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Landing Gear: An airplane that
rolls straight and responds predictably to steering input, especially on
takeoff, will be easier to fly. If you want to look like a hero at the
flying field, die-straight takeoffs and smooth landings that roll to a
straight stop will help.
On the other hand, if you really crave attention, zigzagging
across the runway will have everyone watching you—as they run for cover! That's not how you want to be noticed, so we
will devote some attention to describing good landing-gear setup. Most
trainers are designed with tricycle landing gear, so I will cover models
with that kind first, followed by tail-draggers.
A few problems can
afflict a tricycle-geared airplane. The most common is the use of a
nose-wheel steering linkage that has way too much throw.
The model does
not need to be able to turn within its own wingspan; the minimum turning
radius should be roughly 15 feet with full rudder control applied. That
probably works out to only 5° of turn at the nose wheel.
This is
accomplished by connecting the linkage to the innermost hole of the
servo arm and the outermost hole on the nose-strut steering arm. It is
sometimes helpful to drill a new hole in the servo arm that is as close
as possible to the center post. Too much steering throw not only makes
it difficult to steer straight at speed, but it can overload the rudder
servo and prematurely age or damage it.
The next problem is an overly
flexible steering linkage. You need positive control, and a springy
linkage does not offer that. If the steering linkage has too much give
in it, the nose-wheel may even twist sideways at touchdown (impact?).
This makes the airplane "curtsy" in the middle of the runway and can
even tear out the firewall if repeated often enough.
Some fliers will
tell you that a springy linkage can save the servo, but the best way to
do that is to give the servo maximum mechanical advantage, as described
in the preceding. As for the linkage itself, whether you prefer a wire
in a plastic tube or flexible cable, the linkage should be as straight
as possible.
I prefer a 0.050-inch-diameter wire in a plastic tube,
provided the run from servo to steering arm is almost straight. Sharp
bends are a no-no, and any binding that causes the steering to center
inconsistently must be avoided. It also helps to move the nose-wheel
steering arm up as high as practical, while avoiding interference with
the fuel tank.
The next problem has to do with that nose-wheel twisting
tendency. If, when you look from the front, the contact patch of the
tire is not directly in line with the axis of steering rotation, every
bump in the runway will try to twist the nose wheel to one side. This
really eats servos. This problem is only worsened by a flexible steering
linkage.
Click on photo to view large image with caption
Attitude Adjustment: The nose-high or nose-down attitude of the
airplane on the ground has a strong effect on how much up-elevator
control is necessary to perform the liftoff.
A nose-down attitude forces
the model to accelerate for longer on the runway, until the airspeed is
greater and the elevator control becomes powerful enough to lift the
nose. This is bad because it leads to a sudden leap into the air as the
full up-elevator finally takes hold. What almost always follows is a
too-steep climb and a loss of airspeed and control.
A nose-up stance
causes bouncing on landing when the nose wheel touches down first. On
takeoff the nose-high stance can lead to "wheelbarrowing" at high speed.
Did you ever try to make a gentle turn while running with a fully laden
wheelbarrow? It tried to tip, didn't it? The same is true with a
tricycle-geared airplane if it is running up on only the nose wheel.
The
ideal attitude is with the wing chord line (or flat bottom) within a few
degrees of level with the ground. A well-set-up trainer will lift off
with just a tiny touch of up-elevator when the airspeed is right. For
trainers with flat-bottomed wings, this stance will lift off by itself
when the airplane is going fast enough.
The last tricycle-gear problem
is the fore and aft location of the main gear. If the main gear is
placed too far aft, the airplane has a great deal of weight on the nose
wheel. This also makes the high-speed steering more sensitive and
requires lots of up-elevator input to break ground. Try pushing down on
the stabilizer to lift the nose wheel, to get a feel for how much force
the up-elevator control has to make.
Again, this can lead to an overly
steep departure after an excessively long takeoff roll. It also causes
the airplane to "slap" onto the ground during landing; that can add to
the wear and tear on the nose gear.
The ideal location for the main gear
makes the nose wheel very light when the fuel tank is empty. Either bend
or shim the main gear so that the wheels move forward. The model should
almost sit on its tail when the tank is empty.
There is one problem that
afflicts tail-draggers and tricycle-geared models: overly springy
landing gear. Sometimes the kit comes with wire landing gear that is too
springy for the airplane's weight. That can make bounce-free landings
difficult; anything less than a grease job is turned into a
roller-coaster ride.
The solutions to this problem range from wire and
rubber-band reinforcements to replacing the gear with a beefier aluminum
unit.
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