Turbo - prop/fan/jet ?

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ETOPS
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Turbo - prop/fan/jet ?

Post by ETOPS »

My search didn't turn up much...sorry if this has already been discussed.

Does turbofan time count as "jet" time?
Does "turbine" time incorporate all three groups (prop, fan, jet), or does it just refer to turboprops?

Thanx
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Re: Turbo - prop/fan/jet ?

Post by ahramin »

Turbofan is jet time.

Turbine time is all three.
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Re: Turbo - prop/fan/jet ?

Post by ETOPS »

But turboprop is not "jet" time?
I'm pretty sure the answer is NO.

What I'm trying to get at is: why does a turbofan count as "jet" time while a turboprop doesn't?
They both have a jet component producing thrust. So maybe the jet component on the fan produces a greater percentage of its total thrust, but where do you draw the line and why? :?

I'm guessing the distinction comes from the operation.
What exactly is so different (more complex) about the operation of a turbofan vs. a turboprop?
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Re: Turbo - prop/fan/jet ?

Post by Apache64_ »

no, turboprop isnt jet time. While both are driven by turbines there is somewhat of a difference between the two.
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Re: Turbo - prop/fan/jet ?

Post by AuxBatOn »

The F-18 has 2 turbofan engines. Would you count that as prop time? Or Jet time? Straight jets are not produced anymore and are actually not allowed to certain airports because of noise.

Go read about the subject. You will understand a little more how things work.
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Re: Turbo - prop/fan/jet ?

Post by pika »

What I'm trying to get at is: why does a turbofan count as "jet" time while a turboprop doesn't?
Maybe pounds of thrust vs horse power? N1 section is just a shrouded prop but without the shroud it's still a prop. Operation is important. Speed, altitude, etops, rvsm, rnp, etc.
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Re: Turbo - prop/fan/jet ?

Post by ETOPS »

First of all AuxBatOn, I understand the principles of the three types.
Second of all, don't try and confuse the topic by throwing an F-18 vs. prop comparison in here. I'm trying to find out why they're not both considered jets (turbofan and turboprop).

What makes operating this airplane (PW300 turbofan)
Image
so much more "special" then operating this airplane
Image
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Last edited by ETOPS on Thu Jun 10, 2010 11:40 pm, edited 2 times in total.

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Re: Turbo - prop/fan/jet ?

Post by ETOPS »

pika wrote: N1 section is just a shrouded prop but without the shroud it's still a prop.
Exactly!
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Re: Turbo - prop/fan/jet ?

Post by AuxBatOn »

ETOPS wrote:First of all AuxBatOn, I understand the principles of the three types.
Second of all, don't try and confuse the topic by throughout an F-18 vs. prop comparison in here. I never suggested a turbofan is a prop. I'm trying to find out why they're not both considered jets (turbofan and turboprop).
THere is no jet in a Turbo-Prop. There is a turbine but no jet. A jet is when air coming off the back of the engine is used to produce thrust. Turbo-Prop is when the energy from the turbine is used to turn a prop to produce a torque, which in turn produces traction (on most turbo-props). The thrust produced by the exhaust is negligible compared to the traction produced by the prop. The operation of a Turbo-Prop is very, very different from the operation of a jet engine.
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Re: Turbo - prop/fan/jet ?

Post by ETOPS »

There is a jet on a turboprop. Weather its negligible or not is arbitrary.

Once again, where do we draw the line, and why? What makes it so special from the point of view of cockpit operation?
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Re: Turbo - prop/fan/jet ?

Post by AuxBatOn »

ETOPS wrote:There is a jet on a turboprop. Weather its negligible or not is arbitrary.
It's so negligible that it's not taken into consideration in the performance calculations. The point is that the way to propulse the aircraft is with the prop, not the jet. The considerations on the operations of a prop are very different than a jet.
ETOPS wrote: What makes it so special from the point of view of cockpit operation?
I have 1 lever per engine. I do not have to worry about torque and all what comes with a prop. If anything, a jet is easier to operate than a turbo-prop. Where it gets harder (and I believe that's where companies requesting jet time are coming from) is generally, jets operate faster and higher than turbo-props. That aspect is more challenging, thus jet time shows that you have experience in a faster environment.
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Re: Turbo - prop/fan/jet ?

Post by Apache64_ »

There's good info on the net out there. Nasa, wikipedia and others have it all covered. Without the shroud the propeller is limited to subsonic flight. With the shroud the engine can go supersonic because the shroud slows the airflow down to subsonic speeds. The turbofan is more fuel efficient at higher speeds while the turbo prop is better at lower altitudes and lower speeds. Both run with nearly the same core design, and nearly same principles. I think the major differences between the two lie in the operations.
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Re: Turbo - prop/fan/jet ?

Post by AuxBatOn »

Apache,

All the energy not used by the compressors is directed to the Shaft of the prop (on a Turbo-Prop). On a turbo jet, all of it is used to produce thrust. On a turbo fan, part of it is directed to the shaft of the fan and part of it used to produce thrust. I think they are pretty big differences in design.
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Re: Turbo - prop/fan/jet ?

Post by Apache64_ »

AuxBat that is true, what I meant by core engine design, is compressor, hot section, and exhaust. Nearly the same. The main difference between them is what you stated.

Also like to add, that the engines create hot exhaust to be used as thrust. How the energy from the hot exhaust gas is translated into thrust is different between the three models.

turbojet - uses direct energy from the expanding gases through the exhaust jet to create thrust

turoprop - uses the energy of the exhaust gases to turn a turbine to power the propeller to create thrust

turbofan - the exhaust gases are used to drive the fan and produce a core exhaust jet to create thrust

I think we are on the same page Aux
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Re: Turbo - prop/fan/jet ?

Post by ETOPS »

AuxBatOn wrote:Where it gets harder (and I believe that's where companies requesting jet time are coming from) is generally, jets operate faster and higher than turbo-props. That aspect is more challenging, thus jet time shows that you have experience in a faster environment.
So thats it. A bit faster and higher (mainly in cruise), meanwhile a turboprop is harder to operate...
This whole distinction seem like a load to me.
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Re: Turbo - prop/fan/jet ?

Post by x-wind »

Operational consideration: Do jets and turbo-fans generally have longer spool up times?

Propellers have better acceleration principles (in both respects) and are therefore easier to manage, apparently arguable. Which would be the reason for the experience distinction IMhO.

It's more efficient to move a large mass of air "backwards" to achieve the opposite reaction- moving the airplane forward, then to move a tiny amount of air (jet) really fast backward (for the same reaction to occur). Hence, the turbo-fan is the modern design.

I wasn't aware there was a consequential distinction to jet and turbo-fan time. Knowing a F-18 has afterburners it'd be plausible to count both times, know? :P Like IFR flying but logging the IMC time.
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Re: Turbo - prop/fan/jet ?

Post by Panama Jack »

Can I log my Piper Navajo time as "Turbine" or "Turboprop" time, since the Lycomings have turbochargers (compressor, etc.) on them? The hot exhaust gasses drive the compressor just like on jets, and it has a prop (unducted fan). :wink:

Guys, I recommend and old and dry book called "Handling the Big Jets" by D.P. Davies if you want to understand the differences more deeply. There are additional factors such as momentum, high-speed and low-speed buffet, swept-wing aerodynamics and other factors that come into play when you change the powerplant from a turboprop to a turbojet. I have never had to worry about "coffin corner" flying a Dash-8. By the way, the pure "jet" is a rarety, somewhat of an early-technology design. Even the straight-piped JT8D's and JT3D's that you'd see on B-727/737/DC-9's and B-707's respectively are turbofans rather than pure jet.
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Re: Turbo - prop/fan/jet ?

Post by Troubleshot »

oh yeah what about this guy...lol
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Re: Turbo - prop/fan/jet ?

Post by Jaques Strappe »

ETOPS wrote:My search didn't turn up much...sorry if this has already been discussed.

Does turbofan time count as "jet" time?
Does "turbine" time incorporate all three groups (prop, fan, jet), or does it just refer to turboprops?

Thanx
I am assuming by your question that you are referring to jet time vs turbine time as time for your log book? For some reason, this country is obsessed with different variants of "time" when it comes to logging it. A quick review of types flown will tell any prospective employer what he or she needs to know.

Your two Dorniers are a great example of that. There is virtually no difference at all systems wise, however they are logged differently if you were to actually track jet time vs turbine time. One rule of thumb I would use would be: If there is no prop hanging off the accessory gearbox and it burns kerosene, then log it as jet. If there is a prop hanging off the exact same turbine section, then log it as turbine. Should anyone really care? Not in my opinion. An HS748 or CV580 demands more from a pilot than say a slowtation and a non turbine powered C46 demands more than all three.

This is why a quick review of types flown will give someone a better snap shot of experience, rather than a number in a column.

Hope this helps
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Re: Turbo - prop/fan/jet ?

Post by flying4dollars »

ETOPS wrote:There is a jet on a turboprop. Weather its negligible or not is arbitrary.

Once again, where do we draw the line, and why? What makes it so special from the point of view of cockpit operation?
Because in a turboprop, the propeller produces primary power through torque. Yes, the exhaust produces some power, but it's not the source of it. Turbofan's produce it's primary thrust through the turbine and exhaust. Yes the compressor blades (not propellers), produce some thrust through the bypass fan inside the shroud, but it's not the primary source of it.

Also consider pilots operating turboprops rely on torque to value their power, where are pilots operating jets rely on N1 and N2 values. As well, prop pilots have a propeller system to deal with, so in some regards, yes they are both turbine engines of some of the same principals of staged compression and power production, but the operations of each differ in their own ways. Where the Dornier 328 Jet and Prop are 2 of the same airplane, they are still different in their systems and operated as such. The SOP's will not be the same, neither will their operating parameters. When a jet engine fails, you're not scrambling to feather the inlet compressor fan blades.
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Re: Turbo - prop/fan/jet ?

Post by ETOPS »

Panama Jack wrote:Can I log my Piper Navajo time as "Turbine" or "Turboprop" time, since the Lycomings have turbochargers (compressor, etc.) on them? The hot exhaust gasses drive the compressor just like on jets, and it has a prop (unducted fan). :wink:
I never suggested this. The engine would still be a reciprocated piston engine and not a gas turbine.
x-wind wrote:Operational consideration: Do jets and turbo-fans generally have longer spool up times?

Propellers have better acceleration principles (in both respects) and are therefore easier to manage, apparently arguable. Which would be the reason for the experience distinction IMhO.
Ok x-wind, spool time and acceleration is reasonable.
Jaques Strappe wrote:Should anyone really care? Not in my opinion. An HS748 or CV580 demands more from a pilot than say a slowtation and a non turbine powered C46 demands more than all three.

This is why a quick review of types flown will give someone a better snap shot of experience, rather than a number in a column.
I agree, but still, I've seen job adds that specifically require "jet" time...
flying4dollars wrote:When a jet engine fails, you're not scrambling to feather the inlet compressor fan blades
No your not, which is another reason why turboprop operations are more complex.

So why is "jet" time held in higher regard?
So far:
-fly higher
-fly faster
-longer spool time
-slower acceleration
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Re: Turbo - prop/fan/jet ?

Post by Strega »

Troubleshot wrote:oh yeah what about this guy...lol

Damn!

you beat me to it!

S
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Re: Turbo - prop/fan/jet ?

Post by crooked timber »

ETOPS wrote:I've seen job adds that specifically require "jet" time...
the disambiguation lies within this context. when an operator is looking for "jet time" (turbofan), they're looking for someone who has experience dealing with the things Panama Jack alludes to (vis-a-vis handling the big jets). as mentioned, some of these considerations are: swept wing characteristics, high alt operation (mach tuck, ozone exposure, etc), flying around jet streams, mountain wave (which becomes a bigger deal when you're snuggled up in the coffin corner), low energy handling, fms experience and international ops. none of these things require any kind of superpowers but if a company is looking to hire someone who they can upgrade quickly then there is value in this type of prior experience.
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Re: Turbo - prop/fan/jet ?

Post by wilton »

it seems to me that you are trying to "UP or PAD" your log book. Turbo-prop is not Jet time.
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Re: Turbo - prop/fan/jet ?

Post by wallypilot »

flying4dollars wrote: Turbofan's produce it's primary thrust through the turbine and exhaust. Yes the compressor blades (not propellers), produce some thrust through the bypass fan inside the shroud, but it's not the primary source of it.
That's true of lower bypass engines, but many high bypass engines today, that's not always the case. The CF34B produces about 80% of it's thrust from bypass air at low altitudes, and at high altitudes, it's about 80% core thrust.
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