Questions on Thrust

[Macgyver]

Hello everyone,

I have a few questions on "thrust," and I hope some of you guys/gals can shed some light. (keep in mind I have no experience whatsoever flying any high-performance or turbine machines).

1. I don't understand why the "thrust available" curve when considering jets, is a straight horizontal line (i.e. y-axis= thrust in lbs, x-axis= velocity). To me, it seems that when a jet wants to go faster, the pilots must increase the thrust setting, thus implying higher velocity= higher thrust available. Obviously, I am misunderstanding something here- perhaps I am confused as to what exactly is meant by "thrust available" and how it is measured??

2. I do not understand why a jet engine has a low specific fuel consumption at higher altitudes. I understand that for the most part, prop a/c lose efficiency at higher altitudes due to the reduced air density; however, some of the books I am reading seem to imply that a jet engine is most efficient at higher altitudes- why is this? Why is fuel flow reduced at higher altitudes? Is it not true that in order to maintain a certain % of power, that you must increase your thrust setting as altitude increases, thus causing higher fuel-flow rates? For example, if you're flying a jet (737, A319, etc), does a thrust setting (assume N1= 65% and EPR= 1.7.... I just made up these numbers, as I have no idea what they may be in a real a/c) at 3000ft ASL have a significantly higher fuel flow rate for the exact same setting at a much higher altitude, like FL330? I don't understand the "why" behind all of this.

I appreciate any input. Thanks in advance, and merry Christmas!

[Tinstaafl]

A somewhat generic answer, I'm afraid. Blame it on too much booze just before xmas!

I'm surmising that you're familiar - and comfortable - with prop/piston performance charts/parameters? As usually presented to pilots, I mean?

Generally a turbine engine takes air (a lot of it!), adds sufficient fuel as is necessary to produce a flame, and thereby increases the overall temp. of the gas. The increased temp. - in other words a conversion of the chemical energy contained in the unburnt fuel into a combination of pressure & kinetic energy in the burnt air/fuel combination - is then converted via a series of divergent and convergent nozzles into throwing the air out of the back of the engine at a greater mass flow than it came in.

Newton showed the principle that a force in one direction produces an equal, and opposite, force ie shove a mass in one direction, get an opposite force in the other direction. This is the same as the 'thrust' that you asked about.

Thrust is equal to a mass (eg air) accelerated in a direction. If that happens to be an engine chucking air in one direction then a force in the opposite direction will be experienced ie thrust.

You may notice that all of this is more or less directly related to buring fuel. Fuel has a limited amount of chemical energy available in it. Burn more fuel --> get more temperature --> get more pressure/velocity --> get more thrust. In jets this is the same as saying "...burn more fuel, get more thrust.".

As it happens this relationship is more or less linear. Double the fuel burnt = double the thrust. For any given speed a certain amount of thrust is required to maintain that speed.

Except in a piston engine.

Piston types introduce a time factor. The fuel must be burnt in a relatively fixed period. This time limit is pretty much the same as saying the energy in the fuel must be released in a certain amount of time. This is also what most people think of as power.

Jet engines are most efficient operating near their maximum operating RPM. To produce a particular required amount of thrust then operating at a lower, or higher, RPM will take more fuel. As the engine gains altitude then the RPM required to produce any given thrust will increase ie at low altitudes a jet engine will need a relatively low RPM to produce a particular thrust. At high altitudes the same engine will need more RPM to produce the same amount of thrust. At some point the most efficienct altitude that produces the desired amount of thrust will be reached. Too low an altitude and too low an RPM will produce that thrust setting. Too high an altitude then too much RPM will be required. Somewhere in between is the 'sweet spot'.

The catch is that a particular airframe at a given weight will have a most efficient speed. This speed, in turn, will require a certain amount of thrust. The trick is to fly at the altitude where the engine RPM happens to be at the most efficient to produce the thrust required for the aircraft's speed.