Reynolds Number

The errors this method actually invites, and what it does about each. Every one of them is already a warning somewhere — in a note that appears when your own figures trip it, in the scope statement, or in the equations behind the page. Collected here so you can read them before you need them.

Multiplying the blade chord by the aircraft's speed. That gives 24,000 on the default trainer, which is deep in bubble country and wrong by a factor of 7.3. At the 75% station the blade has its own rotational velocity: 90 m/s of rotation against 12.5 m/s of flight, and the real figure is 174,000. A blade is a small wing flying very fast, not a small wing flying at the aircraft's speed.

Using the wing's Reynolds number for the tail. The tail nearly always sits lower than the wing it stabilises — 94,100 against 162,600 on the same aeroplane at the same moment. That is why a tail can stall before anyone expects it to, and why flat-plate tails work better on small models than scaled-down wing sections do.

Reading the regime boundaries as exact. They are conventional. Where transition actually happens depends on the section, the surface finish and the turbulence in the air that day, and none of those are a round number.

Expecting a flat plate to predict a polar. The laminar separation bubble — the thing that actually decides low-Reynolds behaviour — is outside a flat-plate model entirely. Use this to know which regime a part is in, not to guess what it will do there.

Forgetting that a hot day pushes it down twice. Density falls and viscosity rises, and the Reynolds number carries both. That is why it drops 16% on a 45 °C day when the density has only lost 9%.