Fig. 12. Tandem Arrangements Used in Eiffel Experiments. (1) Chords in Straight Line, (2) Rear Wing at 2.5°, (3) Rear Wing at 5°.
Tandem Arrangement. In tandem wings as shown by Fig. 9, the downward wash of the front wing (m) will affect the rear wing (n) by causing a change in the relative direction of the flow. If the front wings are at an incidence of (i) degrees (Fig. 11), the deviation or washdown of the air stream to the rear will be expressed by: d = (0.5i + 1). If i = incidence of rear wing measured from the chord of the front wing, then the angle of incidence made by the rear chord to the horizontal will be: I = (i-i’) - (0.5i + 1), where I is the incidence of the rear plane with the horizontal.
Experiments by Eiffel on tandem planes with circular cambered aerofoils gave exceedingly good results for certain combinations (Fig. 12). These arrangements were used, (1) Chords in a straight line (2), Rear aerofoil tilted down at a negative angle of 2.5°, (3), Rear Plane tilted down at a negative angle of 5°. In all cases the camber was 1/13.5 of the chord, and the front and rear wings were spaced two chord widths apart. While the drag did not change much for any of the arrangements, the lifts varied widely, and arrangement (2) is by far the more efficient in lifting capacity. No. 2 is 50 per cent greater than (1), and has twice the lifting ability of (3). For the same angle of incidence, the front wing does the same amount of lifting in all cases, the difference being entirely due to the changes in the rear surface. In (2) the lift of the rear aerofoil is actually 13 per cent greater than the front plane. The following tables give the results:
| ANGLE OF INCIDENCE | ARRANGEMENT (1) | ARRANGEMENT (2) | ARRANGEMENT (3) |
|---|---|---|---|
| 3 | 665 | 1094 | 334 |
| 6 | 987 | 1568 | 703 |
| 9 | 1315 | 2068 | 965 |
| 12 | 1540 | 2326 | 1347 |
| Average at all angles | 1127 | 1764 | 837 |
| Percentage | 0.64 | 1.00 | 0.47 |
Fig. 13. Drzewiecki Tandem Arrangement for Longitudinal Stability.
The lifts in the above table are for the two planes working together, and the angle of incidence is the angle of the front aerofoil, or rather the angle of the combination. The wings were 15 x 90 centimeters, aspect ratio = 6. Fig. 12 shows the construction clearly. This is only true for circular arched surfaces of the camber given.
Fig. 14. Drzwiecki Tandem Wing Arrangement for Stability.
M. Drzewiecki working with Eiffel's results on the above combinations, produced an inherently stable tandem monoplane, in which the front and rear wings were of different cambers and were set at different incidences. The front wing is Eiffel No. 8 set normally at 8° incidence, and the rear wing is Eiffel No. 13-bis (Bleriot 11-bis), set normally at 5°. The center of gravity is approximately half-way between the two wings, and the front is smaller than the trailing surface. Because of the difference in area, the lift of the front wing varies less rapidly than the rear when the angle of the machine changes because of disturbed air. Should the machine "head up," the rear wing increases faster in lift than the front, and hence restores the machine to a horizontal position. Should the front surface drop, the incidence is reduced, but as incidence of the rear wing is less than the front (8°), the rear wing is reduced to nearly a zero angle of incidence—(With little lift). The front wing is still inclined at a considerable incidence: (3°) when the rear is at zero. This drops the rear, and raises the front wing so that the normal attitude is restored. Lateral stability is obtained by moving the two halves of the front wing in relation to one another, the relative movement being similar to that of ailerons.