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Wake-plane dependence of a downwash measurement means that a measured downward velocity depends on where the observation plane lies relative to the lifting wing and its evolving wake. The induced velocity entering a wing-section force calculation is not generally equal to a downstream probe reading. Classical lifting-line theory explicitly adjusts local incidence using the induced flow at the lifting line, under assumptions such as steady flow, incompressible flow and a nearly planar high-aspect-ratio wing.^[ref]

An original straight-filament model isolates one reason for the difference. Consider a vortex filament of circulation magnitude $\Gamma$ starting at streamwise coordinate zero and extending downstream forever. At a point with streamwise coordinate $x$ and perpendicular distance $r>0$, its induced azimuthal speed isThe integral follows by differentiating $(s-x)/[r\sqrt{(s-x{)}^2+r^2}]$. At $x=0$, only one half of the corresponding infinite straight filament contributes; far downstream, the local filament extends far in both axial directions relative to $r$.

For two straight trailing filaments at transverse positions $y=\pm a$ with opposite circulation signs chosen to induce downward flow between them, their midpoint contributions add:With stipulated $\Gamma =20{\mathrm{m}}^2/s$ and $a=5\mathrm{m}$, this trailing-filament contribution is $0.6366m/s$ at $x=0$, $1.0868m/s$ at $x=5\mathrm{m}$, and approaches $1.2732m/s$. This is an idealized geometry calculation, not a measured wake or a universal factor-of-two correction for every wing.

The model omits finite vortex cores, the bound-vortex contribution away from the lifting line, the complete distributed vortex sheet, wake deformation and wake contraction in a downwash survey. Each can change the spatial transfer. In particular, a wing-plane value and a far-wake value must be connected using the same load and wake model before using one to infer the other.

In the small-angle lifting-line model, the wing-plane induced-drag calculation weights downwash by the circulation or local lift. Its source derivation integrates the local induced contribution along the wing.^[ref] Substituting an arbitrary downstream speed into that expression can misestimate drag even if the speed measurement itself is accurate.

A proposed survey measures velocity components at several downstream stations with the same probe calibration and freestream reference, covering both vortex cores and the intervening region. Record the wing loading, probe orientation and cross-plane coordinates, and track how vortex separation and circulation estimates change. A centerline reading alone does not recover the whole momentum field. Matching an axial trend tests the proposed transfer model only over the surveyed region; it does not establish that a rolled-up real wake remains a pair of straight mathematical filaments.