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#21
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"AnyBody43" wrote in message om... (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. Not quite. Gears don't have preferred operating conditions, props do. The engine has its preferred RPM and torque for optimum efficiency and the prop blades have their optimum angle of attack. If the engine/prop combination results in the prop operating at a higher (or lower) angle of attack than optimum to absorb the torque of the engine (Prop governor increases pitch to hold RPM setting.) then the combination operates below optimum conditions. Under some conditions, it would make sense to introduce a third variable i.e. a gearbox between the engine and prop, to allow both the engine and prop to operate at peak efficiency. This was the reason that two-speed grearsets were installed in the nose case of some large radials. This, in turn, allowed the propeller designer to optimize his prop blades for a single AOA, thus gaining still more efficiency. The problem, simply stated was this: How does a heavily loaded, long-range bomber haul itself off a short runway and climb to cruise altitude and then shift to highly efficient, long-range cruise. The answer was just emerging from the labs as the world shifted to turbines. The flight engineer would shift his engines into a "hole gear" by selecting a cam profile and engine timing optimized for the low gear that would let the engines scream at high RPM and pump massive HP into props set for maximum acceleration and climb. Once in cruise, the engineer would shift his engines back to low RPM, high efficiency settings. A propeller is not a gear box analog. It is more like the torque converter in an automatic transmission. A torque converter still needs a gearbox behind it for efficiency. Bill Daniels |
#22
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Either that or a WEP setting (break a wire at full throttle) that
basically says to the microcontroller "Its now or never." Something that indicates that there is a real possibility of loss of vehicle and also disconnects the field current for the alternator. (pacplyer) wrote in message . com... If I had FADEC in a single-engine GA aircraft I would want a non-software override. pacplyer |
#23
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#24
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"Bill Daniels" wrote in message news:5_- 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. Not quite. Gears don't have preferred operating conditions, props do. The engine has its preferred RPM and torque for optimum efficiency and the prop blades have their optimum angle of attack. If the engine/prop combination results in the prop operating at a higher (or lower) angle of attack than optimum to absorb the torque of the engine (Prop governor increases pitch to hold RPM setting.) then the combination operates below optimum conditions. Under some conditions, it would make sense to introduce a third variable i.e. a gearbox between the engine and prop, to allow both the engine and prop to operate at peak efficiency. This was the reason that two-speed grearsets were installed in the nose case of some large radials. This, in turn, allowed the propeller designer to optimize his prop blades for a single AOA, thus gaining still more efficiency. The problem, simply stated was this: How does a heavily loaded, long-range bomber haul itself off a short runway and climb to cruise altitude and then shift to highly efficient, long-range cruise. The answer was just emerging from the labs as the world shifted to turbines. The flight engineer would shift his engines into a "hole gear" by selecting a cam profile and engine timing optimized for the low gear that would let the engines scream at high RPM and pump massive HP into props set for maximum acceleration and climb. Once in cruise, the engineer would shift his engines back to low RPM, high efficiency settings. First time I've ever heard of gear-shifted props in certified engines. Which engines were these? I know that many radials (and other engine layouts) used reduction gearing in the case nose to allow the engine to run faster and produce more HP while keeping the prop within safe limits, and that there were two-speed geared superchargers on many of these engines, but two-speed props? Jim Bede used a snowmobile-type propshaft drive in the early BD-5s but abandoned it as unworkable. It still required a relatively tiny prop to keep the tip speeds subsonic. As far as the propeller pitch angles go, the constant speed prop improves takeoff performance by more than just letting engine RPM reach redline to produce max HP. It reduces the angle of attack so that more of the prop is unstalled and producing thrust in the static condition, improving acceleration and shortening takeoff distance. The inboard sections of a fixed-pitch prop blade have a large angle so that they still produce thrust in faster forward flight even though they don't travel the circumferential distance that blade areas near the tips do, but the large angle means a stalled blade, or at least a really turbulent flow, at low forward speeds. A gear-shifted fixed-pitch prop will still have those problems. Dan |
#25
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On Mon, 01 Mar 2004 12:38:15 -0800, Dan Thomas wrote:
"Bill Daniels" wrote in message news:5_- 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. Not quite. Gears don't have preferred operating conditions, props do. The engine has its preferred RPM and torque for optimum efficiency and the prop blades have their optimum angle of attack. If the engine/prop combination results in the prop operating at a higher (or lower) angle of attack than optimum to absorb the torque of the engine (Prop governor increases pitch to hold RPM setting.) then the combination operates below optimum conditions. Under some conditions, it would make sense to introduce a third variable i.e. a gearbox between the engine and prop, to allow both the engine and prop to operate at peak efficiency. This was the reason that two-speed grearsets were installed in the nose case of some large radials. This, in turn, allowed the propeller designer to optimize his prop blades for a single AOA, thus gaining still more efficiency. The problem, simply stated was this: How does a heavily loaded, long-range bomber haul itself off a short runway and climb to cruise altitude and then shift to highly efficient, long-range cruise. The answer was just emerging from the labs as the world shifted to turbines. The flight engineer would shift his engines into a "hole gear" by selecting a cam profile and engine timing optimized for the low gear that would let the engines scream at high RPM and pump massive HP into props set for maximum acceleration and climb. Once in cruise, the engineer would shift his engines back to low RPM, high efficiency settings. First time I've ever heard of gear-shifted props in certified engines. Which engines were these? I know that many radials (and other engine layouts) used reduction gearing in the case nose to allow the engine to run faster and produce more HP while keeping the prop within safe limits, and that there were two-speed geared superchargers on many of these engines, but two-speed props? Jim Bede used a snowmobile-type propshaft drive in the early BD-5s but abandoned it as unworkable. It still required a relatively tiny prop to keep the tip speeds subsonic. As far as the propeller pitch angles go, the constant speed prop improves takeoff performance by more than just letting engine RPM reach redline to produce max HP. It reduces the angle of attack so that more of the prop is unstalled and producing thrust in the static condition, improving acceleration and shortening takeoff distance. The inboard sections of a fixed-pitch prop blade have a large angle so that they still produce thrust in faster forward flight even though they don't travel the circumferential distance that blade areas near the tips do, but the large angle means a stalled blade, or at least a really turbulent flow, at low forward speeds. A gear-shifted fixed-pitch prop will still have those problems. Dan Some of the supercharged recips had a gear box with two different gear ratios to drive the supercharger. They needed to spin the supercharger at high rpm at high altitude in order to get enough manifold pressure. But if they used the same supercharger gear ratio at low altitude it would produce more manifold pressure than the engine could handle at full throttle. The engine would then have to be run very throttled, and there would be a lot of wasted power used to spin that supercharger at a needlessly high rpm. So, they used a different gear ratio to spin the supercharger at a lower rpm for take-off and low altitude flight. -- Kevin Horton RV-8 (finishing kit) Ottawa, Canada http://go.phpwebhosting.com/~khorton/rv8/ e-mail: khorton02(_at_)rogers(_dot_)com |
#26
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Great Stuff Kevin, thanks for your insight. I have a couple of points
I slightly disagree with however further down in your post. :-) Kevin Horton wrote: 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. Yeah I mixed those up. Thanks for keeping me honest. Those were both IMHO over-reliance in airbus automation accidents IIRC. I saw this frequently with new-to-airbus co-pilots who would stare at the PFD trying too figure out why the last button push on the FCP didn't do anything. Instead of disconnecting everything and regaining control. After you got yourself behind the power curve, however, for whatever reason, I'm essentially talking about old-school guys like me who were used to flying non-FADEC machines capable of "overboost." If you got into trouble, because you were stupid, in say the previous generation of Boeing products: You could always push up and call for power far in excess of limiting max GA epr or N1, N2, EGT limits. (but maybe that's because like you say: old eng's didn't operate so close to the surge/stall margin.) It's unlikely the engines were going to fail like a piston or super/turbo charged engine might. Those old buckets would warp. The blades might creep and stretch and the engines might have to be scrapped (at say 5 mill a copy.) But you had a better chance of clearing the trees by going all way the to the mechanical stops (physical wire to the FCU Hydr/Mech linkage) than you do now with a Throttle resolver / PFM/MEC/ FADEC arrangement. The airbus test pilot may think he's called for Jesus power, but FADEC will not let him have it. This may have saved me a couple of times in my career flying 60's gen aircraft overseas. You smash everything to the wall and only slightly pull back on the engines that are "barking." (compressor stalling.) ATC would steer you into mountains in those days in some places. (more war stories.) 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. I haven't flown any FBW. But we had the predecessor AOA system on the A310 which had a A/T "alpha floor" mode (Vls) which would not allow you to command (not select) a speed slower than 1.2 Vso. Check pilots would scare the **** out of themselves relying on this system, and come back and rewrite the manual! This also lead to a bunch of documented (AWST) vertical tailslides at third world airlines where a little turbulence knocked the A/S below alpha floor for a second. Throttles (sometimes asymmetrically) would in about six seconds from (flight) idle reach G/A thrust: locked into what the french call Thrust Latch mode: meaning if you disconnected A/T's and manually retarded them, and let go, they would re-engauge themselves (without your permission) and smoothly place you back up to full pwr again. (New guys never noticed the uncommanded re-power up. They would fixate on the airplane departing altitude and start ****ing around with the pickle switch trim: which was active!) The auto pilot would respect redline on flaps at all costs. It would pull the airframe up into a 90 degree body angle and then stall the machine into an airshow tailslide just like Art Shoal used to. Even if you disc the A/P on the pull up it's too late. The machine has insuficient down elevator authority now to arrest the pull up(cuz nugget ran the tailplane down and "Auhhto" overshot it the back the other direction to get even; there was no aural stabilizer-in-motion sound in a/p trim so nobody noticed the comming set-up!) Great French design! Hang on Grandma! We called these man vs. machine incidents/accidents. 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. Were the engines in flight idle? Were you guys pulling the ground sensor breaker? Ground idle takes longer. Older High Bypass designs eg: the GE CF6-80 series only take about six seconds from flight idle to reach GA thrust IIRC, but still cannot over boost. But it's more like twelve seconds in profile mode (slow spool up looking at FMS parameters.) So I remembered it wrong. I think he tried to change alt with Level Change and Profile mode engaged first, and when nothing much happened (norm) he smashed the thrust levers to the wall and ate wood. But Kevin, I'll concede the acceleration argument to you. (Older designs were even slower (something like fifteen seconds to spool up (e.g. GE CF700's aft-fans.) You could bust altitudes descending, if you didn't lead with the throttles a couple thousand feet before level off. 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. Yep, you're right. FADEC by itself didn't put him in the trees. But most accidents have "a chain" of factors that cause the accident. If you can break any one of the factorial links the accident would not happen. I submit the inability to get over-boost power was just one of those links. Another was a FBW AOA limit that cannot be temporarily sacrificed to clear obstacles. 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. Well I have to disagree with this. We train annually now to fly below stick shaker to escape microburst wind shear ground contact on t/o. We will go below stall speed (bugged) momentarily in ground effect will full power to avoid contact. We don't care about airspeed. We only look at V/S. If we didn't do this, some dry microbusts would kill us. Risking a stall is always better than contact with hard objects. (remember impact g-force energy goes up exponentially with speed) (besides: most jets don't break fast, they burble and pre-buffet a bit first. After a positive rate is obtained and we're still alive, then we fly on intermittent stick shaker (way higher deck angles than FD/AP AOA limits) until about 1000 ft AGL. AOA FBW autopilots never fly at speeds this low to escape terrain to my knowledge. But I'd have to ask an A320 driver to be sure. The other thing that bugs me about that machine is not being able to bust into a 45 degree bank. (Another thread for that one.) But keep in mind that if you'd advocated these advanced techniques twenty years ago, they would've pulled your ticket. :-( (but you'd still be alive.) :-) Now's its req FAA training. Note: These techniques vary widely from airline to airline and change from Chief Kahuna to Chief Kahuna. YMMV. For the record I think FADEC is great. Do you want it in your GA airplane? Cheers, pacplyer |
#27
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#28
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On Mon, 01 Mar 2004 17:52:00 -0800, pacplyer wrote:
Great Stuff Kevin, thanks for your insight. I have a couple of points I slightly disagree with however further down in your post. :-) Kevin Horton wrote: After you got yourself behind the power curve, however, for whatever reason, I'm essentially talking about old-school guys like me who were used to flying non-FADEC machines capable of "overboost." If you got into trouble, because you were stupid, in say the previous generation of Boeing products: You could always push up and call for power far in excess of limiting max GA epr or N1, N2, EGT limits. (but maybe that's because like you say: old eng's didn't operate so close to the surge/stall margin.) It's unlikely the engines were going to fail like a piston or super/turbo charged engine might. Those old buckets would warp. The blades might creep and stretch and the engines might have to be scrapped (at say 5 mill a copy.) But you had a better chance of clearing the trees by going all way the to the mechanical stops (physical wire to the FCU Hydr/Mech linkage) than you do now with a Throttle resolver / PFM/MEC/ FADEC arrangement. The airbus test pilot may think he's called for Jesus power, but FADEC will not let him have it. This may have saved me a couple of times in my career flying 60's gen aircraft overseas. You smash everything to the wall and only slightly pull back on the engines that are "barking." (compressor stalling.) ATC would steer you into mountains in those days in some places. (more war stories.) OK. I misunderstood your gripe against FADECs in the first message. I too am not happy to have a FADEC limit how much thrust I get out of the engine. I would much rather have some way to override it if the s*** hits the fan. But, I don't think this would have made any difference in the Habsheim accident. All published reports have said either that the engines didn't respond at all, or that they were still spooling up when he hit the trees. The only report I can find that actually quotes an N1 says the engines were at 83% N1, which must be well below TOGA, so the engines would have still been accelerating, and it wouldn't have mattered what rpm was commanded. There was a bit on an internal bun fight at Bombardier when the CRJ-700 was being designed. It has a FADEC engine, and the flight test folks were not happy about the inability to get more thrust if needed. The engine does have an Automatic Power Reserve (APR) that commands a thrust bump if you have an engine failure. The powerplants engineers were persuaded to add a heavy detent that you can push the thrust levers through to allow you to get APR thrust with both engines running if you really need it. The Bombardier Global Express also has a FADEC engine, but there are two little switches behind the thrust levers that allow the crew to manually select a back up N1 control mode. If the engine is in N1 control mode, you are no longer limited except by the overspeed limiter, which allows you to get much more thrust if needed. 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. Were the engines in flight idle? Were you guys pulling the ground sensor breaker? Ground idle takes longer. Older High Bypass designs eg: the GE CF6-80 series only take about six seconds from flight idle to reach GA thrust IIRC, but still cannot over boost. But it's more like twelve seconds in profile mode (slow spool up looking at FMS parameters.) So I remembered it wrong. I think he tried to change alt with Level Change and Profile mode engaged first, and when nothing much happened (norm) he smashed the thrust levers to the wall and ate wood. But Kevin, I'll concede the acceleration argument to you. (Older designs were even slower (something like fifteen seconds to spool up (e.g. GE CF700's aft-fans.) You could bust altitudes descending, if you didn't lead with the throttles a couple thousand feet before level off. The 8 second requirement is for an acceleration from flight idle. There is typically some worst case condition (specific bleed configuration, altitude and temperature) where the engine will be close to 8 seconds, and it will be a bit better at other conditions. So I am not surprised if you saw about 6 seconds in many cases. 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. Yep, you're right. FADEC by itself didn't put him in the trees. But most accidents have "a chain" of factors that cause the accident. If you can break any one of the factorial links the accident would not happen. I submit the inability to get over-boost power was just one of those links. Another was a FBW AOA limit that cannot be temporarily sacrificed to clear obstacles. 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. Well I have to disagree with this. We train annually now to fly below stick shaker to escape microburst wind shear ground contact on t/o. We will go below stall speed (bugged) momentarily in ground effect will full power to avoid contact. We don't care about airspeed. We only look at V/S. If we didn't do this, some dry microbusts would kill us. Risking a stall is always better than contact with hard objects. (remember impact g-force energy goes up exponentially with speed) (besides: most jets don't break fast, they burble and pre-buffet a bit first. After a positive rate is obtained and we're still alive, then we fly on intermittent stick shaker (way higher deck angles than FD/AP AOA limits) until about 1000 ft AGL. AOA FBW autopilots never fly at speeds this low to escape terrain to my knowledge. But I'd have to ask an A320 driver to be sure. The other thing that bugs me about that machine is not being able to bust into a 45 degree bank. (Another thread for that one.) Well, the AOA limiter an the Airbus's is set very close to the stall. It is well beyond where a stick shaker would be. The curve of lift vs AOA tends to have a fairly flat top with modern swept wing jets, so once you get up on top of that curve there isn't any benefit to pulling more AOA, as you don't get any more lift. I wish there was some way to get in a FBW Airbus sim with you. We could do two windshear recoveries - one using full aft stick riding on the AOA limiter, and one in Direct Law, with no AOA limiter. I'm convinced you would do better just using the AOA limiter. For the record I think FADEC is great. Do you want it in your GA airplane? Well, I'm a suspicious type, and I want to see some more service history first to assure myself that they've sorted all the bugs out. So not on my RV-8 project, but maybe on the next one. -- Kevin Horton RV-8 (finishing kit) Ottawa, Canada http://go.phpwebhosting.com/~khorton/rv8/ e-mail: khorton02(_at_)rogers(_dot_)com |
#29
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"Dan Thomas" wrote in message om... "Bill Daniels" wrote in message news:5_- 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. Not quite. Gears don't have preferred operating conditions, props do. The engine has its preferred RPM and torque for optimum efficiency and the prop blades have their optimum angle of attack. If the engine/prop combination results in the prop operating at a higher (or lower) angle of attack than optimum to absorb the torque of the engine (Prop governor increases pitch to hold RPM setting.) then the combination operates below optimum conditions. Under some conditions, it would make sense to introduce a third variable i.e. a gearbox between the engine and prop, to allow both the engine and prop to operate at peak efficiency. This was the reason that two-speed grearsets were installed in the nose case of some large radials. This, in turn, allowed the propeller designer to optimize his prop blades for a single AOA, thus gaining still more efficiency. The problem, simply stated was this: How does a heavily loaded, long-range bomber haul itself off a short runway and climb to cruise altitude and then shift to highly efficient, long-range cruise. The answer was just emerging from the labs as the world shifted to turbines. The flight engineer would shift his engines into a "hole gear" by selecting a cam profile and engine timing optimized for the low gear that would let the engines scream at high RPM and pump massive HP into props set for maximum acceleration and climb. Once in cruise, the engineer would shift his engines back to low RPM, high efficiency settings. First time I've ever heard of gear-shifted props in certified engines. Which engines were these? I know that many radials (and other engine layouts) used reduction gearing in the case nose to allow the engine to run faster and produce more HP while keeping the prop within safe limits, and that there were two-speed geared superchargers on many of these engines, but two-speed props? Jim Bede used a snowmobile-type propshaft drive in the early BD-5s but abandoned it as unworkable. It still required a relatively tiny prop to keep the tip speeds subsonic. As far as the propeller pitch angles go, the constant speed prop improves takeoff performance by more than just letting engine RPM reach redline to produce max HP. It reduces the angle of attack so that more of the prop is unstalled and producing thrust in the static condition, improving acceleration and shortening takeoff distance. The inboard sections of a fixed-pitch prop blade have a large angle so that they still produce thrust in faster forward flight even though they don't travel the circumferential distance that blade areas near the tips do, but the large angle means a stalled blade, or at least a really turbulent flow, at low forward speeds. A gear-shifted fixed-pitch prop will still have those problems. Dan The gear shifted prop was the last gasp of piston engine development before the turbine age. Look at the Lycoming XR7755, Napier Nomad or the Rolls Royce Crecy. These were 5000 HP+ monsters that needed every trick in the engineers bag. Piston engines produce more HP at high RPM at the cost of fuel consumption but deliver low fuel consumption at low RPMS. Props produce more thrust at low RPM and most efficiency with the blades at a single best AOA. That AOA must be maintained over a wide range of airspeeds. Just too many variables for a CS prop to deal with alone. The two speed gearbox isn't perfect but it does buy the engineer a bigger range of options. Bill Daniels |
#30
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Kevin Horton wrote in message ...
Some of the supercharged recips had a gear box with two different gear ratios to drive the supercharger. They needed to spin the supercharger at high rpm at high altitude in order to get enough manifold pressure. But if they used the same supercharger gear ratio at low altitude it would produce more manifold pressure than the engine could handle at full throttle. The engine would then have to be run very throttled, and there would be a lot of wasted power used to spin that supercharger at a needlessly high rpm. So, they used a different gear ratio to spin the supercharger at a lower rpm for take-off and low altitude flight. Yes, I knew that about the supercharger gearing to allow different settings at altitude, but the poster I was questioning had discussed (or seemed to hint at) a two-speed propeller drive; in other words, a transmission. I had never heard of it, outside of Jim Bede's belt-driven variable-ratio system in the early BD-5. The only propeller gearing I've ever seen is a fixed reduction as used in many larger radials, all the V-12s except very early ones, and many opposed engines such as the Continental GO-300 (Cessna 175), Lyc's GO-480 (Helio), and GTSIO-540 (Cessna 414 or 421?), and Continental's Tiara engine that never reached significant production. And, of course, all turboprop, turbofan and turboshaft engines. All with a fixed ratio, single reduction. And all to allow the engine to develop high RPM and therefore higher HP, while allowing a larger, slower prop to operate in an efficient range. It seems to me that turning a prop faster in cruise flight is self-defeating, since drag rises as tip speeds rise with forward speed factored into the operation. It's why airplanes with big props like the Dash 8 turn at around 1300 for takeoff and 850 or so in cruise, and why variable pitch of the blade is absolutely necessary. Dan |
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