How to Calculate Moment in Fuselage Frame

[aflick]

How can a moment in a circular frame be derived due to an external couple? I am trying to evaluate mc/i of a frame that has a couple on two external lugs attached to the frame. It is a symmetrical frame.

Thanks,
Aflick

 

[GregLocock]

cut the circular frame in two places and establish equilibrium.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[rb1957]

not quite greg ...
i think the problem is singly redundant, the usual solution is to cut the frame (releasing the internal moment) and apply a moment to remove the dispalcement between the two ends of the frame ... the unit load method.

easier is to refer to books (possibly Roark, but i know it is the Boeing static stress manual) as this is a standard problem.

 

[rb1957]

form boeing, they've used NACA references, TN-929 and -1310, and NASA D-402.

all the NACA reports are available on-line, some NASA ones are available (for free).

assuming your applied moment is reacted by a shear flow in shell (presumable the ring frame is attached to a tube (like a fuselage), from boeing the maximum moment inside the frame 1/2 the applied moment (sort of makes sense ... at the point of application the frame reacts the moment 1/2 going one way round the frame, and the other 1/2 going the other). But this isn't the full story, there are internal shear forces that the frame is reacting.

 

[Kwan]

Exactly Rb,

Roark has good ring analysis. Nasa is a great source also. But don't forget that there is already load in the frame. Your lugs will just add load to the frame.