such as the tendency for the aircraft to slowly spin on its axis while in a vertical hover; its pilot also noted that the vertical speed indicator was unrealistic and that the controls were incapable of steering the aircraft with precision while performing the critical landing phase. Dead-stick landings were deemed to be an impossibility. One of aircraft's flights involved a public display of its hover performance before an assembled audience. The eye-catching design of the Coléoptère rapidly made waves in the public conscious, even intentionally; author Jeremy Davis observed that the aircraft had even influenced intentional efforts, having allegedly motived the United States Navy to contract American helicopter manufacturer Kaman Aircraft to design its own annular-wing vehicle, nicknamed the Flying Barrel. Accordingly, the United States experimented with its own tail-sitters, typically involving propeller-driven design configurations with relatively conventional fixed wings. The Convair XFY Pogo was one such aircraft, featuring a delta wing with cruciform tail configuration; initial test flights were conducted inside of a naval airship hangar at Moffett Field in Mountain View, California. The XFY successfully demonstrated the full transition between flight modes on 5 November 1954. A somewhat similar aircraft was the Lockheed XFV Salmon, which paired a straight wing with an X tail; however, the XFV never achieved the crucial flight transition. However, it became evident during flight testing that such VTOL aircraft would be flown only by the most experienced pilots, even if all technical problems were disregarded; thus, it was not feasible to place VTOL fighters—as previously hoped for—on every ship. Also, whereas jet-engined fighters had top speeds that approached Mach 2, the turboprop VTOL fighter was at a disadvantage due to its maximum speed being below Mach 1. As a result of these circumstances, work on the XFY was halted. During 1955, the United States commenced flight testing of a jet-powered design, the Ryan X-13 Vertijet. Two prototypes were constructed, both of which flew, made successful transitions to and from horizontal flight, and landed. The X-13's final test flight was conducted near Washington DC during 1957. An inherent problem with all these tail-sitter designs was poor pilot visibility, especially of the ground, during vertical descent and landing. Ultimately, most work on applying the concept towards manned aircraft were abandoned upon the arrival of more practical form of VTOL appeared, in the form of thrust vectoring, as used by production aircraft such as the