Tail engines add more weight. Placing the engines in pods underneath the wing relieves the bending moment in flight, allowing for lighter wing root construction. Placing them at the tail requires local beefy frame construction.
Torenbeek mentions the following on this subject:
Empty weight. The following factors have to be considered:
- A wing structure weight saving is possible with wing-mounted engines due to the mass relief effect on the bending moment on the inner wing.
- Engines placed too far outboard increase the landing impact loads and necessitate a large vertical tailplane.
- Engines at the rear of the fuselage require local "beef-up" and lead to loss of useful space in the tail, resulting in added structure weight and a larger fuselage for the same payload.
- Differences in weïght of the tail surfaces depend on various factors which do not permit a general conclusion.
Summing up, we may say without too much emphasis that the empty weight of a CaraveIle-type layout will typically be 2 to 4% more than that of a comparable design with the engines on the wing.