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#41
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PSRU design advantages
"Richard Lamb" wrote in message k.net... Peter Dohm wrote: "Bill Daniels" bildan@comcast-dot-net wrote in message ... The basics: Piston engines produce more power per pound if they rev higher. (HP = RPM x torque/5252) Propellers are MUCH more efficient if they turn slow. This begs for a PSRU. BUT, a PSRU adds weight, cost and complexity. Resonances, particularly torsional resonances are a real problem. Lots of examples of PSRU's on 12, 14 and 18 cyinder engines Few workable examples with fewer cylinders suggesting PSRU's don't like power pulses. If a shaft has a strong resonant fundamental, don't excite it or lower the fundamental below the input frequency. Tuning a PSRU/shaft/propeller system is like tuning a piano - it's an art not a science. The 9 cylinder 1820 and 1840 CID radials used on B-17's were geared approximately 16:9. However, your point is well taken, and I also am unable to name any 4 or 6 cylinder engines that have stood the test of time with reduction drives. I also believe that tuning any drive system, including a PSRU, is a science--when fully understood. And therein lies the rub: There's plenty left to learn--especially if it must also be light. So, in practice, you are right--it is still an art. :-( Peter Rotax - the 912/914 Jabaru - (but the 6 cylinder will be a better seller - IMHO) Believe it or not, a few VW's with belts. And a couple of Subes with Rotax B boxes scabbed on. The one that DIDN'T work was the Geo Metro 3-banger (broke the crank). But that issue was already known - don't cut off any flywheel on 3 holers. With the full flywheel, the 3 cylinder runs fine. Richard OK, you caught me fair and square on poor phrasing. I tend to think of higher power applications, but you are right that some of the more conservative and lower powered systems with flywheels still in place and a little looser coupling seem to run quite reliably. I don't know how much power is lost to friction, but some of the v-belt reduction drives even seem to work quite reliably without any external crankshaft support! Peter |
#42
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PSRU design advantages
"Dan Horton" wrote in message ups.com... There is nothing that eliminates a long shaft from the design of a PSRU. Nonbelievers might be advised to consider ship propulsion; long shafts, low cylinder counts, propellers operating in uneven flow, often via a gearbox. Sound familiar? The important issue is torsional stiffness of the shaft, not length. A long shaft can be torsionally stiff or soft, depending on diameter and material. The engineering process will tailor torsional stiffness of the shaft (along with a number of other factors) to adjust natural frequency. The information you need is found in engineering texts, not on RAH. The subject can be complicated, but there are no unknowns. You will find most of the torsional vibration classics listed in the bibliography of Taylor's "Internal Combustion....". Some texts, like Wilson's "Practical Solution.." (the ultimate reference) will be difficult to locate. Try a large university library. The best readily available text (sort of the ultimate primer on all matters vibrational) is JP DenHartog's "Mechanical Vibrations". You can buy it for less than $15 at Amazon. Here is a short list: CF Taylor, "The Internal-Combustion Engine in Theory and Practice", 1966 (vol. 1), 1968 (vol. 2), MIT Press W Ker Wilson, "Practical Solution of Torsional Vibration Problems", 3rd Ed, 5 Vols., 1956, 0412091100, Chapman & Hall JP Den Hartog, "Mechanical Vibrations", 1956, 070163898, McGraw-Hill My compliments to Mr. Christley, whose comment (re frequency) was a sole beacon of accuracy. Dan Horton You are very probably right--and it won't be the first time that I believed that something was still a "black art" until I found out otherwise. For years after I first became an electronic technician, I believed that about grounding problems--and then I read a book titled "Sheilding and Grounding Techniques in Instrumentation." Even 20 years ago, that book was long out of print; but could still be obtained by special order from University Microfilm. Almost miraculously, the problems went away! After reading your post, I decided to look for the books you mentioned and found that you were correct about the difficulty of locating W Ker Wilson's book. That could indicate that it is the true source, as the dates mentioned for earlier editions suggest, and therefore a custom reprint could be worth every penny and more if a source is known. The other two books seem to still be available, although I have no idea when I might find time to read them... Peter |
#43
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PSRU design advantages
Peter Dohm wrote:
"Richard Lamb" wrote in message k.net... Peter Dohm wrote: "Bill Daniels" bildan@comcast-dot-net wrote in message ... The basics: Piston engines produce more power per pound if they rev higher. (HP = RPM x torque/5252) Propellers are MUCH more efficient if they turn slow. This begs for a PSRU. BUT, a PSRU adds weight, cost and complexity. Resonances, particularly torsional resonances are a real problem. Lots of examples of PSRU's on 12, 14 and 18 cyinder engines Few workable examples with fewer cylinders suggesting PSRU's don't like power pulses. If a shaft has a strong resonant fundamental, don't excite it or lower the fundamental below the input frequency. Tuning a PSRU/shaft/propeller system is like tuning a piano - it's an art not a science. The 9 cylinder 1820 and 1840 CID radials used on B-17's were geared approximately 16:9. However, your point is well taken, and I also am unable to name any 4 or 6 cylinder engines that have stood the test of time with reduction drives. I also believe that tuning any drive system, including a PSRU, is a science--when fully understood. And therein lies the rub: There's plenty left to learn--especially if it must also be light. So, in practice, you are right--it is still an art. :-( Peter Rotax - the 912/914 Jabaru - (but the 6 cylinder will be a better seller - IMHO) Believe it or not, a few VW's with belts. And a couple of Subes with Rotax B boxes scabbed on. The one that DIDN'T work was the Geo Metro 3-banger (broke the crank). But that issue was already known - don't cut off any flywheel on 3 holers. With the full flywheel, the 3 cylinder runs fine. Richard OK, you caught me fair and square on poor phrasing. I tend to think of higher power applications, but you are right that some of the more conservative and lower powered systems with flywheels still in place and a little looser coupling seem to run quite reliably. I don't know how much power is lost to friction, but some of the v-belt reduction drives even seem to work quite reliably without any external crankshaft support! Peter You didn't follow the link that blueskies posted, didja Peter. The BD-5 story - in all it's glory! And a few other odds and ends, That was not a high powered setup, but kicked a bunch of engineers around. http://www.prime-mover.org/Engines/T.../contact1.html Richard |
#44
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PSRU design advantages
"Richard Lamb" wrote in message nk.net... Peter Dohm wrote: "Richard Lamb" wrote in message k.net... Peter Dohm wrote: "Bill Daniels" bildan@comcast-dot-net wrote in message ... The basics: Piston engines produce more power per pound if they rev higher. (HP = RPM x torque/5252) Propellers are MUCH more efficient if they turn slow. This begs for a PSRU. BUT, a PSRU adds weight, cost and complexity. Resonances, particularly torsional resonances are a real problem. Lots of examples of PSRU's on 12, 14 and 18 cyinder engines Few workable examples with fewer cylinders suggesting PSRU's don't like power pulses. If a shaft has a strong resonant fundamental, don't excite it or lower the fundamental below the input frequency. Tuning a PSRU/shaft/propeller system is like tuning a piano - it's an art not a science. The 9 cylinder 1820 and 1840 CID radials used on B-17's were geared approximately 16:9. However, your point is well taken, and I also am unable to name any 4 or 6 cylinder engines that have stood the test of time with reduction drives. I also believe that tuning any drive system, including a PSRU, is a science--when fully understood. And therein lies the rub: There's plenty left to learn--especially if it must also be light. So, in practice, you are right--it is still an art. :-( Peter Rotax - the 912/914 Jabaru - (but the 6 cylinder will be a better seller - IMHO) Believe it or not, a few VW's with belts. And a couple of Subes with Rotax B boxes scabbed on. The one that DIDN'T work was the Geo Metro 3-banger (broke the crank). But that issue was already known - don't cut off any flywheel on 3 holers. With the full flywheel, the 3 cylinder runs fine. Richard OK, you caught me fair and square on poor phrasing. I tend to think of higher power applications, but you are right that some of the more conservative and lower powered systems with flywheels still in place and a little looser coupling seem to run quite reliably. I don't know how much power is lost to friction, but some of the v-belt reduction drives even seem to work quite reliably without any external crankshaft support! Peter You didn't follow the link that blueskies posted, didja Peter. The BD-5 story - in all it's glory! And a few other odds and ends, That was not a high powered setup, but kicked a bunch of engineers around. http://www.prime-mover.org/Engines/T.../contact1.html Richard Actually I did, some months ago following an earlier post, and subsequently also learned that the Contact! article is quite famous. One of the more interesting points was that trying to make the shaft and/or transfer drive more rigid was not helpful on the BD-5. Softening the system eventually did resolve the breakage problem within the drive train; but IIRC the drive system to airframe resonance (evidenced initially by loosened rivets) was not fully resolved during the author's tenure. That was the article that really convinced me that I didn't necessarily know enough to design a clutchless system with a high degree of confidence--even by leaving the flywheel in place. However, the set of books mentioned elsewhere in this thread, by Mr. Horton, could prove to contain the necessary formulas and explanations to reduce this problem to a cookbook science. A quick web search confirmed his belief that one of the books may now be virtually unobtainable. I am willing to entertain his book suggestion because, in my earlier career as an electronic technician, a technical tome entitled "Shielding and Grounding Techniques in Instrumentation" made previously insurmountable grounding problems easy to solve. It is probable that work on mechanical resonance, done for World War II, may have been covered in books published during the succeeding quarter century. Peter |
#45
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PSRU design advantages
"ADK" wrote in message news:WlXYf.4880$4S.2741@edtnps82... Actually I do work in aviation. I am an aviation machinist and aircraft mechanic, I also work on Allison turbines (hercs and convairs) that drive a gearbox via a shaft. My experience is mostly helicopters but being a fixed wing pilot I want to have my own plane for cross country flights. I don't believe any one person can ever learn everything there is to know about a subject and therefore I am was soliciting usefull information on this subject. Thank you! Good for you. Read all you can, talk to some others that have been there, done that. They are not on this group, though. Find the author of the link that was posted on the subject. Then, if your heart is set on it, start experimenting, and be prepared to experiment, a bunch! g Good luck! -- Jim in NC |
#46
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PSRU design advantages
"Dan Horton" wrote There is nothing that eliminates a long shaft from the design of a PSRU. Nonbelievers might be advised to consider ship propulsion; long shafts, low cylinder counts, propellers operating in uneven flow, often via a gearbox. Sound familiar? I think you will find that they do it on ships, with pure weight. A big, heavy, solid steel shaft. Very heavy! That is how they get the stiffness. Also, the shaft turns very slowly, so there are many pulses per revolution; more than you will get with a 4 or 6 cylinder, 4 cycle airplane engine, in most cases. I agree with the rest of your post; dig into the engineering text books. -- Jim in NC |
#47
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PSRU design advantages
Richard Lamb wrote:
Ian Stirling wrote: Peter Dohm wrote: snip The 9 cylinder 1820 and 1840 CID radials used on B-17's were geared approximately 16:9. However, your point is well taken, and I also am unable to name any 4 or 6 cylinder engines that have stood the test of time with reduction drives. I also believe that tuning any drive system, including a PSRU, is a science--when fully understood. And therein lies the rub: There's plenty left to learn--especially if it must also be light. So, in practice, you are right--it is still an art. :-( I suspect that electronics help. Instrumenting the shaft, to measure resonances in real time is no longer prohibitively expensive. I suspect a belt PSRU - if properly configured could act to decouple the prop from the engine/shaft somewhat. Add one or more rotational vibrational dampers - fill the shaft with oil? And trim. Best tool available to the amateur is a variable speed strobe - Party Light! That way you can actually look and SEE what's happening. That'll spot ordinary vibrations. Torsional ones are a little bit harder. Especially if you want, as you probably should, a graph of maximum stress anywhere in the shaft/PSRU/Prop system vs RPM. |
#48
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PSRU design advantages
ADK wrote:
IF you had to design a PSRU, to drive a pusher propellor via shaft, what would your experience dictate? Thinking along the lines of a gearbelt, chain or gear. Please, I would appreciate the collective experience available on this group. I have decided on the aircraft, but want to make it the most reliable and safest it can be. For the sake of Peter, IT DOESN'T MATTER!! For the energy to transfer to the prop, you have to attach the engine to the prop. The engine doesn't produce smooth even power. It produces a series of pulses. If the frequency of the pulses resonates with the prop or shaft, it will store a little bit of each pulse as "spring energy". This type of energy is stored by deflecting (ie, bending) the prop or shaft. The prop stores it and then immediately tries to release it by unbending. If the next engine pulse comes along at just the right time, the new "spring energy" will be added to the previous "spring energy" and the prop will bend a little more. This continues until the prop or shaft has as much "spring energy" as it can phyiscally hold, and then the element just vibrates. Eventually, the prop or shaft gets tired of all the bending and unbending and just gives up (ie, breaks). Making the pulses smaller doesn't help for the most part. All that does is cut down on the amount of "spring energy" added with each pulse. A smaller pulse will take 2000 pulses to fill the prop with "spring energy" vs 1000 with a unmodified pulse. Whoop-te-do! What difference will that make with the engine turning 2000RPM and four pulses per round. Any one of the gearboxes you mentioned made to work safely, and each has a set of advantages and disadvantages that are well known and easily engineered around. The type of gearbox has nothing to do with torsional resonance. Will not mitigate torsional resonance. Will not cure/alleviate/lesson or bypass torsioanl resonance. Torsional resonance is a totally different issue. You didn't tie gearbox type and torsional resonance together directly, but many people have in the past, and it's just self-deception. Any of the gearboxes you mentioned can be as safe and dependable as any of the others, if engineered properly. -- This is by far the hardest lesson about freedom. It goes against instinct, and morality, to just sit back and watch people make mistakes. We want to help them, which means control them and their decisions, but in doing so we actually hurt them (and ourselves)." |
#49
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PSRU design advantages
Ian Stirling wrote:
Best tool available to the amateur is a variable speed strobe - Party Light! That way you can actually look and SEE what's happening. That'll spot ordinary vibrations. Torsional ones are a little bit harder. Especially if you want, as you probably should, a graph of maximum stress anywhere in the shaft/PSRU/Prop system vs RPM. A few fine white lines down the length of the shaft will clear up that problem. -- This is by far the hardest lesson about freedom. It goes against instinct, and morality, to just sit back and watch people make mistakes. We want to help them, which means control them and their decisions, but in doing so we actually hurt them (and ourselves)." |
#50
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PSRU design advantages
"Ernest Christley" wrote in message ... ADK wrote: IF you had to design a PSRU, to drive a pusher propellor via shaft, what would your experience dictate? Thinking along the lines of a gearbelt, chain or gear. Please, I would appreciate the collective experience available on this group. I have decided on the aircraft, but want to make it the most reliable and safest it can be. For the sake of Peter, IT DOESN'T MATTER!! For the energy to transfer to the prop, you have to attach the engine to the prop. The engine doesn't produce smooth even power. It produces a series of pulses. If the frequency of the pulses resonates with the prop or shaft, it will store a little bit of each pulse as "spring energy". This type of energy is stored by deflecting (ie, bending) the prop or shaft. The prop stores it and then immediately tries to release it by unbending. If the next engine pulse comes along at just the right time, the new "spring energy" will be added to the previous "spring energy" and the prop will bend a little more. This continues until the prop or shaft has as much "spring energy" as it can phyiscally hold, and then the element just vibrates. Eventually, the prop or shaft gets tired of all the bending and unbending and just gives up (ie, breaks). Making the pulses smaller doesn't help for the most part. All that does is cut down on the amount of "spring energy" added with each pulse. A smaller pulse will take 2000 pulses to fill the prop with "spring energy" vs 1000 with a unmodified pulse. Whoop-te-do! What difference will that make with the engine turning 2000RPM and four pulses per round. Any one of the gearboxes you mentioned made to work safely, and each has a set of advantages and disadvantages that are well known and easily engineered around. The type of gearbox has nothing to do with torsional resonance. Will not mitigate torsional resonance. Will not cure/alleviate/lesson or bypass torsioanl resonance. Torsional resonance is a totally different issue. You didn't tie gearbox type and torsional resonance together directly, but many people have in the past, and it's just self-deception. Any of the gearboxes you mentioned can be as safe and dependable as any of the others, if engineered properly. -- This is by far the hardest lesson about freedom. It goes against instinct, and morality, to just sit back and watch people make mistakes. We want to help them, which means control them and their decisions, but in doing so we actually hurt them (and ourselves)." Let me try a different idea. Suppose the prop shaft is to be just long enough for the gear belt pulley and the neccessary bearings - say 10 inches. But the engine flywheel pulley is to be 4 - 6 feet below the prop shaft. The idea is to use a very large multi-blade carbon fiber prop turning 800 - 1000 RPM driven by a 4 cyl Soob turning at best power RPM. The idea is to get best thrust in the 0 - 60 knot range. The airframe configuration is a prop over tail boom pusher - an ultralight on steroids. (BTW, I'm not looking for a long engine life under these conditions. I'll treat the Soob as a throwaway power plant.) I'm thinking there isn't too much torsional vibration concern with very short shafts, high reving engine and a stiff carbon fiber prop. The prop will be seeing 6 - 7 power pulses per rev from the high reving Soob. Bill |
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