Another Rudder Question (NACA)

I normally use 63a2, which has no hollow.

 

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I wouldn't mind if anyone could discuss the performance differences some more.

 

perhaps it's 63 2A 00xx?

It's been a while since i looked at the original data - i just use my own notes now

 

I guess the concave tail dampens vortex turbulence. The downside is that the bucket of the 63 series (concave) looks more U-shaped, whereas for the 63A's (convex), the bucket is squarer, the bottom is flatter. That means that 63012 gets very low drag near zero degrees, but more drag around a more typical operating angle of 2-3 degrees. 63012A, on the other hand, may be worse at zero degrees but better at 2-3. I suppose you could compromise and just make it straight.

Edit: Actually, 63012A looks straight as a ruler for the last 25% of the chord. So far, I like that one.

 

The concave contour is a function of how steep the pressure recovery region is. If you look at the pressure distributions in the figures above, you'll see there's a flat "rooftop" region extending from just behind the leading edge to part way back on the chord. This is specified by the second digit in the 6-series airfoil designations. For example, the 63-012 has its rooftop extend to 30% of the chord.

After the rooftop comes the pressure recovery region, where the pressure increases from the rooftop value to near the free-stream value. The NACA 6-series sections all used a straight line for the pressure distribution in this area, at the design angle of attack. It happens that the shape that matches this linear increase in pressure is slightly concave.

Other sections have a much more abrupt pressure increase at the end of the rooftop, where the boundary layer is still fresh and energetic, and then flatten out the pressure increase toward the trailing edge, where the boundary layer is tired and can't be stressed much more. This gives an even more concave contour. See Optimization of Airfoils for Maximum Lift.


If the pressure recovery is convex, you will also end up with a convex contour in the surface. The NACA 16- series sections are a good example. (http://www.piv.de/papers/ceccioasme2001.pdf)

 

Eppler says in "Airfoil Design and Data" that he designed his 836/7/8 series of keel/rudder foils to minimize cavitation as well as optimize the boundary layer. I've never noticed any pitting on the blades of my Laser, and the 83X's look very different from NACA00XX. It's hard to imagine both of those shapes being ideal solutions to the same problem. Eppler's sections have a somewhat rapid pressure drop at about 2/3 of the chord, and a very slight concavity in the shape of the tail.

 

Me too!

 

I have moulds to produce a 63A012 in two sizes but they are just 100mm and 140mm chord length parallel sections.

www.fastacraft.com

 

Eppler also says something about the flat rooftop, that it only applies to one angle of attack, a sharp suction peak develops at a slightly higher angle. Instead, he designs the section so that if you look at a whole range of angles, the low-pressure peak never rises above some max value that is as small as possible. I can see problems with that. Chances are the higher angles will occur at lower speeds, so the peak might not be too bad. But I do like the idea of optimizing performance over a range of conditions.