Why Jets fly high and propellers can't keep up

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Richard6

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Sorry that this is a little long, but I don't see a way to shorten it without losing content.

From the August 2021 edition of Flying Magazine
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I had to read this several times for it to make sense.

Richard
 

rv7charlie

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I hope the editors of Flying are as embarrassed by that article as I am for them.

'A propeller depends on the surrounding fluid medium to produce thrust, whereas a jet does not....'
Really? Try plugging that hole in the front & see how that works out.
 

Voidhawk9

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I think you misunderstood the context, Charlie. It's referring to the air around the engine, not the air going through it.
 

mcrae0104

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I hope the editors of Flying are as embarrassed by that article as I am for them.

'A propeller depends on the surrounding fluid medium to produce thrust, whereas a jet does not....'
Really? Try plugging that hole in the front & see how that works out.
For props, P=TV; therefore, for given power, thrust decreases as velocity increases. Thrust output of a jet, on the other hand, is largely independent of velocity. Still, it's good to know we have people smarter than Peter Garrison posting here to keep us all straight. Thanks for that.
 

wsimpso1

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I could not help myself.

I just wrote a letter to the editor of "Flying" criticizing the above article for technical errors in the third paragraph. Pressure must drop continuously from rear face of compressor all the way aft to the nozzle. Momentum change in the air is reacted to the airframe through not only the spinning compressor blades but through stator blades, the inlet, and the outlet. I followed by suggesting that the author's unneeded detour through some wrong details of the engine may have diverted him as much as us on satisfyingly conveying the topic of how thrust changes with airspeed on jets and props.

Billski
 

rv7charlie

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I didn't even notice that Garrison's name was on the article; that's pretty surprising. The article reads like an attempt to dumb down a technical subject that went so far that it descended into plain dumb. How do jets produce thrust? ans: By spinning the carefully sculpted and angled blades in the compressor. What spins those blades? ans: Torque.

How do piston engines use torque to move the airplane? ans: to spin carefully sculpted and angled blades on a propeller.

I'm not questioning the fact that jets fly higher/faster than props; just that the descriptions lack sense.
 

12notes

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And helicopters don't fly. They beat the air into submission.
That's nonsense. Helicopters work by simple Newtonian physics, they continually throw money at the ground until the weight of the money is greater than the weight of the helicopter. Propeller planes throw money at the wings, they only fly if you use enough money and more goes under the wings than over. Gliders merely set a pile of money on fire, then thermal in the heat for lift. Jets are actually hydraulic in that they set compressed money on fire, use the smoke to run a big fan to cool the burning money, then use more compressed money to raise the temperature again, and are propelled entirely by the tears of your accountant.
 

tspear

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I thought jets flew higher because that is where there is less drag for a given true airspeed, and therefore where the turbofans are mostly tuned for optimal performance.

Tim
 

Dana

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I thought jets flew higher because that is where there is less drag for a given true airspeed, and therefore where the turbofans are mostly tuned for optimal performance.
That's also true, but it's also true for piston engines, up to a point. The difference is that jet thrust doesn't fall off with decreasing air density like propeller thrust does, so you still have enough thrust to fly at the altitudes where drag is reduced.
 

Dusan

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Everything about producing thrust is efficiency. How much of the go-juice you spend to generate thrust. Propellers are better at slow speed as it's more efficient to impart a small acceleration to a large air-mass. At high speed, turbofans are better since transonic blade flow is eliminated by the ducts flow effects.
 

Martin W

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Helicopters cannot actually fly .... it is just an illusion.

You can test this yourself .... find a parked helicopter and attach a crane and ropes to the rotor blades and try to lift it . The blades are not strong enough and will simply bend upwards.
 

Dan Thomas

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IMHO, both jets and props develop thrust by accelerating air. They both need the surrounding air to provide the medium to generate thrust. The whole "pushing against the air" idea is bogus. A rocket makes its own gases and develops thrust in empty space, with nothing to push against. EK=½MV².
 

BoKu

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For props, P=TV; therefore, for given power, thrust decreases as velocity increases. Thrust output of a jet, on the other hand, is largely independent of velocity...
And that in a nutshell is why smooth lines and good energy management are so important in jet racing. With a propeller, if you happen to slow down, you have more thrust available to get back up to speed. With a jet, you have to dig your way out with the same amount of thrust you went in with.
 

mcrae0104

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Edit: Post redacted in the interest of civility.
 
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