Need help with 4 digit airfoil

We were given an outlined code in our engineering class for a 4 digit airfoil. I have input all equations and the program runs, however it doesn't seem right. Any help would be appreciated.

clear all
close all
c=1; %chord length
s=num2str(2412);
NACA=s; %4 digits
d1=str2double(s(1)); % pulls the first digit out of the scalar
d2=str2double(s(2));% pulls the second digit out of the scalar
d34=str2double(s(3:4)); % pulls the third and fourth digit out of the scalar
m=d1/100;
p=d2/10;
t=d34/100;
x=0:0.1:c;
yt =5*t*c*(.2969*(sqrt(x/c))+-.1260*(x/c)+-.3516*(x/c).^2+.2843*(x/c).^3+-.1015*(x/c).^4); 

for k = 1:length(x)

     if x(k) <= p*c
         yc(k)=m*(x(k)/p^2)*(2*p-(x(k)/c));
         dx(k)=(2*m)/p^2*(p-(x(k)/c)); 
     elseif x(k) > p*c
         yc(k)=m*((c-x(k))/(1-p)^2)*(1+(x(k)/c)-(2*p));
         dx(k)=((2*m)/(1-p)^2)*(p-(x(k)/c));
     end
  end
  %upper and lower limits of the airfoil (xu,yu) ; (xl,yl)
  theta=atan(dx(k));
  xu=x(k)-yt(k)*sin(theta);
  yu=yc(k)+yt(k)*cos(theta);
  xl=x(k)+yt(k)*sin(theta);
  yl=yc(k)-yt(k)*cos(theta);
  %plot of airfoil
  plot(xu,yu)
  hold on 
  plot(xl,yl,'r')
  plot(x,yc,'g')
  axis equal

grid on

Answer 1

[Star Strider]

You have an end in the wrong place, and you need to subscript some of your variables.

Correct those, and then it works:

c=1; %chord length
s=num2str(2412);
NACA=s; %4 digits
d1=str2double(s(1)); % pulls the first digit out of the scalar
d2=str2double(s(2));% pulls the second digit out of the scalar
d34=str2double(s(3:4)); % pulls the third and fourth digit out of the scalar
m=d1/100;
p=d2/10;
t=d34/100;
x=linspace(0, c, 250);
yt =5*t*c*(.2969*(sqrt(x/c))+-.1260*(x/c)+-.3516*(x/c).^2+.2843*(x/c).^3+-.1015*(x/c).^4);

for k = 1:length(x)

      if x(k) <= p*c
          yc(k)=m*(x(k)/p^2)*(2*p-(x(k)/c));
          dx(k)=(2*m)/p^2*(p-(x(k)/c));
      elseif x(k) > p*c
          yc(k)=m*((c-x(k))/(1-p)^2)*(1+(x(k)/c)-(2*p));
          dx(k)=((2*m)/(1-p)^2)*(p-(x(k)/c));
      end

      %upper and lower limits of the airfoil (xu,yu) ; (xl,yl)
      theta=atan(dx(k));
      xu(k)=x(k)-yt(k)*sin(theta);
      yu(k)=yc(k)+yt(k)*cos(theta);
      xl(k)=x(k)+yt(k)*sin(theta);
      yl(k)=yc(k)-yt(k)*cos(theta);
  end
  %plot of airfoil
  plot(xu,yu)
  hold on
  plot(xl,yl,'r')
  plot(x,yc,'g')
  axis equal

[Justin Brakeman]

Thank you!!!

[Star Strider]

My pleasure!

I always wondered how the NACA numbers worked. I never knew until I read your script (that I’m keeping in my archive).

[Ganesh Natarajan S]

In line 4 use "s='2412';" instead of "s=num2str(2412);" because in case of symmetric airfoil e.g.NACA0021 the code produces error "Index exceeds matrix dimensions." as the 00 is not considered.