monty
Senior Member
Currently being spanked by mother nature.......
Joined: September 2018
Posts: 400
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Post by monty on Aug 16, 2022 7:35:35 GMT -5
Thats it :-) But something is troubling me :-( If a jet kart has a high mounted engine , lets say 3 feet above the axle and the wheelbase is 6 feet and we have say 100 lbs of thrust , then there'll be an extra 50 lbs of downforce on the front wheels . That 50 lbs must come from somewhere , LOL, it must be tied up in Sine Cos and Tan somewhere , is there a "resultant??" , do we actually lose motive thrust . I've tried looking at aircraft "thrust lines" for inspiration , but its still got me perplexed . Cheers John John,
No loss of thrust, that 50 lbs is just whats needed to keep things in equilibrium. It might mean slightly increased tire drag, but otherwise no loss of thrust. An airplane is a bit different, that moment will need to be reacted out by the tail, which adds drag. Usually the engine is positioned so the thrust line goes through the center of gravity of the aircraft. That way there are no pitch changes with thrust. On a cart, the high thrust line could cause a problem if you lift off the throttle at high speed. All of a sudden the steering may go away or the front lift off. Keeping the engine as low as possible, and the thrust line through the CG of the cart will minimize all these issues.
Monty
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Post by racket on Aug 16, 2022 17:02:28 GMT -5
Hi Monty
Thats a relief :-)
Thanks for that
Cheers John
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Post by racket on Aug 18, 2022 23:20:38 GMT -5
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Post by racket on Aug 18, 2022 23:48:00 GMT -5
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Post by racket on Aug 18, 2022 23:54:41 GMT -5
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Post by racket on Aug 19, 2022 0:58:24 GMT -5
Hi Guys
Well that was a waste of time , only a couple of pounds difference in thrust , well within the scope of "error" :-(
My turb exducer is running choked , so the steam simply displaces gases , ..............same/same mass flow basicaly
Methanol injection might help as it evaporates faster , but again , my exducer is choked.
Thrust at 275 lbs was down considerably as I only took the engine to 3 Bar to pamper the comp wheel , that extra 0.5 Bar in earlier tests added a lot more mass flow and jetpipe pressure .
Need to do more digesting of the data , TOTs vs P2 vs thrust .
LOL....More bits to hang on the wall :-)
Cheers John
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dieselguy86
Veteran Member
Joined: September 2014
Posts: 186
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Post by dieselguy86 on Aug 19, 2022 11:51:57 GMT -5
Hi John,
I noticed nothing really changed with the water, but the gauges all looked really solid compared to the first runs. You definitely got it sorted out, and it's still my favorite sounding startup.
Your making it really tempting to do something with this gt62 rotor!
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Post by racket on Aug 19, 2022 16:28:57 GMT -5
Hi Joe
LOL.......you won't be disappointed with that big wheel , lotsa flow area ;-)
Yep , she roars into life beautifully these days , no dramas , nice and fast ................I could possibly think about cutting back the idle speed a bit , but I like the spoolup as it is
Cheers John
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Post by racket on Sept 6, 2022 4:23:43 GMT -5
Hi Guys Took her out for a testrun with just a "dry" 118 mm jet nozzle to gather some T2 data , the extra sensors confirm poor comp eficiency :-( youtu.be/m5PGPlo4w8UCheers John
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Post by racket on Sept 6, 2022 5:15:16 GMT -5
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Post by racket on Sept 6, 2022 5:15:53 GMT -5
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Post by racket on Sept 9, 2022 1:32:46 GMT -5
Hi Guys
Engine disassembly progressed OK , outer can came off and flametube and turb wheel looked "cool" , no overtemp running , comp removal was standard ,.................BUT , on checking the comp bore :-(
It hadn't stretched as much as previous wheel , but it had only been run up to a 4:1 PR rather than the earlier 4.5 :1 , just a thou or so increase , enough for the 1/2" bore tester to drop in and rattle a tad .
Looks like its only a low pressure wheel and probably accounts for my poor comp efficiencies as well .
Time to checkout the KTS catalogue
Cheers John
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dieselguy86
Veteran Member
Joined: September 2014
Posts: 186
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Post by dieselguy86 on Sept 9, 2022 21:56:11 GMT -5
John,
What did you get for efficiencies? I calculated ~76% at 1 bar, and I think by 2 or 3 bar it was 73ish I think. What efficiency was you aiming for?
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Post by racket on Sept 9, 2022 23:32:54 GMT -5
Hi Joe
It was 73% at 1 Bar , 72% at 2 Bar and only 70% at 3 Bar , it should be the high side of 75% for such a large comp with a vaned diffuser
Cheers John
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Post by finiteparts on Sept 10, 2022 20:10:09 GMT -5
John,
That is a bummer. Are you thinking the reduction in compressor efficiency is due to slipping of the impeller on the shaft? I guess you would see some tell tale marks on the bore of the impeller and shaft, but if that did occur, you could be getting more bore heating due to the friction of the slip and that would cause the bore to be weaker than expected.
Another thought for how we can take techniques from the past...you could drill a small hole in the diffuser housing just downstream of the impeller discharge, then squirt some lamp black in oil through it when you get to a good steady state point, so it leaves surface flow lines on the compressor diffuser entry. That way you could get an idea of how the diffuser entry is working at that condition. Just a thought.
Hopefully, you can find a good replacement for it in the KTS catalogue. Good luck!
- Chris
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