Talk:NACA airfoil

reqdiagram
The German article has good diagrams which would just need the captions translated. --pfctdayelise (talk) 14:25, 2 August 2008 (UTC)

Equation for a symmetrical 4-digit NACA airfoil
The third coefficient, 0.3537, seems to be wrong. Shouldn't it be 0.3516? (Apologies if this is in the wrong place or whatever - wikipedia newbie) —Preceding unsigned comment added by 150.214.142.101 (talk) 10:18, 19 April 2010 (UTC)

Interestingly, the NACA 4-digit series as specified in the referenced source has a finite thickness at the trailing edge: the original equation here on Wikipedia was modified to create a zero-thickness TE. It is probably best to actually mention this in the article, I guess, though perhaps that is getting too technical. The fact of the matter is, the NACA 4-digit series was empirically derived from real airfoils, none of which actually have a zero-thickness trailing edge (obviously). However, most people these days are probably using these airfoil equations for computational studies, where it is best to close the airfoil. However, the choice made by the Wiki author to use the x^3 term to close it is arbitrary, I think. Many times you see the x^4 coefficient modified instead. To ensure a closed airfoil it is only necessary to make sure the coefficients sum to zero. 99.23.91.196 (talk) 04:09, 27 October 2010 (UTC)

It should be 0.3516, nothing else. How people solve the problem with the closing of the trailing edge is very personal choice, some people modify the equation, some people put an extra element behind the trailing edge. It is stated in the article that equation is not closed, and how to modify it to suite your needs if you want to close it. Standards are technical by definition, and the correct equation should be stated. — Preceding unsigned comment added by Sverek (talk • contribs) 03:31, 4 February 2011 (UTC)

Equation for 5-Digit camber line
Should not the conditions for the 5-digit camber line use m rather than p, since m is the chordwise location at which the leading and trailing curves intersect? Using the trailing (linear) curve for an x value between p and m will result in a slight discontinuity of the surface curvature.173.89.170.253 (talk) 13:20, 24 April 2011 (UTC)

a mess
Article contains a bunch of redundant figures. You only need *one* clarifying chord, camber etc. Also it'd be nice if the number of NACA profile for the red lined section was called out. — Preceding unsigned comment added by 66.166.4.118 (talk) 18:37, 6 November 2013 (UTC)

Formulas
The given formulas seem not to be correct, compared to these :. Is there a problem with them or is it just equivalent calculation ? Thanks ! --Badidzetai (talk) 15:09, 2 February 2014 (UTC)

Fixed NACA 5-Digit Description
So, the description of the NACA 5-digit foils was flat wrong and at odds with the cited reference (which was, correctly, the canonical source).

The definition of NACA 5-digit foils is due to NACA Report 537 (Jacobs and Pinkerton, 1935), where they extend the 4-digit scheme introduced in NACA Report 460 (Jacobs, Ward, and Pinkerton, 1933). From that, it is abundantly, absolutely, and 100% clear and unambiguous that the intended semantic for a 5-digit descriptor "LPSTT" is (assuming the usual normalization to chord length):


 * L: a single digit representing the theoretical optimum lift coefficient at ideal angle-of-attack $$C_{L_I}$$= 0.15*L (this is not the same at the lift coefficient, CL)
 * P: a single digit representing the x-coordinate of the point of maximum camber at 0.05*P
 * S: a single digit that says whether the camber is simple (S=0) or reflex (S=1)
 * TT: the usual two-digit maximum camber in percent of chord

Accordingly, the given example “77887” is nonsensical. You simply cannot have a an “8” in the middle position: it _must_ be either “0” or “1”. An 87%-of-chord thickness is also highly dubious: _none_ of the canonical NACA profiles (10 5-digit and 87 4-digit) go beyond 25%.

The intention in the mid 1930's was to provide an ID number with embedded semantics, not provide a specification that would be valid for all possible values of all numbers between 00000 and 99999. The 10 canonical NACA 5-digit foils match the regex: '^2[1-5][01]12$'. Extending it for other thicknesses and lifts is OK within aerodynamic constraints, but altering the semantics is not, particularly when the cited reference gives the correct semantics (of course, since it is the canonical definition).

My edit is better, but still not 100% correct, since it admits confusion between the usual lift coefficient and the optimum ideal lift coefficient, which are different.

--EmmetCaulfield (talk) 02:43, 17 May 2014 (UTC)


 * Very insightful! So is it actually so that the various "NACA airfoil generator" software that the article links to, or one can find on the net (but all of which seem to be ancient, software-wise, and hard to run in a modern environment) are just silly, if NACA defined a fixed set of airfoils and did not intend that one could go the reverse direction, pick some other set of numbers and come up with an airfoil? TorLillqvist (talk) 12:45, 11 November 2023 (UTC)
 * EmmetCaulfield is correct. The important aspect of this work, as presented by Abbott and Von Doenhoff, is the data relating the lift, drag and moment coefficients to a particular angle of attack. This data was originally measured in comprehensive wind tunnel tests using scale models of the airfoil section at particular values of Reynolds number. Data of this kind must be determined empirically - if it is to be credible and reliable it cannot be computer generated. Dolphin ( t ) 20:21, 12 November 2023 (UTC)

Going back to the sources is better
The original reference for symmetrical airfoils must be: "Tests of six symmetrical airfoils in the variable density wind tunnel" NACA Technical Note No. 385 of Eastman N.Jacobs. Let explain myself: the correct definition of the distribution thickness for 4 digit and 5 digit series is very explicit in the article of Eastman N.Jacobs of 1931. This article is the very source of the NACA airfoil investigation (it is chronologically the first). Then, why to cite inexact more recent works that follows the questionable tradition initiated by Ira H.Abbott of writing numbers which are (truncated) numerical solutions of well defined equations?

Thickness distribution is defined by finding the coefficients of the sesqui-quartic polynomial (a0*sqrt(x)+a1*x+a2*x^2+a3*x^3+a4*x^4) subject to the following conditions: a) maximum ordinate of 0.1 at x=0.3; b) thickness of trailing edge of 0.002; c) trailing edge angle defined by slope dy/dx=-0.234; d) nose shape defined by ordinate of 0.078 at x=0.1

Then, numbers written in several references for that coefficients can be missleading because: a) they are all truncated; b) some authors choose to write things very different than the real solutions for different reasons (that I will not discuss here). What I want to remark is that, going to the real source of knowledge leaves clear that NACA airfoils have not 'closed' trailing edge. In fact, only airfoils derived from potential theories have closed trailing edges (some examples: Joukowski airfoil, Karman - Trafftz airfoils, Eppler inverse derived airfoils). The mayority of 'real' airfoils have a thick trailing edge. The myth that closed trailing edges are 'better' for computational calculations must be debunked and not replicated in wikipedia. This myth was initiated in the first days of computational fluid dynamics when solving with very simple panel methods (which needed the closed trailing edge to satisfy the Kutta condition). But I have found that, when solving for real, viscous, fluids, closed trailing edges are not 'better' in any sense because they create a first order singularity at this point, and that can cause problems for some Navier - Stokes solvers. It is my personal experience that thick trailing edges behave better in viscous flow simulations.

If you can take into account some of these comments, I believe that the quality of the article will be enhanced. Thank you Crodrigue1 (talk) 18:02, 29 May 2018 (UTC)

NACA BULBS
It would be useful for readers if you could add a section on calculation for a NACA bulb. With formulas for such calculation's like for volume, wetted surface areas, centre of Mass, length and diameter, etc.

The Bulb being a full rotation of the section.

This has applicability in a number of areas particularly in Marine and Air applications.

180.181.83.220 (talk) 04:56, 26 April 2019 (UTC)

7-series and letters for standard profiles
The 7-series (and therefore, presumably also the 8-series) use a letter, referring to a standard profile. The example uses "A". Nowhere in this article is there any indication of how to look up what the standard profile A would refer to; the only use of profile seems to be the shape defined by the numbers, so that standard profiles would have at least 4 digits, rather than character names.

JimJJewett (talk) 20:33, 1 January 2021 (UTC)