Push-Pull control tube fabrication

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TFF

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There must be more to the reference because you would be in trouble flying about half of all homebuilts. Pitts, Vans, and who knows how many others along with certified planes.
 

wsimpso1

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I saw an EAA reference that said NOT to use threaded rod ends for push-pull tubes more than 8-9” long for the entire assembly. Do you folks agree?
I do not agree. I think we need to see the reference.

There are male and female threaded rod end bearings and eyes and forks for connection to the next elements all available through the aircraft supply houses and widely used. There are male threaded rod ends for installing into some standard size tubing and some are AN standard parts. Then there are lots of made-to-suit tube ends for tubes, both male and female threaded. These parts are all built up into push-pull rods from inches to several feet in length, and widely used.

When someone is designing these parts, I strongly suggest the designer be friendly with Euler calculation of column buckling and know the biggest column load that will be put on this push rod, then check that it is safe against buckling at that load with suitable factor of safety applied. Note that Euler criteria uses the entire length of the rod, from bolt hole in the end bearing to the other bolt hole in the bearing.

Old FAR Part 23 had guidance on min control stick and pedal forces the designer should use absent other info. Many small airplanes have control surface moments too small to get to that level. If one uses the biggest control moment methods, the designer had better also include control stops at the stick and pedals to prevent overloading control system components at travel limits. Control stops at stick/pedals and at control surfaces are a really good idea anyway.

In long slender tubes, the tubes may need guides at intervals to prevent buckling. In some designs an idler or idlers may be placed, breaking one very long tube into two or more shorter tubes, and able to stand the load without buckling.

In really short pushrods, you may not have room for tubes and tube end fittings. The designer may be tempted to design with a short piece of threaded rod or to turn a short piece of steel with threads on both ends to make it work. Again, run a buckling check using the section characteristics of the part at and near mid length to make sure you won't buckle the short rod. Threaded rod used as the rod may be particularly length limited for these purposes. I have two such push-rods in my flap system, and they have been checked against column loads possible from control moments and reviewed for other possible loads such as a person stepping on the flap while embark/disembark.

If the rod is just a little too long for threaded rod, the alternative is to turn a larger rod for threads on the ends. Making the rod from hex stock and using opposite handed bearings makes such a rod particularly easy to adjust when it is time to rig the system.

Billski
 

wsimpso1

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I read it again and my new interpretation is that they were referring to using ONLY the threaded rod and rod ends (with no tubing) as the assembly. In other words, the control rod was just a bolt threaded at both ends with the bearing ends screwed on. Here it is in context:

The author is no less than Tony Bingelis. We should all thank Tony's soul for all that he put down in one set of books for us to learn from.

Tony did not tell us the limit on threaded rods is some number. I quote from the cited article "Threaded rods (male and female) are most effective in shorter lengths ... This fabrication method may be practical for push-pull rod assemblies not exceeding 8-9" in overall length." Tony ended the paragraph there.

We are back to where I started with my comments. The designer of a pushrod should check that the section chosen is suitable for the column load and length of the column using Euler and Johnson column buckling equations. See Shigley to apply both theories. I have two such pushrods each built of a short piece of 1/4" UNF Grade 8 allthread, two AN-316 check nuts, and two female threaded spherical bearings. At the lengths and loads I have for these rods, they calculated out to being quite resistant to buckling, so I did not have to start with a thicker rod and thread the ends.

If we have an airplane already designed and copies have been flying, most of us just build to the plans. If we are designing new or making pushrods longer or changing column loads, we would be wise to run the calculations for pushrod column loading and check against buckling.

Billski
 

wktaylor

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Push-pull control tubes are elegant/simple solutions to systems that require back-n-forth movements... as opposed to cable systems that drive movements by [2] tension cables with lots of moving pieces/parts.

These elements of designing with PPCTs must be accounted for...

Shear-loading of rod-end fastening in high-tension/compression load reversals
Bowing/buckling of tubes in compression
Vibration modes for long unsupported tubes
Internal/external defects that can affect serviceability... such as dents, gouges and corrosion.
Tubes that are NOT made precisely can also be a factor, IE permanent/obvious: lengthwise bowing/distortion [aren't 'arrow-straight']… and, more subtly... have internal/external diameter miss matches resulting in wall thickness differences... that can lead to premature buckling.

In aircraft manufacturing [cost VS value-added] there are special/rigid standards for push-pull control tubes. 'Straightness' and close-wall-thickness tolerance, end-to-end are mandatory. Also, specific aluminum alloys/tempers are often specified, such as 2024-T8511 or 7075-T73511 that are stretched/straightened and resistant to stress and exfoliation corrosion... with very good toughness.
---------

NOTE1. Threaded-rod-ends MUST be secured with a mechanically locked nut... preferably NOT just a 'self-locking nut, IE: double [2] thin plain Jam nuts locked together and marked with compound to show any loosening; a single plain thin jam-nut lock-wired appropriately [lock-wire holes thru hex-corners]; castellated nut with cotter-pins; locking tab [bent] washers; etc.
 
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Heliano

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Yes, wktaylor, Threaded-rod-ends must be secured with a mechanically locked nut, locked with lockwire. The main reason a nut is used is to ensure that the load on the threads are always in one direction. As for the length I can see no reason why threaded terminals should be limited to a certain length. The relevant aspect related to length is, as mentioned by wsimpso1 and others, is column buckling. Minimum certification control forces as per Part 23 (23.397) are 100lb longitudinal, 40lb lateral and - mind you - 150lb for pedals. You bet: a stressed pilot, with a full load of adrenaline in his blood, easily reaches those force levels. As the forces in push pull tubes are the stick forces multiplied by a factor like 4 or 5, forces can easily reach 1000 lb in push-pull tubes. One should used the euler column buckling formula ( F = π2 E I / L2 ) and a factor of safety of 50% - to cover for corrosion, misalignment, dimensional inaccuracies, etc.
 
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