Aircraft systems weight allocation

[myandher]

Does anyone know how in practice the designers define weight requirements for the Aircraft systems (subsystems and components)?
Where one can get the actual or relative data of main systems weight for airplanes like Airbus(let say, A320) or Boeing (737?), or similar...
Is there a Commercial Database when one can get or buy such an information?
Thanks,
M.

 

[der8110]

One of the volumes in Roskam's series of books covers weight allocations, could be dated information but might be relevant to 737 and A320.

 

[myandher]

THank you der8110 for the answer. I will find the book.
Maybe now there is some data and the metodology hint...
Best regards,
M.

 

[KENAT]

I think Raymer may have some stuff on this, can't recall.

As to where to find this information, I believe Janes has a book about avionics etc and information about engines in one or another reference so you may find something there.

KENAT, probably the least qualified checker you'll ever meet...

 

[rb1957]

"how in practice the designers define weight requirements ..."

a mixture of ...
1) methodical, detailed evaluation (and adjustment) of similar airplanes already flying,
2) inspired guesswork, as no doubt each plane has it's own novel features,
3) pressure from the project engineers ("the plane's got to be lighter ... reduce the weight forecast 10% ...")
4) political mascinations of the project engineer "i expect our suppliers (god bless their cheery little hearts) will come in over-weight (and late and over-budget, but that last one's their problem) so i'll broadcast estimates 90% of our best guess, and try to hold them to a 10% weight growth ...")

The texts being mentioned (and I'd add Torenbeek to the list) will all have good info in them, eg the systems generally account for 10% of the airplane's all-up-weight, but little information relevant to current designs. an example, hydraulic systems used to be 3000 psi, new one's are 6,000 psi ... the structure weight would increase with the higher pressure system (heavier tubing).

 

[myandher]

Thank you rb1957 and KENAT for your help. Steel looking forward to get more info, if any...