---------------------------------------------------------- Pietenpol-List Digest Archive --- Total Messages Posted Fri 10/31/08: 7 ---------------------------------------------------------- Today's Message Index: ---------------------- 1. 04:16 AM - Re: Adequate rod ends (Michael Perez) 2. 05:06 AM - Re: Adequate rod ends (Jeff Boatright) 3. 10:39 AM - Re: Adequate rod ends (Bill Church) 4. 11:47 AM - Re: Adequate rod ends (KMHeide, BA, CPO, FAAOP) 5. 11:53 AM - Streamlining Struts (Bill Church) 6. 02:36 PM - Re: Adequate rod ends (Gary Boothe) 7. 02:56 PM - Re: Adequate rod ends (Jeff Boatright) ________________________________ Message 1 _____________________________________ Time: 04:16:28 AM PST US From: Michael Perez Subject: RE: Pietenpol-List: Adequate rod ends Bill, you sir, are right on! That is what-I was looking for. Thank you fo r taking the time and effort to do that for me. (and the list) - I am not a big fan of aluminum fasteners as well, usually. However, they do have there place. Wing struts are not one of them. I may use aluminum on t he center struts, but that is yet to be determined. The aluminum solid ends I have been looking at cost more then their steel twins.- For each strut , I will more then likely use one threaded, solid rod end for each. So, for the center section, I would only need to buy 4. Not sure it is worth the a dded cost to buy 4 aluminum fittings...I doubt the weight savings would be much. But, then again it ALL adds up over the entire project. Like I said, TBD on the center section. You have convinced me on steel for the wing stru ts. Thanks again. --- ________________________________ Message 2 _____________________________________ Time: 05:06:32 AM PST US From: Jeff Boatright Subject: Re: Pietenpol-List: Adequate rod ends Ryan, Thanks for the reply and link. You are describing in part what I'm thinking. The negative g-loading causes compression of the strut. The strut then bows, and like a straw, can eventually snap. Once the bowing starts, loads are no longer 100% in parallel with the former line of the unbowed strut. In a sense, some of the load is now from the side, a bending load. If the rod end shaft is part of the strut diagonal, it will also be subjected to those loads. All of this pondering on my part is because the plane is already built and I'd like to add more dihedral. The easiest thing to do is to back the rod ends out to lengthen the distance between wing as fuselage strut attach points. But easy doesn't mean safe. Like anything on a plane, changing what has proven to be safe into some other configuration demands much consideration. Thanks all for your thoughts, Jeff >Jeff, > >I could be wrong on this, but I believe the "collapsing" that the >jury struts eliminate results from negative g-loading that places >the struts in compression. Let's say the Piet is flying through >smooth air. The wing is making lift, and thusly "lifting" the >fuselage. The struts would then be in tension. If you fly through a >downdraft and experience negative g's those struts would now be in >compression, and that would be where they are susceptible to >failure. Here is a link to a short article about model airplanes: > >http://www.fly-imaa.org/imaa/hfarticles/const/v2-4-69.html > >I know that they are not the same as a real airplane, but it does >simply explain the idea. > >As far as the loads on the strut being the same as the loads on the >rod end fitting....well, they may be transmitted the same but the >piece probably would not react the same. One is a hollow >steel/aluminum tube, the other is a solid metal fitting. > >Finally...I posit a situation. The service manual for a J-3 provides >rigging instructions (according to the table of contents). But they >may not provide such specific data as how far out the fork can be >unscrewed. They know exactly how long the strut/fork combination is, >because they manufacture it. They specify a standard set of >instructions for rigging the airplane, and if things need to be >adjusted they are relatively minute adjustments. They don't need to >worry about thread engagement on the fork end, because they are >dealing with very small adjustments. Thusly, even if engineering >calculated the minimum thread engagement that may never have made it >into the maintenance manuals, as there was no reason for that to >have ever come into play when rigging a Cub (unless it has some >serious issues). > >Maybe that would be the case, I don't know. Just throwing that out >there. As you said, even if you could find the data it's for an >entirely different airplane, and you probably shouldn't use that >standard without some solid analysis. Personally I would think one >of the preferable things to do in the situation of a Piet is to look >at what others have successfully flown, preferably for many hours, >and choose which of those to emulate. Determine how long you want >your struts to be, and build them so that they have maximum thread >engagement. > >Ryan ________________________________ Message 3 _____________________________________ Time: 10:39:10 AM PST US From: "Bill Church" Subject: RE: Pietenpol-List: Adequate rod ends 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. ________________________________ Message 4 _____________________________________ Time: 11:47:25 AM PST US From: "KMHeide, BA, CPO, FAAOP" Subject: RE: Pietenpol-List: Adequate rod ends Well Explained! - Ken Heide - --- On Fri, 10/31/08, Bill Church wrote: From: Bill Church Subject: RE: Pietenpol-List: Adequate rod ends #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 un supported-column is, the more prone to buckling it becomes. If the compre ssion forces on the ends of a column are large enough (based on the specifi c geometry and physical characteristics of the column), once the column sta rts to buckle, it-can suffer a catastrophic collapse. This type of failur e can be avoided by a couple of methods. One is to change the structure of the column (larger profile, thicker wall thickness, different materials, et c), and the other method is to provide some intermediate support(s) to effe ctively shorten the unsupported length of the column. The second of these m ethods is recommended for the Pietenpol. This is achieved by the addition o f 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 rough ly 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 ver y 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 run way. The only possible problem under these conditions would be if the airfl ow over the lift struts were to cause the struts to flutter, or vibrate. Th e addition of the jury struts will help to alleviate this situation. In thi s instance, the jury struts should be attached at a point that is NOT the m idpoint of the strut length, as attachment at the midpoint can allow harmon ic 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 a ir, and occasional hard landings and rough runways (or fields) among other things. These conditions all-have the power to impart negative G forces o n the plane and its occupants. When the Air Camper is put into negative G c onditions, the wings (especially the 3-piece wing) will tend to pivot downw ard, putting the lift struts into compression, rather than tension. When th e lift struts are in-compression, they begin to act like the columns ment ioned above.-Unsupported, the lift struts might collapse when subjected t o 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 det ermine the "side loads" the strut ends might encounter are difficult to det ermine. But, just out of curiosity, I did a few calculations to try to dete rmine 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 dimensio n of 2" for the dihedral (seems about right). Since the lift struts are att ached at roughly-half way between the fuselage and the wingtip, and the s truts 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 fo rces required to bend a 7/16" diameter bolt would be huge. I would think th at 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 ex tra 1/2" is going to put you at risk (provided the threaded rod is long eno ugh to keep-the threads-properly engaged). - Anybody have a differing view on this matter? - Bill C.- - =0A=0A=0A ________________________________ Message 5 _____________________________________ Time: 11:53:48 AM PST US Subject: Pietenpol-List: Streamlining Struts From: "Bill Church" The recent discussion regarding the various methods possible to streamline round struts caused me to remember an article that was published in an issue of To Fly, the magazine that was published by the now defunct Sport Aviation Association that was founded by Paul Poberezny (in an effort to return to the original ideals of the EAA). As the magazine is now out of print, and since the following article used to be available on the SAA website, I figure it would be okay to scan a copy and share it with the List. >From the data presented, one can easily see how beneficial it is to use streamline tube or, if round tube is used, to streamline that round tube. Bill C. ________________________________ Message 6 _____________________________________ Time: 02:36:18 PM PST US From: "Gary Boothe" Subject: RE: Pietenpol-List: Adequate rod ends Bill, You are a treasure! Even I could understand that.. Gary Boothe Cool, Ca. Pietenpol WW Corvair Conversion Tail done, working on fuselage (endless metal parts!) (12 ribs down.) Do not archive _____ From: owner-pietenpol-list-server@matronics.com [mailto:owner-pietenpol-list-server@matronics.com] On Behalf Of Bill Church Sent: Friday, October 31, 2008 10:38 AM Subject: RE: Pietenpol-List: Adequate rod ends 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. ________________________________ Message 7 _____________________________________ Time: 02:56:09 PM PST US From: Jeff Boatright Subject: RE: Pietenpol-List: Adequate rod ends Bill C, Great synopsis of the issues and a very useful analysis of the real-world scenario. Thanks so much, I'll stand you a beer at B'head. 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