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Array indices must be positive integers or logical values.

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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

Accepted Answer

Bjorn Gustavsson
Bjorn Gustavsson on 3 Jun 2020
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

More Answers (1)

Hank
Hank on 3 Jun 2020
Edited: Hank on 3 Jun 2020
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)?
  2 Comments
Hank
Hank on 3 Jun 2020
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
BioZ on 4 Jun 2020
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! :)

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