So you are flying along and your elevator cable snaps

SixPapaCharlie said:
There has to be a linkage.
I move my trim wheel and I can see it move when sitting in the hangar.

I am misunderstanding I think.

Edit: No, I know the elevator is deflected the opposite direction that the antisrervo tab is moved and I get that it is aerodynamic force changing the AoA of the elevator

I guess what my question is how there are 2 separate systems controlling the same moving surface and they do not interfere with one another.
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The yoke controls the stabilator directly through a pair of cables. One cable pulls it up, the other pulls it down.

The trim wheel runs a cable loop that passes over a capstan in the tail. The capstan drives a jackscrew that has a nut that runs up and down on it, and that nut is attached to a rod that works the antiservo tab. The tab moves the stabilator up and down independent of the stabilator's control cables.

You really need to spend some time looking at the guts of your airplane when it's opened up for the annual.

Edit: I think I see your misunderstanding. The trim system is for taking the presure off the controls so you don't have to hold the nose up in the climb or approach or force it down in cruise. The trim tab or antiservo tab will move the elevator or stabilator; if you work the trim in flight and watch the yoke, you'll see it move a bit. The tab is forcing the elevator up and down, and that movement is fed back via the control cables.

In the old days, large airplanes often did not have hydraulically boosted control surfaces, and the air loads were far too heavy to just manually force them to maneuver the airplane. There were two methods used to achieve control, both similar. Each control surface had a tab called a servo tab, and it worked just like a big trim tab. The pilot's controls moved those tabs and the airflow did the heavy work of moving the control surfaces. In some of the systems, all the pilot controlled was that tab, and in others, there was a spring as part of the system to feed a force into the control surface itself for better response at low speeds.



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Dan
 
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Jaybird180 said:
In my POH Emergency Section it says to use the trim for 70KIAS with 10* flaps and adjust power for desired descent rate. It also says to expect to land on the nose if you get the timing wrong in the flare. But it's survivable.

My C-172S has 2 cables for elevator (up/down) and a separate rod for trim.
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That trim rod is driven by a jackscrew that is driven by a pair of cables. At the trim wheel a sprocket drives a section of small bicycle-type chain that is attached to two cables, and at the tail those two cables are attached to another section of chain that drives a sprocket on the jackscrew. Lot of mechanism involved.

Dan
 
Some trim systems use nothing but springs. The Aviat Husky does that. The trim wheel runs a geared affair that pulls alternatoely on a pair of heavy springs that pull on a pair of cables that run back to the elevator. The pilot's stick works another pair of cables to the elevator. The drawback with that system is the pilot's having to fight those springs anytime he wants full up or down elevator, such as raising the tail at takeoff or pinning it down in the rollout. And I have found that the springs weaken and stretch with time and trim authority is lost.

Dan
 
Dan Thomas said:
Some trim systems use nothing but springs. The Aviat Husky does that. The trim wheel runs a geared affair that pulls alternatoely on a pair of heavy springs that pull on a pair of cables that run back to the elevator. The pilot's stick works another pair of cables to the elevator. The drawback with that system is the pilot's having to fight those springs anytime he wants full up or down elevator, such as raising the tail at takeoff or pinning it down in the rollout. And I have found that the springs weaken and stretch with time and trim authority is lost.

Dan
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My Flybaby trim system consists of two adjustable bungee cords that are attached to the stick. It actually works surprisingly well.
 
SixPapaCharlie said:
There has to be a linkage.
I move my trim wheel and I can see it move when sitting in the hangar.

I am misunderstanding I think.

Edit: No, I know the elevator is deflected the opposite direction that the antisrervo tab is moved and I get that it is aerodynamic force changing the AoA of the elevator

I guess what my question is how there are 2 separate systems controlling the same moving surface and they do not interfere with one another.
Click to expand...

The linkage drives the trim tab (or horizontal stab, or entire tail) typically through a jack screw mechanism. It's linkage though has no direct effect on the attitude of the elevator. The elevator is adjusted as a result in the change of lift profile generated by the tab leveraging at the hinge. In other words, you move the tab mechanically, but the control surface is manipulated strictly through aerodynamics.
 
As an aside, in my Sky Arrow they managed to have solid control rods all the way from the side stick to both the ailerons and elevator.

Makes for a solid feel and avoids a lot of these issues. Only the rudder uses cables.

And my elevator trim is electric and the motor is in the elevator, just inches from the trim tab.

Downside is my empty weight is high for an LSA at 861 lbs. I guess that rods are heavier than cables.

I think the linkage used to accomplish the rod thing is both clever and Rube Goldberg-esque at the same time! http://youtu.be/FIMr4pzoY1o
 
FastEddieB said:
As an aside, in my Sky Arrow they managed to have solid control rods all the way from the side stick to both the ailerons and elevator.

Makes for a solid feel and avoids a lot of these issues. Only the rudder uses cables.

And my elevator trim is electric and the motor is in the elevator, just inches from the trim tab.

Downside is my empty weight is high for an LSA at 861 lbs. I guess that rods are heavier than cables.

I think the linkage used to accomplish the rod thing is both clever and Rube Goldberg-esque at the same time! http://youtu.be/FIMr4pzoY1o
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Tubing is by no means failsafe, the exact same failure possibilities exist.
 
Henning said:
Tubing is by no means failsafe, the exact same failure possibilities exist.
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Must disagree.

To be clear, rods are not failure-proof.

But when I think of the myriad issues one can have with cables, I'd say they are certainly not "the exact same failure possibilities".

I mean,

Broken cable
Jammed cable
Loose cable
Tight cable
Frayed cable
Misrouted cable
Chafing cable
Worn or broken pulley
Poorly clamped ferrule

As opposed to what I feel much be a much shorter list of control rod maladies.

Maybe some A&P's can speak up as to the relative troublesomeness of each system.
 
Rods are about as direct-connect as you can get.
Gotta agree with you F.E.B.
 
FastEddieB said:
Must disagree.

To be clear, rods are not failure-proof.

But when I think of the myriad issues one can have with cables, I'd say they are certainly not "the exact same failure possibilities".

I mean,

Broken cable
Jammed cable
Loose cable
Tight cable
Frayed cable
Misrouted cable
Chafing cable
Worn or broken pulley
Poorly clamped ferrule

As opposed to what I feel much be a much shorter list of control rod maladies.

Maybe some A&P's can speak up as to the relative troublesomeness of each system.
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All of those apply except for adjusting tension. Fittings and pins wear, corrosion exists, physical chafing exists jamming exists.... The real advantage to rods comes in on flaps and ailerons when you have folding wings. It makes things way simpler.
 
SixPapaCharlie said:
...I guess what my question is how there are 2 separate systems controlling the same moving surface and they do not interfere with one another.
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Actually they do interact together. In the case of the Pipers you have a stabilator rather than a fixed or trim-able stabilizer and separate elevator. If you move the stabilator up and down you will see that the servo tab aligns with the airfoil in only one position and that position is determined by the trim setting. When in flight, hands off the servo and stabilator airfoil will be aligned in the streamlined position.

In the case of your typical Cessna an anti-servo tab is used on the elevator and it's position, determined by the trim wheel, remains fixed throughout full elevator travel. The deflected tab alters the aerodynamics of the elevator and hands off it will assume a position where the forces on the elevator and opposing tab will balance. The advantages of this system are simplicity and lower cost. The disadvantage is that the anti-servo tab reduces the full elevator authority in the direction of trim. So if you have full nose up trim you will have reduced nose up authority from the elevator.
 
Silvaire said:
Actually they do interact together. In the case of the Pipers you have a stabilator rather than a fixed or trim-able stabilizer and separate elevator. If you move the stabilator up and down you will see that the servo tab aligns with the airfoil in only one position and that position is determined by the trim setting. When in flight, hands off the servo and stabilator airfoil will be aligned in the streamlined position.

In the case of your typical Cessna an anti-servo tab is used on the elevator and it's position, determined by the trim wheel, remains fixed throughout full elevator travel. The deflected tab alters the aerodynamics of the elevator and hands off it will assume a position where the forces on the elevator and opposing tab will balance. The advantages of this system are simplicity and lower cost. The disadvantage is that the anti-servo tab reduces the full elevator authority in the direction of trim. So if you have full nose up trim you will have reduced nose up authority from the elevator.
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He knows this. He's trying to envision HOW the mechanism works to allow it.

John
 
I dug through my manuals and it is way more complex than I imagined.
To clarify the goal is not complex but the connections, hinges, pulleys, etc were designed by someone that worked harder in school than me.
 
jsstevens said:
He knows this. He's trying to envision HOW the mechanism works to allow it.

John
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It's not all that complicated. The elevator control (yoke) pulls up and down on the counterweight forward of the stabilator pivot point in the aft fuselage. The servo tab on the aft edge of the stabilator is connected by a rod to the bottom of the adjustable trim jackscrew which is on a fixed bracket mounted to the aft-most bulkhead of the tail section. So as the stabilator moves up and down the hinged servo tab remains fixed to the jackscrew.

If you go to the tail and move the stabilator up and down by hand it's pretty obvious how it works. The trim adjustment cable only connects to the fixed jackscrew, it does not move with the stabilator.
 
FastEddieB said:
Agree to disagree.

I will yield to any experienced A&P's on this, however.
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The only pushrods I encounter in my full-time work as an AME (Canadian A&P/IA) are the trim tab rod, the elevator pushrod in the Cessna 18X series, the push-pull rods on Cessna ailerons, and the pushrod systems in the Cessna 400. Everything else is cables. Those rods have to be watched as closely as any cable; they are subject to corrosion, worn hardware at their terminals, bending from compression overload (often by strong winds banging unsecured controls around), and so on. Cables do wear, especially at fairleads and smaller pulleys, but I find most of the wear is due to the wind moving "locked" controls just a bit, enough to keep the cable scrubbing away at the fairlead, or to corrosion (same as a rod). The wind can wear out an airplane that never flies. Controls should ideally have external locks on them; shouldn't rely on the typical control column lock. Or keep the airplane in an enclosed hangar, not an open-sided affair.

Once in a while we find cables with broken strands, but they're still a long way from failing. An airplane would need really poor maintenance to get that far.

Dan
 
Thanks.

Maybe my intuition that rods were "better" is not substantiated.

Still, for me it's a personal preference, though it would never affect my choice of planes, let's say.
 
FastEddieB said:
Thanks.

Maybe my intuition that rods were "better" is not substantiated.

Still, for me it's a personal preference, though it would never affect my choice of planes, let's say.
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The reality is they are equally fine systems of getting the job done with an extremely high rate of reliability. To me the control rigging method doesn't even factor in any airplane choice category except like I said before, for a folding wing it really simplifies things.

The important thing though is that you can't consider control rods maintenance free, none of the hardware is permanent, it all degrades, wears out, and can get damaged.
 
I think rods make a more precise control system but among the legacy production GA aircraft Mooney is the only one I can think of off hand that used them almost exclusively. If I recall correctly the only cable in a Mooney is for the trim position indicator.
 
Silvaire said:
I think rods make a more precise control system but among the legacy production GA aircraft Mooney is the only one I can think of off hand that used them almost exclusively. If I recall correctly the only cable in a Mooney is for the trim position indicator.
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If that was their objective, they missed the mark, because a Bonanza with cables beats a Mooney with rods in that department any day.
 
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