¿Question?: Aerodynamics, Parasitic Drag of Exterior Skin Panel Intersections, & TAS

[D Egan]

Speed Question: Does anyone have experience or an opinion whether it would be worth the trouble to use PRC to “smooth” or “transition” the slight reward-facing overlapping ridge where exterior aluminum skins meet and overlap on the wings and fuselage? (Particularly, of course, the intersections that are perpendicular to the wind direction.)

When added up across all the fuselage & wing surface areas, there are many linear feet of this slight “lip” facing rearward (like fish scales I suppose).

As such, if a person where to transition and smooth every inch of that lip with a reward-beveled strip of PRC (like hard caulk), do you think it would measurably reduce parasitic drag by reducing the "tumbling" of air in these areas, and therefore increase TAS?

[Bob Dingley]

This provokes some questions back at ya. What speed range are we talking about. Many 150-200 KT planes have lap joints. The skins usually are only .025 inch. Not much of a bump and the slipstream down there aint going very fast any way.

The Zenith kit planes all have lap joints on the wing near the L.E. The plans call for the top skin to overlap downstream of the leading edge sheet metal. This makes a small .025 bump about where a vortex generator could be placed. The most common error building these is to over lap the joint with the L.E. skin over the more rear top skin. After all, it looks right.The designer, Chris Heintz discusses this on the Zenith site and says "dont sweat it." This seems not to be an issue. The Beech 23, 24s have the cleanest wing around. Its bonded with no butt or lap joints, but its slower than most every thing in the air.

Don't sweat it unlesss you want to go more than 300 kts. Build it light. Bug debris will slow you more.

[D Egan]

This provokes some questions back at ya. What speed range are we talking about.

Thank you, Bob. I read your thoughts and rationale with great interest.

You estimated correctly - our speed range is between 150kts and 180kts.

I suppose we have hoped for either opinion by those with experience, that: A) Yes, it would make a big difference, or B) No, it would make little to no difference. Smoothing every linear foot of these skin intersections would be a lot of work, but we would certainly do if it would have meaninful speed improvements.

Sounds like you suggest that low weight and a bug-free leading edge would be a better focus.

[Bob H]

If the boundary layer is around .10" thick and the skin step is .025", then all of the step is within the layer and fairing it will not make any difference in drag. Air within the boundary layer is slow moving and does not contribute to turbulent drag.

[eiclan]

Gday,because the PR Spitfires during WW11 were unarmed they used to fill all gaps and sheet joins to get more speed and increase the wing efficiency at high altitude.All this and the fact that they were painted a light pink meant they were fast and nearly invisible at height. Cheers Ross

[Bob H]

Let me add something- the boundary layer increases in thickness after about 1/3 of the chord, ~where chord height is greatest. Forward of that point, the skin should be smooth and faired; aft of it, the thicker layer would negate any advantage for fairing in steps.
Bob H