Hinge Moment is defined as the force required deflecting a control surface. In small, relatively slow airplanes, hinge moments are not very large; pilots move the controls with ease. But…
Shape factor is the ratio between displacement thickness and momentum thickness. Its used in boundary layer flow to determine the nature of the flow.
Vertical tail volume ratio is the relation between tail volume and wing volume. Vertical tail contributes positively for Directional Stability. Its SI unit is dimensionless.
Boundary layer thickness is the distance from wall to the point where the velocity is 99% of the free stream velocity. Its SI unit is meter.
Specific weight is the ratio between weight and volume. Its depends on density, area, length, load. Its SI unit is N/m^3.