Trailing Edge Extension

Question

Trailing Edge Extension

Sparweb (Aerospace)

(OP)

25 Aug 04 12:57

I'm comparing two 21" chord airfoils, one is a simple NACA 0012, and the other is a NACA 0012 with a 2-1/2" inch strip on the tailing edge. Literally, it's a strip of flat sheet tacked onto the TE that runs the whole length of the airfoil. It is not bent, therefore does not change the camber of the airfoil. I'm sure it's not there as a trim tab.
Should I treat the airfoils the same?
Should I treat the second airfoil as having 23.5" chord?
Where is the aerodynamic center?

Steven Fahey, CET
"Simplicate, and add more lightness" - Bill Stout

btrueblood (Mechanical) · Answer 1 · 2004-08-25 16:52:22

I think the answers are: no, yes, at the 1/4 chord position. The airfoils are not the same, the foil with a trailing edge extension has a longer chord, and thus (all else being equal) also has a finer "slenderness ratio" or t/c profile.

My $.02 worth.

Ben T.

WhiteRabbit (Aerospace) · Answer 2 · 2004-09-12 21:58:59

The airfoils you described are symmetrical airfoils providing symmetrical pressure distribution across the length of the chord at zero angle of attack.  However, as Ben T. stated "yes, no." there are complications.  The extension may cause the boundary layer to separate depending on how long the extension is and other various flight characteristics.

If the airfoil is not at a zero angle of attack the streamlines may not have the pressure distribution to "reattach" to the upper and lower streamlines resulting in a turbulent region resulting in induced pressure drag at the trailing edge.

I would not extend the chord length for the calculations for a TE extension.  The pressure distribution will be totally different across the chord line.  The net pressure distribution area differential will change also.

What exactly are you trying to do?  It may help with the theoretical analysis of the situation.

Sparweb (Aerospace) · Answer 3 · 2004-09-13 14:44:32

I strongly expect the kind of detached flow you describe, too, WR, considering the manner in which these extensions were attached.  Two straps 0.020" thick are glued on top and bottom of the TE, with about 3/4" overlap, and they add 2-3/8" to the chord.  There is a "step" for the air to travel over which has been blended, but not completely eliminated.

This kind of TE extension was done on the main rotor blades of the Bell 212 helicopter when it was upgraded from the 205A-1 design.

I realized after posting the question that I'm looking at things from a planform standpoint, and I don't need a pressure distribution, therefore the measured chord must be used.

Bell also changed the airfoil section of the tip of the blade, making it thinner and giving it some camber, so the radial distribution of aerodynamic properties is quite complicated.  Considering the speed of the tips is at high Mach numbers when flying forward, it's hard to see the "big picture", aerodynamically speaking.

Thanks.

Steven Fahey, CET
"Simplicate, and add more lightness" - Bill Stout

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Trailing Edge Extension