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The Lockheed Martin X-59 was rolled out yesterday :-)

I am no expert here (and no English native), but to me, I can see the shape more or less according to "area rule" until the engine intake comes up and the wing "is still there".

Does it make any sense? Does the "area rule" not matter after (e.g.) 70% of the aircraft length axis? Does the fact that it is an air intake change the "area rule" effect (i.e. it's sucking air in)?

Also, I see a lot of riveting on the fuselage, I would be expecting a lot "cleaner" surface at the front (check "green area" under cockpit). Maybe it's just because it's a prototype?

Thanks in advance! This is my first question at this site ever! I have been following this forum for long and it's my favorite :-)

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    $\begingroup$ Welcome to Av.SE - nice first question, about a wild-looking machine! $\endgroup$
    – Ralph J ♦
    Aug 5 at 17:20
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    $\begingroup$ Well the engine starts where the wing becomes thinner and thinner so it might be that some area rules is anyway preserved at least until there. Anyway the main purpose of the X-59 is to reduced the sonic boom, not the drag 😉 $\endgroup$
    – sophit
    Aug 5 at 17:59

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One of the weird things about area ruling -- the area that goes 'through' the engine does not count towards your area. So although the engine looks like a large increase in area, it isn't from the fluid's point of view.

In most simple terms, the wave drag is caused by pushing the air out of the way -- the air that goes through the engine does not get pushed out of the way.

As mentioned other places, the X-59 is designed to minimize the sonic boom -- not to minimize the drag. It can't have a horrible area distribution for drag (otherwise the thrust requirement would get prohibitive), but the area distribution is optimal for boom. Fortunately, these shapes are somewhat correlated -- strong shocks that cause lots of drag also cause loud boom. So while there is a drag/noise tradeoff, they aren't entirely opposed metrics.

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  • $\begingroup$ Thanks a lot. So my feeling on the air intake not counting for the area rule was on the right direction :-) After some more googling I read that wing mounted jet engines do not account for the area rule but they do if they are on the fuselage... I would appreciate any link where I can learn a bit more about the topic :-) $\endgroup$
    – Benur
    Aug 6 at 12:07
  • $\begingroup$ I don't have my hands on a good reference for this, but the way you typically model engines (body or wing mounted) in a wave drag code is to take the inlet shape and extend it far in front of the aircraft. Then you take the outlet shape and extend it far aft of the aircraft. When you take cross sections, the first and last cross sections will have non-zero area (they will have the capture and exhaust area, respectively). You can then subtract out the inlet area and handle the exhaust area in a number of ways (it doesn't really matter how). $\endgroup$
    – Rob McDonald
    Aug 7 at 17:29
  • $\begingroup$ The wave drag calculation actually uses the 2nd derivative of the area distribution, so an offset or linear function added to the area won't matter. So you extend the inlet and exhaust tubes to infinity, take the cross section areas, subtract them out, take the second derivatives, and go from there. $\endgroup$
    – Rob McDonald
    Aug 7 at 17:31

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