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The Pitch-Control Balancing Act: Predictable
control is a balancing act. There is a balance of forces always at work
to make the airplane fly straight and level, to climb, and to descend.
When the forces are not precisely in balance, the airplane will be
changing pitch—either nosing up into a climb or dropping into a dive.
The dominant forces are aerodynamics, gravity, and engine thrust. That's
not much of a surprise, is it?
I don't want to give a whole course on
aerodynamics here, so this explanation will not be entirely rigorous,
but I do want to give you a feel for how these forces juggle so that the
kinds of adjustments we make later will make sense.
For almost all
"normal" airplanes the horizontal tail holds the tail end of the
airplane down. The wing makes lift, and the act of making lift creates a
nose-down torque. This is for two reasons, the first of which is that
for stable flight (again, for almost all normal airplanes) the CG, or
balance point, is in front of the wing's center of lift.
The second
reason is that as the wing bends the passing air downward, it can be
said to rotate the airflow; therefore, the air imparts an opposite,
nose-down rotation to the wing and the airplane to which it is attached.
Although it's simplistic to put it this way, the wing pushes down on the
passing air and the passing air pushes up on the wing.
Along with this
nose-down torque, which is a by-product of making lift, add the nose-up
effect of the horizontal-stabilizer incidence angle and the level-flight
trim position of the elevator. Ideally the elevator should be straight,
as compared to the horizontal stabilizer, but sometimes it is necessary
to trim the elevator up or down a bit.
Finally, there is the small
nose-down torque caused by the engine downthrust. That effect is changed
by the engine's throttle setting; at idle the trim force caused by
downthrust is nil, while at full throttle it can be important. This
makes downthrust an important part of the pitch-trim balance "see-saw."
Look at the diagram showing pitch see-saw and the diagram showing
incidence angles and downwash.
There is also a balance of forces in roll
or from side to side, but I'll cover that later.
Click on photo to view large image with caption
Pitch Trim: In the list
of preceding problems, items 3 and 4 were devoted mostly to pitch
issues; we'll start there.
First we should tend to a few details of the
sort that are best taken care of at home, in the workshop. That's right;
trimming (just like charity) begins at home.
To begin with, make sure
the balance point, in the fore and aft direction, is where the plans or
instructions indicate. If the plans show a range of positions, as they
should, shoot for somewhere in the forward half of that range. We call
that a "nose-heavy" CG.
The ideal balance point is not a well-defined
location for a particular airplane design. It can vary a bit depending
on the flying for which your airplane is intended. It also depends on
the all-up weight, the size and location of the fuel tank, and small
differences in building or assembly.
A quarter of a degree difference in
the incidence angle between the wing and horizontal stabilizer in your
airplane compared to the designer's can change the ideal CG location.
For that reason, most designs show a CG range.
As the CG moves aft from
the initial nose-heavy position, the airplane becomes less stable in
pitch. This is not necessarily a bad thing; excess stability makes an
aircraft more sensitive to airspeed changes and makes it less
maneuverable.
On the other hand, if the model is too tail-heavy it tends
to have a short life! Instability, or even near-instability, causes many
crashes.
As an airplane gets close to tail-heavy, the first sign is that
elevator control gets touchy. When a model is set up at the aft end of
its CG range, the elevator control will usually be more powerful. But if
it gets jumpy, or the airplane feels as though the elevator trim is
inconsistent, you are flirting with tail-heaviness.
For more advanced
sport airplanes with semisymmetrical or symmetrical airfoils, an
important factor in where the CG belongs is inverted flight. If it takes
too much down-elevator to fly inverted, the model is likely nose-heavy.
If it takes no down-elevator, or even climbs sometimes, it is definitely
tail-heavy. A jumpy elevator is a sign of near-disastrous
tail-heaviness.
If your airplane always seems to run out of elevator
authority when it comes time to flare for landing, it could be a sign of
nose-heaviness. That is not the only reason for this problem, but I'm
mentioning it at this point for completeness' sake.
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