Author: James Slough Zerbe

Chapter VI the Lifting Surfaces of Aeroplanes

THIS subject includes the form, shape and angle of planes, used in flight. It is the direction in which most of the energy has been expended in developing machines, and the true form is still involved in doubt and uncertainty.

RELATIVE SPEED AND ANGLE.—The relative speed and angle, and the camber, or the curved formation of the plane, have been considered in all their aspects, so that the art in this respect has advanced with rapid strides.

NARROW PLATES MOST EFFECTIVE.—It was learned, in the early stages of the development by practical experiments, that a narrow plane, fore and aft, produces a greater lift than a wide one, so that, assuming the plane has 100 square feet of sustaining surface, it is far better to make the shape five feet by twenty than ten by ten.

However, it must be observed, that to use the narrow blade effectively, it must be projected through the air with the long margin forwardly. Its sustaining power per square foot of surface is much less if forced through the air lengthwise.

Experiments have shown why a narrow blade has proportionally a greater lift, and this may be more clearly understood by examining the illustrations which show the movement of planes through the air at appropriate angles.

STREAM LINES ALONG A PLANE.—In Fig. 22, A is a flat plane, which we will assume is 10 feet from the front to the rear margin. For convenience seven stream lines of air are shown, which contact with this inclined surface. The first line 1, after the contact at the forward end, is driven downwardly along the surface, so that it forms what we might term a moving film.

The second air stream 2, strikes the first stream, followed successively by the other streams, 3, 4, and so on, each succeeding stream being compelled to ride over, or along on the preceding mass of cushioned air, the last lines, near the lower end, being, therefore, at such angles, and contacting with such a rapidly-moving column, that it produces but little lift in comparison with the 1st, 2d and 3d stream lines. These stream lines are taken by imagining that the air approaches and contacts with the plane only along the lines indicated in the sketch, although they also in practice are active against every part of the plane.

THE CENTER OF PRESSURE.—In such a plane the center of pressure is near its upper end, probably near the line 3, so that the greater portion of the lift is exerted by that part of the plane above line 3.

AIR LINES ON THE UPPER SIDE OF THE PLANE.— Now, another factor must be considered, namely, the effect produced on the upper side of the plane, over which a rarefied area is formed at certain points, and, in practice, this also produces, or should be utilized to effect a lift.

RAREFIED AREA.—What is called a rarefied area, has reference to a state or condition of the atmosphere which has less than the normal pressure or quantity of air. Thus, the pressure at sea level, is about 14 3/4 per square inch

As we ascend the pressure grows less, and the air is thus rarer, or, there is less of it. This is a condition which is normally found in the atmosphere. Several things tend to make a rarefied condition. One is altitude, to which we have just referred.

Then heat will expand air, making it less dense, or lighter, so that it will move upwardly, to be replaced by a colder body of air. In aeronautics neither of these conditions is of any importance in considering the lifting power of aeroplane surfaces.

RAREFACTION PRODUCED BY MOTION.—The third rarefied condition is produced by motion, and generally the area is very limited when brought about by this means. If, for instance, a plane is held horizontally and allowed to fall toward the earth, it will be retarded by two forces, namely, compression and rarefaction, the former acting on the under side of the plane, and the latter on the upper side.

Of the two rarefaction is the most effectual, and produces a greater effect than compression. This may be proven by compressing air in a long pipe, and noting the difference in gauge pressure between the ends, and then using a suction pump on the same pipe.

When a plane is forced through the air at any angle, a rarefied area is formed on the side which is opposite the one having the positive angle of incidence.

If the plane can be so formed as to make a large and effective area it will add greatly to the value of the sustaining surface.

Unfortunately, the long fiat plane does not lend any aid in this particular, as the stream line flows down along the top, as shown in Fig. 23, without being of any service.

THE CONCAVED PLANE.—These considerations led to the adoption of the concaved plane formation, and for purposes of comparison the diagram, Fig. 24, shows the plane B of the same length and angle as the straight planes.

In examining the successive stream lines it will be found that while the 1st, 2d and 3d lines have a little less angle of impact than the corresponding lines in the straight plane, the last lines, 5, 6 and 7, have much greater angles, so that only line 4 strikes the plane at the same angle.

Such a plane structure would, therefore, have its center of pressure somewhere between the lines 3 and 4, and the lift being thus, practically, uniform over the surface, would be more effective.

THE CENTER OF PRESSURE.—This is a term used to indicate the place on the plane where the air acts with the greatest force. It has reference to a point between the front and rear margins only of the plane.

UTILIZING THE RAREFIED AREA.—This structure, however, has another important advantage, as it utilizes the rarefied area which is produced, and which may be understood by reference to Fig. 25.

The plane B, with its upward curve, and at the same angle as the straight plane, has its lower end so curved, with relation to the forward movement, that the air, in rushing past the upper end, cannot follow the curve rapidly enough to maintain the same density along C, hence this exerts

an upward pull, due to the rarefied area, which serves as a lifting force, as well as the compressed mass beneath the plane.

CHANGING CENTER OF PRESSURE.—The center of pressure is not constant. It changes with the angle of the plane, but the range is considerably less on a concave surface than on a flat plane.

In a plane disposed at a small angle, A, as in Fig. 26, the center of pressure is nearer the forward end of the plane than with a greater positive angle of incidence, as in Fig. 27, and when the plane is in a normal flying angle, it is at the center, or at a point midway between the margins.

PLANE MONSTROSITIES.—Growing out of the idea that the wing in nature must be faithfully copied, it is believed by many that a plane with a pronounced thickness at its forward margin is one of the secrets of bird flight.

Accordingly certain inventors have designed types of wings which are shown in Figs. 28 and 29.

Both of these types have pronounced bulges, designed to "split" the air, forgetting, apparently, that in other parts of the machine every effort is made to prevent head resistance.

THE BIRD WING STRUCTURE.—The advocates of such construction maintain that the forward edge of the plane must forcibly drive the air column apart, because the bird wing is so made, and that while it may not appear exactly logical, still there is something about it which seems to do the work, and for that reason it is largely adopted.

WHY THE BIRD’S WING HAS A PRONOUNCED BULGE.—Let us examine this claim. The bone which supports the entire wing surface, called the (pectoral), has a heavy duty to perform. It is so constructed that it must withstand an extraordinary torsional strain, being located at the forward portion of the wing surface. Torsion has reference to a twisting motion.

In some cases, as in the bat, this primary bone has an attachment to the rear of the main joint, where the rear margin of the wing is attached to the leg of the animal, thus giving it a support and the main bone is, therefore, relieved of this torsional stress.

THE BAT’S WING.—An examination of the bat’s wing shows that the pectoral bone is very small and thin, thus proving that when the entire wing support is thrown upon the primary bone it must be large enough to enable it to carry out its functions. It is certainly not so made because it is a necessary shape which best adapts it for flying.

If such were the case then nature erred in the case of the bat, and it made a mistake in the housefly’s wing which has no such anterior enlargement to assist (?) it in flying.

AN ABNORMAL SHAPE.—Another illustration is shown in Fig. 30, which has a deep concave directly behind the forward margin, as at A, so that when the plane is at an angle of about 22 degrees, a horizontal line, as B, passing back from the nose, touches the incurved surface of the plane at a point about one-third of its measurement back across the plane.

This form is an exact copy of the wing of an actual bird, but it belongs, not to the soaring, but to the class which depends on flapping wings, and as such it cannot be understood why it should be used for soaring machines, as all aeroplanes are.

The foregoing instances of construction are cited to show how wildly the imagination will roam when it follows wrong ideals.

THE TAIL AS A MONITOR.—The tendency of the center of pressure to change necessitates a correctional means, which is supplied in the tail of the machine, just as the tail of a kite serves to hold it at a correct angle with respect to the wind and the pull of the supporting string.


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Chicago: James Slough Zerbe, "Chapter VI the Lifting Surfaces of Aeroplanes," Aeroplanes, ed. Smiles, Samuel, 1812-1904 in Aeroplanes Original Sources, accessed July 23, 2024,

MLA: Zerbe, James Slough. "Chapter VI the Lifting Surfaces of Aeroplanes." Aeroplanes, edited by Smiles, Samuel, 1812-1904, in Aeroplanes, Original Sources. 23 Jul. 2024.

Harvard: Zerbe, JS, 'Chapter VI the Lifting Surfaces of Aeroplanes' in Aeroplanes, ed. . cited in , Aeroplanes. Original Sources, retrieved 23 July 2024, from