Ever wondered why modern airplanes only have one set of wings? You may have heard that it's more efficient than the old biplanes, but why is that? And why do sail planes have those long thin wings? There are actually two significant reasons that a monoplane is more efficient aerodynamically than a biplane or a triplane (or any number of stacked wings if you're really ambitious), which are discussed below. One small note - the discussion below applies to subsonic flow. Once you go faster than the speed of sound and get shock waves, the aerodynamics associated with the shock waves becomes a big consideration, and low-aspect ratio wings start to look more appealing.

The primary reason that a monoplane is more efficient than a biplane has to do with wing tip vortices. Think about a wing this way - there's high pressure air on the lower surface, and low pressure air on the top surface (more info). Out at the wing tips, since there's nothing there to block it, that high pressure air can "leak" aroud the tip and flow up around to the top surface. That creates what's known as a wing tip vortex.

Wing Tip Vortex - Click on Picture for Larger View

As you can imagine, if air is leaking around the tip, it's not doing its job of pushing up on the wing, so the vortex reduces the efficiency of the wing. That's why for a given wing area, high aspect ratio wings (long thin wings, like on sail planes) are more efficient than low aspect ratio wings (short wide wings, like on the F-104) - less of the wing is being affected by the wing tip vortex. That's also the reason why, for a given wing area, a monoplane is more efficient that a biplane or triplane - not as many wing tips to create vortices. This is also the reason for different wing tip designs, such as the winglets on some airliners, or the Hoerner wing tips on some general aviation aircraft.

The other reason why biplanes are less efficient aerodynamically is less of a factor, and can be minimized with proper design, but it's still there. And it's just basically that the wings interfere with each other. Wings don't just affect the air in their immediate vicinity - they deflect the air for a decent ways above and below them (see the picture below). So, if you have wings stacked, they'll interfere with each other in that way.

Air Being Deflected Above Wing - Click on Picture for Larger View

So, that takes care of the aerodynamics, but everything's always a trade off. First, remember that even though a biplane is less efficient that a monoplane, since you get twice the wing area for the same wing span, you can get by with much shorter wings, and there may be cases where that smaller wing span is more important than cruise efficiency. Consider an aerobatic plane. You may want to maximize the aircraft's roll rate - the shorteryour wings, the lower your inertia, so a biplane/triplane will give you better roll maneuverability than a high aspect ratio monoplane of the same wing area. Another trade-off is structural weight. A biplane with the struts and braces connecting the wings can be made pretty structurally efficient, and weigh a good deal less than a monoplane of the same wing area. So, it becomes a question of, is the increased aerodynamic efficiency of a monoplane worth the increased weight. In general, at the speeds most aircraft fly at today, the answer is yes. But looking back in history, at the speeds aircraft were flying back in WWI, with the materials they had to build their aircraft out of, the biplane was probably the better option. (This structural consideration is also important for determining the aspect ratio of a monoplane airplane.)