Is anyone an aerodynamicist?

[air-cool-me]

Its a long shot but I am taking an aerodynamics class right now and need some help!


My class took reading off of a 24”x24” wind tunnel with a 2D 6”chord/24”span NACA 4412 airfoil.

relatively low Reynolds number (300,000)

When we plotted the CL alpha curve it came out relatively flat on top when compared to the publish NACA chart.

This result is the same as last semesters class so i know i didn't screw up. The proff wants us to write a little report on why our results are different from the published NACA charts






Also the airfoil recovers lift after 22 degrees but i believe that is a result of the small size of the the wind tunnel and interference with the top and bottom.

my Questions are:

Why could my graph be reaching its peak CL at a lower Angle of Attack then the NACA chart? Why will it linger above 1.2 CL for so long before the big stall?

What could be the reason my graph is so flat over the top 9 degrees?

I don't think i would see the recovery in lift pas 22 degrees with a 3d airfoil.. but why? Any idea how\why span-wize flow would affect this phenomenon occurring in our wind tunnel?

[air-cool-me]

ooo found a Plot in my book on scale effect...

that knocks one question down...

[kach22i]

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[onewhippedpuppy]

Just a guess, but I would think that a wing with a 6" chord in a 24" tunnel would start getting interference from the wall at much of an AOA. Like I said, just a guess, I'm in an Aero Theory class right now and hate it.

[oldE]

Boundry layer effect along the tunnel walls affecting the loading of the airfoil at its ends? I also suspect Matt is on to something.
I am not an aerodynamicist, nor do I play one on TV. I just like tech-stuff.
Les

[MotoSook]

Looks like my old fluid flow lab....I'll sell you my old reports.. If I could find them.

...it's been too long..probably can't help without doing some refreshing of my old grey matter.

[MotoSook]

Nick is on with the Reynolds number chart. Your RN in the tunnel is likely different from the NACA chart RN (hard to see all the text in the picture you posted for the NACA chart).

Also, the data on the NACA chart cuts off well before your chart does on the x-axis. Your recovery may be due to flow impinging on the bottom of the wing...and not necessarily "lift" generated by flow around the wing.

In the tunnel, you can continue to increase the angle and continue to measure data....not so in the real world. Your wing in the tunnel also doesn't articulate like a free floating wing, so you can continue to rotate it until your telemetry starts to see enough wind force to push up on your rig. In the real world, that wing would stall and the aircraft would start to do things your "fixed" wing in the tunnel can't do.

Edit: just to clarify...with the rig in the wind tunnel, you will always have a vertical and horizontal force. before stall, you are generating lift and F-v is greater than F-h. After stall and as you continue to log data, the horizontal force is greater than your vertical force....but you are not logging F-h...just F-v, so your data is showing lift. Ever heard the term that one can pilot a brick if you have enough power? (or something like that....someone needs to find and post the picture of that jet fighter that landed with just one wing intact)

[island911]

Originally posted by air-cool-me


ooo found a Plot in my book on scale effect...

that knocks one question down...

There ya go...

If I were your prof, I'd like to see the results graph super-imposed on the expected NACA . . then on the expected with scale effects . ..try to corollate; THEN give speculation about other possible effects (tunel? ...end condition of foil--that is, it's not an infinite lenght... relative surface roughness. .. )

[onewhippedpuppy]

Island makes another good point, a finite wing will always perform worse than a theoretical infinite foil, and never line up with a NACA plot.

[air-cool-me]

thanks guys!

i Incorporated some of your ideas into my report. i will let you know how i do!

thanks again

[BlueSideUp]

I was using that same wind tunnel about ten years ago for my aero class! It's been a long time but I'll look around and see if I saved that report.