While I wait for some help on the bottom skins, I'll just move on to the flaps. I feel a literary digression coming on so I'll offer fair warning to the easily bored. Skip down to the pictures if you already know what a flap is for.
There are several different types of flaps, but generally, the flaps just extend downward at the trailing edge of the wing with the overall effect to increase its effective camber. The flaps on this aircraft will be of the 'plain' type, illustrated below via the ever so convenient Google images.
So the purpose of flaps on an aircraft is to get some additional low speed performance out of your wings, albeit at the expense of a lot of extra drag. The two phases of flight where flaps become advantageous is in take off and landing. Having more lift during these two flight regimes allows the aircraft to use less runway. In ascent, because the increased lift allows the aircraft to depart the runway sooner, and in descent because the aircraft can fly slower, it will contact the runway with less horizontal velocity and consequently use less runway.
Another advantage of having flaps available for landing is that it allows the aircraft to execute a steeper approach which is safer from an "engine out, emergency glide to the runway" point of view. Lastly, the flaps allow the descending aircraft to fly in a more nose down attitude giving the pilot better visibility of the approaching runway.
Now that we've covered flaps 101, let's see what goes inside the RV-7A flap. I start with a quick check of the plans and then go collect the parts from storage. After laying them out it is a fairly simple matter to understand the assembly.
Spar on the left, skins on the right, and ribs in the middle. Yep, that's about it. Then because one needs some boring tasks to balance out the fun stuff, one has to remove all the blue protective film and then smooth the edges of all of the parts.
After all of the edge deburring it's time to assemble the flap. What is visible in this photo is the bottom skin and ribs. The next thing is to fabricate the FL-706B flange. This part transmits the load from the flap actuator to the flap. It is a fairly beefy 1/8" thick affair and is surprisingly resistant to my attempts to impart the specified bend. It seems that the inboard end of the flap is 6.3 degrees off square, probably to follow the taper of the fuselage.
Whatever the cause, the plans call for the end of the FL-706B to be bent at precisely 6.3 degrees. In my mind I could see the result of this exercise as a pile of flanges bent to 6.2 degrees and another at 6.4 degrees, with a single example of a 6.3 degree flange kind of glowing with an aura of correctness in the foreground. I digress. The bend is not really such a big deal except that the section to be bent is only 25/32".
If you're like me, fractionally challenged, you'll be interested to know that 25/32" is actually .78 inches. Somehow the fraction doesn't mean anything to me until I convert it to a decimal. Anyway, that's not much on such a thick piece of metal. After screwing around for about an hour with several failed attempts, I finally gave in and consulted the internet. Being the source of all knowledge, I knew I would find an answer there, but as is customary with the internet, many "solutions" are usually found.
So the problem becomes one of selecting one that sounds reasonable. Sometimes finding a consensus of opinion online can be a challenge. The ideas I rejected were "Use your million dollar numerically controlled 7 ton hydro static home bending brake" And "Hire Uri Geller to bend the piece with his mind."
Fortunately, most online felt that what I was supposed to do was clamp the short end in a vise and then clamp the long end between some wood to keep it straight. Obvious isn't it?
Well, on my next plane, I guess I can skip internet part. And hopefully all of the screwing around wasting time part and just get right to it. So here we are with the FL-706B bent nicely at 6.3 degrees.
Then it's just a matter of match drilling the bent flange to the inboard rib and spar.
Then its time to match drill the piano hinge that attaches the flap to the rest of the wing. That's me in the photo above trying to align the hinge with some precision. I find the magnifying visor to be more and more helpful...
Once the match drilling is complete the flap is disassembled, the holes deburred and the ribs and skins were dimpled.
The last item before priming was the bottom of the spar is machine countersunk rather than dimpled. Three pieces of metal are sandwiched at this point: the bottom skin, the spar and the hinge. Machine counter sinking the spar is possible here because it is fairly thick, unlike the skin, and this allows the dimpled skin to nest in the countersunk spar on one side, but leaves the other side of the spar flat so that the hinge does not require dimpling.
Machine counter sinking the bottom of the spar in progress.
Priming.
Then it's reassembly time. Most of the rivets had to be bucked. I found that it was easier to reach down inside from on top of the table. For the next flap, I think I'll put a back on the cradle so I can lay it over on its side to avoid the gymnastics.
And finally, the last two rows of rivets attach the spar and finish the flap. The bottom row, shown above, is squeezed. The top row had to be bucked.
One flap done.
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