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.Hi All
Can anyone explain why some aircraft have closed loop tension rudder cables and others don'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.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.
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)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.
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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.
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.150/172/182 all have nose wheels with (admittedly) spring connections to the steerable nosewheel that 'sort of' closes the loop... (arrowed)
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.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.
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.Like many design issues, each choice made presents its own set of requirements and another set of choices.
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.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
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.
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.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.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.
... 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.
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