Control Cable specs.

[JimCrawford]

Hi Chaps,

I've searched my documentation and drawings but cannot find the type of cable used for the ailerons and elevator runs. My aircraft is likely to have an original set and I need to find out if they are 10cwt or 3/32" to set up the tension gauge. Can anybody tell me what the original Mk3 (Slingsby built) cables are? Also if they are hemp cored, because if they are they will be replaced.

Jim

[Neil Spooner]

Hi Jim,
I am sure David will come to the fore and provide the cable spec's. Or give Airweld a call. I believe they are or can be 3/32".
I am sure you are aware of the AWN 28/07/1975 for aileron belcrank annual inspection. This was created due to someone overtensioning the aileron circuit and bending the belcranks. I personally would not use a tension gauge but set them up so you have minimal friction consistent with no backlash in the circuit. Make sure you lubricate the pulley bearings, it makes a heck of a difference, I always notice an improvement after an annual service.
Regards,
Neil

[JimCrawford]

Hi Neil,

Thanks for the gen. I've had a call from Dave who confirmed the cables to be 3/32". I'm slowly getting through the permit renewal on GAVTC and I've had a fine time with the aileron circuit. The port aileron has enough free motion, stick fixed, to require attention and it is wear in either the push rod end or it's pivot bolt. I'm going to replace the push rod ends with Rose joints which will also allow me to tighten the pivot bolt and use a stiff nut as the motion is in the ball end.

I've had a lot of head scratching re the control deflections. One aileron was absolutely spot on whilst the other overtravelled significantly in one direction. I can only assume the aircraft was previously signed off by only measuring the one that was correct! After unlocking the turnbuckles -why do they call it safety wire when so much blood is spilled trying to remove it through those small inspection panels? - and spending much time trying to set up the deflections I came to an interesting conclusion: There aren't enough freedoms of motion to guarantee that the ailerons can be set correctly, to explain:

With stick and ailerons locked neutral the power and balance turnbuckles can be easily adjusted so that when the locks are removed, the neutrals are good. You would then think that adjusting the stops is all that's required but that isn't necessarily true. The aileron bellcranks work in the classical differential manner to give more up than down, so far so good. However this circuit is very sensitive to the absolute geometry of the layout. Ideally the bellcrank arms should be positioned parallel with the stick fixed neutral, the push rods than move different distances as the stick is moved to give the differential effect. This is a council of perfection as the major dimensions of the system are set by a wooden wing which will not have been built to the accuracy required.

So, imagine stick neutral, port aileron neutral, port bellcrank spot on, starboard aileron neutral, starboard bellcrank slightly rotated clockwise due to assembly tolerances. The port aileron will move with the correct differential but the starboard will have less. Simples. Slacken off the balance cable and recover the tension in the starboard power cable to get the bellcrank neutral position correct. OOPS, now the stick neutral position has the starboard aileron slightly down. You have now started running around the same circles as I have. I think my aircraft had been set up to try and absorb the tolerances within the available limits but, in my opinion, the neutral position and deflections were unacceptible, even for government work.

I have a solution. I’m using rose joints on the push rods. These are threaded so allow some adjustment in the push rod length which, in turn, will allow me to set stick neutral, ailerons neutral and bellcrank position correct independently, even if it takes several iterations to get there. In retrospect the same problem is inherent in my Taylor Monoplane but wasn’t apparent because I made up the pushrods with rose joints at the end.

I’ll post some pictures and report on success, or otherwise, as soon as I have them.

I'm not at all aware of AWN 28/07/1975 for aileron belcrank annual inspection. Where do these words of wisdom live?


Jim

[Neil Spooner]

Herewith the AD:

AD 28071975.jpg (765.22 KiB) Viewed 8772 times

[Neil Spooner]

Do you have the rudder pedal AD Also?

AD Rudder pedals.jpg (546.62 KiB) Viewed 8769 times

[Neil Spooner]

......another good reason to have the rudder mod' !!

[JimCrawford]

Thanks Neil,

I haven't seen any Nipper ADs except for the two you have copied onto the forum. It seems as if there is a bit of a hole here as there isn't a TADS document for the Nipper, so all the ADs and various mods have disappeared into the bowels of the PFA and aren't published by the LAA. There might be something on the SPARS thing but I believe that is restricted to inspectors. If so then I find that a rather strange mode of operation by the LAA. The BGA site should serve as a model here as all the information about particular aircraft types is available on line and I find that invaluable when performing an annual on a glider.

Jim

[Neil Spooner]

Hi Jim,
No, there is nothing in SPARS #15.
Regards,
Neil