cabin pressure

[vololiberista 205]

Relating to the cabin pressurisation gauges I found (see above) Could someone be kind enough to explain the mathematics of this equation.

<Select>		  <Value>(L:Cabin_Differential_Press, number) 7.82 < if{ (A:Plane Altitude, feet) (L:Cabin_Alt_Ind, number) - 0.00040975 * (>L:Cabin_Differential_Press, number) } els{ 7.8 (>L:Cabin_Differential_Press, number) }</Value>		   </Select>

I did a test flight to FL180(A:Plane Altitude, feet) with a cabin pressure set to 10,000ft(L:Cabin_Alt_Ind, number). When the cabin altitude settled to 10,000ft the pressure differential stayed at 3.28psi. I was expecting a much lower figure of approx. 2.30478psi. Presumably this number 0.00040975 is the "multiplier" and can be tweaked until the figures correspond. But I would like to know exactly how the function works first.vololiberista

[n4gix 4,947]

You've already sussed it out. Alter the scalar value to whatever you calculate it should be to produce the result you expect... :)I'm assuming you set the cabin altitude pressure to 10,000 purposes of simplicity, since your pax would be very unhappy with such a thin cabin pressure! :(

[vololiberista 205]

You've already sussed it out. Alter the scalar value to whatever you calculate it should be to produce the result you expect... :)I'm assuming you set the cabin altitude pressure to 10,000 purposes of simplicity, since your pax would be very unhappy with such a thin cabin pressure! :(

Yes, I've already done that that by a bit of trial and error!! The only remaining problem for the sake of accuracy is that in reality air pressure reduces at different rates in relation to the lower and upper atmosphere. So, in theory I would have to introduce a conditional statement with different values dependent on on say below FL240 or above FL240. As it stands it is correct up to FL180 but by FL420 the differential is greater than it should be resulting in a cabin pressure of 14,000ft.vololiberista

[mgh 89]

Yes, I've already done that that by a bit of trial and error!! The only remaining problem for the sake of accuracy is that in reality air pressure reduces at different rates in relation to the lower and upper atmosphere. So, in theory I would have to introduce a conditional statement with different values dependent on on say below FL240 or above FL240. As it stands it is correct up to FL180 but by FL420 the differential is greater than it should be resulting in a cabin pressure of 14,000ft.vololiberista

There isn't a significant change in the atmosphere at 24,000ft (FL240) so I suggest that can't be the cause of the problem.I assume that Microsoft modelled the Standard Atmosphere in FS. The Standard Atmosphere pressures are:14.696 psi @ sea-level10.108 psi @ 10,000 ft 7.344 psi @ 18,000 ft 5.703 psi @ 24,000 ft 3.306 psi @ 36,000 ftAlso, the upper atmosphere begins at about 36,000ft. Below that altitude temperature decreases as altitude increases. Above that altitude, temperature remains constant until about 65,000 ft when it begins to increase.

[vololiberista 205]

Average decrease Altitude Range per 1,000 feet Feet per in/Hg Sea level to 5,000 ft 1.006 in/Hg 994 5,000 to 10,000 0.862 1,160 10,000 to 15,000 0.740 1,350Sea Level to 10,000 ft 0.934 1,070The problem is the lapse rate which means that it's not possible to use one scalar to model the entire flyable atmosphere. Therefore either one has to take an average or model it correctly. You can see at FL200 there is a marked change which is why when doing an emergency dive from say FL350 you myst start to pull strongly into the buffet at that point in order to pull out of the dive. If you don't do that you risk the a/c breaking up!!!vololiberista

[mgh 89]

Average decreaseAltitude Range per 1,000 feet Feet per in/HgSea level to 5,000 ft 1.006 in/Hg 9945,000 to 10,000 0.862 1,16010,000 to 15,000 0.740 1,350Sea Level to 10,000 ft 0.934 1,070The problem is the lapse rate which means that it's not possible to use one scalar to model the entire flyable atmosphere. Therefore either one has to take an average or model it correctly. You can see at FL200 there is a marked change which is why when doing an emergency dive from say FL350 you myst start to pull strongly into the buffet at that point in order to pull out of the dive. If you don't do that you risk the a/c breaking up!!!vololiberista

There isn't a marked chage at FL200. There only appears to be one because your graph is misleading by using a non-linear altitude scale. It has intervals of 1000 ft rom 0 to 5,000 ft: 5000 ft intervals to 15,000 ft: a 3000 ft interval to 18,000 ft: a 2000 ft interval to 20,000 ft: 5000 ft intervals to 50,000 ft: 10000 ft interval to 60,000 ft: and 40,000 ft interval to 100,000 ft. With a linear altitude scale the variation of pressure with altitude is perfectly smooth.

[mgh 89]

As a follow up, the FSX SDK lists the following XML simulation variables under Aircraft Miscellaneous Data.PRESSURIZATION CABIN ALTITUDE The current altitude of the cabin pressurization.. Feet PRESSURIZATION CABIN ALTITUDE GOAL The set altitude of the cabin pressurization. Feet PRESSURIZATION CABIN ALTITUDE RATE The rate at which cabin pressurization changes. Feet per second PRESSURIZATION PRESSURE DIFFERENTIAL The difference in pressure between the set altitude pressurization and the current pressurization. Pounds per square foot PRESSURIZATION DUMP SWITCH True if the cabin pressurization dump switch is on. None are settable.The following Event ID's and String Names are associated with them:KEY_PRESSURIZATION_PRESSURE_ALT_INC - KEY_PRESSURIZATION_PRESSURE_ALT_INC Increases the altitude that the cabin is pressurized toKEY_PRESSURIZATION_PRESSURE_ALT_DEC - KEY_PRESSURIZATION_PRESSURE_ALT_DEC Decreases the altitude that the cabin is pressurized to. KEY_PRESSURIZATION_CLIMB_RATE_INC - PRESSURIZATION_CLIMB_RATE_INC Sets the rate at which cabin pressurization is increasedKEY_PRESSURIZATION_CLIMB_RATE_DEC - PRESSURIZATION_CLIMB_RATE_DEC Sets the rate at which cabin pressurization is decreasedKEY_PRESSURIZATION_PRESSURE_DUMP_SWTICH - PRESSURIZATION_PRESSURE_DUMP_SWTICH Sets the cabin pressure to the outside air pressureThe first two String Names start with KEY_. This is possibly incorrect because they don't generally start with that. The word SWTICH is as spelled in the SDK! EDITThis is another useful simulation variable in Aircraft Ambient Data.AMBIENT PRESSURE Ambient pressure Inches of mercury, inHgThis is obviously not settable. 1 inHg = 70.72619 lb per square foot

[mgh 89]

I've also found that the following entry is aircraft.cfg seems to be needed[Pressurization]design_cabin_pressure =0max_pressure_differential = 0Helpfully, the SDK doesn't say what the units are or exactly wwhat they are used for!Any ideas?

[_Pv_ 0]

Any time you see KEY_ you substitute with K: which is the intended convention in the FS SDKs.so in XML code:1 (>K:PRESSURIZATION_PRESSURE_DUMP_SWITCH) ...which is interpreted as:1(>K:PRESSURIZATION_PRESSURE_DUMP_SWITCH) Activates the pressure dump switch. In testing this switch seems to work. Yes the command is misspelled in the SDK. The above works. Monitoring the following values:(A:PRESSURIZATION CABIN ALTITUDE,feet)(A:PRESSURIZATION CABIN ALTITUDE RATE,feet per second)(A:PRESSURIZATION CABIN ALTITUDE GOAL, feet)(A:PRESSURIZATION PRESSURE DIFFERENTIAL,psf) ... Ive not been able to set these system variables. The goal always stays -2000 and the PRESSURIZATION CABIN ALTITUDE always tries to obtain -2000.-Pv-

[n4gix 4,947]

... Ive not been able to set these system variables. The goal always stays -2000 and the PRESSURIZATION CABIN ALTITUDE always tries to obtain -2000.-Pv-

You need to set the parameters in the aircraft.cfg for your system: Check the default FA-18 for an example:

 [Pressurization]design_cabin_pressure 	= 10.9    	;PSI (~8000 feet)max_pressure_differential = 4.96    	;PSI

[fxsttcb 1]

Seeing as this has already been Bumped! I'll ask! I could not find the cabin pressure xml that was linked by vololiberista in post #13 over at FFDS.I tried searching their forums for "Cabin", "Pressure", "Altitude", and various combinations of those terms and got zilch, none, nada, results.Anyone know the title or how I can retrieve that info? Thanks...Don [edit]Never Mind! Found it! Thanks anyhoooo...

[_Pv_ 0]

Thanks. I have the FSX Deluxe and didn't have the benefit of the F18 example. Seeing if I can figure out how to set the press inc and press dec rates. Documentation is not much help.By default it's 0008 fps. This didn't have any effect:0010 (>K:PRESSURIZATION_CLIMB_RATE_INC) Rate remained at 0008 -Pv-

[_Pv_ 0]

Actually the rate is 8.32 ft/sec so without using a switch just to see if I could change it I tried:(A:PRESSURIZATION CABIN ALTITUDE RATE,feet per second) 100 <if{ (>K:PRESSURIZATION_CLIMB_RATE_INC) } This runs on a 1 second loop. Still no rate change.I did the same search you did fsxttcb with no hits.-Pv-

[_Pv_ 0]

aha... sorry for the incremental thinking and posting. Got it. (A:PRESSURIZATION CABIN ALTITUDE RATE,feet per second) 105 <if{ 3.23 (>K:PRESSURIZATION_CLIMB_RATE_INC) } This effectively set the rate of compensation at 3.23 ft every second of sim time which would be like clicking a gauge once per second, or ~ 30 clicks to get me to the desired change rate of 105 or ~ 6300 ft/min.I see what's going on now. I can effectively use this to create a useful auto pressurization system. -Pv-

[n4gix 4,947]

I came across -this web page- with regard to cabin pressure.What are your thoughts?vololiberista

Updated link to Willem's pressurization gauges!

http://www.aerodynamika.com/cgi-bin/yabb2/YaBB.pl?num=1262180518