Turbines, Turbofans, Turboprops, oh my!

[SkyWestCRJPilot]

Props are for boats.

 

[LR25]

Chawbien, how did you know I wore glasses?

But you missed on the tape though, I got a new pair, man there expensive.

 

[Deftone45075]

Turboprops fly at lower altitudes than jets, which causes them to pay less. If anyone could invent a real high-altutude turboprop, we could finally get regional pay where it belongs.

Piaggio!

 

[pilotyip]

The secret

The secret of flying is: that jets are easier to fly that T-props, and T-props are easier to fly than recips. Jets may be harder to learn, but eaiser to fly and the engines on a jet are much easier to start and manage than an R-1820 or R-1830. In less than a 1000 hours of round engine flying I have lost 3 engines, and in over 9,000 hours of turbin flying I have lost one engine where the engine quit, almost all of my shutdowns in T-prop airplanes were for prop malfunctions.

 

[Timebuilder]

Eriknorth-

How's the training going? Is she beating you up really bad? Peachtree seems like a nice place to be...

Anyway, there are two basic kinds of turboprops. One has a direct mechanical connection between the gas generator (the heart of any turbine engine) and the propeller. The common version of this is the Garrett engine used on Conquests and the Mitsubishi airplanes. They are VERY loud, so you always hear them coming. After flying, many pilots get out and turn the props for a few revolutions BY HAND in the normal direction of rotation (never the other way) to pull some cool air into the engine. You will see that the props are NOT feathered on these engines as they sit on the ramp.

The other type is a "freewheeling" design that uses a fluid coupling (gases aren't liquids, but they ARE fluids) of the gas generator's turbine and another tubine section which is conected to the propeller system. Those props ARE feathered with the engines off. Mostly, they are a variant of the venerable PT-6 from Pratt and Whitney, and they often have a distictive exhaust "horn" near the front of the engine nacelles, one on each side (think:King Air), except for the Caravan, which has one massive exhast pipe on the right. Larger P&W engines, like on the Dash, still feather at rest.

While turbines are more reliable than pistons, they have a large appetite for money. Inspections and repair are stratospheric, and a very large amount of fuel is used, compared to a piston twin, even before takeoff. Once the igniters light the fire in the turbine engine, it keeps burning continuously, on its own. Since there are no magnetos, you don't need a "run up". The turboprops have an autofeather system. The jets can generate a fair amount of drag as they spool down, buy they have no ability fo be feathered, so you just put in some rudder, trim it, and plan for a landing.

Now you have some basics. You might be interested in
The Turbine Pilot's Flight Manual

Let me know how things are going. Fly safe.

 

[Timebuilder]

You're right, Fly. I'm just exposing Erik to the basics that he will see, most of the time.

I'm not sure of the exact temp, but that shaft DOES get hot, and I often see that shaft being cooled by the pilots. Two or three revolutions and you're done.

The reason you don't turn backwards are twofold:

1) you want to draw cool air in the front

2) the accessories don't like to go backward, and they turn all the time. The gen brushes are mounted at an angle (so I'm told) and the commutator segments can chip and break them if turned backward.

 

[pilotyip]

10,000 degrees?

I don't think they had invented a material that can take the torque of Turbo prop shaft and stand that high of a temperature at the same time. 1st stage Turbine inlet temperatures of 1200C are about high as I have heard of in any commercial turbine engine. Pulling the engines through is a common practice on Garrett TPE engines, we did it on the AC-690 and CA-212

 

[wookiehasbeen]

yeah but how much good can it do?

i have only flown a garrett powered plane once, but when you turn the gas off the whole time it is spinning down, which usually takes about thirty seconds, from 1500rpm or more it is cooling down so what good does six more revolutions really do? think about it.

 

[Archer]

10,000 seems a little impossible. Some of the most advanced jet engines can't have turbine inlet temperatures greater than 2000 C, as the turbine blades would melt.

And those that can sustain 2000C, they have hallow blades with holes in them that allow the flow of cooling air on their surface, and allo them to operate at 2000 C even though their melting point might be 1900C!

So I doubt a prop shaft could handle 5 times that heat.


As for the different engines, the differences between turboprop (turboshaft for helicopters) turbofans, turbojets are mainly:

1. Fuel consumption
2. Drag
3. Thrust
4. Noise
5. Speed

Propeller has been the way to go for decades after the Wright Brothers. But propellers limit the forward speed of the aircraft, as the tip of the blades reaches sonic or supersonic speeds, and you don't want that.

So that's why there are no turboprops that streak across the sky at 450 knots.

Turbofans can easily acheave 450 knots, as they have a "prop" that is smaller called a "fan" that's inbeded in the inlet, which slows the flow down, so that that blades don't reach supersonic speeds, thus the aircraft speed is less dependent on prop tip blade speed.

Turbofans have what's called a bypass ratio. That's the ratio of air that bypasses the core of the engine, to that which goes through it. 747s I think might have a Bypass ratio of 5 or so.

The greater the bypass ratio, the more you can call it a turbofan. The lower the bypass, the more like a turbjet will it be.

So basically you can make a compromise with a medium bypass ratio.

High bypass ratio turbofans emit less noise (jet noise is the loudest, and there is more air going through the fan rather than the core, thus less noise). They are also more efficient at slow speeds as there is a LOT of air going through the turbofan, compared to the turbojet.

A high-bypass ratio turbofan can't be used in supersonic flight regimes though. Low bypass or turbojet are used due to reduced drag.

Drag becomes more important at supersonic speeds (cross sectional area) so you can't have the big area of a high bypass turbofan.



Basically, if it's fat, it's a high bypass turbofan, it's efficient, it's subsonic, it's less noisy.

If it's slim, it's a low bypass turbofan or a turbojet, less efficient, more noisy, but supersonic

Turboprop, highest efficiency, but lowest speed. Also, huge weight penalty as the huge prop diameter requires extra gearing with the turbine shaft...gear boxes I think they are called, and weight a crap load.


Boeing, Airbus - most use high bypass turbofans

Concorde- turbojet (a little modified, at Mach 2 it becomes some kind of a ramjet...so sometiems it's called a turbo-ramjet)

SF 340- turboprop (duh)

Archer

 

[Timebuilder]

from 1500rpm or more it is cooling down so what good does six more revolutions really do? think about it.

After everything stops, there is a "hot soak" that occurs as the components continue to radiate heat into the air inside the engine. The most dense metal parts, like the shaft, hold the most heat. Surrounded by now heated air, the heat from the shaft and other dense parts has no where to go. By the time you make it outside to turn the props, most of the parts have given up the vast majority of their heat into the air, and by pulling the props through you are expelling the heated air and providing a final parcel of cooler air to absorb some of the remaining heat.

I'm told this is a must for good service life.

I think fly just had one too many "zeros" there in that "10,000".