Building the shrecky turbine

Zippiot · Post by **Zippiot** » Wed Jan 17, 2007 6:34 am

Finally got my shiny new paperback copy of Gas Turbine Engines for Model Aircraft, and will immediately start construction.
Prettymuch gonna play this one by the book but had a question.

The compressor is made of wood, could I replace the wood with acrylic or polycarb? I have an endless supply of acrylic in all thicknesses and such, polycarb I have to pay for but its pretty cheap for the amount that would be needed; polycarb is much stronger but flexes more and that prolly isnt good for a high-revving turbine...

Acrylic and acrylic cement makes an incredibly strong bond, the stuff seems far superior to a wooden comrpessor and mills/casts much easier (casting wood is a challenge). The stuff is also more uniform than wood.
Will I be able to substitute acrylic for wood in making the compressor?

I also recently completed the first stage compressor and stator for an axial flow (got to play with the lathe and mill for a day), has anyone incorporated a 1 stage axial w/stator into a centrifugal flow turbine? Might add a little compression.

Zippiot · Post by **Zippiot** » Fri Jan 19, 2007 9:31 pm

After a few tests I have decided that carving out a solid chunk of acrylic was the strongest method, here is my first (nearly complete) impellor.

I plan to make a mold of the finished one and make a few metal castings of it.

(sorry for the blurry pics still trying to figure out this camera)

Johansson · Post by **Johansson** » Fri Jan 19, 2007 10:08 pm

Nice! Are you making it on a mill or by hand?

How much air does the compressor flow according to the book, and at what pressure?

//Anders

Zippiot · Post by **Zippiot** » Fri Jan 19, 2007 10:18 pm

I am making it by hand, with a router...kinda like milling but not quite as accurate...

The one in the book makes 1.4:1 compression, this one should make more but still it wont pass 2:1 b/c its pretty basic. Also this one is much much smaller, hopefully that will make it less unstable at running speeds.
I am mixing a high flow with a high pressure design to hopefully get a decent impellor, with the radial compressors it seems to be either one extreme or the other.

I am now finished and it is all ballanced, and I made another test one to play around with its strength. It is quite sturdy, not as impact resistant as polycarb but way stiffer and stiffness is what I am looking for.

Later today I hope to be done with the stator aswell, that part is much easier as it is plain cut and weld no ballancing no milling!

This is just a test engine, if it sustains than its a success! I did make an axial flow a long time ago, and maybe another day the knowledge gained from this project will help me build another.

Johansson · Post by **Johansson** » Fri Jan 19, 2007 10:36 pm

Anyone correct me if I am wrong, but I believe that a greatly retro-curved compressor like the Schreckling has less need for a diffuser than a turbocharger compressor since most of the diffusion is done inside the wheel rather than in the diffuser section.

Anyway, I like the way you are doing this project. No months of calculating and top dollar materials, just trying to build it to find out if it works. That is what I find appealing about pulsejets by the way, I can wack one together in an afternoon and have it screaming by midnight.

Now you are going to prove that it can be done with turbines too, interesting!

//Anders

Zippiot · Post by **Zippiot** » Fri Jan 19, 2007 10:54 pm

Well there are a bunch of calcs and formulas in the book, but they are just to find the specific specs of you engine. Thrust and weight are the ones most ppl look at and those dont require any calcs to find, also fuel consumption is easy to do with a bottles that has the ml marked.

The shrecky does not require much of a diffuser b/c the impellor is contained (more like covered on both sides) and the area between the blades is increased as it moves through the wheel. All the diffuser is in the shrecky is a bunch of blades welded to a washer...possibly its biggest function is the straighten the flow of air as it enters the CC. It does help slow the airflow to, change in area from the blades to the CC.

There is a bunch of trial and error in these engines too, if the compression/airflow is too high the engine runs too hot, which shortens its life (but gives more thrust). If stuff is not quite right it might not sustain at all...
Usually (in my opinion) it is better to get longevity than thrust, even though with the way the book outlines making turbines it would be easy to make one that lasts for 5 hours at an increase of thrust by 25% compared to one that lasts 10 hours and makes less thrust...

The way the book shows how to make turbine could easily apply to making the compressor of an axial, I will definitely test this in the furutre!!!!!

Zippiot · Post by **Zippiot** » Fri Jan 19, 2007 10:57 pm

Oh look at a pic of a compressor from a turbocharger to one that shrecky designed (should be able to find pics on the int, if not I'll scan one and post it) and look at the tips of the blades.

This is all guessing, but it looks like the turbo is designed to increase pressure a bit but speed up the air mostly (geared towards flow and not pressure like I said earlier). So the diffuser must convert this high speed medium pressure air to 50m/sec air at the highest possible pressure.

Zippiot · Post by **Zippiot** » Fri Jan 19, 2007 11:03 pm

here is a good way to visualize it.
Air enters at the bottom and as it spins is forced upwards (outwards really but upwards in the pic)
the black stuff around it is a bad attempt to make a cover for the compressor

Zippiot · Post by **Zippiot** » Fri Jan 19, 2007 11:13 pm

Ignore that pos picture, here is a better one!

Notice how in the high pressure compressor the backing is much thicker to cope with the high force exterted on it by the large blades at the tip of the comrpessor.

I am building a neutral one just as a test model, will prolly make 1 of each and swap them out to see what works best!

Johansson · Post by **Johansson** » Fri Jan 19, 2007 11:26 pm

How are you going to fit the front cover on the comp wheel?

//Anders

Zippiot · Post by **Zippiot** » Fri Jan 19, 2007 11:32 pm

Good question :)

I dont think this one needs it, but if it does I will cut a piece of acrylic to size and use acrylic cement to secure it in place. Acrylic cement melts the plastic and reforms it in palce, nearly as strong as the cell cast stuff about the same strength joint as a good weld (compared to the strength of the plastic and metal).

The cover supposedly helps by adding another surface for the air to be comrpessed by (pushed against as it is spun outwards) but many jets dont have one, a close gap by the intake works well enough.

You convinced me to make a cover.....

Zippiot · Post by **Zippiot** » Fri Jan 19, 2007 11:52 pm

Little trick I developed :)

There is a different between true and ballanced in a wheel (compressor turbine etc...) and once the wheel has been made true it is time to balance it. This acrylic is very uniform so little ballance was needed, the way the book say to ballance it is to attatch to the shaft and spool it up using an airsource, then feel (holding with your hands is sensitive enough) if it is ballanced. My little trick was made b/c the shaft and bearings are not made yet.

Bolt the thing up and stick that bolt in a hand drill. You can tell instantly whether or not it is ballanced, and it is very easy to fix once you find the heavy part (add glue or grind a small portion off). If you need to add weight to a metal turbine a small tack weld at the shaft can help.

Zippiot · Post by **Zippiot** » Sat Jan 20, 2007 2:38 am

Well I finished the compressor only to realize that the shaft is smaller than the hole that was drilled :/

So I drilled out the hole and filled it with carbon fiber composite. The stuff, when dry, is stronger in all ways than acrylic, so I will drill a new hole through this and make it the proper diameter this time (damn metric to imperial conversions!!!).

This set me back an hour or so but I still plan on tackling the diffuser tonight.

On a side note, always wear gloves... The router shot the compressor out of my hand, the blades of the compressor tore up my left hand and the router bit my right pretty hard...

Zippiot · Post by **Zippiot** » Sat Jan 20, 2007 4:21 am

My lathe cant hold a piece of metal the size that I need for the turbine...so I'm kind of stumped...

I might be able to use a washer, if I could onyl find one with a small enough hole in the middle.

Any ideas where I can get a nicely rounded ballanced circle of steel?

Zippiot · Post by **Zippiot** » Sat Jan 20, 2007 4:45 am

Ok I am just dumb...the book outlines how to do it with a hacksaw!!!

I can manage a hacksaw, its jsut gonna take 8 or 9 hours!!!!!!!!!!!!!!!!!!