Aeroplane Construction and Operation

by John B. Rathbun

Fig. 1. shows a typical wing section with the names of the different parts and the methods of dimensioning the curves. All measurements to the top and bottom surfaces are taken from the straight "chordal line" or "datum line" marked X-X. This line is drawn across the concave undersurface in such a way as to touch the surface only at two points, one at the front and one at the rear of the wing section. The inclination of the wing with the direction of flight is always given as the angle made by the line X-X with the wind. Thus, if a certain wing is said to have an angle of incidence equal to 4 degrees, we know that the chordal line X-X makes an angle of 4 degrees with the direction of travel. This angle is generally designated by the letter (i), and is also known as the "angle of attack." The distance from the extreme front to the extreme rear edge (width of wing) is called the "chord width" or more commonly "the chord."

In measuring the curve, the datum line X-X is divided into a number of equal parts, usually 10, and the lines 1-2-3-4-5-6-7-8-9-10-11 are drawn perpendicular to X-X. Each of the vertical numbered lines is called a "station," the line No. 3 being called "Station 3," and so on. The vertical distance measured from X-X to either of the curves along one of the station lines is known as the "ordinate" of the curve at that point. Thus, if we know the ordinates at each station, it is a simple matter to draw the straight line X-X, divide it into 10 parts, and then lay off the heights of the ordinates at the various stations. The distances from datum to the upper curve are known as the "Upper ordinates," while the same measurements to the under surface are known as the "Lower ordinates." This method allows us to quickly draw any wing section from a table that gives the upper and lower ordinates at the different stations.

A common method of expressing the value of the depth of a wing section in terms of the chord width is to give the "Camber," which is numerically the result obtained by dividing the depth of the wing curve at any point by the width of the chord. Usually the camber given for a wing is taken to be the maximum camber; that is, the camber taken at the point of greatest depth. Thus, if we hear that a certain wing has a camber of 0.089, we take it for granted that this is the camber at the deepest portion of the wing. The correct method would be to give 0.089 as the "maximum camber" in order to avoid confusion. To obtain the maximum camber, divide the maximum ordinate by the chord.

Fig. 1. Section Through a Typical Aerofoil or Wing, the Parts and Measurements Being Marked on the Section. The Horizontal Width or "Chord" Is Divided Into 10 Equal Parts or "Stations," and the Height of the Top and Bottom Curves Are Measured from the Chordal Line X-X at Each Station. The Vertical Distance from the Chordal Line Is the "Ordinate" at the Point of Measurement.

Example. The maximum ordinate of a certain wing is 5 inches, and the chord is 40 inches. What is the maximum camber? The maximum camber is 5/40 = 0.125. In other words, the maximum depth of this wing is 12.5 per cent of the chord, and unless otherwise specified, is taken as being the camber of the top surface.

The maximum camber of a modern wing is generally in the neighborhood of 0.08, although there are several Successful sections that are well below this figure. Unfortunately, the camber is not a direct index to the value of a wing, either in regard to lifting ability or efficiency. By knowing the camber of a wing we cannot directly calculate the lift or drag, for there are several examples of wings having widely different cambers that give practically the same lift and drift. At the present time, we can only determine the characteristics of a wing by experiment, either on a full size wing or on a scale model.

In the best wing sections, the greatest thickness and camber occurs at a point about 0.3 of the chord from the front edge, this edge being much more blunt and abrupt than the portions near the trailing edge. An efficient wing tapers very gradually from the point of maximum camber towards the rear. This is usually a source of difficulty from a structural standpoint since it is difficult to get an efficient depth of wing beam at a point near the trailing edge. A number of experiments performed by the National Physical Laboratory show that the position of maximum ordinate or camber should be located at 33.2 per cent from the leading edge. This location gives the greatest lift per square foot, and also the least resistance for the weight lifted. Placing the maximum ordinate further forward is worse than placing it to the rear.

Thickening the entering edge causes a proportionate loss in efficiency. Thickening the rear edge also decreases the efficiency but does not affect the weight lifting value to any great extent. The camber of the under surface seems to have but little effect on the efficiency, but the lift increases slightly with an increase in the camber of the lower surface. Increasing the camber of the lower surface decreases the thickness of the wing and hence decreases the strength of the supporting members, particularly at points near the trailing edge. The increase of lift due to increasing the under camber is so slight as to be hardly worth the sacrifice in strength. Variations in the camber of the upper surface are of much greater importance. It is on this surface that the greater part of the lift takes places, hence a change in the depth of this curve, or in its outline, will cause wider variations in the characteristics of the wing than would be the case with the under surface. Increasing the upper camber by about 60 per cent may double the lift of the upper surface, but the relation of the lift to the drag is increased. From this, it will be seen that direct calculations from the outline would be most difficult, and in fact a practical impossibility at the present time.