*Biplane Characteristics*. From an aerodynamic standpoint, the monoplane
wing is more efficient than the superposed wings of the biplane type,
since the proximity of the two surfaces in the latter causes a decided
loss in the total lift. Other practical advantages, however, offset the
losses due to the superposed surfaces, and hence the total efficiency of
the complete biplane may be even greater than that of the monoplane. For
the same area the structural parts of the biplane are lighter, and this
advantage increases rapidly with the size of the machine so that when a
span of 36 feet is exceeded, any other arrangement than that of the
biplane or triplane becomes almost a practical impossibility. A biplane
is easier and cheaper to make than a monoplane, since the wing bracing
of the former can be arranged to better advantage, the load-bearing
members can be simpler, and the safety factor made higher for an equal
weight. By suitable adjustments between the wings of a biplane, it is
possible to obtain a very high degree of inherent longitudinal stability
without incurring much loss in efficiency, an arrangement that is of
course impossible with a single monoplane surface. By "staggering," the
view of the pilot is increased, and the generally smaller size of the
machine permits of better maneuvering qualities for a given load.
*Interference*. Due to "interference," or to the choking of the air
stream between the upper and lower surfaces, the lift of both wings is
reduced, with the drag remaining about the same as with a single
surface. This, of course, reduces the total lift-drag ratio at all
except certain angles. The relative lift-drag ratios of the monoplane
and biplane depend to some extent upon the form of the wing.
Interference causes a loss on the opposing faces of the wings, the lift
being reduced on the top surface of the lower wing, and on the bottom
surface of the top wing. Since the upper surface of the lower wing is
under suction, and therefore produces the greater proportion of lift, it
is natural that the lower wing lift should be reduced to a greater
extent than in the upper wing, since it is only the lower surface of the
latter that is affected. At normal flight angles the upper wing carries
about 55 per cent of the total load. At zero degrees incidence, the
upper wing carries as high as 62 per cent of the total load, while at 12
degrees this may be reduced to 54 per cent.
*Gap-Chord Ratio*. Calling the distance between the upper and lower
wings the "gap," it may be said that the ratio of the gap to the wing
chord greatly influences the lift. This ratio is called the "gap-chord
ratio," and may vary from 0.8 to 1.0 in small machines or 1.0 to 1.2 in
slow, heavy aeroplanes. With the drag remaining practically constant,
the lift-drag is of course affected by a change in the gap-chord ratio,
this quantity being diminished at small gap ratios. Compared with a
monoplane, the lift of a biplane is about 0.77 when the gap is 0.8 of
the chord, and about 0.89 of the monoplane value when the gap-chord
ratio is increased to 1.6. In this range the lift-drag approximates 0.82
and 0.89, respectively. The center of pressure movement is not greatly
changed with any gap-chord ratio, and to all practical purposes remains
the same as with the monoplane. It should be understood that these
remarks apply only to the "Orthogonal" biplane arrangement in which the
wings are vertically over one another.
While biplane efficiency is increased by having a large gap-chord ratio
(wing efficiency alone), the total efficiency of the aeroplane is not
always increased by a large gap, principally because of the great head
resistance due to the longer struts and interplane bracing. At high
speeds the longer bracing members often more than offset the gain due to
wing efficiency, and as a result the gap of high speed scouts will
generally be found in the neighborhood of 0.8 the chord. With slow,
heavy machines, where lift is of great importance, and where slow speed
does not affect the structural resistance to so great an extent, the
gap-chord ratio will range from 1.0 to 1.2.
In making the above comparisons between monoplanes and biplanes, equal
aspect ratios have been assumed for both types, but in actual practice
the aspect ratio of biplanes is always greater than with monoplanes, and
as a result the biplane loss is usually less than indicated above. When
correction has been made for the aspect ratio, the disparity in the
monoplane and biplane values of Ky and L/D is not as great as commonly
supposed. "Biplane reduction factors," or the factors used in reducing
monoplane values to those of the biplane, depend to a great extent upon
the wing section as well as upon the gap, and for exact values of the
factors we should have the tests report of the wings in biplane form.
Lacking this information, we can adopt the values obtained by the N. P.
L. for an old type of wing in order to get approximate results. To
obtain the biplane values, multiply the monoplane values obtained by the
wind tunnel test by the factors found under the required gap-chord
ratio. These factors apply to an aspect ratio of 6.
BIPLANE REDUCTION FACTORS (N. P. L.) (At Normal Flight Angles)
───────────────────────────────────────────────────────────────────
Gap-Chord Ratio. 0.8 1.0 1.2 1.6
───────────────────────────────────────────────────────────────────
Ky Reduction Factor 0.77 0.82 0.86 0.89
───────────────────────────────────────────────────────────────────
L/D Reduction Factor 0.82 0.84 0.85 0.89
───────────────────────────────────────────────────────────────────
Dr. Hunsaker conducted experiments at the Massachusetts Institute of
Technology on biplane and triplane combinations, and the results were
reported in "Aviation and Aeronautical Engineering," Nov. 1, 1916. The
R.A.F.-6 section was used with a gap-chord ratio of 1.2. The biplane
portions of the experiments are as follows, the actual Ky and L/D values
and reduction factors being arranged according to the angle of
incidence:
It will be noted that there is steady improvement in the lift factor
with an increase in the angle from 2° up (except at 8°), and that the
same holds true with the L/D factor. That is, the biplane values become
nearly monoplane values at high angles, and in the case of the L/D ratio
the biplane actually is 24 per cent greater than the monoplane value at
an angle of 16°. The lift coefficient Ky above, is not far from the
corresponding Ky, for gap-chord ratio = 1.2 in the first table. The
maximum biplane value of L/D occurs at the same point as in the
monoplane wing, that is, at 4°. The fact that the lift-drag is so high
at 16° is very favorable, since the biplane would be less likely to
stall when flying slowly, and with a big demand on the engine. The range
of angles at the stalling angle is much greater than with the monoplane
wing, and the lift does not fall off so rapidly after the maximum is
reached.
*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.
[Illustration: 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.]
[Illustration: 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.
[Illustration: A Single Seat Biplane Speed Scout with an Air Cooled
Motor.]
In a series of tests made by A. Tcherschersky, the backward stagger as
in Fig. 5 gave about 15 per cent greater lift than the orthogonal
biplane, or about 4 per cent less lift than a monoplane surface of the
same area. The stagger in this experiment was about 0.33C. In default of
more accurate information, it would seem that backward stagger would
give better results than forward stagger, since the air swept down by
the upper surface would pass further to the rear of the lower plane and
hence would not so greatly affect the vacuum on the upper surface of the
lower wing. This would, however, destroy the view of the pilot to a
greater extent than any of the other arrangements.
Stagger always introduces structural difficulties, makes the wings
difficult to assemble, and the wires are of varying lengths. A simple
orthogonal cell is more compact and better from a manufacturing
standpoint, as it simplifies the fittings, and to a slight extent
decreases the weight. When combined with sweep back, the complication is
particularly in evidence. It is pleasing to note the prevalence of
orthogonal cells on modern battle-planes.
*Influence of Camber*. The amount of air swept down by the upper wing is
largely determined by the curvature of the under surface of the upper
wing. By decreasing, or flattening out the curvature of this surface,
the velocity is increased in a horizontal direction and reduced in a
vertical direction, so that the lower wing is less affected. The upper
surface of the upper wing is not influenced by interference. It should
be noted at this point that air in striking a convex surface is
increased in horizontal speed while the reverse is true of the lower
concave surface. If the under surface of the upper wing were made
convex, the down trend of the air would be still further reduced, and
the loss on the lower wing reduced in proportion.
Increasing the camber on the upper surface of the lower wing increases
its horizontal velocity and hence affects the upper wing to a less
extent, but as the upper wing loss is comparatively slight, the camber
increase below is not of great consequence. This has only been tried in
one machine to the writer’s knowledge, one of the Standard seaplanes, in
which the upper wing was an R.A.F.-6 and the lower wing was a deeply
cambered U.S.A.-2 section. The lower surface of the R.A.F.-6 is
comparatively flat.
*Effects of Decalage*. When the upper wing incidence is increased in
regard to that of the lower wing, or is given decalage, the stability is
increased with a slight increase in the power or drag. This angle shown
by (d) in Figs. 7 and 8, must be accompanied by stagger to obtain
stability, the angle (d) ranging from 1° to 4°. With a decalage of 2.5°,
and a stagger of half the chord, a high degree of stability is attained
with a loss in the lift-drag of from 4 to 6 percent. The lift and the
range of the stalling angle are both increased, the former by about 3
percent, while the latter is nearly double. By increasing the decalage
to 4°, the lift-drag is still 4 percent less than with the orthogonal
cell, but the range of the stalling angle is nearly tripled. The 4°
decalage is very stable and is suitable for training machines or for
amateurs. In either case, the stagger-decalage system is usually better
than sweep back, reflex curves or negative wing tips.
Without regard to the stability, and only with the idea of a greater L/D
in mind, it has been usual in several European machines to adopt a
"negative" decalage; that is, to increase the angle of the lower wing in
regard to the upper chord. With the top chord horizontal, a negative
decalage of 4° would make the incidence of the lower wing equal to 4°.
This has not been generally found advantageous in model tests, but in
full size machines there is a considerable increase in the L/D ratio.
The greater incidence of the lower wing also improves the lift of this
surface and thus requires less surface for obtaining the same total
lift, especially when top wing is staggered forward. Incidence of top
wing of Nieuport = 1°-30’. Lower wing is set at 3°.
*Varying Incidence*. With several types of European speed scouts, and in
the case of the old Handley-Page monoplane, the angle of incidence is
reduced from the center of the wing to the tip. Thus in one speed scout,
the incidence at the body is 4°, and 2° at the tips. A decrease in angle
toward the tips has much the same effect as an increase in aspect ratio;
that is, it decreases the lateral flow and end leakage. It also has an
effect in aiding the lateral stability because there is less lift at the
tips, and hence they are less affected by side gusts. "Washed out"
incidence is an aid to longitudinal stability, as the center of pressure
at the tips is moved further back than at the center of the wing, and
therefore the C. P. is distributed over a longer distance fore and aft
than it would be with a uniform angle of incidence.
In driving the propeller, the motor tends to turn the body in a
direction opposite to that of the propeller rotation, and if no other
provision is made this must be overcome by means of the ailerons. The
"Motor torque" on small span machines is particularly difficult to
overcome in this way, owing to the short lever arm length of the
ailerons. To practically overcome the torque, without excessively
loading the ailerons, it is usually the practice to set the lower left
wing tip at a greater angle than the lower right wing. The greater angle
at the left gives a lift that opposes the turning moment of the motor.
This compensation can never be complete, for the motor torque varies
with the motor output, hence an average angle is selected so that the
incidence will cover the usual horizontal flight speeds.
*Triplane Arrangement*. When a biplane exceeds a certain weight the area
required for a given landing speed makes it desirable to increase the
number of lifting surfaces to more than two, if the span and stress are
to be kept down within reasonable limits. Thus the biplane has its
limits as well as the monoplane, and in the biplane this limit is
generally reached when the span approaches 80 feet. In addition to the
increased weight due to spans of over 80 feet, there are other troubles
in regard to the space required for housing, and awkwardness in
maneuvering. On the smaller and faster aeroplanes, the triplane
arrangement permits of space condensation, and also allows of larger
aspect ratios than with the biplane. The greater depth of the triplane
structure makes the interplane bracing even more effective than in the
case of the biplane. For equal spans there is less bracing exposed to
the wind, and the weight of the wing spars and ribs can be considerably
reduced. The shorter ribs of the triplane alone contribute in no small
degree to the saving in weight.
Considering the wings alone, without reference to the head resistance of
the bracing, etc., there is a greater loss of lift and L/D when three
tiers of wings are superposed than with a biplane. In experiments by Dr.
Hunsaker upon R.A.F.-6 and Curtiss wing sections, it was found that at
about 4°, that the triplane required about 6 percent more power than the
corresponding biplane. At this angle, the L/D for the triplane was 12.8,
against the ratio of 13.8 for the biplane. The gap-chord ratio in each
case was maintained at 1.2. Both the R.A.F.-6 and the Curtiss wings gave
results of the same general character, and there was not a great deal of
difference in the numerical values. At very high angles, 12° to 16°, the
lift of the biplane and triplane only differed by about 2 percent, but
at very small angles such as are used at normal flight speeds, the
reduction of lift in the triplane was very marked.
The drag was not greatly different below 12°, but at 16° the
drag-coefficient is less than that of either the biplane or monoplane,
and for machines flying at low speeds, or heavily loaded, this decrease
is of great advantage since it relieves the motor at a time when power
is particularly required. At this point it should be noted that at high
angles, the L/D generally is better for multiplanes in an almost direct
proportion to the number of surfaces. In this experiment, the lift-drag
ratios for a monoplane, biplane, and triplane were respectively 4.5,
5.6, and 6.5. The drop in lift after the point of maximum lift, or the
stalling angle, is not as rapid as in the case of the biplane or
monoplane, and hence there is less danger of stalling the triplane. With
the same area, and loading, the landing speed of the biplane and
triplane will be about the same.
The following tables give the lift, and lift-drag ratios as determined
in these experiments, the factors being in terms of the monoplane values
of an R.A.F.-6 wing. Thus to obtain triplane values, multiply the given
monoplane values by that number opposite the required angle of
incidence. Aspect ratio = 6.
[Illustration: Curtiss Triplane Speed Scout. Note the Great Aspect Ratio
of the Wings, and the Relatively Great Gap-Chord. Ratio. Only One Set of
Struts Are Used in a Single Row, Hence the Head Resistance Is at a
Minimum. The Span Is 25’-0" and the Chord 2’-0", Giving an Aspect Ratio
of 12.5.]
Thus, if the monoplane lift value for the R.A.F.-6 wing at 4° is Ky =
0.001.45, then the triplane value will be 0.00145 + 0.757 = 0.001097 as
given in the table. The monoplane lift-coefficient of any other wing
section can be handled in the same way with fair accuracy. To obtain the
corrected lift-drag ratio for any wing section, multiply the lift-drag
of the monoplane wing by the factor in the above table corresponding to
the incidence of the monoplane test wing.
[Illustration: The Italian Caproni Triplane of the Heavy Lift or Bombing
Type. Motors Are Installed in Each of the Three Bodies, Tractor
Propellers Being Used in the Two Long Outer Bodies, While a Pusher Screw
Is Used at the Rear of the Central Passenger Body. The Enormous Size of
This Triplane Can Be Seen by Comparing it with the Caproni Monoplane
Shown at the Right. Courtesy "_Flying_."]
The upper wing gives the greatest percentage of lift, and the middle
wing the least, since the latter suffers from interference on both
sides. It has been found that the sum of the top and bottom wings of a
triplane group gives the same lift as the two wings of a biplane under
equal conditions. It was also found that the lift-coefficients and
lift-drag of the upper plane alone was very nearly equal to the lift of
the combined effects of all three wings, and at all angles. Calling the
lift of the middle wing 1.00 (4°), the lift of the upper wing will be
1.91 and the lower wing 1.64. Calling the L/D of the middle wing 1.00
(4°), the relative life-drag will be L/D = 2.59 for the upper wing and
1.69 for the lower. With the middle wing still assumed at unity, the
lift of the top plane is at 1.49 at 16°, and the lower wing 1.20. The
liftdrag at 16 degrees will be respectively 1.00, 1.22, and 1.117 for
middle top and bottom. At 0°, the upper wing will carry 2.68, the middle
1.00, and the bottom 1.82. At 0°, the lift-drag of the top is 3.63, the
middle 1.00, and bottom 2.30. These relative figures are only useful in
comparing the loading when computing the strength of the structural
parts. See "Aviation and Aeronautical Engineering" Nov. 1, 1916.
*Overhanging Wing Tips*. In many American machines, and in some European
machines, such as the Farman, the upper wing is given a much greater
span than the lower. Of late, the tendency has been to make the wings of
equal, span and fully 90 per cent of the modern machines will be found
to be arranged in this way. While the overhanging tips may slightly
increase the efficiency of the biplane by reducing interference at the
ends, it makes the span unduly long and difficult to brace at the end.
The added end bracing due to the overhang probably offsets any
aerodynamic advantage to be obtained, although I have no accurate data
on this point. Compactness is certainly not a feature. It is said that
ailerons are more effective when mounted on the upper overhang, and this
may be so, but I note that the area is about the same in any case. With
overhanging tips, the ailerons are generally placed on upper wings, only
while with equal or nearly equal spans, they are placed top and bottom.
The overhanging section and the ailerons form a single detachable unit
as a general rule. With nearly equal spans, the upper and lower ailerons
are generally interconnected with a small strut in such a way that they
act together.
Small speed scouts, rarely if ever, have any overhang since the object
of these machines is to make them as small and compact as possible.