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Which Pitch Adjustment to Make?
• If the
climb or glide is too steep and the elevator trim is up.
The trick in
telling the difference between nose-heaviness and insufficient downthrust in a model that climbs too steeply under full power is to
look at the elevator trim. If the airplane carries up-trim, move the CG
back approximately one-quarter inch, retrim for cruise power level
flight, and do the full-throttle climb and low-throttle glide tests
again.
If the elevator trim is still up compared to the stabilizer, move
the CG back another quarter inch and retrim again until the elevator
trim is level or close. If the climb and glide are acceptable, even
though there is a bit of elevator trim, it is okay to stop there. Even a
bit of down-trim is okay. After all, we are interested in results.
If
you have to move the CG back far enough that down-elevator trim becomes
necessary to achieve trim for cruise power level flight, you really
should move the CG forward the last step and start to add downthrust.
•
If the climb or glide is too steep and the elevator trim is near
neutral.
If the airplane had no noticeable up-trim to begin with, add downthrust, retrim for cruise power level flight, and do the climb and
glide tests again.
Of course it is possible that your model needs both
adjustments. Start by moving the CG back to get rid of excess up-trim.
If the climb is still too steep, add downthrust.
• If things don't
behave.
If at any point in this process you move the CG back and the
model gets touchy in pitch, you need to stop and check the CG location.
Your airplane is almost certainly tail-heavy. Move the CG forward to the
last location where the elevator control felt predictable.
It's rare
that an RC sport model or trainer ever needs the CG to be placed more
than one-third of the way back on the MAC and, as I mentioned earlier, a
normal CG is closer to one-quarter of the MAC. If you have stumbled onto
tail-heaviness using this method, you need to put the CG back at a
position where the elevator control was predictable.
If the airplane
still needs a great deal of elevator trim to fly level, you should look
at changing the wing incidence. If it needs a lot of up-trim, shim the
LE of the wing up on a high-wing airplane. If the model needs a lot of
down-trim, shim the TE of the wing up. When you change the wing
incidence, small steps such as 1/16 inch are best. If more than one
adjustment is necessary, so be it, but drastic adjustments can have
unpredictable consequences.
Again, the goal is to get the model to trim
in cruise power level flight with the elevator closely lined up with the
stabilizer. Any remaining problem with a steep climb and glide is almost
certainly because you need more downthrust.
• If the climb or glide is
too steep and the elevator trim is down.
The likely cause for this is
that the wing and/or stabilizer incidences are wrong. The wing and
stabilizer incidence angles are creating a strong nose-up tendency,
which gets even more powerful at high airspeed.
You either need negative
(TE up) incidence in the wing or positive (LE up) incidence in the
stabilizer. The wing is usually easier to change. This is a sign of a
model that has excessive pitch stability and excess horsepower.
Trainers
are intentionally quite stable, but such designs do not tolerate
overpowering well. In this case the cure is not to have less power, but
to put the airplane in "low gear" with a propeller that limits the top
airspeed.
A larger-diameter, low-pitch propeller or a three-blade
propeller with the same diameter and lower pitch will help limit the
excess speed while harnessing the same horsepower. This
airspeed-limitation trick is typically useful if the full-power climb is
too steep.
Another way of reducing this problem is to trail both
ailerons up approximately 1/32 inch. I will not go into this at length
right now, but it will come up later in the section about improving roll
control on airplanes with flat-bottom airfoils.
The Opposite Situation:
• If the climb or glide is too shallow and the elevator trim is down—even a bit.
If the model climbs well, or even a bit shallow, at
full throttle and then glides nicely or a bit shallow, you want the
airplane to change trim with airspeed more than it already does.
Look at
the elevator trim to tell whether or not you should reduce the downthrust or push the CG forward. If the elevator is trimmed down, move
the CG forward, retrim for cruise power level flight, and redo the
full-throttle and low-throttle tests. Continue making adjustments until
the elevator trim is level, or at least close to level with the
stabilizer.
You may start by moving the CG forward to get rid of the
down-elevator trim, and then reduce the downthrust once the elevator
trim in cruise power level flight is zeroed out.
• If the climb or glide
is too shallow and the elevator trim is level, or even a bit up.
If the
elevator was not trimmed down, the CG position is not the issue. Reduce
the downthrust.
That about covers basic pitch trim. Next month I will
share a method of checking downthrust that is more appropriate for
high-performance sport models and airplanes that are intended to be
flown fast rather than slow, such as trainers. Then we will test for and
adjust right thrust.
Until then, remember that your equipment should be
set up to work with you—not against you!
MA
—Dean Pappas
deanf3af2b@pappasfamily.net
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