What are the effects of using a straight shaft in a homebuilt axial turbojet?

[tc_kid]

hi,
ive started drawing up plans for a small axail turbojet and i was wondering(for financial reasons) what the negative effects vs positive effects would be of having a straight cylindrical rod-like shaft rather than the usual shaft shape which is somewhat representative of a series of cones?...

and would it run very effeciently? its probably going to end up on a gocart or nothing at all so it doesn't need to have much thrust, just so long as it works.

thanks.

 

[FredGarvin]

A series of cones? You've lost me on that part.

By "axial" I am assuming that you mean an axial compressor and turbine.

To be honest, the only advantage I see to this is the ease in manufacture. There are a ton of down sides to doing this.

Down sides:
- Difficult to assemble: With all of the components having the same ID, many components require a press fit on the shaft due to operating loads. It is very difficult to press on a component and have to move it over a long length of shaft. It will mar the shaft.

- Impossible to balance: You leave no stock for dynamic balancing. You need to adjust the mass distribution around the shaft at various planes to ensure a vibration free operation.

- You limit the size of bearings you can use by setting one diameter.

- Very difficult to stretch: In most applications, again due to dynamic loads, you need to stretch the shaft to ensure tight clamp groups. The shaft stretch ensures that groups of components on the shaft stay in contact during operations. With a constant diameter shaft, there is nothing to "force" the shaft to stretch in the particular locations you want it to. You would also have a very hard time ensuring that the stretch is equivalent across the entire length.

- Stress distributions: There are different stresses induced in a shaft at different times. You may be lacking in cross section for certain areas.

 

[tc_kid]

series of cones is just to describe the appearance of the shaft, i guess its pretty vague but i couldn't think of any other way to describe it.
i was actually hoping i could thread the shaft where the blades will be and secure them with knuts and washers but then comes vibrations and warping if there is even a slight displacement of the cg of the shaft cross section not to mention how aerodynamically inefficient this would make the engine.

i guess I am just going to need to build it the usual way then... oh well, too bad for my wallet...

 

[FredGarvin]

It wouldn't necessarily make the engine inefficient just by using that set up. The use of a nut and threaded components on high speed rotating hardware means that you have to have very tight tolerances on the threads themselves as well as on the clamping nuts. The parallelism and flatness is also very much controlled because if the pitch diameter or one of the mating faces is off by just a bit, it will boss around whatever it is clamping and then you will have vibration and run issues.

When you get into things operating at these speeds, it doesn't take more than a thousandths of an inch discrepancy to cause you issues.

Do you know what the operating speed range of the engine will be (standard day)?

 

[tc_kid]

as of now i have no idea what my rpm will be as this engine is still only in the early design stages and i haven't done any testing on prototypes or components.
perhaps i should scan what I've drawn up so far and post it here?

 

[FredGarvin]

Sure you can post it.

 

[tc_kid]

ok... here's the first general diagram, i would show all of them, i got diagrams for each part but with my laggy com and a scanner that scans the pictures into huge 87mb files(changed the format to upload) it took me an hour and a half to get this onto my com.
this is just a general overview.

 

[Danger]

Your explanation was still a bit vague, TC. I assumed from you first post that you wanted to make all of your turbine and compressor wheels the same diameter, rather than tapering the airflow. If that's correct, then you will be losing efficiency.

 

[tc_kid]

ah, yes! it was a bit vague but you understood it perfectly! i do understand there will be a great loss in efficiency but i suppose what I am asking is will it still provide a sufficient amount of thrust to get a small vehicle such as a gokart moving at a decent speed?
i was also thinking of possibly putting it on a model aircraft with about a 2m wingspan and maybe a gross weight of 4 to 10 kg anychance it would provide enough thrust for that?

my main objective is just to build a turbojet engine which works using only parts i designed myself.(efficiency is not such a big deal here)

secondary objective is to use the engine on a vehicle(now efficiency maters a bit more)

the problem i have with a tapered shaft is that to have it made and have the blades fitted for it will cost way more than i can affort...

 

[FredGarvin]

ah, yes! it was a bit vague but you understood it perfectly! i do understand there will be a great loss in efficiency but i suppose what I am asking is will it still provide a sufficient amount of thrust to get a small vehicle such as a gokart moving at a decent speed?
i was also thinking of possibly putting it on a model aircraft with about a 2m wingspan and maybe a gross weight of 4 to 10 kg anychance it would provide enough thrust for that?

my main objective is just to build a turbojet engine which works using only parts i designed myself.(efficiency is not such a big deal here)

secondary objective is to use the engine on a vehicle(now efficiency maters a bit more)

the problem i have with a tapered shaft is that to have it made and have the blades fitted for it will cost way more than i can affort...

There is no way to answer any of these questions. If you are designing these from scratch, I challenge you to even get this thing made let alone get it running.

I got from your original post that you wanted to keep the shaft a constant diameter. Now it sounds like you are saying that you want the rotor disks to be equal in OD? Which one is it?

If you are simply talking about keeping the OD of the rotors equal that has no effect on anything. What you would have to do is to taper the inner flowpath to make sure the flow area decreases as you progress through the stages of compression.

...it will cost way more than i can affort(sic)...

Welcome to the world of jet propulsion. Nothing is ever even close to cheap.

 

[tc_kid]

here, this diagram should explain what i mean better.

i will test all those things out once i build it, i just want to be sure before i have the parts made that the loss in efficiency won't cause the engine to stop or not start at all. If the loss in efficiency will cause these problems then i will go back to the tapered shaft

 

[Danger]

I guess that I didn't understand after all. I thought that you meant...
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[tc_kid]

I guess that I didn't understand after all. I thought that you meant...
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ok, i guess you did not get the exact design but you still got the concept, both your diagrams and mine demonstrate the same compression effects even though obtained by tapering completely different parts... but yes, your first diagram is exactly what i was thinking of making

 

[Plastic Photon]

I think the conical sections help compress the inflowing air and create the expansion chamber for fuel mixture and then compress the mixture for ignition and exhaust. So do you think you are to have anything missing with your altered design?

 

[tc_kid]

I think the conical sections help compress the inflowing air and create the expansion chamber for fuel mixture and then compress the mixture for ignition and exhaust. So do you think you are to have anything missing with your altered design?

yes that's what they do... well i just want to get a turbojet that spins and gets the air in the front and out the back, not looking for much power or effeciency and yes, altho that would be better, it really costs too much for my budjet.

 

[Astronuc]

The tapering of the compressor section would be based on the specific volume of each stage. As air (gas) is compressed, it occupies less volume! So, either one reduces the radial (lateral) dimension(s), or one decreases the axial dimension. For a purely straight section, each successive stage would have to have decreasing length (width).

These maybe helpful.

http://en.wikipedia.org/wiki/Jet_engine_performance

http://www.grc.nasa.gov/WWW/K-12/airplane/nozzleh.html

 

[FredGarvin]

Whoever did that Wiki article did a nice job. It's a bit tedious, but they got a lot of information down there.

 

[jaap de vries]

Yes the volume has to go down either the shaft has to diverge or the casing has to converge.
Why not use a old turbocharger from a car dumpsite. That way you have the compressor and turbine on a single shaft with nice bearings. ussually small turbine engines don't use axial compresors (APU's Turboshafts) because the need for a very high compression ratio isn't there. Also you need some flame holding capability (that is more my field) the way you designed your combustor now will not hold the flame! You need either a dump plane to create recirculation or separate v-shape flame holders.
Have you thought about your igniters yet?

 

[tc_kid]

Yes the volume has to go down either the shaft has to diverge or the casing has to converge.
Why not use a old turbocharger from a car dumpsite. That way you have the compressor and turbine on a single shaft with nice bearings. ussually small turbine engines don't use axial compresors (APU's Turboshafts) because the need for a very high compression ratio isn't there.

my reason for not using a turbocharger is because i wanted to have every part drawn out and put together on my own, more satisfaction that way... also, i want to keep the engine as light as possible and as straight as possible incase i decide to put it on a rc plane. a turbocharger turbojet is way too heavy and it has a very bulky exterior combustion chamber. i suppose i really don't have much of a choise but to taper either the shaft or engine case. any idea which would be cheaper?
hey, while on the topic of the turbo charger engine(incase i need one for a future project) what is the maximum possible thrust i could get out of one of those things?

 

[FredGarvin]

If I may ask, how, exactly, do you plan on making this from scratch?

 

[Astronuc]

If I may ask, how, exactly, do you plan on making this from scratch?

Yeah - turbomachinery requires high precision work with little margin for defects/flaws. The higher the rpm, the greater the requirement for symmetry in mass distribution and minimization of flaw size.

The most critical components are the turbine blades, hub and rotor, then the compressor.

 

[kam213]

Axial is just the name of the compressor type
the other type is centrifugal.
axial flow only works with the cone shapes and is much more complicated,
you can try to design from scratch but it would be a lot easier to modify a car turbocharger.
The spinning parts need to be very precision build because of the very high RPM.

 

[ronnierong]

Im building this I am eager to know if u build ur engine
http://upload.wikimedia.org/wikipedia/commons/2/22/Turbojet_operation-_centrifugal_flow_(ro).png