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The Application: At low and part
throttle the effect of right thrust is minimal. Here's where the
compromise comes in. We set the right thrust to straighten out a
full-power takeoff climb and accept the small, unwanted influence it has
at cruise.
In the glide the right thrust has practically no effect, so
it's no problem. Assuming that the airplane is trimmed to glide in a
straight line in calm air, the job of right thrust is to preserve that
straight flight path under full power.
That's all there is to it. Typically, the right-thrust
adjustment determines how straight the airplane climbs after takeoff.
Good landing-gear setup will reduce the steering workload until the
student gets a model in the air. The landing-gear discussion comes
later.
Let's adjust the right thrust. As you read a bit ago, the
requirement is for the model to go straight in the glide with the engine
idling and at full throttle in a takeoff climb. For now let's assume the
airplane glides without turning since it was trimmed for straight and
level flight at cruise power. (In the next installment we will discuss
making the model fly straight at all airspeeds.)
What we really need to do is adjust the engine right thrust
so it adds the right amount of correction for "engine torque" during
climb. Engine torque makes the airplane turn left. The word "torque" is
a misnomer, but it is a convenient catchall.
The Right-Thrust Test: To start with, let's make sure the model
is trimmed to fly nice and straight at cruise power. Next, I like to set
the airplane up so it is pointed straight away from me and headed either
directly into the wind or directly downwind. You don't want to do this
lined up with a crosswind because the sideward wind drift hides the turn
for which you are looking.
Now that you are lined up, add full throttle and smoothly
pull up into a climb, at the same angle as your steepest post-takeoff
climb. We typically climb into the wind, but doing this downwind also
works, and it allows you to pick the direction so you don't have to fly
over the pits or the safety line at the field.
You don't want to climb so steeply that the model is
stalling, but you do want to climb as steeply as your horsepower will
permit. The airplane will lose airspeed during the climb, and it may
become more easily influenced as the flying surfaces lose some of their
control power. In all likelihood the model will start to turn.
If the airplane deviates to the left, you will have to add
more right thrust. On the next flight retrim for straight and level flight (probably just a click of rudder)
and repeat the test until the model climbs straight.
If the airplane has
too much right-thrust, it will deviate to the right in the climb. That
doesn't happen often.
If the right thrust is close to correct, and if
there is enough wind to make the model bounce around, you may have to
repeat the test a couple times to be sure of which direction the
airplane is turning. That usually means you are getting close.
It is
best to adjust the right-thrust angle one degree at a time and repeat
the process. Most airplanes have, or at least need, 2°-3° of
right-thrust, although a rare few need much more.
Right-Thrust
Measurement: The easiest way I have found to determine the right-thrust
angle is to measure the distance from each propeller tip to the tail
post. With a 12-inch propeller the difference between the two
measurements will be 3/16 inch for every degree of right-thrust. Three
degrees of right-thrust works out to 9/16 inch difference between the
two measurements from the tail post. With a 16-inch propeller this ratio
works out to 1/4 inch per degree.
You might have to readjust the right
thrust a time or two, but if you start with it adjusted as the kit
recommends, you should have to make only a fine adjustment or two. Many
kits and ARFs may not make how much right thrust is recommended entirely
clear, but if you can't find anything on the plans or in the
instructions, start with 21/2° or so.
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