Lift Strut Straightness

[JoeRuscito]

I was working on fitting my lift strut speed fairing yesterday and had the wings back on the fuselage to locate the holes for the jury strut attach points in the fairing. I rigged the jury struts by the measurements I took after installing them. Everything looked good so I went on and attached the fairings. Then I noticed that one of the fairing trailing edges doesn't quite look perfectly straight. Im not sure if the center of the plastic fairing sagged slightly as the hysol dried or if the lift strut is slightly bend in the middle, but the trailing edge it is out of perfectly straight by a little under a 1/4 in. Now Im concerned somehow my jury struts might be out of perfect alignment. To be safe I wanted to get your opinion on whether a 1/4 inch out of straight could be problematic, from a structural standpoint. Im likely overthinking it, but I want to be sure.

Thanks

[Dave S]

Joe,

As far as the lift strut tubes are concerned, I'd go with the statement in the build manual black box which says "The lift struts must be absolutely straight....."

My kit was from the old defunct skystar company; and, I put considerable measuring of everything and modifications into the jury struts to make the jury struts perpendicular with the wing and the lift strut tubes absolutely straight, up and down, fore and aft. I ended up cutting the upper ends off the jury struts and welding in new sections of short tubes to make everything square and straight.

An engineer would be better equipped to give a perfect explanation why this instruction is critical; however, logic would seem to indicate a lift strut that is bowed would have less compression strength, like what a person would get with a negative G load like a hard landing or some interesting maneuvers.

I don't know that measuring off the fairings would be sufficiently precise compared to measuring off the LS tube.

Your measurements off the jury strut and the lift strut before the fairing is put on are what counts, providing the fasteners and clamps are in the same position as when rigged.

[JoeRuscito]

Thanks Dave, I’m positive they were straight when I set them up. Just trying to get a bearing on acceptable tolerances to calm my mind now that the fairings are on and I cannot measure the strut directly. Looking down inside the fairing I do see a slight bow, I may just slide the lift strut attach point down the rod a bit to pull the list strut up slightly and not move the jury strut too out of perpendicular with wing strut and airflow.

[Esser]

I found the struts sag ever so slightly when they are only supported on each end. For me hooking up the Jury strut pulled it to where it was supposed to be

[JoeRuscito]

Agreed, and I remember that when I was rigging the struts. This looks like about that much.

[colospace]

The fairings can easily twist prior to the adhesive setting up. I would first verify whether the lift strut is bowed, or if it is just the fairing trailing edge due to twist.

[David47]

Joe,

As far as the lift strut tubes are concerned, I'd go with the statement in the build manual black box which says "The lift struts must be absolutely straight....."

An engineer would be better equipped to give a perfect explanation why this instruction is critical; however, logic would seem to indicate a lift strut that is bowed would have less compression strength, like what a person would get with a negative G load like a hard landing or some interesting maneuvers.



Dave S/Joe, as an Engineer, your comment re compression strength above is correct. The struts should be as straight as you can make them. Compression strength falls of rapidly with increasing bow, even with the Jury strut support.

[JoeRuscito]

Thanks David. I talked with the factory yesterday and was told that the bow Im seeing would be within safe limits. That being said, I do agree that the bow reduces the compression strength. I could slide the rod attachment point closer to the fuselage a cm or so which should pull the center about about the same when I get around to final assembly. This will however, slightly alter the straightness of the jury strut.

[efwd]

I was stressing out on the Jury Strut installation until I went to Flabob Airport and seen a Piper Cub that had the Jury Strut totally misaligned. Not saying that it was correct by any means but that thing was flying like that with no issues I guess. I was reminded once, by the factory, on a prior build that we aren't building the Space Shuttle. :)

[JoeRuscito]

Thanks Eddie, it certainly is easy for me to let my mind go and get tunnel vision on some of these things. I still going to do my best within reason to straighten it out.

[efwd]

LOL Id do the same. :cool:

[Flyboy66]

Joe, I would stay away from prolonged inverted flight and outside loops.

[JoeRuscito]

Dang! I figured -3 g's was standard for the kitfox ;)