reedy
Veteran Member
Joined: January 2017
Posts: 111
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Post by reedy on Jan 15, 2017 10:20:48 GMT -5
Hi Jeff Glad to here that,I made a lot of scrap early on with jet building, Kamps book does say to leave a expansion gap but they found it does give you problems,when I welded mine i always stress relieved it with a blow touch then m/c out the turbine housing minimum clean up and then m/c the hole out slide fit the bearing tube,if it does distort with the first few runs on gas i just locally grind with a dremel grinder trying to keep the clearance as tight as I can. back tracking a bit through your posts the fit of the bearing needs to be a slide fit on the shaft and in the bearing tube but the compressor wheel should be a shrink fit using heat from a hair dryer the turbine just needs to be a tight slide fit make sure you use grease or something like copper ease,tolerance in Kurts book is 8mm -0.002 -0.005 for the shaft and 22mm +0.01 for the housing. If you don,t get the fit right on the compressor wheel and it ends up a slack slide fit you well never get it balanced,the compressor end is always the hardest to balance as it overhangs from the bearing and you have to take it off to fit it in the engine.
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Post by jetjeff on Jan 15, 2017 17:03:46 GMT -5
Thanks Chris,
Your input is much appreciated.
Regards
Jeff
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reedy
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Joined: January 2017
Posts: 111
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Post by reedy on Jan 16, 2017 5:35:22 GMT -5
some photo,s of my turbine wheel ,shaping and grinding of the wheel makes the most improvement to temp,s of the turbine. The round block is a clamping aid when twisting the blades and helps when grinding out the root stops me going to deep.these where cut at a 40 degree angle which automatically gives you the leading edge chamfer and thins the trialing edge,also widens the cord of the blades now 11.4mm compared to 10.4 in the book. it also means the root is more open.the trailing edge is important too thick gives you drag same as an aircraft wing.the photos are an unfinished wheel roughed out ,still needs final shaping with files. Hope the photos come out Ok, it's the first time i've done this.
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reedy
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Joined: January 2017
Posts: 111
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Post by reedy on Jan 16, 2017 5:39:26 GMT -5
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Post by jetjeff on Jan 16, 2017 17:33:31 GMT -5
Hi Chris,
Mmmm,,,the pictures help. I've watched some of your YouTube vids of your FD3/64 builds. They seem to run just fine.
I've ground the blades on Heineken II down to 1/32" inch thick,,,the thinnest turbine blades I've made so far. I also moved the compressor diffusor blades out a bit.
Regards
Jeff
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reedy
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Joined: January 2017
Posts: 111
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Post by reedy on Jan 16, 2017 21:15:26 GMT -5
I built 4 fd3/64 engines with a friend about 20 years ago just starting to remember the problems and errors with the book ,i never built the original combustor it was redesigned by Mark Drake who produced the Hobbyjet 631 but John Wright of the Wren team did he managed to get it working by blocking off the path through the NGV and bearing tube (think you have already done this). and the inner cone and NGV by slitting it and opening it up (see page 14 diagram of turbo jet) all the air needs to go throught the blades of the NGV and turbine .the only cooling air you need travels down the bearing tube and the slits allow a small flow to cool the roots of the turbine blades. You picked up on this in one of your posts. Chris
will tell you about the compressor error tomorrow.
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Post by jetjeff on Jan 17, 2017 3:59:11 GMT -5
Hi Chris, Nice to have somebody on this forum with Schreckling experience. Pics of my turbine after thinning the blades further. I'm also leaning towards trying another 100% carbon compressor. I've cut two base disks out, and may try to slant the blades on one, nothing radical just 20 degrees or so (or 70 degrees from the disk). I'm not sure if anyone tried forward slant blades on a Schreckling compressor. Regards Jeff
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Post by jetjeff on Jan 17, 2017 4:12:45 GMT -5
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Post by jetjeff on Jan 17, 2017 4:24:41 GMT -5
The completed NGV assembly.
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reedy
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Joined: January 2017
Posts: 111
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Post by reedy on Jan 17, 2017 5:14:09 GMT -5
Hi Jeff The twisted turbine wheel can be made to work well but the twist ends up being less at the root and more at the tip so work is not even as tip travels faster than root,try untwisting the tip by 2 -4 degrees ,I use shaped pliers with a curved jaw ,makes a big difference, I was a aerospace toolmaker for 40 years ,i would not make a carbon compressor wheel too much tooling! but you could try building it the way Kurt say's using laminations of plywood and carbon cloth say 1/8 plywood,the main weakness is the base centre with the slots, got to go will tell you more later Chris
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Post by jetjeff on Jan 17, 2017 5:43:13 GMT -5
Hi Chris,
I'm sure it was the cover plate that failed on my 1st carbon compressor, it was only .050" thick. It failed at 3lbs of case pressure, not sure of the RPM. Oddly enough the base disk was undamaged.
Regards
Jeff
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lofi
Member
Joined: October 2014
Posts: 47
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Post by lofi on Jan 17, 2017 6:24:09 GMT -5
Hi Jeff
Beautiful work, and you're sailing through it!
Regarding your carbon compressor, I was wondering if maybe you'd hit a resonance issue. That super light weight, very stiff structure that the cover plate and blades form seems like an ideal candidate for high frequency resonance.. Or certainly higher than the more supple, heavier ply version. Maybe you hit a specific RPM when it failed? No idea how you'd model or test it, just a hunch and thought I'd throw it in for discussion. It might go some way to explaining why the base was left unscathed.
Cheers
Ian
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reedy
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Joined: January 2017
Posts: 111
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Post by reedy on Jan 17, 2017 9:47:14 GMT -5
Back Again a full carbon compressor will be heavy,did you wrap carbon tows around the edge,that's a must do with the slots in the base only thing holding it together is the carbon tow banding on the edge.using full carbon for the base just increase's the weight and stress on the banding,if you use a thin carbon base then the leverage of the blades will twist them over break off and destroy the blades and top cover.if you plan out the birch plywood and carbon cloth laminate you can wrap carbon tows into the plywood,I have a Mark Drake alloy compressor wheel that use's 14 blades 7 full length and 7 half length at the edge this would increase the centre strength 4 less slots to the centre.the blades could be laminations of 0.8 ply cloth 0.8 ply (on your x1.4)preformed to shape with the outer grain parallel to the axis of the engine use carbon cloth with 2 part epoxy laminating resin,you would need to test all this with a sample to make sure epoxy bond to cloth and plywood was good. pre made carbon is too smooth to glue to ,pre preg will not have enough resin in to bond to the wood.carbon to carbon you will need to use peel ply so you can glue it later. when cutting the blade slots make the disc big enough to drill holes on the outer edge of each blade and use a jigsaw upside down and cut the blade slots from the outside to the centre try using a blade for tile cutting should cope with the carbon and may give you the right size slot. for the standard size wheel the compressor wheel gap at the edge is 7.2 mm the text say's use a 6.5 mm spacer this should be 6.2mm with 0.5 to come off edge face of the disc making 7.2mm finish this is one of the area's that Kurt say's can cause a problem too big of a gap can cause swirl back into the wheel more blades in this area will help (14 blades) especially with a bigger wheel (x1.4 ) you can also increase the diameter of the flange on the bush (T2.2). All above is untried just my ideas feel free to jump on me as in my job i did not work with carbon much,saw plenty of it and filed and drilled it but never laminated or vacuum baged it.most military jet aircraft skins are carbon now ,we called it the black death don't get splinters in your fingers if you do make sure you get them out !
Chris
p.s if the jigsaw will not do the curve grind the back of the jigsaw blade.
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Post by jetjeff on Jan 17, 2017 16:10:46 GMT -5
Hi Ian,
Thanks,,,you may be on to something with the resonance issue. I don't believe the turbine was that well balanced causing a shaft vibration that did the compressor in. That's the reason why I originally wanted to go with a Delrin compressor diffusor, as Delrin absorbs vibration much better that aluminum.
Hi Chris,
No, I did not wrap the ends with carbon tows. I left a ring of carbon about 1/4" around the perimeter thinking it would equate to the carbon tow wraps. My father owned a tool and die shop (small) decades ago, and I've inherited all his tooling. The curved slots on the base disk are cut on a rotary turntable on my Bridgeport. You may think the spindle speed (2720 max RPM) isn't fast enough to cut through carbon (cleanly) with a .040" end mill but it does. I plan to double the thickness of the top cover plate to around .100",,,should be plenty strong.
I know the Schreckling design is dated and most people build newer designs, but the Schreckling design is SO forgiving of manufacturing defects that I've decided to focus on this type of turbine. BTW, I'm not a machinist by trade, I'm an IT guy.
Regards
Jeff
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lofi
Member
Joined: October 2014
Posts: 47
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Post by lofi on Jan 17, 2017 16:47:21 GMT -5
Hi Jeff
Hehe - also an IT guy! I did find someone building a small engine (may even have been a shreckling) who had adapted a magnetic prop balancer - reckoned it was accurate enough to balance the shaft! I'll try and rake out the page if it's still around. Keep up the great work, anyway.
Cheers
Ian
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