What is a laminar airfoil and what are their pros and cons?

Question

I am wondering what is a laminar airfoil and what are their advantages and disadvantages if compared to other designs.

Answer 1

The fist decades of aviation used empirically determined airfoil shapes which usually had most camber near the nose. Such airfoils tend to have pressure peaks near the nose, followed by a long and shallow pressure recovery. The distinction between laminar and turbulent boundary layers was not known, so airfoil shapes did not factor in boundary layer transition.

In the late 1930s, when the first wind tunnels with reduced turbulence became operational, new insights were gained in how to further reduce friction drag. From this NACA history page:

The low drag coefficients achieved by internally braced monoplanes equipped with retractable landing gears suggested that any further large reductions in drag could only be achieved through the maintenance of extensive laminar flow over the surfaces of the aircraft. The boundary-layer flow of contemporary aircraft was essentially all turbulent; and since the skin friction coefficients for turbulent flow are much higher than those for laminar flow, the achievement of laminar flow on the surface of the aircraft would be expected to yield large reductions in drag. For example, the skin friction coefficient on a flat plate is reduced by a factor of almost 2 as the point of transition from laminar to turbulent flow is moved from the leading edge to the 50-percent-chord location.

Better understanding in the causes of boundary layer transition and improved analytical methods allowed researchers to tailor airfoils which showed a delayed laminar to turbulent transition over a small angle of attack (AoA) region, so a reduced drag coefficient in that range could be achieved. This is done by shaping the airfoil surface for a desired lengthwise pressure distribution which shows on the upper side a short, steep pressure drop at the nose, followed by an extended region of shallow pressure drop to about mid chord on both sides. Such a pressure distribution helps to stabilize the laminar boundary layer, thus reducing friction drag, and results in a backward shift of the point of maximum thickness.

Outside of this AoA region, the suction peak on the upper side (at high AoA) rsp. the lower side (at low AoA) forces an early transition on that side which shows up in the drag polar as a sudden jump to higher drag coefficients. The region of reduced drag coefficients between both points is called the laminar bucket of the airfoil polar.

This plot (made with XFOIL 5.4; own work) shows how the laminar bucket is shifted up and down the c$_L$ range by different flap deflections without affecting drag much except for the -20° setting - here, the suction peak on the lower side in combination with the strong flap camber is too much for the boundary layer to remain attached. But this setting is really useable for inverted flight only.

Laminar airfoil pros:

• Low drag at an intermediate AoA range. That is the reason why all modern gliders and most modern GA designs use laminar airfoils.

Laminar airfoil cons:

• They work as intended only at Reynolds numbers between 1 Million and 5 Million - above that the transition happens very early, regardless of pressure distribution. Since the stability of the laminar boundary layer decreases with higher Reynolds numbers, a higher Reynolds number narrows the laminar drag bucket and reduces the advantage of laminar over turbulent flow.

• Susceptibilty to bugs and rain: Early laminar aifoils used the steepest possible pressure recovery after transition, and an earlier transition triggered by surface imperfections like bugs or raindrops would lead to much earlier flow separation.

• Wing sweep will lead to early transition anyway, so large sweep angles will render laminar airfoils ineffective.

• High demands on surface perfection: A wavy surface or a gap in the metal sheets for covering the wing will also cause an early laminar-to-turbulent transition, so again a laminar airfoil will be rendered ineffective when combined with shoddy workmanship.

• At Reynolds numbers below 1 Million the stability of the laminar boundary layer becomes too high and transition needs to be triggered deliberately in order to get the best performance and avoid large-scale laminar separation. Simply put, almost any airfoil is a laminar airfoil at low Reynolds numbers but performs better with a turbulent boundary layer in the pressure recovery region.