RV-3B - Fiddling with Flaps
I told Louise last night that I am really, REALLY enjoying building the -3B with her, as there is just so much more thought required than doing a pre-punched, modern kit. Even though we are working right now on the QB wings (which you’d think would be no-brainers), we are finding that it is still a matter of look at the plans, measure five times, look at the plans again to determine they aren’t exactly right, measure again to determine that the QB’s are a little different from the plans, and then stop to think about what is REALLY the right thing to do! All fun, challenging, and exactly what I was hoping to find in this build as I help Louise in this project.
While the QB kits for the later models of the RV’s are pretty much “production” items, all the same, and made to match, the -3B QB wing (at least ours), is clearly a limited production item, and they aren’t all alike, or matching (yet). I wrote a couple weeks ago about the inboard aileron bracket holes being in the wrong place. Fixed that - no problem. Now we are trying to get the trailing edges of the ailerons and flaps to line up. No, the plans don’t say that they have to, and truthfully, since most RV’s are parked “flaps down”, no one but the builder/owner/pilot will ever know if they don’t. But….it sure is nice if they do! As many builders of -7’s and -8’s know, it is common for the flaps to be “shorter” than the ailerons. (The fix is to use a hinge with a longer flange, so that you can mount the flaps a little farther aft. Well, we have the opposite problem on the -3B!)
With the ailerons mounted per plans, and measuring so, we discovered that the flaps were sticking out close to a quarter inches beyond the ailerons. All the individual measurements shown on the plans look right, but the results just don’t add up! We built a little “flap deflection template” (a 40 degree angle), and discovered that we could slide the hinge a fair amount forward and still get the deflection - this bought us an eighth. The only way to get another eighth was to trim the trailing edge of the lower wing skin. Of course, this would change the alignment of the flap brace, meaning that we needed to change the angle of the pre-bent flanges on that piece. After a couple evening’s work getting the flap braces cut and shaped to fit the spar (it REALLY helps to have the -6 and the -8 in the hangar to refer to, as well as the plans for both those planes…), we figured that we might as well tweak the flanges and trim the skins. A few hours work, and now everything looks good, the trailing edges lined up, and all the mating surfaces are lying true and flat. We effectively shortened the wing chord at the flaps by 1/4”, but I am not sure if we will ever be able to tell (by measuring) if they are now “correct”, or shorter than the goal. And frankly, that is where the “art” of building comes in.
One of the most challenging things about being a teacher on this project is, in fact, helping Louise to understand when a deviation from the plans is OK by inspection, and when we need to tread more carefully. I know, for instance, that I can substitute a flush pop rivet for one of the four round-head rivets holding the trailing edge of a wing rib to the rear spar, in order to make a flat spot for the flap brace to sit. How do I know that? Well….experience tells me so. I know there are more rivets than we need in the kit, I know that a flush rivet is almost as strong as a regular rivet, a pop rivet is pretty good, we NEED a flat spot for the flap brace, and (I think) it was done that way on the -8. No problem. But then we get to the need for a couple of flush rivets at the root end, on the big rear spar yoke parts - now THOSE are going to be Cherry-Max’s, even though we have to order and wait for them to arrive. Why? Structure - BIG structure - which implies that you need to have strong rivets. And….it shows it that way on the plans for the -8! Now how do I know to justify minor mods and changes based on these sources and reasoning? Judgment based on experience (and the aeronautical engineering degree doesn’t hurt either - I know how to look at load paths, and determine what is primary, and what is secondary structure).
I can see why the RV-3B is seeing a resurgence in popularity among repeat offenders, and many are picking up abandoned kits from those who started them without the experience to work beyond the plans. Without a significant support system of advisors, it would be pretty tough to make one in a vacuum, without prior experience. But for those looking for a challenge beyond the pre-punched, QB world, the -3B is really a great option - and when you’re done with the fun of building, you end up with a pretty cool airplane as well!
Paul F. Dye
Editor in Chief - KITPLANES Magazine
RV-8 - N188PD
RV-6 (By Marriage) - N164MS - "Mikey"
RV-3B - N13PL - "Tsamsiyu"
A&P, EAA Tech Counselor/Flight Advisor
Dayton Valley Airpark (A34)