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Old 01-28-2005, 09:11 PM   #1
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Question Jet engine v.s. Prop engine cargo aircraft?

Pro and con comparing between Jet engine and prop engine?

Example: C-130 v.s. C-17 if both have same capabilities specifications which one would benefit the most from operation to capaibilities to quality of engine? I am aware that jet engine can carry more lbs than prop engines but I just ask that question as what if their capabilities is about the same and what the difference to benefit between those two?

Why/what benefit to need a prop engine? Less worry of engine damage on sensitivity envoriment than senstivity jet engine which require clean enviroment to fly? Etc.......

Just curious to know the difference between jet and prop engines of cargo aircrafts.
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Old 01-29-2005, 05:56 PM   #2
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Johnny,
Think of it just like you would regarding Chryslers "Cab forward" design.....................wait a minute don't do that, on second thought that makes no sense at all.

The benefit with a modern fan-jet engine is that it sounds real cool and goes faster.
A turbo-prop plane gets off the ground in a shorter distance (prop gets a better bite than a fan), there is no real economical or reliability benefit either way as you’re dealing with essentially similar engines just that one’s spinning a big fan and the other a propeller.
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Old 12-06-2005, 03:20 AM   #3
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I am sorry to say that is not quite true. Turboprop engines are not as efficient at high altitudes like turbojets are. In order to explain why this happens you must remember one rule; jet engines like high RPM's. when a turbojet is at high altitudes it accelerate the air moving through the jet more because the pressure is less. Therefore it can spin faster.
Now you might ask yourself wouldn't that be the same for the turboprop and it is. The jet section of the turboprop is more efficient at high altitudes. But the jet exaust only accounts for about 20% of the thrust of the engine. The other 80%comes from the prop and props like more dense air because they can take a bigger bite out of it. So turboprops like it better in the mid altitudes.
I hope that answers your question.
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Old 12-06-2005, 06:58 PM   #4
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Quote:
Originally Posted by sciguy
I am sorry to say that is not quite true. Turboprop engines are not as efficient at high altitudes like turbojets are. In order to explain why this happens you must remember one rule; jet engines like high RPM's. when a turbojet is at high altitudes it accelerate the air moving through the jet more because the pressure is less. Therefore it can spin faster.
Now you might ask yourself wouldn't that be the same for the turboprop and it is. The jet section of the turboprop is more efficient at high altitudes. But the jet exaust only accounts for about 20% of the thrust of the engine. The other 80%comes from the prop and props like more dense air because they can take a bigger bite out of it. So turboprops like it better in the mid altitudes.
I hope that answers your question.
sciguy

This post could not possibly be more wrong!
I will address the most obvious errors first:

1. On a Turboprop aircraft engine 100% of the usable thrust is produced by the Propeller! The thrust produced by the core is expended as it passes through the power-turbine section and the energy is converted from thrust to shaft horsepower.

2. Turbojet engines are inefficient and of early design and should have not have been brought up by you, Turbofan engines are used in virtually all commercial jet-airliner applications now and for the past 40 years or so.

3. Your analogy regarding high altitudes and speeds allowing the jet engine to “spin faster” is wrong!
Ram-air effect does have an impact on how a jet engine operates but does not allow the engine to rotate any faster, engines are throttled back during the cruise phase of flight anyhow.
Ram-air effect associated with thinner air at high altitudes allows the jet engine to operate in a more efficient state, jet engines are designed and tuned to operate at peak efficiency during cruise as this accounts for well over half of the hours that the aircraft will be operated (more so on long-haul aircraft).


There are some other inaccuracies that we’ll get to later.
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Old 12-08-2005, 07:57 PM   #5
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Of those three pionts # 2 is correct. I ment to say turbofans. I just made a mistake. But #1 and #3 are absoutley false. Not all of the thrust comes from the prop of a turboprop. Some thrust is generated by the exaust coming from the jet. And now for #3. When a jet is up at high altitudes it is more efficient because the air is less dense and therefore can accelerate the air faster thus giving it more thrust.
Besides the main reason that turboprops are less efficient than turbofans (which you for some reason didn't cover) is that the prop likes denser air found at low altitudes. Props also like it lower because they can spin slower and therefore cause less drag.
And if you reply to this you better have some danm good proof because your previous entrys clearly show you don't know what you are talking about.
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Old 12-08-2005, 11:24 PM   #6
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Quote:
Originally Posted by sciguy
Of those three pionts # 2 is correct. I ment to say turbofans. I just made a mistake. But #1 and #3 are absoutley false. Not all of the thrust comes from the prop of a turboprop. Some thrust is generated by the exaust coming from the jet. And now for #3. When a jet is up at high altitudes it is more efficient because the air is less dense and therefore can accelerate the air faster thus giving it more thrust.
Besides the main reason that turboprops are less efficient than turbofans (which you for some reason didn't cover) is that the prop likes denser air found at low altitudes. Props also like it lower because they can spin slower and therefore cause less drag.
And if you reply to this you better have some danm good proof because your previous entrys clearly show you don't know what you are talking about.
Being new to flying turboprops, I'm probably not as qualified as some on this forum to rebuke your idea that any measurable thrust is generated from a turboprops exhaust. In the C-208B I fly, I can assure you that, aside from leaving a nasty little soot trail down the right side of my cargo pod, absolutely NO measurable thrust comes out of my exhaust stack. Whoever told you that doesnt know what they are talking about.
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Old 12-09-2005, 03:17 AM   #7
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I am sorry for the last paragraph of my most recent entry. my friend typed that as a joke and I accidently pressed enter before I noticed it was there. I am sure you are very knowedgable about the world of aviation and I would like to retract that statement. Once again I am sorry and I look foreward to hearing more of your insights.
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Old 12-09-2005, 04:10 PM   #8
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Quote:
Originally Posted by sciguy
#3. When a jet is up at high altitudes it is more efficient because the air is less dense and therefore can accelerate the air faster thus giving it more thrust.
This isn't what you posted, it's what I posted moron!
You stated earlier that a jet engine is more efficient at altitude because it can "SPIN FASTER" I agree that it is more efficient but it's NOT because it can "SPIN FASTER"!

You obviously don't know your a$$ from your elbow and have proven that very thoroughly with every single claim you have made.

Kids, let this be a lesson to you all;
Stay in school and don't do drugs!
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Old 12-12-2005, 07:50 PM   #9
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I was trying to be a gentleman about this but you are obviously too immature and stupid to show any decency whatsoever.
First of all, I would like to point out that you agreed that a jet is more efficient when it pushes more air out the back. Now I don't know about you but I have observed that if a fan spins faster it pushes more air. So you are the moron who doesn't "know your a$$ from your elbow.".
And by the way I have been in school for the last three years studying aeronautical engineering and I have never touched an illegal substance in my life.
And one last thing;judging by your picture I am guessing that you are so old that when you learned to fly they didn't even have any kind of jet so you never learned about them and therefore have no right to talk about them.
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Old 12-13-2005, 11:10 AM   #10
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Quote:
Originally Posted by sciguy
I was trying to be a gentleman about this but you are obviously too immature and stupid to show any decency whatsoever.
First of all, I would like to point out that you agreed that a jet is more efficient when it pushes more air out the back. Now I don't know about you but I have observed that if a fan spins faster it pushes more air. So you are the moron who doesn't "know your a$$ from your elbow.".
And by the way I have been in school for the last three years studying aeronautical engineering and I have never touched an illegal substance in my life.
And one last thing;judging by your picture I am guessing that you are so old that when you learned to fly they didn't even have any kind of jet so you never learned about them and therefore have no right to talk about them.
Yeah that photo of me on my avatar sure makes me look old don't it there smart guy?

BTW, You need to get a refund from whatever college that you're attending, as a junior shooting for a degree in "aeronautical engineering" you have got to know less about jet propulsion than any person I have ever ran across, I'll take odds that you aren't any older than 15 tops.

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Old 12-15-2005, 03:27 PM   #11
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Well I guess you are going to loose that bet because I am 21 years old. My bityhday was in may of 1984. and besides, I am going to Embry Riddle in arizona, and its ben judged as one of the best aeronautical schools in the country.

Then you're just one dumb sob because if you attend Embry-Riddle then you would know that they do not offer an Undergraduate Degree in "Aeronautical Engineering" as you have stated.
I have taken many classes at E-R Prescott, who are some of your professors?
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Old 12-15-2005, 05:02 PM   #12
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ok you called my bluff. I am a sophmore at a junior college in southern california but I know alot more than a friend of mine who does go to embry riddle. I had a 3.9 in high school and I took many advanced classes. I am certainly smart enough to get into almost any college I want to but my parents need me close to home. I plan to go to an aviation college like embry riddle or SIU in a year or 2.
on the matter of our disagreement of turboprop vs turbofan, I dont think I am going to be able to convince you and I know you are not going to be able to convince me so lets just agree to disagree because I am sick of arguing with you.
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Old 12-15-2005, 07:26 PM   #13
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Quote:
Originally Posted by sciguy
ok you called my bluff. I am a sophmore at a junior college in southern california but I know alot more than a friend of mine who does go to embry riddle. I had a 3.9 in high school and I took many advanced classes. I am certainly smart enough to get into almost any college I want to but my parents need me close to home. I plan to go to an aviation college like embry riddle or SIU in a year or 2.
on the matter of our disagreement of turboprop vs turbofan, I dont think I am going to be able to convince you and I know you are not going to be able to convince me so lets just agree to disagree because I am sick of arguing with you.


Not only 15, but a lying little bastard to boot.

You don’t even know enough to be able to argue about it, maybe after you’re old enough to be able to read you can learn something about jet engines, until then shut the hell up!
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Old 12-15-2005, 07:29 PM   #14
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I think the TU-95 Bear would be a good argument here! Carry on.
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Old 12-15-2005, 09:37 PM   #15
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This is a good subject;
(Craig I realize that you already know this stuff)
Why did the Russians build the Bear with counter-rotating turbo-props instead of turbo-jets?
It was because the Russians could not design jet engines that matched the power of Western built jet engines without burning significantly more fuel and thus hampering range, so they designed the TV-12’s with counter-rotating propellers which provided significant thrust allowing the aircraft to achieve jet like speeds and altitudes but burn considerably less fuel than a conventional turbo-jet of the era (1952).

Even using these tweaked turbo-props this aircraft could never match the capabilities of the B-52 that was designed at about the same time, the B-52 can fly about 75mph faster and almost 10,000ft higher, the airframe weighs less but can carry almost double the payload and can fly about 1,000 miles further on a single bag of fuel, the only drawback to the B-52 is that it uses about 40% more fuel and twice as many engines to accomplish this.

The primary reason that further development of turbo-prop engines of the counter-rotating design has not produced a modern power-plant capable of even better performance is due to two short falls of the design, first to attain jet-like speeds and altitudes the propellers must be accelerated beyond the speed of sound which makes it far to noisy, and secondly the complex design of counter-rotating propeller gear-box adds weight and reduces reliability. These pitfalls along with the advances in fan-jet technology make it unnecessary to design a turbo-prop that has jet like performance now days with most of the advances in modern turbo-props being aimed at enhancing fuel efficiency, noise/vibration reduction and improved reliability with new designs like composite (light weight) 6-bladed propeller systems that use reduction gearbox’s with fewer moving parts.
Modern fan-jets are becoming so efficient that it won’t be long and most all turbo-prop commuter aircraft will be replaced by them, companies like Embraer are now focusing almost entirely all of their efforts developing new fan-jet powered aircraft while other companies such as Saab and ATR are simply doing inexpensive upgrades to dates airframes.

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Old 12-16-2005, 12:57 AM   #16
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First of all I am not 15 I am 18. I am sorry I lied I should not have done it. But I know I am correct. I just have one last question for you: if turboprops are just as efficient as turbofans then why do all the big jets have turbofans and not turboprops. if you take the time to look at some planes of similar designes where one is a jet and one is a turboprop you will see that the turboprop has a lower service cieling.
And by the way I do know enough to argue about it I am just sick of trying to convince a stupid bastard that doesn't know what he is talking about.
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Old 12-16-2005, 01:06 AM   #17
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WHAT THE HELL ARE YOU SMOKING!!! I have been fighting with you for the past week and a half trying to prove to you that turboprops are more efficient and you have been fighting me to the hilt, and then in the last entry you show that I was right all along. you admitted that jets can fly higher and faster, so I guess that makes me right and it also makes you a lying bastard.
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Old 12-16-2005, 02:47 AM   #18
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Quote:
Originally Posted by sciguy
WHAT THE HELL ARE YOU SMOKING!!! I have been fighting with you for the past week and a half trying to prove to you that turboprops are more efficient and you have been fighting me to the hilt, and then in the last entry you show that I was right all along. you admitted that jets can fly higher and faster, so I guess that makes me right and it also makes you a lying bastard.
Scigirl, you have spent all 7 posts so far making a complete and total a ss of yourself. You are a fu cking moron!! Please just go away
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Old 12-16-2005, 10:59 AM   #19
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No im 21 i go to ERAU
ok fine i dont go there
ok i lied im 18 not 21


Make up you mind child. Pick a story and stay with it at least if you have to be an idiot!

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Old 12-16-2005, 02:09 PM   #20
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No im 21 i go to ERAU
ok fine i dont go there
ok i lied im 18 not 21
Make up you mind child. Pick a story and stay with it at least if you have to be an idiot!
Your Idiot-Freak Alarm Is Ringing!



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Old 12-16-2005, 06:27 PM   #21
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What do you say here JJ?

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Old 12-16-2005, 09:29 PM   #22
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Old 12-17-2005, 12:03 AM   #23
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And yeah, I already know the abrv. The sig explains everything.

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Old 01-15-2006, 08:52 PM   #24
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Another argument is that the UDF program was closed. UDF did mimic a counter rotating propeller but ducted high bypass fans were more to favor.
Until the introduction of high bypass turbofans I would have said turboprops were more efficient. Seeing as how most of the direction of the aviation Industry has gone within the last ten years.
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Old 01-20-2006, 05:27 PM   #25
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Lightbulb Hovercraft fan vs propeller

Hey all,
brand new to this forum..
I have a question, I am building a low power hovercraft (a UH13PT from www.hovercraft.com and I have the option to purchase a fan(Hasconwing(simular to Multiwing brand)to use for the thrust or a two blade propeller (www.arrowprop.com "Low Horse Hower Engine" series).
Normally I wouldn't consider a fan for thrust in a direct drive configuration because it has always been my understanding that a prop is much better for this app. than a fan but the cost differential is significant, the fan being cheaper... so to the question..
Which would produce more thrust? the fan or the prop and why?
spec: engine 13 horse power, max rpm 3200, prop and fan dia. 42" and it will be in a duct.
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Old 12-09-2006, 01:42 PM   #26
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Originally Posted by JOHN JOHNSON View Post
Pro and con comparing between Jet engine and prop engine?

Example: C-130 v.s. C-17 if both have same capabilities specifications which one would benefit the most from operation to capaibilities to quality of engine? I am aware that jet engine can carry more lbs than prop engines but I just ask that question as what if their capabilities is about the same and what the difference to benefit between those two?

Why/what benefit to need a prop engine? Less worry of engine damage on sensitivity envoriment than senstivity jet engine which require clean enviroment to fly? Etc.......

Just curious to know the difference between jet and prop engines of cargo aircrafts.
Well you guys should read next text just because it explain benefits of jet engines and prop engines....

Jets vs Props
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Old 12-09-2006, 04:18 PM   #27
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A ducted fan is better for thrust than a propeller...the terms say it all. A propeller is to propel, a ducted fan is better in the downward thrust of a ground-effect vehicle.
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Old 12-09-2006, 04:20 PM   #28
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That's also because of the shape of the blades, a propeller is generally not pitched as steeply as a fan.
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Old 12-11-2006, 06:32 PM   #29
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Default Props vs Jet Engines

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Pro and con comparing between Jet engine and prop engine?

Example: C-130 v.s. C-17 if both have same capabilities specifications which one would benefit the most from operation to capaibilities to quality of engine? I am aware that jet engine can carry more lbs than prop engines but I just ask that question as what if their capabilities is about the same and what the difference to benefit between those two?

Why/what benefit to need a prop engine? Less worry of engine damage on sensitivity envoriment than senstivity jet engine which require clean enviroment to fly? Etc.......

Just curious to know the difference between jet and prop engines of cargo aircrafts.
Why do prop engines have more power at low speeds and appear to lose power at higher speeds, while jet engines behave the opposite way? Why are prop engines measured in terms of horsepower while jets are measured in terms of thrust?

First off, let's clarify the difference between power and thrust. Power is the rate at which total energy is transferred. You have power input to an engine, in terms of the fuel delivery rate, you have total power transferred out of an engine, and you have power transferred to the plane, in terms of making it go faster or higher. Thrust is simply the amount of propulsive force applied to the plane by the engine. From the laws of physics, power delivered to the plane is thrust times velocity.

The power efficiency of different engine types depends on the operating situation. There are multiple effects operating against each other. The common wisdom is that props have higher power at low speeds than jets. But consider power delivered at standstill: Stand on the brakes and rev a prop engine. The engine is delivering lots of power, but none of it is going to the plane because the plane is stationary. The power is all dissipated in the propwash. So what's really going on here is that a prop engine delivers maximum thrust at standstill.

As your speed increases obviously the power delivered to the plane increases. However, eventually you start running into the limitations of a prop. As your speed goes up, the flight path of the prop blade becomes more and more straight ahead and less sideways; consequently, the "lift" generated by the blade more and more turns into rotational resistance and less into thrust. In other words, the thrust generated by the prop falls off as you get faster. So you reach the point of diminishing returns where the engine is no longer imparting any power to speak of into the air and its thrust drops to nearly zero.

A jet engine, on the other hand, imparts a much greater acceleration to a smaller amount of air. (OK - I'm talking about a pure turbojet here. I'll address bypass engines in a moment.) At standstill, this comes in part from just running the air through the compressor and turbine. However, most of the jet's thrust comes from heating the air. The air entering the engine is at atmospheric pressure; the air pressure leaving is only slightly higher, so by the old gas equation PV = NRT the air must be leaving at a velocity that's faster roughly in proportion to the amount by which it's been heated (in absolute degrees).

Just like a piston engine, at standstill a jet delivers zero power to the aircraft. However, as you accelerate, the thrust delivered by a jet does not fall off as fast as that of a piston engine because the exhaust velocity is much higher. As the jet gets faster, the compressor effectively does less work in accelerating the air into the engine - more and more the air just flows in at the freestream velocity. (But the compressor still has lots of work to do to compress the air inside the engine.) So you lose the element of thrust from accelerating the air into the compressor. But most of it is still there from heating the air. At very high speeds the air is actually being decelerated (relative to the engine) at the intake to keep the compressor blades subsonic. This is accomplished by expanding the diameter of the intake duct from the engine mouth to the compressor face. (Constant flow rate with constant pressure into an expanding volume means the flow velocity has to decrease.) This causes intake drag but is still compensated by the acceleration at the other end from heating the air.

With properly designed intakes, jet engines become even more effective at supersonic speeds. The air must be decelerated even more at the intake, but this is now done with supersonic flow rather than subsonic flow. The supersonic flow causes shock waves that compress the air as it decelerates; the compression of the air increases the total mass flow and therefore the engine's thrust. For a while, as your speed increases above Mach 1 the engine's thrust increases. There are a couple of limiting factors:
  • Intake geometry. Above a certain Mach number the air can no longer be decelerated to subsonic speed before it hits the compressor face.
  • Temperature limits. Compressing the air heats it up, so there is less "headroom" for heating the air in the engine and still staying within the engine's temperature limits.
  • Ultimate limits on exhaust gas velocity. Intake drag finally exceeds the thrust that can be developed by heating the air to the maximum exhaust temperature.
One might ask about Mach limits to the jet exhaust velocity. "How can the jet exhaust flow faster than the speed of sound?" The answer is it doesn't. Remember that the speed of sound in a gas is proportional to the square root of the temperature. So the speed of sound is much faster in the hot engine exhaust, allowing it to flow much faster than the nominal 1000fps you get at normal atmospheric temperatures.

Another detail about flow and pressure... The air pressure at the engine exhaust is determined by the exhaust's flow resistance. EPR (engine pressure ratio) is the ratio of air pressure at the exhaust to the air pressure at the intake. It effectively represents the air flow rate through the engine as seen through the engine's flow resistance. Typical EPR for an airliner jet engine at full power is in the neighborhood of 1.4:1.

So what does this all come down to? Ultimately what matters to an aircraft is thrust, not power. You need to accelerate the plane at takeoff and overcome air resistance at cruise. Props and jets are rated differently because they are measured differently. A prop engine is rated in horsepower because that's what you can measure at the shaft: power = torque * rpm. But that's only loosely related to what it's really doing for the plane because the amount of thrust the prop can translate that power into greatly depends on the plane's speed.

Horsepower doesn't make any sense for a jet because most of the power delivered at the turbine shaft is used internally. And its thrust is more constant relative to speed, so static thrust is the more useful metric.

From an energy conservation point of view, props are more efficient at low speeds than jets because almost all the energy goes into accelerating the air. A lot of the jet's energy goes into heat; that part of the energy is effectively lost. The downside of props is that as you approach Mach 1 at the prop blades they can't accelerate the air anymore. Also from the energy conservation point of view: A jet engine typically generates a lot more total power than a prop engine. However, at low speeds a greater proportion of that power is wasted heating air and accelerating it to very high velocities. At higher speeds, more of that power is actually imparted to the aircraft.

Now about bypass engines... In a bypass engine, a portion of the air flowing through the first stage of the compressor simply passes around the rest of the engine (for high bypass engines, as much as 90% of it). Effectively, that part of the compressor is simply acting as a prop. Bypass engines (AKA turbofans) are in effect a hybrid of a jet and a prop engine. You get improved static thrust (vs total energy consumed) and higher energy efficiency at low speeds in return for reduced thrust at high speeds. The bypass ratio determines the extent of the tradeoff. From this point of view, a turboprop is simply the ultimate high-bypass jet engine. High bypass engines are a good fit for airliners because they are most efficient at the airliner's cruise speed of Mach .85. You get maximum thrust for takeoff and climbout, which are done at relatively low speeds, and you need relatively less thrust for cruise at Mach .85.
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Old 12-11-2006, 06:55 PM   #30
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Wow! A mini-course in jet engine mechanics! Cool. But now it think my head is swelling...too much info...can't...absorb... No, really, thanks for the info. Veddy interesting...
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Old 02-01-2007, 10:17 PM   #31
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Default Re: Jet engine v.s. Prop engine cargo aircraft?

And the ducting (the fan cowling) constrains the airflow so it's immensely more efficient than if it weren't there. Attempts at unducted fans have been largely unsucessful.

The Chronic Snoozer (sonic cruiser) design was to use unducted fans in the beginning, but it would have been a situation similar to the RR 211 and the 1011 - the engines (fan state of the art) weren't ready. Earlier attempts at encouraging engine makers (and prop makers as well) to do R&D on the ducted fan were essentially "unsuccessful" (the engine guys weren't willing to spend the money on a "maybe" technology)
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Old 02-06-2007, 08:54 PM   #32
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Default Re: Jet engine v.s. Prop engine cargo aircraft?

The jet has been a loser since it started flying. Hmmm...What made me say that...Let's see. What could it possibly be? Could it be Global Warming?
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Old 02-06-2007, 10:09 PM   #33
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Default Re: Jet engine v.s. Prop engine cargo aircraft?

Well, I think this still stays with the topic and with all the expert knowledge available here, I bring up the cargo operations of Aloha Airlines in Hawaii. They say the CF56s on the Series 300 and 400 Boeing 737s experienced far higher maintenance problems than the JT-8Ds on their series 200s.

They fly in an environment of short (30 minutes or less) flights at no more than 27,000 feet. They just leased a D-Checked Alaska Airlines 200c to replace one of their high time birds.

Why would the CF-56s be less durable?
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Old 02-06-2007, 10:21 PM   #34
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Default Re: Jet engine v.s. Prop engine cargo aircraft?

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The jet has been a loser since it started flying. Hmmm...What made me say that...Let's see. What could it possibly be? Could it be Global Warming?
We could be walking...living in caves, using sticks and stones and hunting with spears. Then WE wouldn't be blamed for global warming...or would we?
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Old 02-07-2007, 12:43 AM   #35
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Default Re: Jet engine v.s. Prop engine cargo aircraft?

Props are better at lower altitudes, jets are better up higher. Engine and airframe manufacturers have known this for the better part of a century already.
Just look around a major airport tarmac and see what kinds of aircraft are using what propulsion and you'll find the answer.

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Old 02-28-2007, 05:34 AM   #36
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Props are better at lower altitudes, jets are better up higher. Engine and airframe manufacturers have known this for the better part of a century already.
Just look around a major airport tarmac and see what kinds of aircraft are using what propulsion and you'll find the answer.
You don't know what you're talking about man...
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Old 08-04-2007, 04:27 PM   #37
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Why do prop engines have more power at low speeds and appear to lose power at higher speeds, while jet engines behave the opposite way? Why are prop engines measured in terms of horsepower while jets are measured in terms of thrust?

First off, let's clarify the difference between power and thrust. Power is the rate at which total energy is transferred. You have power input to an engine, in terms of the fuel delivery rate, you have total power transferred out of an engine, and you have power transferred to the plane, in terms of making it go faster or higher. Thrust is simply the amount of propulsive force applied to the plane by the engine. From the laws of physics, power delivered to the plane is thrust times velocity.

The power efficiency of different engine types depends on the operating situation. There are multiple effects operating against each other. The common wisdom is that props have higher power at low speeds than jets. But consider power delivered at standstill: Stand on the brakes and rev a prop engine. The engine is delivering lots of power, but none of it is going to the plane because the plane is stationary. The power is all dissipated in the propwash. So what's really going on here is that a prop engine delivers maximum thrust at standstill.

As your speed increases obviously the power delivered to the plane increases. However, eventually you start running into the limitations of a prop. As your speed goes up, the flight path of the prop blade becomes more and more straight ahead and less sideways; consequently, the "lift" generated by the blade more and more turns into rotational resistance and less into thrust. In other words, the thrust generated by the prop falls off as you get faster. So you reach the point of diminishing returns where the engine is no longer imparting any power to speak of into the air and its thrust drops to nearly zero.

A jet engine, on the other hand, imparts a much greater acceleration to a smaller amount of air. (OK - I'm talking about a pure turbojet here. I'll address bypass engines in a moment.) At standstill, this comes in part from just running the air through the compressor and turbine. However, most of the jet's thrust comes from heating the air. The air entering the engine is at atmospheric pressure; the air pressure leaving is only slightly higher, so by the old gas equation PV = NRT the air must be leaving at a velocity that's faster roughly in proportion to the amount by which it's been heated (in absolute degrees).

Just like a piston engine, at standstill a jet delivers zero power to the aircraft. However, as you accelerate, the thrust delivered by a jet does not fall off as fast as that of a piston engine because the exhaust velocity is much higher. As the jet gets faster, the compressor effectively does less work in accelerating the air into the engine - more and more the air just flows in at the freestream velocity. (But the compressor still has lots of work to do to compress the air inside the engine.) So you lose the element of thrust from accelerating the air into the compressor. But most of it is still there from heating the air. At very high speeds the air is actually being decelerated (relative to the engine) at the intake to keep the compressor blades subsonic. This is accomplished by expanding the diameter of the intake duct from the engine mouth to the compressor face. (Constant flow rate with constant pressure into an expanding volume means the flow velocity has to decrease.) This causes intake drag but is still compensated by the acceleration at the other end from heating the air.

With properly designed intakes, jet engines become even more effective at supersonic speeds. The air must be decelerated even more at the intake, but this is now done with supersonic flow rather than subsonic flow. The supersonic flow causes shock waves that compress the air as it decelerates; the compression of the air increases the total mass flow and therefore the engine's thrust. For a while, as your speed increases above Mach 1 the engine's thrust increases. There are a couple of limiting factors:



  • Intake geometry. Above a certain Mach number the air can no longer be decelerated to subsonic speed before it hits the compressor face.
  • Temperature limits. Compressing the air heats it up, so there is less "headroom" for heating the air in the engine and still staying within the engine's temperature limits.
  • Ultimate limits on exhaust gas velocity. Intake drag finally exceeds the thrust that can be developed by heating the air to the maximum exhaust temperature.

One might ask about Mach limits to the jet exhaust velocity. "How can the jet exhaust flow faster than the speed of sound?" The answer is it doesn't. Remember that the speed of sound in a gas is proportional to the square root of the temperature. So the speed of sound is much faster in the hot engine exhaust, allowing it to flow much faster than the nominal 1000fps you get at normal atmospheric temperatures.

Another detail about flow and pressure... The air pressure at the engine exhaust is determined by the exhaust's flow resistance. EPR (engine pressure ratio) is the ratio of air pressure at the exhaust to the air pressure at the intake. It effectively represents the air flow rate through the engine as seen through the engine's flow resistance. Typical EPR for an airliner jet engine at full power is in the neighborhood of 1.4:1.

So what does this all come down to? Ultimately what matters to an aircraft is thrust, not power. You need to accelerate the plane at takeoff and overcome air resistance at cruise. Props and jets are rated differently because they are measured differently. A prop engine is rated in horsepower because that's what you can measure at the shaft: power = torque * rpm. But that's only loosely related to what it's really doing for the plane because the amount of thrust the prop can translate that power into greatly depends on the plane's speed.

Horsepower doesn't make any sense for a jet because most of the power delivered at the turbine shaft is used internally. And its thrust is more constant relative to speed, so static thrust is the more useful metric.

From an energy conservation point of view, props are more efficient at low speeds than jets because almost all the energy goes into accelerating the air. A lot of the jet's energy goes into heat; that part of the energy is effectively lost. The downside of props is that as you approach Mach 1 at the prop blades they can't accelerate the air anymore. Also from the energy conservation point of view: A jet engine typically generates a lot more total power than a prop engine. However, at low speeds a greater proportion of that power is wasted heating air and accelerating it to very high velocities. At higher speeds, more of that power is actually imparted to the aircraft.

Now about bypass engines... In a bypass engine, a portion of the air flowing through the first stage of the compressor simply passes around the rest of the engine (for high bypass engines, as much as 90% of it). Effectively, that part of the compressor is simply acting as a prop. Bypass engines (AKA turbofans) are in effect a hybrid of a jet and a prop engine. You get improved static thrust (vs total energy consumed) and higher energy efficiency at low speeds in return for reduced thrust at high speeds. The bypass ratio determines the extent of the tradeoff. From this point of view, a turboprop is simply the ultimate high-bypass jet engine. High bypass engines are a good fit for airliners because they are most efficient at the airliner's cruise speed of Mach .85. You get maximum thrust for takeoff and climbout, which are done at relatively low speeds, and you need relatively less thrust for cruise at Mach .85.



I think what we are witnessing here is called plagiarism. And for those that don’t know what that is plagiarism is “the practice of claiming, or implying, original authorship or incorporating material from someone else's written or creative work, in whole or in part, into one's own without adequate acknowledgement.” The written words between the quotes came from Wikipedia, not me. If you are going to write something then write it yourself or give the author credit. Clicking and pasting, and then trying to accept the credit for it, is irresponsible and childlike. This appears to be the site where you have gotten the material. http://home.earthlink.net/~x-plane/FAQ-Theory-Jets-Props.html
fficeffice" />>>
>>
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Old 08-04-2007, 06:43 PM   #38
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I think what we are witnessing here is called plagiarism. And for those that don’t know what that is plagiarism is “the practice of claiming, or implying, original authorship or incorporating material from someone else's written or creative work, in whole or in part, into one's own without adequate acknowledgement.” The written words between the quotes came from Wikipedia, not me. If you are going to write something then write it yourself or give the author credit. Clicking and pasting, and then trying to accept the credit for it, is irresponsible and childlike. This appears to be the site where you have gotten the material. http://home.earthlink.net/~x-plane/FAQ-Theory-Jets-Props.html
fficeffice" />>>
>>
Dennis
This is another case of "Who Cares?" He's not writing a thesis or publishing a book, just providing information. I'll take information from whatever reliable source is available (Wikipedia being taken with a grain of salt, since anyone can edit it).
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Old 08-12-2007, 03:50 PM   #39
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This is another case of "Who Cares?" He's not writing a thesis or publishing a book, just providing information. I'll take information from whatever reliable source is available (Wikipedia being taken with a grain of salt, since anyone can edit it).
It always amazes me that there are people like you who respond just to argue. What point are you making here? Everyone who has a lick of sense wants to know that the information they are getting is creditable and truthful. Just like so many of the opinions posted here are just that and have no scientific merit. If you have something to say then say it, but if your clicking and pasting then you should also say that. Learn Netiquette and how information should be posted on the Internet before you open your big mouth next time. It sounds like you have a lot of growing up to do son, or you’d know what you just wrote just reeks of stupidity.
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Old 08-12-2007, 03:51 PM   #40
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This is another case of "Who Cares?" He's not writing a thesis or publishing a book, just providing information. I'll take information from whatever reliable source is available (Wikipedia being taken with a grain of salt, since anyone can edit it).
It always amazes me that there are people like you who respond just to argue. What point are you making here? Everyone who has a lick of sense wants to know that the information they are reading is creditable and truthful. Just like so many of the opinions posted here are just that and have no scientific merit. If you have something to say then say it, but if your clicking and pasting then you should also say that. Learn Netiquette and how information should be posted on the Internet before you open your big mouth next time. It sounds like you have a lot of growing up to do son, or you’d know what you just wrote just reeks of stupidity.
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Old 08-12-2007, 05:08 PM   #41
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Default Re: Props vs Jet Engines

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It always amazes me that there are people like you who respond just to argue. What point are you making here? Everyone who has a lick of sense wants to know that the information they are reading is creditable and truthful. Just like so many of the opinions posted here are just that and have no scientific merit. If you have something to say then say it, but if your clicking and pasting then you should also say that. Learn Netiquette and how information should be posted on the Internet before you open your big mouth next time. It sounds like you have a lot of growing up to do son, or you’d know what you just wrote just reeks of stupidity.
The point is, my friend, that you are at the wrong forum if you think what you have to say regarding plagiarism is important. We are not a bunch of stuffy intellectual A-holes (although you may very well fall into that category) who feel it necessary to research the source of all statements made on this informal forum. If I feel an error has been made, I do my own research. If I find that a person is giving me erroneous information, or if I have received such and pass it on, we are free to correct each other, without rancor or the use of demeaning insults (most of us, anyway). You seem to require some growing...why don't you post elsewhere if all you're going to bring to this forum is conflict? I noticed that you have only posted 3 times. Is this because A. You're new to this post, and have been searching for an outlet for your hostility, B. You've been kicked off another forum, and are searching for an outlet for your hostility, or C. You're a fellow DAC'er disguising himself so he can find an outlet for his hostility? This is a tempest in a teacup, bud.
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Old 08-12-2007, 09:02 PM   #42
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Default Re: Jet engine v.s. Prop engine cargo aircraft?

Part two of this response is, plagiarism is bad, and I don't condon it. The theft of intellectual property for notoriety or gain is illegal. But the person who posted had nothing to gain, and was not directly passing it off as his work. You might infer it, since he didn't quote a source, but once again, WHO CARES? Dennis, you dragged up an old thread to make a point that I'll bet almost anyone on this forum could not give a rat's butt about. And you did p.iss me off talking to me like that. How do you like being called stupid and a big mouth? That's not a good way to conduct yourself. And I'm certainly NOT your son. I'd disown you.
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Old 10-28-2007, 08:16 AM   #43
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Question Re: Jet engine v.s. Prop engine cargo aircraft?

Gentlemen lets not forget about epr vs dynamic conpression ratio.

1. Brake specific fuel consumption
2. Thrust ratio
3. Propeller efficiency
4. Is a turbo fan not like a propeller?
5. Do jets use high rpm at high altitudes (epr)
6. Why are airlines not profitable today if jets are that great?
7. Why do small trunk carriers rely heavier on props?
8. What's happening to all the c02 at 40,000 feet?
9. Why are the airlines looking at props today?
10. What about turbo chargers???
11. Will ethanol ever be a fuel for aviation?
12. Will jets be efficient with ethanol?
13. And let's not forget about fuel dumping.
So what do ya think???
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Old 10-28-2007, 10:57 AM   #44
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Default Re: Jet engine v.s. Prop engine cargo aircraft?

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Originally Posted by hatnthering View Post
Gentlemen lets not forget about epr vs dynamic conpression ratio.


1. Brake specific fuel consumption
2. Thrust ratio
3. Propeller efficiency
4. Is a turbo fan not like a propeller?
5. Do jets use high rpm at high altitudes (epr)
6. Why are airlines not profitable today if jets are that great?
7. Why do small trunk carriers rely heavier on props?
8. What's happening to all the c02 at 40,000 feet?
9. Why are the airlines looking at props today?
10. What about turbo chargers???
11. Will ethanol ever be a fuel for aviation?
12. Will jets be efficient with ethanol?
13. And let's not forget about fuel dumping.

So what do ya think???
Hmmm...have you read all the entries in this thread? You know that one of the biggest factors in jet vs props is speed. It's physically impossible for a propeller-driven aircraft to travel as fast as a jet. A jet is far more efficient in that respect. Airlines are having profitability problems because they don't have enough passengers, and fuel costs keep rising. Ethanol does not have the octane rating that a jet engine would require, but alternate fuels are being researched big time right now. I'm not sure what you mean by CO2 levels at 40,000 ft, but turboprops are more affected by high CO2 than jet engines, simply because of the volume of air compressed by a jet vs prop...a jet compresses a massive amount of air, the prop not so much, even with turbo boost. Plus, with thinner air, the prop isn't nearly as efficient...think air density as a performance factor in propeller craft. AFA fuel dumping, not sure how that's a significant factor in props vs jets.
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Old 10-29-2007, 04:50 PM   #45
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Being new to flying turboprops, I'm probably not as qualified as some on this forum to rebuke your idea that any measurable thrust is generated from a turboprops exhaust. In the C-208B I fly, I can assure you that, aside from leaving a nasty little soot trail down the right side of my cargo pod, absolutely NO measurable thrust comes out of my exhaust stack. Whoever told you that doesnt know what they are talking about.
On a side note... the C-130H gets about 10% thrust out of the tailpipe of the exhaust....
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Old 10-29-2007, 04:57 PM   #46
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Pro and con comparing between Jet engine and prop engine?
Example: C-130 v.s. C-17 if both have same capabilities specifications which one would benefit the most from operation to capabilities to quality of engine? I am aware that jet engine can carry more lbs than prop engines but I just ask that question as what if their capabilities is about the same and what the difference to benefit between those two?
Why/what benefit to need a prop engine? Less worry of engine damage on sensitivity environment than sensitivity jet engine which require clean environment to fly? Etc.......
Just curious to know the difference between jet and prop engines of cargo aircrafts.
Back to the almost three year old question here...

The prop has much more instant torque...you push the throttle forward and there is no spool up time..you have that power right then and there....

This is most useful not on take off but on an assault landing... if you pull the throttles back too far or misjudged it a tad and you need to punch the power...you have it as soon as you shove the throttles up (or down) [just don't over torque my engines] The engine is running at 100% all the time (well..not really but we do not need to go there)

About the jet engine being able to carry more weight...what is that in reference to? I don't get it.
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