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Step Three:
Geometry of
Pattern Plane Linkage
Geometry is a
huge part of linkage setup. We're now dealing with precision
setups not 3D. We're talking about using the servo to its
maximum efficiency and using the best mechanical advantage
possible on our linkages.
First step: You
want to choose surface control horns and servo arms that
give you only as much throw as you need. If you're
only going to fly on 10 degs of aileron travel why set up
the linkage for 20 degrees maximum? I never run my flying
rates less than about 80%. Yes that means D/R values are
around 80% this gives me some wiggle room to get a little
more aileron travel, if needed, but still maintain the
resolution of my servos. You want to run the longest control
horn on the surface and the smallest servo wheel you can get
away with.
For example, my
standard setup is to place the aileron's control horn clevis
"pivot point" at 1-3/8" above hinge line. And my servo arms
are about ½" long. This gives a better than 2 to 1 advantage
to the servo for holding the aileron in the right spot. This
leverage helps in loop tracking, rolling, and in getting the
plane to lock on and groove.
I used to be a
big user of aluminum servo arms, and the linkage was really
pretty. Well, pretty doesn’t always cut the mustard. I
like the aluminum servo arms because they're stiff and don’t
flex. But they don’t need to be 1.5" long. I use the
connection hole in the arm that's only ½" from the servo's
center of rotation. A note if you choose to use
plastic arms and wheels with the servos: Use the round
wheels. They're the stiffest with the least amount of flex.
Mechanical
Advantage is the king now on my models. The result has been
models that fly better. And they continue to fly better
because gear slop and pot wear are minimized by a good
mechanical linkage. Lots of guys who set up a 1 to 1 ratio
on the linkages find it really tough to maintain them. Not
to mention the fact that you just don’t need that much
travel on the surfaces. If you're setting your dual rates
down around 50-60%, you're giving up resolution on your
servo.
Make sure your
linkage is symmetrical.
Lots of people
who'll make sure the pushrods are parallel to the servo case
still neglect to look at the symmetry of the linkage. Always
make sure the pushrod is connected to the servo wheel or
horn at the spot that forms a right angle between the servo
arm and the pushrod when connected to the control surface.
Take a look at these illustrations.
The two linkages
with pushrods at 90° angles to the servo wheels (image on
the left) are correct. In the image on the right, the
servos with the pushrods attached at points higher or lower
than a 90° angle are not correct.
How do you get
the servo arm and the linkage to look right, like pictures
on the left above?
First, you must
totally disregard the location of the control surface to be
connected. It need not be centered or inline with airfoil
as of yet. You need only to have the linkage connected in
order to get the pushrod to come off the wheel at the proper
angle, as shown in the diagrams. Notice the servo can
be mounted at an odd angle as long as the pushrod is at a
true right angle to the arm. Notice the angle formed
by the line through the center of the arc’s rotation and
that of the pushrod. This angle should remain at 90° when
we're dealing with small travels like those on pattern
models. When going to larger 3D models we may need to change
our geometry a little bit. But similar principals will still
apply.
Returning to the
question above, how do you get it this way? My servo wheels
don’t give me a hole that sits at this 90° angle location.
So how do I fix it?
There are two
ways to get the geometry correct. First of all, there's an
odd number of splines or "teeth" on servo output shafts.
What does this mean for you and me? Well, if you rotate the
servo wheel 180°, to use the other side of the wheel, you'll
move ½ tooth in terms of the angle. This'll help get it
closer.
The next step is
accomplished by sub-trim. In fact, this is what sub-trim is
for. You all thought sub-trim was a TX’s way of keeping a
trim setting so that your trim tabs were in center, and the
surfaces were aligned with the wing? WRONG. Many guys use
sub trim this way, but until you get into really good
linkage geometry sub trim can actually serve to make your
linkage worse instead of better. The key here is to use as
little sub trim as possible. As in Zero is good. 80% is bad.
But sub trim the servo so that you get the geometry shown in
the pictures. After that you don’t touch sub trim again.
Next thing is to
mechanically adjust your pushrods’ length to get the control
surface centered. I use 2mm threading because this is a very
fine thread. It allows a very fine adjustment on the clevis
and ball links. Remember when I mentioned 4-40 hardware?
Well, 40 threads per inch is what the 40 stands for, or
0.025 inch per full turn of the clevis. A 2-56 thread is
0.0179 inch per full turn of a clevis. The 2mm threads are
even finer than this. Since we're trying to narrow it down
to ½ turns of the clevis, 2mm threading produces less than 9
thousands (0.009”) length change per ½ turn on the clevis.
This makes fine-tuning the linkage easier and more accurate.
Next: Adjusting Servo Travel
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