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A Little Jet Info

Sun Jan 19, 2003 7:51 am

Hello all, I was just wondering about how many times the main fan on a jet engine completes a cycle (Once around) per second? I'm sure it depends on the particular aircraft engine,and how much power is being used. Whats the main source of propoltion (howeverr you spell that) Is it the back of the engine working to turn the big fan, or is it the other way around? also is there any good web sites where you can find out information about jet engines, or even turbo props? I'm sorry you'll have to bare with me here I'm not up on all the names and so on of specific engine parts.
Thanks for any help!
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RE: A Little Jet Info

Sun Jan 19, 2003 7:59 am

You can *kind of* think of it as a perpetual motion machine. The "big fan" pulls air in, the air is combusted, then forced out through the smaller fans in the back. The smaller fan in the back is linked to the big fan in the front, so when the back fans spin because of the combusted air, it spins the big one which draws in air, which combusts, and spins the back ones, which turns the first one, which pulls in air.......................
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RE: A Little Jet Info

Sun Jan 19, 2003 10:05 am

That makes sense, but how do you get it started when its shut down?
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RE: A Little Jet Info

Sun Jan 19, 2003 10:35 am

Another way to look at jet engine operation is that the engine must produce enough energy (the ability to do work) to do two things:

1. Rotate the compressor section of the engine, thus allowing the engine to sustain it's own operation.

2. Produce enough thrust to push the aircraft through the air.

Just sustaining it's own combustion cycle by rotating that big compressor takes up most of the energy produced by the engine... about 75% or so. The rest of the energy is used in making thrust. As for how many times per minute the compressor spins, I have no idea. Some mechanic once told me that in the turbine engine that my Dash 8 uses (PW-120 series) the high pressure compressor spins at around 48,000 RPM. I have no idea if that is correct... but lets just assume it is very, very fast!

So what is the main source of propulsion? The answer is "it depends on the engine." In large turbofan engines (like on the 777) The fan does contribute some of the thrust, which is why they are so efficient. On older jet engines (like on a 707) the thrust is totally due to the high speed expulsion of exhuast gases out of the back of the engine. On turboprops, there is a second set of turbines in the engine which spins a propeller via a reduction gear box.

You also asked, "but how do you get it started when its shut down?" Jet engines have either a pnuematic or electric starter which spins the engine until it reaches a self sustaining speed. Igniters (like spark plugs) are used to start the burning of fuel.

Long post... hope this answers some of our questions.
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RE: A Little Jet Info

Sun Jan 19, 2003 11:16 am

This might help you as well:

It talks about other gas turbine engines as well.


[Edited 2003-01-22 06:09:05]
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RE: A Little Jet Info

Sun Jan 19, 2003 6:13 pm

"In large turbofan engines (like on the 777) The fan does contribute some of the thrust, which is why they are so efficient."

Correct me if i'm wrong but I believe the fan on a high bypass engine produces MOST of the thrust, and that is why thrust is measured as N1 or EPR.
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RE: A Little Jet Info

Sun Jan 19, 2003 8:17 pm

The main power delivering part of any [non-supersonic flying!] turbine engine these days is the cold section of the engine. Reason for this is that hot air holds a lot of energy. Blowing this hot air into the atmosphere through any form of exhaust ruins the efficiency of the engine since a lot of energy is being lost by blowing it over board so to speak. Mind you, [hot] gas-energy basically is present in three forms: temperature, pressure, and velocity.

The hot section of a modern turbine engine produces a lot of hot air, at high pressure and lots of speed. So to make maximum use of this energy, additional turbine stages are being put in place behind what we call the core engine [basic engine - gas generator].
These turbines convert the hot gas energy into mechanical energy [energy which would otherwise blow off into atmosphere, thereby losing a lot of potential energy/efficiency]. That mechanical energy is now being used to drive a large fan [turbofan], or a propeller [turbo prop], or a shaft which can drive anything from a helicopter rotor to a natural gas pumps or electricity generators or even ships [turbo shaft].

Utilizing this energy to drive a fan increases efficiency dramatically, since the lost energy from a fan is much smaller than from the hot end of the turbine. The airflow which the fan is blowing into atmosphere is much cooler and slower, thereby reducing lost energy. The down turn off course is weight. One would like to make the fan as large as possible; a larger fan turns slower and thus less energy loss. But the larger fan is also very much more heavier [which you don't realy want in an airplane]. Losing the shroud would help very much in terms of weight --> propeller/propfan. However due to aerodynamic effects, the shrouded prop [better known as fan] is better suited for high speed [Mach 0.85] where as the prop or propfan is better suited at M0.5 - M0.7.

Now coming to your question, the turning speed of a fan very much depends on the size of the engine. I'm not into big engines, but I believe the fan of a GE CF6-80 on the 744 will turn at approx 3400 rpm [=3400/60 = 56 cycles per second].
On a modern turbo fan, upto 80% of the thrust is being produced by the fan, the balance coming from the hot end.

On a small engine like the 2100shp PW120 series [to be found on Dash 8 and ATR42] the power generating turbine rotates at approx. 20000 rpm. Mind you, this rotational speed is being reduced by a reduction gearbox [RGB] to approx. 1050-1200 rpm, which is the optimum prop speed. The high pressure turbine [which drives the high pressure compressor] rotates at 33000 rpm.

Theoretically, this same hot section and compressor [which are referred to as Gas Generator] can be used to drive a small fan. There are engines where basically the same Gas Generator [aka core engine] is used to drive a prop in one engine model, and a small fan in a different engine model from the same engine family.

Scootertrash is right in stating that 75% of the energy produced by a turbine engine is used to drive the engine itself [i.e. the compressors]. however this might be very misleading without any additional comments. One should not forget that this 75% energy is being returned into the gas stream by the compressors. The only thing that a compressor does is converting mechanical energy [i.e. rotating spool] into gas energy [minus a couple of % efficiency loss].

So lets say that you burn 100 units of energy through fuel. 75 units of this is required to drive the compressor. 75 units are being returned into the gas path, These 75 units enters the engine, and another 100 units of energy are added by burning fuel. So I now have 175 units of energy in my hot section, again 75 units required to drive the compressor. I now have 225 units of energy in the gas path et. etc. etc. End game: all the energy required to drive the compressor is being returned into the gas stream and is NOT lost, meaning that all 100 units of energy inducted through fuel burn [minus a couple of percent due to turbine/compressor efficiency loss] are still available for net power delivery.

Total thermal efficiency of a typical turbofan is around 40-45%, meaning that 55-60% of fuel energy is lost. Compare this to a typical automobile engine which has a thermal efficiency of only 20-25% . . . ! Yeah, that's right folks, 75% of the thermal energy in your auto gas tank is directely converted in heat, warming our global atmosphere, and only 25% is used to move your car and yourself [which off course is also converted in global heat due to friction].

Concluding: on modern subsonic engines, the core engine is working to drive the fan or prop or whatever. Approx 15-25% of the thrust is being produced by the hot section. The main power delivering source is the cold part of the engine.


PS. On a Fokker 50 [which has two 2500shp PW125B turboprops], the hot gas stream is angled 15 degrees or so downward, generating upto 5% of the aircraft lift requirements . . . reducing wing lifting requirement and thus drag by 5%!
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