Array indices must be positive integers or logical values.

Hello everyone, I am new to Matlab and just learning at the moment.

I am trying to calculate a polynomial and then graph it, but I gent an error "Array indices must be positive integers or logical values." on line 23

would appreciate any help

Thank you!

clear

Cd0=0.01
K=0.02
W = 80000
S = 50
q=0.6

dH = 0.1 
Hm = 12
H = 0:dH:Hm;

Pol1 = zeros(4,2);
Pol4 = zeros(4,2);

Pol = K*W^2

q = zeros(1,length(H));
q(1) = 1;

for n=2:length(H)
    q(n) = (20-dH(n-1))/(20+dH(n-1));
    Pol4(n) = Cd0(0.5*1.225*q(n)*50)^2;
    Pol1(n) = ((-1.225*q(n))^2*0.5*50);
    
    r =roots([Pol4(n) 0 0 Pol1(n) Pol])
end

[madhan ravi]

There are lots of bugs , answers below points only one of the mistakes.

Answer 1

[Bjorn Gustavsson]

Most likely you want to multiply Cd0 with the expression in the parenthesis, you try to index the variable with the value of (0.5*1.225*q(n)*50); Try:

Pol4(n) = Cd0*(0.5*1.225*q(n)*50)^2;

HTH

Answer 2

[Hank]

In the line:

Pol4(n) = Cd0(0.5*1.225*q(n)*50)^2;

what you wrote says to index Cd0 at 0.5*1.225*q(n)*50, which is probably not an integer. I think you want to multiply by Cd0 like this

Pol4(n) = Cd0 * (0.5*1.225*q(n)*50)^2;

You're also going to have trouble indexing the scalar value like you are: dH(n-1). Maybe you mean H(n-1)?

[Hank]

As I'm continuing to attempt to fix and run the code. More problems are popping up and I'm left wondering what you're trying to simulate here?

[BioZ]

During a cruise speed, aircrafts can have so called min a max available cruise speed. It's easy to solve for jet engine as it is just a quadratic but for prop engines it is a x^4 polynomial, therefore best to solve it via computational method. It should produce 2 real numbers and 2 imaginary values and when we reach abosolute ceiling 4 imaginary values should appear. My code was intended to solve that until 4 imaginary numbers or max alt specified was reached. then plot it against altitude I fixed the code already, but thank you very much for help! :)