this not my plane ,her maker is @Geebers , I just
Modifyed some parts of plane and let she fly on sky.

plane line:
https://www.simpleplanes.com/a/2HtQ9P/XC-120-Pack-Plane

her maker line:
https://www.simpleplanes.com/u/Geebers

@Geebers

I modified your plane so that she can fly

Modified parts:
-larger wing
-larger propeller engine blade size and larger propeller engine blade area
-reduced weight (removal of the VTOL system and removal of excess oil)
-slightly changed the angle of the tail wing to better level flight (+1 angle)
-changed the control of the lower part of the tail yaw (reversed the direction of control)
-changed the position of the landing wheel
-increased the weight of the cargo

The reason why your plane can’t fly is that it’s too heavy, and then the wing is too small and the speed is not enough. I slightly modified the wing and increased the thrust from the speed source.

There are also the center of gravity (the weight center of the aircraft (red)-can be adjusted by adding weight) and the lift point (the lift center of the aircraft (blue)-the wing can determine where it will be) too close, but your wing Not big enough (if the wing is big enough, even a b25 weight plane can fly at 10 mph in theory), and the speed is not enough (fast enough even a small wing can generate huge lift, so In theory, as long as the speed is fast enough, even a mini wing can fly a heavy aircraft like b25), resulting in the lift that your aircraft’s wing can generate cannot match the agility of your aircraft (center of gravity point and lift point) The closer the aircraft is ,the more agile it can be maneuvering (but if the wings are not large enough and the lift is insufficient, it will cause a gap between the direction of the aircraft's nose and the actual flight direction of the aircraft Fall)

Lowest controllable speed (ias-mph)
When cargo is loaded-160
When no cargo is loaded -145

2- Abandon the goods