112 M ODEL AVIATION
I AM WRITING this month’s column as I
prepare to leave for the Superman Jet Rally.
The routine inspection of systems on my
Razor is complete, and it’s ready for another
major weekend of flying.
I had a nice conversation with Dubb Jett
concerning his ducted-fan engine line, his
philosophy on engines, and what the future
may bring for us ducted-fan modelers.
Dubb has two engines on the ducted-fan
market: the Fan-Jett 50 and the Fan-Jett 95.
These engines are reasonably priced.
Dubb’s primary design goals when
building engines are performance with
reliability, in terms of durability and ease of
operation. Art Arro has been flight-testing
these engines, and he has always had good
things to say about them.
Local flier Joe Lewis has been running a
Fan-Jett 50 in his Jet Hangar Hobbies F-86 Sabre for approximately
two months. Joe saw the success Art was having at the Nationals, so he
decided it was time to upgrade his engine to get the same performance.
Joe is turning a Turbax fan with a MACS quiet pipe, left over
from his K&B 7.5 days. The Fan-Jett’s performance is significantly
better than the old K&B 7.5 engine it replaces.
Joe has roughly 20 flights on the engine, and it is coming up on
pipe nicely. He is on only his second glow plug, and it has more than
a dozen flights on it, with no sign of problems. The Fan-Jett has
turned out to be a friendly engine to tune and fly.
Check out the Jett Engineering engines on Dubb’s Web site
at jettengineering.com; E-mail him at [email protected]; or call him
at (713) 680-8113.
The Internet age is upon us, so I hope you don’t mind my listing
Web sites; they are an effective way to stay in touch with all the new
products coming at us in the ducted-fan and turbine community.
I am not very computer savvy, but even I can find my way
around the Web.
I had to relearn an old trick last weekend.
I have been flying my new jet for a couple months, and I have
been happy with its flying characteristics—but the “dragon” is
always waiting around the corner.
I got low and slow on final, using way too much up-elevator
on the final approach. That resulted in a hard descent to landing,
with no elevator left for the landing flare. No big deal, just a
bump—or it should have been.
I forgot to set up the nose-gear strut with a weak spring. The nosegear
strut spring collapsed, and launched the nose into the air with the
power of a regular-strength main-gear spring still in the nose-gear strut,
leaving me with no airspeed and the nose pointing toward the sky.
The model had a scraped nose, wingtips, and stabilizers.
Remember to put light springs in the nose gear, for the day the
Jim Hiller, 6090 Downs Rd., Champion OH 44481
RADIO CONTROL JETS
Dave Ribbe’s (Marlton NJ) pretty new BVM Bandit, powered by a Sophia turbine. This
combination gets awesome fuel economy for the longest turbine flights.
Initial rollout of CAI’s prototype Predator at 2000 Heart of Ohio
Jet Rally. This sport jet is nearly eight feet long.
Dave Ribbe shares setup points with Gordon Dickens on Gordon’s
new BVM MiG-15, powered by JetCat Hammer P-80 turbine.
blown landing comes. Springs of various
strengths that fit our landing-gear struts can
usually be found at the local hardware store.
Take the time to change that nose-gear
strut spring before your first bad landing
turns into a truly humbling experience.
This past summer was great for flying jets.
I haven’t flown anything but ducted fans
and turbines all summer, and I don’t miss
my propeller twisters one bit.
Sport-flying and jet meets kept me busy.
All this flying does mean one thing—the
need for a good maintenance schedule, to
keep the reliability we demand from our
high-performance models.
Since I started flying ducted-fan
models more than 10 years ago, I have
kept fairly accurate maintenance logs on
them. My original need was to assure
myself they would be ready for the few
opportunities when I found a jet meet or
an adequate field from which to fly.
The first few years were tough. I never
flew my models within 100 miles of home,
because adequate model sites did not exist.
I continue to keep these logs even though I
have access to a couple of local flying
sites now, because the complexity of the
models has increased.
I’m going to discuss living with a
turbine-powered model. I’ll look at turbines
in general, but remember—not all turbines
are created equal, so some issues vary.
The first thing these turbines bring with
them is Jet-A fuel, and the stink of
kerosene. This makes transporting the
model, and the fuel, a memorable
experience. Many modelers have trailers for
this reason; I am also on the lookout for a
trailer for my models and that Jet-A fuel.
Purchasing Jet-A fuel has not been a
problem, except at one location: the major
airport closest to my house. This really
doesn’t matter, though; I get my fuel at
another local airport that welcomes our
model-airplane traffic.
We operate with well-defined rules
for models and how we mix with the
full-scale traffic.
The proper mixture of kerosene or Jet-A
and turbine oil for most turbines is 5%. This
is an easy mix—five gallons of fuel to one
quart of turbine oil.
Find a container with a large-diameter
mouth, to make it easier to fit the commonly
found nozzles on the fuel trucks. The usual
small-mouthed bottles at most department
stores are difficult to fill; the mouth is
smaller than a common Jet-A pump nozzle.
An alternative way to fill your container
is from the bottom tank drain of a fuel
truck, with a small funnel. However, this is
where all the dirt and water collects in the
tank, so I recommend against filling your
container in this manner.
The normal cost of this mix is less than
$20, and it provides roughly 10 flights.
Now let’s talk about the common
flightline equipment for turbines with
autostart systems with ground support to
start the turbine.
The normal turbine requires a starter
box, a compressed-air source, starter gas,
and a fire extinguisher. The starter box can
be obtained from the turbine manufacturer,
and it is a great aid in simplifying the
starting procedure.
The compressed-air source is usually a
scuba tank, and there are small ones
available; the full-size scuba tanks are not
required for field operations.
The starter gas commonly used is a simple
propane canister from a camping store.
The methods for carrying these supplies
vary from simple shoulder packs to
beautifully constructed carts.
Move over NASCAR™ pit crews; the
jet modelers are on the flightline.
Turbines with ground-support
requirements are normally started with the
sequence manually controlled or
controlled by the auto-start box. The
turbine’s ECU (electronic control unit)
requires specific operations after starting,
to warm up the turbine and calibrate
engine parameters.
I fly a Golden West Hammer P-80 with
the onboard auto-start feature, which is
unique in field operations at this time. The
aircraft is fueled with approximately 80
ounces of Jet-A with a butane starting gas.
The turbine’s fuel pump and starting
glow plug get their power from a six-
114 M ODEL AVIATION
cell, 1,200 mAh battery pack on-board
the model.
I am spoiled; all I bring to the flightline
is the airplane, the transmitter, and my fire
extinguisher.
The starting sequence is a simple matter
of turning the radio on, then initiating the
startup with the auxiliary channel and
throttle channel.
The channels are used in a specific
pattern, to avoid accidental starts. The
turbine spins up to 54,000 rpm after
startup, to bring the engine up to
temperature, then it returns to idle before
returning normal operation to the throttle
stick.
This is far below the full throttle setting
of 118,000 rpm, hence the turbine is
reasonably quiet during the starting
sequence. The disadvantage is the onboard
starting system’s additional weight, but
everything has its tradeoffs.
No matter which system you use, the
kerosene fuel will play heck with the
model. The fuel lines cannot be silicone,
and even the best products seem to have
aging effects during the course of a season.
The most critical thing to watch is the
effect the kerosene has on insulation found
on servo wires. Exposure to kerosene, or
sometimes even the fumes, makes the
insulation hard and brittle. Combine this
with servos mounted aft of the turbine
receiving tailpipe heat, and you have the
makings for disaster.
This is where good maintenance comes
in; look for a crisis before it happens.
Normal vibration has not been an
issue on the turbines I have flown. These
engines run so smoothly, vibration is
minimal. I find myself violating many
established rules for installing receivers,
battery packs, and ECUs, with no
negative effects.
However, I still respect vibration because
it is always present with rotating devices.
Watch out for loose items—especially
those located ahead of the turbine—that
may get sucked into the turbine or make
contact with the hot tailpipe.
The general maintenance cycle of a
turbine model beyond these issues is
similar to any ducted-fan model. We fly
rather heavy, high-performance models that
do stress the airframe and landing gears, so
watch critical areas for wear and tear.
The wing spars and wing-mounting
systems should be inspected carefully
for stress cracks as they age. Landinggear
plates and mounting pads are
heavily loaded, so watch them for
impending failures.
And those plastic parts on the retract
units? How many times have I found
them cracked, waiting for that next
blown landing?
It’s time to sign off and hit the road. The
weather forecasts are good, and you have to
take advantage of good flying weather
when you live in the North.
See you at the field. MA
