Adjustable rudder pedals?

Discussion in 'Instruments / Avionics / Electrical System' started by RSD, Sep 7, 2019.

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  1. Sep 7, 2019 #1

    RSD

    RSD

    RSD

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    Has anyone fitted (or found) a set of adjustable rudder pedals that operate the rudders with cables?

    (I couldn't find a section for flight controls so this area seemed like the next best option - mods feel free to move it if you know of a better place).
     
  2. Sep 7, 2019 #2

    TiPi

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    this is the adjustable rudder pedal (option) on the Spacek SD-1 Minisport. A very clever design, basically running the rudder cables through an S-shaped tube to be able to change the location of the rudder pedal assembly along the cables (not tensioned yet).

    upload_2019-9-7_14-6-2.jpeg

    upload_2019-9-7_14-7-10.jpeg

    upload_2019-9-7_14-7-33.jpeg
     
  3. Sep 7, 2019 #3

    RSD

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    Cheers TiPi - those photos answer something that was doing my head in - how to adjust the pedals without needing to re-adjust the cables at the same time - never thought of keeping them in constant tension by anchoring the ends to the wall - d'oh!
     
  4. Sep 7, 2019 #4

    RSD

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  5. Sep 7, 2019 #5

    TiPi

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    very nice, and 1/2 the $ of my complete aircraft kit. It is the same concept of running the cable through an S-shaped tube. Basic geometry is that the entry of the cable is on the pivot line, the exit is where you want the lever point to be. The firewall anchor is then parallel to the cable from the first pulley or sheave entry point to the pedal tube point, in both longitudonal axis. That way, the tension doesn't change with the different positions. It also reduces the amount of cable tension possible, something to be considered (rudder flutter!).
     
  6. Sep 7, 2019 #6

    wsimpso1

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    Look up what Van's has for adjustable rudder pedals. The ones in the RV8 seem to be a pretty good scheme. They appear similar to what you have found and there are a LOT of birds flying this stuff. Lots of sailplanes using this sor of thing too.

    My design is side-by and I am adjusting both sides independently. I just went to the advanced search and checked: use rudder pedal in the subject and my handle in the author and there are threads I ran with a lot of good advice received. Omit my handle, add adjustable to rudder and pedal, you will get a lot more. Also, I have a thread on my efforts to make good S-tubes that may be worth your time too.

    One of the things bothering me is that many systems I have seen have small cables and small radii on the S-tubes and pulleys. FAA minimum for rudder cables is 1/8". You should only go down in control cable sizes from the FAA mins if you can absolutely say that you can NEVER get to tensions that are half the breaking strength of the cable. For each cable size, there are min radii for pulleys and the like. While we "experimental" guys can ignore the design FAR's, I went with "legal" size cable and "legal" radii for my cable sizes, S-tube bends, and pulley sizes. My bird is fast and I can get to big aero forces for control actuation.

    I built my own pedals with toe brakes using flat and tubes and my OA welding stuff. Probably overkill, but I won't break it easily. We can talk more after you read this stuff and do a bit of analysis.

    Billski
     
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  7. Sep 7, 2019 #7

    gtae07

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    You don't necessarily have to have tension on the rudder cables. RVs (and derivatives) don't use tensioned rudder cables; the cable runs from the rudder pedal weldment straight back to the rudder and the pedal weldments are independent. Take your feet off the pedals and the rudder floats free--and on the earlier models (3/4/6) there's not even a counterbalance. That said, the rudder seems to be the first thing to flutter on RVs, particularly if you slather it in bondo/filler to get rid of the rivet lines :rolleyes:

    If you take this approach of non-tensioned cables I'd suggest a fully-balanced rudder, though.
     
  8. Sep 7, 2019 #8

    TFF

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    Like above, most rudders are not closed circuit. Feet keep the cable from slack. How quick an adjustment? A Pitts uses a strap from cable to pedal with multiple holes. Mount pedal bolt to strap position you want. Not fancy but weight is about nothing.
     
  9. Sep 7, 2019 #9

    Dan Thomas

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    The S-tubes could be replaced with pulleys and cable guards to achieve two things: allow pretension on the cables without the resulting friction inside the tubes that makes adjustment difficult, and eliminate the cable bending points at the S-tube ends.

    But you're adding weight.
     
  10. Sep 7, 2019 #10

    Mark Z

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    The slickest adjustable rudder pedals I’ve ever seen are found on the Panther.
     
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  11. Sep 7, 2019 #11

    bmcj

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    What about a system where you can disengage the pedal arms from the cable actuator arms via a lever or pull cable? It would need some form of interface with a positive lock between the pedal arms and the control arms (you could think of it as a no-slip clutch or transmission positrak lock). The interface could take any of several forms like a tooth and cog, square cut mesh gear or pin in hole.
     
  12. Sep 7, 2019 #12

    Dan Thomas

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    The problem with those systems is that the pedal angle changes drastically when you make the adjustment. For a long-legged guy, the pedals end up angled far forward and the feel goes all goofy, with the pedal falling--moving down--as it is pushed forward. For a short pilot, the pedal rises as it goes forward. This sort of messes up everything when you're resting your heels on the floor, and the pedal has to slip against your sole. The S-tube affair keeps the geometry correct, or at least a lot more ideal.
     
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  13. Sep 7, 2019 #13

    pictsidhe

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    I may well copy this idea. I'd liberally grease the cable through the s-tubes to ease adjument. I'd like the ends split and flattened so that the cable rolls off the inside of the tube. Weight increase should be about zero. But, harder to make.
    If you can stomp both pedals and have them fight each other, having pedals and cable that is stronger than you are seems prudent.
     
  14. Sep 8, 2019 #14

    wsimpso1

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    Last edited: Sep 8, 2019
  15. Sep 8, 2019 #15

    wsimpso1

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    The usual way to do this is to use as large a bend radius as is recommended for your cable, and then use Nyloflow tube to line the S-tube for low friction. If you kink the cable or deform it by running too small a bend in it, it will run poorly, no matter what else you do...

    Do your homework on rudder travel, pedal travel, and then the cable travel to do all that, then cable force and what size cable to run.

    I made mine so the cable could run fair throughout the range of motion - I bent the S-tubes around well beyond where they would run out, then split the outermost part, heated it with the torch, and opened it with an old Phillips screwdriver while still red hot.

    Oh, and I posted a thread on how I finally got those nice bends too. https://www.homebuiltairplanes.com/forums/threads/bending-steel-tubing-to-small-radii.19258/

    109.jpg 110.jpg 111.jpg

    Billski
     
    Last edited: Sep 8, 2019
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  16. Sep 8, 2019 #16

    Dan Thomas

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    Very, very nice work, and much better treatment of the cables than tubes just sawn off square. WIll the system have any pre-tensioning to help prevent the cables jumping off the tube flanges? Or maybe the exposed section of cable will be too short to find its way off anyway...

    Nylon tubing as a liner would work much better than grease, which just attracts dust. Nylaflow is a little bit soft, and a harder nylon would resist taking an impression of the cable that might make adjustment sticky. Air brake tubing used on large trucks is a harder nylon. A thinner-walled nylon tubing would allow the use of a smaller steel tubing. Maybe brass tubing would be better? Be lots of fun trying to get it into the bent steel tube, though.
     
  17. Sep 8, 2019 #17

    wsimpso1

    wsimpso1

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    Thanks. The runs are dead straight to the firewall and to fairleads in the seatback bulkhead. Between the guidance we have with the S-tubes and Nylaflow, it works great in a lot of other airplanes. I have worked out a provision for return springs or even bungees and could add it if the system misbehaves.

    Bill
     
  18. Sep 8, 2019 #18

    pictsidhe

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    Exactly what I was thinking. You must be as smart as I am ;)

    Re grease. yeah, it'll catch dust, but it's only there to ease adjustment, there won't be movement during rudder operation. I did wonder about sleeving the cable with air line tubing, but that would make inspection of broken strands difficult. I pondered a plastic s-tube liner, but anchoring it seemed tricky. Stranded cable would likely have higher friction than the s-tube. But with adequately sized s-tubes, this can be played with later. I'd be tempted to use tubing big enough to pass a nipple.
     
    Last edited: Sep 8, 2019
  19. Sep 8, 2019 #19

    pictsidhe

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  20. Sep 8, 2019 #20

    Dan Thomas

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    Yeah. Weight wasn't one of their concerns, was it? Looks like the brake pedals off a bulldozer. A lot of quarter-inch aluminum there. They could have cut the weight by maybe 40% if they'd used 1/8" 7075 instead of the 6061.

    Nice idea, though, the hand-crank adjustment. But it seems to me that an old military airplane, the T-6 maybe, had adjustable rudder pedals that were spring-loaded aft and you pushed them forward until they felt right and locked them there. Less mechanism, and no screw to get dirtied up and seized. One has to watch for that, see? I once built an inboard boat and made all my own steering stuff for it. The steering wheel was on a 3/4" shaft and I made bulkhead mounts and welded bushings to them, and honed those bushings to get maybe .002" or so of clearance between the shaft and bushing. The steering seized solid within a few hours as sand--just a grain or two--got between the shaft and bushing and galled it all, raising the metal and locking it tight. I had fun getting it all apart, even. I cut the shaft off and replaced it with a length of 3/4" threaded rod, which had a bit less diameter and had places for the sand to go and work its way out. One soon learns that tight tolerances need seals to prevent that. The Starflight rudder pedals we're discussing here have a .005" clearance between the slider bushings and shafts, and the only thing that will prevent lockup is the plastic bushings.

    The Gippsland Aero GA8 Airvan (Australian) had a similar problem with its aileron control stiffening or locking up. It was a close-tolerance steel shaft galling and seizing, and the fix (mandated by AD) was to replace it with a bronze shaft, which will scratch without raising burrs.

    Now you know why so many certificated airplanes have tractor-technology control systems, with some slop here and there. It's to prevent binding. Even advanced airplanes like the Cessna 400/ttX, a derivative of the Lancair, has push-pull tubes that run in guides having three plastic rollers. The rollers have been known to seize and start eating into the aluminum tube, aided by abrasive dust that embeds itself in the rollers. One of the annual inspection items is to make sure there is some clearance between the rollers and tubes, and see that the rollers aren't stuck.
     
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