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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 |
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