Fluid Flow & Aerodynamics
why things fly, and why the usual explanation is wrong
Concept

Equal transit time — where it appears

The claim that air parted by a wing's leading edge must rejoin at the trailing edge, so the longer upper path forces faster flow and lower pressure. It is false: the upper parcel arrives well ahead, and the speed difference comes from the circulation the trailing edge sets, not from any meeting.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

Two parcels released together do not arrive together. The most repeated explanation of lift says that air parting at the leading edge must meet again at the trailing edge, so the longer upper path forces a higher speed. Released into the solved field, the upper parcel arrives long before the lower one — the premise is simply false, and the real speed difference is larger than it would require.

The story about air meeting up again

The most repeated explanation of lift says that air parting at the nose must rejoin at the tail, so the longer upper path forces a higher speed. The premise is false, and the speed it predicts is wrong by a factor of twenty.

misconceptions · Equal transit Inside the layer the upper parcel's lead grows with depth. The transit lag, lower parcel minus upper, of parcels held at a quarter, a half and three-quarters of the boundary layer's thickness and just outside it, timed from either side of the attachment point to each chordwise station, on a thin cambered section at zero incidence, up to the upper layer's laminar separation at 62% chord. At mid-chord the lag outside the layer is 0.183 chord-over-speed units; at three-quarters of the thickness 0.195, at half 0.281 and at a quarter 0.695.

Inside the layer the upper parcel leads by more

Two parcels parting at a wing's attachment point arrive at different times, and outside the boundary layer the difference is the circulation over the speed squared. Inside the layer one might expect the slow, sticky fluid to wash the difference out. It does the opposite: deep in the layer the upper parcel's lead is several times the inviscid one, because the suction side's layer is fuller than the pressure side's. Only the layer's outer quarter reads the inviscid lead, and on the suction side the laminar layer gives out before the trailing edge.

misconceptions · Equal transit

Named alongside it

The objects these essays reach for when they reach for this one.

CirculationLiftBoundary layerIntegral methodKutta conditionMisconceptionModel limitSeparationSurface speed

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