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Picking Up Where We Left Off: In
the first installment of this "Trimming From the Ground Up" series we
dealt with the subject of pitch trim. As it turns out, it's a whole lot
more than just moving the transmitter trim lever a few clicks or beeps
until the model flies without climbing or diving.
Airplane trimming is
similar to setting up a race car: even when a crew chief says the "race
car was fast right out of the trailer," he really means that the team
was able to go through the entire list of setup checks quickly. That
usually means few or no adjustments were necessary, but it doesn't mean
every little thing wasn't checked anyway.
As it turns out, there aren't
all that many things to check when test-flying a new airplane, but if
one of the tests described here shows a problem, you are working harder
than you need to when flying your aircraft. If your model is intended
for the all-important one of training mission, that's a bad thing. If
you are a more advanced flier, you are simply missing out on flying and
looking better than you already do.
The mission of this series is to
describe the tests and corrective actions, in a systematized way, to
help you make your airplane fly better. None of it is any great effort,
and you don't have to attack it all at once. Your model's pitch behavior
can be investigated separately from something such as the unfortunate
tendency the airplane has to turn left immediately after takeoff.
Click on photo to view large image with caption
Engine
Right Thrust: I've already written about the adjustment of downthrust,
so now it's time to discuss right thrust.
Some years ago I attended a
Scale Masters Qualifier meet. The airplanes sure were beautiful; there
just aren't enough people who build like that! One of the competitors
was flying a Cessna L-19 Bird Dog: a slow-flying, military,
forward-air-control-type airplane. The same aircraft, in civilian
livery, has been used as a trainer and glider tow airplane for decades.
We were taking off from right to left that day, and every one of the
pilot's takeoffs veered left, over the flightline—and the pits—and the
parking area! Many of you have seen this one, right? No, left! It's
particularly tough for a student to deal with, and it's a serious safety
hazard as well. This is what happens when the right thrust is not
correctly set up.
The full-scale pilots deal with this situation
differently—emphasizing the proper application of right rudder to
counteract "torque" on takeoff. Although that is an excellent skill to
develop, the fix for most aeromodels is to put the proper amount of
right thrust into the engine. This minimizes the rudder corrections that
are necessary during takeoff.
This is important for the beginner pilot,
who is still learning to take off. Should we let the beginner give
everyone in the pits a haircut until he or she learns to use his or her
rudder hand simultaneously and independently of the right hand? I think
not. Actually most model-airplane fliers never really master accurate
and independent control with their rudder hand, but it is a worthy goal.
Before I describe how to put right thrust into the model and how to test
for whether or not the airplane has the correct amount of right thrust,
I need to discuss the nature of the right-thrust trimming adjustment.
Right thrust is a compromise because it is used to counteract an
airspeed-dependent problem. As it turns out, it is usually a good
compromise.
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