Originally Posted by Wilyflier;#1312
Can any one equate the design strength of the VS with the max possible deceleration "G" in any sort of flat ditching, and the forward "G" required to permit a fin and rudder to rip clean off its fittings?
The max possible deceleration "G" in flat ditching is perhaps a separate question, with too many unknowns in the equation to expect a meaningful answer. Flat ditching refers only to attitude: almost wings level, right wing slightly down, some nose-up pitch. Other relevant factors are: high vertical speed, probably low horizontal speed although unlikely to be lower than stall speed (160 kt clean?), and the condition of the airplane immediately after the initial tail-down impact. If one assumes the fuselage broke aft of the wing (crew rest module floating on the surface), the drag of the rear fuselage in the water would have been high relative to its mass, easily resulting in decelerations of the order of 30 – 60 g, depending on the assumptions made.
Just a crude layman’s attempt to relate aerodynamics, structural strength, and crash deceleration:
Aerodynamic load. The area of the VT is estimated as 44 m^2, and the max. rudder angle is 35 degrees up to 150 kt CAS. Assuming cL=2, the sideforce on the vertical tail is 30 t.
Vertical loads on VS attachments. If the VS bending moment is distributed equally over the six attachments, each receives a vertical load of 44 t, tensile on one side, and compressive on the other.
Inertial load compatible with attachment strength. The forward inertia force results in a vertical pull force on the two aft attachments. A pull force of 2*44 t corresponds to 73 t at the c. of g. of the vertical tail surfaces. In his post #1116 (p.56) “cc45” gives the VT a mass of 1800 kg, resulting in an acceleration of 41 g.
Graphic: A ‘back-of-the-envelope’ sketch illustrating the above is available here:
https://docs.google.com/leaf?id=0B0C...MmZhNDI4&hl=fr