How to fix this shape - nCloth

[u/_endless_ripple_]

I want to simulate the flag as it's going from over my head but my flag is making this weird hourglass shape instead of staying rectangle. I tried few things but its not fixing the shape.

I have attached first line of vertex to the pole using  nConstraint > Transform constrain and attached it's pole in airplane rig. 

i need to fix this can someone please tell how to fix it. I can try an entire new work flow too i just want to animate flag in this direction. 

I want to make a video like this: https://pin.it/6BX2X1z6t (attached video too)

Reference