Radio Control Jets - 2008/08

I’M GOING TO address turbine reliability
and a common mistake I have seen: FOD
(foreign object damage) to your engine. The
big stuff is easy; if a bolt gets into the
compressor, you will immediately see the
damage. It’s the smaller stuff you don’t see
that can do serious damage.
Grass, dirt, and sand are some of your
turbine’s worst enemies. Why? Because of
what they can do to the bearings.
Failure of a turbine bearing is among the
most serious and expensive problems that
can occur. The rotating assembly consists of
expensive parts in a turbine: the compressor
wheel, turbine wheel, and shaft. When a
bearing fails, this rotating assembly grinds
to a halt, usually rubbing the turbine wheel
and compressor against things they should
never touch, totally destroying these
expensive components.
Today’s technology gives us ceramic
ball bearings. These lightweight
components are one of the keys to the
success of bearing life and the high rpm
potential of our turbines, but they are
unforgiving.
These bearings must be lubricated and
kept cool. The fuel is used for lubrication,
hence the need for oil mixed into it, but it’s
the cooling that is of interest in this
conversation.
Air is bled off from behind the
compressor wheel to cool the shaft bearings,
entering the bearing shaft tunnel through the
front bearing and exiting through the rear
bearing. This technology has been
developed to a highly reliable state, but it
does require clean air.
The typical air that the compressor
wheel sucks in is plenty clean for cooling
our bearings without contaminating them
under normal flying conditions. It’s the air
sucked in near the ground that changes all
that.
A turbine is like a powerful vacuum
cleaner. When the intakes are near the
ground, they can suck all the dirt, sand, and
grass from the ground into the compressor
and through the bearings. That debris will
destroy our wonderful ceramic bearings in
short order—usually on the next flight when
full power is needed most, costing you not
only an expensive turbine repair, but
possibly an airplane.
So what can you do to preserve your
bearings? Don’t do anything dumb!
Be conscious of the surface on which
you are running up your turbine. If you fly
from grass or sandy surfaces, avoid fullthrottle
run-ups unless they are absolutely
necessary. Why make all that noise,
knowingly cleaning all the garbage off the
tarmac, just to hear your turbine spin up to
full throttle? Save it for takeoff.
The biggest mistake I see is pilots not
shutting down their turbines when they have
trouble with landing gear. Many times I
have seen a modeler land a jet in the grass
with the landing gear up, and the crowd
applauds. The pilot walks over to the model,
picks it up, and then shuts down the turbine.
This looks unbelievable when the model
is an F-15 Eagle with the intakes resting on
the ground, sucking everything it can into
the turbine and those precious bearings. That
modeler is shortly complaining about the
poor bearing life that particular brand of
turbine has and how he or she has never
abused that engine.
I believe strongly in shutting down my
turbine whenever I have a problem with
landing gear, because it does two things.
First, it allows the model to slow to a stop
on landing as quickly as possible, with no
residual thrust and reducing the airframe
damage.
Second, a turbine during the cooldown
process is not moving much air. Hence it
does not tend to suck in all the undesirable
dirt, grass, and sand that is on the ground.
I am not saying that when the nose gear
fails, extend your jet’s climb to 500 feet and
shut down the turbine. Shut it down when
you commit to land. In these situations I like to shut down the turbine just before I pull on
the elevator for the final flare to land. Think
about your turbine and how to avoid
damaging it when you go off the runway.
At the Toledo Show this year, I stopped by
the ElectroDynamics booth to pick up a
servo reverser for my T-33. Late last year I
had a flap-servo failure—the reversed
servo—so instead of replacing this specialorder
part, I purchased a servo reverser and
installed two new standard units.
ElectroDynamics makes a nice product:
the EDR-106 Pro Servo Reverser. It’s
available as both a direct, in-line, plug-in
unit or, at a slight premium, a Y-connector
unit with one plug standard and one
reversed. I chose the latter, which cleaned
up my wiring harness in the T-33 and
allowed me to set up with two fresh servos
on the flaps.
I had originally set up my T-33 with a
10-channel radio. But after converting to the
Spektrum module for my JR 10S
transmitter, I was down to nine channels. So
I adjusted my setup accordingly.
ElectroDynamics’ EDR-125 Gear Mg’R can
help you free up channels by doing some
neat mixing to solve some old problems
most of us have experienced when setting up
landing gear retracts, brakes, and steering.
Setting up aircraft for brake activation
with the application of down-elevator is
convenient and popular with modelers. The
problem is that when doing aerobatics that
require down-elevator, we are constantly
activating the brakes, using precious brake
air pressure.
The EDR-125 handles this problem.
When the landing gear is retracted, the brake
servo does not move; it functions only when
the landing gear is down, eliminating the use
of brake air when the gear is retracted.
The elevator and retract channels from
the receiver are plugged directly into the
EDR-125, and then the elevator servo,
retract servo, and brake servo are plugged
into the unit. It is programmable, to allow
you to match your control directions and
throws as required for the brake function.
The next item the EDR-125 takes care of
is the steering servo. Plug the rudder channel
directly to the unit, and then plug the rudder
servo and steering servo into it. With the
EDR-125, you can reverse the steering servo
if required, but the neat thing is that the
steering servo will be deactivated when the
landing gear is retracted.
This function should help you avoid
some of those nasty situations in which
steering cables get tangled up while
retracting, causing the nose gear to fail to
extend or, worse yet, a broken cable and loss
of steering.
The EDR-125 also saves us from the
dreaded powering-up of the model’s radio
with the transmitter retract switch in the up
position, retracting the gear and damaging
both gear doors and our pride. The unit will
move the retract servo to the gear-down
position on power up, and it will not move
the retract servo to the gear-up position until
after cycling the retract switch through the
gear-down position on the transmitter’s
retract switch.
I have installed the EDR-106 and EDR-
125 in my T-33. So far, so good. I am quite
pleased with them. But hang on, Andy
(ElectroDynamics’ proprietor); this jet is
more than 10 years old, and I am not ready
to quit flying it yet, so we may be doing
some extended testing on your units.
BRUCE THARPE Engineering’s (BTE’s)
ever-popular Reaction 54, built from the
wood kit, is now manufactured as an ARF. It
148 MODEL AVIATION
has a prepainted airframe and the same
great wing, but the smooth, curving lines
are made possible by composite
construction. This model is available from
PST Jets and BTE.
This new version is as large as the
original, with a huge 78-inch wingspan
and more than 10 square feet of wing area.
But the composite ARF version did gain
some weight. Expect weights of 22-26
pounds, depending on turbine and options.
The ARF can handle the 20-poundthrust
turbines, allowing significantly
improved power and speeds but retaining
the sweet landing characteristics of the
original—but at slightly higher speeds
because of the composite construction’s
increased weight. Extra strength has been
added to the ARF, allowing trouble-free,
high-speed flight.
Recommended turbines are in the
sedate 14- to 20-pound-thrust range. But if
you’re brave enough and can control the
throttle well enough, it will fit even more
powerful turbines.
Typical of modern painted-in-the-mold
ARFs, the Reaction 54 features nice paint
schemes that dress up well with militarystyle
decal sets. The wing is two pieces,
joined with aluminum tubes, and features
live skin hinges on ailerons, flaps, and
elevator. I like the fact that the model
comes with a clear canopy, to allow
cockpit detail to be added as desired.
The Reaction 54 ARF is neat. It is
available as a basic airframe, along with
retract, wheel, and brake packages. There
is even a smoke-tank option.
It’s summer, and that means it’s time to
fly—so let’s burn some kerosene. MA
Sources:
ElectroDynamics
(734) 422-5420
www.electrodynam.com
BTE
(800) 557-4470
www.btemodels.com