Sucking the Boundary Layer

From "A History and Philosophy of Fluid Mechanics" by G. A. Toksty

Needless to say, Jacob Ackeret, Albert Betz and Ludwig Prandtl, the three most outstanding aerodynamicists of Germany, knew at least as much about the Rotor as Flettner. But they were much less enthusiastic about its practical value; and subsequent history proved that they were right. Accordingly, they tried to persuade Flettner to concentrate on his metal sails rather than on the Rotor ship, "Dr. Betz and Herr Ackeret actually succeeded in inducing me to drop the idea of the Rotor ship for the time being," he wrote with some bitterness, "and to consider the possibility of participation in another invention for which they themselves, Betz and Ackeret, had made patent application." This was, of course, the idea of sucking off the boundary layer.

On 16th September, 1922, Flettner applied for a German patent for the Rotor ship. Towards the end of 1922, Professor Foettinger published an article called "Neue Grundlage Fuer die theoretische und experimentalle Behandlung des Propeller-Problems," which convinced the inventor that he was moving along the right path. Flettner's first idea was to create the propelling force needed by means of a belt moving around two cylinders (figure 82): he thought (wrongly) that, in this way, a much greater circulation would be maintained. But, after several months of further thought, the belt-idea was abandoned, and the Rotorship concept emerged in the form shown in Figure 83.

In the meantime, Betz's and Ackeret's experiments on the boundary layer sucking proved to be more or less futile, and Flettner's invention began receiving more favourable attention. At long last, his models found their way into the Guettingen wind-tunnels (Plate 7). The experiments proved beyond doubt what was already known: that a rotating cylinder exposed to a wind creates on one side a region of low pressure and a region of high pressure on the opposite side. The force arrising from these pressure differences was the "Magnus Effect," the aerodynamic thrust needed to propel a ship.


We shall meet Froude also in the section dealing with airscrews. Figuratively speaking, what he did in water, a German engineer called Zahm did in the air. Zahm was, perhaps, the first man in the histoy of aerodynamics to undertake a programme of study of the nature and quantitative laws of air friction on wind-tunnel walls. The results obtained by him were similar to those by Froude, but the absolute values were, of course, proportional to the mass densities of water and air, respectively.


The Goettingen Centre of Experimental Aerodynamics tried to solve the problem of prevention of early flow separation by means of removing the low-energy parts of the boundary layer, or of adding kinetic energy to the boundary layer. This could, and can, be done by removing low-energy air through suction slots or a porous surface. Another common method is to blow high-energy air through backward-directed slots (Figure 103). The air handled through either the suction or blowing slots may be carried through the interior of the wing. By far the most extensive investigations along these lines were carried out by Schrenk. Using a single sucking slot, he succeeded in preventing flow separation from a thick wing and thus in increasing its maximum lift coefficient up to about 5, nearly three-times. Similar experiments were also going on in NACA (USA), CAGI (USSR) and other research establishments. The CAGI even built an aircraft (approximately 1935-6), with a full-scale boundary layer sucking in flight.

"The Bat" XP67 aircraft had a laminar flow body with boundary layer sucking.