Control cable tension design guide

[PBPouw]

Hello,

I am working on the design of control cables of a mid size twin engine aircraft. I have a hard time finding data such as :

advisible unsupported cable lenghts / diameter cable

advisible cable tension during rigging operation

advisible minimum distance between cables / structure

a formula with :
input = free length, cable, cable tension, g force
output = sag in the middle

A help will be more than welcome,

Pim Pouw

http://www.Fokker-g1.nl

 

[Sparweb]

My first guess was AC 43.13-1b, but a quick look through that came up pretty dry. It only covers inspection and construction.
Aircraft maintenance manuals will give some information about cable rigging; once cables are removed for maintenance, they must be re-installed with the correct tension.

The rest of your questions could be answered by analysis and tests of your own. Examining a few existing twin-engine aircraft yourself would be of the greatest value.

The mathematical model for a flexible cable under tension and sagging under its own weight is called a "caternary curve". For this purpose a parabola is close enough, but maybe the math for the caternary is easier for you. Don't forget to include g-loads.

Seeing that you're restoring an aircrft (judging by a glimpse at your website), why isn't this already defined?


Steven Fahey, CET

 

[PBPouw]

This project ( Fokker G1 Replica ) is not a restoration project. It's a reverse engineering effort using only photographs and surviving stress reports.
Basically I am designing an aircaft with the difficulty that it has to look a certain way.

Pim Pouw

 

[rb1957]

have you done an analysis of the forces in the control systems, due to the load inputs (pilot force, aero. loads) ? FAR23 is probably a reasonable source for load cases.

with this information i'd say the minimum cable tension would be larger than the maximum compression load you calculated above. cables should always be in tension, if they are in tension they can resist compression loads, so model them in the control system as rods. i suspect that too high a tension load increases the wear on pulleys and quadrants, as well as probably affecting the "feel" of the controls (maybe making them very stiff).

obviously, all the technical (and maintenance) data for this plane has gone missing (as this would probably answer your question).

 

[spider007]

will this help?

http://www.spaceagecontrol.com/calccabm.htm

PBPouw, i can't imagine doing a project that size, even this gets too big sometimes, and it's only paper to cad.

http://www.freewebs.com/caddraftsman/se5a.htm

i wish you luck.

 

[PBPouw]

Thanks both RB1957 and spider007

Remark on RB1957.
When you speak about compression loads on a cable, I think that what you mean is create a FEM model, put pre-tension in the system and then look with maximum control loads in the system if the none actuated cable loads go to zero or compression. If so increase the pre-tension. The problem here is that to create a FEM model of the aircaft is very time consuming and I think the knowledge of creating a wooden wing is none existing.

I have all the original stress reports but these don't mention pre-tension. I think I will look into some maintenance manuals of similair sized aircaft to get some idea were to start.

Remark on Spider007
Thanks for the link this was exactly what I was looking for.
Nice work on your SE5, are there still original drawings around, because I know there are a lot of flying replica's?

My project is indeed huge but my driving force is not the fly one or to see it flying. I love the engineering side of this project. I have developed a huge respect of the engineers before the digital age. Sofar I haven't found many construction solutions with which I disagree. To me it looks that the only field of airframe design which has made real progress since WW2 is stress.

 

[spider007]

Yes, there is only one usable set of drawings (that I know of)....the trouble is, each factory did their own thing, and parts on each plane ware adjusted differently....So trying to fit it all together so that it assembles in catia and at the same time keep it historically accurate can be frustrating sometimes.

Yes, I’m amazed (and a little jealous) of the engineers of that time. Makes me think, we are a little spoiled, with the computers, and all the expensive software (and we still whine and complain sometimes).....
All it takes to remind you is to go to an air show and look at one of those things up close.

 

[rb1957]

hi pim,

actually i meant to analyze the control curcuit by hand. but if you've got the original calcs, then this is gold. this'll show you the loads in the cables. but they might have analyzed the cables as tension only, which is a simplification and imples that there wasn't signifiicant preload.

at least you can do an ultimate stress analysis (control load+preload)*factors < strength of cable, and a fatigue analysis. i'd've thought that one critieria is that the cables need to be "reasonably" taunt; i can't visualise a slack cable being a good thing.

good luck