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#1
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Constant Speed Prop vs Variable Engine Timing
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. |
#2
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"Jay" wrote in message om... 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. Prop blades are just rotating wings. The goal is to run the blades at their most efficient angle of attack for the RPM and aircraft airspeed. The performance of the prop is best at low RPM but the piston engine driving it is likely to be most efficient at a higher RPM. That is the reason that high performance piston aircraft have both PRSU's and constant speed props. Some experimental powerplant/prop systems included a two speed gearbox in addition to the CS prop to run the engine at high RPM at takeoff and low RPM for cruise. These experimental engines also shifted the cam and ignition timing for the two PRSU ratios. This helped the prop blades stay at the best AOA to maximize thrust and optimized the engine at two set points, high RPM for takeoff and low RPM for long range cruise. This was at the very end of large piston engine development and an attempt to wring the last bit of performance out of these monsters. Having an engine with a wide "dynamic range" is nice for a car but less useful for an airplane where it is best to optimize the engine for one RPM and let the CS prop and PRSU operate the prop in the most efficient way. Bill Daniels |
#3
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Thanks for your insight, which raise a few questions:
Apart from the geared Cessna (which isn't all that hi performance) which aircraft have a PSRU AND a CS prop? Which experimental aircraft has 2 speed gear boxes? I heard of a guy flying a WW1 replical biplane on a Honda motocycle engine. He just kept the original gear box and said he actually shifts gears depending on if he's climbing or cruising. My point about using an engine that can operate efficiently over a large range of RPMs (like a modern automobile engine) is that the CS prop is NOT as necessary although it certainly does help, no doubt about it. Certainly you will get you peak horsepower at high revs, but the moderm engine has a fatter torque curve due to being able to change valve AND ignition timing in a manner optimum for the particular revs it is at. The Lyco/Conti design takes a double hit for operating at low revs, its off the peak HP point, and its timing was peaked for a specific RPM. "Bill Daniels" wrote in message ... "Jay" wrote in message om... 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. Prop blades are just rotating wings. The goal is to run the blades at their most efficient angle of attack for the RPM and aircraft airspeed. The performance of the prop is best at low RPM but the piston engine driving it is likely to be most efficient at a higher RPM. That is the reason that high performance piston aircraft have both PRSU's and constant speed props. Some experimental powerplant/prop systems included a two speed gearbox in addition to the CS prop to run the engine at high RPM at takeoff and low RPM for cruise. These experimental engines also shifted the cam and ignition timing for the two PRSU ratios. This helped the prop blades stay at the best AOA to maximize thrust and optimized the engine at two set points, high RPM for takeoff and low RPM for long range cruise. This was at the very end of large piston engine development and an attempt to wring the last bit of performance out of these monsters. Having an engine with a wide "dynamic range" is nice for a car but less useful for an airplane where it is best to optimize the engine for one RPM and let the CS prop and PRSU operate the prop in the most efficient way. Bill Daniels |
#4
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"Jay" wrote in message om... Apart from the geared Cessna (which isn't all that hi performance) which aircraft have a PSRU AND a CS prop? Navions, Helio Couriers, Twin Commanders, Beech Twin Bonanzas, Republic SeaBee, Beech Queen Air all have models that use variants of the Geared Lycomings (435 and 480). They almost always drive a variable pitch (some cases not constant speed ) prop. |
#5
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All moderate to high power geared piston engines and all turboprops have
gear reduction and CS props. It is not just about engine efficiency, it is about prop efficiency over a range of airspeeds and altitudes. As airspeed increases, AOA of the prop decreases so you can not have a prop which works efficiently for takeoff and cruise unless the airplane is very slow. My airplane advances 19 feet for each revolution of the prop at cruise vs. 0 feet at the beginning of the takeoff roll. Similiarly, the prop needs more pitch when the air is less dense to be efficient.. Mike MU-2 "Jay" wrote in message om... Thanks for your insight, which raise a few questions: Apart from the geared Cessna (which isn't all that hi performance) which aircraft have a PSRU AND a CS prop? Which experimental aircraft has 2 speed gear boxes? I heard of a guy flying a WW1 replical biplane on a Honda motocycle engine. He just kept the original gear box and said he actually shifts gears depending on if he's climbing or cruising. My point about using an engine that can operate efficiently over a large range of RPMs (like a modern automobile engine) is that the CS prop is NOT as necessary although it certainly does help, no doubt about it. Certainly you will get you peak horsepower at high revs, but the moderm engine has a fatter torque curve due to being able to change valve AND ignition timing in a manner optimum for the particular revs it is at. The Lyco/Conti design takes a double hit for operating at low revs, its off the peak HP point, and its timing was peaked for a specific RPM. "Bill Daniels" wrote in message ... "Jay" wrote in message om... 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. Prop blades are just rotating wings. The goal is to run the blades at their most efficient angle of attack for the RPM and aircraft airspeed. The performance of the prop is best at low RPM but the piston engine driving it is likely to be most efficient at a higher RPM. That is the reason that high performance piston aircraft have both PRSU's and constant speed props. Some experimental powerplant/prop systems included a two speed gearbox in addition to the CS prop to run the engine at high RPM at takeoff and low RPM for cruise. These experimental engines also shifted the cam and ignition timing for the two PRSU ratios. This helped the prop blades stay at the best AOA to maximize thrust and optimized the engine at two set points, high RPM for takeoff and low RPM for long range cruise. This was at the very end of large piston engine development and an attempt to wring the last bit of performance out of these monsters. Having an engine with a wide "dynamic range" is nice for a car but less useful for an airplane where it is best to optimize the engine for one RPM and let the CS prop and PRSU operate the prop in the most efficient way. Bill Daniels |
#6
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#7
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#8
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This is directed more at the original poster,
Latest Kitplanes, March 2004 top of page 25 under heading of "Performance". Comparisons of 6 constant speed props and one fixed pitch prop on a RV-8. Speeds within 4% fastest to slowest, Constant speed props are very useful for take-off and speed reduction to pattern, but counter-intuitively, not much difference in top speed. Given that the piece that takes the bite out of the air would be difficult to optimize more than it is, it's hard to figure how current engine tweaking could do any better and I see all the bells and whistles in New Model Training all the time for cars. There are practical limits with what you can do with add-on doo-dads. Dale Alexander Velocity Stealth RG Gullwing Toyota Master Tech Mazda Master Tech Been working on cars WAY too long... "Dan Thomas" wrote in message om... (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. 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? So, we only have about 60 percent of the engine's power in the takeoff. Worse yet, this diminished power is going into a propeller that is largely in a stalled condition at the beginning of the takeoff roll (because of high blade pitch angle and low forward speed) and is producing much less than its max thrust as a result, and acceleration is pretty dismal. What can we gain by fooling with valve or ignition timing in a situation like this? Not much. We add weight and failure points, neither of which are welcome here, and gain very little in performance. So the constant-speed prop was invented. It is controlled by a governor so that the engine is allowed to reach full rated RPM, which produces full rated HP (if at sea level and standard temp), and because the propeller's pitch is much lower in this mode, much more of it is producing thrust instead of useless stall turbulence. In cruising flight, the pitch increases to keep the engine RPM within limits while still producing more thrust and a higher cruise speed than a fixed-pitch prop can. 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. Dan |
#9
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#10
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"Jay" wrote My point about using an engine that can operate efficiently over a large range of RPMs (like a modern automobile engine) is that the CS prop is NOT as necessary although it certainly does help, no doubt about it. Certainly you will get you peak horsepower at high revs, but the moderm engine has a fatter torque curve due to being able to change valve AND ignition timing in a manner optimum for the particular revs it is at. The Lyco/Conti design takes a double hit for operating at low revs, its off the peak HP point, and its timing was peaked for a specific RPM. 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 at high speed and high altitude, so the engine can loaf along at really slow and low HP output, to keep the thrust up, while at the low engine RPM'S. Most of the successful auto conversions tend to keep it simple, and variable valve timeing is not in that spirit. YMMV. -- Jim in NC --- Outgoing mail is certified Virus Free. Checked by AVG anti-virus system (http://www.grisoft.com). Version: 6.0.594 / Virus Database: 377 - Release Date: 2/24/2004 |
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