How to Compare Airfoil Shapes and Lift/Drag Polars for Multiple Airfoils

Use Airfoil Tools' Airfoil comparison application to plot two or more airfoils on the same plan for shape comparison, and to compare lift and drag polar diagrams across a range of Reynolds numbers. The plan can be downloaded or printed full size for better resolution. This works when the airfoils you want to compare are already available in the site's database (1,638 airfoils listed) or have been added to "My airfoils" for the current browser session.

Loading airfoils for comparison

The site tracks how many airfoils you have loaded — the homepage shows a counter such as "You have 0 airfoils loaded." Before comparing, you need to bring the airfoils you care about into that loaded set.

• From the database: Use Airfoil search to find candidates, filtering by thickness and camber, with preview images of the sections. From a result you can open the airfoil details page, which includes polar diagrams for a range of Reynolds numbers.
• From your own data: Use My airfoils to paste DAT file data into a form. The airfoil then appears in the drop-down menus used by the tools. Note that these custom airfoils are only stored in your browser session and must be re-added when the browser is reopened.
• Generated sections: The NACA 4 digit airfoil generator (and a 5 digit generator) can produce sections to compare against database airfoils.

Comparing shapes on one plan

The Airfoil comparison tool plots two or more airfoils on the same plan. This is the core of shape comparison: differences in camber line, thickness distribution, and leading-edge radius become visible when the sections are overlaid at the same chord.

To keep the comparison meaningful, hold the reference conditions constant:

• Chord width — plot all sections at the same chord so thickness and camber differences are not confused with scale.
• Camber and thickness adjustments — the plotter allows these to be adjusted; if you modify one airfoil, apply the same treatment to the others or note the change.
• Pitch — the plotter can set pitch to account for wing angle of attack, wash out, or wind turbine blade angle. Use the same pitch setting across airfoils unless pitch itself is the variable you are studying.

Exporting and printing

The plan can be downloaded or printed full size for better resolution. The plotter produces an SVG (Scalar Vector Graphics) plan that can be printed full size or across multiple pages for large sections — useful when you want to check the overlay at full scale rather than on screen.

Comparing lift and drag polars

Shape similarity does not guarantee similar performance. The comparison tool also lets you compare lift and drag polar diagrams for a range of Reynolds numbers, so you can judge candidates on aerodynamic behavior rather than outline alone.

Practical points:

• The site's default Reynolds number range is shown as 50,000 to 1,000,000, and this can be changed via the "( set )" control. Set the range to match your application before reading the polars.
• Use the Reynolds number calc application if you need to work out the value for your chord and speed.
• Compare polars at the same Reynolds number across airfoils; a polar at Re = 100,000 is not comparable to one at Re = 1,000,000.

Reading the comparison against your design goal

Combine the two views — shape overlay and polars — with the numeric parameters:

If two airfoils look nearly identical on the plan but diverge in the polars, the polar comparison is the deciding evidence. If they perform similarly in the polars but differ in thickness, the thicker section may offer more structural depth for the same aerodynamic result.

Common snags

• Custom airfoils disappear. Airfoils added via "My airfoils" live only in the browser session; reopening the browser clears them. Re-paste the DAT data before starting a new comparison.
• Mismatched Reynolds numbers. Polars are only comparable at the same Re — check the range setting before drawing conclusions.
• Scale confusion on the plan. If airfoils are plotted at different chords, thickness differences are exaggerated or hidden. Match chord first.
• Screen resolution limits. For fine differences in leading-edge shape, download or print the plan full size rather than judging from the on-screen plot.