RC Jets - 2012/10

Many modelers enjoy fl ying
Scale models—recreating the
fl ight of full-scale aircraft they
remember so well. With that in mind,
let’s discuss what it is like to set up and
build a modern ARF jet model. I’ll use
the setup and fl ying of my Der Jet de
Havilland Vampire as an example, and
present some insight into why it was set
up in such a manner.
The Vampire is a British jet fi rst
fl own before the end of World War II.
It is a vintage jet from the early days of
jet power, but one that carried on for
years—serving throughout the world
in several air forces. Its classic lines
and sound make the Vampire instantly
recognizable. The aircraft is from the age
when jets were not powerful, but still
fast.
The Der Jet Vampire’s setup is
straightforward and typical for an ARF
jet model. It comes painted and ready
for system installation. The ailerons
and elevator are hinged with Kevlar
cloth molded in, providing a clean
and solid hinge. The fl aps, rudders,
and speed brakes need to be hinged
using the provided hardware. All of
the control horns must be installed.
Modern ARFs such as this typically
have the control linkages hanging
outside—practical, but not clean, for a
Scale model.
The Vampire is unique in the quantity
of servos required in the wing. Eight are
required—one for each control surface.
This aircraft has ailerons, an air brake
that rotates approximately 90°, an outer
fl ap, and an inner fl ap.
I elected to set up this Vampire with
a JR 12X radio and use JR servos. The
four fl aps and two air brakes have the
economical JR821 servos. There is
plenty of power at a good cost, but a
servo reverser for left versus right-side
installation, with plenty of Y connectors on the fl aps, is required.
Selection of the elevator servo is infl uenced by the size of the surface and the speed
of the model. A JR8611 has plenty of power, but for a few dollars more, you can
purchase the monster power of the JR8711 servo.
It is nice to know the elevator control will not go soft coming off the back side of a
loop, so I went overboard and installed the JR8711; that’s one less doubt when fl ying
this grand aircraft. The rudders are limited by the available space, so narrow servos are
required. Staying with the JR theme, I selected JR8321s.
Some of the aspects that make putting these complicated models together fun are
the optional equipment setups. I prefer to set up the nose gear steering servo on the
rudder channel and use an auxiliary channel (normally Auxiliary 4) for the rudder
servos. This allows me to turn off the rudder trim to the mixed Auxiliary 4 channel,
thereby permitting me to use the rudder channel trim to accurately trim for straight
taxi as I back-taxi for takeoff.
The disadvantage is that I do not have rudder trim available in fl ight. I have to land
and sub-trim Auxiliary 4 to dial in the rudder trims.
The landing gear doors are all actuated with air cylinders and controlled by a Jet-
Tronics dual solenoid valve. This device eliminates the need for a separate servo and
air valve, which are all in one neat package.
Use a servo slow in the up direction to control the timing for when the gear doors
come up after the landing gear has fi nished retracting. A 3-second delay should be
adequate. No servo slow is programmed for landing gear down, so open the doors as
the gear extends.
Setup of the fl ap-to-elevator trim mix requires test fl ights to dial in. The Vampire
is typical of many jets—requiring approximately 3% down-elevator trim with full
fl ap extension. Not all models require the same amount of trim, or even the same
direction. If no recommended values are available, you have to fl y the airplane,
determine the direction, and determine how much you need.
I just returned from a test fl ight on a new jet. It required plenty of down-elevator
trim. Flaps down with no elevator mix resulted in a slow glide speed, so I made it
with no fl aps to keep the fi rst landing safe. The next fl ight got it close. I did some fi nal
adjusting following the third fl ight.
I have the airplane systems installed. Now comes the part we all love with Scale
models, whether turbine-powered or pulled by a propeller: fl ying. Most Scale
modelers try to fl y their models in a manner similar to the full-scale aircraft, and here
is where the Vampire does well. Many
of the characteristics of the full-scale
aircraft carry over to the model.
I like to read flight reviews of the fullscale
aircraft prior to flying the model
so I know what to expect and how to
simulate full-scale flight.
My Vampire is powered by a Jet
Central Rabbit producing 22 pounds
of thrust. It has an empty weight of
33 pounds and weighs approximately
39 pounds when loaded with fuel.
The model’s power-to-weight ratio is
comparable to the full-scale aircraft’s
power-to-weight ratio. This requires you
to fly the model on the wing similar to
how the full-scale aircraft is flown.
Takeoff commences with half flaps,
pulling plenty of elevator. By transferring
the weight to the main gear, the model
tends to hold a straighter line down the
runway. Too much weight on the nose
gear results in erratic steering.
The second reason is that there is
nothing prettier than hitting that right
point when you raise the nose gear off
the ground. Relax
the elevator slightly
to hold the nose
gear 1 to 2 inches
off the ground,
rolling along for a
couple of feet, until
the aircraft lifts off.
Once clear of
the ground, it is
easy to maintain
a gentle climbout.
Remember, it
doesn’t have the big power of modern
fighters, so a gentle climb is correct.
Quickly pull the landing gear up to aide
in acceleration and climb. When the
airspeed is up and the climb established,
begin milking off the flaps.
Using the servo slow feature of my
JR12X, I can set the timing to take
nearly three seconds to retract the flaps.
This allows the flaps to be retracted
without that big bobble, because the
aircraft has to increase the angle of attack
to maintain lift as the flaps retract. It’s all
part of duplicating the look of the fullscale
Vampire flight.
In the air and cleaned up, the Vampire
accelerates well, hitting roughly 130
mph at top speed and full throttle. At
1/5 scale, that would equate 650 mph for
the full-scale aircraft. That’s too fast, but
you don’t cruise around at full throttle
with Scale models. Most of my flying is
done at 50% throttle, which is plenty
of speed to fly comfortably, yet look
good. I carry more throttle and speed for
aerobatics.
Rolls with the Vampire will never be
axial on a straight line. It will not hold
knife-edge; it just drops out of the sky.
Rolls are best performed on a light arc—
typical of early jet aircraft. It is best to
carry some speed for slow rolls and point
rolls because it fl attens out the required
arc.
The knife-edge, even with full rudder,
is weak so watch out for the ground
approaching fast when coming through
the last quarter of the roll. One neat
thing about the Vampire is that point
rolls are sweet. It stops on points well,
and once dialed in for rudder-to-aileron
mix, four-, eight-, and even 16-point rolls
come off nicely.
Loops with the Vampire require
planning. You need to pick up speed, pull
full throttle early, and remember it’s a
turbine. Get up from cruise throttle to
full throttle before you pull the elevator
for the loop. The Vampire’s big wing
holds speed well through the loop. Float
the aircraft across the top by easing up
on the elevator, and round loops you can
fl y well.
I have found that the Vampire can
perform nice dead-stick aerobatics. Loops
come off nicely because it does not bleed
off much speed since you load up the
wing with elevator while entering a loop.
It’s great fun with energy management.
When setting up to land the Vampire,
think back to those fl ight reports on the
full-scale Vampire because it handles
similarly. My routine is to slow the
Vampire down, lowering the fl aps to
takeoff position. Slow fl ight with fl aps
down, even at takeoff position, improves
the feel of the aircraft in slow fl ight and
makes it steady.
Once speed is decreased, lower the
landing gear. Immediately bring on the
throttle as the drag increases. All of this is
taking place in the downwind leg of the
landing approach.
Before turning into base leg to land, I
like to lower the fl aps to full position—
again bringing in additional power. As I
begin the turn to fi nal approach, I bring
out the air brakes and add plenty of
power. I’m carrying 40% to 50% on the
throttle by now, and there is plenty of
drag in the landing confi guration.
The full-scale Vampire was designed to
have plenty of drag to allow the pilots to
carry power to control the approach. Fly
the model the same way and it will work
well.
Early jet engines were slow on throttle
response, especially coming off of idle.
Throttle response from mid-throttle was
much better. Again, fl y it like the fullscale
aircraft, which is fun.
The power does not have to be pulled
off until the main gear tires touch the
ground. This gives you more time to
perform a smooth fl are to touchdown
if some power is carried throughout the
maneuver. On dead-stick landings, I don’t
like using the air brakes because the
angle of descent for landing gets steep.
Duplicating the appearance and fl ight
of the full-scale aircraft to the best of our
ability is likely why so many of us enjoy
Scale models. I’ll always enjoy building,
setting up, and fl ying Scale models,
and when one fl ies well, every fl ight is
rewarding.