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Ignorant Turbojet/turbofan Questions...

Mon Apr 04, 2005 2:43 pm

Hello everyone, your enlightening help would be appreciated in clearing up some questions I have about these two types of jet engines...
First of all, why did any engine manufacturer create low-bypass turbofans like those on the early 737's and DC9's and the like? It seems to me that once the turbofan was first used, it would have been logical to immediately go for the sort of high bypass engines that we have today, as they must be far more effecient and creat much more thrust? Also, the early jets- When did they turbofan first appear? I know that some later 707's had turbofans and I think the DC8 did as well, but did the earliest versions of these planes have turbojets and did any other larger commercial aircraft ever use turbojets? I suppose the comet did...
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RE: Ignorant Turbojet/turbofan Questions...

Mon Apr 04, 2005 8:19 pm

Hi there!!!

In the late 50s early 60s when the 707, DC-8/9 were being designed, I hardly think that reliable "high-bypass" turbofans were around to use.  Wink

The JT3D was introduced in 1959 on the 707-320B, if I remember correctly, and were the first turbofans...as opposed to the JT3C...which was a turbojet engine (again...if I am correct).  Smile

Since at that time there were no "high-bypass" fans...those and the JT8s were used to power the early jetliners...

The first real "high-bypass" fans were the P&W (or GE) for the 747, I think...and that beauty took to the air in 1969...

Noise and fuel consumption were not really big factors back then...  Wink


"That's the good thing about the 707...it can do anything, but read!" Joe Patroni, Airport '70
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RE: Ignorant Turbojet/turbofan Questions...

Mon Apr 04, 2005 9:36 pm

The first by-pass engine in commercial service was the Rolls-Royce Conway. It had a very low by-pass ratio and had the same stove pipe exhausts as the JT3C and JT4A powered airplanes.
The first high by-pass ratio engines were developed for the Lockheed C-5A by both GE and P&WA. GE was teamed with Lockheed and P&WA was teamed with Boeing.
With Lockheed winning the production contract for the GE powered C-5A, P&WA teamed with Boeing on the 747.
P&WA has a system of designating their "paper" engines, i.e. engines that are design studies only and never built. The study engine that was proposed for the USAF heavy lift plane was built and tested.
The data from this engine was the basis for the JT9D for the 747. There were a couple of early big problems with that combination; first P&WA, after making their performance guarantees, found out that there were errors in the test data from the study engine which affected their guarantees greatly; second the 747 came in much heavier than Boeing had estimated. These two items results in a badly underpowered 747 with the JT9D-1 engine.
P&WA rushed the development of the JT9D-3 engine in order to allow Boeing to keep its delivery schedule. This rushed development resulted in an engine with a lot of problems. The first major problem was that the engine would warp longitudinally on the pylon (this was not seem in the test stands due to the stiffness of the engine mounts there) at high engine power. This warping caused excessive blade clearances in one sector of the engine and heavy blade rubbing in the opposite sector. P&WA designed a wye shaped external structural member, called a yoke, that installed on the outside of the engine to prevent or reduce the warping.
All available production capacity was dedicated to building these yokes including the shops that produced parts for the various other development programs that P&WA had going on then. The program I was on was heavily impacted. We were having our own problems and between us the JT9D program, we had several nasty engine failures in the experimental test stands.
It was a busy and intense time at P&WA (1968-1970).
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RE: Ignorant Turbojet/turbofan Questions...

Tue Apr 05, 2005 12:58 am

While the RR Conway was the first turbofan in commercial service, Frank Whittle patented the concept back in the 1936 (patent 471,368). A prototype (the Power Jets LR.1) was actually built, but the project was cancelled before it could be run. He was way ahead of his time. Also, Griffith at RR designed a BPR=7 engine in the 1940s (the CR.1), but it was never built.

Its often said that RR lacked courage when they designed the Conway, as its bypass ratio is very low (.3). Gunston says that the bypass air on the Conway does little but cool the outer case.

There was a lot of engineering development required to get where we are today. The engine manufacturers couldn't just leap off the fence and get to high bypass ratio engines. They had to crawl and walk before they could run.
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RE: Ignorant Turbojet/turbofan Questions...

Tue Apr 05, 2005 2:49 am

The Conway was the first bypass jet into service, in 1960, but most people don't call it a turbofan. The JT3D entered service in March 1961 on American's 707s. Lots of fan-powered DC-8s too, but don't recall offhand when the first one entered service-- probably later in 1961, and certainly by 1962.
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RE: Ignorant Turbojet/turbofan Questions...

Tue Apr 05, 2005 2:56 am

The answer to the main question here is essentially that you have to walk before you can run.

The bigger a step you make from what has been done before (And what you have solid research regarding), the greater the design risk.
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RE: Ignorant Turbojet/turbofan Questions...

Tue Apr 05, 2005 3:18 am

I guess I just never considered the fact that it was such an egineering feat to go from the early low bypass ratio engines to those on the 747 and C5... as you can tell, I'm no aeronautical engineer! Are there any turbojet powered DC8's or 707's still in cargo or military use? I know I see lots of DC8's without the CFM high bypass engine upgrade flying into MIA for arrow air (and fine air in the past, plus some others) but I assume these don't have turbojets, just low bypass ratio fans? Thanks for everyone's input, much appreciated!
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RE: Ignorant Turbojet/turbofan Questions...

Tue Apr 05, 2005 3:53 am

I suspect part of the problem was with controlling all that air. Modern FADEC systems can all but eliminate compressor stalls and other problems associated with high bypass ratios. Other design enhancements, such as variable pitch guide vanes, probably also contributed to the increasing bypass ratios.
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RE: Ignorant Turbojet/turbofan Questions...

Tue Apr 05, 2005 6:18 am

One major limiting factor on the bypass ratio is the EGT (exhaust gas temperature).

All the power is generated in the turbines which must take all the heat from the core.

To create high bypass ratio engines the turbine technology had to be taken to entirely new levels. New and more heat resistant materials and new and sophisticated cooling systems had to be invented.

That technology was simply not available around 1950 to such an extent that the extra complexity of a separate fan stage would really pay off.

But as AeroWeanie pointed out, the concept of the fan engine is at least 20 years older than its commercial application.

In the perfect world where we have a metal which will take any heat - where we can forget about EGT, we can go and produce an engine with a 1:20 bypass ratio and with a significantly improved fuel efficiency.
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RE: Ignorant Turbojet/turbofan Questions...

Tue Apr 05, 2005 8:39 am

Take a look at this:

It gives you a idea what the metals and the heat they are subjected to.

Look at the text window as it plays through, it says things such as "Fuel burns in the Trent engine's combustion chamber at temperatures up to 2,000* C"
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RE: Ignorant Turbojet/turbofan Questions...

Tue Apr 05, 2005 7:47 pm

Quoting 777WT (Reply 9):
It gives you a idea what the metals and the heat they are subjected to.

Look at the text window as it plays through, it says things such as "Fuel burns in the Trent engine's combustion chamber at temperatures up to 2,000* C"

No metal or metal alloy can resist 2000°C. These high temperatures inside the combustor are possible only thanks to a very good cooling technology. Without cooling the combustor liner and the turbine the alloy would simply melt. Beside that it is still very impressive, what these parts of a jet engine are made to withstand!
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RE: Ignorant Turbojet/turbofan Questions...

Wed Apr 06, 2005 2:47 am

The upper temperature limit that jet fuel can theoretically reach is about 3500 degrees F or about 1950 degrees C.
Currently newer turbine engines can have a maximum turbine inlet temperature up to 2500 degrees F or about 1370 degrees C.
Usually, the engine would only see these temperatures when running at very high power on a very hot day. This would happen if the airplane is heavy, or the runway is relatively short, or the airport is at a high altitude.
If none of these conditions are present, the crew can select a reduced power setting, which reduces the turbine inlet temperature resulting in longer turbine life and lower fuel burns.
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RE: Ignorant Turbojet/turbofan Questions...

Wed Apr 06, 2005 8:55 am

I love this Thread!!!

Here's a REALLY DUMB question!!!

I was reading an Alleged A-10 Flight manual and it said something to the effect of you don't have to worry about going from full/cruise power to idle @ altitude (no shock cooling worries).

Now, I don't remember what the ceiling was, whatever whatever what ever.. Assuming it's greater then FL250, how is it possible to not worry about shock cooling? I can speculate there is a difference between Flight idle and full fuel cutoff, where at flight idle at least there is still heat being generated by the core. No, I don't think it said you could shut them off, but even going to FI seems a bit odd to me..

Are there any civilian engines that can manage this and how? The thermal dynamics blow this BADLY failed physics student away!!
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RE: Ignorant Turbojet/turbofan Questions...

Wed Apr 06, 2005 9:37 am

On any aircraft, you will go straight from cruise power to idle at the top of descent. Some pilots, mindful of extending engine life, will do this fairly slowly, but you're unlikely to do any immediate damage by closing the throttles quickly. Basically, you are generating a lot less heat but much less air is flowing through the engine so things balance themselves out. You might even see a temporary rise in internal engine temperatures because of the thermal capacity of the components. It's certainly quite common to see a rise in oil temperature, particularly if oil temperature is unregulated.

On a military aircraft like the A-10, the last thing the pilot needs to worry about is engine management niceties.
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RE: Ignorant Turbojet/turbofan Questions...

Wed Apr 06, 2005 12:47 pm


Nope, not a dumb question. Shock cooling is an air-cooled reciprocating piston engine phenomena. Liquid cooled recips are not affected, nor are jets. The reason is that when you throttle back from high power in an air-cooled engine, the heat in the cylinders drops, but the rate of cooling from air flowing around the cylinders increases (less heat from within). So the possibilty of scuffing the the cylinders from cold cylinders/hot pistons rears.

On liquid cooled engines, the thermostat and the decrease in water pump speed negate these problems. On turbines, you worry about tip-to-shroud clearance. But the turbine, relieved of hellish heat of the power setting, contracts first, the case (shroud) later.

As always, you people in physics (or just damned smart!) feel free to correct me.

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RE: Ignorant Turbojet/turbofan Questions...

Sun Apr 10, 2005 5:48 pm

To add to the discussion on how quickly to throttle back from cruise power to flight idle, it is always better (from a compressor surge margin view-point) to decelerate slowly, (as one would in auto-throttle operation). A faster deceleration from high power to idle can result in lower compressor transient stall margins and can cause a top-of-descent surge, especially in deteriorated, higher-time engines. This compressor working line;s transient excursion toward the surge line is more and more pronounced as the by-pass ratio increases. Hence you can decelerate a military jet engine faster without surging than you would want in a high-bypass engine.The process has little to do with heat-soaking especially when we speak of high-bypass jet-engines.
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RE: Ignorant Turbojet/turbofan Questions...

Sun Apr 17, 2005 6:29 am

Lots of good stuff here. Thanks! The term "shock cooling" has been used around the patch of aviation I'm currently working. Good to hear it's a bit of a modern myth. At least as far as the PW4060 is concerned!
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RE: Ignorant Turbojet/turbofan Questions...

Sun Apr 17, 2005 4:14 pm

Shock cooling is definately not a modern myth with regards to your typical IO-360 or TSIO-540/550. You have to be careful with them, especially if it's a turbo.
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RE: Ignorant Turbojet/turbofan Questions...

Sun Apr 17, 2005 7:50 pm

Understood. I remember while flying in (not as pilot, then) the venerable DC3 how delicate one had to be with power reductions. We nearly paid the ultimate price one night in 1983 when, after a 5 hour night flight to Resolute Bay in February, the left engine exploded as the pilots reduced power to descend for the approach. The next twenty minutes were "interesting" and we were all very lucky to live through it.
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RE: Ignorant Turbojet/turbofan Questions...

Wed Apr 20, 2005 1:53 pm

Also another thing that hasn't been mentioned that you would have noticed in the Rolls Royce video is that as the air is compressed going into the engine, it builds up heat to the point it is already at around 200 degrees just from going through the compression stages, and then that 200 degree air passes into the combustion chamber, so that will keep the engine warm.


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RE: Ignorant Turbojet/turbofan Questions...

Fri May 20, 2005 11:42 pm

At the moment, I believe bypass ratios are something around 9:1. Is there any prospect for that changing?

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