"Fat Boy" 12/118 thrust engine

[racket]

Hi Smithy

Things are looking promising ................I did a rough truing up of the commutator and put the starter back together , old brushes , bearings etc etc , and now have rpm on the output shaft of ~36,000 rpm , an ~90% increase :-)

A new starter with good bearings, brushes , commutator etc should do even better.

It took several seconds for my roughly reconditioned commutator to "run in " to the old brushes, with rpm rising from an initial 25,000 until it reached the 36,000, with plenty of "sparks" obvious thru the drain hole near the brushes.

At 36,000 rpm the armature is doing ~12,500 rpm , this is pretty fast but still below the ~20,000 rpm I have seen for an unloaded starter shaft , so hopefully a new starter will produce the goods.

I'm encouraged :-)

Andrew is parceling up some new bits that he has and is sending them up from Melbourne , I'll give them a test next week, in the mean time I'll make up a heavier duty earth strap and see what else can be improved on the starter , thankfully the starter was still fairly vibration free at 36,000.

Cheers
John

[stoffe64]

Good work there John, dont give up that interesting project, with those new bits it Will be perfect,another option might be such brushless motor that both Andy Morris and Anders Johansson use...
Cheers /stephan

[racket]

Hi Stephan

Thanks for the suggestion , yeh , I know that motor well , I purchased a comparable one a couple of years ago, its been used by Andrew on the 10/98 engine here in Oz , though with some controller issues .

Having made this current gearbox I won't be concerned about making another one for a large brushless outrunner if needed .

Cheers
John

[stoffe64]

No they have plenty of rpm in Them!
Cheers /stephan

[racket]

Hi Stephan

The INrunners have lots of RPM, the OUTrunners generally run a lot slower , and its the outrunners that mostly have the biggest power outputs, they don't seem to make an ideal inrunner for my potential needs.

Cheers
John

[stoffe64]

Apr 14, 2016 16:40:01 GMT -5 racket said:

Hi Stephan

The INrunners have lots of RPM, the OUTrunners generally run a lot slower , and its the outrunners that mostly have the biggest power outputs, they don't seem to make an ideal inrunner for my potential needs.

Cheers
John

Sky rc Toronto beast
Sparka for that John
30000 rpm

[stoffe64]

Toro beast it should be

[racket]

Hi stephan

Andrew has been blowing up 200 amp speed controllers so they've made their own 400 amp job .

The Toro stuff is the same as the unit I purchased , just a different name..............our engine start requirements are a lot harder than a 1/5th scale buggy unfortunately :-(

Cheers
John

[racket]

Hi Guys

A bit of an update .

Andrew ( Jettoymaker) sent me some starters that he'd been playing around with on the 10/98 engine and I decided to do some modifications to the starter gearbox to accommodate the larger starter he sent me.

It appears my old starter was only 1 Kw on 12 volts , the bigger new one is hopefully the 1.4 Kw unit.

I did some comparison testing of the two motors by getting them to spin up the engine under full 70psi oil pressure with ~20 deg C oil temp .

The old starter on 12.8 Volts produced 6,200 rpm , on 25.3 Volts 10,000 rpm

The new bigger unit on 12.7 Volts gave 7,700 rpm , on 25.2 Volts 11,750 rpm and with nil oil pressure 13,000 rpm .

Now at first glance this wasn't much improvement in rpm from the bigger unit , but power required by the compressor goes up as the square of the tip speed.

With a "medium" exducer diameter of ~157 mm giving a circumference of ~1.618 feet I did some calcs .

The old starters 6,200 rpm= 167 ft/sec , 167 squared = 27,890

10,000 rpm = 270 ft/sec, 270 squared = 72,900 , or ~2.6 times more tip speed "energy" required

The new bigger starters 7,700 rpm = 208 ft/sec , 208 squared = 43,265

11,750 rpm = 317 ft/sec , 317 squared = 100,490 , , or 2.3 times more tip speed "energy" required

Now if we compare the 12 Volt figures of the two starters things come into focus .

27,890 vs 43,265 represents a 1.55 times more energy being supplied .

On 24 Volts .......72,900 vs 100,490 represents a 1.38 times increase by the larger unit , thats just about spot on the 40% increase expected between a 1 Kw motor vs a 1.4 Kw motor allowing for some variability due to lube drag and possible air flow variations at the different rpm .

Hopefully the new starter will get the engine over the hump to self sustain , thats the next test

Cheers
John

[racket]

Ooops forgot the pic

[smithy1]

Hmmm....1.4kw is still only ~1.9hp....the 6041 uses nearly all of ~3hp supplied by the blower....but it isn't direct drive and I suspect a lot of the power is lost beating the air to death....you should be close to getting over the hump John.... Fingers crossed..!

Cheers,
Smithy.

[racket]

Hi Smithy

LOL, 1.4 Kw on 12 Volts , so lets double that for my 24 Volts , so getting close to 4 HP , I'm hopefully of at least getting plenty of rpm this time , if not , it looks like another battery and try 36 volts for ~5.5 HP :-)

But then , maybe the engine isn't capable of running with the components I'm using ............ah , the unknowns .

Cheers
John

[racket]

Hi Smithy

Getting close youtu.be/02kK7DKk-1Q .

Starter applied until ~30,000 rpm , needed more fuel I think , it was just on the cusp of continuing to run , but then died away .

At idle the comp will be sucking ~45 HP from the turbine , so the 10% that the starter is providing could easily be replaced by a tad more fuel I feel .

The usual stuffups during the start , starter not fully engaged , then there was no oxygen in the FT for the next try for ignition with the LPG , but eventually away she went , needing extra fuel at ~25,000 rpm to get up to 30,000.

Maybe next time :-)

Cheers
John

[madpatty]

Hi Racket,

Its getting so interesting that its getting very hard waiting for the next try.

All the best for the next run.

Cheers,
Patty

[racket]

Thanks Patty

I get the feeling its almost there , I might make up a diffuser cone exhaust to try and turn some of that high speed gas energy into static pressure to increase the pressure ratio across the turbine stage .

The diffuser will be discarded once I workout the start technique and get some running data .

Cheers
John