to report that there
was something for everyone this year.
Spectators enjoyed the E-TOC (Electric
Tournament of Champions) for two nights,
and the best of the best showed off their 4-
D flying skills. The PERF group sponsored
the micro indoor fly again this year, and the
facility was fantastic.
The weight class was limited to less
than 3 ounces, which reduced the overall
number of models present but there were
some beautiful things there. It’s relaxed and
limited to invited pilots, but that’s a loose
rule from what I’ve seen. Congrats on a
super job and a great evening of flying.
Another site offered two nights of
indoor flying with lax requirements. RC
Groups sponsored (along with many minor
sponsors) an event that allowed you to fly
almost anything you wanted. There were
some snafus on the radio requirements, but
it was the first year and there was a
learning curve that will make next year
even better.
Thanks to Jim Bourke for putting this
event together and the Flying Tigers club
for administering it. The frequency control
and rotation schedule was flawless and kept
things moving along nicely.
The flying sessions were split into
groups of helicopters, 3-D, and trafficpattern
types so all those present had a
chance to fly and could see on the big
board how long it would be before their
turn came up. Great job!
I’ve thrown in some photos and
captions to tease you if you missed the
show.
Back to Class: You’re at the point of
selecting your power system, and you
must make some decisions about how and
what you’re going to electrify. Is it a
trainer? Is it going to be aerobatic? How
aerobatic?
How is your wing loading? How that
affects your airplane, whether it’s glow,
gas, or electric, can be a whole lesson in
itself. I’m going to assume you’re
comfortable with your proposed airplane’s
wing loading and focus on the appropriate
power for it.
For down-and-dirty guidelines you can
check out the chart I’ve included. This is
information that will get you in the
ballpark and information with which I feel
safe.
Different people can do a lot with less
power than others. If you watch some
fliers, you see that they never use anything
less than full throttle and they bull their
way through the air. That uses a great deal
of power and decreases duration.
If you get to watch someone such as
Keith Shaw, you’ll see him do aerobatics
most think take much more power than he
has in the system. Fliers such as Keith
understand throttle control and using
airplanes’ characteristics to perform their
routines.
How do you fly? Learn throttle control
and “flying on the wing,” and you’ll find
that your model performs better than
projected and your equipment will last
longer.
If you haven’t learned yet, electric
power is measured in watts by multiplying
voltage and current (amp) draw. If we
have 10 volts and the motor/propeller
combination will pull 25 amps, our total
input power is 250 watts.
Notice I used the term “input power,”
which is fine for our purposes. If you want
to get into the output power, you’ll have to
take into account all the losses in the
system and do the math. That’s where
programs such as ElectriCalc and
MotoCalc come into play. I’ve always
found that input power will suffice for
basic planning.
There is an old argument about whether
we are better off using high amps or high
voltage to get our power. It has been
debated forever, and there are many
schools of thought on it.
We settled into a higher-amps mode of
thought for years, only because we didn’t
have the ability to pack the volts into the
model. Ni-Cds and NiMH cells were heavy
and brought approximately a volt per cell
to the table. Since sub-C cells weighed
roughly 2 ounces apiece, we found that
getting 36 volts weighed almost 5 pounds.
We started using fewer cells and
drawing more amps to get the same power
and save weight. That thought process has
continued, and there’s even been some
taunting of modelers who are “afraid” to
get more than 40 amps, then 60 amps, and
now even more.
In our example we’re looking at 30
amps and 36 volts to get 1,080 watts, but
the pack weighs 4.5 pounds. Let’s assume
that airplane is aerobatic, so we’re
planning on 80 watts per pound. With this
in mind our model also weighs 13.5
pounds using the Ni-Cd/NiMH option.
Now we have Li-Poly cells on the
market, and suddenly getting 36 volts
requires only 10 cells assuming a nominal
voltage per cell of approximately 3.7. With
that voltage we can get the same power
(watts) with a battery pack weighing less
than 2 pounds. The cells would have far
more capacity than the former Ni-Cds,
roughly the same as the most recent NiMH
cells, and they wouldn’t even be breathing
hard at 30 amps.
What’s the downside? The cost is more
than double, and the Li-Polys haven’t
shown to have a comparable number of
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106 MODEL AVIATION
duty cycles. There’s always something,
isn’t there?
Going back to our example, there’s
more we can look at. Since we want 80
watts per pound and our airplane is going
to lose 2 pounds, we’re actually going to
need only 920 watts (11.5 pounds
multiplied by 80 watts equals 920).
If we wanted to stay with 30 amps, we
could drop to an 8S pack. That would save
a few more ounces and maybe a little
money. The other option would be to stay
with the 10S pack and see the current drop
to 25 amps.
Let’s go back to deciding on volts or
amps and see what these options do for us.
With the 8S pack and 30 amps using the
Thunder Power 3850 mAh Li-Poly pack,
we can see that our duration at full throttle
would be roughly 71/2 minutes (3850 mAh
= 231 amp minutes that we divide by 30
amps to get 7.7 minutes).
Now let’s say we stay with the 10S
setup and drop the current to 25 amps. We
take the 231 amp minutes the Li-Poly pack
has, divide by the lower 25-amp current, and
see that the full throttle duration is now 9.25
minutes. So we get a couple more minutes of
duration by using the higher-voltage setup and our current drops
to make it easier on the equipment.
Now for the clincher. The problem we have run into with
trying to get higher voltage in our setups and using lower current
is that the equipment on the market today isn’t designed for it.
We usually hit the controllers’ voltage limit before we hit the
current limit.
Plenty of controllers will handle 20 volts and as much as 80
amps, but getting one that will take 36 volts and maybe only 50
amps has been a problem. It’s beginning to change, but change
comes slowly. Fortunately manufacturers such as Castle
Creations are working on this.
Final Approach: I’m running out of space, and you might think
you’re not any closer to making your decision than you were. I
hope you’re beginning to see how the thought process goes,
though, and I assure you we’ll clear it up before we’re finished.
In the meantime, you can use the time-honored method of
copying the setup someone else is using until you learn it all.
I’ve had a major computer crash that took out my two hard
drives and a motherboard. That’s bad, but the worst part is I’ve
lost everything from the last six months.
If you have sent E-mail or photos and didn’t get a reply,
that’s why. It’s a real lesson in backing things up regularly, and I
can guarantee that I’ll be better at it in the future. MA
Sources:
AstroFlight Inc.
13311 Beach Ave.
Marina Del Rey CA 90292
(310) 821-6242
Fax: (310) 822-6637
[email protected]
www.astroflight.com
Castle Creations
402 E. Pendleton Ave.
Wellsville KS 66092
(785) 883-4519
Fax: (785) 883-4571
[email protected]
www.castlecreations.com
TeeRific Jets (Attn.: Terry Nitsch)
3450 Harrisburg Geo. Rd.
Grove City OH 43123
[email protected]
www.terrificjets.com
Stevens AeroModel
1528 S. Nevada Ave.
Colorado Springs CO 80906
(719) 393-0830
Fax: (208) 692-6855
www.stevensaero.com
Winged Shadow Systems
Box 432
Streamwood IL 60107
(630) 837-6553 (8 a.m.-4 p.m. Monday through Friday)
[email protected]
www.rcreporter.com
Ryan Aircraft
6941 Rob Vern Dr.
Cincinnati OH 45239
(513) 729-3323
Fax: (513) 729-3323
[email protected]
http://home.fuse.net/ryan
Bill Stevens takes care of customers at his Toledo booth. He knows how to design
great kits that go together easily and have all the power options you need.
Power Loading for Electric-Powered Aircraft
Trainers, Old-Timers, sport models 40-60 watts per pound
Mild aerobatic types 60-70 watts per pound
Strong Aerobatics 80-100 watts per pound
Extreme-performance types More than 100 watts per pound
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