Control surfaces actuators design/preferences

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geosnooker2000

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What are the specific reasons torque tubes are preferred over tension wires? I know one reason would be, you never have to worry about checking the tension on a torque tube, and torque tubes can't "fray". What else?
 

Jay Kempf

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They don't tighten and loosen as the airframe flexes or changes dimensions with temperature changes. But they are super light for crossing long distances.
 

TFF

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Wires are lighter weight and have more precise control because the control is always pulling to move it. Runs can be long without problems. Requires lots of planning to get the runs perfect.
Control tube is simple and almost as precise but bearing wear looses some precision over time. Just about anyone can make the tube with the right parts cheaply. Good cable tools are expensive, especially for one time. Pulleys have gotten expensive too for cables. There usually is one that will fit an application better, than the other.
 

geosnooker2000

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What about flutter? I seem to remember a discussion about the Cri-Cri having a problem with an undersized torque tube for the ailerons? Would cables have eliminated that problem or exacerbated it?
 

TFF

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Torque tubes on ailerons can be an issue on some planes. There is always twist and you don’t want any. Simplicity like a Tailwind or a Grumman it makes sense. Make the tube big enough and most pilots don’t ever notice. Planes that put a lot of loads on the ailerons like the Stephan’s Acro pushed torque tubes to the limits and skilled pilots could feel them wind up. Cri Cri are built two ways. The original complex bonded and the rivet aluminum style. The aluminum version was under designed in a few ways from the space shuttle complex original. The aluminum was an adaptation not from the original designer.
 

Jay Kempf

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Flutter is not related to cables or pushrods. One or the other can be a root cause but neither is apparently flutter proof or worthy. Slop can cause flutter in a badly maintained control system.
 

Map

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As it was already said in other posts, torque tubes will twist under load. The longer a torque tube is the worse it gets. So it is ok to use them if they are short, made from a stiff material (carbon, steel..) and have a large diameter.
It is easier to make a stiff control system (requirement for avoiding flutter and getting control surface deflections that match the stick input) by using push-pull tubes or cables. But the design of the system also has a big influence on stiffness. Attaching a support to a soft piece of structure can have the same undesirable effect as a long flexible torque tube.
 

wsimpso1

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What are the specific reasons torque tubes are preferred over tension wires? I know one reason would be, you never have to worry about checking the tension on a torque tube, and torque tubes can't "fray". What else?
First off, you are attempting to get us to discuss cables vs torque tubes. What about push-pull tubes? Torque tubes are appropriate in few places and are used sparingly by airplane designers for a variety of reasons.

I did a quick analysis of all three as we might apply them to ailerons, using a wing span of 24 feet, chord of 5 feet, ailerons 5 feet by 1.25 feet, airspeed of 180 knots. The ailerons were all hinged at their leading edges to keep this comparison straight up.

Push-pull tubes - They are usually sized by buckling limits. Mine turned out to need 1-3/4 x 0.035 aluminum tube, lost motion due to control circuit deflection is about 0.5 degrees on a 15 degree throw. This will be low friction, and relatively bug free to design and implement, with ratio changes made at idlers and bellcranks if you need it. System will have a number of rod ends in it, so the system should be designed so that air loads take the slack out in nominal flight;

Torque tubes -They are usually sized to get reasonable circuit deflections. I stopped at 3 x 035 aluminum tube (172% of push-pull tube weight), lost motion is 3.3 degrees. These can be modest friction if wing deflections are small and executed properly. Steve Wittman used them. Wing deflections must be low and system must be really excellently executed, or they can be sticky. Ratio changes can be made inside the fuselage if you need them. Cautions about loading rod ends apply here as well;

Cables - Are sized for adequate strength. I needed a 3/16 cable for this job with arms sticking outside the airfoil profile to keep weights low. These usually have numerous pulleys needed and with excellent alignment required of cables to pulleys just to get down to decent friction. Muddy feeling controls can result with cables if much in the way of misalignment or fairleads are needed. I got 4 degrees of lost motion on my 15 degree control throw. The cables themselves are lighter than push-pull tubes (69% of push-pull rods), but the devil is in the details. Many pulleys are sometimes needed, and this can make system weight approach push-pull tubes on weight.

Cables require preload to keep slop down and make the system feel responsive. Ratio changes can be fussy because the net change in cable length over the control throw needs to be kept very small or the system will bind or go slack as the stick moves. Design it to be perfect, then change one lever length, and now it is not so good. Additionally, steel cables change length less with temperature than aluminum and less again than wood or fiberglass. Get into carbon fiber, and well, maybe the cables should be the same material and schedule as the structure. Story is that Paul Allen's heavy lifter airplane - correction - White Knight 2 has carbon fiber rudder cables for just this reason.

Billski
 
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Hot Wings

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Attaching a support to a soft piece of structure can have the same undesirable effect as a long flexible torque tube.
This highlights one of the misconsecptions of good design practice. There is no one single best method. There are no universally specific reasons one chooses a particular method over another. You have to think in terms of systems, not isolated parts. Add in parameters that are external to the actual engineering, as bean counters like to do, and you may end up with a completely different solution.
 

Marc Zeitlin

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Get into carbon fiber, and well, maybe the cables should be the same material and schedule as the structure. Story is that Paul Allen's heavy lifter airplane has carbon fiber rudder cables for just this reason.
I believe you're conflating WK2 with Stratolaunch.

WK2 was an unboosted aircraft with a 140 ft. wingspan and two fuselages, so all the control surface cable loops were incredibly long. It has carbon fiber "cables" fabricated specially so that the CTE is the same as the structure, and all cables were run as close to the neutral bending axis of the wings and fuselage as possible.

Stratolaunch's control system is basically a cannibalization of two 747-400's (along with 6 of the 8 engines from two 747's). So it's a boosted plane, working pretty much as does a 747.
 
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