Unidirectional Carbon Tube as Control System Rod

[wantobe]

Bob, I do care about all of these! Thanks!

I do not think that a contol system has significant impact loads. Anyone has specific number on this?

I think some considerations are similar to those in determining design allowables for composite structures, namely, the effect of temperature, moisture and impact dammage. It would be wise to check that neither maximum tension nor compression strains of the tube exceeds the design allowables. I expect these tubes to be buckling limited, not strain limited.

[Frank Giger]

I don't know anything about composites, but if you want to know about controls and impact forces you should come watch me do touch and goes in a Champ.

[tonycondon]

It looks to me like Dick Butler used carbon rods for the controls on Concordia. One of the pictures at least showing the final control setup appears to have a metal rod end bonded and riveted to the carbon push rod, similar to as described earlier. It also shows how they set up a load test on the system to ensure the strength of the system.

http://soaringcafe.com/2011/04/build...ontrol-system/

[58boner]

OK just jumping in here with a bucket of cold water. Every composite structural or control surface part on certified aircraft I inspect on a regular basis (Gulfstream) has a mandatory periodic inspection for DE-lamination. Would you include such an inspection program for your push pull tubes?

[Bob H]

You would do a visual check periodically on composite tubes, much like you do on metal parts looking for corrosion or deformation. Delaminations on large area parts like a control surface are initiated by interply separation from resin matrix breakdown usually due to an external impact force or very high cycle fatigue forces. Normal commercial aircraft inspection techniques use ultrasonic interrogation to map delams or disbonds, followed by a judgement as to repairs/replacement. On a homebuilt plane with composite tubes, the primary loading is axial tension/compression with buckling as critical mode of potential failure. If tube is designed correctly, I would not expect normal operations to produce delams and a visual check during annual inspections should be ok.

[Thomas Stute]

I just dropped into this thread and would like to add some comments based on my composite engineering experience from the last 30 years in aerospace structures.

In general a major consideration should be given to the diameter of the push/pull tube. This defines stiffness and buckling resistence. The larger the diameter the better the tube can withstand buckling. However, pushrods in an aircraft control system usualy are not exposed to ery high loads, so this problem should easy be solved by a simple calculation.

In order to stay on the safe side the amateur or homebuilder with no experience in composites should stay with standard carbon fibers like the Toray T300 or similar since the higher the Youngs-Modulus of the fiber the lower is the allowable elongation until rupture and the more critical it is to process.

Commercially pultruded tubes/rods have a mainly unidirectional fibre orientation and hence are critical when torsional loads would be applied, e.g. during screw tightening at integration steps. To be safe you should design your tubes with some +/- 45° layers. Braided carbon sleeves come in handy when opting for angles different from the tube central axis.

When you want to manufacture the tubes by yourself use an aluminium mandrel and cure the tube at elevated temperature (e.g. 60°C or whatever temperature is best suited for your resin system). After curing and cool down to room temperature the aluminium mandrel will shrink and will enable de-moulding. The fibre volume content and the consolidation of the laminate can easily be achieved by finally wrapping the lay-up with a band of peelply under tension. Surplus resin will be forcd out and with it air and voids.

Tube end fittings is a theme: This are the points wher the loads are introduced and they need to be designed with sufficient care. The easiest way is to design the fittings with the inner diameter of the tube and to foresee a circumferential glue pocket for bonding layer thickness of 0.1mm to 0.2mm. A major problem can be the material of end fitting due to the CTE mismatch between CFRP and the end fitting. Avoid aluminium (CTE = 23.5 E-6 1/K) and take stainless steel (CTE = 13 E-6 1/K).

When selecting the adhesive for bonding the fitting into the tube don't use a mixture of epoxy resin with some additives for making it tixotropic but take a certified and proven adhesive like HYSOL EA 9321 or Scotchweld 9323 or something similar. These adhesives are designed especially for structural bonding and have some additives to improve their toughness. Epoxy resins are too brittel for critical bondings like in the aircraft control system. For calculating the necessary bonding area don't use the nominal lap shear value given by adhesive manufacturer (usually in the range of 20 MPa to 30 MPa) but use a reduced value to account for irregularities like poor bonding surface treatment, etc.

Don't rely on a single load pass (bonding) but use an additional one, rivetting. Rivets should be used only after the adhesive has completely cured. In order to avoid corrosion don't use aluminium rivets or steel, but titanium rivets. Apply some glue when inserting the rivets.

Once ready you should expose your tubes to proof loading for being sure your workmanship is fine. A good load is the design load. Don't overtest, since you want to have the inbuild margin for emergency cases.

The are just a few aspects to avoid the major mistakes.

[wantobe]

The depth of this forum always surprises me!

In order to stay on the safe side the amateur or homebuilder with no experience in composites should stay with standard carbon fibers like the Toray T300 or similar since the higher the Youngs-Modulus of the fiber the lower is the allowable elongation until rupture and the more critical it is to process.

This is kind of a compromise between process and performance. Young's Modulus is what really matters in buckling resistance, yet we can not use higher ones due to process sensitivity.

Don't rely on a single load pass (bonding) but use an additional one, rivetting. Rivets should be used only after the adhesive has completely cured. In order to avoid corrosion don't use aluminium rivets or steel, but titanium rivets. Apply some glue when inserting the rivets.

Where can titanium rivets be found? I can not find it on aircraft spruce website by searching it on that website.

[hogheadv2]

Diameter is a factor over thickness of the tubing. I would not recommend "Rod". The carbon rod I have seen, (extruded) was stiff bit also flexed with compression forces. A carbon arrow tube 1/4 the weight was far more stiff and rigid. I understand this is not control actuator size but my feeling is the same would be true. *** I have considered making and testing custom carbon tubing using a pipe or bar as a mandrel tape wax paper the length needed wrap it and the wrap with the carbon / epoxy (pre-wet) squeeze out wrap the pea-lply and vac. [this may be much easier with a few helping hands. /// Hoping compressed air would release the tube and non-destructive and destructive testing with flex and strength would begin. [ work = money = weight ] Is it worth it,,, or just go without the French fries ....

I stand to correct myself, Looking further I have found CF tubing of a far greater strength and quality than any I've seen before. Not that I can endorse the use of these. http://www.cstsales.com/Carbon_Fiber_Tubes-ss3.html I could see these used as control rods.... With proper testing for application.

[Bob H]

There are appications where composites make sense and some that don't, especially if you are homebuilding and have limited equipment resources. If you are playing around, have money to spend and want to practice some composite fabrication techniques, then making tubes is a learning exercise and the first few may not turn out just right. When measuring the overall weight savings, you need to include the end fittings which usually makes up over half of the strut weight. Assuming rod end bearings that have to be attached via an insert bonded into the tube end, the tube dia may have to be increased just to accommodate the fitting. And you would want the fittings to be removeable to change rod ends should they wear. By the time all is done, it's easier to use an aluminum tube off the shelf, slide it into a female rod end and fasten.

Back in the late 1970s, I made parts for the Gossamer Condor, the human powered vehicle that went across the English Channel. These guys were extremely weight consicious where saving a fraction of an ounce was a big deal. I redesigned the pedaling hub out of composites to reduce the weight by 2 oz and thought it was not worth the expense but McReady's guys thought it was a major savings. For most engineering weight reduction choices, the cost/lb has to enter the decision process and sometimes forcing a composite solution just isn't worth the effort.

[Thomas Stute]

I just don't know where in the US you can find Ti rivets. I think you have to check in the internet. Manufacturers are Hi-Lock, Shure-lock, Huck and some others. For my needs I would get a few of them directly from my company. A good source could also be suppliers for aerospace industry. There should be many in the US.