Hughes H-4 Hercules utilizes the following NACA airfoils
NACA 63(420)-321 and NACA 65(420)-415

What NACA series airfoil are these and what do the numbers in parentheses & dash number identify/mean?
Examples: NACA 4 digit
NACA 6 digit


Thanks in advance,
Dave

These seem to be NACA 6-series modified in the third digit (related to low drag).

Here you have two explanations, Dave:

http://en.wikipedia.org/wiki/NACA_airfoil

http://www.desktop.aero/appliedaero/...lgeometry.html

What would be a close 6 digit NACA no. ?

Thanks in advance
Dave

Seems like we had this sort of thread show up about one of your previous projects. At the time I seem to remember that it was suggested that an airfoil that is more friendly to the airflow for model size Reynolds numbers would be a wise choice. If you build fairly light you can't go wrong with a ClarkY and if you tend to build heavier then the Selig 8036 woudl be a wise option.

I know that these are not scale but the airflow around 6 series airfoils likely doesn't scale well down to our sizes.

From how I'm reading the numbers according to that Wiki page;

NACA 63(420)-321- 6 series with minimum pressure at 30% (more or less meaningless to us modelers) with the optimum lift coefficient at 0.4 and 20% thick. That tapers (as suggested by the brackets and final numbers) to an optimum lift coefficient of 0.3 and 21% thick.

NACA 65(420)-415 - 6 series with minimum pressure at 50% and design lift coefficient of 0.4 and 20 and 15% thick for the two ends of the panel.


For example look at the gif images of airfoils at this site.

http://www.ae.illinois.edu/m-selig/a..._database.html

For the NACA 63(4)-421 airfoil I'm guessing that the (4) takes the place of the lift coefficient range subscript number and this is a close approximation to the 63(420)-321 airfoil. Either way 21% thick is far too thick to work at our speeds.

If I'm right about these then the thickness %'s other than the 15% thick option are not going to work well at model size and speed Reynolds numbers. Again the suggestion would be to go for the Selig 8036 or 8037 airfoil which is designed to work well at model speeds and have a nicely controllable stall point. At 16% thick these are still rather thick for our range of flying speeds and wing chords but they make up for it by having been carefully designed to achieve the optimum airflow around the shape. This airfoils are a hit on heavily loaded warbird scale models so you know that they handle higher wing loadings more easily.