Aeroplane Construction and Operation

by John B. Rathbun

Center of Pressure Movement. As in the case of the flat plate the center of pressure on an aerofoil surface varies with the angle of incidence, but unlike the plate the center of pressure (C. P.) moves backward with a decrease in angle. The rapidity of travel depends upon the form of aerofoil, in some types the movement is very great with a small change in the angle, while in others the movement is almost negligible through a wide range. In general, aerofoils are inherently unstable, since the C. P. moves toward the trailing edge with decreased angles, and tends to aggravate a deficiency in the angle. If the angle is too small, the backward movement tends to make it still smaller, and with an increasing angle the forward movement of the center of pressure tends to make the angle still greater.

Fig. 4 is a diagram showing the center of pressure movement for a typical aerofoil with the aerofoil at the top of the chart. The left side of the chart represents the leading edge of the aerofoil and the right side is the trailing edge, while the movement in percentages of the chord length is shown by the figures along the lower line. Thus figure ".3" indicates that the center of pressure is located 0.3 of the chord from the leading edge. In practice it is usual to measure the distance of the C. P. from the leading edge in this way.

Fig. 4. Chart Giving Relation Between Incidence and C.P. Movement.

For an example in the use of the chart, let us find the location of the C. P. at angles of 0°, 3° and 7°. Starting with the column of degrees at the left hand edge of the chart, find 0°, and follow along the dotted line to the right until the curve is reached. From this point follow down to the lower row of figures. It will be found that at 0° the C. P. lies about half way between 0.5 and 0.6, or more exactly at 0.55 of the chord from the leading edge. Similarly at 3° the C. P. is at 0.37 of the chord, and at 7° is at 0.3 of the chord. From 11° to 19°, the C. P. for this particular aerofoil moves very little, remaining almost constant at 0.25 of the chord. Reducing the angle from 3° causes the C. P. to retreat very rapidly to the rear, so that at –1° the C. P. is at 0.8 of the chord, or very near the trailing edge of the wing.

Other Forms of Charts. The arrangement of wing performance charts differs among the various investigators. Some charts show the lift, drag, lift-drag ratio, angle of incidence, center of pressure movement, and resultant pressure on a single curve. This is very convenient for the experienced engineer, but is somewhat complicated for the beginner. Whatever the form of chart, there should be an outline drawing of the aerofoil described in the chart.

Fig. 5 shows a chart of the "Polar" variety in which four of the factors are shown by a single curve. This type was originated by Eiffel and is generally excellent, except that the changes at small angles are not shown very clearly or sharply. The curve illustrates the properties of the "Kauffman" wing, or better known as the "Eiffel No. 37." A more complete description of this aerofoil will be found under the chapter "Practical Wing Sections." A single curve is marked at different points with the angle of incidence (0° to 12°). The column at the left gives the lift-coefficient Ky, while the row at the bottom of the sheet gives the drag-coefficients Kx. At the top of the chart are the lift-drag ratios, each figure being at the end of a diagonal line. In this way the lift, drag, liftdrag and angle of incidence are had from a single curve.

Take the characteristics at an angle of 10 degrees for example. Find the angle of 10° on the curve, and follow horizontally to the left for Ky. The lift-coefficient will be found to be 0.0026 in terms of miles per hour and pounds per square foot. Following down from 10°, it will be found that the drag-coefficient Kx = 0.00036. Note the diagonal lines, and that the 10° point lies nearest to the diagonal headed 7 at the top of the chart. (It is more nearly a lift-drag ratio of 7.33 than 7.) In the same way it will be found that an angle of 8 degrees lies almost exactly on the lift-drag diagonal marked 9. The best lift-drag is reached at about 2 degrees at which point it is shown as 17.0. The best lift-coefficient Ky is 0.00276 at 12 degrees.

Fig. 5. Polar Type Chart Originated By Eiffel.