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Vne defines the maximum indicated airspeed which should not be exceeded. And clearly, doing so would result in "bad things" happening like flutter, etc. Presumably manufacturers do some sort of design work to determine likely Vne but how do they test for that? Exceeding Vne would put the aircraft into a structural failure situation would it not?

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$V_{ne}$ is typically set to about 90% of $V_D$, the maximum design dive speed. This difference provides a safety margin. Above $V_D$, damage may occur due to flutter or structural failure, or controllability considerations may make the aircraft unflyable. The aircraft is not guaranteed to fall apart at $V_D$ due to additional structural and manufacturing margins.

During flight test, the aircraft is flown above $V_{ne}$, up to, or close to, $V_D$. Appropriate safety precautions are taken, e.g. parachute. Any failure below $V_D$ indicates a design or manufacturing issue that must be resolved prior to certification.

The limitation to flight below $V_{ne}$ (as opposed to $V_D$) during regular operations provides protection against airspeed measurement errors, pilot errors and some margin against structural degradation over the lifetime of the aircraft.

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It is exactly as pericynthion says. VD is one of the so called design speeds defined at design time. Many of the structural/aeroelastic characteristics have to be demonstrated at speeds up to VD during flight test. However VD is for certification flight testing only. Normal operation is limited to VNE. However VNE is for light aircraft only. Larger aircraft have VMO instead.

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The generic answer is:

  • Design Cruising Speed (Vc) = 33 * Sqrt (wing loading)
  • Design Dive Speed (Vd) = 1.4 * Vc
  • Never Exceed Speed (Vne) = 0.9 * Vd

Then empiric testing (wind tunnel or test pilot) is done to test and expand those.

Actual formulas are in the F.A.R. FAR/JAR-23 and a reference page with this question answered is in this forum.

Military and advanced civil design does finite analysis based on the material properties in a computer simulation to calculate failure at stress points based on the materials used and predict flutter but that's beyond any simple answer and in industry is done by entire teams of individuals.

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