Eliminating Play from the 4-Place Control Sticks

[AZBearhawk272]

I second the “do no do that “vote.
Control system components need to be securely attached. Any looseness is potential contributor to flutter. As far as vibration goes, chase the source and eliminate it whenever possible.

Kevin D
#272

[marcusofcotton]

I've placed a piece of silicone "baffle" material as a dampener in-between the airframe and the rudder pedal attach point, and plan to do the same to the stick assembly - trying to isolate some vibration from that direction. My line of thought was that the cable itself is a dampener, so with dampening the airframe attach point, I may reduce the rate of play increase. 2nd line of thought is that with the areas of rotation won't be perfectly in line, (once attached to airframe) and our friendly tolerances, the dampener would allow things to line up. I've had difficulty seeing any failure modes introduced by doing that... Thoughts/feelings/concerns?

(warning: there is no reason to do what I posted, I am not a licensed professional, barely even a bearhawk builder so far)

Nice idea, but I fear that give may increase the chance of flutter.

[Battson]

I think that is very unlikely. Maybe if you disconnected the cables...

[triumphantduke]

thank you for the feedback! I wish we could conference call this -
based on what was brought up, some clarifying/additional statements:


1) Degradation of Material Over Time:
I thought about degradation of the material over time - and thought it wouldn't be a sudden thing (noting its cooler and not in UV light, cabin env vs cowling) - therefore any degradation or slop would be discovered same as it is now, without action gradually over time. Setting a replacement cadence is also a risk mitigator. You could reach down anytime and "poke" it to see if it is getting brittle. The material is quite dense, and not so "spongey" that it feels like slop was added. But it does allow the concentric circles, to be more concentric where there is rotation around the frame attach points. Safety wiring could mitigate concerns about the nut backing out.

2) Flutter (critical to prevent!):
I could not feel additional "play" with this on while I was 'testing' with/without - and at least for the rudder this seems moot since there is a designed spring in that cable system.
For the control stick - when you preflight and do a wiggle test of the control surfaces(I think elevator would be the highest risk for this) - is there "no" play what so ever? If that's the standard then ok, that finding would introduce some hesitation to me - but then again a dampener shouldn't excite an oscillation (flutter) it should do the opposite.


3) Don't Fix What's Not Broken:
I perceived this as a problem to initiate consideration for what could work as prevention. Are there flyers that have no growing slop-from-vibration issues over time?

<I do have it written down as a question to Bob, with intention to follow his guidance, I am a beliver in the Mess Around - Find Out phenomenon >

The surplus quotations are to try to emphasize subjective words..

some EAA reference, utilizing my PHD in google-lytics: "Avoid free play or slack in the control cables. Stiffness in the control system does have a useful damping effect on the control surfaces further inhibiting flutter tendencies. However, this should not be completely relied upon as later, in service, the wear and occasional lubrication could free the system of much of its original friction and result in an increased risk of flutter." - https://www.eaa.org/eaa/aircraft-bui...utter-problems


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[Battson]

Jonathan,

Can you elaborate a bit more on where your tensions were and what the issue was?
I have been tightening up turnbuckle by fingers until the deadband slop just goes out of the system, have not yet put a tension meter on the cable run.

Kevin D
#272

Control cables can wear out alarmingly quickly if there's too much tension, such as at the pulleys under the floor or perhaps a fairlead.
As little as around 100 hours operation with the cable tension a little too high, can cause strands of the cable to wear through and break - enough to force replacement of the cable.

My main concern was some of the control cable wear points are hard to see without considerable disassembly of the floorboards, and a casual inspection could easily miss the wear and tear, with a possible consequence of more serious damage occurring to the elevator control cable before the issue gets picked up. I consider that fairly serious.

I always advocate for running a cotton rag over the cables near turnbuckles, it always catches on any broken strands. Often times it finds issues when a visual check would have missed them.

[gregc]

A research paper that supports the negative effect of excess tension on cable wear is here for those of you that like to see numbers.

https://www.researchgate.net/publication/334590273_The_Fatigue_Life_of_Cables_in_Aircraft_Flight_Control_Systems

[triumphantduke]

Spoke with Bob today- if reiterating conversations with Bob is not appropriate then please delete..
-he has not heard of or had this issue himself
-if you start with tight tolerances/fit then a problem shouldn’t develop
-the assembly is not built for bushings, recommend making/buying a bolt/using washers/etc to get the dead zone one out (for top left right pivot point)
-he saw no reason to add anything below, conceded if it was to get better alignment (my rudder pedals had some binding without adding shims for better alignment of the rotational attach points), in that case best to make sure proper torque can be achieved and monitored for replacement when needed

[Nev]

Today I removed the standard AN4-23 bolt from the left control stick and replaced it with a close tolerance bolt.

The original bolt measured .2455 inches, the close tolerance bolt measured .2485 inches. The difference was enough to remove most of the play to a point where I'm pretty happy with it. I'll do the right stick as well,

There is still a small amount of play in the front bell-crank rod ends (between the seats) coming from those AN3 bolts so I think replacing those AN3 bolts with close tolerance bolts would improve that too.

This then just leaves the main bushing blocks lateral movement which is more noticeable now with small lateral stick movements.