AirTalk.org Aviation discussions newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Advancement of prop blade in flight, new information Goto page 1, 2, 3  Next          AirTalk.org Forum Index -> Aviators (General Discussions) View previous topic :: View next topic   Author Message Corky Scott Guest Posted: Mon Aug 16, 2004 4:37 pm    Post subject: Advancement of prop blade in flight, new information Sorry, I got my information wrong when I stated that a three bladed prop advanced 15 inches during each revolution at 200 mph. I now have the article in front of me and the exact quote is as follows: "At 200 mph and 2,800 rpm, the blades on my three-plade prop follow three distinct helical paths through the air, and each blade is 25" ahead of the previous blade at the same point of rotation." I repeat that I am not a prop engineer nor do I have any formal training in aerodynamics but it appears to me that by advancing 25" during it's revolution, the affect of one blade might have upon the next one would seem to be pretty inconsequential. Corky Scott Back to top AJW Guest Posted: Mon Aug 16, 2004 5:16 pm    Post subject: Re: Advancement of prop blade in flight, new information Quote: Sorry, I got my information wrong when I stated that a three bladed prop advanced 15 inches during each revolution at 200 mph. I now have the article in front of me and the exact quote is as follows: "At 200 mph and 2,800 rpm, the blades on my three-plade prop follow three distinct helical paths through the air, and each blade is 25" ahead of the previous blade at the same point of rotation." I repeat that I am not a prop engineer nor do I have any formal training in aerodynamics but it appears to me that by advancing 25" during it's revolution, the affect of one blade might have upon the next one would seem to be pretty inconsequential. Corky Scott For what it's worth, at 150 kts and 2500 RPM means the airplane advances about 6 feet per prop rev. A two bladed prop means each balde is in air 3 feet ahead of the prior blade. Back to top Roger Long Guest Posted: Mon Aug 16, 2004 9:04 pm    Post subject: Re: Advancement of prop blade in flight, new information If you look at a diagram of the streamlines around a wing, which is all a prop is, you'll see that the velocity and direction of the air is changed a surprising distance above and behind the wing. One of your prop blade tips would be like a wing flying two feet above and behind another. The effect at this distance would not be significant but would exist. However, the same prop climbing out at 90 mph and 2800 RPM would put each blade only 11.15 inches "above" the preceding. This is close enough that each blade will encounter air that already has some component of motion to the rear. This reduces the change in velocity (lift) that the blade can impart. The three blade prop will be less efficient per unit of area than the two blade where it counts, near Vx with trees in the windshield. Given a limitation on length however, the extra blade area of the three blader can easily offset the efficiency loss by a substantial margin. Another factor in the efficiency equation is the tips. The tip losses and vortexes are a big factor in wings which is why there is such emphasis on making tips small (high aspect ratio) and things like winglets. A three blade prop has an extra tip which will effect the effeciency without any help from the blades ahead. -- Roger Long "Corky Scott" <charles.k.scott (AT) dddartmouth (DOT) edu> wrote Quote: Sorry, I got my information wrong when I stated that a three bladed prop advanced 15 inches during each revolution at 200 mph. I now have the article in front of me and the exact quote is as follows: "At 200 mph and 2,800 rpm, the blades on my three-plade prop follow three distinct helical paths through the air, and each blade is 25" ahead of the previous blade at the same point of rotation." I repeat that I am not a prop engineer nor do I have any formal training in aerodynamics but it appears to me that by advancing 25" during it's revolution, the affect of one blade might have upon the next one would seem to be pretty inconsequential. Corky Scott Back to top DanH Guest Posted: Mon Aug 16, 2004 9:35 pm    Post subject: Re: Advancement of prop blade in flight, new information AJW wrote: Quote: [snip] For what it's worth, at 150 kts and 2500 RPM means the airplane advances about 6 feet per prop rev. A two bladed prop means each balde is in air 3 feet ahead of the prior blade. That's the same number I came up with, but that assumes there's a one-to-one ratio between engine RPM and prop RPM. Is that true of all single engine piston aircraft? I'm obviously not an AC mechanic, but I thought I could see a reduction gear in the cowl. DanH Back to top AJW Guest Posted: Mon Aug 16, 2004 10:13 pm    Post subject: Re: Advancement of prop blade in flight, new information Quote: AJW wrote: [snip] For what it's worth, at 150 kts and 2500 RPM means the airplane advances about 6 feet per prop rev. A two bladed prop means each balde is in air 3 feet ahead of the prior blade. That's the same number I came up with, but that assumes there's a one-to-one ratio between engine RPM and prop RPM. Is that true of all single engine piston aircraft? I'm obviously not an AC mechanic, but I thought I could see a reduction gear in the cowl. I don't know of a SEL airplane in general use that uses reduction gearing between the shaft and the prop, Dan. Back to top Dale Guest Posted: Tue Aug 17, 2004 12:10 am    Post subject: Re: Advancement of prop blade in flight, new information In article <20040816181349.12901.00001819 (AT) mb-m06 (DOT) wmconnect.com>, [email]ajw27703 (AT) wmconnect (DOT) comr[/email]emove (AJW) wrote: Quote: I don't know of a SEL airplane in general use that uses reduction gearing between the shaft and the prop, Dan. Cessna 175 -- Dale L. Falk There is nothing - absolutely nothing - half so much worth doing as simply messing around with airplanes. http://home.gci.net/~sncdfalk/flying.html Back to top Mike Rapoport Guest Posted: Tue Aug 17, 2004 3:59 am    Post subject: Re: Advancement of prop blade in flight, new information Any airplane with a "G" in its engine designation. Mike MU-2 "AJW" <ajw27703 (AT) wmconnect (DOT) comremove> wrote Quote: AJW wrote: [snip] For what it's worth, at 150 kts and 2500 RPM means the airplane advances about 6 feet per prop rev. A two bladed prop means each balde is in air 3 feet ahead of the prior blade. That's the same number I came up with, but that assumes there's a one-to-one ratio between engine RPM and prop RPM. Is that true of all single engine piston aircraft? I'm obviously not an AC mechanic, but I thought I could see a reduction gear in the cowl. I don't know of a SEL airplane in general use that uses reduction gearing between the shaft and the prop, Dan. Back to top Thomas Borchert Guest Posted: Tue Aug 17, 2004 8:20 am    Post subject: Re: Advancement of prop blade in flight, new information Ajw, Quote: I don't know of a SEL airplane in general use that uses reduction gearing between the shaft and the prop, Dan. All Thielert Centurion driven aircraft - way over 100 and counting. -- Thomas Borchert (EDDH) Back to top Dylan Smith Guest Posted: Tue Aug 17, 2004 9:14 am    Post subject: Re: Advancement of prop blade in flight, new information In article <20040816181349.12901.00001819 (AT) mb-m06 (DOT) wmconnect.com>, AJW wrote: Quote: I don't know of a SEL airplane in general use that uses reduction gearing between the shaft and the prop, Dan. Many hundreds of examples of Europa aircraft. My friend's Europa runs the 4-cylinder, opposed, liquid cooled 914S engine at something like 5400RPM in cruise. I think the prop turns less than half that RPM. -- Dylan Smith, Castletown, Isle of Man Flying: http://www.dylansmith.net Frontier Elite Universe: http://www.alioth.net "Maintain thine airspeed, lest the ground come up and smite thee" Back to top Dylan Smith Guest Posted: Tue Aug 17, 2004 9:22 am    Post subject: Re: Advancement of prop blade in flight, new information In article <Nj9Uc.30292$Kt5.2240 (AT) twister (DOT) nyroc.rr.com>, Roger Long wrote: Quote: The three blade prop will be less efficient per unit of area than the two blade where it counts, near Vx with trees in the windshield. Given a limitation on length however, the extra blade area of the three blader can easily offset the efficiency loss by a substantial margin. Anecdotally, I'd say that the extra blade does easily offset the losses. You see quite a few glider tugs (lower powered ones especially) like the Ralleye with a 4-blade prop to improve climb performance (and reduce noise due to shorter blades) -- Dylan Smith, Castletown, Isle of Man Flying: http://www.dylansmith.net Frontier Elite Universe: http://www.alioth.net "Maintain thine airspeed, lest the ground come up and smite thee" Back to top Paul Sengupta Guest Posted: Tue Aug 17, 2004 10:59 am    Post subject: Re: Advancement of prop blade in flight, new information "Dylan Smith" <dylan (AT) vexed3 (DOT) alioth.net> wrote Quote: In article <20040816181349.12901.00001819 (AT) mb-m06 (DOT) wmconnect.com>, AJW wrote: I don't know of a SEL airplane in general use that uses reduction gearing between the shaft and the prop, Dan. Many hundreds of examples of Europa aircraft. My friend's Europa runs the 4-cylinder, opposed, liquid cooled 914S engine at something like 5400RPM in cruise. I think the prop turns less than half that RPM. Or any Rotax engined plane. Paul Back to top tscottme Guest Posted: Tue Aug 17, 2004 11:18 am    Post subject: Re: Advancement of prop blade in flight, new information "DanH" <danh (AT) NOSPAM--delete_this___dragontracks (DOT) org> wrote Quote: AJW wrote: [snip] For what it's worth, at 150 kts and 2500 RPM means the airplane advances about 6 feet per prop rev. A two bladed prop means each balde is in air 3 feet ahead of the prior blade. That's the same number I came up with, but that assumes there's a one-to-one ratio between engine RPM and prop RPM. Is that true of all single engine piston aircraft? I'm obviously not an AC mechanic, but I thought I could see a reduction gear in the cowl. DanH In addition, the term "slippage" comes into play. That's the difference between the theoretical distance the prop should advance with each revolution and the actual. And besides a reduction gear creating a difference between engine and prop RPM, a constant speed prop gives control of the prop speed to the pilot or the prop governor mechanism. -- Scott Back to top Corky Scott Guest Posted: Tue Aug 17, 2004 12:03 pm    Post subject: Re: Advancement of prop blade in flight, new information On 16 Aug 2004 22:13:49 GMT, [email]ajw27703 (AT) wmconnect (DOT) comr[/email]emove (AJW) wrote: Quote: I don't know of a SEL airplane in general use that uses reduction gearing between the shaft and the prop, Dan. They aren't in general use anymore, but ALL the big engined WWII aircraft used some sort of gear reduction unit to turn the prop, every single one of them. You don't see that so much anymore in certified airplanes, but in the world of experimental homebuilt aircraft where auto conversions are not uncommon, they are in use. Corky Scott Back to top Ron Natalie Guest Posted: Tue Aug 17, 2004 1:23 pm    Post subject: Re: Advancement of prop blade in flight, new information "AJW" <ajw27703 (AT) wmconnect (DOT) comremove> wrote Quote: I don't know of a SEL airplane in general use that uses reduction gearing between the shaft and the prop, Dan. My old Navion, Helio Couriers, Republic Seabees, Cessna 175's... Lots of Rotax powered light planes... Not overly common, but they are out there. Back to top G.R. Patterson III Guest Posted: Tue Aug 17, 2004 1:36 pm    Post subject: Re: Advancement of prop blade in flight, new information DanH wrote: Quote: I'm obviously not an AC mechanic, but I thought I could see a reduction gear in the cowl. That was probably the flywheel -- they have toothed edges to mesh with the starter. If gears are used for a reduction system, they are likely to be enclosed in a housing; you wouldn't be able to see them. George Patterson If you want to know God's opinion of money, just look at the people he gives it to. 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