In most of the biplanes, the top wing is located a little forward of the lower wing. What is the use of this? Also, how does this affect stability of the aircraft.

Placing the top wing ahead of the bottom wing in biplanes is called (positive) stagger. It is mostly used in small biplanes and improves pilot vision.

In order to accommodate a variation of pilot weights and to reduce accelerations in maneuvers, it is advisable to place the pilot very close to the center of gravity. If the upper wing were in the same lengthwise location, it would obstruct the forward and upward vision of the pilot.

Why is upward vision so important? Because of the bank angle, in turns you need to look up in order to see what is ahead. A high mounted wing right at the center of gravity would rob the pilot of forward vision in turns.

Therefore, in small biplanes the top wing is moved forward while the bottom wing is moved back correspondingly, so lift is created close to the center of gravity, but the top wing is out of the way.

Large biplanes do not need this arrangement because the pilot sits ahead of the wing. The Antonov 2 below is an example (source):

Antonov 2 biplane side view

In many designs, the center wing has a cutout at the rear in order to give the pilot a better field of view. Even more interesting is what had to be done to improve pilot vision in parasol monoplanes. The Focke-Wulf 56 used slight wing sweep in combination with a reduced root chord in order to reduce the obstruction to the pilot's field of view of the rear wing center (picture source).

Focke-Wulf 56 high-wing monoplane

It's called a staggered wing and is done to reduce aerodynamic interference between wings in certain circumstances. A wing with positive (forward) stagger is most common because it improves both downward visibility and ease of cockpit access for open cockpit biplanes.

In the case of the agricultural plane, pictured, the stagger increases the nominal CG range, a bit. Finally, the upper wing will have a slightly greater angle of incidence, enhancing its stall and stall recovery characteristics.

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    "Answer above" isn't much use on SE sites where answers move up and down by vote or user sorting preferences. – Transistor Jul 10 at 18:37
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    Please edit this answer to indicate the author of answer you are referring to. There are (currently) three answers "above". – Martin Bonner Jul 10 at 20:27
  • Now only two answers "above" (at least, when ordered by votes), proving the point even more! – David Richerby Jul 11 at 12:07
  • Sorry. Still a little new to this. Fixed it. – Walker Jul 11 at 18:45
  • @Transistor There is an exception, though. If Peter Kampf wrote the answer, it's safe to assume it will remain 'above.' ;) – reirab Jul 11 at 20:34

Wing efficiency.

Here is an analysis of biplane wing efficiency relative to separation as a percent of chord.

The reference states that positive stagger (upper wing ahead of lower) by 0.4 chord gives 5% better efficiency, so advancing the upper wing 0.4 chord would be beneficial compared to the vertical stacking shown.

enter image description here

Remember that a wing works by having high pressure below it and low pressure above it.

So, looking at the top wing, it has low pressure above it - OK. And it has high pressure below it, BUT... that high pressure is then affected by the low pressure of the bottom wing.

Now looking at the bottom wing: it has high pressure below it - OK. It has low pressure above it, but that is affected by the high pressure below the upper wing.

So the high/low pressure areas of the two wings interfere with each other if they're mounted vertically above each other. But if you move one wing foreward or aft of the vertical, then the high/low pressure areas affect each other less.

You can move the top wing forward, which is normal. Or you can move the top wing backwards as on the Beech Staggerwing. Either way works.

  • "areas of the two wings interfere with each other" How much are we talking about? – Mast Jul 11 at 17:48
  • Although it is true that the wings influence each other, this is a far too simplistic explanation. The wings don't somehow magically make the air below have a higher pressure and above lower pressure, instead what they do is they deflect the stream of air downwards from its (relative to the plane) straight-backwards movement. The air right next to the wing thus needs to flow in a curve, and that creates a pressure gradient: the pressure is higher right at the lower wing surface, but as soon as you move away from the surface it drops again. – leftaroundabout Jul 12 at 14:13
  • In a positively staggered configuration, the lower wing is probably influenced more by the upper than in a straight-stacked one, because the downwards wash passes by closer. – leftaroundabout Jul 12 at 14:13

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