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#21
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"AJW" wrote in message
... Wouldn't you think the more serious interference would be the prop wash beating against the airplane itself? I'd have thought we'd see more pusher props, but that's not a common SEL configuration either. I wonder why? Pusher props are only *theoretically* more efficient. Since the airframe disturbs airflow into the prop, you just wind up with a different kind of inefficiency. Also, there's the problem of how to protect the prop from ground strikes, engine cooling, CG and the like. All solveable, of course, but the engine/prop in front configuration actually works pretty well in most cases. Pete |
#22
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In article , AJW wrote:
I'd have thought we'd see more pusher props, but that's not a common SEL configuration either. I wonder why? If you crash, it's better to hide behind a big heavy engine instead of being sandwiched between said engine and ground. ;-) -- Janne Blomqvist |
#23
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"David CL Francis" wrote in message
... The extreme of this is the enclosed fan where the enclosure markedly reduces tip losses. The fan runs nicely along like this with a high blade area and little daylight visible through the disc. Whatever happened to the concept of piston engines running a ducted fan? That aerocar thing has them, but what about on other more normal planes? How efficient is a ducted fan compared to a prop? I seem to remember hearing in model aircraft settings, a prop is more efficient. Paul |
#24
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serious interference would be the prop wash
beating against the airplane itself? I'd have thought we'd see more pusher props, but that's not a common SEL configuration either. I wonder why? Pusher props are only *theoretically* more efficient. Since the airframe disturbs airflow into the prop, you just wind up with a different kind of inefficiency. Also, there's the problem of how to protect the prop from ground strikes, engine cooling, CG and the like. All solveable, of course, but the engine/prop in front configuration actually works pretty well in most cases. I agree with some of your observations, but re efficiency -- the airflow into the loow pressure area around the prop comes from pretty much everywhere, but the exit flow is directed backwards. I think props don't get much thrust from 'suck' as opposed to 'push'. I suppose my model's example would be to stand a few feet in front of a fan, then a few feet behind it. Didn't the Skymaster do better with the rear prop, and the Rutan around the world airplane? The other issues re having the engine visit the cockpit during a crash surely bear thinking about. It's also true that propwash does a good job of keeping the Mooney's windscreen clear during rain. |
#25
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"AJW" wrote in message
... I agree with some of your observations, but re efficiency -- the airflow into the loow pressure area around the prop comes from pretty much everywhere, but the exit flow is directed backwards. I think props don't get much thrust from 'suck' as opposed to 'push'. I never said they did. However, an airplane flying 100mph through the air WILL necessarily have significant flow through the prop from the front. If an airframe is in the way of that airflow, it affects the airflow and in turn the prop. [...] Didn't the Skymaster do better with the rear prop, and the Rutan around the world airplane? I don't know much specific about Voyager. I'd say the fact that it was the rear engine they used in cruise, not the front, says something about that particular design. Note, of course, that the rear engine of Voyager was a smaller engine; it was the one used in cruise for fuel efficiency reasons, and its location may have been dictated by CG issues or something else, rather than efficiency per se. Only Rutan could answer for sure why exactly the lower horsepower engine was put at the back, and whether that was a significant issue or not. As far as the Skymaster goes, everything I've heard about the 337 was that the rear engine/prop was always a problem. Thrust was worse and the engine had cooling problems. In any case, as I said before, it's not like rear engines are impossible. There are numbers of aircraft out there flying with rear engines. It's just that a rear engine is not the miracle worker one might think it is. The other issues re having the engine visit the cockpit during a crash surely bear thinking about. Certainly a concern, but I'm not aware of any data that indicates rear-engine aircraft are significantly less crash-worthy. In a crash where the engine is likely to actually shift all the way into the cabin, the cabin is not likely to have survived the crash in any case, whether the engine is in front or the rear. It's also true that propwash does a good job of keeping the Mooney's windscreen clear during rain. This is only a concern during ground operations. In flight, and in fact quite early in the takeoff run, the relative wind due to the aircraft's movement is sufficient for keeping the windscreen clear. Pete |
#26
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The rear engine supplies more thrust on a Skymaster.
See: http://www.skymaster.org.uk/perform.asp Karl "Peter Duniho" wrote in message ... "AJW" wrote in message ... I agree with some of your observations, but re efficiency -- the airflow into the loow pressure area around the prop comes from pretty much everywhere, but the exit flow is directed backwards. I think props don't get much thrust from 'suck' as opposed to 'push'. I never said they did. However, an airplane flying 100mph through the air WILL necessarily have significant flow through the prop from the front. If an airframe is in the way of that airflow, it affects the airflow and in turn the prop. [...] Didn't the Skymaster do better with the rear prop, and the Rutan around the world airplane? I don't know much specific about Voyager. I'd say the fact that it was the rear engine they used in cruise, not the front, says something about that particular design. Note, of course, that the rear engine of Voyager was a smaller engine; it was the one used in cruise for fuel efficiency reasons, and its location may have been dictated by CG issues or something else, rather than efficiency per se. Only Rutan could answer for sure why exactly the lower horsepower engine was put at the back, and whether that was a significant issue or not. As far as the Skymaster goes, everything I've heard about the 337 was that the rear engine/prop was always a problem. Thrust was worse and the engine had cooling problems. In any case, as I said before, it's not like rear engines are impossible. There are numbers of aircraft out there flying with rear engines. It's just that a rear engine is not the miracle worker one might think it is. The other issues re having the engine visit the cockpit during a crash surely bear thinking about. Certainly a concern, but I'm not aware of any data that indicates rear-engine aircraft are significantly less crash-worthy. In a crash where the engine is likely to actually shift all the way into the cabin, the cabin is not likely to have survived the crash in any case, whether the engine is in front or the rear. It's also true that propwash does a good job of keeping the Mooney's windscreen clear during rain. This is only a concern during ground operations. In flight, and in fact quite early in the takeoff run, the relative wind due to the aircraft's movement is sufficient for keeping the windscreen clear. Pete |
#27
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I think props don't get much thrust from 'suck' as opposed to 'push'. Props are like wings - the "upper" surface, i.e. the front prop surface is critical to good performance. The pressure differential between the front of the prop and the back is what produces the force that moves the aircraft forward, and most of that differential (when compared to static pressure) is due to the lower-than-static pressure on the front/upper surface of the prop/wing. Todd Pattist Actually, I think the current thinking is change of momentum in the downward air direction. there was an extensive thead on this in the newsgroup. Think about this, standing by a fan: you feel more force downstream, where the air is moving in a fairly well defined column, rather than upstream, where in fact the air is drawn in from all directions. |
#28
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"Peter Duniho" wrote in message As far as the Skymaster goes, everything
I've heard about the 337 was that the rear engine/prop was always a problem. Thrust was worse and the engine had cooling problems. You've heard wrong. The overheating problem was quickly corrected. Single-engine climb-rate was 100 fpm better at sea-level on the rear engine. D. |
#29
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"Capt.Doug" wrote in message
... You've heard wrong. The overheating problem was quickly corrected. Single-engine climb-rate was 100 fpm better at sea-level on the rear engine. Thank you kage and Doug. I stand corrected. Still, that doesn't mean that a rear-engine is the perfect solution for every airplane, and of course it's similarly not true that no airplane should have a rear engine. |
#30
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"Peter Duniho" wrote in message Still, that doesn't mean that a
rear-engine is the perfect solution for every airplane, and of course it's similarly not true that no airplane should have a rear engine. Which is the reason Rutan doesn't put pusher engines on all of his designs. It's my understanging that the C-337 performs better on the rear engine because the rear prop sucks air in behind the blunt end of the fuselage thus decreasing drag. On a slick low-drag design, I imagine the difference between front and rear drive would be much less pronounced. D. |
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