Gentlemen,

What formula, method, or technique is used for determing the Rib Spacing in a wing?

Will the Rib Spacing change for a wing if it is fully covered with silkspan (open bays) or if the wing is fully sheeted with 1/16 inch balsa?

It seems like the criteria depends of the strength of the wing spars. 

My wing spars are 1/4 by 1/4 spruce and the wings are D tube construction with 1/16 inch lite balsa sheer plates.




t michael jennings               
knoxville, tn

I do not know if there is any equation that would apply to answer the question about how wide should wing ribs be.

What follows essentially agrees with what Ty Marcucci suggested above.

My comments below apply to stunt designs in the 35 to 50 size (550 sq in to 650 sq in)

For fabric/tissue covered wings, with leading edge sheeting (like a D-tube or C-tube structure), rib spacing from around 2 inches to not more than 2 1/4 inches seems to be a general rule of thumb that is seen on most designs.  This sort of assumes that there will be 1/4 inch capstripping on each rib behind the LE sheeting.  If there is no LE sheeting, like on an I-Beam structure, rib spacing for the LE portion should probably be not more  than 1 inch to minimize the scalloping of the covering between the ribs on the LE portion of the wing.  Half ribs can be considered in such a situation.  Then, if the ribs are capstripped (not often seen on I-beams, but there are other non-I-Beam designs without LE sheeting), then there needs to be only about an inch between the edges of the cap strips which would mean the ribs could be on 1 1/4 inch centers.

If the entire wing is sheeted (like with 1/16 sheet) and the sheet wood is fairly firm (as in stiff), then  rib spacing could go as high as 2 1/4 inches and not show much "saggaing" of the sheeting from the leading edge to the max thickness of the airfoil.  This sagging, even on sheeted surfaces, will show up if you use a high shrink paintover those sheeted surfaces.

The wider the rib spacing in any of the situations described above will result in increased "sagging" of the covering to various extents.  Also, the rib spacing is a function in the stiffness of the wing because of the manner the ribs and spars interact to form a structure.  If there is a full depth spar for the entire span (a D-Tube or sheer webbing or a full depth spar as in a true I-Beam) then  the rib spacing is not so critical from a structural standpoint.  Basically, the rib spacing should be at a minimum to minimize the "scalloping" or "sagging" of the covering (either fabric, tissue or wood sheeting).

My experience when covering with silkspan or any other material that has a longitudinal grain to it (including silk and some of the iron on woven coverings)  is to align the grain span wise to minimize the "scalloping" or "sagging between the ribs where there is maximum curvature of the airfoil section.

The spacings described above seem to work even on the larger 750 sq in ships and for those designs that might use 3/32 inch LE sheeting.

As designs get smaller (as in 1/2A stunt ships - 200 sq in plus or minus) with 18& or thicker wing sections should have correspondingly narrowere rib spacings, again to minimuze the sagging of the covering between the ribs where there is maximum curvature of the wing section.

Combat ships get away with 5 inch rib spacing behind the LE primarily because there is not much curvature to the wing sections these things use aft of the LE structure and the LE structure carries most of the loads.

Keith

If you use the typical C or D tube costruction and nitrate or butyrate dope, there will be some sagging of the sheet between the ribs, amount depending on stiffness of the sheet and how much plastciser one puts into the dope. I cover the open bays with silkspan but use carbon fiber mat on the sheet and find there is very little to no sagging from applying dope. This also applies to any other sheeted areas of the model where the pulling tight of doped silkspan could cause sagging.
Don

Assuming that the LE is sheeted, it depends some on the airfoil.  What you don't want is a bump at the back of the sheeting.  The more the rib spacing, the bigger the bump will be.  However, if the airfoil is a straight line from 20% chord to the TE, and the aft edge of the sheeting is at 25% chord, the rib spacing can be wide.  A bump in the neighborhood of the max thickness point of a wing reduces its max lift capability.  One way to look at that result is that to turn a good corner, you'll need a lighter wing loading, hence an airplane that's more susceptible to turbulence. 

My own experience is that rib spacing is a function of with whom I have lunch.  While I was designing the Snort, I was having lunch with an aerodynamicist.  He coerced me into putting the ribs pretty close together. 

We can consider using a rather wide rib spacing, if we, 1, do not sheet the leading edge. 2, Use half ribs to keep the entry portion of the airfoil truer. 3, If leading edge sheeting is used, extend the aft edge back far enough to where the curve of the airfoil is less. This helps get rid of the "bump". 4, Use half ribs under the sheeting, for the same reqason as above.