the first in class design challenge 2/20/2017
function dudt=phugoid(t,u)
g = 9.81 ; % gravity in m s^(-2)
v_t = 5.5; % trim velocity in m s^(-1)
C_D = 1/5.2; % drag coefficient --- or D/L if C_L=1
C_L = 1 ; % for convenience, use C_L = 1
pi = 3.1415926;
v = u(1); theta = u(2); x = u(3); y = u(4); dudt = [-gsin(theta) - C_D/C_Lg/v_t^2v^2, -gcos(theta)/v + g/v_t^2v, vcos(theta), v*sin(theta)]';
end %ode45