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So Why Are You Dragging Your Tail
Around? Tail-draggers have different versions of the same problems
as tricycle-gear airplanes, with one interesting difference: the
fore-and-aft location of the main gear.
If the main gear is mounted too far aft, the airplane tends
to nose-over easily. That's embarrassing at the very least.
What is not as often
appreciated is that if the mains are mounted too far forward, you get
that high-speed wheelbarrow problem I previously discussed. The airplane
will be difficult to keep straight at high speed just before liftoff.
The "ideal" location is just far enough forward to not nose-over easily. It's that simple.
This means that if your home field is paved, you will
be setting up your landing gear farther aft than someone who calls a
grass field home. Since most aluminum landing gear are made with a small
amount of rake, or angle, you can move the wheels a half inch or so just
by turning the gear around. You can also change the wheel position with
tapered shims between the landing gear and fuselage.
Toe-in and camber
adjustments are interesting, but they are secondary to the position of
the main gear itself. Keep the wheels lined up straight, and you'll do
fine.
Tail-wheel linkages are simpler than nose wheels. Too much control
throw is rarely a problem, but sloppy or overly springy connections to
the rudder can make precise corrections on takeoff almost impossible.
You want just enough give to act as a servo saver.
The other trick that
helps save servos is to put less caster in the tail-wheel assembly.
Although it looks "real" to have the wire tail-wheel strut bent way
back, it also strengthens the mechanical advantage of any sideward
landing impact to rip the guts out of the rudder servo, beat up the
bottom rudder hinge, and mangle the rudder pushrod or cables.
A short,
nearly vertical strut with a small spring coil works well. For airplanes
weighing 5-10 pounds, a 3/32-inch-diameter-music-wire 1/2A nose-wheel
strut works beautifully.
If you still find it difficult to keep the
airplane straight on takeoff because of overcontrol, the tail-wheel
throw needs to be reduced. This is actually easy to do. For those of you
using the "two springs"-type steering linkage, all you need to do is
hook up to the inner end of the rudder horns and the outer end of the
tail-wheel horns.
If you used the "tiller arm"-type linkage, where a
single piece of wire runs along the bottom of the rudder and is attached
with some kind of clip, it's a bit tougher to do this unless you are
still assembling the airplane; then it is easy.
All you need to do is
move the tail-wheel pivot forward and find a location for the clip on
the bottom of the rudder where the steering throw is reduced. This is
simple and offers positive steering control.
Click on photo to view large image with caption
Takeoff, Climbout, and the
CG: Let's cover what happens on takeoff when the airplane is nose-heavy.
A severely nose-heavy model will require lots of up-elevator to lift the
nose wheel and break ground. The problem could also be landing-gear
position, the ground stance, or the CG.
The last two are easy to
eliminate, but you need the information you gathered in the air to tell
whether to move the landing gear or not. If the CG is in the right spot,
holding a constant climb angle is easier. If the airplane is nose-heavy,
you will find yourself needing a quick elevator adjustment a split
second after liftoff.
Let's look at the other, more urgent side of the
problem. On takeoff, tail-heaviness often shows itself as climbouts that
quickly become too steep, even when they did not start out that way. If
you find yourself chasing the elevator in a pilot-induced oscillation (PIO),
you've probably got a tail-heavy airplane.
Tail-heavy airplanes tend to
snap roll too, and that is usually how they get turned back into their
component parts. Try moving the CG forward temporarily, and see if it's
easier to fly a smooth departure climb.
Pitch Trim Revisited: Now that
the airplane is departing nicely, it is time for Part 2 of the "From the
Ground Up" series on basic trim to depart as well. I'll land back here
next month and wrap things up.
MA
—Dean Pappas
deanf3af2b@pappasfamily.net
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