If It Flies ... 2008/06

The Hangar 9 Funtana 90’s structure is plenty light to start with, but every bit helped
when converting to electric. Battery cooling requires a fresh-air inlet, so a lightening
hole doubled as a cooling inlet.
Electric and glow conversions (both ways), Part 2
June 2008 95
If It Flies ... Dean Pappas | [email protected]
The lightening hole serving as a cool-air
inlet to the battery box is in the middle
of the firewall. There is still plenty of beef
left to support a vibration-free motor.
When Quique Somenzini first flew his Brio in competition, it had electric power. The
design is usually powered with a YS DZ four-stroke engine, as Don Szczur’s (shown) is.
This is an electric-to-wet conversion!
COMPARE AND CONTRAST: The
process of converting existing airframes and
familiar designs to electric power highlights
something a few of us have observed, not
just in RC but in CL and maybe FF as well.
Although there are many little details to tend
to that are particular to this kind of power,
the basics are the same. Torque turns the
propeller; when torque and rpm are present,
power results; and power must be harnessed
to the airplane by a propeller in the
appropriate size range.
If there is a difference to be found in this
basic relationship, it is that the rpm
characteristic of the electric motor’s torque
curve means that the range of “useful”
propellers is a bit more restricted.
In the last installment of this column, I
promised to discuss the “domino theory of
weight” as it applies to airplanes. Once it is
described, your response will probably be
much like mine the first time I heard it
described in any detail: a slap to the
forehead, followed by an almost Homer
Simpson-like “Doh”! This is a normal
response when you are told something you
already knew in some form but had never
grasped the big picture.
I knew this bit of wisdom as a youngster,
but former CL Precision Aerobatics
columnist Windy Urtnowski came up with
the ideal name for it slightly more than 20
years ago. The domino theory has nothing to
do with Geopolitics and everything to do
with cause and effect. I’ll get to that shortly.
For the sake of continuity, I’ll recap a few
key aspects of what “well powered” means
in a propeller-driven airplane.
06sig3.QXD 4/24/08 10:47 AM Page 95
• Electric power and gas have fundamentally
different “grunt vs. go” behaviors.
• A gas/glow engine typically gains
horsepower with rpm over an extremely
wide rpm range, but the torque curve is
almost flat over an exceptionally wide range
and falls off at both high and low rpm. The
size of propeller that allows the engine to
reach its sweet spot in flight is very much
dependent upon the airframe’s weight and
drag.
• The motor makes both more torque and
more power when you load it down with a
bigger or higher-pitch propeller, as long as
the voltage or throttle is unchanged.
Assuming that the voltage does not
change, the torque drops with rising rpm,
unlike with the engine. This means that the
electric power plant inherently draws more
power in an attempt to try to maintain
airspeed when the load is increased during a
climb.
• The electriclike characteristic of torque
and power increasing with added load can
be replicated in gas/glow by using an
intentionally mistuned exhaust system.
Although useful, this produces less than
optimal horsepower. This was done to create
the CL Precision Aerobatics tuned-pipe
setup.
• The motor displays much less flexibility in
its personality, but that basic personality is
adaptable to almost everyone, provided we
do our homework.
• The process of picking the correct
propeller for both types of power plants is
similar, but still different enough to give the
user fits, depending on the type of airplane.
What if the dominoes aren’t lined up? In
the April column we powered a hypothetical
5-pound airplane with three different
packages, each with more power than the
one before. The first was a trainer with
something like 300-400 watts. Think of it as
roughly the equivalent of a strong 25-size
glow engine.
A reasonable guess is that a motor sized
for that load will weigh 5 or 6 ounces, and a
battery that is capable of delivering that
much power for takeoff and a few
maneuvers during an eight- to 10-minute
flight would weigh maybe 8 ounces (4S
2200-2500 mAh). The total is roughly 14
ounces including a speed controller.
Now let’s bump up to almost double that
much power in the 40-size example that
followed. We are looking for 700 watts.
That much power delivery will almost
certainly require an 8- to 10-ounce motor,
depending on the type chosen.
The battery would weigh either 12 or 16
ounces, depending on whether or not you
would be happy with short flights. Throw in
an ESC and you have a power plant that
weighs 1 pound, 10 ounces.
The last combination we looked at was
the equivalent 60 setup. We figured that
such an engine in muffled sport trim was
equivalent to roughly 1,000 watts. This will
typically require a 13-ounce motor and
approximately 20 ounces worth of battery
(5S 5000 mAh or 6S 4000 mAh), yielding a
power plant that weighs 34 or 35 ounces!
It’s all in how you line up the dominoes.
Let’s examine these individual examples a
bit further.
In the last column the idea was to look at
three different airplanes that weighed
roughly the same 5 pounds. They ranged
from a 25-powered trainer to a highperformance
60 model. For the three aircraft
to weigh approximately the same, the
airframes plus radios would have to have
been lighter.
A muffled 60 weighs roughly 14 ounces
more than a muffled 25 engine, so the 60-
powered hot rod would have to have been
built for performance, while the Kadet LT-
40-like trainer would have had a bit more
beef in it for the inevitable student landing.
It would take a bit of effort to build or
assemble a 600- or 700-square-inch design
to remove 14 ounces, but it is doable.
Let’s try to do the same with the
electrified versions of the same airplanes.
The difference in power-plant weights is
somewhat more than with wet power,
although the difference doesn’t seem to be
dramatic. I’ll have more about that later.
Instead of roughly three-quarters of a
pound difference between the small and
large power plants, we end up with
something closer to 1.25 pounds. We would
have to take yet another half pound out of
96 MODEL AVIATION
www.modelaircraft.org www.masportaviator.com
ClickOn!
Basic Trainer
The glow-enginepowered
Hobbico
NexSTAR is a great
basic trainer, and Sport
Aviator has
recommended it for
several years. Rip out the engine and replace it with a
honking big Rimfire electric motor, and you get the
NexSTAR EP Select.
The NexSTAR EP has the same airframe, Pilot Assist
stabilization system, LE droops, and speed brakes, but it
has a computer-transmitter upgrade featuring in-flight
Pilot Assist adjustment. Its electric power plant is clean
and quiet.
You can find a review of this model in the On the
Flight Line section at www.masportaviator.com.
Sport Aircraft
The 1/4-scale Piper J-3 from Hangar 9 comes
complete with working cockpit controls. This giant
ARF Cub flies like a trainer on its 106-inch-span
wing. Built stock, it can be an attractive balsa
overcast that any sport pilot will love flying.
We turned this remarkable model into an airshow
performer by powering it with a Mark III
2.10 glow engine. It offered gobs of power that,
although isn’t required, makes this airplane a
delight for
spectators as
they watch
the RC pilot
giggle
constantly.
The
review
article is complete with more than 100 photos
and a great how-to field-assembly video. You
will find it in Sport Aviator’s On the Flight Line
section at www.masportaviator.com.
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