Rudder Cables

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Ozarea5140

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Hi All
Can anyone explain why some aircraft have closed loop tension rudder cables and others don't ..
Joe
 

Dan Thomas

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Hi All
Can anyone explain why some aircraft have closed loop tension rudder cables and others don't ..
Joe
Well, the Cessna singles have open-loop systems unless they have rudder trim, which involves a bungee system working on one of the rudder bars. Need closed-loop to make that work. A three-axis autopilot would also need the closed loop.

Some other aircraft might have closed-loop to help suppress flutter.
 

N804RV

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I've got 2 different homebuilts now that don't have return springs on the rudder pedals. The cables go slack when you're not on the pedals. On both, there's room to instal simple return springs. But, I'm so used to it, I see no reason to. I secure the rudder from wind damage with a physical gust lock between the rudder and vertical stab.
 

TFF

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There isn’t a good reason except designer preference. Regular GA airplanes, not WW1 or P-51s or airliners. Some is space and simplicity, full circuit takes more room. The rudder is not supposed to flutter no matter, so let’s take that out as a reason. As a personal note, when you do find a plane that is closed, it feels good. Mine isn’t.
 

Dan Thomas

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There isn’t a good reason except designer preference. Regular GA airplanes, not WW1 or P-51s or airliners. Some is space and simplicity, full circuit takes more room. The rudder is not supposed to flutter no matter, so let’s take that out as a reason. As a personal note, when you do find a plane that is closed, it feels good. Mine isn’t.
Oh, there's a good reason. If you want a rudder trim that's bungee-based instead of tab-based, you have to have a closed system or that bungee will only pull the rudder one direction.

This is the 180/185 trim system. Rudder bars shown, but not the pedals. The trim wheel screws a spring-loaded bungee forward and aft via a bellcrank, and the other end of the bungee is attached to just one of the rudder bars. The loop is completed by the cable around the horizontal pulley aft of the rudder bars.

1660258240309.png

The Cessna Corvalis/350/400/ttX had a closed-loop system. There was a cable that ran from the pilot's left rudder bar to the copilot's right rudder bar, and that cable ran across the firewall and through an electric brake. You held the rudder where you wanted it, and actuated a switch that clamped the brake on the cable to hold it for you. IIRC, there was a microswitch that sensed the pilot's pressure on the system, and it would trip the brake off if the pilot pushed hard enough.
 

pylon500

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Oh, there's a good reason. If you want a rudder trim that's bungee-based instead of tab-based, you have to have a closed system or that bungee will only pull the rudder one direction.

This is the 180/185 trim system. Rudder bars shown, but not the pedals. The trim wheel screws a spring-loaded bungee forward and aft via a bellcrank, and the other end of the bungee is attached to just one of the rudder bars. The loop is completed by the cable around the horizontal pulley aft of the rudder bars.

View attachment 128695

The Cessna Corvalis/350/400/ttX had a closed-loop system. There was a cable that ran from the pilot's left rudder bar to the copilot's right rudder bar, and that cable ran across the firewall and through an electric brake. You held the rudder where you wanted it, and actuated a switch that clamped the brake on the cable to hold it for you. IIRC, there was a microswitch that sensed the pilot's pressure on the system, and it would trip the brake off if the pilot pushed hard enough.
I think the 180~185 have this closing pulley because they're tail draggers, 150/172/182 all have nose wheels with (admittedly) spring connections to the steerable nosewheel that 'sort of' closes the loop... (arrowed)
Rudder_pedals.png
Nose_wheel.png
That said, they also have return springs
 

Dan Thomas

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150/172/182 all have nose wheels with (admittedly) spring connections to the steerable nosewheel that 'sort of' closes the loop... (arrowed)
The 150/152/172 have an open-loop system. There are no cable tensions given for those systems, a giveaway that they're open-loop. The rudder bar springs contribute the only cable tension in those systems. You can pull both rudder pedals back at the same time, impossible with a closed-loop system. The rudder bars are connected to the nosewheel steering via spring-loaded bungees. The nosewheel is locked straight ahead when the weight is off it, and it acts as a centering device for the rudder in flight. When you push rudder, one bungee or the other compresses its spring to allow rudder movement.

The 182 has a "wiffletree", a bellcrank that the rudder bars are connected to via pushrods, that makes the system closed-loop. Cable tensions are given for that rudder system. The rudder trim is an adjustable spring bungee connected to the wiffltree.

1660327952718.png

The R182 (182RG) is also closed-loop, but instead of a cable loop or a wiffletree, there are gear sectors on the rudder bars that only allow them to move in opposite directions, a more compact system. Cable tensions apply.

1660328057263.png
 

wsimpso1

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On any rudder control design, follow the cables through, look for how much cable you move as one pedal moves and see if the other pedal moves the same amount in the opposite direction. If they move the same amount, you can run a closed loop. If they move a different amount, it has to either be open or you adjust design so pedals move the same amount... Also important if you are running a rudder bar between pedals instead of independant pedals. Angles of run and arm angles have to be coordinated for a closed loop. And yes, that facilitates bungee trim...

Many of us use fixed pedals, but for those of us who run adjustable pedals, an S curved tube on the pedal allows adjustments while flying, and so two cables, with one end anchored forward of the pedal and the other end on rudder or intermediate linkage elements are needed.

Seriously, follow the cables on existing systems and see how they are run... The reasons for design choices should become obvious.

Billski
 

Ozarea5140

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On any rudder control design, follow the cables through, look for how much cable you move as one pedal moves and see if the other pedal moves the same amount in the opposite direction. If they move the same amount, you can run a closed loop. If they move a different amount, it has to either be open or you adjust design so pedals move the same amount... Also important if you are running a rudder bar between pedals instead of independant pedals. Angles of run and arm angles have to be coordinated for a closed loop. And yes, that facilitates bungee trim...

Many of us use fixed pedals, but for those of us who run adjustable pedals, an S curved tube on the pedal allows adjustments while flying, and so two cables, with one end anchored forward of the pedal and the other end on rudder or intermediate linkage elements are needed.

Seriously, follow the cables on existing systems and see how they are run... The reasons for design choices should become obvious.

Billski
The reason for my question ...Jurca warbird designs are optional ..fixed hinge on floor pedal with a tensioned cables or billycart bar style pedals with tensioned cables. I would prefer hung pedals..FEW Mustang has hung pedals with a balance cable behind them (what I call closed loop)...as did the SAL Falconar P-51 design I think... these are all older designs.. My RV6 has hung pedals with no balance cable (loose cables if your feet aren't on the pedals). The RV8 and Lancair Legacy has S type cable as you suggested for adjustment, still fixed forward of the pedal but loose to move the rudder if your feet aren't on the pedals.
My question..is there a scientific reason why some have balance cables and need to be tensioned , and others don't. Loose cables would be easier to maintain as there is no checking tension and pulley systems...Just wondering if I can fit RV style rudder pedals and cables to my Jurca P-40..
Joe
 

wsimpso1

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RV taildraggers with adjustable pedals anchor the cable forward, so closed loop is not a universal requirement of taildraggers.

In order to use a closed loop, the system length can not vary as the pedals move, so the pedals must either move independently or the system be designed very carefully to maintain constant “cable length”. Rudder bar systems usually can not be designed this way, but independent pedals can take all sorts of liberties with “constant length”, which MUST be maintained in closed loops and in rudder bars.

With independent pedals, they can go slack with your feet off them, so some folks put in springs, and some avoid springs because they make the “feel“ poorer.

This whole topic really comes down to much of the fuss with cable systems in general. They can either be open and artificially balanced, closed and must be rigorously designed so as avoid going slack one way and tight the other way… Like many design issues, each choice made presents its own set of requirements and another set of choices. Welcome to the Monkey House.

Billski
 
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BJC

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Like many design issues, each choice made presents its own set of requirements and another set of choices.
One thing the designers and builders should consider with an open system, which is the most common in E-AB aircraft in the USA, is something to keep a rudder pedal from falling flat on the floor should a spring come loose or break. Typically, toe brakes serve that purpose, but there have been incidents of setups with heel brakes where a pedal ended up flat on the floor.


BJC
 

Dan Thomas

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In essence, you could fit whatever you want but the requirements of a taildragger might just demand a closed loop type as well as a tail locking function
The taildragger doesn't need closed loop unless you want rudder trim using a bungee. The 180 didn't have a closed-loop system unless the optional rudder trim was installed.

Tail lock, for most, is done at the tailwheel itself, not via rudder cables.

1660524863746.png

McCauley made the locking tailwheels for the 180 and 185. They are rare and awesomely expensive. Before I retired four years ago it cost around $13K for a new one, in Canadian dollars. Maybe $11K USD.

Some cropsprayers use a locking, free-castering tailwheel that unlocks when you push full forward on the elevator, which you'll only do when taxiing.
 

flitzerpilot

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Striving for simplicity, the Flitzer uses an open rudder cable system, closed only be foot pressure against the floor-mounted pedals. But due to the relatively steep ground angle, without light return springs from pedal to firewall, the pedals would fall backwards when the a/c was unoccupied. Later variants use a lockable/swivelling tailwheel for operation from hard surfaces.
 

PTAirco

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I've got 2 different homebuilts now that don't have return springs on the rudder pedals. The cables go slack when you're not on the pedals. On both, there's room to instal simple return springs. But, I'm so used to it, I see no reason to. I secure the rudder from wind damage with a physical gust lock between the rudder and vertical stab.


As long as your pedals can't flop backwards on the floor, that it fine. There was fatal accident in an aerobatic aircraft where this happened. He was unable to get out of a spin because he could not get the pedal off the floor to push it.
 

BJC

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As long as your pedals can't flop backwards on the floor, that it fine. There was fatal accident in an aerobatic aircraft where this happened. He was unable to get out of a spin because he could not get the pedal off the floor to push it.
In a Pitts, the pilot can reach down by his side, grab the cable, and pull it up to apply rudder in that direction. But the toe brakes should prevent it from happening in a Pitts.

BTW, I grew up on heel brakes, and thought that toe brakes would adversely affect the feel of the rudder. It does not, at least with my feet, on the Pitts.

BJC
 

N804RV

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As long as your pedals can't flop backwards on the floor, that it fine. There was fatal accident in an aerobatic aircraft where this happened. He was unable to get out of a spin because he could not get the pedal off the floor to push it.
On both of mine, the geometry of the rudder pedals/toe-brakes & master cylinders prevents this. I cannot imagine having a setup in which the rudder pedals could fall as you describe.


... BTW, I grew up on heel brakes, and thought that toe brakes would adversely affect the feel of the rudder. It does not, at least with my feet, on the Pitts.

BJC

I got my TW endorsement in a 7ECA, hated the heel brakes at first. But after awhile, it didn't bother me at all.

Both of our club's Citabrias were totaled in nose-over accidents. I suspect a lack of proficiency with heel-brakes was a factor in both.
 
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