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Checking the CG: To
find the balance point, you need to hang the airplane from somewhere
above its three-dimensional CG. All that really means is that if your
airplane has a high or shoulder-mounted wing, you can hold it up using
one finger on each hand under the wing. If you have a low-wing airplane,
you may find it easier to do this with the model upside-down. A photo
illustrates this technique.
Make sure to place both fingers the same
distance back on each wing panel, and move back and forth until the
airplane hangs level. A typical safe starting point for almost any
airplane is if the CG is placed at 25% of the mean aerodynamic wing
chord (MAC). The farthest back the CG usually gets on a typical trainer
is 33%, or one-third, of the MAC.
Flying wing and tailless models
typically fly with the CG at 15%-20% of the MAC. On a constant-chord
wing, the 25% point is exactly one-quarter of the way back from the LE
to the TE. Most trainers are designed with constant-chord wings.
Once
you have found the starting balance point, move equipment if necessary
to make the airplane balance properly.
When the balance point is
incorrect, the first thing that typically gets moved is the battery pack
for the radio. Most often the battery has to be moved forward under the
tank to move the balance forward. If that isn't enough, you may even
consider using a heavier, larger-capacity battery. After all, nickel and
cadmium are useful heavy metals, and lead is just dead weight.
If you
must add nose weight, place it as far forward as practical so that less
is necessary. The weights that mount to the crankshaft are not generally
recommended.
If, on the other hand, your airplane is nose-heavy to start
with, it is slightly easier to move the battery and receiver aft. The
receiver is relatively fragile in a crash and expensive compared to the
battery, so keep the receiver behind the battery! If you must add tail
weight, place it as far aft as you can, on the fuselage, because less
will be necessary.
One more thing: take a good look at your airplane to
make sure the wing and stabilizer are mounted exactly as described on
the plans. You are looking for incorrect incidence angles, which could
force you to counteract them with excessive amounts of elevator
deflection.
Oh yes, one more thing. Make sure the elevator trim on the
transmitter is centered and the elevator control surface is straight.
That will require a control-linkage adjustment. You don't want to run
out of trim-lever movement because you didn't set the elevator straight
to begin with. That goes for all the other control surfaces too!
Click on photo to view large image with caption
Going
Flying: Most trainers are designed to climb at full throttle and fly in
level cruise at a power setting just above half throttle without having
to change the elevator trim. On takeoff your test pilot will take this
into account and wait until the airplane is throttled back to cruise
power before making any fine elevator-trim adjustments for level flight.
Now, the importance of knowing that the elevator was straight with the
trim lever centered will become apparent. As you first put trim into the
airplane, you already have some idea of what you are dealing with. Does
it need up or down from the ideal, and roughly how much?
That's better
than waiting until after landing to look and see that all that furious
wiggling of the transmitter trim lever was just to get things straight!
Pitch Flight Testing: Now that the airplane is trimmed for level cruise,
let's do a couple simple tests. Smoothly advance the throttle to full.
Without making elevator corrections, but still keeping the wings level
with minimal, smooth aileron control inputs, watch the climb that
results.
Is the climb too shallow and fast? This might be ideal for an
advanced sport airplane, but for a trainer you want a solid climb with
adequate airspeed.
Is the climb too steep? Watch to see if the climb is
so steep that the airspeed has decayed.
Has aileron control become
sloppy? Is it difficult to promptly correct the wind's effects? If so,
that is a sign that the airspeed is too low because of the steepness of
the climb. In that case, you can do one of two things: make the airplane
less speed sensitive by moving the CG aft and adding down-elevator trim
or add more downthrust. If the airplane climbs too shallow, you would do
the opposite.
How do you decide whether to change the engine downthrust
or the balance of aerodynamic trim and balance point? Maybe you should
use a combination of the two. You need more information, and to get it
we use the low-throttle glide test.
For the low-throttle test, set up a
straight and level pass, parallel to the runway and roughly 100 feet up.
Trim for cruise power level flight and with your hand off the elevator
stick, quickly reduce the power to maybe one or two "clicks" above a
dead idle just before the airplane passes you.
This is the throttle
setting that most of us use for the all-important final approach. Near
the threshold of the runway, the engine is slowed to low idle.
Watch the
glide slope that results, again keeping the wings level but making no
elevator corrections. Does the model settle into a nice glide angle or
does it come down like a space shuttle?
Maybe the glide slope is too
shallow and the airplane wallows along in a near stall; that is, with
the airspeed too low. In that case the directional control will get
sloppy too; the ailerons may get sluggish or the airplane will slowly
drift off to one side even though it was trimmed for straight and level
flight.
Sometimes poor aileron control manifests itself as what feels
like a time lag between when the aileron control is applied and when the
model actually starts to roll in the desired direction. It will get
better if you push the nose down a tiny bit. That's another hint that
the glide slope is too shallow.
Now that you've done both tests, it's
time to assemble what you have observed and make a change to the setup.
If the model has insufficient downthrust, the elevator would have to be
trimmed level or slightly down for level flight compared to where it
would be with the correct downthrust. Alternatively, the airplane would
have to be nose-heavy. See the pitch see-saw diagram.
If the airplane
needs more downthrust, at full power it will climb too steeply because
the nose-up engine thrust is great. It will also glide too steeply when
the nose-up engine thrust is missing and the down-trim or nose-heaviness
takes over.
It is also possible that the airplane climbs too steeply
under full power and glides okay or a little steep if the model is
nose-heavy. That means it is overly stable in pitch and responds to the
added airspeed by trying to climb too much.
How can you tell if this is
the case? If the elevator is trimmed up for level flight, even a bit,
this is a hint that the airplane is nose-heavy and the aerodynamic trim
was necessary to counteract it.
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