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#12
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On Fri, 27 Feb 2004 23:43:47 -0500, "Morgans"
wrote: IMHO, to take advantage of the auto engine's characteristics, you need a CS prop, even more. Flat pitch for takeoff, then really get the course pitch....... ++++++++++++++++++++++++++++++++++++++++++++++++++ ++++++ COURSE? Sorry, 'Teach'. It's your turn in the barrel. g One need not be gifted or an English major to be educated in the basics of our native tongue. Just being a teacher should induce one to become somewhat less of an embarrassment to this noble vocation.... by osmosis or a remedial 'course', if nothing else. COARSE adj. - Consisting of large particles; not fine in texture COURSE n. 1. a. Onward movement in a particular direction; progress: the course of events. 2. a. A complete body of prescribed studies constituting a curriculum: a four-year course in engineering. b. A unit of such a curriculum: took an introductory course in chemistry; passed her calculus course. Barnyard BOb - |
#13
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Those numbers can not be correct. The power curves in the Lycoming operator's manual show that in standard sea level conditions at 2,300 RPM full throttle, a 150 hp Lyc (O-320 A, E) will produce 132 hp or 88% of full rated power. Interestingly, your 92 hp figure closely matches the propeller load curve at 2,300 RPM. The propeller load curve, however, is not a full throttle curve. Rather, it is a variable throttle static run-up curve using a fixed pitch test prop (or club) chosen to achieve max rated engine RPM at full throttle. If your C-172 POH says that the 150 hp Lyc produces only 92 hp at 2,300 RPM full throttle in standard sea level conditions, then it is wrong by a wide margin. Same thing in my manual. While I'm here, I'd like commend you on your typically spot-on explanations and your generosity in frequently answering questions here. Unfortunately, business and other matters keep me from participating here as much as I'd like. It is folks like you who make the difference here, not the... (well, I'll let that go)....... David O -- http://www.AirplaneZone.com +++++++++++++++++++++++++++++++++++++++++++++++ Bring it up.....and then let it go? g What the hell.... Is this an attempt to get in touch with your 'feminine side' or what? Barnyard BOb - |
#14
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RU ok wrote: Bring it up.....and then let it go? g What the hell.... Is this an attempt to get in touch with your 'feminine side' or what? Just seeking some balance. David O -- http://www.AirplaneZone.com |
#15
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David O wrote in message . ..
(Dan Thomas) wrote: Horsepower is a function of torque multiplied by RPM. A Lycoming engine in an older Cessna 172, for example, produces 150 HP at 2700 RPM under standard conditions (sea level atmospheric pressure and 59°F). In the takeoff roll with the fixed-pitch prop, RPM will be around 2300 RPM, which, according to the POH, would indicate a horsepower output of about 61% of 150, or about 92 horses. Not very good, is it? snip Dan, Those numbers can not be correct. The power curves in the Lycoming operator's manual show that in standard sea level conditions at 2,300 RPM full throttle, a 150 hp Lyc (O-320 A, E) will produce 132 hp or 88% of full rated power. Interestingly, your 92 hp figure closely matches the propeller load curve at 2,300 RPM. The propeller load curve, however, is not a full throttle curve. Rather, it is a variable throttle static run-up curve using a fixed pitch test prop (or club) chosen to achieve max rated engine RPM at full throttle. If your C-172 POH says that the 150 hp Lyc produces only 92 hp at 2,300 RPM full throttle in standard sea level conditions, then it is wrong by a wide margin. Right you are. The 92 HP figure is taken from cruising charts, less than full throttle. My mistake in assuming that the 2300 RPM would have a consistent HP. We once did some physics calcs regarding the acceleration to takeoff speed for the 172. We found that the energy to accelerate that mass to that speed came to 24 HP, demonstrating the enormous losses to prop and airframe drag and wheel rolling friction. Wouldn't it be great if we could reduce those to a fraction of what they are and make truly efficient flying machines? Dan |
#16
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#17
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the test pilot was
interviewed in the hospital. He stated that nothing happened when he called for max power. -------------------------------------------------------------- I hate it when that happens :-) -R.S.Hoover |
#18
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(Dan Thomas) wrote in message . com...
(Jay) wrote in message . com... Seems to me that some of the benefits of the constant speed prop were based on the limitiations of timing (ignition and valve) of the Lyco/Conti engines. If your engine was designed to have a large dynamic range of efficient operation, you won't need the articulated prop as much. .. . . snip . . . A fixed-pitch prop is a compromise and is like having only second gear in your car: lousy acceleration, lousy highway speed. Could this be fixed with fancy engine doodads? Nope. More gears are needed, and the constant-speed prop is the airplane's transmission. It seems to me that the gear analogy is spot on. A variable pitch prop has EXACTLY the same function as the gearbox on a car. |
#19
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It seems to me that the gear analogy is spot on. A variable pitch
prop has EXACTLY the same function as the gearbox on a car. Dumb newbie question here... If you have a prop that is best for cruise....am I right in assuming it has "too much of a bite" on the air when the aircraft is sitting still...therefore the engine doesnt have enough torque...and therefore the prop cant spin quite as fast as it would otherwise...and both these lead to less low speed thrust than you would like? And if that is the case...could you not use something like water mist injection or nitrous oxide to temporarily increase the torque the motor produces? Probably wount make much sense if you really wanted it for many minutes of climbing....but it might make sense if all your trying to do is shorten your takeoff distance..... take care Blll |
#20
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On Sun, 29 Feb 2004 18:52:16 -0800, pacplyer wrote:
(Corky Scott snip That will likely change when auto engines, complete with the computerized ignition and fuel injection, and all the sensors to make it work properly get into the air. But then again, the Lycomings and Continentals would also benefit from such treatment. Variable timing and fuel injection is coming, it's already running on several models, it's called FADEC for Fully Automated Digital Electronic Control. Corky Scott I think you are right Corky. FADEC (Full Authority Digital Engine Control) has been around on jets since the 70's. It is unquestionably the best way to reach TBO and optimum burn performance for an individual engine. It however has resulted in unforeseen accidents (e.g: Airbus 330 in Toulouse, France, where test pilot got behind power curve, then pushed throttles to the wall, and FADEC refused due to thermal spool up considerations. Its programming decided that full power would be available to the crew in something like five seconds. This saves millions for the fleet every fiscal year. Problem was: The prototype hit the stand of trees in something like six seconds… This was caught on video, and the test pilot was interviewed in the hospital. He stated that nothing happened when he called for max power. If I had FADEC in a single-engine GA aircraft I would want a non-software override. pacplyer Two comments: You've mixed up two different accidents here. The 330 at Toulouse was a loss of control due to the aircraft (on autopilot) going way below VMCA with one engine at idle and the other at full take-off thrust. The sat and watched until it was too late to recover. The accident you are referring to was the A320 at Mulhouse-Habsheim. The pilot did a very low (30 ft AGL) pass with the thrust at idle. The speed decreased til he was at full aft stick, riding on the AOA limiter just above the stall. Then he realized that what he had thought were just low bushes when he was looking down on them as he descended, were actually trees that were higher than he was. He couldn't raise the nose, as the fly-by-wire (FBW) was already on the AOA limiter, so the only way to climb was to get more airspeed. He slammed the thrust levers forward, and the FADEC accelerated the engine on its normal acceleration schedule. Turbine engines run more efficiently if they are running close to the surge line (i.e almost ready to compressor stall). But the engine has to come closer to the surge line to accelerate. So the closer you run to the surge line the slower acceleration you'll have. FAR 25.119(a) requires go-around performance to be calculated using the thrust that is available 8 seconds after a throttle slam from idle. Manufacturers want the engine to run as efficiently as possible, but they don't want to take a hit on the AFM go-around performance. So, they typically design the fuel controls to allow full go-around thrust to be reached in just less than 8 seconds from a throttle slam from idle. I've done tests to check the acceleration on many transport category aircraft, and the result is usually somewhere between 7 and 8 seconds, and this is the same no matter whether the engine has a FADEC or an "old fashioned" hydro-mechanical fuel control unit. So don't blame the FADEC for the A320 accident at Mulhouse-Habsheim. It was caused by a pilot who had way too much confidence in the low-speed protections of the FBW. Fortunately the FBW prevented him from raising the nose, as then the aircraft would have stalled, any many people would probably have died. As it was "only" three live were lost. -- Kevin Horton RV-8 (finishing kit) Ottawa, Canada http://go.phpwebhosting.com/~khorton/rv8/ e-mail: khorton02(_at_)rogers(_dot_)com |
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