Hey @Todd Coburn, question. Let's say my fuselage has known stringer & frame spacing & geometry and I am trying to write the overall fuselage margin to known section loads. Your previous lectures state how to write margins for each stringer with some effective skin width, and from this method I might have written a section's capability as that of the lowest margin stringer. Would we consider the curved panel between each stringer in our analysis, or neglect it since the stringer and it's effective skin will carry load past the curved panel's buckling allowable? Another way to pose this question is: when and why would we apply these equations in a structure if the buckling capability of the stringers neighboring the panel is much greater?
Ted, good question. One underlying principle is that we must first determine loads for the skin panel. That can be a challenge in itself. The reason is that the loads that the skin change based on how much skin is effective. If we assume full effective widths then we tend to over-predict the load in the skin, especially if the skin is in compression. Lecture 25 in the Stress Analysis III playlist explains a simple way of accounting for this (although i need to record a more-clear explanation of what you seek). Once you have determined the appropriate effective skin for a given set of loads, then as you said, the assumption that the effective skin is included in the stringer and that the rest of the skin is unloaded does in fact kind of deal with the skin. However, it is best to also evaluate the skin separately. First, you need to recaclulate the skin load. You can do this by multiplying the stringer/skin stress by the area of the effective skin. Pskin=f_stringer&skin (w_eff)(t_skin) Then divide by skin area in bay for stress… f_skin=Pskin/(skin width between stringers)(t_skin) Then check that skin for buckling. ( It shouldn’t be based on the other assumptions, especially since the load is based on a flat skin assumption and the actual buckling capability is a curved panel value. Of course, if it ever did buckle, that is not the end of the story since a diagonal tension analysis can then be performed. However, that introduces a number of additional checks that can get tricky to deal with properly. After you do this a time or two, you may decide you can stop at the stringer evaluation since it generally will cover the skin itself if the skin loads are properly unraveled from the prior assumption values. Make sense? This is super-fun stuff, but requires careful stepping thru the process.
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Hey @Todd Coburn, question. Let's say my fuselage has known stringer & frame spacing & geometry and I am trying to write the overall fuselage margin to known section loads. Your previous lectures state how to write margins for each stringer with some effective skin width, and from this method I might have written a section's capability as that of the lowest margin stringer. Would we consider the curved panel between each stringer in our analysis, or neglect it since the stringer and it's effective skin will carry load past the curved panel's buckling allowable?
Another way to pose this question is: when and why would we apply these equations in a structure if the buckling capability of the stringers neighboring the panel is much greater?
Ted, good question.
One underlying principle is that we must first determine loads for the skin panel. That can be a challenge in itself. The reason is that the loads that the skin change based on how much skin is effective. If we assume full effective widths then we tend to over-predict the load in the skin, especially if the skin is in compression. Lecture 25 in the Stress Analysis III playlist explains a simple way of accounting for this (although i need to record a more-clear explanation of what you seek).
Once you have determined the appropriate effective skin for a given set of loads, then as you said, the assumption that the effective skin is included in the stringer and that the rest of the skin is unloaded does in fact kind of deal with the skin.
However, it is best to also evaluate the skin separately.
First, you need to recaclulate the skin load. You can do this by multiplying the stringer/skin stress by the area of the effective skin.
Pskin=f_stringer&skin (w_eff)(t_skin)
Then divide by skin area in bay for stress…
f_skin=Pskin/(skin width between stringers)(t_skin)
Then check that skin for buckling. ( It shouldn’t be based on the other assumptions, especially since the load is based on a flat skin assumption and the actual buckling capability is a curved panel value.
Of course, if it ever did buckle, that is not the end of the story since a diagonal tension analysis can then be performed. However, that introduces a number of additional checks that can get tricky to deal with properly.
After you do this a time or two, you may decide you can stop at the stringer evaluation since it generally will cover the skin itself if the skin loads are properly unraveled from the prior assumption values.
Make sense?
This is super-fun stuff, but requires careful stepping thru the process.