Biplane Arrangements. In the foregoing data we have assumed that the upper wing was placed directly above the lower, and with the leading edges on the same vertical line as shown by Fig. 3. This is known as an "Orthogonal" biplane, and the gap is indicated by G and the chord by C. In Fig. 4 the forward edge of the top wing is advanced beyond the lower, or is "Staggered," the amount of the stagger being indicated by S. This allows of better view, and slightly increases both the lift and L/D values. With a comparatively large stagger the range of the stalling angle is increased, and the lift does not fall off as rapidly after the maximum is reached as with the orthogonal type. In Fig. 5 the top wing is given a backward stagger, but the exact effects of this arrangement are not generally known. There are few machines using the reversed stagger, the only example, to the writer's knowledge, being the De Havilland speed scout. By staggering, the resistance of the interplane bracing struts (3) is somewhat reduced, because of their inclination with the wind, although they are longer for the same gap than in Fig. 3.
Fig. 6 shows the chord of the lower wing (C’) shorter than the upper chord, a type used in the Nieuport speed scout. In effect, this is a form of stagger, and it undoubtedly widens the view of the pilot, and to some extent increases the efficiency and the range of the stalling angle. Neither the stagger in (4) nor the small lower chord alone improves the stability to any extent. To obtain any marked advantage with the short lower chord, the chord C’ must be very much shorter than the upper chord, say from 0.80C to 0.50C. The loss of area is so great that this would not be permissible on any except the fastest machines, where lift is not a primary consideration. The pilot's view, however, is very much improved with the short lower chord, and in battle this is an important consideration.
Fig. 7 shows the chord of the upper wing inclined at an angle with the lower chord by the amount (d). This is known as "Decalage" and is productive of a great degree of longitudinal stability when taken in combination with stagger. The stability attained by decalage and stagger is without a great loss in the L/D ratio, while the lift and stalling angle range are both increased. This latter stable combination is shown by Fig. 8, in which the wings are given both stagger and decalage.
Slow Speed, Two-Seat Biplane, with a Large Gap-Chord Ratio. The Large Gap Is Permissible in a Slow Machine, as the Strut Resistance Is Less Than the Gain in Lift-Drag Ratio Obtained by the Greater Gap. It Will Be Noted That These Wings Have a Considerable Amount of Stagger. The Position of the Bottom Wing Allows the Observer to See Almost Directly Below.
A High Speed, Two-Seat Fighting Biplane, with a Small Gap-Chord Ratio. In This Case, the Strut Resistance Would Be Greater Than the Aerodynamic Gain of the Wings with a Greater Gap Chord Ratio. The Gunner Is Located in the Rear Seat, and Behind the Trailing Edge of the Lower Wings. He Has a Clear Field to the Rear an Over the Top Wing.
Forward Stagger. Eiffel performed experiments with Dorand wings, and found that when the top surface was staggered forward by 1/2.5 of the chord (0.4C), and with a gap-chord ratio of 0.9, an increase in lift of from 6 to 10 per cent was obtained. The L/D was the same as with no stagger. With thin circular plates, 1/13.5 camber, and a gap-chord ratio = 0.66, the lift-drag was better (than with no stagger) only when the value of Ky was greater than 0.066 (metric). Then the L/D improved progressively with the amount of stagger. Ky was improved by 5 per cent when the stagger was equal to half the chord, and by 10 per cent when the stagger was equal to the chord. The N. P. L. with a Bleriot wing, aspect ratio=4, found that Ky was increased by 5 to 6 per cent with a stagger of 0.4C, and the L/D was increased by about 4 percent. The gap-chord ratio was 1.00.
A Single Seat Biplane Speed Scout with an Air Cooled Motor.