January 05, 2018, ©. Leeham Co: In the last Corner we described a dominant drag component affecting the Wright Brothers’ Flyer, Form drag. The many wires and braces on the Flyer created separations and a high Form drag was the result.
At the time, Langley and others thought friction drag could be neglected. Now we describe how it was discovered one couldn’t and how it gradually made its way to the top of the drag contributors.
When the Wright’s did their first flights at Kitty Hawk in North Carolina, a young Professor at the Technical University of Hannover (Germany), Ludwig Prandtl, was studying how friction affected the flow of a fluid when it passed over a surface.
At the third International Mathematics Congress in Heidelberg, the year after (1904), the 29 years old Prandtl presented his theory of what happened when a fluid with a viscosity (a sticky fluid) flowed over a surface. He presented the first boundary layer theory.
Prandtl said; “there is a thin layer on top of a surface where the fluid stream goes from zero velocity due to friction against the surface, and then flows faster and faster until it flows with full free stream velocity at the edge of the boundary layer” (Figure 2).
The standing still of the air in direct contact with the surface was because of the air’s stickiness (viscosity) causing friction against the surface thus slowing the air to a stand-still. As the air came further from the surface the friction would slow the air less, until it kept its full speed a few millimetres out from the surface. The quickly changing speed layers causes shear stresses between the layers, which are the source of Friction drag.
Prandtl also postulated “what is happening in this layer will affect the whole fluid stream over the object”. In fact, he predicted there would be situations where the boundary layer would separate from the airfoil and the smooth flow would break down. This was the first explanation on what happens at a wing’s stall, more of below.
The proceedings from the 1904 conference, with Prandtl’s paper, were distributed in 1905. The presented theory was the trigger for aerodynamic boundary layer research and measurements over the next 50 years.
After the presentation in Heidelberg, Prandtl was made the Director of the prestigious Institute for Technical Physics at the University of Göttingen. Prandtl and his Göttingen Institute (which he elevated to world fame) would dominate the world of aerodynamics for the next 40 years.
He and his students researched a number of subjects and developed theories and calculation methods, not only for friction and the boundary layer effects, but also for wing lift and the physics behind induced drag.
A famous result of Prandtl’s theory of induced drag and how a wing should be designed to minimize it, was his elliptical lift distribution theory. To minimize induced drag for a given span, a wing should be designed to have an elliptical lift distribution.
Reginald Mitchell, the Supermarine Spitfire designer, used Prandtl’s friction and induced drag theories when he designed the high-performance Spitfire wing, Figure 1. The smoothly shaped wing had a form to create a true elliptical lift distribution. While beautiful and efficient, it wasn’t easy to produce.
Prandtl in his 1904 theory predicted the mechanism behind the stall of an aircraft’s wing.
Subsequent detailed research by Prandtl’s team at Göttingen showed what happens at boundary layer separation and stall, Figure 3.
When air has passed the crest of a wing profile, the way to the trailing edge is flow against gradually higher pressure (the lowest pressure point is at the profile’s crest). When the wing is given increased incidence against the air (increased alfa angle) to create more lift, the adverse pressure gradient increases.
This increasing pressure causes the boundary layer to slow down. It can then no longer force its way against the higher pressure. It doesn’t want to flow back, instead, it flows beyond the back-pressure air, it separates from the surface. The boundary layer from the bottom surface of the wing is the blue trail at the trailing edge. Between the two separated boundary layers is the turbulent wake, destroying the lift of the wing and causing high Form drag.
In the next Corner, we will describe more about the research into the boundary layer and the effects of skin friction drag.