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Does glide ratio change with altitude?
During a recent discussion about calculating glide ratio, I began to wonder
what the effects of pressure altitude were on the glide ratio of an aircraft. Since air is 50% as dense at FL180 as at sea level, would the glide ratio increase (glide further) as altitude decreases? AND if there is a difference in glide ratio as altitude changes, then what values do most manufacturers use when they publish their numbers (if they do)? Oh yeah, I do understand that glide ratio changes to 0:0 upon impact, so the wags can skip that part of the reply... -- Jim Carter Rogers, Arkansas |
#2
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Does glide ratio change with altitude?
Jim Carter wrote:
During a recent discussion about calculating glide ratio, I began to wonder what the effects of pressure altitude were on the glide ratio of an aircraft. Since air is 50% as dense at FL180 as at sea level, would the glide ratio increase (glide further) as altitude decreases? AND if there is a difference in glide ratio as altitude changes, then what values do most manufacturers use when they publish their numbers (if they do)? Oh yeah, I do understand that glide ratio changes to 0:0 upon impact, so the wags can skip that part of the reply... Ratio math is good only if terrain is perfectly flat in a perfect world. You got to figure in the geography to get the actual ratio. Or...... splat A little calculus anyone??? |
#3
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Does glide ratio change with altitude?
"Jim Carter" wrote in message t... During a recent discussion about calculating glide ratio, I began to wonder what the effects of pressure altitude were on the glide ratio of an aircraft. Since air is 50% as dense at FL180 as at sea level, would the glide ratio increase (glide further) as altitude decreases? AND if there is a difference in glide ratio as altitude changes, then what values do most manufacturers use when they publish their numbers (if they do)? As you descend, drag increases. Remember that the purpose if having an engine (thrust) is to overcome weight, but also DRAG. Oh yeah, I do understand that glide ratio changes to 0:0 upon impact, so the wags can skip that part of the reply... Okay...I'll skip that point! :~) -- Matt Barrow Performance Homes, LLC. Cheyenne, WY |
#4
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Does glide ratio change with altitude?
"FredGarvinMaleProstitute" wrote in message
... Jim Carter wrote: During a recent discussion about calculating glide ratio, I began to wonder what the effects of pressure altitude were on the glide ratio of an aircraft. Since air is 50% as dense at FL180 as at sea level, would the glide ratio increase (glide further) as altitude decreases? AND if there is a difference in glide ratio as altitude changes, then what values do most manufacturers use when they publish their numbers (if they do)? Oh yeah, I do understand that glide ratio changes to 0:0 upon impact, so the wags can skip that part of the reply... Ratio math is good only if terrain is perfectly flat in a perfect world. You got to figure in the geography to get the actual ratio. Or...... splat A little calculus anyone??? Ok Fred - that's a new one. What does terrain have to do with glide ratio? I already noted that impact has an impact on the ratio, but only at point of impact. Other than that one point on the continuum, what effect could terrain have on glide ratio? And don't get started on turbulence, or wind shear, or any number of other weather anomalies, because I'm asking about glide ratio, not glide distance. -- Jim Carter Rogers, Arkansas |
#5
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Does glide ratio change with altitude?
"Matt Barrow" wrote in message
... "Jim Carter" wrote in message t... During a recent discussion about calculating glide ratio, I began to wonder what the effects of pressure altitude were on the glide ratio of an aircraft. Since air is 50% as dense at FL180 as at sea level, would the glide ratio increase (glide further) as altitude decreases? AND if there is a difference in glide ratio as altitude changes, then what values do most manufacturers use when they publish their numbers (if they do)? As you descend, drag increases. Remember that the purpose if having an engine (thrust) is to overcome weight, but also DRAG. Yes, (induced) drag increases as a factor of lift caused by the higher air density, and parasitic drag increases as a factor of speed which isn't changing. So back to my original question - sort of, does the lift created by denser air at lower altitudes vary the glide ratio for any significance? Or is the airfoil performance at a given speed static across the entire flight profile? Oh yeah, I do understand that glide ratio changes to 0:0 upon impact, so the wags can skip that part of the reply... Okay...I'll skip that point! :~) -- Matt Barrow Performance Homes, LLC. Cheyenne, WY -- Jim Carter Rogers, Arkansas |
#6
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Does glide ratio change with altitude?
"Marc J. Zeitlin" wrote in message
... Jim Carter wrote: During a recent discussion about calculating glide ratio, I began to wonder what the effects of pressure altitude were on the glide ratio of an aircraft. Since air is 50% as dense at FL180 as at sea level, would the glide ratio increase (glide further) as altitude decreases? To a first order approximation, the maximum glide ratio (L/D) does not change with altitude. Since what's changing is the density of the air, and both the lift and the drag are linearly related to the air density, the effect cancels out. Since the max. L/D occurs at a given IAS for any particular weight and CG position, the TAS will change as the altitude changes, but the L/D will not (again, to a first order approximation). -- Marc J. Zeitlin http://www.cozybuilders.org/ Copyright (c) 2007 I was not sure the relationship between lift and drag was linear, so thanks for clearing that point up. You've also answered the question very well. Thanks. -- Jim Carter Rogers, Arkansas |
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Does glide ratio change with altitude?
"FredGarvinMaleProstitute" wrote: Ratio math is good only if terrain is perfectly flat in a perfect world. You got to figure in the geography to get the actual ratio. Or...... splat LOL. Still got a way to go to catch Mxsmanic, though. -- Dan "Did you just have a stroke and not tell me?" - Jiminy Glick |
#8
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Does glide ratio change with altitude?
Jim Carter wrote:
I was not sure the relationship between lift and drag was linear, *nitpick* That's not what he said. He said that both lift and drag are linearly related to air density, so that if the air density changes, the L/D stays constant (ceteris paribus, of course). Ad. -- The mail address works, but please notify me via usenet of any mail you send to it, as it has a retention period of just a few hours. |
#9
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Does glide ratio change with altitude?
Adhominem writes:
Jim Carter wrote: I was not sure the relationship between lift and drag was linear, *nitpick* That's not what he said. He said that both lift and drag are linearly related to air density, so that if the air density changes, the L/D stays constant (ceteris paribus, of course). What does Airbus certification have to do with it? Ad. |
#10
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Does glide ratio change with altitude?
"Adhominem" wrote in message
... Jim Carter wrote: I was not sure the relationship between lift and drag was linear, *nitpick* That's not what he said. He said that both lift and drag are linearly related to air density, so that if the air density changes, the L/D stays constant (ceteris paribus, of course). Ad. -- The mail address works, but please notify me via usenet of any mail you send to it, as it has a retention period of just a few hours. *nitpick with finer granularity* ....and if lift is linearly related to air density and drag is linearly related to air density, then aren't lift and drag in a linear relationship to each other? -- Jim Carter Rogers, Arkansas |
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