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The first XV-15 prototype aircraft, N702NA, was transferred back to Bell for company development and demonstration use. On 20 August 1992, the aircraft crashed while being flown by a guest test pilot. He was lifting off for a final hover when a bolt slipped out of the collective control system on one pylon, causing that rotor to go to full pitch.
The FAA states "The height–velocity diagram or H/V curve is a graph charting the safe/unsafe flight profiles relevant to a specific helicopter. As operation outside the safe area of the chart can be fatal in the event of a power or transmission failure it is sometimes referred to as the dead man's curve."
In this test, a 40% scaled BO-105 rotor model along with a fuselage is used, a range of sophisticated measurement techniques are introduced to measure the noise level, blade surface pressure, tip vortices, blade motions, and structural moments with and without the application of HHC (Higher Harmonic Control) pitch control inputs. [7]
Helicopter rotors are designed to operate at a specific rotational speed. The throttle controls the power of the engine, which is connected to the rotor by a transmission. The throttle setting must maintain enough engine power to keep the rotor speed within the limits where the rotor produces enough lift for flight.
XV-3 in forward flight Bell XV-3 in a hover, 1955. This was first version with 3-blade rotors and crashed. XV-3 test, in vertical flight with 2 blade rotors. In 1951, the Army and Air Force announced the Convertible Aircraft Program and released the Request for Proposals (RFP) to solicit designs from the aircraft industry.
The fenestron consists of 11 blades spinning inside a circular housing at the base of the helicopter's tail fin. Certified for single-pilot instrument flight rules (IFR) operation, the HH-65A was the first helicopter certified with a four-axis autopilot, allowing for hands-off hover over a pre-determined location.
The American Helicopter Society (AHS) International's Igor I. Sikorsky Human Powered Helicopter Competition was a competition to achieve the first human-powered helicopter flight to reach an altitude of 3 m (10 ft) during a flight lasting at least 60 seconds, while remaining within a 10 m (32.8 ft) x 10 m (32.8 ft) square, and complying with other competition requirements. [1]
Even when hovering, the rotor tips may be travelling at a significant fraction of the speed of sound. As the helicopter accelerates, its overall speed is added to that of the tips, meaning that the blades on the forward-moving side of the rotor sees significantly higher airspeed than the rearward-moving side, causing a dissymmetry of lift.