Gas Turbines ?

[Grapes]

Anyone with a link etc. that shows the shematics and the way they work ?

Cheers.

[Smoketoomuch]

Try: http://www.rolls-royce.com/education/jetengine/default.htm

"The Trident was also very quick but the Comet was only a .72/.75 airplane. "

[SRR99]

http://www.pwc.ca/www/en_markets/demonstration.asp

[Lu Zuckerman]

In a previous post about rocket and jet engines I made the claim that it was not the exhaust gasses that resulted in thrust but it was the force exerted on the forward wall of the combuster which is mechanically attached to the airframe structure via the engine assembly. I used a balloon analogy as well as a rocket engine description. I was told that I was full of it. Well it seems that Pratt and Whitney agrees with me even if the supporters of the Roll Royce textbook don't. I would strongly suggest that you look at the above P&W website. You might learn something.

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The Cat

[Smoketoomuch]

Lu, I agree with you entirely but I don't see where the contradiction between P&W and RR is. From the RR site;

"...This demonstration shows that the reaction is an internal function and does not, as is often assumed, result from the pressure of the gas stream on the atmosphere.
The reaction to the gas stream which is ejected from the nozzle is felt on the parts of the engine opposite the nozzle, mainly the front of the combustion chamber and the tail cone.
This force, termed thrust, it then transmitted from the engine casing to the airframe, through the engine mountings, to propel the aircraft."

Surely they're both in agreement?

Regards.

[This message has been edited by Smoketoomuch (edited 08 March 2001).]

[Lu Zuckerman]

To: Smoketoomuch


In the referenced posts everyone that disagreed with me would refer to the Rolls Royce book as their frame of reference. I just assumed, never having read the book that it too was in disagreement with my theory. Maybe those individuals that said I was wrong never read the book either.


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The Cat

[This message has been edited by Lu Zuckerman (edited 08 March 2001).]

[captain marvellous]

Um, isn't it that the expanding gases push on both the forward wall of the combustion chamber and the atmosphere behind the tail pipe?

Isn't it this combination which propells the aircraft forwards?

Then again, what about the 40 or so rocket thrusters on the space shuttle which operate in the vacuum of space?

[DoctorA300]

Captain M,
I belive that the force exerted on the atomosphere by the exhausts gasses are neglible. That theory/roumor came about from a bad training film used in the RAF and certain other european airforces. It showed a man holding a garden hose, and when he put his hand into the waterstream it exerted an extra force. This cannot be transferred to jet engines because water is not compressible and air is.
Brgds
Doc

[capt waffoo]

OK, here's grist for the mill. ;)

So what does the fan do?

Does it blow or suck?

Is thrust derived from massflow displaced in an opposite direction or a reduced air pressure in front of the fan/engine?

[Ceppo]

Capt waffoo,

Good point. I think that the others are talking about the actual jet part of the engine. i.e. in the combustion chamber.

[Danish Pilot]

Older P&W JT3 and JT8 allso refferd to as "fuel to noise converters".... :)

capt waffoo:

The Fan is basicly just a big "blower" so to say. At low level the trust increase can be as much as 20-30% compared to a straight turbojet. The fan allso rotates at lower rpm and makes less noise.

[SRR99]

http://www.grc.nasa.gov/WWW/K-12/airplane/shortp.html

[DoctorA300]

To keep it short. A Jet engine does neither blow or suck or anything else, it work by accelerating mass, in most cases air. The acceleration of air backwards, causes and equal and oppesite force forwards ei. thrust, if anyone thinks they have heard this before , its probably because you have heard of Newtons third law of physics: " An action causes and equal and opposite reaction"
Brgds
Doc

TcasClimb: Næste gang du har problemer med din støvsuger, kan du jo prøve at skifte posen > :)
DanishPilot: JT8 har givet dig brød på bordet i mange år, så sig ikke sådant noget pjat :mad:
To all others: Just slagging my fellow vikings

[This message has been edited by DoctorA300 (edited 14 March 2001).]

[Danish Pilot]

Doc A300, many things have been putting bread on my table, but the JT8 is NOT one of them!! (at least not direct)

And is makes a lot of noise, nomather what you say!!!..... http://www.pprune.org/ubb/NonCGI/tongue.gif

We can continue on the Scandinavian Forum if you want...?

[Flight Safety]

DoctorA300 is correct, it's Newton's third law of physics that's responsible for jet engine trust.

The example I like is the "jumping from a rowboat" example, where someone jumps off the back of a rowboat into a lake. The energy of the jump into the water, causes the acceleration of each mass in two opposite directions. The swimmer is accelerated away from the back of the boat (so he lands in the water), and the rowboat is accelerated in the opposite direction away from the swimmer.

The energy exerted in the jump "pushes" the two masses apart. If it were possible to measure both acceleration (change in speed) and the mass of both objects being moved in this example, you would see that the product of the acceleration of one object multiplied by its mass, is equal the product of the acceleration and mass of the other object. Newton's third law requires that the products be equal, because the energy being exerted is divided equally between the two objects. It's just that when one object has a much greater mass than the other, the lighter object has to be accelerated to a much highter velocity for the products to be equal.

In a jet engine, thrust is created when mass (air molecules) is being ejected out of the back of the engine. Depending on the design, either a lower mass of air is being accelerated to a very high velocity (as in a turbojet) or a much larger mass is being accelerated to a lower velocity (as in a high-bypass turbofan).

I'll use the original V2500 high-pybass turbofan engine for the A320 and the ion drive engine of the NASA spacecraft "Deep Space 1" as extreme opposite examples to illustrate Newton's third law.

The V2500-A1 has 25,000 lbs. of takeoff thrust at SL, has a bypass ratio of 5.4, and most importantly moves 792 lbs. of air through the engine every second at takeoff power.

The "Deep Space 1" ion drive engine by contrast uses Xenon as the ejecta mass, which is 4 1/2 times heaver than air. The engine produces a total of 1/50th of a lb. of thrust at full throttle. The spacecraft launched in 1998 with a total of 181 lbs. of Xenon on board. Mission status as of today, shows that 97 lbs. of Xenon has been expelled by the ion engine so far, and this has imparted a total velocity change of 5,800 miles/hour to the spacecraft (which weighs 1,080 lbs).

Both engine types rely on Newton's third law of physics. The thrust of a turbofan engine is measured in lbs of thrust, which is really a product of the mass of the air being moved through the engine, and the acceleration imparted to that air mass as it moves through. The core ejects a much smaller air mass but at a higher velocity, while the fan ejects a much larger air mass at a lower velocity.

Contrast this to the ion drive of "Deep Space 1". The Xenon gas molecules are accelerated by an electro-static field (energy supplied by solar panals), which accelerates each Xenon molecule to a velocity of 62,000 miles/hour, or 17.2 miles/second, or 90,933 feet/second. The acceleration of the spacecraft is not due to the great mass of molecules being ejected by the ion engine (as in the turbofan engine), but instead is due to the extremely high velocities being imparted to a very small ejecta mass. BTW, there is no pressure inside the ion engine in the vacuum of space to push against the forward chamber wall.

Either way, Newton's third law of physics is at work in both types of propulsion systems.

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Safe flying to you...


[This message has been edited by Flight Safety (edited 14 March 2001).]