NASA GAW Foil Section
With regards to the GAW foil sections, which I have come across in various articles.
What is their history vis-vis the following sections
LS-0013
LS-0413 GA(W)-2
LS-0413MOD
LS-0417 GA(W)-1
In published articles (by Vacanti) the GAW section is referred to, which one of the above sections was converted to a symmetrical section for use on yachts and why.
There seems to be no published data (pressure and l/d) for the symmetrical GAW section, how does this section compare to the NACA 00, 63, 64, 65 series of sections. What characteristics (good or bad) has the GAW got over the standard NACA sections and why would you choose to use this in lieu of any of the NACA series.
In addition to use on keels would the GAW section be suitable for use on rudders.
Can the GAW be scaled up or down in thickness and by how much if any.
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Dey
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Old 06-22-2007, 07:42 AM
tspeer tspeer is offline
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Quote:
Originally Posted by Dey View Post
With regards to the GAW foil sections, which I have come across in various articles.
What is their history vis-vis the following sections
LS-0013
LS-0413 GA(W)-2
LS-0413MOD
LS-0417 GA(W)-1
From NASA’s history pages,
“The Low-Speed Airfoil Program was initiated in 1972 with the development of the GA-1 airfoil, which was analytically developed by Whitcomb with the previously mentioned computer code developed at Lockheed-Georgia under Langley contract. This 17-percent-thick low-speed airfoil exhibited low cruise drag; high climb lift-drag ratios; high maximum lift; and predictable, docile, stall behavior. National interest in this new airfoil rapidly accelerated as the data were disseminated in a NASA report. In fact, a rare second printing of the technical report was required because of the unanticipated demand. An entire series of airfoils with varying thickness ratios was subsequently developed for low-speed applications by Whitcomb’s team, including a new GA-2 airfoil. The GA-2 section employed a 13-percent-thick profile and generated considerable interest within the general aviation community. This low-speed family of airfoils also included 9-percent- and 21-percent-thick airfoils that were designed for fully turbulent boundary layers (negligible laminar flow) and Mach numbers below about 0.50. Langley conducted both wind-tunnel and flight research to support the development and application of these airfoils. Wind-tunnel tests were conducted to develop trailing-edge flaps and control surfaces, and flight tests were conducted to evaluate performance, stall characteristics, and handling characteristics.”
…
“With the expansion of the airfoil family from the low-speed to the medium-speed airfoils, a new airfoil designation system was put into effect by Langley in 1977. The airfoil designations were changed to the form LS-xxxx for the low-speed series. LS indicated the first series of low-speed airfoils, the next two digits designated the airfoil design lift coefficient in tenths, and the final two digits gave the airfoil thickness in percent chord. Thus, the GA-1 airfoil became LS-0417 and the GA-2 airfoil became LS-0413. A similar designation system was developed for the medium-speed airfoils of the form MS-xxxx.”
Quote:
…There seems to be no published data (pressure and l/d) for the symmetrical GAW section, how does this section compare to the NACA 00, 63, 64, 65 series of sections.
That’s because the GAW/LS sections were not designed the same way as the NACA 6-series sections. The 6-series basically bridged the older approach that was based on empirical thickness distributions and camber lines, and the modern approach of defining the pressure distribution based on boundary layer considerations and using an inverse method to find the shape that provides the specified aerodynamics. The NACA designed the 6-series sections using the method of Theodorsen and Garrick, and used thin airfoil theory to design the camber lines. Calculating the thickness distributions was a heroic effort using the machines like the Frieden mechanical calculators that were available during WWII.
The GAW/LS sections were designed using inverse methods that could design and analyze the whole airfoil at once instead of building it up from a thickness distribution plus camber line. So there was no need to test the symmetrical thickness distributions separately. This airfoil was what it was.
Of course, you can always find the camber line for any shape and flatten it out, which is what Vacanti has done. Since that wasn’t done by NASA, there are no NASA data for the symmetrical sections.
Quote:
What characteristics (good or bad) has the GAW got over the standard NACA sections and why would you choose to use this in lieu of any of the NACA series.
You can best answer this question yourself using XFOIL. XFOIL has a more modern boundary layer method than the Eppler code or previous airfoil programs, and it is well suited to low speed applications like keels because it’s one of the few that have the capability of calculating the effect of laminar separation bubbles.
With XFOIL it’s a simple matter to read in the coordinates for a section from a text file and calculate the drag polars. You can also overplot the drag polars from multiple sections for comparison.
Quote:
In addition to use on keels would the GAW section be suitable for use on rudders.
You need to define the requirements for your particular application. Only you can decide, for example, how much you are willing to trade minimum drag for a wider drag bucket. Or if you will be operating in the drag bucket at all, especially when going upwind.
Quote:
Can the GAW be scaled up or down in thickness and by how much if any.
Another good question for XFOIL.
Perhaps a better approach than flattening out the GAW section is to simply reflect the upper surface about the chord line. I think you’ll find this does a better job of capturing the nature of the upper surface behavior in the symmetrical section.
The best approach is to use the GAW section as a starting point in XFOIL, then use the MDES or QDES methods to modify it. MDES is an inverse method that reshapes the whole section, and you can constrain it to only produce a symmetrical section. QDES is used if you want to only modify a specific portion of the shape and leave the rest alone. You alternate between OPER mode to analyze a candidate section, GDES to scale the thickness up and down, and MDES to define a modified shape.
There’s also a graphical front-end to XFOIL called Profili that many people find easier to use than XFOIL’s command-line interface.
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Tom Speer
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