What about the DC-9's wing makes wingletting it so difficult? A common way of making older airliners more efficient without increasing their wingspan is to add winglets, either as a retrofit for existing aircraft or as a new, wingletted derivative of a preexisting non-wingletted model.

One particular older airliner for which this doesn't work, however, is the DC-9; winglets for the DC-9 have been studied, but never made it into production.  One of the main reasons for this (alongside the relative smallness of the benefit that could be gained by wingletting a DC-9, compared to that gained by wingletting other types of airliner, and the increasing smallness of the world's DC-9-classic1 fleet) is that the DC-9's wing structure makes it unusually difficult to safely and successfully winglet:


  APB has investigated a retrofit design for the DC-9 family. Those design studies have not been successful in creating a viable business case. Projected block fuel burn reductions of less than 2 percent are offset by substantial modification costs. The limited potential for the DC-9 is a result of the existing wing structure, which hinders installation of a large winglet, as was possible on the Boeing 737 family. Since the DC-9 has been out of production since the early 1980s, the fleet size has shrunk and the fleet has aged, making the business case for a retrofit winglet or wingtip not as attractive as that for the Boeing 737 and 757 families.  [Emphasis added.]


What is it about the DC-9's wing that makes it so hard to winglet, compared to (say) a 737?



1: The DC-9-10 through DC-9-50, but not the DC-9-80 or DC-9-90.
I do not know, and I have looked for answers for a little bit. Interesting question. I have seen it said that the DC-9 has a high aspect ratio, which may be a factor -- resisting a wing-axial torque is easier with a longer chord, which all things being equal, would be a lower aspect ratio.  Still, a split winglet (part up, part down) or tip fence like on the A320 could mitigate that particular torque. Ground clearance comes into play then, with the low wing and very low dihedral.  

So, my other suspicion is that resistance to flutter might be an issue -- podded engines (podded correctly) combat flutter. Rear-mounted engines do not. If the winglet tended to induce flutter, it might be tougher to suppress. Nobody wants to re-engineer the spar box for the scanty gain offered.