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#41
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more confusion on cessna performance chart
On Jan 16, 9:55 pm, terry wrote:
On Jan 17, 8:18 am, "Ken S. Tucker" wrote: On Jan 16, 12:22 pm, terry wrote: On Jan 17, 4:16 am, "Ken S. Tucker" wrote: On Jan 15, 5:59 pm, terry wrote: On Jan 16, 7:05 am, "Ken S. Tucker" wrote: Humidity feeds into "density altitude" because water vapour molecule H2O has density ~ 10 compared to Nitrogen N2 ~ 14 *at equal pressures* Not quite. The density is proportional to molecular weight, which would be in the ratio of 18 for water to 28 for nitrogen ( g /mol ) But of course we are really interested in the density ratio between water and air which would be 18 to 28.9 Ths simply comes from rearranging the Gas Equation we all learn in high school PV =nRT substiute n =m/M where m is mass and M molecular weight , you rearrange to get m/V = PM / RT m/V of course = density ( assuming ideal behaviour exists which is a pretty good assumption at the pressures and temperatures involved in flying light aircraft ). I'm guessing: but I get the impression that the onset of turbulence over wings was also dependant on temp- erature, even when the density altitude is the same. In Quantum Theory that makes sense. To start, warm air is more chaotic than cold air at the molecular level, and the chaos *seeds* the turbulence. You know, hot fluids are less viscous than cold and so less sticky. That's likely a secondary correction. Regards Ken- Hide quoted text - So if warm air is more turbulent ( I think I can accept that ) wouldnt that mean that at higher temperatures for the same density altitude you would get less lift and require longer take off distance? " As previously stated the results are the other way around." Cheers Terry I checked what you "previously stated", and the words "correction" and "difference" didn't have the usual "+/-" in them. Is the Cessna handbook online, that will save time, I'm interested. I coulnt find it on line, but I would be happy to scan and email the page, or even email you the Excel spreadsheet with the data and my calculations. Then you can do all the quantum mechanics, vector analysis, euclid geometry and reverse differentiation your heart desires, and report back to us. Just let me know if you want to recieve this info by email. terry Let's see the posted info for all to see and then we all may examine the data equally, otherwise, shut the **** up. Don't waste our time. Regards Now no need to be rude Ken, its your choice how you spend your time, its not me that is wasting it. I posted a question which I thought would be of interest to pilots of real aircraft for whom understanding (or lack thereof) of takeoff performance data can literally be the difference between living and dying. I didnt post the data set because it is large and it was not my intention to have others spend hours analysing it. ( although anyone is welcome to it , and my full analysis of it, but offf line) I found something that didnt gel with my understanding, and sort reasons for the potential discrepancy. As for some of your suggestions Ken, all I can say mate is that you need to come out of the clouds a little, most things in life are not as complicated as you seem to think. I am sure you could come up with a thousand brilliant ideas, that even your Mensa friends would be impressed by, as to the possible reasons why a car might go put put splutter , splutter and then stop.. but just checking the gas first makes a lot more sense. I like physics and maths too and whilst I do have a PhD in science ( physical chemistry) I am certainly no genius. I have spent many many hours mathematically analysing a lot what is involved in flying, from navigation problems, density altitudes with corrections for humidty, wt and balance, radius of a turn with correction for crosswind ( thanks Cain Liddle) , I even wrote a program for an air traffic controller who wanted to be able to predict the wind speed and direction from radar tracks and flight plan information for multiple aircraft..... But you know what, none of this stuff has ever required anything I didnt learn in high school, like a good grasp of trigonometry, solve a quadratic here and there, Newtons Laws, the gas equations etc. For christ sake leave the quantum mechanics and anti viscosity discussions to your Mensa meetings. Terry Well heck, I didn't get a High School Diploma! What *****es me off* is the take-off roll for the C-152 is ~ 735', so although your using a C-172, the variation of 300' between 0C and 40C at constant density altitude is rather high. Regards Ken |
#42
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more confusion on cessna performance chart
On Jan 15, 10:24*am, quietguy wrote:
Your PA-to-DA calculations clearly differ from Cessna's, probably because they used a different standard atmosphere. *There are plenty to choose from: International SA, U.S. SA, ICAO SA (all revised over the years) and some others, some of which are no longer used. *You'd need to find out which SA was used by Cessna when the 172N was built. Good luck with that project. *I would just plot some points from the POH and draw a smooth curve connecting them; I'd be conservative in my choices of data points and call that good enough. Some very sensible suggestions quietguy, I know I could just be conservative and plot a curve through the higher set of data on the graph, but the curiosity in me just wants to find the reasons. I think you are probably right about the standard atmospehere and i have had no luck yet finding out exactly what Cessna used, but I can tell you after some manipulation of the data by just changing the correction factor for pressure to density altitude of 120 feet to 80 feet per 1 degree off isa standard atmosphere temp, the points revert to the single smooth line of takeoff distance vs density altitude that I was expecting. Does that ring any bells with anyone re some other version of a standard atmosphere? Terry |
#43
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more confusion on cessna performance chart
On Jan 18, 6:10*am, "Ken S. Tucker" wrote:
On Jan 16, 9:55 pm, terry wrote: On Jan 17, 8:18 am, "Ken S. Tucker" wrote: On Jan 16, 12:22 pm, terry wrote: On Jan 17, 4:16 am, "Ken S. Tucker" wrote: On Jan 15, 5:59 pm, terry wrote: On Jan 16, 7:05 am, "Ken S. Tucker" wrote: Humidity feeds into "density altitude" because water vapour molecule H2O has density ~ 10 compared to Nitrogen N2 ~ 14 *at equal pressures* Not quite. *The density is *proportional to molecular weight, which would be in the ratio of *18 for water to 28 for nitrogen ( g /mol ) But of course we are really interested in the density ratio between water and air which would be 18 to 28.9 Ths simply comes from rearranging the Gas Equation we all learn in high school PV =nRT substiute n =m/M * where m is mass and M *molecular weight , you rearrange to get m/V = PM / RT m/V of course = density ( assuming ideal behaviour exists which is a pretty good assumption at the pressures and temperatures involved in flying light aircraft ). I'm guessing: but I get the impression that the onset of turbulence over wings was also dependant on temp- erature, even when the density altitude is the same. * In Quantum Theory that makes sense. To start, warm air is more chaotic than cold air at the molecular level, and the chaos *seeds* the turbulence. You know, hot fluids are less viscous than cold and so less sticky. That's likely a secondary correction. Regards Ken- Hide quoted text - So if warm air is more turbulent ( I think I can accept that ) wouldnt that mean that at higher temperatures for the same density altitude you would get less lift and require longer take off distance? " As previously stated the results are the other way around." Cheers Terry I checked what you "previously stated", and the words "correction" and "difference" didn't have the usual "+/-" in them. Is the Cessna handbook online, that will save time, I'm interested. I coulnt find it on line, but I would be happy to scan and email the page, or even email you the Excel spreadsheet with the data and my calculations. *Then you can do all the quantum mechanics, vector analysis, euclid geometry and *reverse differentiation your heart desires, and report back to us. *Just let me know if you want to recieve this info by email. terry Let's see the posted info for all to see and then we all may examine the data equally, otherwise, shut the **** up. Don't waste our time. Regards Now no need to be rude Ken, *its your choice how you spend your time, its not me that is wasting it. *I posted a question which I thought would be of interest to pilots of real aircraft for whom understanding (or lack thereof) *of takeoff *performance data can literally be the difference between living and dying. *I didnt post the data set because it is large and it was not my intention to have others spend hours analysing it. ( although anyone is welcome to it , and my full analysis of it, but offf line) *I *found something that didnt gel with my understanding, and sort reasons for the potential discrepancy. *As for some of your suggestions Ken, all I can say mate is that you need to come out of the clouds a little, most things in life are not as complicated as you seem to think. *I am sure you could come up with a thousand brilliant ideas, that even your Mensa friends would be impressed by, as to the *possible reasons why a car might go put put splutter , splutter and then stop.. but just checking the gas first makes a lot more sense. *I like physics and maths too and whilst I do have a PhD in science ( physical chemistry) I am certainly no genius. I have spent many many hours mathematically analysing a lot what is involved in flying, from navigation problems, density altitudes with corrections for humidty, wt and balance, radius of a turn with correction for crosswind ( thanks Cain Liddle) *, *I even wrote a program for an air traffic controller who wanted to be able to predict the wind speed and direction from radar tracks and flight plan information for multiple aircraft..... But you know what, *none of this stuff has ever required *anything I didnt learn in high school, like a good grasp of trigonometry, solve a quadratic here and there, Newtons Laws, the gas equations etc. * * For christ sake leave the quantum mechanics and anti viscosity discussions to your Mensa meetings. Terry Well heck, I didn't get a High School Diploma! What *****es me off* is the take-off roll for the C-152 is ~ 735', so although your using a C-172, the variation of 300' between 0C and 40C at constant density altitude is rather high. Regards Ken- Hide quoted text - You didnt get a high school diploma, now I am impressed! (i am not being sarcastic either) but I still go with what i said about searching for the simple answers first. The 300 ft variation between 0 and 40 C was at 8000 ft density altitude where the takeoff distance was around 2600 ft, so percentage wise it is not huge but enough to be concerned about. |
#44
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more confusion on cessna performance chart
On Jan 17, 11:40 am, terry wrote:
On Jan 15, 10:24 am, quietguy wrote: Your PA-to-DA calculations clearly differ from Cessna's, probably because they used a different standard atmosphere. There are plenty to choose from: International SA, U.S. SA, ICAO SA (all revised over the years) and some others, some of which are no longer used. You'd need to find out which SA was used by Cessna when the 172N was built. Good luck with that project. I would just plot some points from the POH and draw a smooth curve connecting them; I'd be conservative in my choices of data points and call that good enough. Some very sensible suggestions quietguy, I know I could just be conservative and plot a curve through the higher set of data on the graph, but the curiosity in me just wants to find the reasons. I think you are probably right about the standard atmospehere and i have had no luck yet finding out exactly what Cessna used, but I can tell you after some manipulation of the data by just changing the correction factor for pressure to density altitude of 120 feet to 80 feet per 1 degree off isa standard atmosphere temp, the points revert to the single smooth line of takeoff distance vs density altitude that I was expecting. Does that ring any bells with anyone re some other version of a standard atmosphere? Terry Yeah, the usual is Lift =~= density * (air speed)^2, (all other things being equal). However Mr. Logajan provided this chart, http://www.engineeringtoolbox.com/ai...c-viscosity-d_... and I was surprised to find a Viscosity diff of 8% between 0C and 30C. The reason I went to Quantum Theory is because it' s sometimes easier to go down to the basement to figure out why the house is sinking. QT can be easier terms than high level *classical gas* physics. ((yes the song)). Regards Ken |
#45
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more confusion on cessna performance chart
On Jan 15, 10:24*am, quietguy wrote:
Your PA-to-DA calculations clearly differ from Cessna's, probably because they used a different standard atmosphere. *There are plenty to choose from: International SA, U.S. SA, ICAO SA (all revised over the years) and some others, some of which are no longer used. *You'd need to find out which SA was used by Cessna when the 172N was built. Good luck with that project. *I would just plot some points from the POH and draw a smooth curve connecting them; I'd be conservative in my choices of data points and call that good enough. Just when I thought this was the correct reason. I have now further analysed the data in the flight manual and looked at the landing distance required data which was in exactly the same form , ie a table of distance required as a function of different combinations of pressure altitude and temperature. With this data table after converting to density altitude, i get a nice smooth curve of landing distance required vs density altitude ( as I would have expected with the take off distance data). This would seem to eliminate the use of a different standard atmosphere as the cause of the discrepancy. Whilst I will certainly take your advice and use the conservative line, my curiosity ( and stubboness) wont rest until I understand the reason for this. I am sure someone at Cessna would be able to explain it. Anybody know who I should contact? terry |
#46
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more confusion on cessna performance chart
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