What to consider when choosing Control mechanisms?

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yankeeclipper

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Why choose cables, or push-pull cables, or torque tubes, or push-pull rods, or....? I mean, cables stretch, rods are heavy, push-pulls are...I dunno...frictiony? So how would you make your choice if starting a design from scratch? I'll narrow this a bit and assume we're not aspiring over 200 mph, nor over 2000 lbs (2+2 or less).
 

Dana

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Pushrods give a smoother feel. I would choose pushrods wherever there's a reasonably straight shot from the cockpit to the surface and the distance is not too great. If you have to go around lots of bends, or the distance s great, cables may make more sense.

-Dana

Grow your own dope! Plant a politician!
 

Michealvalentinsmith

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I wonder why not, or if, anybody has tried using low stretch kevlar lines as control cables. There's some kevlar, aramid, dyneema, spectra etc composites now with very low weight high strength and low stretch characteristics that seem ideal to me, all without the hassles and problems of nicos, tangs etc.

I'm considering a combination in my design since the wings need to fold cable out to the aileron horn but a pushrod from the horn to the control surface so I don't need two sets of cables upper and lower.
 

yankeeclipper

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I like the idea of push-rods as well, but how to cleanly deal with the translations from linear to angular motion? Won't the change in the mechanical advantage of, say, a bell-crank return a less fluid feeling (i.e. resistance & travel proportionate to the sine of the linkage angle)? In considering this, I've wondered if there are light rack & pinion type linkages to keep the linkage system tight, and the M.A. consistent.
 

Dana

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I wonder why not, or if, anybody has tried using low stretch kevlar lines as control cables. There's some kevlar, aramid, dyneema, spectra etc composites now with very low weight high strength and low stretch characteristics that seem ideal to me, all without the hassles and problems of nicos, tangs etc.

I'm considering a combination in my design since the wings need to fold cable out to the aileron horn but a pushrod from the horn to the control surface so I don't need two sets of cables upper and lower.
Even low stretch kevlar is stretchier and weaker than steel cable, and much less abrasion resistant.

Pushrods can actually be easier to disconnect for folding wings. On my Kolb, I simply remove two safety clips and clevis pins and the aileron pushrods are disconnected from the horns (which are at the wing root) and the wings are ready to fold.

I like the idea of push-rods as well, but how to cleanly deal with the translations from linear to angular motion? Won't the change in the mechanical advantage of, say, a bell-crank return a less fluid feeling (i.e. resistance & travel proportionate to the sine of the linkage angle)? In considering this, I've wondered if there are light rack & pinion type linkages to keep the linkage system tight, and the M.A. consistent.
If the control stick and control surface are both moving through arcs, then the loss of motion on one end is equal to the loss of motion needed on the other end, so they cancel out. A cable system works the same way, unless you had a drum or sprocket at one end and arms at the other.... which is not uncommon. My old Taylorcraft had sprockets on the control wheels connected to cables going out to the aileron bellcranks. The control feel was fine. However, I did get the chance to fly an Interstate Kadet once, a similar aircraft but with all pushrods and Heim joints... the control feel was silky smooth and delightful.

In the end, it's whatever makes more sense in the particular aircraft, with my preference being for pushrods. In my new design, the ailerons will be pushrods, cables for the rudder, elevator TBD.

-Dana

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wsimpso1

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Differential ailerons are easy to set up and tune with push rods (only one control path to each surface, angles can be played with, etc.) while with cables, well, it is harder to do. Progressive feel elevators - ditto;

Control feel is excellent with push rods (low friction) when all you have is a couple bellcrank bearings and rod ends. Cables mean a lot of pulleys, and springy system parts that muddy up the feel;

Cables go around corners with simple pulleys. You gotta build bellcranks and idlers and stuff to make pushrods go everywhere;

Cables can change tension with hot or cold (different thermal expansion rates than the primary structure). Pushrods don't change tension when the airplane sees hot or cold, although they might change rigging a bit (an excuse to make your pushrods out of the same materials as the wings and fuselage);

Cables are light and sized based on tension loads. Pushrods are sized based upon column loads, and thus can end up pretty substantial. Breaking a pushrod into more than one part may allow the rod to be lighter, but now you have to add an idler and two more rod ends;

My choices for my fiberglass low wing is pushrods for flaps, ailerons, and elevator. The Rudder is cables. The long pushrods are all fiberglass tubes, while the short pushrods are metal. I am thinking about synthetics (Vectran, Dyneema) for the parts of the cables shielded from sunlight. The reasons for fiberglass tubes are multifold. Minimizing aileron rigging changes with temperatures is one issue, but so is allowing my internal antennas to work as well as possible and the desire to build a fiberglass airplane with as little metal as possible. Same deal on synthetic cables, plus they may shave some weight (transition from steel to Vectran will cost something in hardware).

Billski
 
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Lucrum

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Having flown both cable and push rod/torque tubing rigged A/C I certainly prefer the feel of rods over cables.
 

Michealvalentinsmith

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I think Dana has a point about kevlar type line. Though touted as stronger than steel that's the strength to weight ratio.

To get the same strength, you need a much thicker diameter - even though the weight is the same. Though large diameter is not an issue for me (other than availability)

Given the consequence of failure, a bit of extra weight and hassle with steel is worthwhile.

I might look into folding pushrods as well.
 

Dan Thomas

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Cessna uses steel cable throughout, except for pushrods from the aileron bellcrank to the aileron (to enable differential) and, in the 180 thru 185, an elevator pushrod from the cable-driven bellcrank so a downspring can be used.

When the Cessna is new the feel is great. As it ages, the pulley bearings wear and will get stiff if they're not regularly lubed with the right stuff (LPS and a bunch of other lubes are the wrong stuff) and the cables wear where they pass over the pulleys and fairleads. The tension required to reduce slop and control flutter places even more loads on the system, making the bracketry more complicated and heavier.

Still, cables are great. They can be run where anything else gets way too complicated and heavy.

As others have noted, pushrods can get big just to resist buckling. Torsion tubes might work but they can fail, too, and unless they're larger diameter they'll be springy.

Dan
 

yankeeclipper

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So if I can ask a potentially meaningless question--how many links from input to surface are too many for a push-rod system in a small, 2-4 seater? Obviously, I know full well there won't be a magic number, but would 5 rods, 4 cranks, and 3 90-degree angles start to get sluggish?

I've noticed that none have mentioned hydraulics. Is that because of weight, or because they are to disaster what the splashy sound of a single malt against a nice Steuben tumbler is to my brain?
 

Hot Wings

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So if I can ask a potentially meaningless question--how many links from input to surface are too many for a push-rod system in a small, 2-4 seater? Obviously, I know full well there won't be a magic number
There is no magic number, but keep in mind that every time you make a connection you add some slop to the system that only increases as things wear.

My opinion is that once you use more than one relay to break up pushpull system you had better rethink using cables. At least you can take up wear with the turnbuckels at every annual.

Hydraulics work, but for small planes they tend to be heavy. They also add more sources of single point failure. There are ways around that but they can get complex in a hurry.
 

wally

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As the designer, you get to decide just how complicated/simple/light/heavy/cheap/expensive your control system is.

As an example on my Pitts the rudder cables go from the peddals to the rudder bellcrank almost a straight shot through some nylon lined guides. The elevator has a rod from the bottom of the joystick to an idler then to another rod to another idler and then to the elevator bellcrank. The aileron is a rod (on each side) to an idler to a bellcrank to a pushrod to the aileron. A slave pushrod goes to the upper wing to the other aileron.

My suggestion is make it as simple, as straight, as robust, and as light as you can. Try to keep it easy to inspect too - you bet your life on it each time you fly.
Wally
 

PTAirco

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A lot of the perceived flying qualities of your design depend on smooth, low friction controls. Like somebody mentioned earlier, you can fly two otherwise identical Cessnas, one with new and decent controls and the other an old hack and the difference is like night and day.

I was seriously tempted by push-pull cables too, but the friction really messes up the feel and control feedback.
One of the nicest controls I have experienced was in a Fairchild 24, where even though it had a mixture of cables and rods, everything ran on ball bearings and what a difference that makes.
 

Jan Carlsson

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Tubes! anyone have ratings for AN946-RA3M4-2 and AN946-RE4M? or recommended replacement?

Jan
 

Jan Carlsson

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No takers on this?
Anyone have ratings for AN946-RA3M4-2 and AN946-RE4M? or recommended replacement?
Aurora maybe?
It isa male rod ends.

Jan
 
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