United States Patent Office
3,521,837
Patented July 28, 1970
3,521,837
AIRFOIL
Hermann Papst, Karl-Maierstrasse 1, St. Georgen,
Black Forest, Germany
Filed June 30,1967, Ser. No. 650,462
Claims priority, application Germany, July 1, 1966,
1,268,979
Int. CI. B64c 21/06
TJ.S. CI. 244—42 21 Claims
ABSTRACT OF THE DISCLOSURE
An arrangement for controlling the boundary layer which develops along the outer surfaces of airfoils and profiled bodies which come in direct contact with fluid streams. The airfoil is provided with one or several recesses which extend transversely of the direction of relative movement between airfoil and stream. The recess is covered by a longitudinal thin metallic strip so that a longitudinal and nearly completely closed channel is formed. The strip has a sharp upper edge at its leading flank and is fixed within the recess so that between sharp edge and opposite part there is a longitudinal narrow slot extending transversely of the direction of said relative movement. The boundary layer is compelled to enter the interior of said channel and the sharp edge peels off the stratum of the boundary layer which is adjacent to the surface of the airfoil. The sharp edge may be elevated above the opposite edge of the slot to improve the effect of "peeling off" the adjacent strata of the boundary layer. The strips are prefabricated and may be composed of two single strips with a narrow longitudinal slot between.
CROSS-REFERENCES TO RELATED
APPLICATIONS
The problems of boundary layers developing at airfoils and boundary layer control means have been described in my copending application Ser. No. 332,619 filed on Dec. 23, 1963, now abandoned, and in the divisional applications divided out of said application, Ser. Nos. 534,732, now Pat No. 3,435,654; 534,733, now Pat. No. 3,410,510; 534,809, now Pat. No. 3,407,539; 534,889, now Pat. No. 3,346,235; 534,906, now Pat. No. 3,319,593; and the continuation-in-part application Ser. No. 615,828, now Pat. No. 3,471,090.
BACKGROUND OF THE INVENTION
The present invention relates to airfoils and profiled bodies which come in direct contact with fluid streams, and more particularly to improvements in means for controlling the boundary layer which develops along the outer surfaces of such airfoils and profiled bodies. The invention also relates to an improved method of producing boundary layer control means for use in aircrafts, gliders, watercrafts and other automotive vehicles and many other types of machines wherein at least some component parts come in direct contact with fluid streams.
The problems which arise as a result of the fact that a thin layer of air or water will develop along the exposed surfaces of fuselages, wings, propeller blades, vanes and similar profiled bodies are well-known to designers of vehicles, blowers and turbines.
These problems, which have long been recognized in the art, are particularly critical in vehicles which are required to attain a very high speed in a fluid stream. This is true of fast-flying aircraft, and most particularly of flying craft which must pass through the earth atmosphere at extremely high speeds, for instance rockets required to attain escape velocity. In flying craft of this type any reduction which can be accomplished at all in
the drag resulting on the craft from the boundary layer developing along the skin of the craft can mean a saving of fuel amounting to hundreds and perhaps thousands of pounds. Evidently, a reduction in the amount of fuel
5 the craft is required to carry will permit an increase in the payload, an important consideration in the present relatively unsophisticated state of extra-terrestrial flight.
It is therefore evident that the elimination of the boundary layer which latter causes this drag, is of prime
10 importance in the industry.
Attempts to utilize porous skins have met with little success because the pores will become clogged after comparatively short period of exposure to a fluid stream and because such pores reduce the smoothness of the skin so
15 that the fluid stream meets greater resistance even if the boundary layer is eliminated. Also, a porous skin is weaker than a nonpermeable skin.
Several methods have already been proposed to make slots into airfoils and profiled bodies for sucking off the
20 boundary layer but highly skilled personnel and special tools and machines are necessary to put these methods into practice.
SUMMARY OF THE INVENTION
25 The present invention overcomes the disadvantages set forth above, and provides the advantages and conditions which have been outlined as desirable.
In particular, the present invention makes it possible to provide an airfoil with boundary layer control means
30 that can easily be manufactured by placing prefabricated parts into recesses of the airfoil.
This is accomplished by making at least one longitudinal recess in the outer surface of the airfoil extending transversely of the direction of said relative movement,
35 then inserting a longitudinal thin metallic strip thus covering said recess and making a channel with a longitudinal narrow open slot whose width increases in a direction inwardly from said outer side and finally fastening said strip within said recess.
40 The inserted strip has a sharp upper edge at its leading flank wtih an angle of less than 60 degrees between outer surface and inwardly directed flank and is arranged within said recess so that between said sharp edge and the opposite metallic part there is a slot which
45 extends transversely of the direction of said relative movement of said airfoil and the adjacent fluid through which the boundary layer may enter the interior of said channel. There are also means to convey the boundary layer from said channel to the outside of the airfoil.
50 In accordance with one feature of my invention said sharp edge is so fixed within said recess that between said sharp edge and an opposite edge a slot of at least .1 mm. but not exceeding .3 mm. width is formed. Owing to this rigid construction the width of the slot is not
55 affected by stresses acting on the airfoil. In a preferred embodiment the sharp edge facing the fluid stream is elevated above the opposite edge, the elevation being about equal to the width of said slot. The sharp edges of the skin at both sides of each slot will actually peel
60 off the boundary layer and will compel it to penetrate through the slots and into the interior. If the outer side of the skin is coated with one or more layers of lacquer, paint or the like, such materials cannot clog the slots because the width of the slots essentially increases in a
65 direction from the inner side of the skin.
In accordance with another feature of my invention the inserted strip consists of two single strips arranged parallel and side by side with narrow slot between said single strip being firmly connected by metallic elements
70 crossing said slots and being fixed to both single strips. The metallic elements are fastened to both single strips on the inner side of said strips thus providing an un
