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As I indicated in the Spit test no attempt at converting to a standard atmosphere was made
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There is no question or issue here. The atmospheric model is not correct or is using a very non-standard lapse rate.
That is fact. You cannot have an OAT of 19C @ Sea Level, a pressure of 992mb, and have a density altitude that equals 17500 ft with any standard lapse rate.
Find Pressure Ratio at Sea level,
992mb = 29.29inHg
Pressure ratio, delta , at sea level is 29.29”/ 29.92” = .978944
Find Pressure Altitude,
Usually we only know the pressure at sea level. We can use charts to find pressure ratio if we know pressure altitude. So Let’s determine Pressure Altitude. Using the rule of thumb, 1” Hg =1000 ft. This is an approximation, but it works pretty good. So with a barometric altimeter setting of 29.29 we can see how far off we are from Standard day. 29.92-29.29 = .63 “ which roughly equals 630 ft. Now, do we subtract or add? This is how I look at it, If I roll the altimeter setting to 29.92 what happens to my altimeter. Since I am increasing the setting, then I am increasing the altitude to get to pressure altitude. SO Pressure altitude is 17500 + 630 = 18130 ft
Find Pressure Ratio at Altitude,
Look in a STD Atmospheric table, 18130 ft equates to a pressure ratio of .4967475
Our value lies 130 feet above the 18000 ft level. You should be smart enough to figure out how to extrapolate it.
Find Temperature Ratio,
19C + 2 degree per 1000 ft lapse rate = 17500/1000 = 17.5*2 = 35 = 19C - 35C = -16C
-16C = 3.2F
Theta = T/To at sea level , where T = Actual Temperature in deg K or Deg R and To = Standard sea level Temperature in deg R or deg R.
At altitude Theta = (3.2F + 460R)/(59F + 460R) = .892486
Find Density ratio,
You can’t just read off the table. Temperature influences density. We use the ideal gas law relationship to find density ratio.
Sigma = Delta / Theta = .556589
Look in a STD Atmospheric table, 18000 ft equates to a density ratio of .56991
In case you don't know how to extrapolate on the table:
.56991-.556589 = .013321
19000 ft density ratio = 0.55112
.56991 - 0.55112 = .01879
.01879/1000 = .000019
.013321/.000019 = 709ft (Ti-89 uses 1.879e-5)
Density altitude at 17,500 ft indicated is 18709 ft
Do you think any Merlin II or III engine had a FTH of 18700 feet?????
NO...
Does anybody have weather reports from the Battle of Britian? They only possible explaination the data is correct is to change the lapse rate adding more moisture to the air. If it is dry, the FTH is even more out of wack.