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on 2/13/03 10:14 PM, [EMAIL PROTECTED] at [EMAIL PROTECTED] wrote: > In a message dated 2/13/03 2:57:41 AM Eastern Standard Time, > [EMAIL PROTECTED] writes: > >> The eye may be on the wrong ball here. No argument that turning a given >> prop at a given rpm yields identical results in climb and speed from >> identical horsepower. You very seldom actually get 85 hp from a 85 hp >> (operate at that narrow point on the power curve). > > Again, fixed pitch props are strange beasts. Assume for a moment you have a > C85 and a prop/airframe combo that yields exactly redline RPM at full > throttle at sea level on a standard day. Power under those conditions is 85 > nominal horse power. Pitch the nose up and the RPM drops off, as does the > power. Pitch down and you exceed redline. Game over. > Hi John, Hope you're enjoying this exchange of ideas as much as I am! Above, you've created the very situation I previously stated "very seldom" happens. In the "real world" because, as I also stated, "the C-85 will only reach 2575 rpm in cruise with a cruise prop (and reduced takeoff performance), we base decisions on assumptions and examples more "real". Most of us opt instead for the more useful performance compromise where we can't cruise as fast, but have more rpm at takeoff to give decent climb...or climb at all on a high density altitude day and at full gross from higher and/or hotter airports. > Now you are turning 2575 legal RPM at full throttle, 28.92 inches of manifold > pressure, and climbing. Power output is more than 85 hp. (but less than 95 > hp, which is what the C90 can do at 2675 RPM and full throttle.) That's only one choice. You can also level out and keep revving. The Continental power curve for the C-90/O-200 mechanicals of the STC engine show it putting out 85 hp at 2395 (or so) rpm. When it reaches 2575 rpm, I see around 97 hp. If 85 hp will maintain that rpm and speed, then 97 hp will allow that engine to rev past peak cam power output to whatever rpm available power can maintain (but not increase further). The STC engine has a C-85 cam with peak power and torque characteristics optimized for 2575 rpm. The C-90 has a cam with peak power and torque characteristics optimized for its maximum continuous hp of 90 at only 2475 rpm. So the STC engine should be capable of producing more power than even the C90 curve shows from 2475 rpm up! I also don't think you're "legal" at 2575 rpm. The moment you allow this powerplant/prop combination to exceed 2395 rpm, your C-85 produces more than rated hp - the FAA regards that as a no-no. The maximum static limit for the C-85 on an Ercoupe with a McCauley 1A90CF or 1B90CM is 2225 rpm. If the STC does not require replacement or repitching a C-85 prop to conform, the pilot decides to throttle-limit this rpm to 2225 rpm until the wheels move (after that, you're not "static", right?) or not. The STC engine starts with and sustains increased power. C-85 power and rpm limitations are as unreasonable and arbitrary today as the ones for the C-75 were in yesteryear. We already know mechanicals with longer stroke (and higher stress) of substantially identical material and production process are FAA APPROVED for sustained 2750 rpm (O-200). Static limits merely give the FAA a measurable parameter of (in)efficiency...to make sure that new Cessna 172 with 160 hp (you earlier used as an example) can't go as fast as a 172 SP...different redline and prop AS APPROVED. Does the FAA (or anyone) really believe in such instance the less-than- optimum prop and lower redline limitation in any way protects the pilot, serves his best interests, or even the interests of safety? Or are they merely numbers to pompously enforce on hardware they (should) already know to be safe, reliable AND APPROVED BY THEM for higher performance? > The reality, however, is that one never climbs at that speed. Back at a more > realistic climb speed, say 75 - 85 mph, RPMs are much lower, and less than > the 85 hp point on the C90, ergo, you are making less than 85 hp. More than > the unmodified engine, though. I think we start out in agreement here, but at your "more realistic climb speed", while neither engine is putting out 85 hp, the STC engine will be performing 10+ percent better which will show up in the rate of climb. So, is the STC engine (in operation) the 85 hp on the the paperwork, or a "closet hot rod" of proven design with 10+% greater usable power? The "real" answer is NOT dependent on who is flying it and how. We're talking about available capability, and that's clearly the latter. > Now, take that prop and airframe and put a C90 (or O200) behind it. At full > throttle you will exceed redline under the same conditions. Again, game > over. Once again, the pilot with the STC engine has choices. He can exceed the "paper" (and unrealistic) C-85 redline (with complete mechanical safety) as far toward the O-200's maximum APPROVED sustained rpm of 2750 as possible and ride his little rocket up as fast as it will carry him, or he can voluntarily limit his rpm/power output so he never exceeds 2575 rpm or 85 hp output. EITHER WAY the STC engine seems to offer substantial advantage. Regards, William R. Bayne <____|(o)|____> (copyright 2002) ========================================================================== ==== To leave this forum go to: http://ercoupers.com/lists.htm Search the archives on http://escribe.com/aviation/coupers-tech/
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