Can Multi-Wing Aircraft Designs Improve Lift and Efficiency?

[petterg]

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?

 

[CWatters]

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).

 

[petterg]

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?

 

[rootone]

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.

 

[olivermsun]

The Wikipedia article on https://en.wikipedia.org/wiki/Biplane seems to have some interesting discussion.

 

[aerohead_18]

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.

 

[petterg]

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?

 

[aerohead_18]

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.

 

[CWatters]

This problem has to be 100+ years old! Have you tried Google for things like "optimum biplane wing separation" or stagger?

http://www.quest.nasa.gov/aero/question/aerotheory/Optimizing_biplane_Wing_Separation.txt

Edit: One of the links I posted here yesterday isn't working but the same paper is here..

http://www.enu.kz/repository/2009/AIAA-2009-1085.pdf

 

[petterg]

Thanks for the links. They gave answer to all I was wondering, and a lot more!

I'll also take a look at XFLR5