Performance on Jets vs. Turbo Props
 

I have just read a splendid book on jet airplane performance. I currently fly a turbo prop, and I would really like to know a bit more on the difference between a jet engine and a turbo prop engine (performance wise :)).

The only thing I found on turbo props in that book was this statement: "Jet enginges are Force Producers while turbo props are Power Producers."

Why is jet engine performance expressed in Newton (or pounds) and turbo prop performance expressed in KW (or horsepowers)?

I can of course just accept the fact, but I would really like a comparison in some way.

Can anybody enlighten me on this one?

Nick

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"I have found that alcohol taken in sufficient quantity produces all the effects of drunkenness."
~ Oscar Wilde

As you will probably have discovered, a turboprop powerplant has two distinct units:

1. The 'Gas generator'

2. The other bit, with no specific name

The gas generator is almost analogous to the engine in a piston aircraft up to the end of the crankshaft. (Although strictly speaking, the gas generator does not include the 'power turbine', which extracts the useful energy from the gas stream to drive the prop). Thereafter, the rest of the powerplant is somewhat flexible. The Astazou engine, for instance, was used on both the Jetstream and the Gazelle (Hardly 'peas in a pod', you might say). It would therefore be nonsense to suggest a particular thrust value, since this would depend vastly upon the propeller (which could come from a different manufacturer to the GG).

Jet engines, on the other hand, although they work on the same principles (particularly high BR turbofans) are generally marketed as a single unit from one manufacturer, and since thrust is a more convenient commodity to work with than power, it is far easier to bypass an intermediate stage of calculation (via prop effeciency, which could not apply in any case) and cut to the chase. Indeed, in order to gauge the useful power output of a jet engine, one would have to bultiply the thrust by the speed of the aircraft, which raises the question 'what speed?'. Jet engines (especially old turbojets) also have a much flatter thrust profile with respect to speed, so thrust is 'more constant'.

In conclusion, I would suggest that the main reason for the difference is simply convention. Whereas from an aircraft designer's point of view, turboprops have developed as a descendant of the piston engine, with which they are directly compared, jets have a pedigree of their own, and as such, have their own set of engineering conventions. Might I also add that jet engines are a lot easier for design engineers to deal with than propellers, which present nothing but grief and hardship to all concerned.

Sorry if this upsets you as a turboprop driver!

Pie

[This message has been edited by Pielander (edited 03 July 2001).]

Might I suggest that the distinction may have more to do with the practical manner in which grunt can be measured ?

The turboprop unit, in the same way as for piston power units, is most conveniently graded according to brake shaft horsepower (or whatever units you might prefer) where the grunt can be dispersed via the means of some external load hooked onto the shaft (hence "brake"). The propeller then converts most of the power coming up the shaft into thrust, which is what the pilot wants. Thrust ties the engine and the propeller controls together and is not simply a matter of what the engine is doing. Hence propeller power units are most conveniently rated in terms of the available power - kW, HP, whatever ...

For the jet/fan, where the useful output is via the exhaust, it is more convenient to measure the thrust, hence kN, lbf, whatever...

The other concern is that the analysis of the aircraft/engine combination is rather easier with a jet/fan unit.

For the benefit of those of us who read books occasionally, what was the book ?

[This message has been edited by john_tullamarine (edited 04 July 2001).]

Thanks for the replies!

I had to go through some card board boxes to find the book (I just moved), and I found it at last.

The book is "Aerodynamics for Airline Pilots" (ISBN 87-16-10557-5) by Niels Peder Ternvig. He's an SAS captain with a BSc in Mathmatics/Physics.

What is so excellent about the book, is that it is very thorough (lots of formulas and graphs, for those who like that :)), but still easy to follow for any average airline pilot. It is also composed in a very simple manner, which gives you a good overview of the subject even if you skip the really heavy stuff.

On the back side it says: "This volume bridges the gap between basic information for light aircraft pilots and technical litterature for aeronautical engineers." And I believe these words hold true.

Enough advertising...

Anyway, the only thing I am missing is a thorough discussion on why the diagrams used to explain jet performance and turbo prop (or piston engine) performance (in other books) differs. Even though the book is discussing it a bit more than just the sentence I quoted, it's about the only thing that is not crystal clear in it.

I understand from your two answers, that engineers have "chosen" to use power when discussing propeller performance and force (thrust) when discussing jet performance because the diagrams and "models" get simpler and easier to understand/work with. ...due to the fact that propeller thrust decreases with increased speed, and jet thrust remains (fairly) constant with increasing speed.

Am I close?

And Pielander - no offence taken. :) I guess I'll spend more than 30 years in a jet, so a few years of propeller experience won't do me any harm. :)

I am actually really happy with the PW125B on the Fokker 50. The design engineers have done an excellent job with those engines, despite any grief and hardship they may have gone through designing it!

Nick.

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"I have found that alcohol taken in sufficient quantity produces all the effects of drunkenness."
~ Oscar Wilde

Nick,

Thanks for book details ...I must see if I can locate it and have a scan ....

The approaches come down to two main considerations -

(a) prop installations lend themselves to power gauges of whatever sort ..and it is lovely if you have a couple of multi-thousand HP fans either side .. memories ... jets use thrust gauges, usually EPR or N1 presentation or similar.

The performance, however, is still a matter of what thrust the installation can push into the airflow, regardless of how the thrust is obtained.

(b) the sums for jets and pistons are much simpler than for turboprops.

Can't comment on the pictures in the book without seeing them .. if I can locate it I might venture a comment or two ...



[This message has been edited by john_tullamarine (edited 04 July 2001).]

John

Would you be referring to the off-design performance of turboprops? I have to admit that apart from some of the finer points of error analysis in CFD, that was the only thing that's had me totally flummoxed. :rolleyes:

Pie

My engineering predates CFD (although my son impresses me with his incomprehensible knowledge of the subject) and I haven't had any need to get involved.

Not being an engine design man, I can only go on my reading on the subject. My limited understanding is that any turbine has a comparatively narrow useful operating envelope, regardless of whether it is driving a shaft or throwing air out the back.

So far as the performance is concerned, the jet can be described in a reasonably closed form manner, whereas the turboprop needs a rather messier approach.

Nick,

Turboprops are characterized by high mass flow through the props, low velocity increase. For a prop aircraft maximum thrust (of the engine prop combination aka power plant) is at zero speed.

The power plant power output (prop + engine) increases with airspeed to a point then decreases.

The turboprop engine is a power producing device, with most of the work done by the gas stream converted in shaft horse power for transmission to gearbox then to the prop.

The turboprop power plant gets most of its thrust from accelerating air through props, only about 10-25% total thrust from the power plant comes from the jet exhaust.

As most of the useful energy output from the engine is in the form of shaft horse power (ie torque and RPM) it makes sense to classify its ability in terms of shaft output, as only a small fraction (10-25%) is gas stream output

As thrust drops off with speed its not a very useful operational parameter. i.e imagine trying to set a thrust figure while the aircraft is moving on a takeoff roll, its decreasing as the speed builds up, much easier to set a torque and temperature to get the power required.

Jets, characterized by low mass flow of the air stream with a high velocity increase. The thrust available through a range of speeds is fairly constant for a jet. To get the performance you want set the thrust via fuel flow, shaft RPM, or engine pressure ratio. This sets the thrust, as airspeed increase, thrust remains fairly constant, and power increases.

The turbojet derives most of the engine performance from the high velocity exhaust gas in the form of jet thrust, with only a small percentage of the gas stream energy used to drive the compressor.

The jet exhaust is doing most of what is useful, hence the jet thrust is used figure people look at, plus its operationally easy to determine.

Now for something to really confuse you, a turbofan derives most of its performance from the larger bypass fans at low altitudes (the high mass flow bit on the fan), and the jet thrust (the high exhaust gas stream velocity) at high altitudes.

Z

I'd have to disagree with your last statement there Zeke. The proportion of thrust attributed to the fan will decrease with speed, but it is still designed to provide most of the thrust of the engine in the cruise. (Depending on the engine, of course).

:) :) :) :) :)

Thanks a lot, Zeke!

This statement is just what I have been looking for! What a simple and clear explanation :). Of course you can explain both jets and turbo props in terms of both power and thrust (depending on what we're discussing), but this clearly explains why power is most commonly used for TP and thrust (force) is used for jets.

God I like this forum!

Nick.

I have tried to find out where y'all can get "Aerodynamics for Airline Pilots."

I found a "flightshop" link in Denmark with this email address: [email protected]

The book costs 382 DKR ('bout 45$).

Surely it will be expensive to order it from Denmark, but if any of you have a special interest (or an employer who would pay for it :)), it's really worth every penny.

Nick.

[ 09 July 2001: Message edited by: Nick Figaretto ]

'Ello Nick!

Think of it this way:

In a straight jet engine, fuel is sprayed into the chambers, combusted and expelled out in the back. This means that (for all practical purposes) all fuel is used for thrust production (thrust producers)

In a prop, after the fuel is ignited, a lot of the heat energy is wasted throught gears/transmissions etc before it is transferred to the propeller disc (which isn't realy that efficient either) which in turn produces thrust, hence it is more convenient to talk about power (Watt)!

So, it is convenient to relate fuel flow to thrust output in a jet, and power output (internally) to fuel flow in a prop.

It is difficult to know exactly how much thrust a propeller generates, but for a jet engine it is quite simple.

Since FF is important for endurance/range calculations, drag (thrust required/FF) curves are used when talking about jets and power curves (FF) is used for props!

Fanjets have thrust curves that look like combinations between straight jets and props, because of the huge bypass fans that act as proopellers.

Are you inbound ENDU Aug.11???

Thanks for another explanation. I think I'm getting it right now.

And regarding inbound ENDU: I just might be!

Originally I thought I would be working on Aug. 11th, but now it turns out I end up at ENEV on the 10th, with a passive transfer to ENGM on the 11th. So if there's still room for another boofhead, I'd really like to come.

I think I could always get a passive transfer on Sunday instead.

Any chance for some aerobatics on Saturday morning... :)? I'm probably not the first one asking for that! :D :D :D

Nick.