[1] Univ Naples Federico II, Dipartimento Informat & Sistemist, Naples, Italy
[2] Univ Bristol, Dept Engn Math, Bristol BS8 1TH, Avon, England
[3] Univ Politecn Catalunya, Inst Org & Control Sistemes Ind, E-08028 Barcelona, Spain
[4] Univ London Imperial Coll Sci Technol & Med, Dept Aeronaut, London, England
[5] Univ Bristol, Dept Mech Engn, Bristol BS8 1TH, Avon, England