We all know that increasing the wingspan of an aircraft tends to increase the glide ratio. Instead of increasing the wingspan, could an extra wing of the same size or less as in a bi-wing configuration also increase the glide ratio? Which bi-wing configuration would be most ideal for this purpose?
Glide ratio is determined by L/D. If we can increase lift without increasing drag, or decrease drag without decreasing lift, we have a better glide ratio.
Increasing the wing span (for a given wing area => reduce the chord) helps: the longer wing span spreads out the lift creation over a longer distance, thereby decreasing induced drag. Keeping the flow laminar for as long as possible also helps.
Your question is about adding a second wing to an airplane. The problem with biplanes is that they were mainly constructed decades ago, and we associate them with older construction methods and mainly lots of drag, such as addressed in this question. It seems that the classical biplane has no aerodynamic advantages, and always has a lower L/D ratio than a comparable mono wing.
However there are now Computed Fluid Dynamics. There has been work done on biplane configuration with low drag in recent years - many are behind a pay wall, this one is not. Their work found two configurations with high L/D ratios:
- Upper wing AoA = 13º
- Upper wing forward sweep = 30º
- Lower wing AoA = -2.8º
- Lower wing backward sweep = 3.5º
- Wing profile of both wings = NACA 4412
- Both wings 6 ft span and 6 inch chord
- Computed L/D = 32.3
The other configuration they found with a high L/D was a canard configuration: one wing in front of the other. If you insist on using two wings, this may be your best bet: one wing mounted in front of the other, in such a way that the back wing is out of the way of the wake of the front wing, or mounted at the optimum AoA for the disturbed flow. Their canard configuration used two different wings, resulting in an L/D of 35.8:
Note that none of the configurations included a fuselage of any kind, so their CFD work would have to be repeated with the fuselage and with interference computations. Fortunately computing power has increased dramatically in recent years.
Instead of increasing the wingspan, could an extra wing of the same size or less as in a bi-wing configuration also increase the glide ratio?
To create lift (over finite span), which is upward force, the wing apply downward force to the air by principle of action and reaction and since the air is free to move, it will accelerate downward. This will increase its energy, therefore the wing must do some work on it and this work manifests as induced drag.
Now because kinetic energy is proportional to mass and square of velocity, while momentum is only proportional to mass and velocity, it is more efficient to accelerate more air to lower speed than less air to higher speed.
At given flight speed, more air can be affected by increasing the wing span, or by having two wings far enough to affect different air. Since the wing affects air to height comparable with the span, the wings would have to be quite high above each other for the construction to be efficient—but such construction would not be efficient mechanically, since it would need very high pylons above and possibly below. Increasing the span is easier.