"Fat Boy" 12/118 thrust engine

[racket]

Hi Guys

Boredom has set in from all this self isolating so decided to refit the 107mm dia jetnozzle to the engine and measure actual thrust vs the theoretical "number" calculated from jetpipe pressures and temps in conjunction with the nozzle area .

I've got the teststand back on the car trailer and will fit an anchoring fitting to the trailer for the 200 lb "butchers shop" spring balance to hook up to , the spring balance gave a fairly accurate measure of my weight when compared to the electronic bathroom scales , within 1% , so close enough for some rough thrust measureing, the test stand will be free to roll forward on its wheels whilst gently tethered to prevent wandering.


Theres plenty of time to finish the job as I don't want to incur a $1,000 fine for leaving home without a valid reason, the Govt is talking of easing restrictions in the next few weeks as we seem to be getting new cases of the Virus under control , only 20 or so yesterday across the whole country with total cases to date under 7,000 with only ~70 deaths , not too bad for 25 million of us , hopefully we've dodged the bullet that has afflicted so many countries.

A couple of pics




Cheers
John

[enginewhisperer]

Let me know if you want a hand for the test run

[racket]

Hi Guys

Getting closer :-)






Cheers
John

[racket]

Hi Guys

Test run this morning

youtu.be/sG7hYvoCLwM

some more gauges

youtu.be/JzlBh3Mu5ZI

Now to digest the results



Rolling resistance of test stand ~5 lbs which needs to be added to thrust measurement

Cheers
John

[pitciblackscotland]

Hi John,
another nice run Looks like there was a different video camera being used?

Cheers,
Mark.

[racket]

Hi Mark

Yep , Linda did the P2 and spring balance with her camera , I did the crappy one with my old one , I really should get a better unit for videoing .

There was a bit of a lengthened spoolup , don't know what cause that , maybe the cold morning , but she was hummin' at full throttle :-)

Cheers
John

[enginewhisperer]

throttle response is pretty nice!

[racket]

Hi Andrew

LOL............not much "turbo lag" :-)

Cheers
John

[racket]

Hi Guys

I’m currently fitting some flow straighteners into the jetpipe along with a central tube to fair the hub .

The jet nozzle sizing is giving me headaches as the calculations resulting from the thrust levels when taken in conjunction with the P4t and TOT aren’t making sense :-(

I think I’ve got the same scenario I had 20 years ago with the TV84 before I clipped its turbine wheel , the TV84 turb exducer was running choked and I had the same thrust levels pretty much irrespective of jet nozzle sizing as the “expansion” and thrust production was taking place at the exducer outlet rather than downstream at the jetnozzle because the jet nozzle was oversized and not controlling the expansion.

I’m going for a 10% reduction in nozzle area compared to the previous 107mm nozzle , the calcs are indicating a mass flow of only ~3.25-3.3 lbs/sec rather than my 3.6 lbs/sec ,~10% reduction, the exducer simply can’t flow it despite the fact that I'm feeding the gases into the turbine wheel with as little pressure/density drop as is possible , then having max velocity increase in the wheel to produce reaction power rather than inlet impulse power , its a juggling act .

The fact that the exducer is choked means my temps are increasing rapidly once past ~3:1 PR to try and provide sufficient gas deflection from the higher sonic velocity the temp increase produces, but the hotter gases are less dense so mass flow is getting reduced , ........................I’m on the wrong side of the curve. :-(

Hopefully the changes will produce a bit more thrust , firstly from the flow straighteners feeding the gases into the jet nozzle in a more axial direction , theres possibly ~20 degrees of swirl in the gases exiting the choked turb wheel exducer and secondly from a shortened jet nozzle with less surface area in contact with high velocity gases which invariably produces losses, I only need ~15 lbs more thrust to achieve my original 206 lbs I had calculated when I started the build .

Even if I don’t get any extra thrust , the data collected will help explain whats happening and provide max flow data for the G Trim turb wheel .

Cheers
John

[enginewhisperer]

how long till you try the old turbine wheel?

[slittlewing]

Hi John,

Those numbers are nice info, thanks for sharing! If I understand correctly, by clipping the turbine wheel you have made it more free flowing and are taking less temp/energy out of the gas hence the quite high TOTs?

Am I correct in thinking the final application for your engine is still with the free power, and this test reverting to nozzle was purely to verify the nozzle thrust calculations? I have been wondering whether there is a "general rule" on nozzle sizing for peak performance (i.e. a % of inducer area). I guess its slightly self balancing whereby if you go for a smaller nozzle, there is higher gas velocity but more back pressure reducing overall flow - just wondering how wide the variance in size is for similar thrust figures!

Cheers

Scott

[racket]

Hi Andrew

Not sure about when , it'll depend on what happens with the next test of the current one, hopefuly next week

Cheers
John

[racket]

Hi Scott

Clipping the turb wheel exducer does increase flow potential but reduces the amount of gas deflection , other things being equal ,.......... the reduction in deflection reduces the power produced , power being a product of gas deflection and mean blade speed .

Clipping might not always produce higher temps especially if it allows the comp to flow at higher efficiences.

Yep, the 12/118 is still a freepower candidate , getting thrust readings is extremely important , everyone should do it as its the only way of getting hard data and determining if the engine is working to its full potential, noise alone won't do that .

Once we have a thrust level we can use the jetpipe total pressure and temperature along with jetnozzle size to do the calculations to find mass flow per second .

In the case of the 12/118 in the data above , at full power with a 2.8 Bar P2 the average jetpipe temp is ~840 C with 0.78 bar of total pressure .

The 1.78 PR across the jetnozzle from 840 C - 1113K at 90% efficiency will produce ~133 C deg drop , that 133 degrees equates to ~1830 ft/sec velocity at a static temp of 1113 minus 133 = 980K, with density of ~44.4 cubic ft/lb

Now thrust = mass times velocity divided by gravity ( 32.2) , so with 190 lbs of thrust = mass X 1830 div 32.2

Mass = 190 X 32.2 div by 1830

mass = 3.34 lbs/sec

To find the jetnozzle area required for 3.34 lbs/sec at 1830 ft/sec at a density of 44.4 cubic ft/lb,........... cubic flow is 3.34 X 44.4 = 148 cu ft/sec , Area = 148/1830 =0.0810 sq ft or 11.67 sq ins , or a nozzle of 3.854" dia or 97.9 mm , I'm currently running a 107 mm dia nozzle of 13.94 sq ins area , nearly 20% greater .....................somethings wrong , even allowing for some boundary layer adjustments .

With my TV84 engine back in 1999 I was running jet nozzle diameters from 76mm up to 85 mm with the same thrust levels , it was very troubling because the texts said big jet nozzles reduce thrust and temperatures , it didn't.

It was only after I clipped the turb exducer and unchoked it that the theory began to apply, I ended up having to use the tightest/smallest scroll A/R to limit compressor flow , the scroll became the choke point .

As for a rule of thumb for nozzle size , nope , too many variables from turbos of unknown configuration, take your's for example , the turb stage is oversized , so the comp will flow more to the choke side of the map , your flametube wall hole sizes are based on the "average" flow from a comp with that inducer diameter , but you're flowing extra so really need a tad more hole area.

Your nonafterburning jetnozzle size will be "off design" due to your lower comp efficiency needing more power to drive it, so less energy in the jetpipe and lower gas velocity thru the nozzle , so your nozzle size will need to be greater , firstly from the extra mass flow but also from the lower velocities , your actual thrust level could be "on design" though.

Now when it comes to afterburning size compared to non A/B , that easy if we've developed the engine first as a non afterburner , our absolute gas temperatures are roughly doubled , so densities are halved , but gas velocity only increases as the square root of the temp chage , so square root of 2 = ~1.4 , so a 40% increase in thrust .

The non afterburning jet nozzle area size needs to firstly be doubled to account for the halving of density , but then divided by our 1.4 to account for the increased velocity .

eg, assume a non A/B nozzle of 2.5" dia - 4.9 sq ins , first double to 9.8 sq ins , then divide by 1.4 to get ~7.0 sq ins for an A/B , 7 sq ins = ~3" dia

This will produce a ballpark figure but due to a multitude of variables with regards afterburner performance , that size may need to be adjusted until you get your 40% increase in thrust over the non afterburning figure, thats why its imperative to do thrust testing , without hard numbers we're flying blind , the A/B may appear to be doing all the right things but unless its producing that 40% increase in thrust its just all show and no go .

Bugger me , I've rambled on , my apologies

Cheers
John

[racket]

Hi Guys

New jetnozzle and flow straighteners added









Hopefully a little more thrust produced, ~103 mm for the jetnozzle with a 33mm centre tube , so ~97.5 mm effective diameter , it might be a tad "tight" so I'll watch my temps .

Cheers
John

[madpatty]

Hello racket.

An out of context question here.

The DC starter motor that you are using for your engine's startup, Do you power it directly (through relays?) giving it full 12 or 24V through the battery or do you have some built in Soft start or a PWM control that limits the initial high current draw and slowly ramps the speed up?
Does it get really hot that you can't handle it after the startup?

I have a dc motor as a starter for my RC jet engine and every-time I plug it to direct 12volts it gets too hot to keep holding it in a few seconds so I have to switch it off to avoid damage it from overheating. I suspect it is due to high initial current draw of the rotor.

Cheers.