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It seems like making the A380 a double-decker increases the cross-sectional area of the fuselage, thereby increasing drag on the airplane. Wouldn't it be more efficient to fit so many seats in a longer airplane rather than a wider airplane?

So, is the A380 a double-decker because there is a limit to how long an airplane can be, would it be impractical to make a longer airplane, am I wrong about the cross-sectional area/length trade off when it comes to drag, or is there some other reason?

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That's right, having a longer plane would reduce drag by lowering the cross section (altho there is also drag linked to the surface area of the plane).

There is a few reasons why you don't want to do it:

  • Cylinders don't scale this way:

Take a cylinder with a radius of 1 and a height of 10.

Volume is ≈31.41

Double the height, volume is ≈62.83

Double the radius, volume is ≈125.66

Increasing the length scale linearly and increasing the volume scales at π * n * n

  • You will need a stronger structure to support the additional length and surface area of your pressure vessel, oscillations during turbulences, takeoff rotation, ...
  • All your cables, hydraulics, fuselage, will have to be stretched
  • Passengers will take forever to reach the back of the plane when boarding
  • Maneuvering in the airport will be impossible, you will need new airports (credit apple maps)
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    $\begingroup$ TODO: add area comparison ( a good proxy for weight?), $\endgroup$ – Antzi Sep 21 '18 at 3:59
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    $\begingroup$ Skin friction drag is not a marginal factor. $\endgroup$ – Pilothead Sep 21 '18 at 14:36
  • $\begingroup$ Though WRT structural strength, I can't help but wonder about lengthening the aircraft by adding another set of wings... $\endgroup$ – jamesqf Sep 21 '18 at 18:12
  • $\begingroup$ @jamesqf Stop playing KSP :) The main issue will be a disturbed airflow for the back wings $\endgroup$ – Antzi Sep 22 '18 at 4:36
  • $\begingroup$ Would not mind somebody stepping up and designing a modern biplane. Dunne was on the right track with swept wings and closing the ends by placing vertical stabilizers there. Some designers want to bring the second wing back to the tail, but this may be less stable than good old Tiger Moth. A biplane, with GE fan jets, could carry a lot of payload, or go higher. $\endgroup$ – Robert DiGiovanni Sep 30 '18 at 13:44
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When designing the A380 Airbus made the decision that it would have to be able to use existing airport infrastructure, so the footprint could not be much larger than that of a Boeing 747. If they made it longer or gave it a wider wingspan airports would have to be redesigned to serve it, for instance gates would have to be spaced farther apart, taxiways would have to be changed to give clearance and accommodate a wider turning radius, These costs would have been prohibitive and the airplane would not have found customers.

Given the footprint constraints a double deck concept was the only workable solution for the number of passengers they wanted to fit.

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    $\begingroup$ Even with staying in the 747 configuration, airport gates had to be redesigned to accommodate the A380. At many airports closes the adjacent gates when boarding because of wingspan. Nobody would fly it if they had to reengineer taxiways and runways. It's an amazing plane, but issues like this have hampered sales. $\endgroup$ – gwally Sep 21 '18 at 15:53
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Your assumption that a longer fuselage has lower drag is incorrect.

Optimal pressure drag (form or diameter driven) vs skin friction drag (surface area driven) can be determined using fineness ratio. From Roskam and considering subsonic aerodynamics only, the optimal ratio is 6:1. Including other factors such as ease of use or manufacture, the ratio can go higher.

enter image description here

The A380 fuselage at 7.15m wide and 8.4m tall is 7.77m average diameter. It is 72.7m long so even as a double decker it is more than 9:1, higher than optimal. The curve is very flat beyond 4:1, so the length of the A380 is driven by considerations other than fuselage aerodynamics which mildly argue for an even fatter fuselage.

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