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Reynolds Number
The Reynolds Number is a dimensionless quantity that can describe (in aviation terms) the movement of an aerofoil through the air. It is quantified by the equation:
Re = Vx / ν
where:
- V is velocity (m/s)
- x is distance from the leading edge (m)
- ν is the kinematic viscosity (m2/s)
From the formula it follows that when Re is low, viscous or friction forces are significant. These result from small chord surfaces, low velocities, and at high altitudes (high ν). When Re is high, dynamic or inertia forces are significant. High values are obtained with large chord surfaces, high velocities, and at low altitude.
Use of Re
A direct use of Re is in the indexing or correlating the skin friction drag of a surface. For a smooth, flat plate in an airstream, the boundary layer is entirely laminar if Re < 500,000, and entirely turbulent if Re > 10,000,000. For Re between these values, the airflow transitions from laminar to turbulent. In order to produce a low drag aerofoil, this transition must be delayed.
Re can also be used when testing scale models. Wind tunnel testing involves scaling parameters in order to maintain the same Re that the full scale aerofoil will be subject to inflight. For example, to test a 1:10 scale model in wind tunnel, the wind speed should be 10× the flight speed of the airplane.