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optimal altitude calculations?
hi: I have a simple question for the piloting physics majors. we all know that planes have less air resistance to overcome at higher altitudes, but that normally aspirated planes have less power at higher altitudes. presumably, both are proportions of what happens at sea level, and are hopefully not too plane dependent. That is, I would guess that a 160hp engine would lose about the same proportion of power as a 320hp engine. for lycomings, at 10,000', this proportion is about 50%. something similar [proportional reduction] may also happen to air resistance, regardless of whether the plane is a cub or a lancair. this leads me to a very simple question: on a standard day, without any winds, what would be the optimal altitude for [cruise] speed in a normally aspirated airplane? is this best altitude dependent on aircraft to a first-order, or is it fairly constant across airplanes? sincerely, /ivo welch |
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#3
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"john smith" wrote in message .. . wrote: this leads me to a very simple question: on a standard day, without any winds, what would be the optimal altitude for [cruise] speed in a normally aspirated airplane? is this best altitude dependent on aircraft to a first-order, or is it fairly constant across airplanes? I pick the "full-throttle" altitude, usually between 6000 and 7000 feet MSL, depending on temperature and the engine. 8,000' is a figure I see quoted frequently, which is roughly the highest altitude a non-turbocharged engine can maintain 75% power, which is normally considered max cruise. KB |
#4
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Yes and no. If you're measuring efficiency in terms of range, once you've
found your best cruising airspeed and engine speed, altitude won't affect range much, though you'll cover the distance faster the higher you go. If your best range is at 55% power, then you'll probably do best to climb as high as 55% will allow (assuming still air) and lean for best economy. See http://142.26.194.131/aerodynamics1/...nce/Page7.html That said, the differences between airframe and prop designs are significant. Light weight, better L/D and a constant speed prop mean better range at all altitudes. A Mooney is more efficient than a 172 with the same engine. wrote in message oups.com... hi: I have a simple question for the piloting physics majors. we all know that planes have less air resistance to overcome at higher altitudes, but that normally aspirated planes have less power at higher altitudes. presumably, both are proportions of what happens at sea level, and are hopefully not too plane dependent. That is, I would guess that a 160hp engine would lose about the same proportion of power as a 320hp engine. for lycomings, at 10,000', this proportion is about 50%. something similar [proportional reduction] may also happen to air resistance, regardless of whether the plane is a cub or a lancair. this leads me to a very simple question: on a standard day, without any winds, what would be the optimal altitude for [cruise] speed in a normally aspirated airplane? is this best altitude dependent on aircraft to a first-order, or is it fairly constant across airplanes? sincerely, /ivo welch |
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George Patterson wrote:
It varies from aircraft to aircraft; it's basically the point at which the engine is putting out 100% rated power at full throttle. It tends to be somewhere between 6,000' and 9,000'. From memory of the manuals, it's 6,500' for a Cessna 150J and 8,600' for a Maule MX-7-160. Me thinks that full throttle power in the 8000' area is 75%. Ron Lee |
#7
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The Bonaza I just got checked out in Burns 13 gah @ 8500 and will cruize 165 knots. If I go to 17500 it will burn 10 gah and cruize @ 160 knots. This is by memory so it may not be exact. |
#8
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Ron Lee wrote:
Me thinks that full throttle power in the 8000' area is 75%. You're right - I should have said 75% power at full throttle. George Patterson Give a person a fish and you feed him for a day; teach a person to use the Internet and he won't bother you for weeks. |
#9
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this leads me to a very simple question: on a standard
day, without any winds, what would be the optimal altitude for [cruise] speed in a normally aspirated airplane? That's not really a simple question. You haven't really defined optimal. If by optimal you mean highest available cruise speed, then it's sea level. That's because most normally aspirated engines may be run at full power continuously. If by optimal you mean highest available cruise speed at maximum RECOMMENDED (not allowed) continuous power, it's the highest altitude at which the engine can develop maximum recommended continuous power. Depending on the prop, that might be anywhere from 6000 to 8000 ft (density altitude of course) for 75% power. If by optimal you mean best fuel efficiency, it's the altitude where maximum available power produces an indicated airspeed equal to the best glide airspeed (unless the engine/prop happens to be inefficient in that condition) - in other words, this will vary with the airplane, the engine, and the operating weight. Michael |
#10
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gentlemen---thank you for your answers. let me mean "highest continuous cruise speed." michael--I find it hard to believe that even highest available cruise for short periods would be ground level. the air resistance is pretty high down there. I think most people are recommending 6000' to 8000', because it gives 75% power. is this just "experience" or "rule of thumb that I learned somewhere" ? in other words, why is 6000-8000' where the air resistance vs. power curves cross? or, why is 4000' not better? there is more power. why is 10000' not better? there is less air density. are there some rough equations that can show that 6000' to 8000' is about optimal? regards, /iaw |
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