Rudder Cable Systems Used in Modern Sailplanes

On May 15, 12:50*pm, Bob Kuykendall wrote:
On May 15, 12:35*am, Ramy wrote:

Checked the rudder cables on my 27 - looking good. But as JJ described,
loosening the tension on one side causes a hard over to the other side!

I do not necessarily agree with JJ's synopsis of the situation. Yes,
on the ground removing the tension of one spring will cause the rudder
to go to the opposite stop. However, in the air there is airflow over
the rudder that would counteract the force of the spring. How much
effect this has depends on the strength of the return spring as well
as other aerodynamic factors. In all of the sailplanes I've flown, the
rudder springs are pretty wimpy in relation to the aerodynamic forces
involved. 14CFR23 and JAR22 dictate that the rudder circuit of light
aircraft is to be designed to react at least 150 lbs per pedal and a
combined force of 300 lbs on the pedal pair unless a lower force can
be rationally justified.

On that basis, it is my conjecture that there is an additional factor
at work in the JS1 incident besides a broken rudder cable. I am
standing by to see if such a factor comes to light.

I would like to hear how glider manufactures defends this design!

As a glider designer, I defend it so:

Cable actuation systems are a simple and effective approach to the set
of problems at hand. They are easy to inspect and service, and
problems are easy to detect. The 1/8" (3mm) cables commonly used have
about a 4x safety factor over typical maximum control forces, so they
will take a lot of abuse before failing. Every experienced A&P and IA
knows to inspect cables in their areas of tightest curvature, and
these inspections bring to light the vast majority of potential
problems long before they become critical. As typically implemented in
sailplane fuselages (including the three I have so far built), the
cable-in-tunnel system has the additionally compelling advantage of
adding increased rudder damping when the pilot applies pressure to
both pedals. This feature has been successfully used to damp incidents
of rudder flutter in quite a number of incidents that might otherwise
have eventually resulted in resonant failure of the aft fuselage.

The one issue I have with typical sailplane rudder cable systems is
that the S-tube on the side of the pedal that allows for pedal
position adjustment can cause a short-radius curvature of the rudder
cable at extremes of pedal deflection. The improvement I will try to
make in my next set of rudder pedals is to try to add an exit radius
to the ends of the S-tubes so that they look like tiny trumpet bells
in side view. This will increase the radius of curvature in the cable,
and hopefully decrease the wear and fatigue in the cable at that
point.

As a counterexample rudder actuation system, I submit the Diamant. The
makers went to heroics to reduce rudder actuation friction, using push-
pull tubes in linear roller bearings with many ball bearing pivots and
a rather complicated pedal adjustment system. What they got was rudder
flutter, and they ended up having to incorporate a hydraulic shock
absorber in order to apply damping to the system. So they started with
a complicated system and ended up having to make it more complicated
yet before it was fully functional. Think of all the things they could
have done with their energy had they just used a standard cable system
and moved on.

*I wonder how many were killed by this design, giving many unexplained spins into
the ground from higher altitude.

My guess is few to none. Here in the US, crash investigations of light
aircraft, especially those of gliders, do tend to be less systematic
than those of larger aircraft. However, in my experience crash
investigators are fully competent at recognizing the signatures of
wear and fatigue failures in cable-actuated control systems. Where
such signatures are found, they are usually announced prominently in
the accident synopsis.

Thanks, Bob K.

"The improvement I will try to
make in my next set of rudder pedals is to try to add an exit radius
to the ends of the S-tubes so that they look like tiny trumpet bells
in side view. This will increase the radius of curvature in the cable,
and hopefully decrease the wear and fatigue in the cable at that
point. "

Apparently this is the solution SH used on at least some of their
sailplanes.
Upon recent inspection on my Mini Nimbus there is no wear after many
years.