Radio Control Jets-2007/12

Thoughts about converting older ducted-fan models to turbine power
Also included in this column:
• JetCentral’s turboprop in a
Great Planes CAP 232
December 2007 147
[[email protected]]
Radio Control Jets Jim Hiller
The conversion to a BVM T-33 from ducted-fan to turbine
power has breathed new life into this airframe.
Mike cut off the inlet ducts on his Viper to make room for the
fuel tank, keeping as close as possible to the CG.
THIS YEAR I have been flying my old Lockheed T-33 built from a
Bob Violett Models (BVM) kit in 1995. The advent of our reliable,
lightweight, 14-pound-thrust turbines convinced me to pull out my
T-33 and convert it from ducted-fan to turbine power. Let’s look at
my conversion as a method to consider for others who may want to
convert one of their ducted-fan models.
I chose a Wren 54 Mk 3 with full autostart, assembled from the
kit, to power this model. The kit version of the Wren 54 is a nice
package. I am familiar with the electronics—a Full Authority Digital
Engine Control (FADEC) unit—and comfortable with its
programming. It’s a real confidence builder. The fuel pump and
control valves are of good quality.
My Wren 54 came from Jet Hangar Hobbies: the US distributor
of Wren turbines. I selected the kit version strictly for the challenge
of assembling my own turbine. It was a pleasant and rewarding
experience.
I spent two evenings assembling and mounting it on a test stand,
and then I had it running without problems. If you have the
mechanical skills to replace the bearings on a four-stroke engine,
you have the mechanical aptitude to assemble a Wren turbine kit. It
is available completely assembled and ready to run if you want to go
that route.
My conversion to the BVM T-33 was a simple setup. I chose a
Wren tailpipe, designed and built for use with the Wren 54. This is a
typical dual-wall pipe that is intended for open mounting of the
turbine; it is not for use in a fully ducted system. The pipe is 24
inches long, which is ideal for the T-33.
My setup has a 7/8-inch gap between the tailpipe and the turbine
exhaust opening for optimum performance for both thrust and heat.
The concept is that high-velocity turbine exhaust will pull cold air
from the fuselage into the tailpipe, increasing the mass of air in the
tailpipe.
The tailpipe itself is not a simple straight design; it starts as a
straight pipe, allowing the cold air drawn in to be heated by the hot
exhaust. At a tuned distance the tailpipe tapers and accelerates the
air out the aft end, thereby increasing the turbine’s thrust. Properly
Mike Kirby built this gorgeous BVM Viper and converted it to
turbine power. It is a powerful-flying model.
done and installed, this tailpipe design can
actually increase a turbine’s thrust compared
to the 5% loss that is typical of a straight
tailpipe.
My T-33 is covered in chrome Coverite
Presto to simulate a buffed-aluminum finish.
This is a Mylar product, so it is not very
resistant to heat. The Wren tailpipe has
proven to run extremely cool; it is hands
down the coolest installed tailpipe I have
ever run or seen run.
I mounted the tailpipe by adding a partial
former back in the fuselage, with its location
based on the mounting tabs supplied on the
tailpipe. I mounted the turbine
approximately 5 inches aft of the original
ducted-fan system and still under the
original hatch opening, so no major changes
were required.
I extended the original intake duct by
adding a fiberglass sleeve to provide outside
air directly to the turbine. It extends 2 inches
beyond the compressor intake, ensuring that
the turbine will not be a vacuum cleaner
sucking up everything in the fuselage.
I made the sleeve using a simple mold. I
created an aluminum sleeve from some old
roof flashing that was the right diameter,
waxed it, and then laid two layers of
fiberglass cloth over it. I used a layer of 4-
ounce cloth followed by a layer of 2-ounce
cloth. It is slightly flimsy, so if I were to lay
up another one I would use two layers of 4-
ounce cloth for additional strength.
I replaced the two 11-ounce saddle fuel
tanks mounted alongside the intake ducts
with two 14-ounce fiberglass tanks from my
old CAI Razor. They fit, but it’s tight. I
added a 24-ounce square Sullivan tank
between the inlet ducts, which gives me 52
ounces of fuel.
This has proven to be adequate, but I set
my timer at takeoff for five minutes, so
flights are a bit short. I should get ambitious
this winter and mold a new set of saddle
tanks to take full advantage of the space
available to increase the fuel capacity.
I used a two-cell, 2100 mAh Li-Poly
battery pack for the turbine in place of the
normal six-cell Ni-Cd pack. This lighter Li-
Poly battery is the right weight to get the CG
dead on when mounted in the nose of the
aircraft, along with an 1800 mAh five-cell
Ni-Cd pack for the radio.
My experience with two-cell Li-Polys for
powering the FADEC ECUs has been
excellent on this Wren and on my larger
JetCentral Eagle powered by a 28-poundthrust
turbine. Power for starts is excellent,
and at roughly 300 mAh usage per flight, the
2100 mAh capacity makes for plenty of
flying between recharges.
The all-up weight of my turbine T-33 is
now slightly less than 18.75 pounds; that’s
roughly 1/4 pound heavier than the ductedfan
setup. That’s nice.
You can achieve a turbine conversion
without significant weight increase and
without too much complication. I hope this
story helps you convert one of your old
ducted-fan models to turbine power and
continue its life with more power.
I chose not to use my BVM conversion kit
for the T-33 because I really wanted to try
the Wren tailpipe. The BVM conversion is
an efficient system that can yield great
airspeeds, and I wanted my T-33 to fly
slightly slower. I ended up with a somewhat
slower T-33, yet I still have strong push for
takeoff and climb.
The BVM T-33’s conversion from
ducted-fan to turbine power can be made
with the quality conversion kit the company
has available. It includes a fully ducted flow
system, quality fuel tanks, and is fully
engineered for success. I have one sitting in
the box along with my new T-33 kit that is
waiting to be built.
The turbine T-33 served me well all
year. I’ve enjoyed flying it again, and with
the power and reliability of the Wren 54
turbine … What can I say? It is much
improved.
The climb after takeoff is better, and
landing with the greater residual thrust at
idle of a turbine has not been an issue. The
sound and smell of a turbine—that’s what it
is all about. It’s a sweet package.
Mike Kirby, our Blue Grass Jet Jam CD,
has a nice converted BVM Viper that flies
well. He approached his conversion in a
similar fashion as I did.
Throughout the years I have seen some
nice-flying turbine conversions to other
ducted-fan models including Starfires,
Mavericks, F-86s, and F-80s. The key is just
to do it and don’t let the weight get out of
hand by overcomplicating your installation.
Eric Clapp of JetCentral has been promoting
the company’s turboprop in a Great Planes
CAP 232, and it is the quietest one I have
heard running. This engine is based on the
highly successful Super Bee turbine core
driving a power turbine and gearbox off the
exhaust end. This is quickly becoming the
standard layout for model turboprop engines.
Eric’s installation on his CAP 232 is nice
because he runs a three-blade propeller and it
is quiet. The three-blade propeller has a
smaller diameter that reduces its tip speed to
quiet levels, and the turbine’s exhaust is
muffled by driving the power turbine. It is
unreal.
Most turboprop setups running today on
two-blade propellers are loud because of the
propeller’s high tip speeds. Well done, Eric.
Installing a turboprop on the Great Planes
CAP 232 takes removal of the firewall and
installation of supports farther aft because the
engine is long. Another consideration is
allowing air to the inlet of the turbine located
on the backside of the engine. Air must be
brought into the main fuselage interior to feed
into the turboprop compressor.
Additional air is required in the front to
control temperatures under the cowling. Any
turbine engine does run hot, so air movement
is important.
So much for this month! I am about to head
off to the Cleveland National Air Show to
show off my T-33 to the big boys. It’s so
cool to be invited to fly there. MA