Turboprop Engines

[PA31Ho]

Hey all, I'm trying to learn more about turboprops and turbine engines. I took out the C90 Manual I have at the house that a friend gave me from Simcom, but I got lost when it started talking about Two-Stage Axial flow and Centrifigul flow. So I was wondering if anybody could give me a good web site to read read about it. I thought about ordering the book "Turbine Pilot's Flight Manual" does anybody have any thoughts on this book?

Thanks.

 

[350DRIVER]

Great book... You can order it from barnesandnoble.com


Turbine Pilot's Flight Manual


well worth it.

3 5 0

 

[chqflyer]

I think that's a pretty good book to learn the basics of jet engines. A turbine engine is a helluva lot easier to figure out than any type of piston engine. Gearboxes and overspeed governors can be somewhat mindboggling as well, but the turbine engine is beautifully simple.

An axial flow turbine just means the air flows straight through the engine. A centrifugal engine uses a compressor similar to what you'd find turbocharging your car engine. The air takes a much more circular route through the engine which allows for a much shorter engine. The book goes into a lot more detail, but it's still easy to follow.

 

[LearLove]

While its been noted around here that I usually recommend against spending $$$ on some aviation stuff such as Sporty's flight bags, knee boards and such, The Turb. Pilots Flight Manual a very good book and one that many recommend. It worth the cash, you'll refer to it many time in the years to come.

other book, these 2 can be found in the local Lib. so you may not have to shell the cash first.

Aerodynamics for Naval Avaitors (classic)

The Illistrated Guide to Aerodynamics (very very very simple well written book on overall aircraft design)

 

[avbug]

The compressor can be thought of as a compactor, with a series of fans pushing air, one after the other, to a higher, and higher pressure. This is accomplished by the fans themselves, and the shape of the duct through which it passes.

An axial compressor is much like a household fan, in a tube. It blows air straight through.

A centrifugal compressor is a slinger. Think of it as the blades in your garbage disposal. Drop in the garbage, the blades spin around, toss the garbage outward, away from the blades. Or the little impeller in your blender. Only in this case, it's air.

A centrifugal compressor produces a much greater pressure rise than an axial compressor; a single stage centrifugal compressor does the work of several axial compressors. The number and location are determined by the design of the engine. Usually a series of axial compressors preceed a centrifugal compressor, and the centrifugal compressor stage is the last one in the compressor section.

From there, the air simply flows into a diffuser ring, which is nothing more than another part of the engine with a different shape. Throughout the compressor section, the duct narrowed, causing increased velocity. In the diffuser section, the ducting opens up, causing a drop in velocity and an increase in pressure.

This high pressure air is routed into burner cans (and mostly around burner cans), where it's mixed with fuel, and ignited. From there, it passes from the burner to the turbine wheels...it blows on the wheels, causing them to turn like windmills. These wheels are mechanically connected to the compressor; the first of these wheels are what make the compresors turn.

In some engines, there are multiple wheels. Some are connected to the compressor. Some are connected to a propeller or fan. The exhaust then exits the engine, serving to propell the aircraft by producing thrust.

If you're familiar with a piston engine, you're familiar with the cycles of intake, compression, combustion (power), and exhaust. This is repeated over and over in the piston engine, one step at a time.

In a turbine engine, the same cycles take place, but continuously, each in a separate part of the engine. You have intake at the front of the engine, and it keeps occuring throughout the engine operation (rather than in a series of "breaths" as in a piston engine). Next you have compression, again a continuous action. Combustion takes place continuously, and without need for a spark, as it's a constant-burning fire that occurs as more and more fuel is added...it works like the burner on your stove. Finally, you have exhaust...not a cycle or stroke, but a continuous venting of expanded hot gasses from the rear of the engine (or from the front, in the case of the PT-6.

There's a little more to it, but you get the idea.

Some engines take air in the front and blow it out the back. Others take it in the back, and blow it out the front. This type of engine is called a reverse-flow engine. The PT6A is one such engine.

 

[FL717]

A centrifugal compressor produces a much greater pressure rise than an axial compressor; a single stage centrifugal compressor does the work of several axial compressors.

Excellent post as always Avbug!!

I only have one question about your response. I was always under the impression that the opposite was true regarding your above quote. I always thought (maybe erroneously) that an axial-flow compressor was more efficient, and as a result of greater pressure rise it would/will produce over 2 times the compression ratio of a centrifugal flow?

I don't have my Turbine Pilots flight manual handy, and haven't really had the need to dissect this theory for several years now, and could be wrong.

Great topic!

 

[avbug]

The two typically work in concert. Axial compressors work in multiples to accelerate the air, for maximum mass airflow (volume), while the centrifugal compressor actually slows air but increases pressure dramatically. The axial compressor's effect is as much due to duct shape with decreasing duct area, as the efficiency of the compressor itself.

One for one, a centrifugal compressor provides a much greater pressure rise, though less velocity. Placing a centrifugal compressor after axial compressors makes sense, as a prelude to the diffuser. It's also a good place to install bleed valves (utility bleeds typically at this point or the diffuser case), while acceleration or engine bleeds in the axial flow section.

Axial sections are quite susceptible to compressor stalls, whereas a centrifugal compressor is not nearly so (bearing in mind that a compressor stall is a compromise between airflow inefficiencies due to angle of attack at any given compressor wheel, and excessive back pressure from the diffuser/burner section. Among other things, a centrifugal compressor also serves to reduce the chance of an axial compressor stall.

The compression ratios vary, but generally multiple stages of axial flow compressor are required to produce the same increase as a single stage centrifugal impeller.

 

[Flying Illini]

Great post Avbug.
Buy the Turbine Pilots Flight Manual, I don't know what the cost is now but it contains some of the best descriptions/pictures you can find on the subject of turbine engines. Mine came with a CD which was pretty helpful. Great book, money well spent.

 

[PA31Ho]

Wow... impressive replies. Makes it a lot simpler to have anologies (thanks Avbug)

So what I get out of this is that axial-flow compressors come before centrifigul-flow compressors. Now, when it says tw-stage, does that mean 2 compressors? That's just what I'm assuming, because I can't think of anything else that 2-stage would mean. Now, you explained what a diffuser ring is, but where is it located? I'm just trying to get a mental picture in my head of the inside.

Thanks Once Again

 

[turning2]

As the saying goes " a picture is worth a thousand words"

Buy the book you certainly will not be disappointed.

If you're looking to save a few bucks you can pick it up here both used or new.

Enjoy

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