Hurricane Speed Test in game

[Crumpp]

As I indicated in the Spit test no attempt at converting to a standard atmosphere was made

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.

[bongodriver]

992mb = 29.29inHg

Wrongggg! 1013.25 Mb or Hpa = 29.92 inHg, yet another example of lack of basic knowlege for an apparent professional pilot.

29.29”/ 29.92” = .978944

Wronggg!! 29.92 divided by 29.92 = 1

Why do I get the feeling the rest is pure comedy?

[Crumpp]

Wrongggg! 1013.25 Mb or Hpa = 29.92 inHg, yet another example of lack of basic knowlege for an apparent professional pilot.

Our pressure at sea level is not 1013.25Mb....

It is 992Mb......

992mb = 29.29inHg or more precisely 29.2938592inHg

http://www.csgnetwork.com/pressinmbcvt.html

Why do I get the feeling the rest is pure comedy?

Start laughing.....

[bongodriver]

992mb = 29.29inHg or more precisely 29.2938592inHg

first of all I think you must mean 29.921373825 inHg

And how exactly do you figure? I suggested in a thread that perhaps the atmospheric model was porked by someone equating 29.92 to 992, but that does not mean it is equal to 29.92 inHg.

Start laughing.....

Chuckle worthy as it is

[bongodriver]

Ok....I see my mistake, forget it .92 looks an awfull lot like .29 when surrounded by numbers.

see that...humility, try it sometime.

And I will ease on the itchy trigger finger.

[phoenix1963]

Amazingly, I do think Crumpp has a valid subsidiary point about rather oddly low pressure at sea level in CloD.
Most of BoB was fought in good - high pressure - weather.
Apart from one period of low pressure in August, I think.

Look here for great historical weather charts http://www.wetterzentrale.de .

56RAF_phoenix

[phoenix1963]

Now if I could find my old link for the historical surface pressure charts we could work out the actual lapse rate...

(of course the 500hPa charts kind of measure atmospheric thickness, i.e. some sort of temperature variation)

56RAF_phoenix

[bongodriver]

Annoyingly without going into graphs it seems Crumpp is right, if FTH in standard atmosphere is 17,500ft density altitude, then if ambient temps are ISA + then density altitude is higher than pressure altitude therefore boost drop off should happen lower than 17,500 ft on the altimeter.

There are simpler graphs showing a density altitude conversion from pressure altitude just using ISA deviation temperature, again Crumpp may have a point about dry lapse rates as I think density alt is based on it, relative humidity comes in and complicates matters.

Crumpp, your working out was done with a standard 2 degree lapse rate, I believe a 3 degree lapse rate would work in 'favour' of performance in that case.

19C + 3 degree per 1000 ft lapse rate = 17500/1000 = 17.5*3 = 52.5 = 19C - 52.5C = -33C

using -33C at 17,500ft pressure altitude I find a density alt of 16,000ft so in effect I could go to more than 17,500 pressure alt before boost drop off.

I will look at this again tomorrow.

p.s. I forgot to add....the key to this would be to test and find the in game actual OAT (outside air temp) at 17,500 ft pressure altitude and see what the deviation is from a 3 degree lapse rate.

p.p.s. I think the test is worth doing using standard setting on the altimeter 1013mb, which RAF aircraft use an OAT gauge?, might be worth doing the test as a co op so someone can check temp at various pressure altitudes.

P.p.p.s reverse engineering the games atmosphere model at 1 in the morning is nuts

[Crumpp]

Find Pressure ratio,

Delta at sea level is 29.29”/ 29.92” = .978944

Find Temperature Ratio,

19C =66.2 F

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 = (66.2F + 460R)/(59F + 460R) = 1.01387

Find Density ratio,

Sigma = Delta / Theta = .965552

Find Standard Means Of Evaluation:

SMOE = 1/SQRT(sigma) = 1.01768

Hurricane Mk I sea level TAS @ standard conditions = 263mph TAS = 263mphEAS

Under game atmospheric conditions with a standard lapse rate:
263mphEAS * 1.01768 = 268mphTAS @ Game Sea Level

The Hurricane Mk I should be in the vicinity of 268mphTAS at sea level in the game under the atmospheric conditions recorded.

[bongodriver]

With a decent graph you don't need to find any ratios, some geek has already done it for you.