From the Wikipedia article on parasitic drag:

a designer can consider the fineness ratio, which is the length of the aircraft divided by its diameter at the widest point (L/D). It is mostly kept 6:1 for subsonic flows. Increase in length increases Reynolds number. With Reynolds no. in the denominator for skin friction coefficient's relation, as its value is increased (in laminar range), total friction drag is reduced.

The higher fineness ratio gives a longer flowpath, slowing the air more (=higher Reynolds). A slower boundary layer creates less friction downstream.

But increasing the fineness ratio increases the total surface area as well. And that is a recipe for increased friction. From the Wikipedia article on fineness ratio:

As the name implies, this is drag caused by the interaction of the airflow with the aircraft's skin. To minimize this drag, the aircraft should be designed to minimize the exposed skin area, or "wetted surface"

So, what am I missing?

  • $\begingroup$ Done, though these quotes are from the current article. $\endgroup$ – Abdullah May 18 at 15:13

Basically, your confusion comes from not knowing what needs to be held constant. When you increase the fineness ratio without increasing wetted-area, i.e. you hold wetted-area constant, the overall drag goes down. You can see this in racing shells (sculling). The shells (boats) have an absurd fineness ratio, very narrow and very long. This decreases the overall drag (somewhat, there are other forces at work there).

As you increase the finess ratio, at the expense of increasing the wetted area, you will need to do a trade-study to determine if you gain an advantage. You probably will, up to a fineness ratio of 6:1. Much beyond that the extra drag from the increase in wetted-area starts to dominate any savings in drag due to body fineness.

The inital statement is true, presuming you keep the wetted-area constant as you adjust the fineness ratio.


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    $\begingroup$ then the volume decreases. $\endgroup$ – Abdullah May 27 at 9:02

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