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Would a triplane be way more efficient if the wing tips were joined at the tips to create one wing tip vortex instead of 3, at RE 200-600k? At that low of a speed, I assume interference and form drag are minimal.

I understand biplanes and triplanes are less efficient than a monoplane due to wing root interference drag and wing tip vortices.

Yet..... I understand boeing/NASA are experimenting with a biplane/braced wing airliner configuration, which would have extra induced/inference drag.

So, to reduce those nasty wingtip vortices, couldn't one just intersect the tips?

I'm thinking, the top wing would have anhedral, a straight middle wing, and bottom wing with dihedral, meeting at the tips.

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    $\begingroup$ Are you thinking something like this? How does the wraparound wing design reduce drag? $\endgroup$
    – fooot
    Sep 9, 2019 at 17:26
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    $\begingroup$ It's not a matter of just blocking the flow of the vortice right at the wing tip by having another wing intersect it. There's actually a large flow field extending some ways out, only minimally influenced by wing tip gadgets. It's like you blocked the fastest moving part of a river with a board stuck in the water, but all that slower moving water beyond the board still flows by. $\endgroup$
    – John K
    Sep 9, 2019 at 18:35
  • $\begingroup$ Why do you expect the vortexes to do anything other than reinforce each other, leaving you with the same or larger vortex intensity of a monoplane and more structural complexity and weight? $\endgroup$
    – AEhere supports Monica
    Sep 10, 2019 at 14:28
  • $\begingroup$ Biplanes and triplanes are less efficient than monoplanes because for a given wing area, the monoplane will usually have a wider span. Span drives induced drag, of which the tip vortex is an indicator. $\endgroup$
    – Pilothead
    Feb 7, 2020 at 0:08

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Well, those nasty wing tip vortices really can't be helped if you are creating lift. Amazing that airliner designers in this century are still considering biplane vs strutted mono plane vs cantilever mono plane concepts in their proposals.

Bringing 3 wing tips together will create all kinds of air flow issues (remember to model at all possible AOAs), reducing efficiency.

The most efficient way to create lift remains a high aspect mono wing. I would look to designers actually reducing sweep a little (around 15 degrees for subsonic) and try for as high an aspect wing as possible, just like sailplanes.

However, wing span constraints and support benefits for very large aircraft may yet see the return of the biplane, with very thin, high AR wings. These could be swept a bit more at the tips, and/or endcapped or wrapped. Fuel and landing gear could go in a more drag efficient longer fuselage. Note mounting them in "classic" Tiger Moth style retains the stability benefits of wing sweep, which may be lost in the newer "Prandtl wraparound" proposal.

It seems some designers just can't bring themselves to make a 21st century high AR Jenny. It might be worth a try.

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    $\begingroup$ sweeping high aspect ratio wings at the tip will create a torsion moment which those wings are poorly equipped to withstand. Better to avoid sweep. $\endgroup$
    – Peter Kämpf
    Feb 6, 2020 at 23:32

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