# At what Reynolds Number does laminair airfoils stop working as expected?

I was watching a RC Plane builder on YouTube create an airliner on a 3D printer and noted he used the scale laminar airfoil of a B-727. It seems to me that laminar airfoils don't work at the scale of bugs, probably don't work for birds, and possibly don't work for RC models with less than 2-3ft wing spans.

1. Intuitively, it seems laminar airfoils are tied to Reynold numbers. Is that assumption true?
2. At what Reynolds number or scale (or other limiting factor) are laminar airfoils no-longer operating as expected?

The B-727 does not use a laminar flow airfoil. It may use a supercritical airfoil -- which may get some limited amount of laminar flow, but nobody would consider it a laminar flow wing.

Any body or airfoil designed for laminar flow will have a somewhat narrow operating range where it has favorable characteristics.

Here is a modified figure from this paper on the design of bodies of revolution for natural laminar flow.

Each of the five bodies was optimized for its particular region of Reynolds number. Notice that the shape of the bodies is quite different.

For each one, going above its designed operating range is very bad. Operating below its designed operating range is not catastrophic, but better performance would be possible with a body designed for that (lower) Reynolds number.

Note that the drag coefficient and Reynolds number have been defined in terms of the volume -- to try to keep the comparison of these bodies as 'apples-to-apples' as possible.

1. Yes, certainly Reynolds # is a key parameter effecting airfoils designed to have laminar flow.
2. As a rough guide, for Reynolds #’s < 100,000 the boundary layer is most likely laminar. By Re ~ 1,000,000 the boundary layer is usually fully turbulent. Some factors that can effect the extent of laminar flow include noise, pressure gradients (including shocks), crossflow instabilities (on swept wings), surface roughness or waviness, and wall boundary conditions (e.g., thermal, suction, blowing).