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I came across this beautiful Phantom 3500 egg-shaped fuselage jet design. Phantom 3500
(source)

I've been hearing for a while about laminar flow and how it can significantly increase efficiency with more efficient / less resistant aerodynamics.

I suppose that aircraft designers have known this for decades; it's not like this is new for them, so, the question is why isn't this more common, or why there aren'tt there even any aircraft in the market flying today (as far as I know) with this design?

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I don't expect this to be the final or selected answer, but some conceptual thoughts:

  1. Even in the most favorable conditions, laminar flow can be maintained only for so long. After the air travels a certain distance along a surface, the boundary layer will become turbulent.
  2. That distance depends on the Reynolds number. The higher the Reynolds number, the lower the distance. Implications: The higher the speed, the shortest that laminar flow can be maintained.
  3. Laminar flow doesn't scale. If you have a design that has 25cm of laminar flow and you double the scale to make a bigger model, you will still get 25cm of laminar flow. So less % of the surface will be washed by laminar flow.
  4. The above means that laminar flow give the most benefit in smaller and slower planes, like sailplanes. In faster planes, where parasitic drag is much larger since it grows with the square of speed, the advantage of laminar flow reduces.
  5. Most of the drag in an airplane comes from the wing, where laminar flow airfoils are already in use. So the extra gain with a laminar flow fuselage is marginal. (remember that at the most efficient angle of attack, 50% is induced drag, 100% caused by the wings and 0% dependent on whether the flow is laminar or not).
  6. Laminar flow is very sensitive to a) Having a clean and steady laminar flow incoming (forget about laminar flow fuselages if you have a prop in front), b) Having the perfect shape so you need very good manufacturing techniques and a stiff structure and severe restrictions on functional shapes that depend on what you need to house, where to place vents, etc, and forget about metal construction, rivets, and all that, and c) Having a very smooth surface texture that even dirt, dried raindrops and bugs can spoil.
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    $\begingroup$ Not to forget that a cylindrical fuselage is easier to build and allows variants of the same airplane type by stretching or shortening it. $\endgroup$
    – Peter Kämpf
    Commented Jun 28 at 22:18
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    $\begingroup$ @PeterKämpf, yep, partially covered in point 6.b) $\endgroup$
    – Gabriel Braun
    Commented Jun 28 at 22:23
  • $\begingroup$ 6c is super important. The Phantom is very pretty, but in actual use it's very likely to require constant cleaning to maintain performance. Airlines need robust designs that can handle the dirty, messy real world, not hangar queens that need everything to be perfect in order to function. $\endgroup$
    – Darth Pseudonym
    Commented Jun 30 at 13:40
  • $\begingroup$ WRT 6c and "dust and insects" of other answer, additional real world challenges include steps, gaps, waviness, and/or roughness associated with forward fuselage things like nose landing gear doors, people doors, cockpit windows, sensors, antennae, etc. $\endgroup$
    – AeroAndy
    Commented Jul 2 at 13:13
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Because a cylinder with two "beautiful egg shaped" ends has a lot more passenger space at the expense of very little increase in (viscous) drag.

"Laminar flow" has been a selling point for some (perpetually in development) designs but is difficult to achieve with full scale aircraft, especially after dust and insects become attached to surfaces.

Cylindrical shapes are also much easier to manufacture.

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    $\begingroup$ HondaJet is laminar, but only over the nose. Cylindrical elsewhere also means it's easier to later make a stretched version. $\endgroup$
    – Camille Goudeseune
    Commented Jun 28 at 19:36
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    $\begingroup$ The Piaggio Avanti too. However, in both of these airplane the laminar flow is achieved only in a small % of the fuselage length. So you have a % of drag reduction on a small % of the fuselage where the whole fuselage accounts which account for a % of the total parasitic drag which, at best efficiency, account for 50% of the total drag. It is not a negligible but a very small advantage. And that if the nose is really clean. $\endgroup$
    – Gabriel Braun
    Commented Jun 28 at 22:22
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    $\begingroup$ Also if you ever have to replace a side panel (interior or exterior), the same shaped panel can be used for the entire length of the plane (barring the ends). If the whole thing were egg-shaped, you'd need to have every piece custom built depending on where it is, which is MUCH more expensive. Good non-aircraft example of this logic is in the design of the famous Sydney Opera House. As crazy as it looks, each section is actually cylindrical, so the same panels can be used anywhere on its surface (with minor modifications at the edges). $\endgroup$
    – Darrel Hoffman
    Commented Jun 29 at 4:19
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Just to expand a bit upon the other answers.

Regarding the laminar boundary layer:

Given a surface blown upon with a fluid, the boundary layer that forms on it always starts as laminar, grows in thickness and, after a certain distance from its leading edge, it reaches an unstable thickness and transitions into turbulent. This distance at which the transition happens mostly depends on the local Reynolds number

$$Re_x=\frac{\rho Vx}{\mu}$$

being $x$ the distance from the leading edge of the surface:

enter image description here Evolution of the boundary layer on a flat plate with the local Reynolds number (picture source)

As a rule of thumb, the boundary layer transitions from laminar to turbulent when the local Reynolds number reaches half a milion. A smooth surface might retard a bit the transition and a Mach number higher than 0.7 might anticipate it. At its normal flying speeds and altitudes, a modern polished composite aircraft might have a laminar boundary layer on about 50% of the wing and 25% of the fuselage but, as soon as it enters the transonic regime, those numbers drop to 25% and almost 0% respectively, no matter what the PR department says.

Regarding the egg-shape:

  • In terms of structural efficiency, the best shape for a fuselage that must be pressurised is a cylinder (or a sphere). Any other shape is going to be less efficient i.e. heavier.
  • Going from the nose to the tail of the fuselage, with that egg-shape we can expect the local pressure first to decrease and, after having reached the top of the eggshell (i.e. the part with the biggest radius), to increase again. This increase will most probably make the boundary layer detach somewhere on the rear part with a big increase in drag.
  • As already pointed out in other answers/comments, a cylindrical fuselage is easier to manufacture and to be eventually stretched or shortened (with the A318/A319/A320/A321 being a very good example of this point).
  • Since this aircraft is going to fly transonically, it might be wise to use some kind of area ruling to reduce drag. In this case the radius of the fuselage close to the wing should be actually reduced more than increased, like in the famous F-102. A T-tail won't help in this regard either.

All in all this egg-shape is not going to be efficient by a structural nor aerodynamic point of view, again no matter what the PR department says.

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To add an other important point: manufacturing.

A tube is relatively easy to make in pieces and to join them, with the aviation constraints: quick change in temperature, pressured. Not to forget the shape (fracture/fatigue in metals): you see it also on window shape, so you have constrains on the shape on connecting pieces. With composite materials, we do not have so many constraints, but you still have unique pieces, and so certifying all of them (with quality controls: you have every connection with different number of rivets).

Also part of manufacturing is the transportation: we rely much on rail and truck to move pieces, so with limited size. So egg shaped would have a smaller cross section but in one point. Note: such limitation could be removed (as we do with larger aircraft), but it is expensive (e.g. using Beluga or Dreamlifter, or moving manufacture in one place). Feasible, but costly (and lack of funds for such things).

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