Do you realy need stators?
 

Hi,

I started a project to build my own smallscale Highbypass turbofan. I will be using CNC to craft the core out of steel. A thing that would simplyfy the process is if i could skip the stators.

I know the stators are used to direct flow inside the core, the question is however how much i would loose in performence if i skip these.

What do you think, would this make the engine completly useless or would it be "Ok" anyways?

The "setup" would be a larger fan in front 6 compressors and 2 HP turbines connected on a single shaft with bearings on two connectionpoints to support it.


Cheers.

I doubt you could get the engine to run at all without stator vanes - it is the combination of rotating compressor blades and fixed stators that produce the compression. Getting a miniature axial flow engine to actually run will be an extremely difficult task - the small engines I have seen have all had centrifugal compressors.

Ok thanks for the reply, I know the stators are very important i did not however know that they were so important that the engine would not function without them.

As I understand it, the stators convert the kinetic energy of the gases leaving the previous stage into higher pressure at the expense of lower speed.

AFAIK stators "straighten" the flow and prevent the air from simply going around in circles. The design, convergent/divergent, also enables kinetic energy to be transformed into pressure energy.

Good luck in your build, the tip losses as a percentage will be, I strongly suspect, simply horrendous.

Thats about it - one compressor stage is regarded as being comprised of a rotating assembly plus a stator assembly. Can I suggest you obtain a copy of the Rolls - Royce publication ' The Jet Engine' - the section on compressors will give you a better idea of how it all works - the paragraph on small engines will be illuminating.

Yes that is correct the lower speed is good this means the pressure will be higher just as the law of Bernoulli's state.

I guess i will just have to figure out a way to get some statorvanes into damn thing :).

In principle it's a simple construction.

Will probebly also have a look at that book The Jet Engine thanks for the suggestion!

If you do not want stators, you can use counter rotating blisks.

Also, CNC machining of a small blisk will not work, at acceptable rotating mass and flow characteristics, because every usable material would deform at the blades.

There is a way though, its called Electric Discharge Machining (EDM).

http://www.bladonjets.com/technology/gas-turbines/ probably builds the nicest small gas turbines. They use EDM to to machine their blisks. http://www.bladonjets.com/technology/blisk/

there are several model aircraft jet-engines on the market...a search on "youtube" will find a man in a wing-suit with these engines mounted on his boots. he launched from a drop-sircraft after "lighting up" and achieved climbing/horizontal flight whilst his fuel supply lasted.

Yves Rossi, the cross-channel "birdman" also uses these engines.

Apart from the engineering challenge, I think you are trying to re-invent the wheel.

Be advised that you will probably need some kind of airflow control system-bleed valves or variable stators- to get your engine to start,run and accelerate as axial flow engines have a comparatively small RPM operating range. Please keep us informed as to progress - you have my admiration for even attempting such a project and I wish you all the best.

Steve - RPetersson is trying to build an axial flow engine - the small turbines that are generally available including the so called Birdmans power units are centrifugal flow engines. Axial flow engines in small sizes generally do not work very well.

Quote from rpetersson:
"In principle it's a simple construction."

Please forgive a slightly frivolous comment from a non-engineer, and I realise it's an axial-flow engine you are attempting to build, but no doubt Frank Whittle would have said something similar when he was trying to obtain funding to build his first engine (not sure how his German equivalent would have phrased it)...

Lycka till!

PS: What's the melting temperature of steel, and what materials might you be using for the cans and the turbines?

The only way to retain a rotating mass inside a sleeve, (case) is with bearings connected radially to the case. Why not mount stator vanes on these required "webs"? A Twofer...

:ok:

bcgallacher - The melting temperature of steel is around 1400 degrees celcious. The only problem with steel in a jet engine would be the weight, tho this is not an issue if just keep it on my workbench :)

Aiming for a combustion tempereature of around 900 degrees, the bypass air from the front-fan will allow for some cooling on the exterior of the core also.

Lyman - Yes a web inside the core to house the bearings is one option i have looked into. But then again a web would cause alot of turbulence. So im trying to find a more streamlike design.


The goal with this project is just to get a self-sustaining HB Turbofan running. Any thrust it generates is a pure bonus.

Have a look at a Dart 7 engine. Basically an external chamber is the stator.

Garret APU's have something similar as well.

What is your reasoning to not have stators? Size, manufacturing problems,or are you trying a different design altogether.

Gas turbine engines all need a stage of compression.

mainwheel - My reasoning for not having stators is simply to make it easier to construct. I understand the need for compression. The question is if it would get enough compression without the stators to be self sustaining. There must atleast be some compression without the stators when the air goes thru multiple "compressors" into the divergent part where i will have the combustion.

I will probebly as an experiment try to build it and try to get it running without the stators first, record the result and then add the stators and record again to see what/how much improvment i got.

This sounds really interesting - good luck with it and let us know how you get on. If you're making compressor discs then presumably the stator discs would be fairly easy to make too, if you have to?

Did you ever see - I think it was a French guy - made a working scale model of a racing Ferrari - complete with fully working flat 12 engine! Absolutely amazing. The engine poppet valves were about the size of matchsticks, and it sounded like a real Ferrari when it was going.

Incredible!

You will get little if any compression without stators - a compression stage is rotor+stator, All you will do is stir the air up somewhat and I would think that all the rotating blades apart from the first row will be operating in a stalled condition. As a matter of interest how will you calculate compressor blade profiles,blade twist,chord and height? Do you know what compressor stall is? To be honest I think you are operating well outside your knowledge of axial flow compressors or you really would not have posed the question that you have.

@uplinker...IIRC he was a postman....now a full-time Engineer building models which sell for the appropriate price and he has a waiting-list so can afford to take commissions that interest him Even made the moulds for his own exact scale tyres!


@ R Petersson...Sorry, my apologies...you are indeed taking on a real challenge.

I would work on the principle that if it wasn't cost-effective, the full-size builders would not put them in! ;)

As the size goes down, the scale of your losses and juggling clearances/thermal distortion and expansion/balancing....just grow!

Good luck with this enterprise and I hope you keep us posted.

Pratt & Whitney PW1000G | A320 NEO - Airbus A320 New Engine Option

Im i blind or why can't i see any stator-vanes in the PW-1000 picture linked above?

Yes - it can be that i do not have sufficent knoweledge or correct tools to make a good jet engine this however does not matter so much for me. It will be a fun project anyway.

The only way to get rid of the stators would be to use alternate counterrotating rotors....
Unfortunately that would require the same number of shafts as rotors, but as the cross section gets smaller with each stage, there would be enough space for a shaft in shaft in shaft in shaft .... design.
This would allow for a suprisingly short housing.
(and probably for a surprisingly short life as well)

Good luck, and stay away from the path of the high speed debris.

As I recall, had a set of plans to build a simple ‘jet’ engine using an old car turbocharger. Here is a link to one of MANY on youtube.
No stators.

It's a Cut-a-way

They jusy just cut-a-way the stators in the drawing

amazing what one can do in this new fangled digital cad-cam age

For a pure tubojet with statorless counter rotating adjacent rotors I wonder how they will react out the postitive thrust component to push the plane

Pratt & Whitney PW1000G | A320 NEO - Airbus A320 New Engine Option

N1 is geared, contra LPT. The other three sections all contra rotate, in trail.

A conventional propeller may be thought of a single-stage compressor rotor sans stator. If you were to conduct a tuft test, of a smoke-stream test, you'd find the slipstream is a vortex about the propeller-shaft axis. This vortex represents energy (static pressure) losses that can readily be recovered with either a stator assembly, or with a contra-rotating prop system.

But worse, if you tried to put a second prop in series, behind the first prop and turning the same direction, it would have difficulty achieving a similar pressure rise, because of the poor match (low alpha) between the airflow vector and the second prop blade angle. This is the situation you face in a statorless axial compressor.

Don't forget that steel may not melt before it reaches 1400 degrees C, but most ordinary alloys will be pretty soft and oxidize rather quickly at 900 degrees.

The PW1000.

No stators shown. In this cutaway.

Imagine the engine as it is, and then somehow turn it over to around 5000 PM, the add ignition and fuel.....whats going to happen?

Fuel will be added to an ambient pressure airflow and burn a bit like a blow torch, for a while.

After the starting source is gone, the engine will not produce the airflow to turn the turbine..efficiently enough to self sustain. It will wind down to zero RPM.

The air has to be compressed.

PW1000

Like a stuck record - its a cutaway. If you look closely you can see the gaps where the stator stage or blisk would sit

Dog Driver - car turbo chargers use centrifugal compressors (and centripetal turbines ) They do not use stator vanes - the equivalent on this type of compressor is diffuser vanes. On the exit from the rotating section they guide the flow through divergent passages slowing the airflow and increasing the pressure.

Volume - I am at a loss as to how you would drive contra-rotating stages on two concentric shafts - I am not saying it cannot be done it is just that I cannot work out how to do it with more than one stage.

I am a retired line maintenance engineer -qualified on RR,GE and P+W big fan engines and this thread has made me get out the books and go back to first principles,trying to bring back stuff that I was taught 30 - 40 years ago and have enjoyed it very much -keeping the brain active! I hope our friend can overcome all the difficulties and get his engine to run and keep us informed as to progress.If he runs into problems I am sure there is enough knowledge both practical and theoretical on Pprune to advise him. Once again I advise him to obtain a copy of the Rolls-Royce publication 'The Jet Engine' there is no better book available that I know of that can give such an insight into the construction and principles of operation of gas turbine powerplants.

Best of luck rpetersson :ok:

CNC machines were way out of our reach when I was having ideas like that. Rather than re-invent the wheel why don't you try doing something that has never done before (everything's been done before) with building a jet engine.

Here are some ideas, and I'm not promissing they are any good.
1) Forward swept axial flow compressor blades. Look cool too.
2) Outer rim air bearing. To prevent tip path losses the blade tip is connected solidly or cast to an outer rim which has tiny holes in it to let air pass through from centripital forces and form a boundary layer of air to act as a lubricant.

A friend of mine (ex aircraft engine maintenance engineer) who is now in the auto turbocharger reconditioning business told me a story about a buch of visiting scientists who in a laboratory were 'growing' turbine blades one crystal layer at a time and were looking for a machine that could balance their spining fan at 200,000 rpm. Incredible stuff.

Have fun.

Single crystal turbine blades have been in use for at least 20 years in production gas turbine engines

bcgallacher

Just for ducks. Stators are used to keep the gaspath in an axial flow turbojet.....erm.....axial.

Without, the gaspath develops the familiar vortex. With that in mind, this vortex is bound by the engine's case, and can be described as a "helix". Which is simply a vortex with cylindrical shape, bound.

What purpose can the energy thus acquired be harnessd to do. What design change relative to the geometry of the gaspath would be required to hybridize this engine as a partial "Centrifugal/Axial" engine.

There are axial/centrifugal flow engines around - the old Bristol Proteus was one such and I think some of the smaller French engines.The axial stages are in front of a final centrifugal stage - about the only advantage I can see is that the engine can be built shorter. Centrifugal compressors are very robust - I worked for about 13 years with RR Dart engines and never saw one pulled for compressor damage but have seen many axial flow engines with sometimes extreme damage from bird ingestion. FOD and even damage due to surge and stall. Centrifugal compressors work over a larger rpm range although they run at higher rpm -up to 15000rpm on RR Dart and a whopping 42000 rpm on the Garrett TPE 331. They do not need airflow control devices such as bleed valves or VGVs. An axial flow engine without stators would not work - you need both to get compression.

Thats the second time in this thread I've seen "high" RPM mentioned. Actually when it comes to aero it's more a case of sonic velocity boundaries (tip dia being as important as RPM).

Not to forget the ubiquitous Pratt & Whitney JT15D, which is also an axial / centrifugal design.

Animation
 

The GEnx Theatre

GE aviation web site. Click on compressor and it will show an animation of an axial compressor turning, stator vanes included.

rpetersson. The best of luck with your project. Stay safe.