Why do wings prefer laminar flow if cd is lower for turbulent flows?

Can anyone please explain to me why we prefer to maintain laminar flow over wings, despite the fact that the overall coefficient of drag appears to reduce as the Reynolds Number increases? I have read that turbulent flows lead to increased skin friction drag, but this is hard to internalize given that the cd is lower overall in turbulent flows for the same angle of attack.

I've been trying to do some research to better understand why aircraft are designed this way, but I haven't had much luck so far. If anyone can offer me some insight here, it would be greatly appreciated!

As an example, I thought it would help to include the drag polar for the NACA 0012, obtained from airfoiltools (http://airfoiltools.com/airfoil/details?airfoil=n0012-il).

Laminar flow and turbulent flow are not directly linked to Reynolds number, even though Reynolds number is a heuristic indicator of when the flow may transition from laminar to turbulent. However, the transition is actually not well-understood and is difficult to model/predict. That's why you will see cases where, for the same Reynolds number, one flow is laminar and another is turbulent.

Let's take a flat plate for an example. The flat plate boundary layer in laminar flow actually has a closed-form solution; the skin friction drag is given by:

$$C_{f}=\frac{1.328}{Re_c^{1/2}}$$

As you can see, as Reynolds number increases, the drag coefficient decreases.

Turbulent flow, on the other hand, has no closed-form solution. An approximation for a smooth plate is given by:

$$C_{f}=\frac{0.074}{Re_c^{1/5}}$$

The two lines are plotted in the figure below (cited from this Penn State course, which has a pretty good summary of boundary layer results for flat plate). As you can see, at the same Re, laminar flow has less skin friction drag than the turbulent counterpart.

From a skin friction drag perspective, if you can keep the wing in laminar flow, then all the power to you. In practice, however, this is very difficult. Any un-eveness on the wing (e.g. rivets, steps and gaps) could transition it to turbulent flow downstream. Any contamination, such as insects, could potentially ruin it.

Another consideration is whether staying laminar for the whole wing is desirable from a flow separation perspective, since laminar flow separates easier than turbulent flow. There will be a trade-off amongst stall speed, high AOA handling and drag.

• Ah - I see now. This helps immensely! Thank you so much :) – sroger13 Oct 22 at 0:02