Hey Ted,

I'm currently using a 6 cell 7.2v NiMh 4600mah pack for my JJ1800 test bed and I couldn't be happier, the thing lasts for ages and will run my 1800 for 20 mins with no probs at all, on average my pump is pulling 1.1-1.8 amps during normal running from idle and mid range rpms, and up to ~2.6amps @ max rpm (115,000rpm), I don't know what amps the starter is pulling as I haven't checked as yet. I can do this twice before I really need to recharge, so I'm getting the best part of 40mins out of it, I haven't run it untill I get rpm degradation from dropping voltage as yet so it may well run for longer...... I'm impressed.

Chris,
I don't see that using a 7 cell would be a major problem as the ECU will limit the current and/or voltage going to the pump to obtain the required engine rpm etc...likewise with the starter, the ECU will only run the starter up to the rpms set in the parameters, I have run my engines with a 7 cell 8.4volt 1800mah NiCd pack with no ill effects at all. However, as usual, I stand corrected and will accept thoughts and ideas from those more learned than myself!

Cheers,
Smithy.

In error...

Smithy, real nice close-up shots of your set up. I see the filter, and I'm finally getting the other components sorted out. I can't thank you enough.

What is that between the gas solenoid and the engine. Just a manual shut valve off maybe ?

> Jim

Brett:

Can you please tell me the size or diameter in mm of the JJ1800 starter motor can? I'm doing an autostart conversion of a big turbine, I tried the starter of my JJ1400 but the 300 series motor is not powerful enough to rotate the turbine shaft as required. I need to validate it's a 400 series, 28mm in diameter, if so I will order a starter pod from Gabe.

Thanks

The starter motor is just a tic under 28 mm.

Jim,

The valve between the gas solenoid and engine is just a Festo regulator or more correctly, an adjustable restrictor, I use it to regulate the amount of gas during the start sequence, I find it's very usefull when changing out empty gas cans, once I had the adjustment set correctly, (took me three or four starts to get it right), I can now just change gas cans and not worry about adjusting gas flows, pretty much a set and forget affair, works a treat too! Alternatively, you could fit a fixed restrictor in the gas supply line, something like 0.5mm or so, I believe some engine manufacturers do this allready. I like to turn my gas supply off when the engine isn't being used, simply because if a solenoid fails to close completely after shutdown, you'll get small amounts of gas leaking through the engine, not only is it a fire hazard, it wastes your gas too!!

I guess it comes down to the type of gas you're using and the pressures supplied by the can itself, no two set-ups would be the same I expect so each individual system would need it's own fine tuning to get it right.

Cheers,
Smithy.

Hey Ruiz,

As FW has mentioned, the starter is indeed "a tick under 28mm" something like 27.85mm or so. They seem to turn the 1800 quite nicely, when mine finally gives up the ghost I think i'll get a good quality Graupner to replace it, unless anybody has any suggestions othwise!

Cheers,

Smithy.

Thanks for the battery info guys I appreciate it, I am going with (for now) a 3300 mah NiMH simply to keep the cost down. In case anyone is interested I am using the handy wand to start my engine very successfully, it is exceptionally easy to use.

Jim, what propane are you using, it looks really great and probably cheaper than I what I am using at $8 for 8oz of propane butane mix. Where did you get it?


Thanks
Ted

Ted,
I think you might be talking about the tall black can you can see in my pics, it's actually a 220gram can of butane, I bought 4 at once so I should be right to go for the forseeable future..... I think they're for use on those little portable gas powered BBQ hotplates, it screws straight onto the fitting which was supplied with my engine, cost me all of $5 aussie each at my local Bunnings, for those overseas people, Bunnings is a large hardware type warehouse here in Aus, they sell nearly anything to do with household/gardening/camping etc... these little butane cans last for yonks, I've done well over 50 starts so far on the one can, it's nearly empty now so I think that's about its limit. I've had no issues with my JJ1800 lighting off on these so I can recommend them, still on the original glow plug too.

Cheers,
Smithy.

Thanks so much Brett for the info on the start gas, and sorry for calling you the wrong name!

Ted

The problem with brushed motors is that the brushes spark and get worn out if the applied voltage exceeds that of the motor's nominal voltage.
The ecu and in fact any brushed speed controller is controlling the power to the motor using the PWM method which does not regulate voltage
but power by varying the time that this voltage will be present to the motor.
The voltage applied to the motor's brushes will always be Vdc max and current will always be Imax thus bigger sparks, more EMI, faster worn out brushes.
Also because the voltage and the current are both more than the nominal values efficiency will be lower increasing heat generation.
When you read the current draw using say the Hyperion e-meter you are reading the rms current and not the peak current which really put stress to he motor.
Using a 7 cell pack where a 6 volt motor exists will degrade motor life, not much but it will.
An eight cell pack will burn the motor(s) very quickly.
If you have a friend with an electric airplane which uses a brushed motor, he will tell you that if you use a three cell lipo instead of a two cell in a 6 volt motor
will make the motor run hotter and eventually fail no matter how low you set the throttle.
I dont know if i was clear enough because this is all technical stuff but very easy to prove both mathematically and in real life.

hendrix, im a student for electrical engineering, and i enjoyed your explaination very much, i didnt realize youre reading Irms rather than actual current when you attach a watt meter, interesting - thanks for that !

Hi.
Well it is not that simple but this is the general idea that a brushed dc motor will work best if you don't exceed max operating voltage.
I was at work writing this post and i didn't had any time so i was very brief.
A brushed dc motor has a rotor with a few coils (electromagnets), a collector which has a set of contacts on it and some permanent magnets.
Let's say that the motors windings have an ohmic resistance of 0,1 ohm and the voltage applied is 6V.
The inductance charging is done very rapidly in low rpm because the inductance is very small so we can omit this aspect.
Initially when the motor is at rest and we apply the voltage one winding gets all the power instantly and it is immediately subjected to
6/0,1=60Amps which generates a lot of temperature in the collector with the possibility of scorching the usually copper made collector contacts.
After the rotor acquire some speed (rpm) the coils move close around the permanent magnets acting exactly like a generator
and this creates an EMF (electromagnetic force) of opposite polarity to the winding's voltage which is rpm dependent.
The current starts to go down until it reaches the value of (6v-back EMF)/0,1 Ohm = say (6-5,9)/0,1=1A (this is the idle current).
Obviously the only load to the motor is the rotor friction.
Now when a load is presented to the motor's rotor obviously the rotor will loose rpm and this will lower the back EMF to say 5,5v.
Then we have (6-5,5)/0,1 = 0,5/0,1=5A and the motor will draw 5 amps.
If we completely stop the rotor the current will be 60A!!!
This explains why the motors draw much more current when they start or when the rotor completely stops under excessive load (stall current),
it is because the back emf is zero.
Now if we increase the voltage to 7 volts and keep the same RPM
(the pump for example will need to have the same rpm in order to pump the same amount of fuel)
the motor will draw (7-5,5)/0,1=15A! Remember that the back EMF is only rpm dependent and the Current is the peak current.
The motor speed controller will reduce the time that this voltage is applied by reducing the duty cycle and the average current will be lower and in fact almost the same as with 6 volts but the peak current has jumped from 5A to 15A!
Obviously this is not good for the brushes and the collector thus the words you might have heard "welded or melted brushes"
Not to mention that this amount of overcurrent will probably drive the winding's core in to saturation heating the core and lower efficiency by much
thus crate more heat.
Of course there are more things involved but this is the basic theory as i know it.
The best motor controller would be a true variable voltage source like a buck converter or better a true variable current controller instead of using PWM but it would be rather heavy compared to the present brushed speed controllers.
I have burned the starter motor and spoiled a pump by using an 8 cell NiMh pack with the JJ1400.
The starter motor only lasted a couple starts.
I am not trying to play the expert here i am just sharing what i know and have experienced.
Please excuse me for the long post but now that i found some time i think someone might find the post useful.
Also i apologize for any generalities and errors in my post.
Chris

Great explanation Hendrix,
Just another example of "learn something new everyday", well for me at least, thanks very much indeed.

Cheers,
Smithy.

hey guys i have jj-1400. twice its been back for bearings going out, it always happens on the third flight, i get it back and once again third flight bearings start making noise.
first set was on klotz r-50 synthetic oil.
second time was on shell turbine oil
third time alysyn aerospace oil.
it has had the larger tail cone since the first trip in for repair
any ideas?

Hey Jeff,
Are you able to determine if the preload system is working correctly? I don't really think the type of oil you are using is the culprit, I tend to think it might be a lack of the fuel/oil mixture getting to the bearings to keep them cool and lubed, can you check the lube line restictor isn't partially blocked or possibly the line under the diffuser housing to the bearings may also have a blockage. Filters etc....Just a few thoughts.

Cheers,
Smithy.

hey brett thanks for the reply. i just got it back from gabe and he had done alot of work to the engine. he did say something about some tubes being stopped up. can debris from my flying field be the problem. i get some grass in the fog screen because this time of year we are not mowing so the clippings arent getting blown off? but it happens always on the third flight, thats whats strange. jeff

I've been reading this thread with great interest. Which is the best way for a beginner to go, Semi Auto, or Full Auto start ? I'm thinking about the aircraft weight savings of the Semi Auto start ? Is it that more difficult, or unreliable to use ?

Thanks > Jim

The main difference between the two is semiauto requires you to control the start gas, glow plug, and starter. Really quite simple once you've done it a time or two. Wieght savings would be around 12 ounces give or take a little.

GeeBeeJim,

My first engine was semi-auto start JJ1400. I used a hand wand to start it. It was fairly simple if you followed the the instructions and read the ECU as it stepped through the sequence. The main stubbling block I had was remembering to wind the turbine to the start rpm, 25K, before removing the starter. Secondly, I learned that NiMH wouldn't get to the 25K rpm. They quit at 23K rpm. Then, the fun began. So I always used 2000 maHr Nicads with the hand starter. I also found replacing the chinese motor with a Gruapner Speed 300 made a world of difference. It was a bit spooky to have your fingers and a wand so close to a spinning impellor.

Then, the same engine, I invest $40 in a starter pod. Sure do like the stick up, stick down and off we go. Just watch the ECU for problems. Which, I have not had since the last upgrade.

For the extra few bucks, I'd go full auto-start. Once set-up, it is a breeze. Learning the setting is enough besides learning a manual sequence.

My two-bits,
DW_Crash

I agree with Smithy that your rear bearing failure is most likely down to lack of lubrication reaching the rear bearing (which resides in the very hottest zone). Consider all fuel is dirty from being in the commercial storage tanks with rust and dirt. The bearing lube line has the smallest opening and restriction, even smaller than the 12 fuel injectors and can be blocked in an instant from unfiltered fuel. So as a guide filter the fuel into the main tank, use a filter on the main tank clunk, use a header tank with a filter and then a 'last resort' filter after the pump to collect the gear rubbings. This last filter should in theory be found to contain nothing and be free flowing after a full season of flying.

If its going to back to Gabe ask him to re check the 'flow rate' on the lube line as three flights per bearing indicates the above situation or a major imbalance that would also cause a bearing failure.

Paul
Paul

thanks paul, i will be talking to him today. sometimes these problems need more than just one head doing the thinking! jeff

Hey Jeff,
What you describe about the grass and debris contaminating the lube line is indeed possible but I feel it's quite unlikely, the lube line exits behind the compressor wheel and just in front of the front bearing. When servicing the engines after ~20-25 hours of use we find a lot of the debris normally ingested into these engines shows up on the diffuser vanes and not under the comp wheel. There's not a lot of room between the comp wheel and diffuser for dirt and debris to get in there, plus with the rotatives spinning @ 100k+ all the crud ingested will tend to go out through into the combustion chamber area via the diffuser vanes rather than behind the comp wheel.

The pressure differential between the underside of the compressor wheel and the bearing tunnel is what does the lube work for us, it is possible for your particular engine to not quite have the differential required, but again it's unlikely. I believe it's more likely for any possible blockages to be coming from the lube line fuel/oil supply, all I can suggest is that the fuel and oil you use is maticulously filtered, the lube line restriction is very small and it doesn't take much to cause problems.

Also, as Paul has mentioned, any slight imbalance will cause bearings to fail pretty quickly no matter how good the lubrication is, does the engine tend to "howl" at any rpm region, normally any imbalance will show up at various rpms harmonically rather than at all rpms.

I'm sure Gabe will be able to fill you in on the details, hopefully he'll be able to help you out, he has a great service and does very good work.

Cheers,
Smithy.

Hi Smithy

The JJ-1800 uses pressure differential between the rear of the compressor and the shaft tunnel to get air and fuel/oil flow through the bearings. The JJ-1400 is different and uses pressure differential between the combustion chamber and the shaft tunnel to take cooling air and lubrication to the bearings. There is a ring on the rear of the diffuser into which the lube line is fed (oulet is just in front of the front bearing) and air then feeds into the gap around this line and down the shaft tunnel. It is possible that the hole around the lube line is not large enough and that there is not enough air flowing down the shaft tunnel to push the oil all the way to the rear bearing. Another problem may be if the seal between the rear of the compressor and the diffuser is not good enough, then the air and lubrication could be getting sucked back into the rear of the compressor and is not going down the shaft tunnel. This would normally show up with evidence of fuel/oil on the diffuser.

You're quite right Shadow, my appologies, however the principle is similar, as you say, the pressure differential may be the problem if the hole around the lube line isn't large enough or restricted in some way....or the comp seal isn't as good as it could be.....again my appologies if I have misled anybody.
I should read the post more thoroughly before sticking my foot in my mouth.....!

Cheers,
Smithy.