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Lift Airbus vs Blended Wing

  • The Burnelli machines had an airfoil shaped fuselage;

  • some indicate near 10% of lift in a 21 century airliner comes from fuselage; 'Flying Wing' concept is old, and still attracts many;

  • bottom of SR-71 looks shapen as a high speed race boat;

  • at very high speeds, air is thick, not as a brick, but as 'marmelade', it's a Reynolds Number affair, this figure, you know, expresses relations of inertia and viscosity forces inside a fluid.

Would a flat bottomed fuselage add even more lift than the regular round section, increasing the overall fuel economy of an airplane? Does the concept deserve Aerodynamic Wind Tunnel testing?

[Proposed shape for fuselage, fins may act as 'Winglets']

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  • $\begingroup$ Likely the answers you're looking for: Why are there no blended-wing passenger airplanes in operation? $\endgroup$
    – mins
    Commented Aug 29, 2021 at 2:09
  • $\begingroup$ No, 'Blended wings' are similar to some German Wartime projekts, Delta, Double Delta, with engines on upper side, rear side of wing, what I was asking about is an ordinary airliner with a semicircular section in a common lenght cabin, to provide more lift than the regular cylinder, round or oval section, sorry for not detailing the question enough as to dissipate any doubt. Blessings + $\endgroup$
    – Urquiola
    Commented Aug 30, 2021 at 11:43
  • $\begingroup$ "Would a flat bottomed fuselage add even more lift than the regular round section, increasing the overall fuel economy of an airplane?": In answers you have some clues about the fuselage section, e.g. "The fuselage has a round cross section, because that is the lightest form of pressure vessel we can make. Tilt it, and it provides some lift as well" $\endgroup$
    – mins
    Commented Aug 30, 2021 at 11:50
  • $\begingroup$ Yes, this is known, what I asked is if a flat bottomed, semicircular fuselage section would provide more lift, thus increasing overall airplane efficency and fuel economy, while keeping the passenger and cargo capacity, and the structural strenght. Thanks. Blessings + $\endgroup$
    – Urquiola
    Commented Aug 30, 2021 at 12:12
  • $\begingroup$ Where are you going to put the luggage? $\endgroup$
    – Robert DiGiovanni
    Commented Aug 30, 2021 at 14:37

2 Answers 2

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Draw in the "fuselage section" in red (equal volume) with the flying wing and you have your answer.

At high subsonic speeds, lift isn't the issue, form drag is. Supercritical wings have evolved to the point where they are actually flattened on top to avoid shockwave drag. These aircraft literally "surf" along on their undersides, so flattening the underside of the fuselage has merit.

However, the wing of the average airliner is already too big for high subsonic flight, as it must be engineered for lower speed approaches and landings (and also must be fitted with Fowler flaps and slats).

Drag at cruise speed, carrying the same weight, is why general airliner design differs little from the Boeing 707 of the late 1950s.

Advances have been mainly in increasing wing aspect ratio and engine efficiency. We can see a smaller tail, further back on a longer, thinner fuselage, will generate less drag at high subsonic cruising speeds, until proven otherwise.

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The goal isn't more lift, the goal is high lift-to-drag ratio.

  • Induced drag is minimized by following elliptic distribution of lift over span, that is fairly constant lift over span in the middle, decreasing towards the wingtips. To achieve that the fuselage in the middle needs to only produce as much lift per its width as the wing roots next to it, which means relatively very little for its size. The circular fuselage can already do that.
  • Form drag is minimized by keeping the cross-section low. A tube has low cross-section for volume.

The other goal is to minimize the structural weight so more capacity remains for the payload.

  • Tube has small surface (so less skin needed) and large strength for volume. Any flattening would increase the relative surface and reduce strength, requiring more weight in structural reinforcements.

The traditional design fits these constraints really well.

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