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#1
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In article ,
Orval Fairbairn wrote: They wanted to keep the main landing gear as short as possible, to simplify structural loads. The Hellcat had a relatively long main gear leg. A four-bladed prop for the same power output should be shorter than the three-bladed equivalent. -- |
#2
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In article ,
Steve Hix writes: In article , Orval Fairbairn wrote: They wanted to keep the main landing gear as short as possible, to simplify structural loads. The Hellcat had a relatively long main gear leg. A four-bladed prop for the same power output should be shorter than the three-bladed equivalent. But somewhat less efficient. Each blade added to a propeller knocks the efficiency down. Later model Corsairs with the more powerful R2800 'C' series engines had a 4 blade prop, but the diameter is essentially the same - 13 ' 2". You also have to tune 'J', the Advance Ratio of the propeller (A product of propeller rotational speed, propeller diameter, and the forward velocity of the propeller (and the airplane it's attached to.) in order to get teh best performance. reducing the diameter for a given horsepower may have beneficial effects at high speed, but severely affect the propeller's performance below, say, about 350 mph. It's all a balancing act - but in ggeneral, you're best off going with the largest diameter propeller with the fewest number of blades that you can practically manage. -- Pete Stickney A strong conviction that something must be done is the parent of many bad measures. -- Daniel Webster -- |
#3
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Peter Stickney wrote:
It's all a balancing act - but in ggeneral, you're best off going with the largest diameter propeller with the fewest number of blades that you can practically manage. i) I'm sure I remember seeing, years ago, a picture of a Noorduyn Norseman with a single-bladed prop. Since you seem to know what you are talking about (more than I do, anyway), what factors would drive a manufacturer to adopt such a radical solution? ii) Radical solutions such as the Unducted Fan proposals mooted a few years ago, had many curved blades - any idea what gain they were seeking that justified the loss in efficiency? iii) How does this work with contraprops? On the face of it they must interfere with each other horribly, but they seem to fly quite well. Can you point me in the direction of some clues? |
#4
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In article ,
Alan Dicey writes: Peter Stickney wrote: It's all a balancing act - but in ggeneral, you're best off going with the largest diameter propeller with the fewest number of blades that you can practically manage. i) I'm sure I remember seeing, years ago, a picture of a Noorduyn Norseman with a single-bladed prop. Since you seem to know what you are talking about (more than I do, anyway), what factors would drive a manufacturer to adopt such a radical solution? In a word, efficiency. Note that many of the model airplanes used in free-flight competitions, (Escpecially the rubber powered ones, where the judges issue you your engine (So many strands of Pirelli rubber, of some particular length) and "fuel" it up for you (So many turns of the rubber bands)) where getting the absolute most out of the limited omount of energy you've got means the difference between winning and losing, use very wide chord single-bladed propellers. The downside is that you need a fairly large diameter. That's not much of a problem in a hand-launched model airplane, but it doesn't work so well in Full Scale stuff. ii) Radical solutions such as the Unducted Fan proposals mooted a few years ago, had many curved blades - any idea what gain they were seeking that justified the loss in efficiency? In tha case, what they're trying to do is reduce the effects of the shockwaves that form on the propeller blades as they fly further and firther into the transonic region. It's not unlike sweeping a wing back to delay the Mach Number that the drag rise occurs at, and the magnitude of the drag rise. Above about Mach 0.65, the efficiency of a straight propeller drops off alarmingly. At typical airliner cruise speeds, (Mach 0.78-0.85) efficiency would be down around 60% at teh low end of the speed range, and 50% at the high end. That's not very useful at all - there are some measures that you can do to cut the tip speed down - for example, the Tu-95 Bear (Russian turboprop transonic bomber) uses a very high step-down gearing from the engines to the propellers - the props rotate at 750 RPM, vs, say, 1500 or so for that of a P-51, and a very clever variable pressure ratio compressor system in its engines that essentially "supercharges" them to deliver sea level power at 40,000'. (About 3 times what you'd get from a typical turboprop). The swept propeller blades supply efficiencies in the Mach 0.78-0.85 range of between 75% and 70%. Using many blades allows the diameter to be cut down from, say, 22 ft for our notional conventional propeller to 13 ft. This gives a lower airspeed at the propeller tip than a large diameter propeller, thus delaying the transonic effects. (Note that the entire propeller doesn't go transonic - the airspeed at the propeller blade is a product of the propeller's rotational speed, and teh forward speed of the airplane. The rotational speed of the propeller in ft/sec or m/sec increases as you move outward along the propeller blade. So, a propeller will start having supersonic flow appear at the tips, with the supersonic flow field moving inward as speed increases. A smaller diameter and a slower rotational speed are helpful in delaying the formation of these shock waves. (transonic/supersonic flow). You do lose efficiency in the lower speed ranges, but you get big gains at what your desired cruise speeds are. iii) How does this work with contraprops? On the face of it they must interfere with each other horribly, but they seem to fly quite well. Can you point me in the direction of some clues? A contraprop does lose some efficiency by placing one propeller behind the other, and it requires a more complex drive system. (Which gave fits on several early U.S. contraprop-equipped aircraft, most notably the XB-35 Flying Wing, where they never got the contraprops doped out, and the Hughes XF-11 recon machine (looked like a hyperthyroid P-38). which was lost on its first flight becasue the aft bladeset in one of the contraprops went into reverse pitch at low altitude. (This is the crash that nearly killed Howard Hughes, and led to his drug addiction (painkillers) and fear of infection.) What you gain is a greater ability for a propeller of a particular diameter to absorb power, adn the elimination of torque and P-factor (destabilization of the airframe due to the rotating airflow from the propeller affecting the airframe). P-Factor is a Big Deal, with a high-powered airplane. For example, with a P-51 or a Corsair, you have to be careful with throttle movement at low speeds, or on takeoff. If you jam the throttle to it too fast, you'll either swing off the runway or roll the airplane inverted. -- Pete Stickney A strong conviction that something must be done is the parent of many bad measures. -- Daniel Webster |
#5
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Peter Stickney wrote:
In article , Alan Dicey writes: Peter Stickney wrote: It's all a balancing act - but in ggeneral, you're best off going with the largest diameter propeller with the fewest number of blades that you can practically manage. a Noorduyn Norseman with a single-bladed prop: what factors would drive a manufacturer to adopt such a radical solution? In a word, efficiency. Hmm. Efficiency in the sense of translating engine power to thrust? I can't see it being aimed at top speed, so I guess it would give more range for a given fuel load? ii) Radical solutions such as the Unducted Fan proposals mooted a few years ago, had many curved blades - any idea what gain they were seeking that justified the loss in efficiency? In tha case, what they're trying to do is reduce the effects of the shockwaves that form on the propeller blades as they fly further and firther into the transonic region. It's not unlike sweeping a wing back to delay the Mach Number that the drag rise occurs at, and the magnitude of the drag rise. [...] You do lose efficiency in the lower speed ranges, but you get big gains at what your desired cruise speeds are. Of course - tip speed and transonic drag rise. To get more airscrew in the airflow /and/ keep the tip speed suitably subsonic,the only answer is more blades - with sweepback to delay the drag rise. I should have remebered that from the discussions at the time. None of the Unducted Fan experiments seem to have made it into a production implementation. I guess the aim was a cheaper powerplant - propellors being cheaper than ducted fans - but the loss of efficiency was too great. iii) How does this work with contraprops? On the face of it they must interfere with each other horribly, but they seem to fly quite well. What you gain is a greater ability for a propeller of a particular diameter to absorb power, adn the elimination of torque and P-factor (destabilization of the airframe due to the rotating airflow from the propeller affecting the airframe). So, for an increase in power turned into thrust there's an improvement in flyability and the ability to make the airframe lighter because it doesn't have to absorb the stresses - they're balanced out at the source. That explains to me how the Fairey Gannet was able to shut off one half of the Double Mamba powerplant, feather one half of the contraprop and achieve better endurance at patrol speed. Thanks very much for taking the time to give me some pointers. Do you do this for a living? :-) |
#6
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![]() hobo wrote: In article , "Geoffrey Sinclair" wrote: Corsair propeller diameter 13 feet 4 inches, ground clearance 9.1 inches, engine R-2800-8. The corsair used a 3-blade prop. Why didn't they use a smaller 4-blade prop if ground clearance was such an issue? Check again, please: http://www.warbirdalley.com/f4u.htm | George Ruch | "Is there life in Clovis after Clovis Man?" |
#7
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In article ,
George Ruch wrote: hobo wrote: The corsair used a 3-blade prop. Why didn't they use a smaller 4-blade prop if ground clearance was such an issue? Check again, please: http://www.warbirdalley.com/f4u.htm The F4U-4 had a four-blade prop, but earlier versions had three blades, and include a large part of what saw combat in WW2. -- |
#8
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In article ,
George Ruch wrote: hobo wrote: In article , "Geoffrey Sinclair" wrote: Corsair propeller diameter 13 feet 4 inches, ground clearance 9.1 inches, engine R-2800-8. The corsair used a 3-blade prop. Why didn't they use a smaller 4-blade prop if ground clearance was such an issue? Check again, please: http://www.warbirdalley.com/f4u.htm The link you provided has no textual information regarding the prop, but there is a picture, dated 2001, of a surviving Corsair with a 4 blade prop. This prop may not be the original factory issue. When this question was first posted the first website on the Corsair I found was this: http://www.nasm.si.edu/research/aero...t/voughtf4.htm This is the Smithsonian's website and has a photo of a Corsair with a 3 bladed prop and this text: "The R-2800 radial air-cooled engine developed 1,850 horsepower and it turned a three-blade Hamilton Standard Hydromatic propeller with solid aluminum blades spanning 13 feet 1 inch." This website was my sole source for the claim that the Corsair had a 3 blade prop. Perhaps a 4 blade was later added, but it seems odd that a 3 blade was ever used if ground clearance was so pivotal to the whole design. -- |
#9
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In article ,
hobo writes: The link you provided has no textual information regarding the prop, but there is a picture, dated 2001, of a surviving Corsair with a 4 blade prop. This prop may not be the original factory issue. When this question was first posted the first website on the Corsair I found was this: http://www.nasm.si.edu/research/aero...t/voughtf4.htm This is the Smithsonian's website and has a photo of a Corsair with a 3 bladed prop and this text: "The R-2800 radial air-cooled engine developed 1,850 horsepower and it turned a three-blade Hamilton Standard Hydromatic propeller with solid aluminum blades spanning 13 feet 1 inch." This website was my sole source for the claim that the Corsair had a 3 blade prop. Perhaps a 4 blade was later added, but it seems odd that a 3 blade was ever used if ground clearance was so pivotal to the whole design. Hobo, All the F4U-1 models had 3-blade propellers, with a 13'1" diameter. The later production models, the F4U-4 and F4U-5, with higher-powered engines, had 4 blade props with a 13'2" diameter, to absorb the extra power. (More than 800 HP in some versions.) So, its fair to say that they didn't go to a 4-blade prop to decrease ground clearance. -- Pete Stickney A strong conviction that something must be done is the parent of many bad measures. -- Daniel Webster -- |
#10
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![]() On Sun, 4 Jul 2004 21:46:07 +0000 (UTC), hobo wrote: This website was my sole source for the claim that the Corsair had a 3 blade prop. Perhaps a 4 blade was later added, but it seems odd that a 3 blade was ever used if ground clearance was so pivotal to the whole design. Prop design is extraordinarily complicated. The Corsair, like several of the high powered, high speed fighters of WWII had a high enough performance to reach the boundaries of propeller powered design. The problem was how to harness all that power. You can use a multi blade prop with a smaller diameter, but acceleration and climb may be compromised. The people who designed the Corsair understood that you loose whatever thrust was being developed by the inner diameter of the prop because the thrust is masked by the cowling housing the engine. One way of getting around the large cowling is to make a large prop. The large prop allows good takeoff and climb performance. The ability of the Corsair to haul large loads into the air was likely one of the reasons it was still flying for the Navy by the time of the Korean war, even though it had been designed in 1938. There were actually several reasons for the inverted gull wing design: This was to be a Navy carrier fighter. Carrier fighters have to land on board aircraft carriers and this landing is often so harsh that it's been likened to a barely controlled crash. The landing gear had to be very very sturdy to take the severe G forces when the airplane smacked down on the deck. The design of the fuselage, as was typical for the day, involved a round cross section. Mating a wing to a round cross section required a large fairing to reduce drag at the wing to fuselage intersection. The fairing was not necessary if the wing could be mated at a 90 degree angle to the fuselage. Finally, the prop being proposed was the biggest ever attached at the time to a fighter, because the design was to use the Pratt and Whitney R-2800 engine which at the time was one of the most powerfull ever developed. The elegant solution to all three problems was to use the inverted gull wing. This kept the landing gear short, or at least shorter than it would have been with a straight wing, made the wing to fuselage intersection possible without a fairing, and gave the necessary clearance for that huge prop. It was not without it's problems however. Corky Scott |
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