We focus our discussion on the bi-planes of the early flying days, which always had more then one wing by default, to achieve the required lift. Some of them can be seen in images below.

The question is why use two or even three wings ? The usual explanation given for this type of double winged airframe are as follows:

Biplanes had two wings primarily for structural and aerodynamic reasons, especially in the early days of aviation. Here are the main reasons:

1. Structural Strength: Early materials used for aircraft construction, such as wood and fabric, were not very strong. The biplane configuration allowed for a more rigid and sturdy structure because the two wings could be braced against each other with struts and wires, providing greater strength and resistance to bending.
2. Aerodynamic Lift: With two wings, biplanes could generate more lift without increasing the wingspan. This was particularly important when engine power was limited, as it allowed for better performance at lower speeds.
3. Wing Loading: By having two wings, the wing loading (the weight of the aircraft divided by the wing area) was reduced. This was advantageous because it allowed for lower takeoff and landing speeds and improved maneuverability.
4. Manufacturing Limitations: At the time, the technology and materials available for constructing long, single wings were limited. Biplanes provided a practical solution to build effective and efficient aircraft within these constraints.
5. Compact Design: The biplane design allowed for a more compact aircraft, which was useful for operating from small airfields and for storage.

As materials and construction techniques improved, and more powerful engines became available, the advantages of biplanes diminished, leading to the adoption of monoplanes, which have a single wing and offer better aerodynamic efficiency and higher speeds.

But if we consider all the reasons closely, only reason No.2 is the most applicable; all others are just a matter of choice even by today’s standards. It all started with the Wright Flyer when they experimented to develop a flying machine based on the airfoil of birds. They observed that a typical airfoil had a thinner profile and overall proportions were such as to have a large thickness to chord length ratio. Judging from the airspeeds achievable at that time, such a wing could not generate enough lift to get the Wright Flyer and its pilot airborne. They dared not fiddle with the proportions of the bird airfoil and made it thicker, and instead they logically added a second wing to increase lift. Thus due to its success, the Wright Flyer two wing design became Gospel for all subsequent aircraft of that time, until it was learnt that lift could simply be increased just by increasing the angle-alpha (i.e. make airfoil thicker). Figure below shows the working surfaces and dimensions of a typical airfoil. The height h and the angle alpha define the lift-speed relationship. The thicker the airfoil at point C the greater will be angle alpha. Such and airfoil will produce greater lift at slower airspeeds, and vice versa.

Modern day Bush Planes with thicker airfoil wings prove that it is possible to have much higher lift at very slow speeds, enabling even metal airframes to fly with ease without adding a wing. Image below shows a very popular American Champion Scout bush plane. Notice the thick airfoil wing.