# Carbon fiber lift strut attach points

### Help Support Homebuilt Aircraft & Kit Plane Forum:

#### flyingforx

##### Active Member
As I have been browsing around for some lift struts for my pacer project, I came across carbon fiber masts for sailboats and have wondered if there has been any tested carbon struts on aircraft and what the most acceptable process to create attachment points to each end. I was originally thinking that one could potentially use milled aluminum or steel blocks attached by bolts, but have read that may create corrosion and weakness around the bolts or rivets. It appears that belite has tested carbon struts on the bench, but I was unable to see the ends. I searched around for other sources but was unable to find anything regarding this.

#### rv7charlie

##### Well-Known Member
To each his own, but I were building I would see that as a corollary to the old 'if you gotta ask, you can't afford it'.

I wouldn't know anything about, for instance, buckling strength (negative G situations) of a shaft designed for pure bending loads.

That assumes we're talking about a homebuilt version of a Pacer, of course.

#### wanttobuild

##### Well-Known Member
My favorite subject

Attachment points have most people scratching their head, including me. Very little information available about the holy grail of carbon tube attachment points, for good reason, it is a very technical subject.

A lift strut out of carbon, for me, would not be the place to start. I would stick to Aluminum.

A decent place to start with carbon tube would be Elevator push tube. Bearing attach is easy and the 4130 method of small tube through a larger tube is doable.

Fiber path and fundamentals.

#### raymondbird

##### Well-Known Member
HBA Supporter
Mike Patey has a great video you might find interesting. Way stronger than the metal rod it's replacing and way lighter.

#### wsimpso1

##### Super Moderator
Staff member
Log Member
The big picture question is "how do you do hard points in composites?" It has already been answered many times, and, probably can be revealed by use of the Advanced Search tool.

The headaches in composites include connecting them together. Hardpoints in composites tend to be heavy, and can result in going with more traditional materials unless you have other compelling reasons for their use.

Threaded fasteners bear on metals most lightly and compactly. Where volume is available and large diameter thinwall tubes are used as pins, stuff like Garolite and some fancy polymeric bearings can be used. All must be sized with generous FOS.

Rods can be bonded to many metals, and you are wise to use steel, stainless, and titanium. Highly reactive metals (mostly alloys of aluminum and magnesium) can particularly problematic for long term bonding and for contact with carbon, but can be mechanically locked together. Think bolts and rivets with suitably large FOS in all failure modes.

For things like lift struts, where they tend to be large thinwall sections to give enough buckling resistence under compression loads, I would be likely to investigate Garolite and thinwall tubes plus nice fairings. Search the design space by writing all of your stress equations, FOS, weights, etc, in Excel, then run through standard bolt and tube sizes, lengthening them as needed. Flag the adequate ones and compare to conventional aluminum design for the same situation. If it saves a bunch of weight and/or drag, it might be worth building that way. If you must match thermal expansion or solve some other exotic problem, by all means, go to the fuss. Just remember that most of the time, we only need to do simple stuff the lightest overall way. WEIGHT IS THE ENEMY.

Billski

#### wanttobuild

##### Well-Known Member
raymondbird

I would like to thank you for posting that video.

Mike is my hero! I watch his videos, but I missed the one you posted, and man oh man, it is full of useful information.

An interested person can learn ALOT watching that video!

I am gonna take some time to digest what I have learned.

I missed the failure points of the steel flying wire and the load where it would yield.

Thanks again man!

#### wsimpso1

##### Super Moderator
Staff member
Log Member
Mike Patey has a great video you might find interesting. Way stronger than the metal rod it's replacing and way lighter.
Actually it is way stronger at about the same weight. Mike figured another 0.9 oz for the foil shape, then there is the resin and rivets at each end. If it were same strength, yes, way lighter, but the parts are same weight, much stronger, which he needed.

#### flyingforx

##### Active Member
Thanks for the replies everyone. I haven't been able to get on here recently due to work. The attach points have been my biggest concern, there is a sailboat mast company that is willing to design and build me a set. $5k for about 10 lb savings vs$2k for steel or aluminum. I don't think the cost and pioneering is worth the weight savings so I might stick with stock steel or Aluminum.