October 06, 2017, ©. Leeham Co: We introduced the Airbus “Blade” laminar flow research program in the last Corner. Now we continue to look into what this research is all about.
We will explain the difference between laminar and turbulent flow (really, laminar and turbulent “boundary layer” flow) and discuss some of the work that has been done with laminar flow structures.
Last week we said the friction part of the drag due to parasitic drag was the most important drag force. The size of the friction drag will be determined by the behavior of the boundary layer of the aircraft.
Boundary air is the air layer a few millimeters out from the aircraft surface. Because of the air’s viscosity (its stickiness), it has zero speed at the surface and the speed of the air free stream a few millimeters out.
It’s in this thin boundary layer the air friction drag is decided. As long as the flow is laminar (orderly, with no turbulence), the friction force is low. This is the state at low airflow speeds.
As the speed of the free flow increases, there comes a point where the smallest roughness trips a transition to a turbulent boundary layer and the friction level increases stepwise.
The figure below shows research results as published in the classical book, Hörner “Fluid dynamic drag”. It plots results from experiments done as early as before WW2. The research done then is still valid today.
The y-axis shows Cf, the friction coefficient, and the x axis has the speed of the airflow as main parameter. But the axis is graded in the Reynolds number, Re. The Re is used to make the friction coefficient independent of the size of the body it is used for.
By dividing the inertia related forces of the flow with the viscosity forces, the independence of size is achieved. It’s not too complicated. The air’s speed is multiplied with the air density and then the test body’s characteristic dimension (for a wing, the cord). This is then divided with the viscosity (the stickiness) of the air (look it up in Wikipedia for more info).
What we can see is that around Re 1000,000 (10^6), the friction jumps from low laminar flow values to high turbulent flow values. It’s this jump that laminar flow wings like the “Blade” try to avoid, for as long as possible along its surface. The longer, the lower the overall friction drag of the body.
The aircraft is flying at Re above 10^6, so the flow tends to convert to turbulent boundary layer as soon as a little roughness in the aircraft skin is passed. So the surface has to be very smooth.
Another characteristic of laminar flow surfaces is a very gradual increase of the bodies thickness. This is to avoid the adverse pressure gradient that comes with strong curvatures of the surface. The curvature triggers back pressure close to the surface, which triggers transition to turbulent flow. Laminar flow wings tries to avoid back pressures until about half the wing,
There were several aircraft designs that applied laminar flow wings during WW2. The P51 Mustang is the most well-known, but the P-63 Kingcobra and B-24 Liberator also had laminar flow wings.
None of these aircraft had the production methods to make these wings really laminar for any length of their surface. The aim is to have the transition to turbulent flow after at least 30% of the wing, the longer the better. In practice these wing had the transition already after 10%, but the standard wings had the transition even earlier.
After realizing how poorly the early laminar flow wings worked, the aerodynamicists focused on making good normal wings and crack other problems like the transonic flow problems. Recently, manufacturing methods have enabled the smooth surfaces needed for laminar flow. So interests in laminar flow structures has resufaced.
Boeing has introduced laminar flow profiles to leading edges of nacelles (787) and winglets (737 MAX). It has also implemented a laminar flow leading edge of the 787’s vertical and horizontal stabilizer. (Only the vertical stabilizer is kept in production today).
The tripping to turbulent flow is delayed on the 787’s tail surfaces by sucking off boundary layer air that gets slowed down and therefore sensitive to back pressure.
The laminar flow is kept for rather short distances on these implementations, but it’s still worth doing.
To-date no more ambitious applications of laminar flow than the Boeing ones have been used. The Airbus Blade project is about finding ways to allow more elaborate implementations in the future.