www.rolls-royce.com/education/schools/journey/flash.html

Suck - Squeeze - Bang - Blow!

aha, it really does get more thrust per squeeze

Cool.  I still struggle to understand the difference between a turbojet and a turbofan though.

Quoted: Cool.  I still struggle to understand the difference between a turbojet and a turbofan though.

How much air is bypassed around the power section of the turbine and then back into the exhaust.

It can get tricky, the FA-22's F119 turbofan for example, bypasses only a few percent more air than the old F-4 Phantoms J79, considered a classic example of a turbojet.

Wow. I wonder if the poor sucker who got fed into that A-6 engine got to see all that....

Quoted: Cool.  I still struggle to understand the difference between a turbojet and a turbofan though.

In a turbojet ALL the air inducted passes through the gas producing portion of the engine. In other words, all the air going in is used in combustion. In a turbofan some of the air inducted passes throught the gas producing portion of the engine but more is propelled AROUND the gas producing portion by a lower pressure/higher volume fan driven by the gas producing portion. Modern turbines are capable of producing more thrust from less fuel by being able to withstand higher temperatures and pressures. The excess power is better used to accelerate unheated air. Planerench out.

That is SO COOL!

As a visual learner I thank you for sharing that!

I'm a Nerd and Nerds just LOVE THAT STUFF!

That was one of the first things we learned in tech school. I found turbines easier to understand than radial engines.

Quoted: www.rolls-royce.com/education/schools/journey/flash.html Suck - Squeeze - Bang - Blow!

VERY cool link.  My company makes forgings for some of the parts you go past in there (mostly for Pratt&Whitney, though).  Neat to clearly see what's going on in there.

Thank you for such a cool link! Now they need to do the journey when a frozen chicken is shot into it!I wonder if those guys get tired of doing that ?Take care

ROLLS ROYCE…BEST OF BRITISH AIRCRAFT ENGINES

Were would the P51 Mustange have been without the wonderful Rolls Royce Merlin engine… music to the ears

Andy

Quoted: Thank you for such a cool link! Now they need to do the journey when a frozen chicken is shot into it!I wonder if those guys get tired of doing that ?Take care

Actually the bird is NOT supposed to be frozen...

"Rolls Royce. When your Mig 15 needs the very best".

Turbine engines 101.

Three types of turbines.

Centrifugal Flow
Axial Flow
Centrifugal-Axial Flow

All modern high-performance turbine engines are Axial Flow engines.

Centrifugal Flow engines in the modern world are limited to small auxiliary power unit or ground support type rolls.

Centrifugal-Axial Flow engines are used in helo or light fixed wing aircraft, such as a King Air 200 or older Cobras.

Axial Flow = airflow is straight through the engine.

Centrifugal Flow = airflow is directed around the outside of the case via impellers from the core to the outer edges.

Centrifugal-Axial Flow = airflow goes in a straight through path in the axial section of the engine and then follows the path as directd by the case / impellers. The Pratt & Whitney PT-6A is an example of this engine, the airflow actualy reverses it's path once it hits the centrifugal turbines.

The Axial Flow engine is broken down into 4 different types:

Turbojet
Turbofan
Turboprop
Turboshaft

A turbojet engine uses 100% of the inducted fresh air in the production of power.
Since a high " jet " velocity is required to obtain an acceptable of thrust, the turbine is designed to extract only enough power from the hot gas stream to drive the compressor and accessories .
100% of thrust produced by a turbojet engine is derived from exhaust gas.
The J-79's of the F-4 Phantom are a prime example of turbojet engines.

A turbofan engine uses only a small percentage of the inducted fresh air in the production of power.
The turbofan engine has an enclosed duct fan mounted at the front of the engine and driven either mechanically by the compressor section or by an power turbine located to the rear of the compressor drive turbine .
The fan air can exit seperately from the primary engine air , or it can be ducted back to mix with the primary's air at the rear .
The core section of the engine is what drives the compressor and accessories.
The CFM-56 is a prime example of a turbofan engine. The fan section produces 80% of the total engine thrust while the core engine only produces 20% of the thrust.

The turboprop engine derives its power by conveting of the majority of gas stream energy into mechanical power to drive the compressor , accessories , and the propeller.
The shaft on which the turbine is mounted drives the propeller through the propeller reduction gear system .
The C-130's T-56 engine is a prime example of a turboshaft (Dooh!) TURBOPROP engine. The T-56 is a constant speed engine, meaning that it only hase one in flight operating RPM. The power is set by the pitch of the props and the amount of fuel being consumed by the engine.
Approximately 90% of thrust comes from propeller and about only 10% comes from exhaust gas.

The turboshaft engine derives its propulsion by converting the majority of gas stream energy into mechanical power to drive the compressor , accessories , just like the turboprop engine, however,  the shaft on which the turbine is mounted drives something other than an aircraft propeller such as the rotor of a helicopter through the reduction gearbox .
The T64 engine of the H-53 is a prime example of a turboshaft engine.
The T-64engine can produce 4,380 shaft horsepower.
Depending on the location of the tail pipe a turboshaft engine can produce up to 10% of forward thrust or it can produce no forward thrust.

Quoted: Cool.  I still struggle to understand the difference between a turbojet and a turbofan though.

LOW BYPASS


HIGH BYPASS

blue is fresh air
yellow is fuel
red is exhaust

Quoted: Quoted: Cool.  I still struggle to understand the difference between a turbojet and a turbofan though. How much air is bypassed around the power section of the turbine and then back into the exhaust. It can get tricky, the FA-22's F119 turbofan for example, bypasses only a few percent more air than the old F-4 Phantoms J79, considered a classic example of a turbojet.

They are called low bypass Turbofans.  The civlians such as that RR is a high bypass design.

The C-130's T-56 engine is a prime example of a turboshaft engine.

Did you mean turboprop? (since you mention it in the par. about turboprops)

Quoted: Quoted: www.vaq34.com/junk/turbojet.jpg www.vaq34.com/junk/turbofan%20(military).jpg www.vaq34.com/junk/turbofan%20(airline).jpg blue is fresh air yellow is fuel red is exhaust So what exactly is the difference between the military and airline versions of the turbofan? The military version seems to have 2 combustion chambers, larger fans, and a cooler exhaust, is this correct? Ist it more efficient?

diff between military engines and civillian ones is mostly the amount of air bypassing the combustion chamber.

Oh, and kickass link KA3B! Thanks!

One of the best threads ever. Learned a hell of a lot in almost no time! Thanks!

Yes, corrected mistake, thanks!!

Quoted: The C-130's T-56 engine is a prime example of a turboshaft engine. Did you mean turboprop? (since you mention it in the par. about turboprops)

The pictures I posted are only examples.
They are also cross sections.
Don't take them for gospal!

There are many different types of turbine engines from the different manufacturers, and many different types of engine configurations within the same manufacturers engine family.

Yes, a high-bypass (civilian) engine is more efficent.
Again, please don't take that as gospel either, there are PLENTY of military aircraft that use high-bypass engines (KC-135R, A-10, C-17, ect.)

The J79 turbojet engine of the F-4 made about 11,000 lbs of thrust on each engine without afterburner, and about 18,500 lbs of thrust on each engine with an afterburner.

The F119 turbofan engine of the F-22 makes more than 22,000 lbs of thrust on each engine without afterburner, and "about" 35,000 lbs of thrust on each engine with an afterburner.

The TF39 turbofan engine of the C-5 makes about 43,000 lbs of thrust on each engine.

The Rolls Royce Trent 800 turbofan engine is rated at 95,000 lbs of thrust.

The GE90 turbofan engine reached 127,900 lbs. of thrust during certification testing.

Quoted: So what exactly is the difference between the military and airline versions of the turbofan? The military version seems to have 2 combustion chambers, larger fans, and a cooler exhaust, is this correct? Ist it more efficient?

Tag

Many different engines are used on 747's.
Here is one list. You could probably Google up more.

Rolls Royce - RB211-524G
Rolls Royce - RB211-524D4  
Rolls Royce - RB211-524B2
Rolls Royce-  RB211-524

General Electric - CF6-80C2B1F  
General Electric - CF6-80C2B1  
General Electric - CF6-50E2  
General Electric - CF6-50  
General Electric - CF6-45A  

Pratt & Whitney - PW4056  
Pratt & Whitney - JT9D-7R462
Pratt & Whitney - JT9D-7Q  
Pratt & Whitney - JT9D-70A  
Pratt & Whitney - JT9D-7F  
Pratt & Whitney - JT9D-7 1971

The E-4B uses the General Electric CF6-50E2 turbofan, 52,500 lbs pounds thrust.
The VC-25A (Air Force One) uses the General Electric CF6-80C2B1 turbofan, 56,000 lbs thrust.

Yes, helo turbine engines are called turboshaft engines.

Quoted: What is on 747's?    Helos use the turboshaft version, right?

Quoted: The pictures I posted are only examples. They are also cross sections. Don't take them for gospal! There are many different types of turbine engines from the different manufacturers, and many different types of engine configurations within the same manufacturers engine family. Yes, a high-bypass (civilian) engine is more efficent. Again, please don't take that as gospel either, there are PLENTY of military aircraft that use high-bypass engines (KC-135R, A-10, C-17, ect.) The J79 turbojet engine of the F-4 made about 11,000 lbs of thrust on each engine without afterburner, and about 18,500 lbs of thrust on each engine with an afterburner. The F119 turbofan engine of the F-22 makes more than 22,000 lbs of thrust on each engine without afterburner, and "about" 35,000 lbs of thrust on each engine with an afterburner. The TF39 turbofan engine of the C-5 makes about 43,000 lbs of thrust on each engine. The Rolls Royce Trent 800 turbofan engine is rated at 95,000 lbs of thrust. The GE90 turbofan engine reached 127,900 lbs. of thrust during certification testing. Quoted: So what exactly is the difference between the military and airline versions of the turbofan? The military version seems to have 2 combustion chambers, larger fans, and a cooler exhaust, is this correct? Ist it more efficient?

IIRC, the low-bypass is more efficient at high speeds, hence the military usage - the military flight envelope goes much faster than civilian airliners, so you see fighters with low-bypass fans to hit the higher speeds, and slower planes with high-bypass fans.  As you speed up, the compressor becomes more of a hindrance than an aid - the air is rushing in fast enough to support combustion without the aid of a supressor. Hence the use of ramjet designs (i.e. a turbojet without compressor or turbine) for high-speed applications.

Tagged!

That's why the F-4, F-14 and the F-15 use inlet doors, to limit the velocity of the intake air.
That's also why the F-16 and the F-18 are limited to a "low" top speed.

F-4
The intakes had originally had a fixed geometry, but it was now decided to fit movable ramps to the sides of the air intakes. These ramps could be adjusted in flight to admit the optimal airflow to the engines at various speeds and angles of attack.

F-14
The intakes are of multi-ramp wedge configuration and offer a straight path for the air entering the engines. Each intake has a pair of adjustable ramps attached to the upper part of the inner intake. Hydraulic actuators in the upper part of the intake adjust the positions of the first and second ramps in the upper surface of the inlet and of the diffuser ramp located further aft, reducing the inlet air to subsonic velocity before admitting it to the engine. A gap between the back edge of the second ramp and the leading edge of the diffuser ramp allows bleed air to escape from the inlet, passing overboard via a bleed-air door in the outer surface of the inlet. The inlet ramps are under the automatic control of a computer, which calculates the optimal position for the ramps based on engine speed, air temperature, air pressure, and angle of attack. At supersonic speeds, the hinged panels narrow down the throat area while diverting the excess airflow out of the ducts through aft-facing spill doors at the top of the intakes. At low speeds (especially during takeoff) when more engine air is needed, this airflow is reversed and extra air is sucked in via the spill doors.

F-15
The engines are fed by a pair of laterally-mounted straight, two dimensional external compression air intakes. The intakes are swept forward from bottom to top, in order to ensure that an adequate amount of air is admitted to the engines at high angles of attack. The intakes are pivoted at their lower edges and can be adjusted in flight to angles of as much as 4 degrees above or 11 degrees below the horizontal. The air intakes "nod" up or down under the control of an air data computer to keep the aperture facing directly into the airstream in order to maintain a smooth flow of air into the engines. The angle of the intakes can also be adjusted to prevent more air than necessary from being taken in to the engines. The intake surfaces have an additional function in providing extra maneuvering control, in a manner similar to the function of the canard foreplanes fitted to aircraft such as the SAAB JAS-39 Gripen. At supersonic speeds, the effectiveness of the "nodding" intakes is almost a third of that of the horizontal stabilators.

F-16
The air intake is located underneath the fuselage, at a point just below the cockpit. The ventral location of the air intake subjects it to minimal airflow disturbance over a wide range of flight conditions and aircraft maneuvers, since the forward fuselage tends to shield the intake from the full effects of aircraft maneuvers, minimizing the effects of sudden changes in the angle of attack on airflow into the engine. At an angle of attack of 25 degrees, for example, the air flows into the intake at an angle of only ten degrees with respect to the aircraft's longitudinal axis. The lower edge of the intake lip is only 38 inches above the ground, but, surprisingly, FOD problems caused by the ingestion of runway debris into the engine have been relatively minor.

The intake is of fixed geometry type, which saves on complexity, weight, and cost. A fixed-geometry boundary-layer splitter plate separates the upper lip of the intake from the lower fuselage. There is a separation strut mounted inside the intake for additional tunnel rigidity.

The Block 30 F-16 was powered by the 28,984 lb.s.t. General Electric F110-GE-100 engine. This engine is somewhat larger than the F100 and about 771 pound heavier. However, the F110 provides about 5000 pounds more thrust than the F100. For this reason, it requires a larger amount of air. This in turn required that the area of the air intake be increased to admit the extra air. However, this change was not made at first, and early F-16C/D Block 30s (Block 30A and 30B) are "small inlet" aircraft, the large inlet being made standard for F110-powered Fighting Falcons from serial number 86-0262 onward. The "large-mouth" intakes allows air mass flow to increase from 254 to 270 pounds per second. The "large- mouth" intakes can be distinguished from "small-mouth" intakes by the presence of a ECS ram air inlet duct below the fuselage which is canted slightly forward. In addition, the engine exhaust nozzle for F110-powered aircraft is slightly shorter and more round than that of the F100-powered F-16s. Because of the higher thrust, the Block 30 F-16 is a better performer than the Block 32.

The Block 32 F-16 was powered by the F100-PW-220 engine, which offered a thrust of 23,770 pounds. The F100-PW-220 was slightly less powerful than the F100-PW-200, but had a new, longer-life compressor, a more stable augmentor, and a digital engine control system which made the engine more reliable and less prone to stagnation stalling. Blocks 25 and 32 are alost identical in external appearance except for the latter's ducts for the ASPJ. Unfortunately, air intake shapes could not be standardized on the production line because the lower-thrust F100 engine could not accommodate the additional air, and the F100 powered F-16s in Block 32 retained the original smaller intake shape. A kit has been developed to bring earlier -200 engines up to a standard nearly equivalent to -220, these converted engines being designated F100-PW-220E.

F-18
The intakes are set well back underneath the LERXes, the cobra-shaped extensions protecting the engine intakes somewhat from the disruption of the airflow caused by the effects of high angle of attack flight. Since there is no requirement for the Hornet to exceed Mach 2, the aircraft does not need sophisticated variable-ramp air intakes. The two-dimensional D-shaped intakes thus have a simple, fixed splitter plate mounted next to the fuselage. The only moving parts are two ducts cut into the top of the LERX which permit bleed air to be ejected upwards into the airflow generated by the LERX. The intake ramps/boundary layer splitter plates are solid at the front end, with perforations directly ahead of the inlet to permit sluggish boundary layer air to be bled away and dumped via spill ducts on top of the LERX.

Super Hornet
A completely re-designed engine air intake of trapezoidal configuration replaces the D-shaped intakes of the earlier Hornets. These intakes will provide 18 percent more air to the uprated engines and will give better performance at high speed.

Quoted: IIRC, the low-bypass is more efficient at high speeds, hence the military usage - the military flight envelope goes much faster than civilian airliners, so you see fighters with low-bypass fans to hit the higher speeds, and slower planes with high-bypass fans.  As you speed up, the compressor becomes more of a hindrance than an aid - the air is rushing in fast enough to support combustion without the aid of a supressor. Hence the use of ramjet designs (i.e. a turbojet without compressor or turbine) for high-speed applications.

Wow, just wow. Love reading your posts, KA3B.

You want to get into STRANGE look at the SR-71!!  That thing uses the supersonic shock wave to make it behave much like a pure RAM jet.  Kelly Johnson was THE MAN IMHO!


The Allison (now Rolls Royce) 250C20C in our OH-58C is an axial / centrifigal design as is the Lycoming T53 in the Huey (some Hueys use different engines).  A Cobra is a Huey with different "hot rod" airframe.  (Narrower, mostly.)  

The Allison 250C20C flows air back into the axial stage, out via the single centrifigal stage, back to the burner can through two tubes then FORWARD to the 2 stages of N1 turbine and 2 stages of N2 turbine.  Exhaust is near the middle, just rear of the gearbox and out the top.  Combustion, N1 & N2 comprise the "hot" section.  N1 drives compressor and accessories, 100% RPM is 58000!!  N2 is a free turbine not shaft connected to the gas producer section but rather geared to the output shaft that drives the main and tail rotor transmissions at about 6500 RPM.  Govenors and fuel control are mechanical / pneumatic devices using bleed air from the compressor.  Engine can produce 425 shaft horsepower from some 30 gallons per hour of JetA yet weighs only 158 LB!!  Time Between Overhaul  (TBO) is 1800 hours on N1, 3500 on N2 & compressor.  OH cost is ~ 30-40k @ 1800, 60 -80k @ 3500.

A friend bought a Huges 500 (Allison 250C20) a couple years ago that had  belonged to a police dept.  They had been a bit careless and "hot started" it, once or more.  Rear bearing shroud was damaged along with 1st stage of N1; caused bearing to fail 3 hours after he bought it.  $30,000 surprise!!

I have worked on radials, horizontally opposed and turbines.  The turbine is very precise in manufacture but much simpler than either of the aforementioned recips.  The fuel control on the jet is highly complex and expensive but the rest of it is pretty simple.  Exotic metals, complex manufacturing processes and precise tolerances make the jet horribly expensive but for the power it generates and overall reliability it just blows away the recip!!!!


KA3B, very outstanding links and details of various engines.  Keep it up!!

Have you ever looked inside of a mechanical fuel control?
I did once.

The "sea story" goes that the guy who designed the first one was insane before he started!

Quoted: The fuel control on the jet is highly complex and expensive but the rest of it is pretty simple.

Only looked at the parts breakdown on a mechanical one.  No doubt the guy was certifiable!!!  OH on that puppy for a bellows AD was about 6 grand!!

In 68 to 75 I serviced ELECTRONIC fuel controls for "Twin Pac" generators.  Used a P&W engine like on the 707 with a free turbine added to the exhaust, making it a turboshaft.  Two of these were backed up to a BIG alternator.  All automatic in operation, un-attended.  Start sequence was by Agastat pneumatic timer / relays.  Fuel control was a drawer aboutr 2' square; I did component level repair on those.  These things were supposed to start, run up to engine govenor RPM & slowly accelerate to 3600 generator RPM (60hz) then syncronize to the line and close main breaker, load up at rate of 1 megawatt per min.  Yea, right!  Company was always in a hurry so that was defeated - breaker closed, black smoke poured and you saw around 42 MW on the meter, IIRC!!  Just about that fast too - at least until the first time they pulled the windings out of the exciter/field rotor!!  Guys who worked on the engines were from an old steam plant.  No concept of safety wire or aircraft components.  Used pipe wrenches on aluminum AN fittings.  Yep, plug fell out of the fuel system and engine bay lit off.  Fire dept took one look and said YOU put it out!!

Same plant had a Westinghouse POS turbine that had compressor, turbine and generator on one shaft.  Don't remeber RPM but think it was 3600.  Used a 750HP Waukeshaw (sp) diesel to wind it up to light the fire.  (P&W used compressed air.)  Geniuses got a "deal" on some fuel.  Turns out they bought 4 million gallons of VERY high sulfur stuff - but they got it cheap so what the hey!!  P&W's ate it without complaint but the We Sting You burned up the hot section.  Nearly lost the building when it torched out the sides!!

Quoted: Turbine engines 101. Three types of turbines. Centrifugal Flow Axial Flow Centrifugal-Axial Flow All modern high-performance turbine engines are Axial Flow engines. Centrifugal Flow engines in the modern world are limited to small auxiliary power unit or ground support type rolls. Centrifugal-Axial Flow engines are used in helo or light fixed wing aircraft, such as a King Air 200 or older Cobras. Axial Flow = airflow is straight through the engine. Centrifugal Flow = airflow is directed around the outside of the case via impellers from the core to the outer edges. Centrifugal-Axial Flow = airflow goes in a straight through path in the axial section of the engine and then follows the path as directd by the case / impellers. The Pratt & Whitney PT-6A is an example of this engine, the airflow actualy reverses it's path once it hits the centrifugal turbines. The Axial Flow engine is broken down into 4 different types: Turbojet Turbofan Turboprop Turboshaft A turbojet engine uses 100% of the inducted fresh air in the production of power. Since a high " jet " velocity is required to obtain an acceptable of thrust, the turbine is designed to extract only enough power from the hot gas stream to drive the compressor and accessories . 100% of thrust produced by a turbojet engine is derived from exhaust gas. The J-79's of the F-4 Phantom are a prime example of turbojet engines. A turbofan engine uses only a small percentage of the inducted fresh air in the production of power. The turbofan engine has an enclosed duct fan mounted at the front of the engine and driven either mechanically by the compressor section or by an power turbine located to the rear of the compressor drive turbine . The fan air can exit seperately from the primary engine air , or it can be ducted back to mix with the primary's air at the rear . The core section of the engine is what drives the compressor and accessories. The CFM-56 is a prime example of a turbofan engine. The fan section produces 80% of the total engine thrust while the core engine only produces 20% of the thrust. The turboprop engine derives its power by conveting of the majority of gas stream energy into mechanical power to drive the compressor , accessories , and the propeller. The shaft on which the turbine is mounted drives the propeller through the propeller reduction gear system . The C-130's T-56 engine is a prime example of a turboshaft (Dooh!) TURBOPROP engine. The T-56 is a constant speed engine, meaning that it only hase one in flight operating RPM. The power is set by the pitch of the props and the amount of fuel being consumed by the engine. Approximately 90% of thrust comes from propeller and about only 10% comes from exhaust gas. The turboshaft engine derives its propulsion by converting the majority of gas stream energy into mechanical power to drive the compressor , accessories , just like the turboprop engine, however,  the shaft on which the turbine is mounted drives something other than an aircraft propeller such as the rotor of a helicopter through the reduction gearbox . The T64 engine of the H-53 is a prime example of a turboshaft engine. The T-64engine can produce 4,380 shaft horsepower. Depending on the location of the tail pipe a turboshaft engine can produce up to 10% of forward thrust or it can produce no forward thrust. Quoted: Cool.  I still struggle to understand the difference between a turbojet and a turbofan though.

Great info!  Thanks for the education!

Now is it true that the turbofans are quieter for the same power output levels?

tag  (darn slow moden connection - I'll try later.)

Thanks for the link!

Yes, a turbofan can be quieter for similar power.  BIG difference in fuel consumption!!!

Guy who is in the OH58 with me owns land adjacent to GE test facility at Peebles OH.  We get a free tour and good feed as a result now and then.  Awesome!!  I used to work on the AC that served the new assembly building.  Got to see some REALLY neat stuff.

Cutaway - F119 Engine

Here's GE's version,
www.geae.com/education/engines101/

PT-6A's ROCK!!



Same smell, reverse flow centrifigal-axial flow engine, no mechanical interface / connection between the compressor section and the power section..



Quoted:
The Allison (now Rolls Royce) 250C20C in our OH-58C is an axial / centrifigal design as is the Lycoming T53 in the Huey (some Hueys use different engines).  A Cobra is a Huey with different "hot rod" airframe.  (Narrower, mostly.)  

The Allison 250C20C flows air back into the axial stage, out via the single centrifigal stage, back to the burner can through two tubes then FORWARD to the 2 stages of N1 turbine and 2 stages of N2 turbine.  Exhaust is near the middle, just rear of the gearbox and out the top.  Combustion, N1 & N2 comprise the "hot" section.  N1 drives compressor and accessories, 100% RPM is 58000!!  N2 is a free turbine not shaft connected to the gas producer section but rather geared to the output shaft that drives the main and tail rotor transmissions at about 6500 RPM.  Govenors and fuel control are mechanical / pneumatic devices using bleed air from the compressor.  Engine can produce 425 shaft horsepower from some 30 gallons per hour of JetA yet weighs only 158 LB!!  Time Between Overhaul  (TBO) is 1800 hours on N1, 3500 on N2 & compressor.  OH cost is ~ 30-40k @ 1800, 60 -80k @ 3500.

Neat! Thanks!

The compressor wheels are on a shaft.  Same for the turbine wheels.  Those shafts can be direct connected to each other or geared together, depending on engine design.  Bottom line is the turbine section mechanically drives the compressor section.  The turbine shaft also drives gears that run accessories such as generator, fuel pump, lubricating oil pumps, govenor(s) Air conditioning compressors, spray pumps, hydraulic pumps etc.

In the PT6 shown above or the Allison 250C** there are additional turbine wheels that are driven by the hot gas stream and in turn drive an independent gear train to an output shaft.  Some high bypass engines use the same scheme.  System is sometimes called gas producer / power turbine.  To vary output shaft power/speed you vary the amount of hot gas produced.  Think of a child's pinwheel out in the breeze - only the breeze is 800 degrees C!!  As that "breeze" flows through the power turbine wheels it expands, cools and gives up velocity to the blades transferring power.  In the 250C** there is a govenor on the output gear train.  That N2 govenor communicates via a compressed air signal  with the fuel control which varies fuel introduced thereby hot gas produced in order to set N2 RPM.  N2 govenor has a linear actuator that controls a lever to permit small adjustment of the governed speed.  Same lever, called "droop compensator" acts like an "anticipator" communicating a change in collective.  IOW, "Hey speed controller, be expecting a change in power requirement real soon; get a head start on it in direction indicated".  Helicopters are very picky about blade RPM so the engine control scheme is designed to provide very accurate output shaft RPM.  If the droop compensator fails to work properly, rotor RPM drops when you pull collective or push in a lot of pedal.  (On a recip. helicopter the pilot handles this by twisting the throttle unless ship has a govenor.  Some do, some don't.)


The compressor in a gas turbine requires a LOT of horsepower!!  Therefore, even at idle, fuel flow is fairly high, indicating the compressor and accessory load.  As the engine RPM increases, so does the power required to drive the compressor - and power delivered as thrust or hot gas to the power turbine.  For example, the 250C** turns 58,000 RPM at 100% output.  It IDLES at 34,800 or 60%.  I would have to look up the fuel flows at each point to be exact but something like 30 gallons per hour at 100%, 17 gallons at 60% (ground idle).  I don't have a clue on the BIG stuff KA3B  works with but I suspect it is similar.  We have to wind it up to about 18% or 10,400 RPM  before introducing fuel for a start and continue cranking to 58%.  Just before lightoff, starter draws about 300 amps at 22 volts, which is over 6KW or nearly 6 HP!  Takes less than 1 min. to achieve idle RPM.

Modern jets lower fuel consumption by several design improvements such as compressors that move more air with less HP input, better airfoil shapes on the vanes in both compressors and turbines, higher temperatures, variations in excess air/cooling air etc. etc.  Early jets were less than reliable and were fuel hogs; they smoked and required lots of maintenance.  Times change!


Edited because I am a lousy typist!

Yep, different engines use different starting methods.  Some can be started by blowing air (a LOT OF IT) into the intake to get things rolling.  Most jets are picky at start time.  SOme establish minimum airflow before introdcing fuel and starting a fire.  Others begin injecting fuel at same time as they begin to turn the wheels.

Mutiengine aircraft often can start from compressed air. They use "bleed" air from another engine that is already running to start the next one.

I suspect the RC engines start on air to save weight and money as air motors are relatively light, high speed and powerful.  While aircraft engines use only a single fuel for start and run (that I know of) the power plant engines I posted about earlier started on Natural gas then either continued on it OR switched to oil.

Interestingly, our current shortage and high prices for natuaral gas are a direct result of so many electric utilites using natural gas fired gas turbines for generation - but THAT is another thread!

I didn't know they used electic motors to get the works of them spinning 10k. (helos and airliners)

Remember, the bigger the engine, the slower it turns.  The 250C** is only about 7" diameter at the axial compressor section, so turns very fast.  An airliner engine only turns the core engine some 9000 RPM or so.  Slower engines start at lower RPMs.

The C-12 / King Air uses a "starter-generator".
It acts as a an electric starter for engine start, then switches to a DC generator once the engine is running.

Quoted: I didn't know they used electic motors to get the works of them spinning 10k.  (helos and airliners)

I am SO taggin' this one.  

The R/C Jets you inject the compressed air at the compressor and once it is spinning, you light the propane, once that is sustained, you switch over to the main fuel which is kerosene.

Ah, the "windmill system" - saves having any starter at all!

never have quite undersood the jet "stall" even though I know much physics. Is it like a prop stall but at high pressure?

Hope I don't screw this one up -  a compressor stall is a disruption of compressor air flow similar to when a wing stalls or quits flying.  Each compressor or turbine blade is really a little "wing" and behaves in a similar fashion.  Disrupt the inflowing air can cause it as well as damaged blades or stators.  A compressor stall can do massive damage - or none.  All depends.  Sounds like a gun shot or loud "bang".

FOD or Foreign Object Damage is a big deal.  Even a tiny hunk of junk can cause a LOT of damage to fragile high speed parts.  Even worse, if it breaks off a hunk of one or more of those parts, the damags keeps getting bigger and more severe as it moves through the engine, multiplying the damage.  Sometimes FOD is evidenced by compressor stall(s).  BIG engines like KA3B deals with have a big problem with it as they have little or no intake air protection like screens.  Most helicopters OTOH use a particle separator (cyclone separator type air filter) which swirls the air and prevents most of the junk from getting in.  Intake air goes in through swirl tubes then into a large plenum where the dirt falls out of the air.  3 Ejector tubes on each side of the OH58 use compressor bleed air to force the junk out.  We have had one tiny nick on 1st stage compressor blade that was easily blended out.  Compressor still requires washing every 50 hours or so to get rid of the tiny dirt that sticks to everything in it.  I pick grass out of the intakes every time we fly.

KA3B, I am curious - how often and what method to do compressor wash on big engines?  We have a built in wash nozzle in the particle separator.  I connect a -4 line to it and a Pepsi syrup tank of wash compound.  Add about 3psi air to the tank while cranking the engine.  Repeat with distilled water.  Motor until dry.  Reconnect bleed air lines, remove wood block from compressor bleed valve and start to fully dry things out.

The C-12 / King Air uses a "starter-generator". It acts as a an electric starter for engine start, then switches to a DC generator once the engine is running.

Same for the Huey, Cobra, OH6/Hughes & MD 500, Bell 206/OH58 and no doubt others.  The Huey has a second generator on the main transmission that does not act as a starter.

The OH58 has a 150 amp / 28volt generator - starter.  Heavy sucker as I find out every 50 hrs / annual when I grease the splines and inspect the brushes!!

I believe the CH-47 Chinook uses a hydraulic starter.  High pressure oil comes from a  pump on the APU (Auxuillary Power Unit).

F-15's use a jet fuel starter (a small turbine engine) to start the engine via a series of mechanical gearboxes.

F/A-22's use an APU to supply bleed air to an air turbine starter to start the engines.