These limitations meant that SST designs were not able to take advantage of the dramatic improvements in fuel economy that high bypass engines brought to the subsonic market, but they were already more efficient than their subsonic turbofan counterparts.
12.
For example, the JT8D, a bypass engine, has a jet velocity of 1450 ft / sec whereas the JT9D, a turbofan, has jet velocities of 885 ft / sec ( cold ) and 1190 ft / sec ( hot ).
13.
A commercial version, designated the CJ805, powered the Convair 880, while an aft-turbofan derivative, the CJ805-23, powered the Convair 990 airliners and a single Sud Aviation Caravelle intended to demonstrate to the US market the benefits of a bypass engine over the existing Avon turbojet.
14.
Another military-to-civilian success followed when GE was selected to supply engines for the S-3 Viking and Fairchild Republic A-10 Thunderbolt II, developing a small high-bypass engine using technologies from the TF39 . The resulting CF34, whose wide variety of models powers many of the regional jets flying today.
15.
The configuration was similar to a 2-spool turbojet but to make it into a bypass engine it was equipped with an oversized low pressure compressor : the flow through the inner portion of the compressor blades went into the core while the outer portion of the blades blew air around the core to provide the rest of the thrust.
16.
Following changes to the ISO standard, D-frequency-weighting should now only be used for non-bypass engines and as these are not fitted to commercial aircraft & mdash; but only to military ones & mdash; A-frequency-weighting is now mandated for light civilian aircraft measurements, while a more accurate loudness-corrected weighting EPNdB is required for certification of large transport aircraft
17.
Although successful in this role, only 37 707s and 32 DC-8s were built with the Conway, due largely to the delivery of the first US-built bypass engines, particularly the Pratt & Whitney JT3D . Nevertheless, the Conway was successful on these designs and was the first commercial aero engine to be awarded clearance to operate for periods up to 10, 000 hours between major overhaul.
18.
In 1954 Rolls-Royce introduced the first commercial bypass engine, the Rolls-Royce Conway, with a 21, 000 RB . 140 / 141 Medway-which itself had been cancelled after British European Airways ( BEA ) had demanded the downsizing of the Trident, the "'RB . 163 "', using the same two-spool turbine system and a fairly small fan delivering bypass ratios of about 0.64 : 1.
19.
Nevertheless, high-bypass engines have a high propulsive efficiency because even slightly increasing the velocity of a very large volume and consequently mass of air produces a very large change in momentum and thrust : thrust is the engine's mass flow ( the amount of air flowing through the engine ) multiplied by the difference between the inlet and exhaust velocities in a linear relationship but the kinetic energy of the exhaust is the mass flow multiplied by one-half the square of the difference in velocities.
