quadcopter vertical dynamics modelling

Hello. I try to get state-space model of my quadcopter vertical motion under gravity.

I can't simulate effect of gravity. For this i try to add matrix K to my state-space model, but without success. There is no difference if i use:

sys = idss(A,B,C,D,K,x0,timestep)

or

sys = ss(A,B,C,D,timestep)

Result same. There is code:

clear all
timestep = 0.002;
m = 0.0027;
g = 9.81;
T_hover = 20000;
Kt = g*m/T_hover;

A = [1 timestep ; 0 1];
B = [+(Kt/m)*(timestep^2)/2 ;  +(Kt/m)*timestep];
K = [ -(g)*(timestep^2)/2 ;   -(g)*timestep];
C = [1 0];
D = [0];
x0 = [0; 0];
sys = idss(A,B,C,D,K,x0,timestep)

length = 2000;
inputm = zeros(length,1);
inputm(300:500) = 25000;
t = 0:timestep:(length-1)*timestep;

[y_m,t_m,x_m] = lsim(sys,inputm,t,x0);
subplot(2,1,1);
plot(inputm);
title('input thrust');
ylim([-1000 26000]);
subplot(2,1,2);
plot(x_m);
legend('coordinate','speed');

Maybe i missed something obvious. Help me please.