Radio Control Jets - 2004/06

138 MODEL AVIATION
THE LATEST JET model from Bob
Violett Models (BVM), arriving in the
hands of the modelers this summer, is the
KingCat. This is a large sport jet, best
described as a large, distant brother to the
BobCat.
The KingCat is a big model—94 inches
in length with an 80-inch wingspan—with
more than 11 square feet of wing. Combine
this with the potential to build and equip
this airplane to weigh as little as 26
pounds, and you have one of the lightest
wing loadings available in a large jetpowered
model. The ample wing area
combined with good wing design makes
this one of the slowest-landing jet models
available today.
The model features an exposed turbine
installation to simplify construction,
reduce components, and consequently
reduce weight. No cockpit details are
required since it features a painted canopy,
again reducing complexity and weight. It is
of the same pod-and-twin-boom layout as
the BobCat, only larger.
The major new feature of the KingCat,
and where it differs so significantly from
the BobCat, is the all-composite
construction with the airplane painted in
the molds. This is a technique that has
become popular with high-end ARF
models. The available color schemes range
from sport-aerobatic styles to military
paint schemes; all aim to please the
modeler.
Control-surface hinging utilizes flex
hinges. The actual skin of the model flexes
to provide the hinging, resulting in an
absolutely smooth surface transition from
the wing to the aileron.
Jim Hiller, 6090 Downs Rd., Champion OH 44481
RADIO CONTROL JETS
The Facet 2300 Sport Jet by Kerry Sterner is for 17-pound-thrust turbines. This simple,
fixed-gear model aims to make turbine modeling more affordable.
The Jet-Tronics proportional brake valve is simple to install: plug
it into your receiver and hook up the two air lines.
The Jet-Tronics brake valve and the BVM Smooth Stop are two
solutions for applying proportional brake control.
A large model such as the KingCat is
quite an investment, and BVM provides
top-quality components to equip such a
large model. It supplies a solid retract
landing-gear unit, complete with wheels
and brakes, to properly support this large
aircraft. The thought and design of the
landing gear is a reflection of the detail
BVM puts into the aircraft. The gear inner
struts are heat-treated and ground to
properly fit the outer gear down tubes,
assuring a slop-free fit. The nose-gear
yoke is a precision steel casting for
durability.
The airframe package includes large
Kevlar fuel tanks and a 16-ounce header
tank, providing a total fuel capacity of 4.8
liters, or more than one gallon of fuel.
Why so much fuel? It takes at least a 27-
pound-thrust turbine to power such a large
model, and these big turbines do use fuel.
This quantity of fuel will provide a
comfortable 10 minutes or more of flying
time under normal throttle usage. This
model follows the trend, as jet modeling is
maturing, of providing fuel-tank systems
large enough to allow reasonable flight
times, comparable to those of pistonpowered
aircraft.
That is enough about the statistics. How
does it fly? The KingCat is large, with a
wing area and weight comparable to many
06sig5QXD 3/24/04 11:46 am Page 138
1⁄3-scale models, so that sums up its lowspeed
range; it is one slow-flying jet. The
wing design, airfoil, and washout are
designed to provide gentle stall
characteristics to allow you to drag this
model in without fear of a sudden,
vicious stall. Sweet is what it is.
When moving out, such a large model
can turn tight aerobatics without fear of a
high-speed stall, so it can be pushed
around in comfort. The top speed is not
as fast as many sport jets because of its
size, generous wing and airfoil, and lack
of trick speed items. This isn’t a bad
thing because the art of aerobatics and
general flying is much more relaxing at
these moderate speeds, but don’t kid
yourself; it is a jet-powered model, so it
still moves out.
Large jet aerobatics are a joy with this
model, and when you tire of the big,
smooth maneuvers, you can slow it down
and fly a low, slow, and tight aerobatic
routine, complete with snap rolls and
spins. It offers a wide speed range for
aerobatic enjoyment.
The strangest thing about the
KingCat, particularly for some F-4 jet
pilots I know, is learning to land it. This
aircraft flies final approach slowly
compared to typical jets, and the final
flare takes so much time that you have to
learn to treat it more like a sailplane than
a jet when landing. When you do, you
will be rewarded with one of the
sweetest, smoothest, and shortest-landing
jet models on the field.
The difficulty in landing this model,
as compared to that of a 1⁄3-scale piston
model, is the clean airframe combined
with not having an idling propeller to
slow down. It is up to the pilot to
maintain and control airspeed on final.
Done well, landings are a breeze. Once
you master airspeed control on final, it is
cool as can be to land; it makes one want
to shoot touch-and-gos all afternoon.
Overall, the KingCat is expanding the
range of available jet models that
emphasize low-speed handling and
aerobatic capabilities instead of pure jet
speed. The cost of this model is on the
high end, but BVM is providing the
modeler with a quality product, and that
comes at a cost when done right. Check
out the BVM Web site at www.bvm
jets.com for the latest information on this
exciting new jet.
You have heard about brakes on jet
models, and you’ve seen them stop, but
how is it done so smoothly by some and
seems so tough for others? The key is
proportional brake systems and
understanding how to use them.
Basically, there are two types of
proportional brake controls for use on the
pneumatic brake systems in typical use
today: pulsed-air control and pressureregulation
control. I fly both systems and
like them both, but each has its own
characteristics, and limits.
The pulsed-air valve, such as the Jet-
Tronics unit and the MiniHobby ATL
brake valve, are simple on-off valves,
pulsed at a rate proportional to the brake
slider switch position, releasing air in the
brake lines at a controlled rate.
At full travel the air valve is held on, and
full air from the tank is supplied to the
brakes. In partial brake positions, the valve
is pulsed on and off, air is bled out of the
brake lines during the valve-off time, and air
is supplied during the brake-on time. The
rate at which air is bled off versus the time
air pressure is supplied determines the
average air pressure in the brake lines and
consequently the proportional brake control.
Air is continually being bled out of the
brake air tank while partial braking is being
applied, so you only have a limited amount
of total time available during which partial
braking can be utilized before you run out of
air. The pilot must remain conscious of
partial-brake-usage time to judge how much
air pressure is available for stopping the
aircraft.
Full-brake application results in 3-5 psi
loss of pressure in the air tank but can be
held for extended periods without additional
air loss, such as when holding short of the
runway awaiting clearance for takeoff.
Setting up a pulsed-air valve is simple.
Merely plug it into the receiver like a servo,
hook up the air lines, and you are ready to
go. This is the best part of these systems:
ease of installation. To date, I have not had
to perform any routine maintenance on my
Jet-Tronics air valves; I just plug them in
and forget about them.
The regulated-pressure brake valve—
such as the BVM Smooth Stop and the Ultra
Precision U.P. 6 valve—regulate the
pressure applied to the brake system. A
servo is used to position a floating piston
which acts as a pressure regulator to control
the air pressure to the brake system.
Care must be taken when setting up the
pressure-regulator valve to get the right
adjustment of the servo linkage to achieve
the proper brake release and brake
actuation. The O-ring seals of the valve
must be airtight, yet allow freedom of
movement for proper operation of the valve.
Occasionally you need to lubricate the valve
O-rings to maintain the O-ring integrity and
assure freedom of movement.
The servo controlling the brake valve
does not experience great loads, so a
microservo is adequate to move the piston.
Air pressure in the tank supplying the valve
must be quite high—typically higher than
65 psi for the regulated pressure valve to
function properly.
The nice thing about these systems is
their lack of air usage. Each brake
application cycle uses roughly 3-5 psi of air
from the air tank, but the length of time the
brakes are held at partial position does not
use additional brake air.
A technique that has become popular
lately to aid in landing models is to apply a
small amount of brake pressure prior to
landing. This acts to help stick a model on
landing. Upon touchdown, the light braking
action on the main tires pulls the nose gear
down tightly to the ground, reducing the
tendency of the model to skip back into the
air if landed slightly fast. Without this,
those little skips can easily develop into
some destructive bounces if they are not
stopped quickly. This technique can save
landing-gear damage on a tricky-to-land
model.
Another advantage is that when landing
on a short runway, the brakes are already
applied at touchdown, so the model
immediately begins slowing down without
losing the time it takes the pilot to transition
from flying the aircraft to braking the
aircraft. Much shorter landing rollouts can
be achieved with this technique.
This technique can only be done with a
regulated-pressure brake-valve system
such as the Smooth Stop or U.P. 6 valve.
A pulsed-air valve will have bled off all
your brake air by the time you land the
airplane, leaving you with no brake
pressure available to stop the model by
the time you get to the ground. I don’t
use this technique on my model
equipped with a Jet-Tronics brake valve,
but it lands so sweetly that it is not
required on this airplane.
I hope this helps you understand what
is available to get the most from your
proportional braking system and
provides some insight about which is the
best system for you. MA