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.