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1

There are several effects which in combination make constant thrust a good approximation at subsonic speed. Thrust is created by accelerating a working mass in opposite direction. Net thrust is the difference between the impulse of the air flowing towards the engine and the combined impulse of burnt fuel and the air exiting the engine (and propeller, if one ...


3

Assuming that the net thrust of a turbojet is constant is not correct. It is assumed to be constant (for simplicity by the aircraft performance engineers and usually valid for low subsonic speeds), but in reality, the performance is not constant, and it also varies with altitude. This is best shown by a simple simulation of a turbojet engine. The following ...


23

There are basically 3 limits that the engine faces, temperature (maximum turbine entry temperature or maximum compressor exit temperature), pressure (maximum compressor exit pressure) and stress (maximum stress in the blades as a result of spool speed). Varying the OAT for a specific engine design will hit one of these limits. When the OAT increases, the ...


12

What you see is called a flat rated engine. It means the maximum thrust from the engine is constant below the flat rated temperature (usually 30°C). Above that temperature, thrust will decrease due to the EGT (exhaust gas temperature) limit. In order to achieve a constant thrust at lower temperatures, the N1 needs to be decreased accordingly. (CFM56-5A ...


2

First, you should state whether you are designing an engine (increasing the TIT) or if you are doing an off design calculation for an existing engine. In the case of design we gas turbine performance engineers call this design point analysis, increasing the TIT indeed would increase the efficiency, but this would require an increase of the pressure (OPR, ...


0

Well, propellers deliver thrust and turbofan engines deliver power to the fan axis, it is just a matter of choice. Finding the thrust that a propeller generates at a specific airspeed and altitude, can be derived from the drag of the whole airframe in those conditions. Statistically, this sort of data can be found in reference books on pre-design of ...


2

Large airliners don't fly at slower speeds. It's simple economics. If you have to connect two destinations 10,000 miles apart, you'll do maybe 2 round trips a week. Your passengers have no other choice, except a flight that takes even longer. You also have the luxury of flying between two very large cities. If you have to connect two destinations 100 ...


1

Why wouldn't an airline choose the MD-90, which was more efficient? Airliners are usually retired because the airframe reaches the end of its useful life, not because the engines are "obsolete." Towards the end of an airliner's production run, the manufacturing process has been optimized and the cost to the manufacturer is lowest. They are willing to give ...


1

Also define obsolete. The 737, one of the (if not The) most successful airliner in the world was designed about the same time (1960s) and it shows no sign of seeing its production being stopped. About narrow body = obsolete, I think you will find members of the CRJ (four-across seating [1]) and ERJ families that are not only are narrowbodies but look very ...


5

The MD80 was not obsolete at the time of its first flight and the decade thereafter. The market situation for 150 pax aeroplanes around 1980: Boeing B727-200: first flight 1967. Three JT8D-7/9/11 engines. MD-80: first flight 1979. Two JT8D-200 engines. The first generation of JT8D had a bypass ratio of 0.7, the JT8D-200 engines had an increased bypass ...


2

Regarding the aspect of obsolence: in contemporary personal land transport, the Otto engine is, by pretty much all means, obsolete. There are very few cases in which the Otto engine outperforms an all-electric drivetrain, but these cases hardly make up more than 10% of the field. Yet... The Otto engine prevails. Why? The infrastructure is there. The ...


4

There's also benefit to parts commonality between aircraft, where if all of the DC-9 variants that an airline was flying were using JT8D engines, then that's one less airframe difference to be concerned about when it comes to maintenance and parts supplies.


39

It's simple. Cheap (development costs amortized decades before) and reliable. They made money for airlines. Or, you know, they wouldn't have bought them. There's more to operating costs than fuel burn, and in any case, fuel prices in the mid 90s were cheaper than at any time since the 20s in constant dollars. Fuel was so cheap that Air Canada was ...


9

First, the airframe and systems were well-engineered and had few flaws and good aerodynamic qualities. Second, the cost to re-engine it with higher bypass ratio engines would have been high, and as such needed to be included in the cost-benefit analysis and balanced against the costs of operating the plane as-is with its original engines. The expected ...


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