Decompression Analysis

[delforo]

I am looking for references to do decompression analysis of transport category aircraft for compliance with FAR 25.365. Also reference for orifice discharge coeficients, as these will be needed for the analysis.

Thanks in advance for any help

 

[hombre]

This type of analysis is usually kept proprietary by the aircraft companies because in-house code is developed. However, I have an Excel spreadsheet that will time-step a two volume model venting to the atmosphere. I also have a Fortran program that will analyze a multi-volume aircraft that is not FAA approved.

As far as a reference for discharge coefficients, NACA TN 3466 investigates them and lists other references as well. This is available on the NASA Tech Report Server.

However, it is usually assumed that the discharge coefficients between volumes is 0.65 and the discharge coefficient for the exit orifice is 0.75.

 

[decomp]

As hombre stated the OEMs keep this close and each one does it differently, even by model. Boeing does it one way Airbus does it another ... this includes how they model discharge coefficients and how integrate the mass flow equations in time.

I have seen Boeing refer to Perry (1949 transaction of the ASME) for some discharge coefficients AIRBUS favors a different approach. Also, discharge coefficients for non standard flow paths are frequently determined by test ... at least at the OEMs.

I do and certify decompression analysis per 25.365(e).

 

[wodi]

Hi All,

we do rapid decompression calculations for an aircraft vendor, and we do use rather conservative discharge coefficients, i.e. discharge coefficients between volumes are 0.45-0.6, and the discharge coefficient for the exit orifice is 0.42.
So, you are referring to a NACA TN 3466.
Unfortunately, I can't get access to the NASA tech report server. Do you have this TN 3466 available? Do you have any other sources investigating discharge coefficients for rapid/explosive decompression scenarios in aircrafts?
Do you have an idea in what the rapid decompression algorithms of major aircraft vendors differ?

 

[hombre]

Wodi,

If you supply me with an e-mail address, I will attempt to send you an electronic copy of TN 3466. If not, I would be more than happy to send you a copy through the mail if you provide an address.

Other sources of info include papers by NASA involving discharge coefficients for space craft venting. I have these as well since I have worked for an aircraft manufacturer as well as a spacecraft manufacturer.

Having worked for only one of the major aircraft OEM's, I can tell you that the algorithms probably differ in the assumptions used for the analysis (adiabatic, isothermal, etc.)

Hope this info is helpful.

Hombre

 

[wodi]

Hello hombre,

thanks very much for your interesting comments. Yes, I would be very glad to get TN 3466 electronically, as well as maybe some other usefull overviewing information.
My private email-address is:
Wolf-Dietrich.Kunze@t-online.de
I am quite familar with programming and underlying physics of the RD algorithm which is used here. I guess that RD algorithms used by different aircraft manufactures wouldn't result in rather different results, but it would be interesting to know how much they differ.
More critical seems to be the choice of appropriate discharge coefficients which have a rather strong influence on results.

 

[wodi]

Hi All,

first of all, I would like to wish everyone a happy and successful new year 2004!

After this, I have a new question concerning rapid decompression analysis:

Do you know wether someone already considered the "huge volume" effect (for large aircrafts like B747 or A340) in calculating rapid decompression pressure histories?

When a huge volume (e.g. maindeck of B747 or B340) is experiencing a rapid decompression event (large hole to ambient), its volume decompression will be limited by the speed of sound. Volumes which are in close neighbourhood to the primarily affected volume will not "start" with decompression until the decompression "signal" has reached the neighbouring volumes. This occurs with speed of sound.
So, in my opinion, the complete decompression of large volumes is limited and delayed by the speed of sound.

Did anybody take this delay into account?

 

[jiqiren]

Hi Wodi, Hombre and Decomp, thanks for the interesting discussion posted in this forum.

I'm currently looking at modification of large transport aircraft, converting from passenger to freighter. I'll like to find out more on the methods in certifying such modification to meet FAR 25.365(e) requirements.

Is there any reference materials which I can read up to understand the kind of analysis involved/ required? What type of engineering tools is required or commercially available for such analysis?

Many thanks in advance.

 

[wodi]

Hi Jiqiren,

as far as I know, there are no "official" or commercial tools available to perform rapid decompression calculations. Each aircraft vendor is using its own tool to calculate rapid decompression scenarios. As far as I know, each aircraft vendor has to certify its rapid decompression tool in front of responsible certification authorities. A detailled rapid decompression analysis and its results have to be explained in front of certification authorities, as well. So, the tool is not so important but the consistent results. As far as I know, all rapid decompression tools are rather similar calculating the pressure vessel discarge equation. There are some differences in boundary conditions.

Wodi

 

[Aeromark]

FYI, NASA TN3466 is available at

http://naca.larc.nasa.gov/reports/1955/naca-tn-3466/

I have worked with several modifications to transport category aircraft and a couple of decompression DERs. Those guys don't let tell too much about how they work their black magic.

To get an analysis accepted by the FAA you will probably have to show similarity with an accepted code the first time, in addition to a rigorous discussion of the analysis technique.

 

[boilrbukeye]

Just to throw in a side note regarding vent opening coefficients (often called "discharge coefficients"), the FAA recently directed me to an Advisory Circular that clearly states the FAA will accept .50 for a coefficient for a "hole caused by failure of the fuselage" and .75 for a "hole caused by loss of a window".

To me and the Atlanta ACO, that meant 0.50 for any damaged structure and .75 for anything designed as a clean hole (round/square/etc).

The AC is AC25-20.

Hope this helps. It's always good to use the FAA's own words when justifying analysis to them.

 

[Jamman]

Hi,

New to the forum here. Sounds like some pf you have experience with the FAA. Boilrbukeye, glad to hear the Atlanta ACO has given you some guidance regarding discharge coefficients at the exit opening. For years there has been talk here in Seattle about getting some guidance in this area but to no avail. Did the guidance from Atlanta originate out of the Directorate in Seattle or is it more or less a commonly accepted practice in Atlanta ?????. I know LA has in the past used a Cd of 1.0 for many Douglas projects or so I'm told....I've had them accept .65 which is traditionally been the standard accepted in Seattle.

Regarding an earlier post, about 90% of the FAA accepted analysis I've done has been Isothermal and constant Cd. The 737 for whatever reason is the only plane I'm aware of that was originally certified using Isentropic pressure decay.

 

[boilrbukeye]

Jamman,

Actually, the guidance was from a Fort Worth ACO Engineer and was accepted by the Atlanta ACO recently when I reviewed some data for approval in mid-2003.

A Cd of 1.0 would be the most UNCONSERVATIVE value to use on internal venting but most CONSERVATIVE on the damage opening size so I guess it would depend where LA was using the 1.0 value whether it made sense or not. Knowing the FAA, they were probably imposing it on the external opening thus making it a true worst case scenario. Lacking data, they like to do that.

This entire subject makes me think however, because the FAA has become so fastidious for things that used to be "obvious to the casual observer"....how does ANYONE certify to 25.307 (proof of structure) for decompression??? The ONLY test data I've ever seen dates back to L-1011 and is from the 1950's where they actually tested their decompression features FULL SCALE in a decompression chamber! As far as I'm concerned, most people since have done the cert. only via analysis with no real test basis. If they ever had a test basis, it was lost in the translation twenty or thirty years ago.

Oh well, guess it's not the first time there's a slight disconnect.

 

[martin54]

Hello!

We just come to discover the forum and we work too on the rapid decompression. We try to find the equation giving the temporal variation of the pressure in the case of an impact by taking into account only two volumes. Could give us ourselves tracks please.

Thank you in advance and good afternoon!!