Interesting aerodynamic point to ponder.

[glenmthompson]

While scalding my hand and frying some eggs on Jim's RV-10's tunnel today (Long story, see other thread in the 10 section), I noticed a very curious thing as we flew through some rain, I noticed that the top of the flap was dry.
Now Jim has a pretty tight fit , and has teflon tape at that intersection. But I noticed as the rain hit the trailing edge of the top skin, it disapeared! Leaving the top of the flap totally dry. Now, the trailing edge of the flap had copious amounts of rain trailing off, either from the bottom of the wing, and/or from the rain that maybe dropped under the flap at the wing top skin/flap intersection. BUT..If the rain DOES indeed drop below the flap at this intersection, then how is that possible, seeing as the bottom of the wing is "high pressure"?
My question is 2 parted..**
1. Where does the water go?
2. Is this an area where say, maybe vortex generators might allow the boundry layer to reattatch, allowing for less drag?
Come on my fellow engineers!! Wadda ya think?
Glen

[rv8ch]

Is it possible the flaps are too low? Were the flaps dry all the way out to the aileron? The VG idea sounds good to me.

[keen9a]

I would guess that the rain is being blown upwards off the wing by air leaking through the flap/top skin intersection, and is just staying above the flap.

It may be getting sucked into the gap though. Keep in mind that the pressure delta toward the trailing edge of most airfoils becomes quite small, and on some even reverses.

As far as VGs go, I doubt that there is any separation in that area unless you were flying at a high AoA.

I'd love to see some video. Putting some tufts across the gap might be interesting to see too.

[tin man]

Tuft it. 50 cents of yarn and a little tape will answere all your questions. Tuft the top skin in front of the flap and a row or two on the flap. My guess is that you have a very slight upturn of the top skin just in front of the flap and its causing the rain to detach. BTW I have spent many hours tufting my RV3 wings, flaps, ailerons and wing tips. Very interesting at high aoa.
Tom
RV3 2000ish hours