From Army idea, 4 mt. Diamond monocoque with Sika adhesived 0.7 mm stainless steel

The comment was made regarding the statement that 'small obects have an inherent structural rigidity' which is just not true. You are simply doubling down which might work in politics, but not here.

 

In engineering assumptions and frames of references must be stated. The claim that small objects have an inherent structural rigidity has no evidence of being true.

 

I think it would be fair to say that it is easier to make a small object rigid than a large one.

 

No, it wouldn't. Rigidity is a function of the material properties and the design. This is one of the situations where size doesn't matter.

 

The flexural rigidity of a beam is usually defined by the young's modulus of the beam material, the moment of inertia of its cross section, and the length of the beam. Accordingly, saying that size does not matter does not seem very accurate.

 

From your point of view, size matters. However, the larger the size the more rigidity, which shows the OP is wrong.

 

For flexural rigidity, the larger the size the less rigidigy.
Rig = E*I/l

 

For a rectangular beam I=bh^3. E increases to the cube, while l increases linearly. Ergo, rigidity increases with size.

 

No Gonzo, the "I" (moment of inertia of the cross section) does not change when the length of the beam is increased. Talk to your advisors, I know you have good friends at the University, tell them what we are talking about and that they explain it to you in simple terms.

 

If all changes maintaining the same ratio (getting bigger as related to the post) the depth and width increases as the length does.

 

No

 

This is another example of needing to state assumptions.
You two have different assumptions for a not very accurate statement = because it was not stated well.

Just answering No was one of the more juvenile statements I've seen here in a while.

Like two 10 year olds repeating - is too, is not, is too, is not, etc.

 

Since I quoted cracked ribs on my answer, it isn't confusing that I was answering his statement. Nobody answered "no" however.

 

That's correct.
However, it does need a little further explaining, as this is a blanket statement.

That does it nicely.

It is misleading however, to say this:

Clearly "size" does matter.
Since size is not related to just its length - the out-of-plane length of the sectional shape. Size infers all 3 dimensions: length, breadth and depth.

Because the frame of reference - the dimensions - have now been changed, from what Gonzo referred to, as the sectional shape (the depth and breadth of the section for the "I") to this:

Gonzo did not state the length - just one dimension out of 3 possible - remains the same. Since it is clear he is referring to the product the - EI - which is the flexural rigidity and the shape of the section.

The moment of inertia of a section does not have length as an input. It has height (the L, or which ever letter you prefer) and width (the B), as per clearly defined in each figure that outlines the 'shape', the nomenclature is for each figure and must not be inferred to be anything else. Mischaracterisating the "L" as length of section to be the out-of-plane dimension - L - belies a lack of understanding and comprehension.

The 'Length' that Tansl is referring to....is the 'length' which is the out-of-plane perpendicular axis... but has nothing to do with the sectional shape Gonzo clearly noted.
Hence the mischaracterisation for perhaps some cheap shot?? This is an engineering/boat forum, not the school playground gents.

For those that do not understand here are typical second moments of inertia of sectional shapes, clearly showing "length of section" is not an input. Despite nomenclature sometimes using "L".
Such as a full range of shapes HERE, (where they use 'h' as the depth)
or
a page out of any text book, shown below (where they use d as depth):



Definitions mean everything in engineering.

 

Should be enough detail for anyone.
Perhaps assuming Gonzo had one definition for the length element was an assumption not warranted.

Enough from me.