Flex cables vs. Push rods

[Avnx EO]

I am trying to separate fact from fiction (or friction as the case may be.) In looking at various options for fixed controls in new designs, the debate comes up about using push-pull rods or push-pull cables (Teleflex or similar) for primary flight controls.
As an avionics puke, this is really outside my area, but it has some avionics repercussions. (automatic flight controls, etc.) But mostly, I’m interested. I’ve heard all kinds of stories, but I’m not sure which are true and which are lore.

1) First, do pilot’s and maintainers even care which they have? Or is this issue as important as the color of primer paint used under the floor?

2) I’ve been told Flex cables can freeze up. If you get moisture into them, and then get into a freezing environment, they can literally ice up. (I would think that would be a severely bad thing.)

3) I’ve been told that the cables stretch and give – making the control somewhat loose, and requiring more frequent rigging maintenance and adjustment.

4) I’ve been told they require more frequent lubrication, replacement, etc.

5) I’ve been told that cables have to have boost, that they can’t handle the control loads directly, even in a limp-home scenario. That it’s a crash-and-burn situation if you lose boost.

6) I’ve also heard that the push rod designs are more the result of the ballistic tolerance required by the “military heritage” of most helicopter designs, and that in a pure commercial / non-combat environment, cables are just as good.

Fan or Flame away..

[Avnx EO]

:confused:
Judging from the total lack of response, I guess the "paint" answer wins. Gee, some of our company pilots were pretty hard over on wanting nothing but push rods. I guess the real world couldn't care less. I am surprised that in this forum, absolutely no one had an opinion.

[Dave Jackson]

Erik,

Some of the following comments may be incorrect, but for what there're worth;

A few homebuilts have used push-pull cables.

The Rotorway or Scorpion uses or used two push-pull cables for the collective, both as pull.

The CarterCopter uses a new type of ball bearing push-pull cable, which has very low friction. They used to use hydraulic.

Perhaps some reasons for rods and bellcranks are lower friction, minimal play, and easy inspection.

Hope this is of some value.

Dave J.

[[email protected]]

Erik, I think purely in terms of maintenance required, proper push/pull rods with bellcranks and levers are better than cables. Cables stretch and need a tensioning device or regular servicing - they can also saw through their guides and into other parts of the airframe. As long as you have a good system of NDT (Non Destructive Testing) to check for cracks, control rods are the way to go.
I have only experienced teleflex type cables in engine controls not as flying controls and they are notorious for seizing, chafing and being difficult to set up.
I think the final clincher is applying hydraulic assistance to push/pull rods - easily done compared to cables.

[sprocket]

Teleflex cables up to a hydraulic servo may work, but because the inner working part of the cable is hidden inside an outer sheath it makes it virtually impossible to detect wear during maintenance, unless complete disassembly is carried out.
On plank wing a/c Teleflex type cables have been used on trim controls between actuator mechanisms, but I have not seen it used on primary flight controls. I guess the use of actuators, can be a safety net, ie; if cable breaks the actuator will stay in its position and not move, which is probably OK for a trim system. The disassembly and rerigging of these type cables just to inspect for wear/lubrication etc makes them a pain in the a*se for maintainers even though they make for a lightweight and apparent simple design.

Cable (open) controls in helicopters (as far as I’m aware) have only been used on tailrotor control systems where loadings are lighter than main rotor controls. Also cables (depending on how the system is routed and loaded) can wear a lot quicker than control rods.
Control cables are harder to inspect for serviceability than the average control rod and sometimes require removal to enable proper inspection.

Control rods are not perfect either as rod ends are continually wearing but it is usually easier to replace a rod end than to replace a control cable.

Now then, back to the colour of the paint under the floor ……
:)

[heedm]

You could show me all the engineering charts you have, but I will still feel more confident with rods controlling my rotors over cables. Confidence becomes a real factor when you try to sell your helicopter.

I imagine rods come with a weight penalty, especially for the tail rotor. Also, it seems that FOD would present more of a problem with rods than with cables.

As far as the paint under the floor, just don't make it the same color as any of the fluids that may leak down there. Kinda like those ambulance companies that use red sheets...can you be sure you've got a clean set????

[Avnx EO]

I had heard, and then later saw a cut-away showing that the EC-145 uses push-pull cables for primary controls. The drawing indicated that they run a set of cables up the windshield center post to a set of boost cylinders in the overhead. I suppose you have to do something like that to get that kind of a wide-open cabin. I’m not sure whether that’s inherited from BK-117, or whether any other Eurocopter models do the same. Is that innovative or scary?

[chuckolamofola]

If they do use teleflex cables in the EC145, don't forget this aircraft is hyd boosted so normal control forces would not be seen on these devices.

I have seen teleflex cables used in the N2 governor controls on the BO-105, but standard push/pull tubes are used for primary flight controls. Same goes for the BK-117

I don't think you would want to use teleflex for primary control, there are other factors involved besides freezing, as they do corrode and wear over time thus causing restricted movement.

You did not mention if the aircraft is to be certified or not, if it is suggest you look at FAR part 27 or 29.

Its hard to beat the push/pull tube method of control, especially if you plan on using armstrong steering.

[sprocket]

I’m not sure whether that’s inherited from BK-117, or whether any other Eurocopter models do the same. Is that innovative or scary?

I am not familiar with the Eurocopter heli's either, but with hydraulic servos used in flight controls, the input from the cyclic and collective needs to be as 'friction free' as possible to prevent jerky control movement. A Teleflex type cable has virtually all its surface area in contact with the outer cable, giving it maximum opportunity to bind if lubrication is lost or if contaminated with grit etc. and we all know how well helicopters can stir up dust.

I would have to guess that the push/pull cables in the 145 have a better or new type design than the standard cables currently in use on other A/C. Perhaps they have a throwaway life.

[John Bicker]

Push Rods versus Cables

With regard to the use in the EC145 etc. The AS365N models have had cables all along. In these aircraft the difference between push rods and cables is somewhat to do with the cabin layout. In most of these types it is a moot point because they all have boosted hydraulic controls. None of these types can be flown without boost so the cable or rod only controls the servo pilot valves and nothing more. Without hydraulic fluid or pressure from at least one system you are toast. One thing that is frowned upon in these aircraft is how much you do in the way of training with one system switched "off". You need to be very sure that the other system is fully functional and need to restore the other system very quickly if a problem arises. Definitely "fly by fluid". The PP rods in an Agusta 109 or S76 etc. are just big enough to control the pilot valve and are in fact very small. There is no way they could transmit any control loads to the main rotor. As far as both are certified and have been used for years with no problems it appears to be in this case a moot point. Cables may be easier to run in difficult runs than rods. As for manually controlled aircraft rods would appear to be more favorable as already discussed from an inspection/maintenance point of view.

[Rob_L]

I'm surprised that no one has mentioned that the SA 365C Dauphin had a single channel push pull (Richoux) cable to the tail rotor controls. The servo was a single system only therefore the cable was also the manual reversion in the event of hydraulic failure. The only problem I ever encountered with this during 4 years was where the cable covering had been allowed to deteriorate allowing water into the cable inner. Corossion caused stiff controls. Basically poor maintenance.
The design of this cable was in effect a solid steel flexible blade running in a series of small bearing.

Interesting that Aerospat returned to conventional rods with the 365N series. I suspect this was purely a commercial rather than a safety issue.

[3top]

Hi,

as long as you keep anything there to control, but wires or light!

Check out the following:

http://commercial.visi.net/eaglesperch/ (http://)

Last time I read about, they used a dual hydraulic system based on car brake systems ("when did you hear last time from some one that had an accident, because the brake system failed..." - worn brakepads or overheated don´t count!!), Interesting concept (Dave...?). The cyclic controls hydraulic cylinders which push hydraulic fluid through tubing and hoses and hydraulic cylinders again, directly to the rotor....

What I don´t like on this machine is the inability to autorotate (fixed pitch rotors...)

3top,:cool: