"Fat Boy" 12/118 thrust engine

[racket]

Hi Smithy

The "big gear" that I fitted to the motor is the original intermediate gear in the starter's gearbox , it normally runs on roller bearings so has a decent sized ground bore , ideal for the pressed in steel flanged bush I fitted , the flange on the bush ( ~1mm X 1mm) was then TIG welded to the gear's side .

In the bush's bore , which is a firm sliding fit over the 9 tooth motor armature gear, I milled 3 semicircular groves into which I slipped 3 X 10 mm long lengths of 1/8" brass welding rod which projected into the space between every third gear tooth on the pinion , the whole assembly is then held in place by a 6mm screw and washer screwed into the end of the motors rotor , I needed to drill and tap the rotor.

The output shaft is a similar armature with all the bits removed , its a tad under 12mm diameter , but where the "steel bits" were pressed into the armature its been "grooved" and the projection is a tad over 12mm so needed machining down a tad to fit through the 12 mm bored bearing on the end of the gearbox snout , the snout section of the gearbox is something I had cast up a long time ago when I was thinking of making a gearbox for a large RC brushless outrunner motor I purchased to start the 10/98 engine with .

So far everything has gone a bit too easy , LOL....... I'm waiting for something to go wrong .

Cheers
John

[smithy1]

Ah...I thought it was something you had laying around in the shed...a good use of "local" bits..

Just give it a bit of time...something will go wrong eventually..! ;-)

Cheers,
Smithy.

[racket]

Hi Smithy

Starter works



With 70 psi oil pressure and 25 Volts it takes ~4 seconds to get from zero to 9,100 rpm , this is a couple of seconds slower than the direct drive , but understandable considering the ~3:1 speedup gearbox.

The starter is now pulling lotsa Amps , both motor and leads were hotish to the touch after a 10 second test run .

A second test was conducted and the same rpm achieved , but at only 9,100 output rpm actual armature rpm was only ~3,150 , now with the normal ~4.5:1 overall reduction gearing for this starter that would only be achieving an output rpm of ~ 700 rpm , this is way below its normal output , so hopefully once I start adding some fuel the rpm will increase into the optimal range .

All looking good so far

Cheers
John

[smithy1]

Maybe your starter is getting a little tired?? Brushes/commutator a tad poorly maybe??

Yep...she should come good with a bit of fire in her belly, The 6041 is similar, with the blower I can only get ~6,000rpm with the full 70psi oil pressure...but as soon as the fire is lit she comes to life and accelerates pretty quickly....I like to keep tabs on the spoolup speed and temps using fine adjustments of the fuel tap.... she seems to be happiest with a nice quick-ish warm spool-up to idle, maxing out at ~700-750c during spool-up is my preference.

You'll be fine as long as you keep your good eye on the gauges, adjusting as you go..!

Cheers,
Smithy.

[racket]

Hi Smithy

Yep , the motor could be getting tired ,it was only a $60 one from the wreckers .

With your RC engines , what sort of relationship is there between their "static" rpm ( nil fuel) vs when the fuel is being added at spoolup as they are disengaged ??

I'm currently making mount for the hand held tach so that I can keep an eye on rpm as I feed in the fuel as I don't have any idea what its requirements will be like to produce a reliable idle .

Thankfully I'm a little more prepared for the next spoolup attempt than I was last time .

Cheers
John

[smithy1]

Apr 10, 2016 23:13:21 GMT -5 racket said:

Hi Smithy

Yep , the motor could be getting tired ,it was only a $60 one from the wreckers .

With your RC engines , what sort of relationship is there between their "static" rpm ( nil fuel) vs when the fuel is being added at spoolup as they are disengaged ??

I'm currently making mount for the hand held tach so that I can keep an eye on rpm as I feed in the fuel as I don't have any idea what its requirements will be like to produce a reliable idle .

Thankfully I'm a little more prepared for the next spoolup attempt than I was last time .

Cheers
John

The engine in my little dragster is actually "lit" at ~5000rpm...it then has electric help to ~20,000 where the starter dis-engages...it then ramps up to idle at 33,000rpm. She'll actually self sustain reasonably well at ~18k, only (~14-15%) but struggles to accelerate quickly away from that. I have the max rpm set to 125krpm.

Cheers,
Smithy.

[racket]

Hi Smithy

Have you ever tested the dragster starter without any fuel to the engine to see what sort of engine rpm it can produce on its own ??

Cheers
John

[smithy1]

Apr 11, 2016 3:43:27 GMT -5 racket said:

Hi Smithy

Have you ever tested the dragster starter without any fuel to the engine to see what sort of engine rpm it can produce on its own ??

Cheers
John

The dragster starter is "ECU" controlled, so it only does what the ECU program tells it, the fuel pump essentially "chases" the starter as it ramps up ...the starter is common to most of the engines made by Kingtech...my new tiny K45G happily spins to 30k on the starter before it disengages during spoolup..I'll be bringing it with me for you to peruse....it's 170k max rpm is pretty impressive too..! I re-balanced it last week as it had a very slight "buzz" at some RPM settings, it's now as smooth as a baby's bum...! I think the starter is just a basic 7.2v brushed motor. Some of the newer European RC jet engines are using brushless starters these days....still ECU controlled via an esc of course.

Cheers,
Smithy.

[racket]

Hi Smithy

Thanks for the info :-)

I'm trying to get an idea of what sort of "power to airflow" requirement there is for the starter .............LOL, theres something in my head thats trying to be reconciled , I think I might just have to do a spoolup instead .

Cheers
John

[smithy1]

Yeah...I think you'll just have to "suck it and see"...Pretty sure if you can get 9k+ out of it static....she should get over the hump ok with the fire going..! I think you'll be quite surprised at how low the "hump" actually is....I'm guessing it's gunna be close to or around 20k.

What you set your idle rpm/P2 to is a different thing all together.

Cheers,
Smithy.

[racket]

Yeh , hopefully ~20K , I needed ~22K with the 10/98 engine to get over the hump , this larger dia wheel should knock some rpm off .

Idling setting is gunna be a big unknown , if the components aren't in a good match it might need to be a tad high to keep things spinning around .............yep , "suck it and see" , ..........I'll follow your good advice , maybe this arvo , just tidying up a few things on the test stand ;-)

Cheers
John

[smithy1]

Take your time and take it easy John....you don't have to answer to anyone but yourself..!

The 6041 is a "big easy" engine, your very generous flame tube helps greatly with it's "low rpm" abilities.

Cheers,
Smithy.

[racket]

Hi Smithy

youtu.be/nlwsiHk6lRk

Unsuccessful again :-(

21,500 rpm , 900-1,000 C TOT , nearly there , but rpm slowly decayed once the starter removed.

Video is only ~20 seconds long as the camera battery went flat .................some days it doesn't pay to get outa bed .

One thing that I can improve on is the earth cable back from the starter , it melted its coating , need a bigger cable to handle more amps .............maybe go to 36 volts as well.

I feel the comp efficiency at low rpm is just too poor , requiring more horsepower from the "undersized" turb wheel than it can supply , maybe with more rpm things will come into balance ................LOL, dream on John ;-)

Cheers
John

[Johansson]

Bugger! It must be very near now, isn´t there anything that can be done to lower the air load on the compressor during starting like blanking off a section of the inducer inlet?

You were at least dressed for the occation John.

Keep the spirits high, I know you will figure it out.

Cheers!
/Anders

[racket]

Hi Anders

LOL, I have thought of maybe fitting a sleeve into the inlet , but might leave it a tad longer before getting that desperate .

It was very close , there were several seconds of running that I thought were idling , but then rpm slowly decayed and flames appeared out of the turbine wheel and I had to shut her down .

I'm still not sure the starter motor is functioning to its best as that ~21,000 rpm didn't change even when more fuel was added , I'd have expected rpm to have gone higher, a bit more static testing required I feel .

Yep , I'm feeling a tad deflated after the test , but not ready yet to turn the engine into a coffee table ;-)

Cheers
John