Steve,

For a flat plate 90 deg to the flow, the drag
coefficient is about 1.17. The drag is equal to the
drag coeff. times the plate area time the dynamic
pressure. For a fixed true airspeed of 200 mph, the
dynamic pressure varies with altitude. It is 0.71 psi
at sea level and 0.52 psi at 10,000 ft.

So, the drag force is 12.5 lbs at sea level and 9.2
lbs at 10,000 ft. Hope that helps.

Ron