Cesium如何加载样例中的飞机，并且随机模拟飞行？

这个问题涉及到Camera，如果能熟练的使用Camera，就可以解决这个问题

举个例子：

// camera fly
var forward = 0;
var path_length = [1320, 200, 200, 200, 300]
var path_angle = [0.39269908125, 0.78539816339744830961566084581988, 0.78539816339744830961566084581988, 1.5707963267948966192313216916398]
var path_dir = ['u', 'r', 'r', 'r'];
var path_speed = [10, 5, 4, 2, 2];
var path_i = 0;
var fly_handler = null;
function freeFly(){
    viewer.camera.moveForward(path_speed[path_i]);
    forward += path_speed[path_i];
    if (forward >= path_length[path_i]) {    
        if (path_i + 1 >= path_length.length) {
            path_i = 0;
            viewer.zoomTo(tileset, default_HeadingPitchRange);
            return;
        }
        forward = 0;
        if (path_dir[path_i] == 'u')         
        	viewer.camera.lookUp(path_angle[path_i]);
        if (path_dir[path_i] == 'r')         
        	viewer.camera.lookRight(path_angle[path_i]);
        path_i++;
    }
}
function startToFly() {
	require(["Cesium"],function(Cesium){
		fly_handler = setInterval(freeFly, 100);
	});  	
}