why do I get the error "Error using atan2 Inputs must be real"

c = 1;
tau = 0.12;
Npanel = 200;
for i = 1:Npanel/2+1
    eta(i) = pi*(i-1)/(Npanel/2) ; 
    x(i) = c*(cos(eta(i)+1)/2); 
end
x = sort(x) ;
T = 10*tau*c*(0.2969*sqrt(x/c)-0.126*(x/c)-0.3537.*(x/c).^2+0.2843*(x/c).^3-0.1015*(x/c).^4);
X_u = x - T/2.*sin(atan(0)) ;
X_l = x + T/2.*sin(atan(0)) ;
Y_u = Y_bar + T/2.*cos(atan(0));
Y_l = Y_bar -T/2.*cos(atan(0));
X_u(1) = [] ;
Y_u(1) = [] ;
X = [fliplr(X_l) X_u];
Y = [fliplr(Y_l) Y_u];
for i=1:Npanel
    X_bar(i) = (X(i+1)+X(i))/2;
    Y_bar(i) = (Y(i+1)+Y(i))/2;
    theta(i) = atan2((Y(i+1)-Y(i)),(X(i+1)-X(i)));
    l(i,1) = sqrt((X(i+1)-X(i)).^2 + (Y(i+1)-Y(i)).^2);
end

Answer 1

[Cris LaPierre]

As the error states, you are trying to use atan2 on complex data, which is not allowed.

The issue starts with how x is calculated. I had to make up a value for Y_bar, but the resulting values were all negative. Then, in your calculating T, you take the sqrt of x. The sqrt of a negative number results in a complex number.

I'm not sure what you want, but function like abs, real, and imag may help you out.