Exploring the Possibility of Multi-Wing Aircraft Designs

In summary, the article discusses how different parameters can affect the lift/drag ratio of an air foil, and how this could be used in designing smaller, more efficient aircraft. The article points out that there are disadvantages to having a large number of small air foils, such as interference and the increased risk of structural failure. However, CFD software could be used to study the effects of different parameters on the lift/drag ratio of an air foil.
  • #1
petterg
162
7
I played around with various parameters to air foils using Foilsim
http://www.grc.nasa.gov/WWW/k-12/airplane/foil3.html

What I found was that for a given wing length, a smaller chord gives a better lift/drag ratio. This will make the wing area small, hence the total lift will be small and there will be structural challenges in making a long but small wing. But what about having a lot of such small wings put together in an array? Together they would give larger wing area hence more lift, and they would give structural strength if they were connected together at the far end.
There are bi-planes and tri-planes that seems to kind of use this idea. Why don't larger airplanes use this idea? Why aren't there something like "sexagint-planes" (60 pair of wings)?

I'm guessing the answer is that wings interfere with each other. So then my question is, how far apart must wings be placed to not interfere with each other? What if their placement is shifted a bit forward?
 
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  • #2
One problem is that wing tips are less efficient than the rest of a wing. So aircraft with a high aspect ratio (span/chord) are more efficient than other options (low aspect ratio or multiple wings).
 
  • #3
Good point.

Imagine a sailboat - the sail has some similarity to a wing.
What would happen if I replaced the sail with a shutter like the one on the picture below? (Assumed the area of the shutter was the same as the sail, ribs placed vertically, each of the ribs has the shape of an air foil and the length of each rib had the same length as the mast.) Would the shutter or the sail provide the most lift/drag ratio?

traditional_oak_2_panel_short_1.jpg
 
  • #4
I think the lifting efficiency would not be very different either way assuming the total surface area is the same.
However there could be interference effects between adjacent slats which could add to drag.
The multiple slats idea also presents the problem of there being multiple units making up the system, and thus increased overall risk of a structural failure.
More parts = more things that could fail.
 
  • #6
Some early airplanes had multiple wings in the "shutter" configuration, but I'm not sure if any of them ever flew successfully. The big problem would be structural integrity; the one in the pic attached reportedly broke up on takeoff. Having a bunch of wings stacked on top of each other just isn't a very strong configuration. I bet it could be done with modern materials and engineering, but it still couldn't be full cantilever, so the weight and drag penalty from struts and supports would be considerable.

Another potential problem might be surface area. You'd probably get less induced drag with several small high-aspect ratio wings as opposed to one with the same planform area, but I think the total surface area of all those wings would be higher, so the increase in parasite drag might cancel it out. Someone could do the math and figure out if the surface area would really be higher (I'm too lazy to do all that at the moment ;). At any rate, this would be fun to play around with in CFD.
 

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  • #7
From reading the wiki of biplanes (and its link to tandem wing) I get the impression that biplanes advantages are when engine power is low. That makes sense - when engine power is low a better lift/drag ratio is needed. I'm thinking the struts is what makes up the problem when a stronger engine is used and you expect higher speed. Also the interference between wings becomes more significant with higher speed. At low speeds the structural load will also be lower, hence less need for struts. I introduced the sailboat because it would be an experiment that could be tested with a dingy. Now I realize the concept probably requires the speed of a sailboat to make sense.

I'm still wondering; How far apart must wings be placed in order to not interfere significantly? I'm not looking for a formula (all tho it would be great). I'm more looking for an understanding of what are the major factors in the answer and how do they scale? I'm guessing speed is a squared factor, chord length is maybe linear, and maybe even thickens is playing a major role?

Are there any CFD software available for 30-day demo that is capable for simulating this?
 
  • #8
XFLR5 is a really good free CFD software, but you'll be limited to either one or two wings, and the data is only reliable below Re=500k or so.

As far as interference goes, I'm not sure if there are any formulas that relate separation to drag or lift. You could try looking through rcgroups.org, there might be something on there.
 
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  • #10
Thanks for the links. They gave answer to all I was wondering, and a lot more!

I'll also take a look at XFLR5
 

1. What are the advantages of using a multi-wing aircraft design?

One of the main advantages of multi-wing aircraft designs is increased lift and stability. With multiple wings, the aircraft can generate more lift, allowing it to carry heavier loads and fly at slower speeds. The additional wings also provide more stability, making the aircraft easier to control in flight.

2. How does a multi-wing aircraft design differ from a traditional single-wing design?

A multi-wing aircraft design has multiple wings stacked on top of each other, while a traditional single-wing design has just one wing. This allows for a larger surface area and increased lift. Multi-wing designs also typically have a narrower wingspan and can fly at slower speeds compared to single-wing designs.

3. Are there any drawbacks to using a multi-wing aircraft design?

One potential drawback of multi-wing aircraft designs is increased drag. The additional wings create more surface area for air to pass over, resulting in more drag and potentially decreasing the aircraft's speed. Additionally, the complexity of multi-wing designs may also increase the cost and maintenance of the aircraft.

4. What types of aircraft are best suited for a multi-wing design?

Multi-wing aircraft designs are best suited for slower, lighter aircraft such as biplanes, triplanes, and gliders. These types of aircraft typically do not require high speeds and can benefit from the increased lift and stability provided by multiple wings.

5. How do multi-wing designs impact fuel efficiency?

Since multi-wing aircraft designs typically have increased drag, they may not be as fuel-efficient as single-wing designs. However, the increased lift and stability can also allow for more efficient use of fuel by flying at slower speeds. Overall, the impact on fuel efficiency will depend on the specific design and purpose of the aircraft.

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