Rudder pedal S-tubes

What is the purpose of S-tubes on rudder pedals?

On Saturday, May 11, 2019 at 7:14:59 AM UTC-8, jp wrote:
What is the purpose of S-tubes on rudder pedals?

So the pedals can be adjusted without changing the cable length.

On pre flight it is good to move the pedals for and aft and look at the cables. Sometimes the cables will fray at the entry and exit points of the S-tubes.


J Banks

Correct.
If adjusting rudder pedal setting a lot, not a bad idea to run pedals through full range of movement and check cables for wear (during preflight checks) as well as some lube (3&1, silicone, whatever....depends on what the service manual states....).

Let me add one little tid-bit, here. The rudder cables enter the top of the S tube and exit at the bottom, then forward to a cable anchor point. The angle of the rudder pedals can be adjusted by making the cables longer (some pilots prefer pedals that lean forward, rather than sitting straight up). A simple way to lengthen the cables is to add a 1� steel carabiner in the system between the cable end “eye� and its anchor point. Check the new pedal position to see if you prefer it and don’t forget to check that full rudder can still be achieved with the new forward pedal position.
You may find you like it, I did!
JJ

The lower exit of the S tube should be on the axis of rotation of the rudder pedals so that when the rudder is used the distance between the exit and the forward attachment point stays the same. If not the cable slides slight to and fro through the S tubes giving the prospect of wear especially if there is no plastic sleeve insert extending out of the tube - worse still if the ends of the tube are sharp and not bell-ended. Schempp-Hirth gliders have this right as the exit is on the rotation axis outboard of the axle. Most other makes have the lower part of the S tube welded to the vertical of the pedal some close and some not so close to the axis. At least one design has the tube exit projecting forward from the pedal so that not only does the cable move in the S tube it is also angled slightly on full rudder deflection.

On Monday, 13 May 2019 05:21:05 UTC+3, wrote:
Let me add one little tid-bit, here. The rudder cables enter the top of the S tube and exit at the bottom, then forward to a cable anchor point. The angle of the rudder pedals can be adjusted by making the cables longer (some pilots prefer pedals that lean forward, rather than sitting straight up). A simple way to lengthen the cables is to add a 1� steel carabiner in the system between the cable end “eye� and its anchor point. Check the new pedal position to see if you prefer it and don’t forget to check that full rudder can still be achieved with the new forward pedal position.
You may find you like it, I did!
JJ

What an awful advice. Fiddling with control system in first place, and secondly rudder pedals would hit cockpit wall almost certainly in forward position when pushed to limit. Please don't never ever do this.

On Sunday, May 12, 2019 at 10:21:05 PM UTC-4, wrote:
Let me add one little tid-bit, here. The rudder cables enter the top of the S tube and exit at the bottom, then forward to a cable anchor point. The angle of the rudder pedals can be adjusted by making the cables longer (some pilots prefer pedals that lean forward, rather than sitting straight up). A simple way to lengthen the cables is to add a 1� steel carabiner in the system between the cable end “eye� and its anchor point. Check the new pedal position to see if you prefer it and don’t forget to check that full rudder can still be achieved with the new forward pedal position.
You may find you like it, I did!
JJ

An AN 115-21 cable shackle will accomplish this also using a standard aircraft part. It gives about 3/4 inch extension. The only other part needed is an AN 393-11 clevis pin with cotter pin for retention.
UH

65 years ago a Boeing Tec-Rep told me; “Aircraft were designed by geniuses to be operated by idiots�! Looks like nothing much has changed at Boeing.
Questioning a manufactures design decisions is not a bad thing, even with something as simple as altering rudder pedal angle.
JJ

On Tuesday, May 14, 2019 at 12:11:43 AM UTC+3, wrote:
65 years ago a Boeing Tec-Rep told me; “Aircraft were designed by geniuses to be operated by idiots�! Looks like nothing much has changed at Boeing.
Questioning a manufactures design decisions is not a bad thing, even with something as simple as altering rudder pedal angle.
JJ

Yes it is. Rudder, cables and springs are a system that has eigenfrequency. They can and will flutter at some point. I bet next someone will come up with idea that you can actually cut off the control stick and welding a steering wheel in place. So much more comfortable to fly!

My experience and training tell me rudder flutter is caused by rudder imbalance and sometimes slop in the hinges or drive. It has nothing to do with rudder pedal angle! I know of 346 people and a whole bunch of airline pilots that wish someone had questioned Boeing’s design decisions on the 737 Max.
JJ