Grumman F4F: Wing Question

[Hugh_T]

I am currently studying the F4F wing structure and noticed that the wing ribs are not perpendicular to the wing chord as you would normally expect. Certainly, this is not something that I have come across before. I had previously studied the F6F and although another of the Grumman 'cats' the wing ribs are perpendicular to the chord as expected.
I am at a loss trying to fathom why Grumman designed the F4F wings in this manner as there does not appear to be any performance advantage or manufacturing benefit in doing so.
I wonder if anyone has any idea why?
This drawing shows how the rib is actually perpendicular to the ground (in-situ) and not the wing chord.

 

[Paul E.]

I am currently studying the F4F wing structure and noticed that the wing ribs are not perpendicular to the wing chord as you would normally expect. Certainly, this is not something that I have come across before. I had previously studied the F6F and although another of the Grumman 'cats' the wing ribs are perpendicular to the chord as expected.
I am at a loss trying to fathom why Grumman designed the F4F wings in this manner as there does not appear to be any performance advantage or manufacturing benefit in doing so.
I wonder if anyone has any idea why?
This drawing shows how the rib is actually perpendicular to the ground (in-situ) and not the wing chord.

View attachment 508754

The F4F had wings that folded at an angle that caused them to lay flat along the fuselage. The detail in the drawing is showing the hinge design that causes that to happen. Quite an interesting design.

 

[MIflyer]

That approach is not unheard of for that era. This is called a Warren Truss design.

 

[MACHIA]

That approach is not unheard of for that era. This is called a Warren Truss design.
View attachment 508793

This truss configuration also used in bridge building and wooden timber construction .

 

[Dana Bell]

Hi Hugh,

I'd never noticed this before, but you're right. With your observation in mind, I went back to my drawings to see if there was any evidence of just what those ribs were parallel to. With my current limited access to drawings, it appears the inboard ribs followed the taper of the fuselage, with all other ribs parallel to the inboard ribs. Am I certain? No, but it appears this is where some further investigation should start...

Cheers,


Dana

 

[Hugh_T]

The F4F had wings that folded at an angle that caused them to lay flat along the fuselage. The detail in the drawing is showing the hinge design that causes that to happen. Quite an interesting design. View attachment 508792

Thank you, actually the question related to the wing ribs (highlighted) themselves not the intersection of the wing subassemblies. It was a poor choice of image selection on my part.

 

[chuter]

That is an interesting detail. Maybe it was easier for the draftsmen to loft the rib profiles? The difference in strength was likely deemed trivial.

 

[Hugh_T]

Thanks, Dana and Chuter; I am continuing my investigation which seems to throw up more questions than answers. I looked at the centre section structure as well for some clarification but that is extraordinarily complex with a multitude of datum lines! I have added some clarification on my query on my blog. Grumman F4F Wildcat: Wing Ribs:

 

[MIflyer]

Note that for the Ercoupe the wing structure is entirely self-supporting because it was not a stressed skin design. The wing originally was covered with fabric, which only kept the wind out. The leading edge was not a full D-section, the AL skin only wrapping partway around from top to bottom.

 

[Paul E.]

Note that for the Ercoupe the wing structure is entirely self-supporting because it was not a stressed skin design. The wing originally was covered with fabric, which only kept the wind out. The leading edge was not a full D-section, the AL skin only wrapping partway around from top to bottom.

I think many of you don't understand is that the drawing shows the wing joint from the front. the round objects you se are .50 cal machine guns and the seemingly tubular things snaking to the right are ammo feed chutes. The angled lines are actually the joint seam, not a front views of the airfoil ribs. The two red arrows are showing the locations of the structural ribs.

 

[James Duff]

For what it's worth, my hunch is that the guns are perpendicular with "the ground" as that is the way g forces would be felt by the various mechanisms including the shell feed mechanisms and they wanted to ensure reliability of the guns in combat operations where g forces would be encountered. Bearing in mind this was one of the early all metal designs in the USA and Grumman. These guns were the primary reason this aircraft existed and were of primary importance in the design.
I suspect the ribs were slanted to give maximum support for both the guns and wing fold mechanism and it was easier to maintain this right along the wing structure.
I also suspect that with experience gained by the various operators and manufacturers I suspect it was noticed that the guns being perpendicular as they thought is was and simplicity became more important.
This is just a guess from someone who has spent 50 years in the aviation industry as a licenced engineer. I may well be proved wrong.

 

[Hugh_T]

Thank you all for your contributions and good advice. It would appear that this is not as straightforward as I first thought. Investigating this further it transpires that the BaseLine; from which the wing ribs are dimensioned from; is not as I initially thought parallel to the ground, it is in fact 1.5 degrees from a Wing datum Line. This Wing datum line is parallel to the ground. Given that they have this BaseLine in the first place; to which the rib dimensions are related; does it, therefore, imply that the ribs are perpendicular to the BaseLine and consequently 1.5 degrees offset from ground perpendicularity (Wing Datum Line)?


UPDATE:
Further research has prompted a solution. It turns out we have 5 datum/reference lines for the wing setout. Chord Ref Line, Base Line, Datum lIne, Thrust Line & Horizontal Ref Line. The ribs are perpendicular to the Datum Line which is set 2 degrees from the horizontal (ground).
View attachment 509747