Ever heard of the Northrop X-21A aircraft? This program used modified WB-66s to study a neat concept called laminar flow control, which can reduce drag and improve range/efficiency. It even kinda worked! More details in the thread... #AvGeek

To understand laminar flow control (LFC) you first have to get the concept of a boundary layer. Basically when something moves through the air, air sticks to the surface in a thin layer called the boundary layer. This creates friction and drag.

Boundary layers are initially small and “smooth”—what we call laminar. And don’t generate much drag. But as you move faster or move further down the wing the boundary layer grows, becomes turbulent, and “pulls” much more on your vehicle thus increasing drag.

So now back to the X-21A. This program was designed to test laminar flow control (LFC) in practice. What’s LFC? You’re basically trying to force the boundary layer on your wings to stay laminar to reduce drag. One way to do that is boundary layer suction.

The idea is that you can use suction to pull the turbulent boundary layer back through the wing to keep the surface laminar. The X-21A had 240 suction slots per wing (half on top, half on bottom) that were between 3-10 thousandths of an inch wide each.

And it actually worked! The X-21A demonstrated LFC on 38 flights for over 95% of the wing surface, providing significant boosts in range or payload (estimated in the 50-70% ballpark)

So why don’t all our airliners use LFC? Here’s the catch: those tiny suction slots have a tendency to get clogged with just about everything. Bugs. Dirt. Rain. Even ice from clouds. Fly through a cloud and it doesn’t work. Hours of meticulous cleaning after every flight.

Funnily, the ice would melt once the flow went turbulent (increased heating from friction) and LFC would resume!

There are also some problems with 3D turbulence.

Nevertheless, a cool plane that demonstrated LFC can work at scale. We mostly need a way to keep the wings clean.

And here’s a nice reference where I borrowed some of these figures, with lots more info on LFC (Kosin, 1965, @aiaa J of Aircraft)
https://arc.aiaa.org/doi/abs/10.2514/3.43672?journalCode=ja