1. Prandtl, L.: Induced drag of multiplanes. National Advisory Committee for Aeronautics, Technical Note No. 182, From Technische Berichte, vol. III, no. 7, pp. 309–315 (1924)
2. Demasi, L., Dipace, A., Monegato, G., Cavallaro, R.: Invariant formulation for the minimum induced drag conditions of non-planar wing systems. AIAA J. 52(10), 2223–2240 (2014). doi: 10.2514/1.J052837
3. Demasi, L.: A theoretical investigation on the conditions of minimum induced drag of closed wing systems and c-wings. J. Aircr. 44, 81–99 (2007)
4. Kroo, I.: Nonplanar wing concepts for increased aircraft efficiency. In: VKI Lecture Series on Innovative Configurations and Advanced Concepts for Future Civil Aircraft, 6–10 June 2005. https://store.vki.ac.be/lecture-series-monographs/transport/innovative-configurations-and-advanced-concepts-for-future-civil.html
5. Von Karman, T., Burgers, J.M.: General aerodynamic theory- perfect fluids. In: Durand, W.F. (ed.) Aerodynamic Theory, vol. II, pp. 201–222. Springer, Berlin (1935)