Safety Item - Rudder Springs and Hard Over

[Chris Burgess]

I received another report where a rudder cable failed with a resulting crash because the rudder went hard over to one side. Pilots, please make sure your pedal return springs are not so strong that they will make the rudder go hard over in the event one cable fails. IMHO they should be no stronger than to return the pedal to neutral and nothing more. "I'm just say'n"

Planting the tail first may help and putting any crosswind on the side to help your situation may also help. In some gyros the aircraft will yaw one way with throttle application and the other when you take throttle away.

Practice on this scenerio may also be a good idea. Check your springs. Thanks

[Mike484]

Excellent post Chris, something everyone should be aware of.

[Earthboundmisft]

Chris, in my opinion, its more than just the springs causing the rudder to go "hard over", depending on the design. Foot pressure resting on the intact pedal / cable will cause it to go "hard over", despite the rudder being hinged at 1/3 chord. A push / pull cable system may offer a chance to hook your toe under the pedal and return the rudder to center.

[Chris Burgess]

Earthboundmisft - Certainly if you keep pushing on the "good" cable, it will go hard over to that side.

What perhaps some do not know is your rudder should remain "in trim" at cruise power if you take your feet off the pedals. You rig it that way for just this failure. Unless you are at the critical "touch down" phase in your landing, a cable failure should not result in an accident.

[Earthboundmisft]

"Touch Down Phase"......... BINGO.

[M._Springer]

when I had already flared for landing and was maybe a foot or less off the ground. I touched down OK, exactly 180 degrees from my original heading.
Marion

[Hardy]

Earthbound
I had a cable failed on a push/pull air command cable system, it's a real thrill trying to get your foot under the pedal and pull it back. After landing off airport and walking back I added straps across the rudder pedals to alleviate that from happening again.

[GrantR]

Is it common for rudder cables to fail? That seems odd to me being that they are not under that much load. Mine are pull pull running over plastic pulleys.

In the fixed wing world if you break a critical control cable you would be dead

[Earthboundmisft]

I used 1/8, 7x19 stainless 'ultraflex' cable. Rated at 1700 lbs. from East Coast Aircraft Supply. Phenolic a/c pullies, with stainless cable stays surrounding the pullies. Most important, I used a/c fittings, swedged with a hydraulic machine. Cable, fittings and swedging service came to roughly $100. The pedal pressure on my gyro is in the order of FIVE pounds I would guess? One could use monofilament fishing line if properly designed....I'm just saying....:noidea:

[JRB549]

I use sleeved cables and have removed my spring after seeing what happens in the event of failure of one spring. I have a vertical stab in front of my rudder so I dont have to worrie about issues of uncontrollable yaw.

[GyroRon]

Who banged up their gyro Chris? I know of one recent one that has remained top secret due to the owner/pilot not wanting folks to know, not sure if it is the same guy

[Doug Riley]

The pedal pressure needed for normal rudder control inputs isn't much, especially if the rudder is properly designed, with some servo area ahead of the hinge. Some people may lean into both pedals at the same time because of nerves, hard landings and so on. It's fairly easy to do a seated leg press of 200 lb. or more. The gym rats can press 300 lb. or better.

Even the 1/16" cables called out by Bensen are good for 500 lb. each if undamaged (and there are two of them to absorb your best seated leg press). If the cables get frayed -- well, shame on you for not catching that on preflight.

Nicopress fittings are rated for the full strength of the cable if swaged properly. You can't just smash them in with a hammer and expect them to hold. Use a proper tool, compress them fully and they will be highly fool-resistant (nothing's truly fool-proof).

Often the attachment points on the rudder or pedals are weaker than the cables themselves. Examples: the early Snobird rudder horns were 1/16" thick or so, and would bend. Doug O'Connor cracked up his Snobird when the horns bent and he couldn't get out of a twirling vertical descent. Ken Brock's formed sheet-metal rudder pedals had integral cable horns that also were around 1/16" thick. No seated leg-pressers allowed.

A cable can jam if it hops off a pulley. Use close-fitting cable keepers on all pulleys.

The FAA requires 1/8" cables on all primary controls. I guess you could argue whether the rudder is "primary" on a gyro -- but it can be messy if the rudder locks hard over.

A tall tail eliminates the need to hold hard rudder at takeoff, since it compensates for torque roll and yaw effects. You barely touch the pedals when flying a Dominator, except when doing intentional slips and twirls.

[bmoore2156]

With the anti-servo tab on my rudder, I really have to push to get it to move. Then it snaps right back to center when you let go. I wish it was easier to push. Any suggestions?
Brad

[Doug Riley]

First, check the leverage ratio. The pedals should swing 35-40 deg. stop-to-stop (that's about as much as a non-arthritic human ankle can take). The rudder should swing 30 deg or so either side of neutral, stop to stop.

If the ratio checks out, see if springs in the system are adding to the centering force. If they are, you could switch to softer springs.

Reducing the size of the anti-servo tab is a last resort. You dont want a tail that fails to center itself aerodynamically.

[Trez]

When I saw a 'parted' cable recently, I was struck by the condition of two ends that 'parted'. They were dark (oil and metal and grit?) and when I ran my fingers over the ends, then rubbed by fingers together, it felt like valve grinding compound or hand cleaning creme that has pumice in it.

Here are my thoughts on this issue from my previous post..........:

..........Also, I would like to add to your very thorough thoughts on 'cable-ing', the issue of 'grit' that's picked up by oil or whatever, that lives in the cable, usually at the pulley(s) located beneath the engine/gearbox, etc. and starts working on the cable as it flexes and changes when it's moving for and aft around a pulley.

I have come to believe that it's like little cutting knives constantly working on and between the strands, weakening and possibly eventually causing the cable to part.

On my machine I noticed one particularly suspect length of cable in an area like that. It looked just fine to the naked eye, just a little darker than the straight runs of the same cable. But I removed the pulley and, with a rag under it, flooded the cable with alcohol and started working it into the strands. I was amazed at the amount of 'grit' that kept coming out of cable. Not in large amounts, but more than I ever suspected might be in there.

So, now, I'll be looking for 'dark' cable which to me will mean there's something going on in that area.

And remember, the area I'm addressing here is likely the hardest part of the run of cable to see since it's 'hiding' in the pulley groove (in my case, underneath) and covered by a 'keeper' and is only visible with full rudder throw one way or the other.

Sorry to post this here but thought of it as an expansion of what you had already said.

BTW guys, I got to see Rick's creation this past weekend. It's gonna be a stunner!

Trez

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[mgk]

A cable is a working mechanical device. Any dirt that gets in will eventually become an abrasive agent.

Also, while stainless steel aircraft cable looks spiffier (I hope that's a real word!) galvanized cable has slightly higher ratings.

For 1/8" 7x19 GAC, the breaking strength is 2000lbs, for 1/8" 7x19 SSAC, the breaking strength is 1760lbs.

[okikuma]

I agree, this is an excellent post with real world information. It does make one think and stay alive. It has been said that the best teacher is a mistake that one has made. I say, the best experience is to learn from other's mistakes and not dulicate your own.

Thanks Chris!

Wayne