The Rabe Rudder
Not many weeks go by that someone doesn't ask me for
information on setting up a "Rabe rudder" properly. In the past,
I've loaned out countless video tapes showing how I did this, so maybe it's time
for a review of this topic in Stunt News. Why not just leave the rudder
offset the same amount all the time? With a low-RPM motor or small,
lightweight prop, you can do that without a severe penalty. When we all
flew Fox .35-size ships, the only "variable" was whether to use a
cambered vertical stab or use rudder offset, and how effective to make it.
But time and progress brought thicker airfoils, bigger displacement motors,
larger diameter props, longer tail moments, etc., and problems started to become
obvious: a 'softness' on the top of the hourglass, the outside part of the
square eight, and the overhead eight...especially with .60-powered ships. The
'band-aid' fixes were shorter lines and/or extra tip weight...but these 'fixes'
caused their own problems in other parts of the pattern. Al Rabe is generally
credited as the originator of the 'variable kickout' rudder that's so
widely used now-it's almost universally called the "Rabe rudder."
A ship with a big displacement motor spinning a heavy, carbon (or even worse,
multi-blade) prop is more prone to need the variable rudder kickout on outside
turns. The heavier the prop, higher the RPM, and sharper the outside turn,
the more 'kickout' is needed. Mike Rogers came up with the "ball link
on the elevator edge" system shown in the drawing, which allows several
adjustments to accommodate different airframe/motor/prop combinations.
This linkage is easy to install, and you can leave it off the ship until it's
completely buffed out.
This linkage is shown on the Typhoon 60 drawing, so if you'd want to use it for
a smaller ship, all the dimensions would be proportionally smaller. The
nominal settings shown will get you in the ballpark. I have bench-trimmed
many "Rabe rudder" set ups and like to start with linkage in the
'slow' movement hole (furthest from rudder). I set it so movement is
neutral on rudder when the elevators are in neutral. This is too
little travel most times, but I fly the ship anyway to get a benchmark.
From there I go one turn more (shorten rod) and watch line tension at top of the
hourglass and the outside part of square eight increase until its equal to
"inside manoeuvre" tension. As you shorten the rod, the rudder
kicks in sooner, and as you move to a hole in horn closer to rudder, travel
increases (more total rudder effect).
Many theories abound as to how and why a "Rabe rudder" works-I'll
leave that explanation for the scientific-minded. I don't worry about the
science, frankly-I just know from flying that it does work. My first .60
design, the Sweeper, goes back 30(!) years, and starting with the Sweeper, any
of my .60 ships that didn't have a "Rabe rudder" required more than
the optimum tip weight to counteract the tendency to turn in on certain parts of
the stunt pattern. Jim Greenaway convinced me to try one on the Red Baron,
and I was shocked at how effective this trim feature was. I was so
impressed that I retrofitted all the planes in my air force, and every one of
them showed a big gain in performance. In 1996, when I was developing the
5-blade props for the Spitfire, I found the "Rabe rudder" extremely
effective in counteracting the forces of a very heavy prop. Joe Adamusko
has had similar experience, as have Dave Midgley, Mike Rogers, Bob Baron, Jose
Modesto, Jim Greenaway, and many other top fliers...including Al Rabe, of
course.
Obviously this can be custom-tailored to your flying style. I prefer to
trim for solid line tension-it's one less thing to think about during the
flight-while some like to back up during parts of the pattern or use their arm
like a fishing pole to 'whip' model. Watch the expert pilot of your
choice, and you can generally see how he prefers his trim. '96 NATS winner
Bob Baron had solid line tension everywhere, especially in the wind. Bob
has a very relaxed style-he seldom whips or backs up. If you design your own
ships, try for 10-15 sq. in. of movable rudder, deducting any 'counterbalance'
area as not part of the measured area. If you use more area than that,
make a proportionally longer horn or move the horn back from the hinge line.
Both will reduce travel and increase leverage. If you're retrofitting an
old ship, you can usually sink a piece of tubing into the bottom of the outer
elevator and repaint this small notch without anyone even knowing it's a
retrofit. I've done it many times in only a few hours. Once you've
tried a "Rabe rudder", you will probably include one in all future
ships. If you've already tried one and been unconvinced, though, maybe you
just didn't have it adjusted in its usable range. Of course, some people
prefer no adjustable rudders or rudders that are adjustable but are not linked
to the control system. That's fine-there's always more than one way that
can be made to work, for sure. Even if you install and adjust a "Rabe
rudder" on a ship, if it turns out that it really doesn't fit your needs,
it's a simple thing to make it 'static', rather than dynamic...with the bonus of
built-in, precise offset adjustment.
Over the last ten years, countless words and pages have been published on the
benefits of .40-size motors vs. .60s. It was an era of everyone trying to
prove their system "best", and in many ways all the arguing had a
negative impact on the hobby. Time has a way of evolving the breed, and I
suspect that with so many fine .60s available now, that displacement size will
dominate. It's been a long time coming, but more and more fliers seem to
have arrived at the same conclusion, including World champs, NATS champs, local
contest winners, and sport fliers. There is room in the hobby for
everyone-including those who prefer .60s-and so the "Rabe rudder" will
become an even more common trim feature. (By the way, it can definitely
improve the handling of a .40-powered ship!) As more and more pilots
learn to exploit its potential, feedback from other pilots sharing their
experiences will benefit all. If you'd like a more solid feel to your setup,
this is one modification I think is really worth trying.
Windy Urtnowski