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It is usually stated than a tear drop shape is a good shape for aerodynamic efficiency:

enter image description here
Adapted from Nasa

On the other hand, fuselage on an airliner has most of the time a long cylindrical section with round/tapered ends. I understand an aircraft has other constraints than just having the lowest possible drag. It comes to mind a cylindrical shape allows the same number of seats in most of the rows and also prevents to increase the section of the forward cabin.

Without further explanation, it seems curious the fuselage has not a shape between a rounded cylinder and a tear drop shape which is often seen for gliders:

enter image description here
Source

Without going into deep details, what are the most important reasons preventing such design?

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    $\begingroup$ Note that even raindrops aren't raindrop-shaped. $\endgroup$ – David Richerby Dec 29 '16 at 19:34
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    $\begingroup$ I'd think the primary reason for not using this shape is the necessity of carrying passengers and cargo. $\endgroup$ – Michael Hampton Dec 29 '16 at 21:40
  • $\begingroup$ There are different bullet designs. Some have a parabolic nose. Others have noses shaped from two intersecting circles and have sharp points. Many airplanes have parabolic noses too, as it is very efficient. $\endgroup$ – posfan12 Dec 30 '16 at 19:58
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On the other hand, fuselage on an airliner is most of time bullet shaped.

No, it is not. A “bullet shape” has a flat end, which is where most of the drag is generated (at subsonic speeds). But that shape is never used on flying vehicles except on rockets where the end is occupied by the rocket engines.

Instead, airliners always have a tapered “boat” tail, sloping gently enough to avoid separation (sometimes with help of strakes). The resulting shape is as if you split a droplet in the widest point and insert a piece of cylinder. And the difference in drag coefficient between pure droplet and droplet with cylinder section inserted is very small.

Still, a continuously varying shape is slightly better than the cylindrical section. And the early aircraft did have that. Likely the last one of those was the Lockheed Constellation. The shape was great aerodynamically, but it was expensive to build, it made it more difficult to ensure all elements have proper strength to withstand the pressurisation, every time they derived a larger version they had to redesign all frames and in operation it was not exactly convenient for loading either.

In contrast the tubular section is much easier to design, easier to make and when they want to make a bigger version of the plane, they just insert a couple more frames and mount stronger engines to match the increased weight—between A318 to A321 or between all variants of B737-MAX, vast majority of parts is the same and there is just different number of frames and different engines. And it's simple to load—seats are mounted on straight rails and containers simply slide in to the cargo bay. And the difference in drag is small enough that the cheaper manufacturing and practicality will easily offset the increase in consumption.

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    $\begingroup$ That's an interesting angle. $\endgroup$ – mins Dec 29 '16 at 19:28
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    $\begingroup$ @mins: And deserves to be accepted. Again, the most votes don't show which answer is spot on. $\endgroup$ – Peter Kämpf Dec 29 '16 at 21:58
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A few things

  • One of the main objectives for glider flight is making it as aerodynamically efficient as possible. For airliners, it is to make it as economically efficient as possible. This means that it has to carry as much as passengers (or cargo; sometimes, there is little difference) for the given fuselage volume.

  • Glider type designs in airliners will increase the possibility of tail strike significantly.

  • What is good for low speed flight is not exactly great for the high speed flight regime. The loads associated are different.

  • The aerodynamic design of glider is based on one thing- there is no engine. This means that the drag has to be kept an absolute minimum for any meaningful flight time. The airline designer, however, has the luxury of engines- he/she can pay some penalty in the form of drag to improve other performance characteristics.

  • Any design is a tradeoff- the structure at the rear of a glider eats into its carrying capacity. It is tolerated because gains from reducing drag outweigh the added weight. However, this is not the case in an airliner.

  • Aerodynamic efficiency is not everything- there are aircraft which are blatantly inefficient aerodynamically, but fly anyway because they serve the purpose.

  • The wing carries the fuel in an airplane. A teardrop shaped fuselage would require the wing to be well ahead of the cg (or you have to strengthen and increase size of the fuselage where the spar passes through). This would increase the size of the horizontal stabilizer (or require lengthier fuselage), increasing weight.

  • There are practical issues too- fuselage shaped like this would increase the boarding/deboarding time, reduce potential for further development (its difficult to plug fuselage sections in that case) among others.

One important point is that you're considering only recreational gliders. If you consider gliders designed for transport, you can see that their shape resembles airliners- a good example would be the troop carrying gliders of WWII.

Horsa

Airspeed Horsa; Public Domain, Link

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    $\begingroup$ Additionally I believe fuselage shape is also influenced to some degree by area rule. $\endgroup$ – TomMcW Dec 29 '16 at 21:10
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    $\begingroup$ Downvoted because you buried the only real reason why airliners have constant cross section fuselages in so many superfluous words. Look at @Jan Hudec's answer for enlightenment. $\endgroup$ – Peter Kämpf Dec 29 '16 at 21:56
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Airliners do actually have an elongated teardrop shape to them in a side profile. In aerodynamics this is generally referred to as 'boat tailing' an object.

The glider has much more of a tear drop shape - and by that I take it you mean the tadpole like shape of the fuselage with a long slender tail - empty because there is no need for additional systems which are voluminous. The pilot, avionics, controls, landing gear, etc. all fit into the forward pod without a problem. All the tailboom really needs to carry are the control linkages and structure. Here it makes sense to have a long slender tail, just strong enough for structural purposes while minimizing skin friction drag.

Now most of the fuselage of an airliner is taken up by the passenger carrying pressure vessel which has to remain very large in order to do that job effectively. It's also long enough that it will reach from nose to tail of the aircraft such that very little additional length is required to provide effective tailplane authority for weight and balance purposes. A long slender tailboom is. not necessary here.

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    $\begingroup$ "NACA/NASA diagram you showed has the relative wind striking the object from a side profile" Not sure I understand what you mean and why this reservation. It seems the appropriate way to compare airfoils. $\endgroup$ – mins Dec 29 '16 at 18:16

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