Biplane

A biplane is a fixed-wing aircraft featuring two main wings stacked one above the other.

This configuration was foundational to early aviation history. The Wright Flyer, the first powered and controlled aeroplane, used a biplane wing arrangement. While this design offers structural advantages, it also introduces aerodynamic challenges. The primary drawback is increased drag, which is the resistance an object faces when moving through air. This drag occurs because the two wings interfere with each other's airflow. Specifically, the high-pressure air under the top wing and the low-pressure air above the lower wing can cancel each other out.

To understand how a biplane generates lift, one must look at the interaction between the wings. Both wings contribute to the total lift, but they do not provide twice the lift of a single wing of the same size. This is because they occupy nearly the same portion of the atmosphere. In a design with no stagger, the lift coefficient is reduced by 10 to 15 percent compared to a monoplane.

To maintain the gap between the wings, engineers use interplane struts. These are vertical supports positioned symmetrically on either side of the fuselage. Additionally, rigging wires are used to prevent the structure from flexing. These wires include lift wires to prevent wings from folding up and landing wires to prevent them from sagging.

Engineers often use different wing arrangements to improve performance or visibility. One common method is called stagger. Positive stagger occurs when the upper wing is moved forward relative to the lower wing. This can increase lift and reduce drag by lessening aerodynamic interference. It also improves access to the cockpit for the pilot. Negative stagger is the opposite, where the lower wing is moved forward. This is often done for structural reasons or to improve the pilot's field of vision.

Designers also organize the wing structure into sections called bays. A bay is the space enclosed by a set of interplane struts. A single-bay biplane, such as the Fokker D.VII, uses one set of struts on each side. Larger aircraft, like the Curtiss JN-4 Jenny, may use two bays to prevent the structure from flexing.

The history of the biplane is closely tied to the evolution of engine technology. In the early years of flight, engines provided relatively low power. This meant aircraft had to fly at low speeds and required a low stalling speed. To achieve this, planes needed low wing loading, which combines large wing area with light weight. A biplane allowed for a large wing area without requiring excessively long, flexible wings. Because of these benefits, most fighter aircraft remained biplanes until the mid-1930s. As engine power and speeds increased, the drag from struts and wires became a major disadvantage. This led to the rise of the monoplane, which uses thick cantilever wings that are more efficient at high speeds.

A unique variation of this design is the sesquiplane. The term comes from a Latin word meaning "one-and-a-half wings."

In a sesquiplane, one wing is significantly smaller than the other, usually the lower wing. This arrangement aims to reduce drag and weight while keeping the structural benefits of a biplane. The Nieuport series of aircraft, including the Nieuport 17, were famous examples of this design. These aircraft were so effective that German manufacturers produced copies, such as the Siemens-Schuckert D.I. Other examples include the Albatros D.III and D.V, which were praised for their maneuverability and climb rates.

Rigging plays a vital role in the stability of a non-cantilevered biplane. Engineers use several specific types of wires to manage different forces. Drag wires run inside the wing bay from the forward inboard corner to the rear outboard corner to prevent the wings from folding back. Anti-drag wires run in the opposite direction to prevent the wings from moving forward when the aircraft stops. There are also external wires, such as lift wires that run from the underside of the outer wing to the lower wing root. These complex systems of wires and struts allow the biplane to remain a rigid, strong structure despite its light weight.

While the biplane became obsolete for most military and transport purposes by the late 1930s, it has not disappeared entirely. The design remains relevant in specific niches. For example, specialist sports aerobatic biplanes are still manufactured in small numbers today. The biplane configuration serves as a fascinating study in the balance between structural strength and aerodynamic efficiency. It shows how early aviators solved the problem of creating large lifting surfaces with the limited materials and engine power of their time. From the first flights of the Wright brothers to the high-performance sesquiplanes of the First World War, the biplane was a central part of the story of flight.