RADIO CONTROL JETS - 2005/05

Jim Hiller, 6090 Downs Rd., Champion OH 44481
RADIO CONTROL JETS
This AMT AT450 on Larry Kramer's model is picture perfect. The
bullet on the front is the electric starter motor.
The ECU, fuel pump, two fuel solenoid valves, and the manualshutoff
valve are readily available for maintenance.
May 2005 129
I HAVE FLOWN jets for a number of years and I have come to
the point I only fly turbines and electrics. I didn’t even take out
any of my piston-powered airplanes this year—only my
daughter’s Seniorita.
What is it like to fly jets? The best place to start is to look at
what you get when you buy your turbine.
Today’s turbine engine is a complete package including the
necessary accessories to start and run it. From a control
standpoint, the heart is the electronic control unit (ECU). This
monitors the engine’s temperature, rpm, and position of the
throttle command. The ECU then controls all the other accessories
necessary to run your turbine safely.
The ECU is an onboard, programmable computer-control
system. Usually the manufacturer allows you to change some
parameters such as the maximum rpm of the turbine to suit your
aircraft installation and flying skills.
The ECU is a speed controller for the fuel pump and the
starter, it’s a glow-plug driver, and it controls two solenoid valves
for the fuel and start gas shutoffs. An additional function of the
ECU is to inform you what went wrong when your turbine didn't
start or why it quit running.
Turbines today can be bought with a ground-support unit that
plugs into the ECU, allowing you to read important information
about the status of your engine. With some turbines the unit is
standard; with some it is an option.
I strongly recommend that you purchase the ground-support
unit. It is a great aide in troubleshooting problems because it
allows you to read fault codes. Typically, information stored in the
ECU includes total run time, number of starts, and last flight data.
You can read real-time data such as rpm, exhaust-gas temperature,
and fuel-pump voltage. This is great information for maintaining
your turbine.
Throttling is controlled by the rate of fuel flow into the engine.
A speed controller built into the ECU drives an electric fuel pump
controlling the flow rate.
The fuel system itself is slightly more involved. The fuel line
from the tank plugs into the pump. From the pump it goes to the
turbine, but it’s not that simple. Appropriate filters are necessary
because any contaminated fuel that gets to the turbine will clog the
fuel nozzles.
How small are these nozzles? Often they are made from
hypodermic needles because the needles already have a small hole.
A clogged needle is not easily repaired; typically it must go to the
manufacturer. I suggest that you always take your fuel filters
seriously.
The fuel solenoid is a key item in the fuel system. This simple
shutoff valve is controlled by the ECU. Its function is to assure a
positive shutdown of the turbine should the fuel pump run away
because of an electrical problem.
A manual fuel-shutoff valve is in place to avoid accidentally
allowing kerosene fuel to feed into your engine. If the model were
carried nose high, gravity would cause the fuel to flow into the
turbine. On the next start you would experience a hot start. All of
that unburned kerosene would blow out the back of the turbine and
burn—outside and highly visible. That little fuel valve was
introduced for a good reason.
What about those funny push-to-connect fittings, often referred
to by the brand name Festo? They are easy to use: simply push in
your tubing and small fingers grip the tube. To remove the tubing,
push down on the colored plate and pull out the tubing. You must
use the correct tubing for the fitting. The supplied kind is the
correct item and additional tubing is readily available from turbine
suppliers.
These fittings work well, but it is recommended that you cut off
the end of the tubing and push in a fresh end to ensure a good seal.
I have seen many air leaks caused by using push-to-connect fittings
on the suction side of a fuel system. I generally check for air
bubbles in the fuel tubing leading to the fuel pump and fuel leaks
from the pump to the turbine during the first run-up of the day.
I can’t say enough about the header tank. You don't need one if
you don't do any aerobatics, but if you fly jets and you loop and
roll your airplane, you want to install a header tank. Turbine
engines are sensitive to air bubbles. When the combustion flame
loses fuel, the flame goes out and it will not relight by itself.
The telltale sign of the turbine engine
shutting down because of an air bubble is a
two-second smoke trail. The fuel behind
the air bubble hits the hot combustion
chamber and turns into smoke. Generally
the fault codes generated by the ECU are
either low temperature or low rpm.
The solution to this problem is to install
a header tank between the fuel tank and the
fuel pump. There are effective units
available commercially or you can make
your own for roughly $10. There is no
excuse not to use a header tank.
I’ve covered typical flight problems,
but what about startup? The manufacturers
have done a great job setting up start
sequences, and they provide informative
and effective start-sequence faults to guide
you through problems. If you fly your
turbine without an operator’s manual and a
ground-support unit to read these faults,
you’re asking for disappointment. These
manuals have good troubleshooting guides
that will walk you through the fault codes.
I keep my manual in my transmitter case
because I still use it.
My most frequent start problem is
because of old glow plugs. I get 40-50
starts on a glow plug before it becomes
coated with a white frost, and then I don’t
get reliable starts.
My old JetCat P-80 quits igniting the
starting gas when this happens. I replace
the plug and then go. My Artes Eagle is
different. It starts by missing ignition on
the first try, but lights successfully on the
second attempt. I change the plug and it is
back to starting the first time.
I haven't yet seen an effective fault
code for this issue, and a common mistake
is to turn up the voltage to the glow plug
until you simply burn it out. If you leave
the voltage turned up, you’ll burn out the
new glow plug too. If an engine that starts
well quits lighting, change the plug.
What about flying? With a turbine
engine, you will lose the joy of the instant
throttle response that comes with
propeller-driven models, but you get
incredible power at speed. A propeller
limits the available thrust at higher
airspeeds.
A function of pitch and rpm, the air
comes off of a model’s propeller at
approximately 120 mph. The air out of the
turbine’s tailpipe can exceed 400 mph.
Beyond 120 mph, that propeller becomes a
big drag brake while your turbine is still
pushing air out of the tailpipe at speeds
greatly exceeding the speed of the model.
That high-speed push allows for those
huge jet loops that are limited only by your
eyesight and common sense.
This same push means you don't have
the luxury of flying around at full throttle
unless you want to fly ridiculously fast.
You must learn how to and use the throttle
to control airspeed. This can be tough for
some experienced modelers to learn, but it
is the reason many Giant Scale 3-D pilots
transfer easily into jets. They know how to
use the left stick for speed control.
The throttle stick is as important and
used almost as often as the elevator stick.
Cruise at partial throttle and then open
the throttle as you pull on the elevator to
climb or loop. Pull back on the throttle in
dives or the backside of a loop. Once
back to level flight, you move the throttle
back to the cruise setting. You
choreograph throttle usage with elevator
usage to maintain airspeed throughout the
flight.
One reason why I fly my turbine more
often than my piston models is because of
ease of operation. I buy my kerosene at the
local gas station on the way to the field. I
have also used Jet-A and hardware-storepurchased
kerosene with great success.
Jet-A fuel is readily available at many
airports, but be prepared to pump it into
your fuel can. The fuel nozzles for Jet-A
are much larger than the holes in our fuel
cans. Bring a funnel or find a large-mouth
bottle. The alternative is to draw fuel from
the bottom drain of the fuel truck, but
that's also where most of the dirt settles,
and the drain is not filtered.
At the Ashtabula, Ohio, airport—my
regular haunt—the staff does a great job of
maintaining its truck, and the fuel has
always been clean. Not all of my
experiences at other airports have been as
good with this technique, so be careful. I
suggest that you filter your fuel into another
can if you use Jet-A. Don’t clog your fuel
nozzles with dirty fuel.
The next alternative is hardware-storepurchased
kerosene. It is usually a low-odor
fuel—nice compared to Jet-A which has a
smell that lingers for weeks. Prepackaged
kerosene from hardware stores is the cleanest
fuel available, but surprisingly, my only
experience with algae growth in kerosene
came from a store-bought variety that was
packaged in clear plastic bottles. I have had
no problem with the kerosene in metal cans.
For the last year I have been flying with
kerosene that I purchased at local gas
stations. They all carry the same low-odor
brand. It has been trouble-free to date, but I
do have double filters on my fuel can and in
my airplane. I have not experienced any dirt
issues or algae-growth problems, and the
convenience of this fuel source means that I
seldom have fuel that is more than two
weeks old. I usually purchase my fuel for the
day on the way to the flying field.
I hope this column helps some modelers
understand what it takes to fly a turbinepowered
aircraft. The big message is that the
development of turbines has reached a point
where they make great engines for sport
modeling and are no longer limited to
“Sunday at the jet meet.”
I fly my jets roughly once a week,
summer and winter, and I live in a coldwinter
climate. MA