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What is multi-wing or multi-plane:

For those who may not know, multi-wing air planes or multi-planes are aircraft that exceed the three sets of wings.

Photography of a multi-plane air plane

The idea is while increasing the number of wings, you increase the lift at the cost of drag.

This is also true for multi-element wings, or flaps.

Illustration showing a multi-element wing with 8 flaps

The question:

What should be the minimum distance between multiple wings in a multi-plane configuration in order to avoid the airflow generated by the wings to interfere with one another?

Of course, I imagine that this depends on some factors, such as wing dimensions, speed and turbulence in the air.

But taking into consideration something like a the wings of this glider under speeds around 100 kph, what would be the minium distance between wings in that stair-like configuration?

Or the most practical and/or efficient way of doing that is putting the wings one under another like in a conventional biplane?

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    $\begingroup$ A multiplane might work fine at a specific AoA, but be a total (airflow) mess at another. If AoA is within a narrow range, multiplanes can provide greater lifting force. Jet engine compressors and fans are multiplanes. Not as efficient as monoplanes, but with plenty of power, very effective. $\endgroup$
    – Robert DiGiovanni
    Feb 13 at 0:12
  • $\begingroup$ @RobertDiGiovanni What is "AoA"? $\endgroup$
    – Fulano
    Feb 13 at 0:18
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    $\begingroup$ AoA is Angle of Attack $\endgroup$
    – Robert DiGiovanni
    Feb 13 at 1:25
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    $\begingroup$ The glider in your link has only one wing 🤔 How many wings do you have in mind? A couple or a dozen? Flaps and bi (or tri or multi) planes work in a very different way. $\endgroup$
    – sophit
    Feb 13 at 18:47
  • $\begingroup$ @sophit at least a dozen $\endgroup$
    – Fulano
    Feb 13 at 18:59

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Rule of thumb is at least 1 chord length of spacing. "Staggering" the wings can also be helpful. The aviator/inventor pictured in your question seems to be off to a good start. Multiple wings can be cross-braced and can very made very strong, which was an early advantage for biplanes.

However, once more powerful engines made higher speeds available, advances in building materials and internal truss structured spars made the less draggy monoplane design the future of aviation.

The relationship of thrust and drag is best understood using a simple rock and parachute model. Double weight (propulsive force), and speed increases by ... $\sqrt{2}$, or only around 40%. This is why streamlining becomes imperative as speeds increase. More wings simply become more weight and more drag.

The good news is that Lift also increases by velocity squared, and higher velocity gets you there faster (especially into a head wind). So the strategy becomes less wing area and more speed.

Lifting factors are explained in the Lift equation

Lift = air density × Area × Coefficient × Velocity$^2$

Reynolds number has a strong effect on lift to drag ratios. Effects of airspeed on L/D ratios for various airfoils can be studied here.

Reynolds Number = Velocity×Chord/Kinematic Viscosity of air

Faster speeds give higher Reynolds numbers for a given wing. Effects on L/D become very significant for an Re > 1 million vs 20,000.

Multiwings live on to extend the low speed range of STOL aircraft and very heavy airliners. The ability to retract these devices greatly lowers drag at higher speeds.

One notable exception to all this is the eagle/vulture design, which favors very slow flight, minimal rate of descent, and the ability to ride even the slightest of updrafts. Here, the thought of a modern glider sprouting multiple heavily cambered wing tips, slats and flaps in order to slow way down and ride the thermal seems a possibility. Again, extra weight of these devices would have to be considered. It would be fun to try.

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  • $\begingroup$ I've seen a photo of an experimental plane with eagle/buteo/vulture-wingtip-like wing extensions, like an array of winglets aimed mostly horizontally but tilted somewhat upwards. The ones near the TE of tip were angled up higher than the ones further back. At least 3 or 4 total. I think the wing was a rectangular wing of not too extreme aspect ratio, and my recollection is that it was a low-wing plane resembling a crop-duster, and this may have been the intended use (not sure). Would make a nice addition to this answer if a photo could be found but I can't recall where I saw it. $\endgroup$
    – quiet flyer
    Feb 13 at 15:31
  • $\begingroup$ Of course, what the actual birds do, is open up those wingtip slots maximally when flying slow-- at increasingly higher airspeeds the whole wing is progressively more and more folded but the tip area is basically the first to fold, closing the slots-- $\endgroup$
    – quiet flyer
    Feb 13 at 15:37
  • $\begingroup$ @quietflyer back at R/C groups on of my early mentors marveled at the ability of old-time undercambered gliders to do well in competition with very light updrafts. $\endgroup$
    – Robert DiGiovanni
    Feb 13 at 15:50

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