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Time to Go Flying Again: In trimming for good directional control we
have two main goals, the first of which is to trim the (now sealed)
ailerons and rudder so that the model is not crosstrimmed and flies
straight at all speeds from slow to fast.
The second goal is to achieve predictable aileron response at all
speeds—especially slow. The two critical flight regimes are the steep
climb right after takeoff and the critical low-speed turns used to line
up with the runway for landing and to counteract wind on final approach.
Aileron and Rudder Trim: I should point out at the start that this topic
overlaps the right-thrust adjustment discussion. There was no
straightforward way to get a handle on both subjects at one time, but we
will combine the tests and adjustments at the field.
When an airplane is crosstrimmed it behaves differently turning left vs.
turning right. Let's say the model has the rudder offset to the right.
The ailerons will have to be trimmed left in cruise flight to fly a
straight line. In fact, the aircraft will be crabbing to the right in
straight flight. The same sort of thing happens when a car has the rear
axle bolted in crooked.
When this airplane is turned to the left it will tend to hang its nose
"out of the turn" and may even constantly tend to roll back to level
flight. When turned to the right, this model will tend to "wind into the
turn" and even try to roll over into a spiral dive.
You already know the test to detect a crosstrim: make left and right
turns, always using the same bank angle, and adjust the rudder trim away
from the direction of turn that winds in. Everytime you adjust the
rudder, go back to trimming the ailerons for straight and level flight.
As are many other trimming adjustments, it's an iterative process and
you'll have to go back and forth a few times to get it right.
When you think you have it right, try a long glide at idle power as a
fine-adjustment test. Set up with the airplane flying straight into the
wind, and repeat the hands-off glide test a few times if there is any
kind of wind out. If the model wanders off to one side, tweak the rudder
trim to correct and retrim the ailerons again.
Any difference between this test and the turn test is generally caused
by subtle wing warps or other assembly issues. You'll have to accept any
difference that remains between left and right turns, although nine out
of 10 times the glide and turn tests agree.
Your aircraft is now really trimmed to fly straight. Landings can be
prettier, and more effort can be put into that picture-perfect
three-point flare rather than fighting to keep the model from veering
off the runway.
Click on photo to view large image with caption
Rock and Roll—Making the Ailerons Work Well at All Speeds: Do you
remember the anecdote about the L-19 Bird Dog from Part 2 of this
series? That airplane had a bad adverse yaw problem, as do many high-
and shoulder-wing models with high-lift airfoils.
During the takeoff climb that turned left over the pits and spectators,
the pilot had gobs of right aileron control cranked in but the airplane
kept wandering off to the left. A lack of right thrust might have been
partly to blame, but the aileron control should have worked well enough
to turn the airplane right. It didn't, and the reason was severe adverse
yaw with aileron application.
There's another scenario. You throttle back and initiate the turn to
your final approach for landing. As the model lines up with the runway,
you apply opposite aileron to level off and stop the turn, but the nose
keeps cranking around for just a heartbeat longer and the ailerons don't
work immediately.
There is a time lag, and when the airplane finally responds it wallows
as it rolls. That's right; it's adverse yaw. We have already sealed the
aileron hinge lines, but ...
Adverse Yaw Is Fundamental: Adverse yaw is not just a problem caused by
aileron hinge gaps; even with perfect gaps there will be adverse yaw.
Again, the problem gets worse at low speed and at high angles of attack.
Now we need to look at what is called "aileron differential." It's time
to go back to the theory book.
Let's say you want your airplane to roll right to exit a left turn. The
right aileron is raised and the left one is lowered. The desired result
will be to lift the left wing and lower the right.
The last time I looked, lifting was work—especially when you're lifting
furniture. Wingtips aren't that heavy, but they do count. So we are
asking the left wing to do more work and the right wing to do less work.
The energy needed to do this work comes from the creation of drag.
The force of drag multiplied by the distance through which it is applied
equals work. This means the wingtip being raised has more drag than the
wing being lowered. That drag imbalance tries to yaw the model the wrong
way compared to the desired roll.
How do we fix this? After all, its cause is buried in the physics and
energetics of flight. It's not a workshop problem such as hinge gaps.
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