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Acceptable surface deviations from an ideal airfoil (Wing construction)

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Tinbuzzard

Member
Joined
Feb 15, 2020
Messages
16
Location
Southern California
My project, the Tinbuzzard, is a reasonable fifteen-foot airplane (Looks fairly good from that distance). On closer inspection however, my metalwork is far from perfect. On wings primarily, what deviations from the ideal airfoil like high or low spots, imperfect seams, and ripples between supporting ribs and spars are considered acceptable? I know this depends on the size and location of the flaws, and the expected performance of the aircraft. I'm using a mid-range laminar flow GA35-215 airfoil, but expect that much of mine will remain in turbulent flow. Here are some illustrative examples from planes at my local apt, and of my project. They seem to indicate that I'm worrying too much.

Musketeer2 (S).JPG
Inner left wing of a Musketeer in morning light to accentuate metalwork contours. The aft-facing lap joint behind the prominent row of rivets is at about 45% of the chord, and has a height of nearly 1/8" in places. Is this buried in the boundary layer and thus has little effect on performance? Would this be enough to trip separation at high AOA?

Cherokee (S).JPG
So as not to pick on Beechcraft, here's the same area on a Cherokee. The worst spot was a step of about 5/32"

Rockwell Lark (S).JPG
Wing root of a well-worn Rockwell Lark forward of the plain flap. This is well aft of the mid chord point, with the maximum depth of the depressed areas being more than a half-inch below contour. (Lean on that screwdriver) The round gascap and the sealant protruding through holes won't promote smooth airflow either. Now on to my worries.

Buzzard Wing 1 (S).JPG
I have a few rivets just behind the leading edge that are depressed by 40 to 50 thousandths due to slight mismatches between ribs. (Tapered wing, all different) This is the worst one. There are also a few smoothly depressed spots further back on both wings (mostly the same spots; I seemed to have been symmetrical with my mistakes) around 30 to 50 thousandths deep and up to a few square inches in size. There are of course also a few random smiled and wrinkled rivets. Will these have any significant effect on airfoil performance? Somewhat more worrisome is a spanwise hump on both sides of the wing center section over the main spar caps. It is at about 25% of the chord, and about the 40 thousandths skin thickness high. There's also a slight depression ahead of it (to the left in the below image) that will definitely promote turbulent flow! Might this promote separation unless I fill it in? And which tends to be worse, a bulge or a depression? I was concerned because I have a more highly-loaded wing than the above aircraft (19# per sq. ft.) and expect a 180+ mph cruise speed.

Buzzard Wing 3 (S).JPG
Forward to the left. This is just outside the wing to fuselage fairing. The hump is from 290 to 320 on the ruler and extends for about two ft. spanwise. Ruler not held down firmly on the right side. That gap is more like the one at the 280 mark.
 
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