The Avro Lancaster was a British four-engined strategic bomber that was used as the RAF's principal heavy bomber during the latter half of the Second World War. The typical aircraft was powered by an arrangement of four wing-mounted Rolls-Royce Merlin piston engines, each of which drove a 13 ft (4.0 m) diameter de Havilland Hydromatic three-bladed propeller. While not optimal, the Lancaster was capable of flying the return journey home on only two operational engines, along with very limited distances on a single running engine.[20] Aviation authors Brian Goulding and M. Garbett have claimed that experienced Lancaster pilots were often able to out-manoeuver Luftwaffe fighters. It possessed largely favourable flying characteristics, having been described by Goulding and Garbett as being: "a near-perfect flying machine, fast for its size and very smooth...such a delightfully easy aeroplane to fly...there are instances of Lancasters having been looped and barrel-rolled, both intentionally and otherwise".