Testing your engine’s performance

What really goes on with engines when they are tearing
around the skies? I have always wondered. Have you?
For more years than I want to reveal, I have read
the various “engine columns” of my heroes, many of whom I
consider to be legends in their own time. When I was asked
to do “The Engine Shop,” I was uncertain whether I could
meet their levels of excellence and technical knowledge about
things such as metallurgy. I was sure I couldn’t replicate their
efforts.
I started thinking about what the new generation of RC
pilots wants or needs to know and maybe things they might
not yet know. When it comes to engines, there is much
misunderstanding about propellers, airspeed, climb rates, etc.
I see it regularly. I visit a large number of fl ying sites in my
travels and see engine/fuel/propeller “folklore” in action.
This gave me the idea of testing engines in a slightly
different manner than the customary bench-running method.
There is no doubt that running engines on test stands with
different propellers satisfi es a certain appetite for information,
but does it really tell you what will happen in the air?
Testing
Airborne performance has been diffi cult to measure
and the data equally hard to collect. There are some good
systems available, but none are as easy to use as the Spektrum
telemetry system. This new technology became a game
changer when it came with the DX8.
Spektrum and its telemetry have now made it easy to set
up and capture data while you fl y. Throw in an iPhone with a
plug-in Spektrum receiver module, and the Spektrum STi app,
and it becomes cool indeed. Not only cool, but you can use
any JR or Spektrum transmitter and receiver with a data-port
socket.
Some of you may have read about the engine test that I did
on the O.S. FS-95V. It was cold at that time of the year, so I
rigged the engine test stand with a DX8, a throttle servo, a
receiver, and a telemetry module. This allowed me to monitor
all of the necessary bench-run data from the warmth of my van!
The free Spektrum STi iPhone app is easy to read and records
data that you can review later. For what I had in mind, I snagged
a volunteer to write down all of the data as it happened on the
ground, and as I fl ew certain speed and climb tests.
The basic plan was to read engine-performance data on the
ground and in the air. This was later expanded to include fl ight
performance such as airspeed and altitude change.
The main idea was to see the difference in engine
behavior between running on the ground and
running in the air.
I am inviting RC pilots to try this at home by
explaining how easy it is! The equipment is readily
available and easy to set up. 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
steady.
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
comparison data.
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-
300° mark.
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
lead.
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.
An appointment was made with
“Captain Dremel,” and soon a cutout in
the cowl gave three times more air-exit
area than air-entry openings.
I think that I hurt the Mintor 33
during the early fl ight tests, but then I
remembered how cold it had been. The
engine handled my abuse without any
problems.
A New Dawn and a New Day
The next test session was satisfactory.
This time the Mintor 33 did not need
the carburetor to be primed and I started
the engine with three fl ips
and opened the throttle, but
the engine stopped because
of the choke’s auto release
when I applied some throttle.
Engine temperature
readings stayed at or below
the 200° mark and started
to make sense. The increased
cooling air—infl ight versus
on the ground—was
effective. I was still using
break-in fuel and because
the Mintor 33 was now
cooler and happier, I elected
to proceed with some more
serious test work.
I tried four propellers
during a two-hour period.
The last propeller, a 20 x 8
Xoar, was not suitable
for the airplane. It would
probably be good for a scale
application, but the rpm was
too low to pull the aerobatic
airplane vertical and the
engine began to overheat.
Flight Log Discoveries
This could almost be a column in
its own right. Looking at the results
table, it is easy to see that you get
signifi cantly more rpm in the air than on
the ground. Engines become cooler in
the air, and as I experienced, you need
adequate cooling as soon as the outside
temperature gets warmer than 35°.
Comments
Smaller propellers provide more
rpm and make a louder sound, but
these only give the illusion of more
speed with typically low-revving gas
engines. You might also see that too
much propeller diameter reduces
performance more radically than pitch
increases/changes.
My attempts to measure climb rate
were inaccurate. The altitude readings
have some latency in reaching the
iPhone operator, so the time to climb
vertically to 400 feet is relative to each
propeller rather than a true rate-of-climb
indicator.
There is so much more that you can
do with this telemetry. I would like to
measure and monitor airspeed going
up, across, and down in a square loop.
This is a Precision Aerobatic pilot’s
dream! I would say that airspeed
telemetry is slightly more practical
than trying to use a radar gun during an
aerobatic maneuver.
Just for grins, I would love to hear
speed measurements for a spin entry.
How about doing a stall turn while
listening to a readout as opposed
to only visual timing? You could go
vertical, keep in some throttle, wait for
zero vertical airspeed, and then execute
a perfect hammerhead. Is it possible?
One thing I want to do, after the
engine has been broken in, is some
hovering to fi nd out what that does to
the engine temperatures. This could
result in some myth busting!
For me, this type of testing has only
just begun and will remain a major
“tool” in my engine workshop. I would
like to hear from anyone else who is
doing this kind of work.
Correction
In my June 2012 column, I indicated
that a 1/4 x 32 tap could be used to clean
out your cylinder head threads. Modern
glow plug threads are 1/4 x 28. Do not
use a 1/4 x 32 tap to clean out your
cylinder head threads.