Original Posted By: "Bill Church"
#yiv179383659 BLOCKQUOTE {PADDING-BOTTOM:0px;PADDING-TOP:0px;}#yiv179383659 DL {PADDING-BOTTOM:0px;PADDING-TOP:0px;}#yiv179383659 UL {PADDING-BOTTOM:0px;PADDING-TOP:0px;}#yiv179383659 OL {PADDING-BOTTOM:0px;PADDING-TOP:0px;}#yiv179383659 LI {PADDING-BOTTOM:0px;PADDING-TOP:0px;}Jeff,-I think I understand the question now. You are concerned about buckling.In compression, the strut assembly will act like a column. The longer an unsupported-column is, the more prone to buckling it becomes. If the compression forces on the ends of a column are large enough (based on the specific geometry and physical characteristics of the column), once the column starts to buckle, it-can suffer a catastrophic collapse. This type of failure can be avoided by a couple of methods. One is to change the structure of the column (larger profile, thicker wall thickness, different materials, etc), and the other method is to provide some intermediate support(s) to effectively shorten the unsupported length of the column. The second of these methods is recommended for the Pietenpol. This is achieved by the addition of jury struts. The lift struts on the Air Camper are about eight feet long, but with the addition of jury struts, the unsupported length becomes roughly half of that. Since the relationship between buckling force and column length is an inverse square, reducing the length by half means that the forces required to buckle the new shorter length increase by a factor of four (likewise, if the length of a column is doubled, the force required to buckle the longer column is only one fourth).- The jury struts do very little in-ideal flight conditions - straight, level flight, in calm air, followed by a perfect, soft-as-a-feather landing on a smooth-as-glass runway. The only possible problem under these conditions would be if the airflow over the lift struts were to cause the struts to flutter, or vibrate. The addition of the jury struts will help to alleviate this situation. In this instance, the jury struts should be attached at a point that is NOT the midpoint of the strut length, as attachment at the midpoint can allow harmonic vibration to occur, and multiply. Unfortunately, most of us have to live in the real world (except for one weekend in July each year, at a small airport in southern Wisconsin). That real world includes turbulent air, and occasional hard landings and rough runways (or fields) among other things. These conditions all-have the power to impart negative G forces on the plane and its occupants. When the Air Camper is put into negative G conditions, the wings (especially the 3-piece wing) will tend to pivot downward, putting the lift struts into compression, rather than tension. When the lift struts are in-compression, they begin to act like the columns mentioned above.-Unsupported, the lift struts might collapse when subjected to these negative G forces. The consequences are nasty if it happens on the ground, but likely fatal if occuring in flight. So, jury struts are a good thing to have on an Air Camper.-Now, with respect to your question, as I mentioned, the calculations to determine the "side loads" the strut ends might encounter are difficult to determine. But, just out of curiosity, I did a few calculations to try to determine how much longer your lift struts might need to be extended, in order to give the wing the dihedral you are desiring. I randomly chose a dimension of 2" for the dihedral (seems about right). Since the lift struts are attached at roughly-half way between the fuselage and the wingtip, and the struts are angled at approximately 30 degrees, the resulting lift strut will only need to be extended by approximately 1/2". I'm not sure, but I think that Cub lift strut ends are 7/16" (correct me if that's not right). The forces required to bend a 7/16" diameter bolt would be huge. I would think that your strut attachment brackets would get ripped out of the plane before the strut ends would fail (the old "weakest link in the chain", once again).-The short answer is that I don't think that extending your strut ends an extra 1/2" is going to put you at risk (provided the threaded rod is long enough to keep-the threads-properly engaged).-Anybody have a differing view on this matter?-Bill C.--=0A=0A=0A ________________________________________________________________________________Subject: Pietenpol-List: Streamlining StrutsDate: Fri, 31 Oct 2008 14:52:08 -0400
Pietenpol-List: Streamlining Struts
RE: Pietenpol-List: Adequate rod ends
Original Posted By: dralle(at)matronics.com