How Thick Is The Boundary Layer?

Boundary layer phenomenon are complex and still a subject of active research but we can ge some kind of idea of how thick the boundary layer is from results of many experiments made in both Europe and the Americas that had consistent reslts. These were measurements of boundary layer thickness over an absolutely smooth flat plate with constant pressure.

The variable velocity and pressure over an aircraft that may have a rough surface will make things more complicated, but these results and give you a feel for how thick the boundary layer can be before any possible flow separation.

Calculations using the results of these tests were made for two atmospheric conditions:

1. Sea Level Standard: 0% humidity, sea level pressure and 59^o F temperature.
2. "High and Hot":    60% humidity, 6000 ft altitude pressure and 80^o F  temperature.

"High and Hot" Is my own made up atmosphere designed to represent summer model flying at a high altitude and high temperature for that altitude. These atmospheres have different boudary layer characteristics. See the article in the Index on the Atmosphere for more details.

The graph below shows the boundary layer thickness in millimeters against the chord on millimeters. 25 millimeters is about an inch so you can use the tic marks as inch markers. Twenty mils (0.020 inches) is 0.5 mm, the smallest tic size on the vertical axis. Note that "High and Hot" atmosphere and slow speed make thin boundary layers at large distances from the start of the flow and vice-versa. Both of these conditions lower the Reynolds numbers.

In addition the same effect goes with low velocity. This translates to the rule that surface finish must be better at high speeds, and from these numbers it seems that the best finish is required for the first several inches of chord. For speed an racing this is nearly thw whole wing. For flying wing configurations as are popular in F2C the finish at the rear of the wing is not so important.