 Wing loading

Wing loading is the gravitational weight of an aircraft divided by its reference wing area:It is a whole-aircraft loading measure. In SI its units are $N/{\mathrm{m}}^2$, not $kg/{\mathrm{m}}^2$. The latter units describe mass per wing area, $m/S$, and require multiplication by gravitational acceleration to obtain $W/S$.

For an original unit check, $m=500\mathrm{kg}$, $S=10{\mathrm{m}}^2$ and $g=9.81m/{\mathrm{s}}^2$ give $m/S=50kg/{\mathrm{m}}^2$ but $W/S=490.5N/{\mathrm{m}}^2$. Although this has the units of pressure, it is not the local pressure distribution on the wing surface.

In straight level flight with negligible vertical thrust, lift balances gravitational weight.^[ref] Combining this with the definition $L=\frac{1}{2}\rho V^{2}S{C}_L$ of the whole-wing lift coefficient givesAt fixed air mass density and required coefficient, airspeed therefore scales as the square root of wing loading. For stipulated $W/S=600N/{\mathrm{m}}^2$, $\rho =1.2kg/{\mathrm{m}}^3$ and $C_L=1$, the result is $31.62m/s$. Doubling only the loading gives $44.72m/s$, a factor of $\sqrt{2}$.

If a model assumes a maximum attainable $C_{L,\max }$, the same equation gives a corresponding minimum speed for that straight-level condition. It is not a universal speed for every attitude or configuration: a turning condition can require $L>W$, and a changed airfoil, surface condition or Reynolds number can change the attainable coefficient.

A geometric and weighing check determines $m$, the relevant loading state and projected wing area. Those measurements establish $W/S$; a separate aerodynamic experiment is needed to establish the applicable lift coefficient. Equal wing loading does not establish equal lift-to-drag ratio or identical aerodynamic stall behavior.