74 Model Aviation AUGUS T 2012 www.ModelAviation.com
THE ENGINE SHOP
The First Try
For my first foray into flight
comparison data collection, I chose
the Redwing RC MXSR aerobatic
airplane that I had recently reviewed,
using the Mintor 33cc gas ignition
engine imported by Top Dawg Aviation.
(Different airplanes will give different
results; a scale biplane will have more
aerodynamic drag, but could be lighter.)
I measured the receiver and ignition
pack voltages and looked for radio
communication losses that could have
been caused by things such as the
ignition systems. However, the main
impetus of these tests was to measure
head temperature, rpm on the ground,
and rpm changes in the air.
The goal was to obtain meaningful
comparative data. As with all testing,
the methodology will have some flaws.
There is some inherent delay in the
telemetry readings on the iPhone.
Whenever possible, I allowed sufficient
time for the readings to stabilize before
they were recorded.
Airspeed was measured with the
aircraft flying straight and level with
a non-diving entry, in both wind
directions. Level flight was maintained
for more than 200 yards. The head
temperatures and rpm had similar
durations and were recorded when
Recording consisted of a volunteer,
a ballpoint pen, and my printed data
sheet. This is not true empirical data,
but it was done the same way for each
propeller and is reasonably accurate as
The Next Set of Tests
A month later (April 13 to be precise),
the first telemetry tests were resurrected.
The first propeller I tried was a wooden
18 x 8 Xoar. All seemed well, but the
engine temperatures were slightly high.
I made a few flight tests, but the engine
temperatures kept soaring to the mid-
A super feature of the iPhone
Spektrum application is that it can be
set to talk to you, depending upon what
alarm settings you have told the app to
look for. I use zero rpm to tell me if, for
example, the engine has quit. When the
engine got too hot, I heard “Warning
temp too high” or words to that effect
from my iPhone.
No problem, I thought. I throttled back
and put the MXSR into a dive to get a
cooling effect—wrong The temperature
reading actually went up. Opening the
throttle to the three-quarters position, I
got the engine to cool down and make
landing more comfortable. Again, I
abandoned the telemetry testing.
Driving home, I was thinking that
the windmill effect of the propeller was
blocking the cool air getting to the engine
in the dive. Then it hit me! The Mintor
33 is a compact engine, which allowed it
to fit perfectly inside the MXSR cowl—it
didn’t even need a cutout for the plug
There was already a rear cooling-air
exit in the MXSR cowl, but the data told
me that there was not enough air getting
in or out of the cowl when the power was
cut for the cooling dives.
The ultra-lightweight muffler has a flat-flange O-ring
and four-bolt retention for a clean seal.
I should apologize to Top
Dawg Aviation for abusing the
company’s lovely little engine,
and to RedwingRC for being
mean to its excellent 3-D
airplane, but I found out what I
always wanted to know!
The initial test did not go
well—or not quite as planned.
The test schedule was too
ambitious to do in one winter
afternoon. The Mintor 33 did not
start easily at first. The engine
comes with a semiautomatic
choke. The spring-loaded choke
is held in the closed position
when the throttle is in the low/
idle position. When you open the
throttle, the choke automatically springs
To get fuel into the carburetor, I had
to resort to manually holding the choke
closed with the throttle set wide open
with the ignition turned off, then slowly
hand crank the engine approximately
six times to get gasoline to the pumped
Afterward, three swift propeller
flicks—with choke back on—fired it up.
Open and close the throttle from the
transmitter to release the choke and give
approximately three more flicks and the
Mintor 33 is purring!
The new engine was given a full tank
run on the ground with engine installed
in the securely tethered MXSR. The
engine ran fine and idled well. Then it
was time for some review flying of the
MXSR. The flight testing ate up the
it was getting
dark and cold.
a spring system
on the throttle
allow the user
to engage the
it’s in the low