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Questions from material specs to skin thickness

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rapidshot

Well-Known Member
Joined
Jan 31, 2015
Messages
69
Location
ankara
Hi,
New here and this is going to be my first message, by the way English is not my native language, so from time to time it may sound weird, sorry for that,

Half retired structural engineer designed tall and/or wide steel structures all life long, basically aeronautical engineer, amateur pilot, aircraft owner etc etc,

Nowadays without being sure that it will fly or it will be permitted to fly i decided to design and build an aircraft with MTOW 750 kg within next 5 years, 2 seated , twin tail boom (now it is time to learn and study flutter ) , pusher type,

Simply the specs i have chosen is wing area around 12 m2, 10m x 1.2 m, all metal structure with Naca 2412 airfoil,

Material i have decided to use is 2024 T3, the first question comes here, there is a huge difference between materials physical properties i have found in the internet, i compared the strength given in Lightaircraft Association Guide and Mil Specs where i saw these differences, so the question is are there two groups of materials in the market ,one is commercial with no guaranteed strength while the other is produced and treated as per specs when ordered by big aircraft companies, but i decided to go with Mil specs to save some weight,

The second question is the skin thickness of the wing, i decided to use a single spar at Aerodynamic Center and a false spar close to ailerons to transmit the forces arising from moment to the fuselage via a single shear pin, by this way i got a torsionally stiff box including the D section of the nose, as a result i got stresses from torisonal shear and the pressure on the skin, i assumed the skin takes no compression or tension from bending of the wing, upto here everything was so familiar with me, i calculated chordwise load distribution using three methods, one from thin airfoil theory, second one was a polynomial curve fit , then finaly i i decided to go with a simple triangular distribution with 2/3 rds of load acting on upper skin, so far so good, i have ribs, some stringers as usual but whatever i do it ended up with a skin thickness around 1 mm (0.04 inches) at the boom junction which is quite thick when compared to the skin thicknesses of such light airplanes ,

Although i have a considerably stiff torsional box , a closed area, the shear buckling of the skin governs the design and leads to relatively thick metals

So the question is, is there any rules of thumb simplifying the analysis and the design of such stressed skin type wings, i double, triple checked the calculations and found no mistakes, went into many different sources for buckling of thin metal plates, it is all the same , no different simplifying approach is made since Bleich, Timoshenko )) So far i have done the calculations by hand with simplified boundary conditions but unwillingly i am now ready to put all data in a structural software to capture the real boundary conditions, stringer stiffness etc,

Greetings
 
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