US2257260A - Airfoil - Google Patents

Description

Sept. 30, 1941. 2,257,260

A. KARTVELICHVILI, NOW BY JUDICIAL CHANGE OF NAME A. KARTVELI AIRFOIL Filed Aug. 27, 1958 2 sheets-Sheet 1- ATTORN EY.

Patented Sept. 30, 1941 AIRFOIL Alexander Kartvelichvili, North Hempstead,

N. Y., now by Judicial change of name Alexander Kartveli, assignor to Seversky Aircraft Corporation, a corporation of Delaware Application August 27, 1938, Serial No. 227,099

4 Claims.

'Ihis invention relates to airfoils. and its object is to provide an airfoil having an improved prole i hood of the burble-point will be smoother, than u that of the'ordinary airfoil.

A further particular object of the invention is to provide an airfoil which will havea greater operative range of angles of attack from zero lift to maximum lift than the ordinary airfoil, yet which will nonetheless have very little travel of the center of pressure. Y

The other objects and advantages of the invention will become apparent as this disclosure progresses.

The presently-preferred embodiment of these and other inventive concepts is illustrated in the accompanying drawings and expounded hereinafter, but it is to be understood that the invention is limited in its embodiments only by the scope of the sub-joined claims. In the said drawings,

Figure 1 is a composite diagram showing a typical subsistent-airfoil-section and the mode of laying it out;

Figure 2 is a diagram for use in showinghow the present airfoil-section is derived, and

Figure 3 is a scale drawing of the profile of the airfoil derived by the present invention.

To clearly differentiate the present concept from all prior concepts in its category, it is necessary to set forth the prevalent method by which ordinary airfoils are laid out. Referring to Figure 1, a median line, AabB, is arbitrarily or empirically chosen. This line has its chord, L, equal to that desired of the final airfoil. A symmetrical section, CcDdC, called the thickness-distribution curve, is then arbitrarily or empirically selected,

ni and nz will completely define the airfoil; thesev points are located by moving point M along the straight line CD and repeating the previous construction-procedure, giving the curve ApBqA.

For an airfoil thus generated, the aerodynamic characteristics will depend upon the shape of the median line and the shape of the thicknessdistribution curve. With a givenmedian line, these characteristics will depend solely upon the conguration of the thickness-distribution curve. For an ordinary airfoil, this thickness-distribution curve is arbitrarily chosen.

In the present invention, the thickness-dis,- tribution curve used is an ideal mathematical streamline for which an analytical expression can be given. According to this invention, the thiclmess-distribution curve is a plane closed contourfthe rectangular coordinates, X and Y, of which, have the following analytical relations in terms of a parameter, 0:

Where a, S and i. are constant quantities for a given airfoil.

Referring to Figure 2, the contour CcDdC is the line determined by Equations 1. It can be seen that the X axis is directed along the line of symmetry CD, the Y axis is perpendicular to "it and the origin O is inside the segment CD.

and it is hence by no means .an ideally stream- When 0 is varied from zero to 1r, the 'point dev The quantities is and L are chosen differently for every usage to which the airfoil is to -be adapted, and when thus known, the relationships 1 and 2 permit the accurate construction of the present thickness-distribution curve, Equations 2 giving immediately arithmetical values for a, S

and A; then, replacing the values a, S and i in system 1, we obtain or, in other words, we obtain the two coordinates of a point on the thickness-distribution curve in terms of-a parameter 0. y

The thickness-distribution curve thus obtained, from which theA present novel airfoil is derived, differs radically from the ordinary in that its entering edge section is substantially' sharp, and in that its trailing edge section is of a Vnovel feathering shape; that is to say, at the rear,

' ythe upper and lower branches of it come tangent to the axis of symmetry, thereat, thus forming an angle of zero therewith, whereas in the ordinary thickness-distribution curve, its upper and lower branches form an angle differing greatly from zero degrees.

The outstanding advantage of the present thickness-distribution curve is that an infinite cylinder having a cross-section of this shape, if

placed in a stream of incompressible and frictionless fluid, will produce a mathematically perfect streamline flow in which the drag will be z'ero. Experiments have shown that even in the. case of a compressible and viscous fluid the`ow around the cylinder of the present cross-section will approach a mathematically perfect streamline flow far more closely than in the case of the arbitrarily or empirically chosen distributioncurves universally employed heretofore, in determiningl ordinary airfoil-sections.

After this novel shape of the present thicknessdistribution-curve has been thus determined, a median line I-I-H of the desired'iinal airfoil section, and which may be ldetermined arbitrarily, empirically, or analytically, is laid down, and the final shape of the airfoil-section is obtained from `the novel thickness-distribution-curve and enfolded around this line in the manner previously described, thus adapting the novel method of the present invention to median lines of any desired kind. When the thicknessfdistribution curve is transferred onto the skeleton curve, such as H-H, all the essential and beneficial geometrical andaerodynamic characteristics of the present novel thickness-distribution curve will reappear in the nal airfoil; the leading edge will retain the novel shape, with a downward reflex, and the trailing edge surfaces will be curved, downwardly and upwardly, respectively, bitangent to the rear edge of the skeleton curve.

It is to be understood that by variously chang--l ing the shape of the skeleton curve I-I-H and the thickness ratio k, an infinite number of section and by using the thickness-distribution curve of this invention combined with an ordinary skeleton curve.

It is to be observed that the airfoil, in prole outline, includes a sharp, downwardly flexed entering edge. It has been found that the radius of the leading edge of any airfoil constructed byl e ness'of the, lift curye.

the method conforming to this invention ca be expressed by the formula:

v rr=.69,1l' where t is the maximum thickness of the airfoil and L is the chord. The entering edge is continued by a central and aft-central porition having, on the top and on the bottom, a shape approaching that of a parabola. The contour of the airfoil is completed by a doubly re-curved feathering trailing edge portion osculatory to the central portion and tangent to the median line at the trailing edge at zero degrees.

Wind tunnel vtests run on airfoils conforming to the invention have conclusively demonstrated their superiority to an ordinary airfoil in regard to maximum lift, minimum drag and the smooth- By simply varying the various pertinent factors in the equations above, theairfoil may be adapted over a Wide range of uses, from high speed airplanes with theminimum compatible lift, to low speed, high lift airplanes.

In employing the method of the present invention in constructing an airfoil, there can be rst built a basal symmetric thickness-distribution section, made--without bracing, etc.of a recurved, bent strip of material and having abscissae and ordinates of lengths expressed by the mathematical equations given hereinabove, and having its chordwise length equal to the chord of the desired ultimate airfoil; and having any desired spanwise length; vthen a structural member, such as a bent strip of material, can be built and laid down which will act as the median member of the ultimate airfoil; on this member can bearranged land attached, successive members, such as strips of metal, perpendicular thereto,

the respective lengths of said members respecy tively corresponding to the respective ordinates of the thickness-member; then by laying or wrapping sheet-material continuously about the previously-formed-skeleton in such manner as to connect the ends of the perpendicular-members, the airfoil can be completed.

Having now made certain the nature and purposes of my invention, and at least one manner of executing same, according to the statutes, wh-at I claim as my property and desire to secure by Lettersr Patent of the United States, is:

1. An airfoil-section having an essentially sharp entering edge; bi-convex forward, central and ait-central portions, and a bi-concave trailing edge portion.

2. An alrfoil-section having an essentially ysharp entering edge; bi-convex forward, central` and' aft-central portions, and a bi-concave trailing-edge portion, the upper and lower branches

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