Difference between a jet and a turbo-prop

[FlyingForFun]

Ok, this question isn't quite as dumb as it sounds from the thread title, but probably not too far off! It's another case of me sitting at work bored, and my brain wandering off thinking about flying.

Throughout all of my studies of principles of flight and performance, I've learnt that propellor-driven aircraft and jet aircraft behave slightly differently. For example (to use the simplest example I can think of), max endurance on a jet is Vimd, and on a prop it's Vimp. This makes perfect sense... more thrust requires more fuel in a jet, and more power requires more fuel with a propellor.

Then I started thinking about the difference between a jet engine and a turbo-prop. Forgive the extreme simplifications, but I'm tryign to concentrate on the differences:

High bypass ratio jet engine: fuel is burnt, and used to turn several turbines. The turbines are used to drive compressors (which feed air into the engine for burning more fuel, and produce a small amount of thrust), and, more importantly, turn a big fan at the front of the engine which forces air backwards very quickly and generates lots of thrust.

Turbo-prop: fuel is burnt, and used to turn several turbines. The turbines are used to drive compressors (which feed air into the engine for burning more fuel, and produce a small amount of thrust), and more importantly, turn a big fan (aka propellor) at the front of the engine which forces lots of air backwards and generates lots of thrust.

So - what's the difference? What properties of the fan/propellor produce the differences in theoretical performance, since they essentially both do the same thing?

Thanks!

FFF
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[Keith.Williams.]

Tripple F,

The (very) short answer to your question is the turbofan air intake!!!

As with all such things the requirement for brevity precludes total accuracy, so what I am about to say is not entirely accurate (and the purists will probably argue that it is simply untrue).

If we ignore the fraction of turbofan thrust that is produced by parts other than the fan, we can say that the fan is essentially a propeller with a very large number of blades. So you are correct in stating that both systems employ a power generator, to drive a thrust producer.

The main problem with propellers is that the efficiency with which they convert engine power into thrust is very sensitive to TAS. Their efficiency is very low at low TAS, increases quickly up to a maximum value at a fairly low TAS, then decreases very quickly as TAS continues to increase. Because of this highly variable efficiency, there is no reasonably constant relationship between the thrust produced and the fuel burned. It is for this reason that we use the Power:Fuel flow relationship to predict overall fuel efficiency for piston/prop combinations.

Turbofans (and indeed all jet engines) exhibit a generally similar efficiency:TAS curve, but are much less sensitive to TAS. This means that the efficiency curve is spread over a much wider airspeed range. We tend to represent the power available from a jet as a straight line angled upwards with increasing TAS, but if we follow the line for long enough we will see that it eventually falls away much like that for a propeller. At very high airspeeds the fan produces very little thrust, and we must rely on that generated by the rest of the engine.

The main factor in reducing the turbofan's sensitivity to TAS, is its air intake. Although the TAS of the aircraft will vary in flight, the TAS at the fan face varies much less. This is because the intake decelerates the incoming air, converting much of the kinetic energy into static pressure. This not only reduces the efficiency losses in the fan, but the ram compression increases the overall efficiency of the engine. Other benefits of the intake include such things as a reduction in blade tip losses by surrounding the fan with a circular duct. The overall effect is a more or less constant thrust:fuel flow relationship over a reasonably wide airspeed range.

This enables us to simplfy matters in POF and PERF by pretending that the thrust is constant at all airspeeds, and that fuel flow is directly proportional to thrust. We can then say that for maximum endurance we must look for minimum thrust required, which is of course at VMD.

But we cannot employ this simplification with a propeller aircraft. By the time we get to VMD the propeller efficiency has already dropped far below its optimum, so the fuel cost of each pound of thrust is high. We get the bast compromise fo thrust:engine power and engine power:fuel flow at VMP.

As with most things in this business, what we take to be true is just a reasonable approximation.

[FlyingForFun]

Hi Keith,

Thanks for an excellent reply to a rather obscure question.

What I am about to say is not entirely accurateProbably not, but I'm sure it's a whole load more accurate than some of the stuff we have to learn for the ATPL exams! And, accurate or not, it's answered my question!

Cheers,

FFF
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