Why Jets fly high and propellers can't keep up

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Erik Snyman

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View attachment 114840
fun historical fact - the current world altitude record of 56,047 ft. for a manned piston-powered propeller-driven aircraft was set way back on October 22, 1938 by an Italian Caproni Ca.161 piloted by Lt. Col. Mario Pezzi.
View attachment 114841
Fairly aggressive pitch on those prop blades. Looks like the pitch could be fixed, as well. Talk about a runway lover....
Erik in Oz.
 

raumzeit

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Closer vehicle's propellant exhaust velocity is to vehicle's actual velocity closer it is to ideal where momentum sums to zero. Always figured jet ev being higher and massflow lower promotes a higher altitude with less air/lower drag for ideal operation.
 

Toobuilder

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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.
Not sure what you are saying here. How much thrust does a CFM-56 produce with an air density of "0"?
 

b7gwap

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I’m curious if the thirty pound brains can answer this one: why is it that a high bypass turbofan doesn’t suffer from the same issues as a prop at extreme altitudes? Is the bypass airflow also locally slightly compressed around the core engine? I’ve heard there’s a thrust gain and a core cooling effect, but a 777 nacelle is bigger around than a 737 fuselage. Those fan sections are bigger diameter than many props and I believe have fixed geometry. I’m sure someone can set me straight.

For my hairy knuckle brain, I’ve always answered the question of the article with the idea that forced induction is essentially an “air squeezer.” Grabs all the air it can and squeezes it to get pressure and heat, mixed with fuel -more fuel- to make more power than natural aspiration. But the cold lonely propellor only gets more torque out of that. It doesn’t get to live in the “hot tub” of the engine intake. Mr Turbojet is essentially a rocket engine that doesn’t carry its oxidizer around, it flies through it and sucks it out of the air in front of the engine, squeezes the crap out of it, mixes Jet A into the flame front and spits all that hot expanding mass rearward.

Hey what’s Garrison got that I ain’t got? :)
 

TFF

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A turboprop core will do the same, but the prop can’t keep up. Shrinking the prop diameter and putting it in a shroud makes it possible.
 

Dan Thomas

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A turboprop core will do the same, but the prop can’t keep up. Shrinking the prop diameter and putting it in a shroud makes it possible.
The inlet of a turbofan is a divergent duct, meaning that it gets larger as you go in. That increases the cross-section, slowing the air and increasing its pressure. The airplane's 600-mph speed is not suitable for the tip speeds of a big fan, necessitating that divergent duct, especially at altitude where the air is cold and the speed of sound is lower.

There's also the misconception that the jet's exhaust must be travelling faster than the airplane. Nope. Anything accelerated backward will produce forward thrust. Even a passenger, standing in the aisle, throwing a baseball down the aisle toward the back of the airplane, will impart a tiny bit of forward thrust to the airplane, even if the ball only travels at 60 MPH and the airplane is doing 600.
Newton's Second and Third Laws.

Now, that propeller needs some angle of attack in free air to produce thrust, and the result is a propstream a bit faster than the airplane. That's a quirk of an open prop, not a requirement of physics to produce thrust. Can a rocket travel faster than its exhaust in the atmosphere or space? - Quora
 

JMyers1

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How does the Avanti do 380 knots at FL380 and use so little fuel doing it with its 5 bladed Hartzells?
 

Rik-

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I’m curious if the thirty pound brains can answer this one: why is it that a high bypass turbofan doesn’t suffer from the same issues as a prop at extreme altitudes? Is the bypass airflow also locally slightly compressed around the core engine? I’ve heard there’s a thrust gain and a core cooling effect, but a 777 nacelle is bigger around than a 737 fuselage. Those fan sections are bigger diameter than many props and I believe have fixed geometry. I’m sure someone can set me straight.

For my hairy knuckle brain, I’ve always answered the question of the article with the idea that forced induction is essentially an “air squeezer.” Grabs all the air it can and squeezes it to get pressure and heat, mixed with fuel -more fuel- to make more power than natural aspiration. But the cold lonely propellor only gets more torque out of that. It doesn’t get to live in the “hot tub” of the engine intake. Mr Turbojet is essentially a rocket engine that doesn’t carry its oxidizer around, it flies through it and sucks it out of the air in front of the engine, squeezes the crap out of it, mixes Jet A into the flame front and spits all that hot expanding mass rearward.

Hey what’s Garrison got that I ain’t got? :)
What you have to realize is that the turbo fan design is not meant to be the "Fastest" design, just a more efficient design of a propulsion device than a pure jet for this application.

Also, note that the aircraft speeds that airlines fly at has actually decreased as we come to present day vs the originally concept of jet powered aircraft in the past. Partially to blame is fuselage size but to a degree, they are running the aircraft at a speed whereby the turbo fan design is most efficient and when we are looking at $/mile it all adds up to big profits or bigger losses for the air carriers.
 

challenger_II

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Well stated, and spot-on.

What you have to realize is that the turbo fan design is not meant to be the "Fastest" design, just a more efficient design of a propulsion device than a pure jet for this application.

Also, note that the aircraft speeds that airlines fly at has actually decreased as we come to present day vs the originally concept of jet powered aircraft in the past. Partially to blame is fuselage size but to a degree, they are running the aircraft at a speed whereby the turbo fan design is most efficient and when we are looking at $/mile it all adds up to big profits or bigger losses for the air carriers.
 

TarDevil

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What you have to realize is that the turbo fan design is not meant to be the "Fastest" design, just a more efficient design of a propulsion device than a pure jet for this application.

Also, note that the aircraft speeds that airlines fly at has actually decreased as we come to present day vs the originally concept of jet powered aircraft in the past. Partially to blame is fuselage size but to a degree, they are running the aircraft at a speed whereby the turbo fan design is most efficient and when we are looking at $/mile it all adds up to big profits or bigger losses for the air carriers.
Also, less wing sweep, greater span.
Cost per seat mile, folks.
 

b7gwap

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All true statements, about the market, the efficiency, the speed, but why doesn’t the big fan stall out at high altitude like a prop?
 

mcrae0104

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All true statements, about the market, the efficiency, the speed, but why doesn’t the big fan stall out at high altitude like a prop?
The fan blades are operating at higher angular velocity—and therefore higher dynamic pressure—than a comparatively slower prop. They are able to manage the supersonic speed (relative to the fan blades, not necessarily forward inlet velocity) because of the shroud and careful blade design, whereas a prop just gets woefully inefficient under this condition.
 

JMyers1

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Magic, and the huge amount of cash required to keep fixing that plane.

Tim (could not resist)
Owners on Beechtalk seem to report lower costs than a similar jet, but the question remains, how does this propeller fly at jet altitudes and perform just fine?

This would seem counter to much of the information in this thread.
 

tspear

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Owners on Beechtalk seem to report lower costs than a similar jet, but the question remains, how does this propeller fly at jet altitudes and perform just fine?

This would seem counter to much of the information in this thread.
It is all about the design. Current turbofans are designed for high altitude. Most turboprops are designed for low twenties.
The engines for Avanti were tweaked to allow for high altitude and to perform very well there. How it was done,. I have never seen the answer. Just that the engines are not the same as you see on other turboprops; my guess is something to do with the gearing and likely a much slower turning prop comparatively.

Yeah, per BT and other forums, the OpEx is driven much more by temperamental MX costs versus fuel; while in most METP fuel is a large driver.
Supposedly the 180 II is cheaper to maintain, but fewer built.

Tim
 
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