From this review paper Page 11:

Strut-bracing for the SST allows very signi®cant reductions in both vortex and wave DDL via an extreme arrow con®guration and could be favourable to LFC via reduced chord Reynolds number

All right. Low chord=low Reynolds=more laminar flow. But arrow wing=high sweep=less laminar flow.

Another paper says: Page 20:

The friction drag reduction concept is a world-first technical approach that obtains a natural laminar flow wing with a subsonic leading edge at supersonic speed. An ideal pressure distribution is first designed to delay boundary layer transition even on a highly swept wing, then an

The screenshotted figures from the paper show the aircraft, an arrow-winged with a high sweep.

enter image description here enter image description here

Here is an image found on the web.

enter image description here

As you can see, it has a cranked arrow wing with a nearly straight leading edge. This wouldn't even give much vortex lift, if I understand correctly. (The aircraft shown is NEXST-1 from JAXA)

All of this is claiming that a highly swept wing can achieve laminar flow. Yet that is not what I am commonly told. So, what am I missing?

  • $\begingroup$ This must be a black program – your picture is a black rectangle on my screen. $\endgroup$ – Peter Kämpf May 29 at 21:54
  • $\begingroup$ @PeterKämpf it appears to be the same here, i just realized. Am trying to fix. $\endgroup$ – Abdullah May 30 at 8:26
  • $\begingroup$ Now the pictures are visible and I agree with the incompatibility of high sweep and laminar flow. I was always told that the change in flow direction over the boundary layer near the leading edge of a swept wing triggers transition to turbulent flow rather soon. Maybe it is the availability of public grants which allows to maintain laminar flow in theory. $\endgroup$ – Peter Kämpf May 30 at 9:54
  • $\begingroup$ Notice how much area ruling helps. The purple curve is warped arrow, what is the light blue? $\endgroup$ – Robert DiGiovanni May 30 at 10:35
  • $\begingroup$ The arrow seems to produce a little more lift. This is consistent with a longer leading edge creating a stronger vortex. The Concorde prototype was a nose to tail "slender" delta, not unlike a common paper airplane. They may have gone with the ogee to mitigate the tornado-like vortexes (enough not to destroy every airborne object nearby), or noise abatement. Would like to see slender delta data there too. Laminar flow across that type of wing seems laughable. $\endgroup$ – Robert DiGiovanni May 30 at 10:44

The first thing to note is that these papers are from 2003 and 2013 respectively. Since then, they have not set the world on fire.

One concern over their quality arises when I recall that the "Warped NFL wing" is typical of the reflex-cambered supercritical wings first developed around the 1970s. If you check out the inner section of say the Airbus A380 (and I suspect any modern jetliner) you will see a certain amount of that technology incorporated. But laminar flow notoriously vanishes once you fly into a few bugs or some mechanic leaves an oily smear and, despite being touted back then and as far back as the late 1930s, it has only ever produced modest results on any flyable airplane.

Next, what about this braced wing concept. The linked source discusses it on Page 10:

"A possibly viable alternative configuration for the conventionally sized long-haul CTOL transport mission is the strut-braced wing. Strut-bracing allows thinner, smaller chord, lower sweep and higher aspect ratio wings. The smaller chord, leading edge radius and sweep have a favourable influence upon HLFC, increasing the amount of wetted area laminarized and reducing suction mass ̄ow and roughness sensitivity as well as increasing attachment line stability. Other `strut-related’ benefits include large drag due to lift reductions from the wing tip engines and greater span enabled by the strut, as well as large wing weight reductions. Circulation control, powered by the auxiliary power unit, could be utilized on a conventionally sized tail to work the engine-out problem. All these benefits produce very large increases in the lift±drag ratio and range at cruise."

It also notes on Page 5 that:

"More unconventional supersonic applications of [laminar flow control], again by Dr Pfenninger, to a strut-braced high aspect ratio arrow wing configuration indicates phenomenal performance levels, L/D on the order of 15-20 versus the 9-11 typical of advanced conventional configurations (with LFC)."

Now I don't know about you, but when I read of a "phenomenal" 40% drag reduction sought 17 years ago but never realized, I smell a rat. I wonder what software modelling the researchers used - 2D or 3D? At sharp sweep angles it totally matters. Were they naive enough to assume flawless laminar surfaces maintained throughout a transcontinental flight? And engines at the tips of the wings, evidently in a kind of sesquiplane variant of the Busemann biplane? Oh, spare us the FAA's demand for directional stability combined with altitude maintenance and adequate range in the mid-Atlantic one-engine-out condition.

So I think you can see where this is going.

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  • $\begingroup$ Engines at the tips of the wings? It sounds like you misread it. nclude large drag due to lift reductions from the wing tip engines This is meant to show how placing the engines at the tips of a low AR, cantilever wing is bad, and not doing so on a strut-braced wing is good. $\endgroup$ – Abdullah May 31 at 16:34
  • $\begingroup$ gliders use bug wipers....? $\endgroup$ – Abdullah May 31 at 16:37
  • $\begingroup$ aviation.stackexchange.com/questions/78471/… $\endgroup$ – Abdullah May 31 at 16:38
  • $\begingroup$ @Abdullah, I think maybe "Other `strut-related’ benefits include large drag due to lift reductions from the wing tip engines and greater span enabled by the strut" is defeating your reading skills rather than mine. Do gliders have bug wipers over the whole wing surface? No, I didn't think so, You really don't need many bug encounters to screw over a scheduled commercial service. $\endgroup$ – Guy Inchbald May 31 at 19:20
  • $\begingroup$ Other `strut-related’ benefits include large drag due to lift reductions from the wing tip engines and greater span enabled by the strut notice the quotes around 'strut-related'. And if you look in the whole paper, you will see only one strut-braced concept, and that will not have it's engines anywhere near the tips. And why would they consider drag and loss of lift a benefit? Though I do think they could have been more clear. $\endgroup$ – Abdullah Jun 1 at 4:48

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