Jet engine thrust

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Ron LaDow
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Jet engine thrust

#1 Post by Ron LaDow »

'OK, the exhaust rushes out the rear of the engine, and the airplane goes forward, but what does the exhaust 'push' on?
https://www.youtube.com/watch?v=uP_G3m0cTiw
In the comments, there is an attempt to distribute the forward thrust to the various physical parts of the engine, but like a rocket engine exhaust doesn't 'push' on anything, it seems Newton's insight is all it takes; the exhaust goes out and the engine goes forward.
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Wes Bender
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Re: Jet engine thrust

#2 Post by Wes Bender »

The "exhaust" doesn't push forward on anything, but the combustion itself does. In a semi-automatic pistol, the bullet exiting the muzzle isn't what causes the action to cycle. It's the combustion of the smokeless powder that has introduced force in all directions, including to the rear. Load a dummy round with no bullet and the action will still cycle when the gun is fired.

In the B-47, we had weakie, weakie J47 engines, but they worked. How? By having the force from the combustion putting a forward force on internal portions of the combustion chambers.

In the video, the main point he is making is that the "thrust bearing" isn't really receiving any "exhaust thrust" and in that point he is correct. It handles the thrust of the compressor trying to move forward, not the output thrust of the engine.

(Or are you pulling our collective legs here, Ron????)

Cheers,
Wes
Some days it's just not worth chewing through the restraints.....

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Ron LaDow
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Re: Jet engine thrust

#3 Post by Ron LaDow »

Wes, I know the exhaust doesn't 'push on' anything and I'd hope most readers here know that and accept a bit of humor; it was a paraphrase of Agent JZ's 'headline'.
But if you check the link in the comments ( http://i.imgur.com/QHx25oK.png ) you'll see the answer is now not as simple as it was presumed to be; the forward thrust it expressed through various engine parts rather than only through the combustion chamber structures.
And yes, he spends too much time in my opinion explaining the function of the thrust bearing, but he's obviously gotten many questions about that. And for obvious reasons.
Here's the point: The guy is a skilled turbine engine mechanic. He's willing to spend time explaining how the things work, and I'm happy to watch him do so even if he's a bit 'elementary' from time to time.
Maybe you knew how the engine thrust was transferred to the airframe, but I didn't and I'd bet I'm not alone.
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Wil Mittelbach
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Re: Jet engine thrust

#4 Post by Wil Mittelbach »

Ron LaDow wrote:'OK, the exhaust rushes out the rear of the engine, and the airplane goes forward, but what does the exhaust 'push' on?
https://www.youtube.com/watch?v=uP_G3m0cTiw
In the comments, there is an attempt to distribute the forward thrust to the various physical parts of the engine, but like a rocket engine exhaust doesn't 'push' on anything, it seems Newton's insight is all it takes; the exhaust goes out and the engine goes forward.
Quite right Ron -
per our buddy Newton, F=ma.
Such as Jet engine forward Force = (Mass of combusted hot gases) times (their Accelerated exit from the combuster per their expansion), resulting in a forward force on the combuster and jet engine, a-la a rocket engine chamber.
Alternatively, perhaps also equivalent to Newton's "For every action there's a reaction", such as to your post. :) :wink:

Brian Jones
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Re: Jet engine thrust

#5 Post by Brian Jones »

Enough of jet engines,! here is a question that has bugged me since schooldays ,oh so long ago . The question is , does a vaccum have volume ?
Looking forward to your answers ,

Brian Jones #8448

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Dennis ODonnell
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Re: Jet engine thrust

#6 Post by Dennis ODonnell »

Volume is the measurement in 3 dimensions of a space. Vacuum refers to the contents of that space. A perfect vacuum (with no contents) would still have the volume described by its container.

Brian Jones
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Re: Jet engine thrust

#7 Post by Brian Jones »

Thats the volume of the container, not the vaccum .

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Wil Mittelbach
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Re: Jet engine thrust

#8 Post by Wil Mittelbach »

Brian Jones wrote:Thats the volume of the container, not the vaccum .
- With avg air density/wt at sea level, its volumetric pressure is approx 14.7 psi.
- With a vacuum having no density/wt, its volumetric pressure would be zero psi.
As taught in proper Jr. Hi science classes years ago.

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Dennis ODonnell
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Re: Jet engine thrust

#9 Post by Dennis ODonnell »

"Thats the volume of the container, not the vaccum ."

Volume is the quantity of space within a container, whether that space is full, e.g. solid, or empty, e.g. perfect vacuum. Volume is the measure of that 3D space. Vacuum as Walt says, is the measure of pressure within it. To rephrase your original question a vacuum occupies the volume of its container.

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Wes Bender
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Re: Jet engine thrust

#10 Post by Wes Bender »

....or maybe vacuum is the lack of occupancy of the volume of a container. I'm thinking Ron is chuckling at us all while he gets ready for cocktail hour in S.F.

Cheers,
Wes
Some days it's just not worth chewing through the restraints.....

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Dennis ODonnell
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Re: Jet engine thrust

#11 Post by Dennis ODonnell »

Right, same thing. It's almost always cocktail hour in SF, chuckles fill the vacuum between.

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