Tail Question

[Dert Merchant]

Hello all. I have been viewing & reviewing the plans & drawings for about a week now. I have been making notes to help me along the way from info I have found on here. I have a question about the tail section I haven't found an answer for. The plans call for the .020 ribs to be installed "bent sides facing down". As I would look at them from the end should they look like an upside down "u" or should it look like a "c"? If it is the "c" shape should the ribs be sheared at a taper so that after being bent they match the forward/aft tube diameter?
Thanks,
Derek

[Sam Buchanan]

Upside-down U. This will prevent moisture from being trapped by the rib.

Yes, the ribs need a taper so they will fit properly at each end.

[Dert Merchant]

Thank you sir. Now it makes more sense.

[Jerry Carter]

Upside-down U. This will prevent moisture from being trapped by the rib.

So, if we could revisit this:  if the ribs on the stabilizer and elevators are open side down, how does the gusset on the bottom side rivet to the rib? It seems if the rib were turned sideways that the gussets could be riveted on both sides. Also, the drawings for the stabilizer and the fin indicate "flat side" of the root ribs.  I assume this means the rib should be open end up on the fin and open end facing outboard on the stabilizer. Correct? Thanks.

[Sam Buchanan]

Upside-down U. This will prevent moisture from being trapped by the rib.

So, if we could revisit this:  if the ribs on the stabilizer and elevators are open side down, how does the gusset on the bottom side rivet to the rib? It seems if the rib were turned sideways that the gussets could be riveted on both sides. Also, the drawings for the stabilizer and the fin indicate "flat side" of the root ribs.  I assume this means the rib should be open end up on the fin and open end facing outboard on the stabilizer. Correct? Thanks.

Jerry, as best I can tell you have it backwards.

Face the middle part of the U (closed side) toward the root of each horizontal control surface.

Now, imagine you are a drop of water inside the vertical control surfaces. We don't want you to get trapped so the closed side of the U faces up. This always gives you a flat on which to attach each gusset.

In the photo below the top of the rudder is to the left.



If this was the right elevator, the root end would be to the left.

No need to over think this.  

[Jerry Carter]

Thanks, Sam. I understood that the U should face down on the verticle surfaces. I was a bit confused by the indication of "flat side" on the fin and stabilizer drawings. I was planning to substitute 1/2" tubing at the root rib of the fin anyway per your suggestion.

[Sam Buchanan]

Ok...glad we got that straightened out. 

[Jerry Carter]

One final thing. It's obvious to me now that "flat side" just means that side of the control surface is not tubular - not that it is flat on the outside. This seems ridiculously obvious to me now, but when one is struggling to understand something simple things can be difficult. Maybe this will help someone else in the future.

[dz1sfb]

The upside down U on the vertical stabilizer and trapping moisture does not make sense to me. Unless the rib tips are sealed to the leading edge and hinge tubes moisture will pass through and the entire component will breath. Makes more sense on the vertical stabilizer as called out on the plan that the "flat side" is the bottom of the U. Similarly on the horizontal stabilizer that the flat (bottom) side faces toward the root end of the flying surface to close it off, and flat side on the rib next to the wire bracing positions. In this way the you are more directly transfering the wire brace load into the structure against a solid surface.

Just figuring this out as I am studying the plans.

[dz1sfb]

Just got done talking to Leonard on the phone. Seems it is true open side of the C section faces down for all but the root rib. This one can have a hole added for a drain. This makes better sense to me. Having the root rib as a close out and easier to cement the fabric to as well. That means the horizontal stab they all face inward except the root rib again for the same reasons.