Modern symmetrical airfoil shapes

[Speng]

Hello,
In looking the typical yacht design literature such as Larsson and Eliasson's "Priciples of Yacht Design" or Fabio Fossati's "Aero-Hydrodynamics and the Performance of Sailing Yachts" they tend to show a lot of information about NACA 4 digit (eg 0012) or 65 or 63 series airfoils. Considering these profiles were all designed before the 1940s what newer symmetrical airfoil profiles are available to chose from today? For an asymmetrical foil I would not expect to use a profile from these generations so I'm sure there's been some development in symmetrical foils since then as well.

[ancient kayaker]

I went through the same process in this thread Sailing Kayak Project

I came to the conclusion that the more modern profiles offer only marginal improvement. A bit better lift/drag ratio tended to be achieved at the cost of less tolerant stall characteristics.

[LarryMcI]

NACA foils are still the best. Sure, there's some fancy shapes for supersonic aircraft and bizarre application-specific shapes but the NACA family of profiles are tried and proven.

[Speng]

I was able to find a NASA report comparing the NACA 0012 with a NASA LS(1)-0013 which showed the newer design was a little better and the extra thickness was good for what I wanted to do structurally. Also the stall characteristic is smoother as well.

If I wanted an assymmetric foil I would definitely have gone for one from the LS(1) or NLF series. These newer NASA foils are much better than the older NACA (assymmetrical) foils and often at fairly high thicknesses which is good for when you want to stuff them with structure.

BTW if you want to see NASA and NACA reports go to http://ntrs.nasa.gov/search.jsp ridiculous numbers of papers on just about anything for free. Your tax $$ at work.

[tspeer]

Originally Posted by Speng

Hello,
In looking the typical yacht design literature such as Larsson and Eliasson's "Priciples of Yacht Design" or Fabio Fossati's "Aero-Hydrodynamics and the Performance of Sailing Yachts" they tend to show a lot of information about NACA 4 digit (eg 0012) or 65 or 63 series airfoils. Considering these profiles were all designed before the 1940s what newer symmetrical airfoil profiles are available to chose from today? For an asymmetrical foil I would not expect to use a profile from these generations so I'm sure there's been some development in symmetrical foils since then as well.

The NACA 4-digit thickness distributions were derived from sections that had proven to be good performers at low- to mid- Reynolds numbers. Their front-loaded pressure distributions didn't promote a lot of laminar flow, but the long gentle adverse pressure gradients promoted short laminar separation bubbles that were quite robust when laminar flow was present. These characteristics have made them good all-round sections.

There have been a number of more modern sections designed. Wortmann designed some sections for sailplane rudders that might be suitable. Wortmann, F. X, "Stuttgarter Profilkatalog I", Friedr. Vieweb & Sohn, 1981.
A couple of sections that might be of interest are FX L V-152 K25 and FX 71 L-150/20 .

Eppler designed sections specifically for use as yacht keels, the E836, E837, E838. See his "Airfoil Design and Data", published by Springer-Verlag. Coordinates are available from the UIUC airfoil database.

More recently, Paul Bogataj has designed some keel shapes for classes that specify thin keels.

Two problems with improving on the NACA 4-digit sections are the fact that (for whatever reason) transition occurs earlier at the same Reynolds number in water than in air, and many classes specify fairly thin sections. The earlier transition means that the big gains from laminar flow that are possible from modern airfoil shapes aren't going to be realized in hydrofoil sections. The thinner a section gets, the more it acts like a flat plate. A symmetrical section only has thickness to use to determine its characteristics, limiting what can be done, and any drag bucket is necessarily centered about zero lift. Most often the section is operating at lift coefficients that are way outside of the drag bucket.

Today there are several inexpensive or free airfoil design programs that can be used to create a section tailored to your specific requirements. In the linear lift range, the predictions from these programs are probably about as accurate as what you'd get from the wind tunnel, allowing you to go directly from the computer to full scale with a reasonable amount of risk.

[ancient kayaker]

Tom, I believe many of the modern sections are more sensitive to minor profile errors than the oldies. When I was building model aircraft, a very long time ago, I quickly realized that the classics were very tolerant in this regard. That is the main reason for my use of sections like Clark-Y and NACA-xxxx.

This may have been because most of them dated back to the days when full-sized aircraft were still fabric-covered like my models. With fabric covering the profile in practice is merely an approximation to the intended section and highly variable along the span. Much the same can be said about my wooden daggerboards and Bruce foils.

[ancient kayaker]

The Bogataj link was thought-provoking, for such an apparently self-evident topic. It made me re-think some assumptions.

In the case of an aircraft the weight and therefore the lift is relatively constant and the range of speed no more than an order of magnitude.

The case of a boat keel (or sail for that matter) is quite different. Speed range starts at zero and levels off at hull speed, planing speed or who knows what if we are speaking of foilers. Lift also varies wildly. The keel of a boat starting in a goodly breeze (or going about hard) approximates to a plane trying to loop from standstill! In a light breeze the keel needs to perform like a glider wing but in a decent wind the same keel needs to perform like a mid-air refueling tanker.

Lift is also (generally) one-way for a wing, not so for a keel. There has been more emphasis, I suspect, on asymmetrical section development than for symmetrical airfoils.

Which prompts me to ask if airfoil sections are really the place to go to? I know that the same theory applies, but the requirements are very different.

[tspeer]

Originally Posted by ancient kayaker

...Which prompts me to ask if airfoil sections are really the place to go to? I know that the same theory applies, but the requirements are very different.

Exactly. Which is why one should define the requirements and then design a section suited to those requirements.

If one doesn't have specific requirements, I think the NACA 4-digit sections are going to be hard to beat for most keels and rudders of modest thickness.

[ancient kayaker]

If I may change the subject slightly:

A sail would intuitively take a shape that kept it at constant front-to-back tension, which should generate an airfoil section that would depend on the sail's (side) profile. It seems a reasonable assumption anyway, if it is also reasonable to ignore the effect of vertical and diagonal tension. This would allow the section and profile to be related mathematically with some assumptions about differential air pressure across the thickness of the sail, which in turn could perhaps be constant to a first approximation.

I haven't tried it yet as a model but it sounds like it would be fairly easy to do.

A further assumption would be to have the angle between the sails surface and the wind direction constant with height. This would generate a degree of twist that could be adjusted with fore-and-aft tension in the sail, especially in a leg of mutton design with an elliptical boom (sprit). it seems like a mathematically elegant way to approach sail design and optimization.

Does any of this have anything to do with (theoretical) sail design? How far back did I leave reality far behind in the mist? Just wondering if there is any point in pursuing the chain of thought further.

[tspeer]

Originally Posted by ancient kayaker

If I may change the subject slightly:

A sail would intuitively take a shape that kept it at constant front-to-back tension, which should generate an airfoil section that would depend on the sail's (side) profile. It seems a reasonable assumption anyway,

Yes, this is true.

if it is also reasonable to ignore the effect of vertical and diagonal tension.

Now you're departing from real sails. A number of investigators have made this assumption to look at 2D aerodynamics, but for a real sail, the spanwise tension is important.

This would allow the section and profile to be related mathematically with some assumptions about differential air pressure across the thickness of the sail, which in turn could perhaps be constant to a first approximation.

I haven't tried it yet as a model but it sounds like it would be fairly easy to do.

Easy if you're Peter Jackson, perhaps!

A further assumption would be to have the angle between the sails surface and the wind direction constant with height. This would generate a degree of twist that could be adjusted with fore-and-aft tension in the sail, especially in a leg of mutton design with an elliptical boom (sprit). it seems like a mathematically elegant way to approach sail design and optimization.

This is basically the idea behind the Princeton Sailwing. However, they used the spanwise tension in a cable running up the trailing edge to control the shape. The leech had a hollow cut (negative roach), so that it flattened out when tensioned and created chordwise tension in the sail.

Does any of this have anything to do with (theoretical) sail design? How far back did I leave reality far behind in the mist? Just wondering if there is any point in pursuing the chain of thought further.

If you really want to know, you can start with these:

Thwaites, B., “The Aerodynamic Theory of Sails,” Proceedings of the Royal Society of London, Vol. 261, No. 1306, 1961, pp. 402-422.

Nielsen, J., “Theory of Flexible Aerodynamic Surfaces,” Journal of Applied Mechanics, Vol. 30, No. 3, 1963, pp. 435-442.

Haselgrove, M., Tuck, E., “Stability Properties of the Two-Dimensional Sail Model,” Society of Naval Architects and Marine Engineers New England Sailing Yacht Symposium, New London, CN, January 24, 1976.

Murai, H., Murayama, S., “Theoretical Investigation of Sailwing Airfoils Taking Account of Elasticities,” Journal of Aircraft, Vol. 19, No. 5, 1982, pp.385-389.

Jackson, P., “A Simple Model for Elastic Two-Dimensional Sails,” AIAA Journal, Vol. 21, No. 1, 1983, pp.153-155.

Sneyd, A., “Aerodynamic Coefficients and Longitudinal Stability of Sail Aerofoils,” Journal of Fluid Mechanics, Vol. 149, No. 7, 1984, pp.127-146.

Cyr, S., Newman, B., “Flow Past Two-Dimensional Membrane Aerofoils with Rear Separation,” Journal of Wind Engineering and Industrial Aerodynamics, Vol. 63, No. 1, 1996, pp. 1-16.

Smith, R., Shyy, W., “Computational Model of Flexible Membrane Wings in Steady Laminar Flow,” AIAA Journal, Vol. 33, No. 10, 1995, pp. 1769-1777.

Newman, B., Low H., “Two-Dimensional Impervious Sails: Experimental Results Compared with Theory,” Journal of Fluid Mechanics, Vol. 144, 1984, pp. 445-462. 172

Smith, R., Shyy, W., “Computation of Aerodynamic Coefficients for a Flexible Membrane Airfoil in Turbulent Flow: A Comparison with Classical Theory,” Physics of Fluids, Vol. 8, No. 12, 1996, pp. 3346-3353.

Lorillu, O., Weber, R., Hureau, J., “Numerical and Experimental Analysis of Two-Dimensional Separated Flows over a Flexible Sail,” Journal of Fluid Mechanics, Vol. 466, 2002, pp. 319-341.

Jackson, P., Christie, G., “Numerical Analysis of Three-Dimensional Elastic Membrane Wings,” AIAA Journal, Vol. 25, No. 5, 1987, pp. 676-682.

Boudreault, R., “3-D Program Predicting the Flexible Membrane Wings Aerodynamic Properties,” Journal of Wind Engineering and Industrial Aerodynamics, Vol. 19, No. 1, 1985, pp. 277-283.

Holla, V., Rao, K., Arokkiaswamy, A., Asthana, C., “Aerodynamic Characteristics of Pretensioned Elastic Membrane Rectangular Sailwings,” Computer Methods in Applied Mechanics and Engineering, Vol. 44, No. 1, 1984, pp. 1-16.

Sugimoto, T., “Analysis of Circular Elastic Membrane Wings,” Transactions of the Japanese Society of Aerodynamics and Space Sciences, Vol. 34, No. 105, 1991, pp. 154-166.

Charvet, T., Hauville, F., Huberson, S., “Numerical Simulation of the Flow Over Sails in Real Sailing Conditions,” Journal of Wind Engineering and Industrial Aerodynamics, Vol. 63, No. 1, 1996, pp. 111-129.

[Paul B]

Originally Posted by tspeer

If one doesn't have specific requirements, I think the NACA 4-digit sections are going to be hard to beat for most keels and rudders of modest thickness.

If you mean the NACA 00xx sections most would agree with your assessment regarding rudders, but there are much better NACA choices for fin keels.

[ken61137]

I suggest you look for Dr. Hugh Elkaim on the web.
The YRA did a short article on his computer developed findings using a reynolds number suitable for low wind speeds as compared to high wind speeds for nasa foils. Since then nasa have published a similar form to his for low wind speeds. i.e. less than 100mph at around 10mph