RAF Typhoon training flights grounded due to ash deposits,,,

[thatone1967]

Just seen that pop up on the BBC news website, not sure if damage found or not... I guess we will find out in due course.....

The Ministry of Defence said that RAF training flights on Typhoons based at RAF Coningsby in Lincolnshire have been suspended, after checks on one aircraft found ash deposits in one of its engines

Edited by thatone1967 on Thursday 22 April 15:01

[Riggernut]

http://news.bbc.co.uk/1/hi/uk/8637978.stm

[Riggernut]

No they are turbofans, They run at a higher pressure ratio then civilian turbofans.

[dilbert]

But they are turbojets, and not turbofans? No?

[Taffer]

Riggernut said:

No they are turbofans, They run at a higher pressure ratio then civilian turbofans.

Also a much lower bypass ratio, so a greater proportion of the airflow is getting combusted. Cold bypass air with ash in it will still have abrasive properties, but not as problematic as heated ash which will melt and then start to collect on turbine blades.

[Riggernut]

Taffer said:

Riggernut said:

No they are turbofans, They run at a higher pressure ratio then civilian turbofans.

Also a much lower bypass ratio, so a greater proportion of the airflow is getting combusted. Cold bypass air with ash in it will still have abrasive properties, but not as problematic as heated ash which will melt and then start to collect on turbine blades.

As you alluded to, greater combustible airflow, greater thrust, increases the risk for damage. A large proportion of bypass air on civilian jets is used to quieten the engine for noise regulations and fuel economy.

[Eric Mc]

dilbert said:

But they are turbojets, and not turbofans? No?

No, they ARE turbofans - but with a much lower bypass ratio than an airliner type turbofan engine.

[navier_stokes]

Taffer said:

Riggernut said:

No they are turbofans, They run at a higher pressure ratio then civilian turbofans.

Also a much lower bypass ratio, so a greater proportion of the airflow is getting combusted. Cold bypass air with ash in it will still have abrasive properties, but not as problematic as heated ash which will melt and then start to collect on turbine blades.

Yes, but you get substantially more air (and therefore ash) through a civil core compared to military, even though the bypass is higher also.

[dilbert]

navier_stokes said:

Taffer said:

Riggernut said:

No they are turbofans, They run at a higher pressure ratio then civilian turbofans.

Also a much lower bypass ratio, so a greater proportion of the airflow is getting combusted. Cold bypass air with ash in it will still have abrasive properties, but not as problematic as heated ash which will melt and then start to collect on turbine blades.

Yes, but you get substantially more air (and therefore ash) through a civil core compared to military, even though the bypass is higher also.

But does it matter if the turbine is cooler?

[navier_stokes]

dilbert said:

navier_stokes said:

Taffer said:

Riggernut said:

No they are turbofans, They run at a higher pressure ratio then civilian turbofans.

Also a much lower bypass ratio, so a greater proportion of the airflow is getting combusted. Cold bypass air with ash in it will still have abrasive properties, but not as problematic as heated ash which will melt and then start to collect on turbine blades.

Yes, but you get substantially more air (and therefore ash) through a civil core compared to military, even though the bypass is higher also.

But does it matter if the turbine is cooler?

Military run hotter but I'm sure they're both easily hot enough to cause problems. It's not just the heat, it's the turbine blade cooling that will get choked up. In addition, the compressor rotors will be getting a bit of a drumming and they obviously run cooler, but dictate the operability/stability of the whole engine. I.e. rapid deterioration of your compressor blades means you're more likely to surge.

[Riggernut]

Actually looking in to it my first statement was slightly outdated, GE90 is producing a pressure ratio of 45:1 compared to the EJ200's 26:1, however the bypass ratio is a whopping 9:1 on the GE compared with 0.4:1 on the EJ200.

[dilbert]

Eric Mc said:

dilbert said:

But they are turbojets, and not turbofans? No?

No, they ARE turbofans - but with a much lower bypass ratio than an airliner type turbofan engine.

Yep. Understood. I think the site was having problems. That post of mine should have been ordered differently.

[911newbie]

Hmmm, I begin to wonder what the difference really is between these 4th gen turbofan engines with v low bypass ratios and turbojets.
How much exactly of the thrust is being generated by the front fan vs the jet action ? On modern civilian turbofans it's about 75% but with a bypass ratio of 0.4 one has to wonder if its much mre than a few percent.

Does anyone know how they have managed to get over the fact that turbofans don't like supersonic air intake ? Or is this why teh front fans are relatively small ?

[dilbert]

911newbie said:

Hmmm, I begin to wonder what the difference really is between these 4th gen turbofan engines with v low bypass ratios and turbojets.
How much exactly of the thrust is being generated by the front fan vs the jet action ? On modern civilian turbofans it's about 75% but with a bypass ratio of 0.4 one has to wonder if its much mre than a few percent.

Does anyone know how they have managed to get over the fact that turbofans don't like supersonic air intake ? Or is this why teh front fans are relatively small ?

I'm no expert, but it sounds more like "bleed air". Certainly I am aware of a couple of aerodynamic uses for this. The other thing that strikes me as odd is the compatibility of a full on turbofan, and reheat, which the Typhoon is certainly capable of.

I think there is some ambiguity, but I think that might be intentional.

I'm pretty sure that turbojets don't like supersonic air intake either.

Edited by dilbert on Thursday 22 April 20:56

[zippy500]

They are not true turbofans as such. It is a very small bypass used mainly for cooling and fresh air systems. Modern military engines are running close to melting temperature at the core and without this tiny amount of cool air to flow over the blade surfaces, blades and combustion chamber would melt. The only way to get the power required these days is to run hotter and hotter, which needs cooling. The power comes wholly from the combusted mixture, which is dry or reheated on the EJ200.
They dont like supersonic air as you say. Intake profile slows air down. As in concorde intakes, but a different principle.

Edited by zippy500 on Thursday 22 April 21:11

Edited by zippy500 on Thursday 22 April 21:18

Edited by zippy500 on Thursday 22 April 21:19

[Riggernut]

The answer to your questions are in here.
http://www.turbokart.com/about_ej200.htm
Traditionally the design of the intake and the ability to vary the shape allows air speed to remain subsonic. Recently engine design as shown on the website above allows LP Turbofans to operate efficiently at supersonic speeds.

[zippy500]

Forgot to add the cooling holes in blades, nozzle guide vanes etc are less than 0.5mm in some cases in diameter, so volcano dust being basically silica, will melt, and form a glass over the holes, causing overheating and melting.

[RDE]

navier_stokes said:

Military run hotter

As I understood it, efficiency was directly linked to turbine entry temperature, which in modern engines was as high as it could be with current materials (~1700K). How do military engines run hotter? Or have I only learned about idealistic engines that no one actually uses?

[zippy500]

I can only comment for military as that is what I have worked on. But I would say as they need to get their power from purely hot gas as opposed to byassed air, like an airliner. They need to be run that way to get the desired power levels. Hotter the gas, the more thrust out the back which is what the aim of it is at the end of the day.

Also an airliner, once level and on it's way to NY, is running at a sensible throttle position.
Not constantly on full power, which is not fuel effective. This also keeps core temperature lower as a by product.

For military the flight envelope involves a varied selection i.e reheat for take off and climb, level a bit, full power to attack etc. So like your car engine, if you were cruising at constant speed, it would be cooler than if you were tracking it for an hour.

[911newbie]

My understanding is that the newer civilian engines are also running at gas temperatures above the alloy's melt temperature and using intake air to wrap the blades in cooler gas.

So am I right in thinking that the only difference between a turbojet and a turbofan is the presence / absence of bypass flow ??