Original Posted By: Larry Neal
Pieters-At the risk of sounding simplistic, it's not a real tough task to do the analysis to backward-engineer a wood box spar equivalent to a solid spar. Since the original Pietenpol spar was routed at the webs for lightness, a box section designed for torsional, bending, and shear strength equivalent to a solid section would actually be a bit over-designed. It would also be considerably over-designed relative to the later 3/4" thick spar. As mentioned, solid pieces would be needed at the fitting attach points to properly transfer the loads.Analysis and design of an I-section with solid flanges and a plywood web would be similarly straightforward.The KR, for one, uses box section spars. It's not rocket science, but I'm not real sure how willing I would be to design one and put the design out there for others to build by, unless I either load-tested one both as a bare spar and in a wing, or else built and flew one to the full limits of the design.With a parachute on, of course. ;o) Anyone for some +6G pull-outs from a split-S?Oscar ZunigaMedford, Oregonmailto: taildrags(at)hotmail.comwebsite at http://www.flysquirrel.net________________________________________________________________________________Date: Fri, 30 Nov 2001 09:54:55 -0600
Pietenpol-List: alternative spars
Re: Pietenpol-List: alternative spars
Original Posted By: Matt Dralle
Re: Pietenpol-List: alternative spars
Original Posted By: "Jack Phillips"
Pieters:Been following the discussion about alternative spars and wonder if we havebeen concentrating too much on the strength of the spar itself andforgetting to figure in the terminals (fittings), struts, and of course thejury struts??? Seems to me to be the possible weak points in the design.Comments?John Dilatush, ready to fly except for the prop.Salida, Coloradodilatush(at)amigo.net>>________________________________________________________________________________
Pieters:Been following the discussion about alternative spars and wonder if we havebeen concentrating too much on the strength of the spar itself andforgetting to figure in the terminals (fittings), struts, and of course thejury struts??? Seems to me to be the possible weak points in the design.Comments?John Dilatush, ready to fly except for the prop.Salida, Coloradodilatush(at)amigo.net>>________________________________________________________________________________
RE: Pietenpol-List: alternative spars
Original Posted By: "Doug Sheets"
Hi Larry,I may have written the numbers you are referring to - I did write an articleon the need for jury struts that appeared in an issue of the BPA newsletterseveral years ago.To clarify, I ran a stress analysis on MY PIETENPOL, i.e., certifiedaircraft spruce spars and ribs, aircraft hardware, aircraft grade 4130steel, etc. and concluded that with the 3/4" solid spars that I used, thewing was good for about +/- 4.9 "g's", assuming a gross weight of 1050 lbs.Note that this is the ultimate load which the spar should be able towithstand, as you say once in its lifetime. A good theoretical limit foractual flight loads would be 80% of that, or about 3.9 g's. Interestinglyenough, when you calculate the loads on the struts, the lift struts carryalmost everything (not surprising since they attach near the center of thewing panels). The cabane struts carry only a few pounds each from the outerwing panels. Of course, they carry almost all the lift loads generated bythe centersection, but that is not much compared to the outer panels.As you point out, jury struts should be on every Pietenpol. Without them,the lift struts can buckle at -1.14 g's, which can be produced by moderateturbulence.I intend to carry out a pretty thorough investigation of the flight envelopeduring my flight test program, including "g" loading up to about 3.2 g's,and including spins and accelerated stalls. I will certainly wear aparachute for such testing, but I can't imagine owning an airplane withoutknowing its stall/spin characteristics and knowing that it was strong enoughto withstand any loading I might put on it in maneuvers. I will probablyloop it, since it doesn't have much more drag than the old J-3 Cub that Ilearned aerobatics in. I doubt that I will roll it, unless it rollsconsiderably faster than a J-3 (shouldn't be difficult). I probably won'tdo really stressful maneuvers like snap rolls, although I used to snap thatold Cub.I absolutely do NOT say that any Pietenpol is good to +/- 3.9 g's. For onething, I don't want a lawsuit, but there is too much variation betweenbuilders in the type and quality of wood used, and the skill and care usedin construction. As someone else recently pointed out, we are concerningourselves with the basic strength of the wood here, without considering thestrength of the fittings that hold everything together. For myself, Itotally redesigned the landing gear/lift strut attach fittings for the solidaxle wire wheel design that I'm building because I didn't like the way theoriginal was designed. In many ways BHP was a genius, but some of hisdesigns and construction practices (like his spar splicing methods) are notthe best way to do things.Thanks for a thoughtful letter.Jack PhillipsOscar,Interesting thought. Being a competent aerobatics pilot (yes, I have my ownchute), I've deliberated about this same issue over the last few years.I'd love to do some mild positive g maneuvers with my Piet. From thehistory ofthe Piet as well as the fact that I can routinely do 2.5 - 3 g loops inotheraircraft my thinking is that it's practical. However, here are a fewthoughtsbefore anyone launches off into the delirious burning blue. Ramble modeon...1. There are some discussions stating that a person (I don't know who) hasevaluated the design and showed it good for +5.25 -2.5 or there abouts, I'llsayI don't remember exactly. I also don't know that person. Piet's don't falloutof the sky, but who can verify these numbers?2. No Piet should be built without the jury struts regardless of usage. Forexample the lack of these structural elements drops the negative loadto -1.5 gin the above reference.3. Any aero maneuvers with a converted auto engine should only be done overalanding field, with lots of altitude, of course. Many float carburetorswillstarve or flood the engine in rough maneuvers and the small props used oncorvairs or others will probably not be able to air-start the engine. AsktheVolkswagen guys!4. I'd rather have my airplane fail expectedly at 9,000ft rather than in aguston downwind. My preference would be an air test. I think though that I'dtrustthe calculations for the full design load and take her up to say 4g as aconfidence check. This would be enough to proof against any gust ormaneuveremergencies.5. But how to test this? The best way would be to pull the g's and thenuncover the wing. You sure as hell don't want to bust something and notknow.Remember too that such a test will test the entire aircraft, not just thewing.Howdya like to fail a seat bottom at 6 g's?6. Remember that design load is max, the limit load is the stress that thestructure is "expected to survive once in it's lifetime"! Don't even gothere,not even daydreaming in email.7. I 'm not suggesting that anyone take any risks. Also, even if I decidedtodo some mild acro, I would never do this with a passenger in this airplane.Theweight and load increases rapidly in small planes and you're going to needtwochutes as well. I would not trust the person up front to be able to get outinan emergency. Don't do this.8. This is not a negative g airplane. Period.9. The built up spars are likely to cost close to solid once you'vecalculatedthe wood, ply and glue. Also what's your time worth?10. I think a design for a box spar would be a valid contribution for thePietmovement (background music please). Some day we might not be able to find 1inch spars in good wood. Also, any strength advantages would be a validsafetyissue, regardless of whether you want to loop a 70 year old airplane.11. If anyone is going to attempt this kind of stuff get training, check themath and build strong. Don't put a Pietenpol on the front page of thepapers,please. That's all we need.So, for me I think I'll do some calculation and would like to see sometests,then I may decide on some light loads. Very cautiously and with a plan.One thing we might consider is to design the box spar and test one (you'donlyneed one side) to destruction. If anyone's game on this, I'll contribute mytwospare ribs and make up an extra set of strut attach hardware. If everyonechipped in, this could be a great project.Larry________________________________________________________________________________
Hi Larry,I may have written the numbers you are referring to - I did write an articleon the need for jury struts that appeared in an issue of the BPA newsletterseveral years ago.To clarify, I ran a stress analysis on MY PIETENPOL, i.e., certifiedaircraft spruce spars and ribs, aircraft hardware, aircraft grade 4130steel, etc. and concluded that with the 3/4" solid spars that I used, thewing was good for about +/- 4.9 "g's", assuming a gross weight of 1050 lbs.Note that this is the ultimate load which the spar should be able towithstand, as you say once in its lifetime. A good theoretical limit foractual flight loads would be 80% of that, or about 3.9 g's. Interestinglyenough, when you calculate the loads on the struts, the lift struts carryalmost everything (not surprising since they attach near the center of thewing panels). The cabane struts carry only a few pounds each from the outerwing panels. Of course, they carry almost all the lift loads generated bythe centersection, but that is not much compared to the outer panels.As you point out, jury struts should be on every Pietenpol. Without them,the lift struts can buckle at -1.14 g's, which can be produced by moderateturbulence.I intend to carry out a pretty thorough investigation of the flight envelopeduring my flight test program, including "g" loading up to about 3.2 g's,and including spins and accelerated stalls. I will certainly wear aparachute for such testing, but I can't imagine owning an airplane withoutknowing its stall/spin characteristics and knowing that it was strong enoughto withstand any loading I might put on it in maneuvers. I will probablyloop it, since it doesn't have much more drag than the old J-3 Cub that Ilearned aerobatics in. I doubt that I will roll it, unless it rollsconsiderably faster than a J-3 (shouldn't be difficult). I probably won'tdo really stressful maneuvers like snap rolls, although I used to snap thatold Cub.I absolutely do NOT say that any Pietenpol is good to +/- 3.9 g's. For onething, I don't want a lawsuit, but there is too much variation betweenbuilders in the type and quality of wood used, and the skill and care usedin construction. As someone else recently pointed out, we are concerningourselves with the basic strength of the wood here, without considering thestrength of the fittings that hold everything together. For myself, Itotally redesigned the landing gear/lift strut attach fittings for the solidaxle wire wheel design that I'm building because I didn't like the way theoriginal was designed. In many ways BHP was a genius, but some of hisdesigns and construction practices (like his spar splicing methods) are notthe best way to do things.Thanks for a thoughtful letter.Jack PhillipsOscar,Interesting thought. Being a competent aerobatics pilot (yes, I have my ownchute), I've deliberated about this same issue over the last few years.I'd love to do some mild positive g maneuvers with my Piet. From thehistory ofthe Piet as well as the fact that I can routinely do 2.5 - 3 g loops inotheraircraft my thinking is that it's practical. However, here are a fewthoughtsbefore anyone launches off into the delirious burning blue. Ramble modeon...1. There are some discussions stating that a person (I don't know who) hasevaluated the design and showed it good for +5.25 -2.5 or there abouts, I'llsayI don't remember exactly. I also don't know that person. Piet's don't falloutof the sky, but who can verify these numbers?2. No Piet should be built without the jury struts regardless of usage. Forexample the lack of these structural elements drops the negative loadto -1.5 gin the above reference.3. Any aero maneuvers with a converted auto engine should only be done overalanding field, with lots of altitude, of course. Many float carburetorswillstarve or flood the engine in rough maneuvers and the small props used oncorvairs or others will probably not be able to air-start the engine. AsktheVolkswagen guys!4. I'd rather have my airplane fail expectedly at 9,000ft rather than in aguston downwind. My preference would be an air test. I think though that I'dtrustthe calculations for the full design load and take her up to say 4g as aconfidence check. This would be enough to proof against any gust ormaneuveremergencies.5. But how to test this? The best way would be to pull the g's and thenuncover the wing. You sure as hell don't want to bust something and notknow.Remember too that such a test will test the entire aircraft, not just thewing.Howdya like to fail a seat bottom at 6 g's?6. Remember that design load is max, the limit load is the stress that thestructure is "expected to survive once in it's lifetime"! Don't even gothere,not even daydreaming in email.7. I 'm not suggesting that anyone take any risks. Also, even if I decidedtodo some mild acro, I would never do this with a passenger in this airplane.Theweight and load increases rapidly in small planes and you're going to needtwochutes as well. I would not trust the person up front to be able to get outinan emergency. Don't do this.8. This is not a negative g airplane. Period.9. The built up spars are likely to cost close to solid once you'vecalculatedthe wood, ply and glue. Also what's your time worth?10. I think a design for a box spar would be a valid contribution for thePietmovement (background music please). Some day we might not be able to find 1inch spars in good wood. Also, any strength advantages would be a validsafetyissue, regardless of whether you want to loop a 70 year old airplane.11. If anyone is going to attempt this kind of stuff get training, check themath and build strong. Don't put a Pietenpol on the front page of thepapers,please. That's all we need.So, for me I think I'll do some calculation and would like to see sometests,then I may decide on some light loads. Very cautiously and with a plan.One thing we might consider is to design the box spar and test one (you'donlyneed one side) to destruction. If anyone's game on this, I'll contribute mytwospare ribs and make up an extra set of strut attach hardware. If everyonechipped in, this could be a great project.Larry________________________________________________________________________________
Re: Pietenpol-List: alternative spars
Original Posted By: Gary Gower
Pietenpol-List: alternative spars
Original Posted By: Larry Neal
John writes:>wonder if we have been concentrating too much on the strength of>the spar itself and forgetting to figure in the terminals (fittings)>struts, and of course the jury struts>Comments?Yes, comments. My thought was this: we have a design that is tried and true, hundreds of examples are flying, the mission profile for the airplane is known. No need to analyze anything beyond the scope of the element that we are interested in; the rest of the design and construction methods work fine! So when we think about substituting for one element of that known assembly, we look at that one element, not the entire assembly. It's one thing to analyze the airplane, and another to analyze just the spars. If we design an alternative spar that performs equivalent to the base design, we're there.Not to be overly simplistic, but if we provide for the same attach points and load transfer points with the alternative design, we need not look beyond that unless we just want to go ahead and analyze the whole enchilada to determine things that BHP never bothered to determine, like C/L plots, chordwise and spanwise lift distributions, whatever.Oops- now you got me started on enchiladas. If you're ever in San Antonio, you have to have the "enchiladas verdes" (green sauce) at Mi Tierra in the old marketplace in downtown San Antonio. Mmmm, good!Oscar ZunigaMedford, Oregonmailto: taildrags(at)hotmail.comwebsite at http://www.flysquirrel.net________________________________________________________________________________Date: Sat, 01 Dec 2001 14:41:29 -0600
John writes:>wonder if we have been concentrating too much on the strength of>the spar itself and forgetting to figure in the terminals (fittings)>struts, and of course the jury struts>Comments?Yes, comments. My thought was this: we have a design that is tried and true, hundreds of examples are flying, the mission profile for the airplane is known. No need to analyze anything beyond the scope of the element that we are interested in; the rest of the design and construction methods work fine! So when we think about substituting for one element of that known assembly, we look at that one element, not the entire assembly. It's one thing to analyze the airplane, and another to analyze just the spars. If we design an alternative spar that performs equivalent to the base design, we're there.Not to be overly simplistic, but if we provide for the same attach points and load transfer points with the alternative design, we need not look beyond that unless we just want to go ahead and analyze the whole enchilada to determine things that BHP never bothered to determine, like C/L plots, chordwise and spanwise lift distributions, whatever.Oops- now you got me started on enchiladas. If you're ever in San Antonio, you have to have the "enchiladas verdes" (green sauce) at Mi Tierra in the old marketplace in downtown San Antonio. Mmmm, good!Oscar ZunigaMedford, Oregonmailto: taildrags(at)hotmail.comwebsite at http://www.flysquirrel.net________________________________________________________________________________Date: Sat, 01 Dec 2001 14:41:29 -0600
Re: Pietenpol-List: alternative spars
Original Posted By: "Warren D. Shoun"
Pietenpol-List: alternative spars
Original Posted By: "Jack Phillips"
At the risk of whipping the spar horse to death, here is some load test information for anybody interested in continuing study of alternative spars.The airplane I'm building, the Barnard M-19 "Flying Squirrel", is of composite construction. The wing spars are straight, non-tapered, and are of solid wood in the prototype, thus my interest in alternative spars being used or developed for the Pietenpol. Each wing is 11 ft. long, and the chord is about 4'-8". Main spar is 3/4" thick, 7" deep; aft spar is 1/2" thick, 4" deep (airfoil is a modified Clark Y shape). In the prototype, each face of each spar also gets a single layer of bidirectional glass cloth laid at a 45. Wing ribs are 3/4" thick foam, as are the wing skins. Anyway, my interest has been in building a composite wing spar to avoid using expensive solid wood spars. Marvin Barnard, the designer and builder of the prototype, has been running some tests on alternative spars. The results of load tests are available on my site, at http://www.flysquirrel.net/wing/spartest.html and some general information on the wing is at http://www.flysquirrel.net/wing/wing.html and photos of the prototype wing construction at http://www.flysquirrel.net/prototype.htmlOscar ZunigaMedford, Oregonmailto: taildrags(at)hotmail.comwebsite at http://www.flysquirrel.netSend and receive Hotmail on your mobile device: http://mobile.msn.com________________________________________________________________________________
At the risk of whipping the spar horse to death, here is some load test information for anybody interested in continuing study of alternative spars.The airplane I'm building, the Barnard M-19 "Flying Squirrel", is of composite construction. The wing spars are straight, non-tapered, and are of solid wood in the prototype, thus my interest in alternative spars being used or developed for the Pietenpol. Each wing is 11 ft. long, and the chord is about 4'-8". Main spar is 3/4" thick, 7" deep; aft spar is 1/2" thick, 4" deep (airfoil is a modified Clark Y shape). In the prototype, each face of each spar also gets a single layer of bidirectional glass cloth laid at a 45. Wing ribs are 3/4" thick foam, as are the wing skins. Anyway, my interest has been in building a composite wing spar to avoid using expensive solid wood spars. Marvin Barnard, the designer and builder of the prototype, has been running some tests on alternative spars. The results of load tests are available on my site, at http://www.flysquirrel.net/wing/spartest.html and some general information on the wing is at http://www.flysquirrel.net/wing/wing.html and photos of the prototype wing construction at http://www.flysquirrel.net/prototype.htmlOscar ZunigaMedford, Oregonmailto: taildrags(at)hotmail.comwebsite at http://www.flysquirrel.netSend and receive Hotmail on your mobile device: http://mobile.msn.com________________________________________________________________________________