138 MODEL AVIATION
scale jets such as the F-86 Super Sabre have
yaw stability boosters to improve the situation;
on a model airplane we can employ gyros.
Let’s start with a well-documented, widely
used, and effective stability augmentation on a
jet model: installing a gyro on a Bob Violett
Models (BVM) BobCat. This is a popular
older design I have employed many times.
The ones I flew weighed between 15 and 22
pounds empty and were powered with 13- to
30-pound-thrust turbines.
The twin-boom design has the high-speed
turbine exhaust between the fins, a highly
swept wing, and large landing-gear doors that
extend down, create drag, and perform as air
brakes to assist in the landing approach. These
features combine to produce some interesting
yaw characteristics, resulting in a strong
tendency to Dutch roll.
Newer BobCats have adapted traditional
flaps in place of the landing-gear air-brake
doors. Flaps appear to have a
stabilizing effect in yaw on this
model, so gyros are no longer
necessary.
Bob Violett has done a great job
developing a setup on the BobCat
that eliminates the Dutch roll
associated with the yaw-stability
issues. A standard model-helicopter
rate gyro is installed horizontally—
preferably within a few inches of the
CG—and aligned with the direction
of flight.
Bob recommends the use of
high-quality JR gyros because a
reliable gyro is as important as the
rudders. A gyro’s failing could be
just as bad as losing the fin off the
airplane and would be a nasty
situation should it happen in flight!
Don’t go cheap in this area.
The standard BobCat setup is to
The benefits of gyro stabilization for jets
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Radio Control Jets Jim Hiller
Gerry Kerr Sr.’s classic BVM BobCat sports landing-gear-door air brakes that create drag, helping
the pilot slow the jet on landing approach.
Attach a gyro to any flat, horizontal surface parallel to the
aircraft’s centerline to increase yaw stability.
The BobCat’s landing-gear door provides drag when extended but
upsets yaw stability, so a gyro is recommended.
LET’S SPEND some time reviewing
stability augmentation and the use of gyros
on flight control surfaces. Employing gyros
on flight control surfaces was in vogue at
the last World Jet Masters, and for good
reason: the benefits to the competition pilot
are enormous. These benefits can make
flying any model more enjoyable. Using a
gyro can correct inherent design
deficiencies and do much to reduce or
eliminate the rocking about caused by
gusting winds or turbulent air.
Dutch roll is an aircraft’s tendency to
rock its wing in roll. It becomes especially
bad as the angle of attack increases in slow
flight. All wings with highly swept LEs
tend to exhibit this trait to varying degrees.
The wing rocking is the result of the
airplane’s yawing from side to side.
The old rule of thumb is that every 5°
of LE sweep provides an equivalent 1° of
dihedral. This resulting dihedral effect
causes a swept wing to bank effectively
when the airplane is yawed out of square.
Follow this out-of-square yaw with a swing
past center and a roll in the opposite
direction, and the dreaded Dutch roll begins.
It is easy for a pilot to chase this Dutch
roll with ailerons and actually aggravate the
condition. The best pilot technique is to
resist the urge to fight the Dutch roll and
allow it to dampen out by itself.
Something has to cause the airplane to
go off center in yaw to initiate the wing
rocking. It could be as simple as the adverse
yaw of using ailerons while in slow flight
or—as is normally the case—turbulence in
the air.
Jet airplanes typically have swept wings
and large fuselage side areas forward of the
wing. Large fins are typically used for yaw
stability, but this is not always enough. Full-
04sig5.QXD 2/25/08 9:03 AM Page 138
use 25% gain on the gyro for normal gear-up
flying and 55% gain when the landing gear
with the large drag doors is extended. If you
have an older BVM BobCat, use these
numbers. Bob is a perfectionist when it comes
to aircraft setup. Follow his instructions and
you won’t go wrong.
The reason for two different settings is that
in the normal clean mode—landing gear
retracted—a BobCat can be flown without a
gyro on the rudder, but it helps the model on a
windy day. The bouncing is greatly reduced in
turbulent air.
Flight with the landing gear extended is a
different situation with a BobCat that is
equipped with large landing-gear doors. I have
flown BobCats with and without the large
door air brakes; they make a big difference in
drag, aileron sensitivity near center, and the
model’s tendency to Dutch roll during the
landing sequence.
A BobCat without the landing-gear-door
air brakes is quite comfortable without a gyro
on the rudder, but on one with these air brakes
you will really appreciate the added
equipment. The gyro will constantly correct
the airplane’s yaw, including the aileron
adverse yaw associated with slow flight.
Suppose you don’t have a BVM BobCat,
but your scale MiG-15 is always wandering in
roll on windy days and rocks its wings on
landing approach. You want to try a gyro on
your model but need to know how to set it up.
That is not a cut-and-dry deal. You must
dial in the gyro to match your airplane’s flight
characteristics.
I suggest using a gyro that adjusts the gain
rate through a separate channel. The rudder
channel plugs into the gyro between the
receiver and the rudder servo. The adjustable
gain of the gyro is plugged into an available
channel. It is typically the Auxiliary 2 channel
in my JR 10X.
The Code 44 gyro-sensitivity adjustment
feature can be set up to provide the gyro gain
rate adjustments. I set up my Auxiliary 2
three-position transmitter switch with the upposition
zero as my landing-gear down rate;
the midposition 1 as landing-gear up rate; and
the down-position 2 as zero gyro gain.
Using this switch is cumbersome but
necessary while dialing in the appropriate gain
rate. If all goes wrong in flight, I can turn the
gyro gain rate to zero by flipping the
Auxiliary 2 switch down to position 2, which
is set to zero gain. Use the proven gain rates
from this information as a starting point and
work from there.
Before flight, confirm that the direction of
correction from the gyro is right. While on the
ground, set the gyro gain to position zero, pick
up the nose of the model, and move it swiftly
to the right. The gyro should compensate by
moving the rudder to the left. Do this again in
gyro gain position 1 in the same direction but
with less travel. Now you’re ready to become
a test pilot.
Perform the initial takeoff with the gyro’s
gain rate set to zero: position 2. You don’t
need surprises this close to the ground. At a
comfortable altitude with the landing gear
retracted, move the Auxiliary 2 switch to the
mid position and fly the model. If it does not
react as expected, immediately flip the switch
down to the zero gain position.
You are trying to achieve the minimum
gain necessary to provide a locked-in flight
during normal windy weather conditions. Too
much gain will result in a rocking in yaw
when upset by turbulence that continues or
even grows with each correction.
Once you have completed the gear-up
trial, slow the model, extend the landing
gear and the flaps, and then switch the gyro
gain switch up and test the landing-setup
gain rate. Land the airplane and readjust
the gains as necessary until you get it right.
Remember, if you feel uncomfortable with
the gain rates at any time, flip the
Auxiliary 2 switch down to the zero gain
rate, effectively turning off the gyro; don’t
fight a less-than-perfect condition.
Once you are comfortable with your
rudder-gyro gain rates, reprogram your radio
to achieve your two gyro gain settings mixed
off the landing-gear switch instead of the
Auxiliary 2 switch. This will greatly reduce
pilot workload during flight since the landinggear
position will set the appropriate gain
setting. For those who have JR radios, using
flight modes is another excellent way to set up
your model.
Give a rudder gyro a try; you’ll like the
results. Flying a jet on a windy, turbulent day
with a rudder gyro is smooth. The roll is
locked in better and it’s a pleasure to fly. Be
careful and take your time setting up the
direction of the gyro correction. Sneak up to
the desired rate gains. MA
140 MODEL AVIATION
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