UNIT 11
Concorde – first supersonic passenger aircraft
Vocabulary: fly-by-wire flight control system, bypass ratio, inlet, shock wave, ramp, pitch, bank, elevon, anti-lock braking system, lift-to-drag-ratio, long-haul flight, streamlined design.
Concorde was an ogival delta-winged aircraft with four powerful Olympus engines based on those originally developed for strategic bomber. The engines were jointly built by Rolls-Royce and SNECMA. Concorde was the first civil airliner to have an analogue fly-by-wire flight control system. It also employed alowering nose section for visibility on approach. These and other features permitted Concorde to have an average cruise speed of Mach 2.02 with a maximum cruise altitude of 18,300 meters, more than twice the speed of conventional aircraft. The average landing speed was a relatively high 298 km/h (185 mph, 160 knots).
To be economically viable, Concorde needed to be able to fly reasonably long distances, and this required high efficiency. For optimum supersonic flight, the engines needed to have a small frontal cross-sectional area to minimize drag and a low bypass ratio to give a high, supersonic exhaust speed. Turbojets were thus the best choice of engines. The more efficient and quieter high bypass turbofan engines such as used on Boeing 747s could not be used. The engine chosen was the twin spool Rolls-Royce/Snecma Olympus 593.
The inlet design for Concorde engines was critical. All conventional jet engines can intake air at only around Mach 0.5; therefore the air needs to be slowed from the Mach 2.0 airspeed that enters the engine inlet. In particular, Concorde needed to control the shock waves that this reduction in speed would avoid damage to the engines. This was done by a pair of ramps and an auxiliary flap, whose position was moved during flight to slow the air down. The ramps were at the top of the engine compartment and moved down and the auxiliary flap moved both up and down allowing air to flow in or out. During takeoff, when the engine air demand was high, the ramps were flat at the top and the auxiliary flap was in, allowing more air to enter the engine. As the aircraft approached Mach 0.7, the flap closed; at Mach 1.3, the ramps came into effect, removing air from the engines which was then used in the pressurization of the cabin. At Mach 2.0, the ramps covered half their total possible distance. They also helped reduce the work done by the compressors as they did not only compress the air but also increase the air temperature.
Due to jet engines being highly inefficient at low speeds, Concorde burned two tones of fuel taxiing to the runway. To conserve fuel only the two outer engines were run after landing. The thrust from two engines was sufficient for taxiing to the ramp due to low aircraft weight upon landing at its destination.
When any aircraft passes the critical Mach of that particular airframe, the centre of pressure shifts rearwards. This causes a pitch down force on the aircraft, as the centre of gravity remains where it was. The engineers designed the wings in a specific manner to reduce this shift. However, there was still a shift of about 2 meters. This could have been countered by the use of trim controls, but at such high speeds this would have caused a dramatic increase in the drag on the aircraft. Instead, the distribution of fuel along the aircraft was shifted during acceleration and deceleration to move the centre of gravity, effectively acting as an auxiliary trim control.
Due to the high speeds at which Concorde traveled, large forces were applied to the aircraft structure during banks and turns. This caused twisting and distortion of the aircraft structure. This was resolved by the neutralization of the outboard elevons at high speeds. Only the innermost elevons, which are attached to the strongest area of the wings, are active at high speed.
Due to a relatively high average takeoff speed of 250 mph (400 km/h), Concorde needed good brakes. Concorde brakes were one of the first major users of anti-lock braking systems, which stop the wheels from locking when fully applied, allowing greater deceleration and control during braking, particularly in wet conditions.
The brakes were carbon-based and could bring Concorde, weighing up to 185 tons (188 tones) and traveling at 190 mph (305 km/h), to a stop from an aborted takeoff within one mile (1600 m). This braking manoeuvre brought the brakes to temperatures of 300 °C to 500 °C, requiring several hours for cooling.
Concorde needed to travel between London and New York or Washington nonstop, and to achieve this the designers gave Concorde the greatest range of any supersonic aircraft at the time (since beaten by the Tu-160). This was achieved by a combination of careful development of the engines to make them highly efficient at supersonic speeds, by very careful design of the wing shape to give a good lift-to-drag-ratio, by having a relatively modest payload, a high fuel capacity, and by moving the fuel to trim the aircraft without introducing any additional drag.
Nevertheless, soon after Concorde began flying, a Concorde "B" design was produced with more powerful engines with slightly bigger fuel capacity and slightly larger wings with improved aerodynamic performance at all speeds. This would have given 500 km greater range even with greater payload. This was cancelled due to poor sales of Concorde.
The high altitude at which Concorde cruised meant passengers received almost twice the flux of extra-terrestrial ionizing radiation as those traveling on a conventional long-haul flight. Because of the proportionally reduced flight time, however, the overall equivalent dose was lessthan a conventional flight over the same distance. Unusual solar activity led to an increase in incident radiation, so the flight deck had a radiometer and an instrument to measure the rate of decrease of radiation. If the level was too high, Concorde descended to below 47,000 feet (14,000 m).
Concorde's famous drooping nose was a compromise between the need for a streamlined design to reduce drag and increase aerodynamic efficiency in flight and the need for the pilot to see properly during taxi, takeoff, and landing operations. A delta-wing aircraft takes off and lands with a high angle of attack (a high nose angle) compared to subsonic aircraft, due to the way the delta wing generates lift. The pointed nose would obstruct the pilots' view of taxiways and runways, so Concorde's nose was designed to allow for different positioning for different operations.
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