What is the optimal circulation for minimizing induced drag in a biplane?

In summary, it depends on the specific configuration of a biplane how much induced drag is created by the wings. The elliptical distribution is still the most efficient for the upper wing in a biplane configuration because it will still minimize wing tip vortices. But when you think about it, the upper wing is also operating in what is similar to "ground effect".
  • #1
traianus
80
0
Hello,
Does anyone know what is the circulation that gives the minimum induced drag in a biplane? Is it elliptical like a single wing?
Thanks
 
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  • #2
traianus said:
Hello,
Does anyone know what is the circulation that gives the minimum induced drag in a biplane? Is it elliptical like a single wing?
Thanks

Interesting question. And as always, the answer begins with "it depends...". The main thing to understand is that, while induced drag is important, it may not be the most important consideration in a bi-plane design. The aerodynamic interaction of the two wings can become significantly more important depending on the specific configuration. Some analysts have even given a name to a different type of drag in such cases and called it "interference drag". Basically, empirical results have shown that Total Drag <> Drag of Body A + Drag of Body B when the two bodies are flying in vicinity of one another.

In general, the elliptical distribution is still the most efficient for the upper wing in a biplane configuration because it will still minimize wing tip vortices. But when you think about it, the upper wing is also operating in what is similar to "ground effect". Ground effect aerodynamic theory tells us we do get more lift (from the "air cushion" effect) but we also create more drag which (empirically) is a function of the height of the wing above the fixed surface (in this case the lower wing) divided by wingspan.

Predicting induced drag on the lower wing is where things get difficult. Even if you assume the same wing shape, wing span, and dihedral (if any) as the upper wing, one of the biggest parameters that will impact its induced drag is how much stagger (fore/aft) exists between it and the upper wing. And of course the distance between the two wings will also affect the shape of the "lift bonnet" created by the lower wing and its interference with the flowfield from the upper wing.

Like I said, it is an interesting question, but not as straightforward as one would hope.

Rainman
 
  • #3
Thank you for you very good answer.
I think that when the two wings are very very distant from a practical point of view they can be viewed as two independent wings (no interaction) and so the "best" distribution is the elliptical one. What about when the distance is finite? the wings A and B influence each other in a not clear way.
 
  • #4
It seems that I found the answer to my questions. See the article

http://pdf.aiaa.org/jaPreview/JA/2006/PVJA15982.pdf [Broken]
 
Last edited by a moderator:

1. What is induced drag in a biplane?

Induced drag is a type of drag that is generated when an aircraft's wings create lift. In a biplane, this drag is caused by the interference between the upper and lower wings.

2. How does the design of a biplane affect induced drag?

The design of a biplane can greatly impact the amount of induced drag it experiences. Factors such as the size, shape, and placement of the wings can all influence the amount of drag produced.

3. What are the effects of induced drag on a biplane's performance?

Induced drag can have a significant impact on a biplane's performance, as it can decrease its speed and efficiency. Higher levels of induced drag can also lead to increased fuel consumption and reduced maneuverability.

4. How can induced drag be reduced in a biplane?

There are several ways to reduce induced drag in a biplane. These include increasing the wingspan, using wingtip devices such as winglets, and implementing advanced wing designs such as the NACA airfoil.

5. Are there any trade-offs to reducing induced drag in a biplane?

While reducing induced drag can improve a biplane's performance, there are also some trade-offs to consider. For example, increasing the wingspan can improve efficiency but may also make the aircraft more difficult to maneuver. It is important to find a balance between reducing induced drag and maintaining other important aspects of the biplane's design.

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