function xdot = xdot_TC04b_hfb_lon(ts, x, u, param)

% Function to compute the state derivatives (i.e., RHS of system state equations) 
% test_case = 4 -- Longitudinal motion: HFB-320 Aircraft
% Nonlinear model in terms of non-dimensional derivatives as function of
% variables in the stability axes (V, alfa)   
%                  states  - V, alpha, theta, q
%                  outputs - V, alpha, theta, q, qdot, ax, az
%                  inputs  - de, thrust  
% State equations (5.86), (5.87)

% Constants
d2r   = pi/180;
r2d   = 180/pi;

% Geometry data
G0     =    9.80665D+0;
global stables;
U0     = stables(1);  % トリム状態における対気速度[m/s]
W0     = stables(2);  % トリム状態における下方向速度[m/s]
THETA0 = stables(3);  % トリム状態におけるピッチ角(安定軸)

% State variables
U     = x(1);
Alpha = x(2);
Theta = x(3);
Q     = x(4);

% Input variables
de    = u(1);
dt    = zeros(size(u(1)));

% Parameters
Xu   = param( 1);
Xa   = param( 2);
Xdt  = 0;
Zu   = param( 3);
Za   = param( 4);
Zq   = param( 5);
Zde  = param( 6);
Zdt  = 0;
Mu   = param( 7);
Ma   = param( 8);
Mq   = param( 9);
Mde  = param(10);
Mdt  = 0;

% Right sides of state equations (5.86)
Udot = Xu * U + Xa * Alpha - W0 * Q - G0 * cos(THETA0) * Theta + Xdt * dt;

Aldot = (Zu * U + Za * Alpha + (U0 + Zq) * Q - G0 * sin(THETA0) * Theta ...
      + Zde * de + Zdt * dt) / U0;

Qdot = Mu * U + Ma * Alpha + Mq * Q + Mde * de + Mdt * dt;

% State derivatives
xdot(1) = Udot;
xdot(2) = Aldot;
xdot(3) = Q;
xdot(4) = Qdot;

% xdot must be a column vector
xdot = xdot';

return
% end of function