RFguy

Hi Mark. An alternative might be a 2nd-order current control- IE duration x load- IE heat load on the alternator knowing that there is some thermal inertia available. Complicates the design, I agree.

I was reading somewhere the Rotax alt was perm mag ? No brushes so that's cool, but I think I only replaced brushes in my car alternator every 250,000 km ! But..... no field control ? how does that work for control ? (apart from as you said, motos short the alt) .

..I guess a shunt regulator would work, but that's quite alot of extra heat, and engine kW to blow off. ...... and I am sure with a 70kW aircraft motor every 500W is precious.

So then the only (efficient) control you have is phase fire rectifier control....needs to be foolproof. As you know, you can't UNLOAD the alternator, that is bad ....the EMF has to go somewhere. the thing has to stay somewhat loaded as not to send the voltages appearing between the diode stacks in reverse bias beyond the limits. seems that conventional field control alternator is better suited to not wasting power. Although I guess there will always be a decent load on the alternator, so the only time it would be no load would be if all avionics was off and battery was full.... seems reasonable to dump then. ---glen.

Glen

....

The generator and also the original regulator are basically motorbike style ones and we know that they are always crappy. This is why lead acid batteries make a better capacitor..these new batteries do not like what they are being served with from the originals. This is why i dont recommend any lithium batteries on a Rotax. the charging system is just not up to scratch for them

Hi Mark. thanks for the time taken to explain. That is good advice and voice of experience for everyone. Yes ACS770 is many a favourite. I rarely deal with more than 40 amps on things I make at DC , IE no ventures into 2 and 4 ounce copper.... ! but plenty of high voltages high RF current.

Bruce, I have gone back and read a few of your posts, you've done a fair bit of work on this. Yeah those RC LiFePO4 (LFP) packs really pack a punch with all those parallel cells. If you charge them nicely (say at 1/5C or 1/10C the cells stay in pretty good balance.

Thanks for the link Old K on the 12 pole mod. Actually yesterday I did a bit of reading on what mfrs are doing - 3 phase IGBT synchronous rectifiers, dual windings in series/parallel configuration. Frankly I think a 3 phase sync rectifier with computer control is a bit complex compared to 6 or 8 diodes (8 for star connected and harmonic current sink) . But the dual winding alternator was interesting. rather like traction motors in trains, switching of the windings for speed or torque.

I think Mark's idea with the simple voltage/current limited regulator is a good one. And overcoming to low RPM issues would seem a better target than getting dissipation down in the diode pack.

One of the issues with low speed is the stator core saturation due to the low frequency. The output is very NON linear down the low end. It really needs a rotor designed specifically for low speed operation.

Tell me- in these direct drive alternator designs, what happens if a chunk of the rotor breaks and stops the rotor rotating ? I have NEVER seen it happen though in autpmotive alternators, even at crazy RPM. . Rotors are usually well balanced in ones I have seen.

cheers- glen

The HQ is the only car I know of with 'A' pillars narrower than your eyes- no blind spot to hide pedestrians, cyclists, etc. unlike too many modern cars whose'A' pillars are needlessly wide.

Yeah, agreed. I have thought of putting some sort of display or projection on the A pillars so that I could 'see through' them. (IE from external video imaging) .

The low stall speed, and thus landing speed (or TO) buys a ton of survival headroom w.r.t crash energy .

I had a friend who was killed in a low speed vehicle crash, car slid sideways into a barrier and he hit his head on the door pillar... ("collision with door pillar").The car was fine and driveable, door could be opened, but that hard surface next to the head was his undoing...

Yes- fire suppression. In my younger car days, most had plumbed-in fire suppression bottles for the engine bay. If fuel gets shut off early enough, I guess not required if there has been no structural damage. I am not sure how many materials on aircraft are self extinguishing...... glen

Hi Mark,

Thanks for the info. OK.... I gather because that RPM is 50% of the running RPM, that it is more of a lack of field current at low RPM, and that some sort of low RPM field boost might improve that ? IE simply to restore the field current provision, it wouldn't be increasing the max field current IE so no ill effects on the brushes, rotor windings etc.

glen

Mark,

why is this Alternator so lazy at low RPM ? , is this a general character of alternators designed for aviation ? I would have thought pulley sizes could have been made 'ideal' since the motors don't run over such a wide RPM compared to a petrol motor car engine. - glen

Output of the regulator at 11.5 amps load. The first picture is the AC output of the generator you can see how the power is used on each cycle

Hi Kyle. Looking good. I gather the yellow trace is current sensor out and that's a step load change going into I-limit on the 2nd pic ? I notice the harmonic structure (waveshape) of the current sensor changes on the high load- is that a slew rate limit in the current sensor, or the behaviour of the alternator ?

I've use generally Allegro hall effect sensors in this sort of service, mainly to stop semiconductors vaporizing in fault condix where there is a bit of capacitance upstream.. IE keep the semiconductor under its SOAR curve. Hall sensor rise times of around 4uS mean it can work pretty fast...The Allegro sensors were pretty good on temperature stability.

cheers

Hey Bruce, I am confused by all the technical talk going on in this thread.

My Jabiru J170 has the standard Odyssey PC625 Battery and Jab factory Voltage Regulator.

Any cause for concern?.....never had any issues!

Bruce ?

If that is what the manufacturer suggests.... anything else is experimental.

*** I see two immediate important problems changing to a LiFePO4 battery : ***

1) Changing to a lighter weight battery will of course change the weight distribution...

2) Starting cranking current. Airplane batteries are primarily starting batteries, which lead acid batteries do well at (unless they are cold !) .

I see that little Odyssey battery is rated at only 18 AH capacity (20h rate) and 540 crank amps for 5 seconds, 220A CCA . The battery is rated for more than 12x the ampere hour rating for CCA.

The LiFePO4 battery for the same physical weight will be rated at ~ 4x capacity of the lead acid. Although at these small capacities, the difference is more like 3x as case and hardware is a high percentage of construction.

Therefore for the same weight the LiFePO4 battery will have 18 x 3 = 54 AH capacity approx

Now, the starting current for a 1.5kW starter (according to Jab2200 info I found), will be approx 150 amps, based on 10V at the starter itself)

The LiFePO4 battery MUST be rated for that peak current.

Now, the CALB 60AH LiFePO4 batteries I use are rated for max current of 180A.

That sounds inside spec . However the stall current for the starter motor could be much much higher. Kyle ?

So, consideration must be with care ! . The standard lead acid battery for $200 seems a good idea.

----

On the upside, The LiFePO4 battery will have a capacity advantage for the same weight, this may be an advantage if the alternator stops making charge during flight , providing extended use of avionics . for the above example, most likely 4 to 5 x the non charge duration . That could be useful . The other useful thing is, that the LiFePO4 will be happy to provide that starter crank for long periods. Although the starter may overheat. Watch the spec on that. (Kyle ?) .

glen.

In answering that question, the answer is the extra resistance is not useful. This is because the amount of charge , or support the alternator could provide is proportional to the voltage difference between the alternator and the battery, divided by the resistance of the cable. Too much resistance and you cant charge or support the battery.

Kyle's idea of the voltage AND CURRENT limiting regulator is a good one.

I do like to see fuses in series with any battery, usually a separate charging fuse and load fuse,s (including a separate starter fuse) . But I do not know how conventional that is in aviation, I have not taken notice of this in enough aircraft.

In the case of the alternator/charger, ideally the voltage sense wires for the regulator should be on the battery itself, this is known as kelvin connected, so that the connectors and cable that are carrying high current do not affect the battery voltage reading. (since they would cause a voltage error equal to to the current multiplied by the resistance of the joint or cable) . The said voltage sense wires ALSO need to be fused, since they probably run off somewhere in the aircraft, and are liable to damage.

Glen.

I didn't know about point 4. Should I arrange for a heavy-duty resistor in the charging circuit on my Jabiru? By this I mean something like a high-wattage 2 ohm resistor between the voltage regulator and the battery. I thought the voltage regulator would protect the alternator.

got it.

Load steps with poor condition batteries (read: high internal resistance) are the biggest trouble I have found. As long as the regulator does not overshoot too much in the control department should be OK. take a scope probe to the battery terminals, Kelvin connected. set it up to trigger on the load step you will apply (usually a transistor and load bank) , and observe what the voltage does during the load step. Oscillatory behaviour is problematic. Excessive overshoot also. Vary the series resistance (very small) with the battery, essentially place a resistance in series to simulate various battery conditions that result in higher internal resistance. At some unrealistic point, it will oscillate... The practical real world result is that because you cannot control the battery condition, the control loop needs to be slow enough not to oscillate into the worst case battery. Where the voltage is picked off for the regulator feedback matters, also. Fortunately, by the time the series resistance is high enough to be a problem for most designs, the battery wont crank the engine ! but batteries do fail, get boiled etc.

And that is probably a perfect example of old-tech works ! The regulator feedback loop is slow in old designs.

And what is the general helmet feeling ? I was involved in a heavy helicopter landing years ago. and then a few years ago, a direct colleague was killed on the job I was on.

https://www.atsb.gov.au/media/2514320/ao2011110_prelim.pdf

From that point onwards, I always wore helmets on jobs in helicopters.

I would have thought a helmet in LSA was a good thing, based on my personal experiences. I have spent alot of time in helicopters on windy mountains and barely acceptable pads and commercial imperatives.... a bad mix.

-glen

agreed and taken on board..

we should have a sandwich sometime., when cross border sandwiches are permitted.

cheers

Yeah voltage regulators are just that. "voltage " regulators.

The cable between the Alternator diode pack output and the battery would limit the current, but not in a useful manner.

Kyle, be a little careful with high speed diodes, too fast and you will get undesirable ringing, possible VHF oscillation and potentially radio interference. Also, some of the high speed diodes will have undesirable (high) reverse leakage currents when really hot. It depends on the barrier construction. (yes I do this for a living...) . on the other hand, if I am telling you how to suck eggs, I apologise ! Happy to advise, comment or keep my mouth shut, which ever is most helpful.....

They're all "lithium Ion" but vary in their construction, and slight chemistry differences.

The cathodes in LiFePO4 are phosphate instead of Cobalt (LiCoO2), and the phosphate-oxygen chemical bond are tighter than the cobalt-oxygen bonds, which when the battery is damaged or stressed in some way, take your pick, means the oxygen is not liberated as easily. Also the Lithium migrates in the the LiCoO2 battery during charge and discharge which changes its stability. There are a few good writeups on this online.

In practical sense, the LiFePO4 battery of four cells or eight cells, closely approximates a 6 cell or 12 cell (12 or 24V ) lead acid battery. (14.4V top of charge at 25 deg C)

Charging is approx compatible with Lead acid batteries with the exception of :

1) "Boost or equalise" for lead acid must not be used. Lead acid batteries are typically deliberately overcharged to equalise the cells ! Lead -acid "smart chargers" might try and do this...

2) Temperature compensation if present (only in the good stuff with temperature sensors for the battery) should be set to zero

3) Ideally , the initial charge when battery has been over discharged (<2.7V per cell) should be limited to 0.05C until cell voltage reaches 2.7V per cell (10.8V for "12V" pack)

4) LiFEPO4 batteries can charge usually as fast as your can supply current. Charging a well discharged LiFEPO4 battery may stress the charging system, since it will likely to be have been design assumed for a lead acid battery with appreciable internal resistance. As for the aircraft alternator charging question, that requires some care and caution in order to determine if the alternator diode pack could over dissipate.

Alright . what comes first.

Do I need RAA to participate / do the flying training, or is a typical method to run the flying training and then join RAA?

Find instructor that fits me in a suitable location. I've had a bit of advice on this, either immersion and stay on site/in town, or something close to home that is a short drive ... What's the maximum number of days between steps ? I can do a full immersion but young family I really need to go home regularly for 'optional activities' as the wife sees it...

glen

Bruce, you said " As soon as the battery is back to voltage, the charging is switched off and the "battery" is now just a couple of big capacitors which the alternator thinks is the battery. "

I would be a bit careful about that- it depends what the time constant is in the alternator voltage regulator.

If there is insufficient capacity in the accumulator (battery, capacitor) the alternator output voltage will go through the roof potentially damaging all the electronics on board ! This happens because the alternator voltage regulator might have a 1 second time constant , and if the load is removed, or there is insufficient place for the charge to go, the voltage will shoot up before the regulator has time to reduce the field current. The better and modern regulators are generally fast enough , although the idea with the long time constant is that the alternator doesn't try and follow a rapidly varying load

Interesting about the comment about over-heating alternators. The diode rectifier packs in alternators are about the most robust electronics in the whole aircraft. They can be damaged by over voltage (if the load is disconnected and the regulator ramps up the effort) . Good design I would think would shield the diode pack from 'seeing' a hot surface. My Volvo had a sheet shield between the back of the alternator and the exhaust man, even though it was 6" away.

-glen

and " As you are new to aviation, making blanket statements that some readers may take at face value could be dangerous. "

yes, I need to qualify everything and not make rash statements I guess, (like "Lead acid batteries are garbage") since there is a bunch of reasons for everything....

The best advice I got I think was that instructors should be matched to the student. A couple I spoke to said they changed instructors twice... I'll move back into that forum topic for the rest of that topic.

Biggest problem with the world and lead acid batteries- LACK OF TEMPERATURE COMPENSATION

When a lead acid battery gets hot, the terminal voltage goes down.

and so the (non compensated) charging system thinks the battery is not fully charged,

so it charges a full battery even harder.

and then the battery in an overcharge state gets hotter

and you get a runaway. Ever seen bulging lead acid batteries ? And I have NEVER seen ANY temperature compensation on any aviation alternator charging system (or vehicle) .

With LiFePO4, no temperature compensation is required.

I am pleased you have had good results with the LiFePO4. They have a nice flat discharge curve, also.

If you do flatten them, just go really easy on the charge rate until they are back up to 2.7V/cell.

And, if there is not a cell balanced board built in, keep an eye on the individual cell voltage.

If they are old, or absued, or been flattend the cells will vary a little. During charging to full, you'll find maybe you have 3 out of 4 cells at 3.65 and 1/4 dragging its feet at 3.3 etc. in this case, apply current and voltage limited charge across the slack cell until it comes up to full like the others. if you only have a 12V charger, not a cell charger, just put a resistor in series with the 12V charger (to limited the current) and watch the cell voltage regularly... (since the 12V supply is capable of overcharging it given time)

Active battery balances in LiFePO4 have a 3.6V zener diode and a transistor. The idea is as soon as the voltage on the cell gets over 3.6V, charge is shunted into a load.

one quarter the weight and half the volume for the same capacity as a lead acid !

and lead acid batteries are garbage. they are useless below 33% state of charge., when I say 'useless' I mean you should not rely on them delivering that last 33% once they are in a low state of charge (as viewed by terminal voltage, not specific gravity of the electrolyte in the case of a wet cell.)

glen

indeed ! and all true.

yes, I will be pounding the forum with questions relating to getting my certificate.

cheers

Hi Nev and others, thanks for the comments.

Yes agreed. Regular pilots of their own aircraft become well tuned to any changes. and yes there are lots of things that force airplanes down, and I see the engine as being the least developed item.

The idea is to collect lots of data to track changes. and post analysis using machine learning. When something is up, there is often speculation and guesswork on the ground.

There are many low hanging fruit to pick. The idea with the Jab is that I am confident that instrumentation can be such that I can see a problem coming.

As for modifications. I'll have to interpret the regs. Adding instrumentation to the engine I would have thought would be no different to adding an extra cabin thermometer. IE the installation of the instrumentation does not in any way affect the system being observed. I guess its all arguable. And if there is a demonstrable improvement to safety......

There would be things that would certainly interfere, if they were done , with the certification. Like real time chem analysis of the oil, and fuel, exhaust etc. requires instrumentation to be inserted into the line. Well there are ways but lets try and keep it simple on the sensors.

There is quite a bit in the vibration arena. Not only are there the fundamental , dominant notes, but there is much information in the echos (reflections) from discontinuities (crack/fatigue, change in crystal structure ?) in the medium (IE a block of Ally) . Sort of like sub miniature acoustic geo strata surveys. The speed of sound in hard metals is fast so the sensors have to be pretty wide band.

I mean you can really go to town on things you can monitor. acoustic and thermal sensors aside- that's the low hanging fruit. attaching high bandwidth high temperature vibration sensors to the engine components might be a no no, but that's also possible by looking at the light reflected off the component, it has all the same information as a attached sensor, maybe with exception of being able to differentiate between various types of waves in the medium. I am sure a geologist geophone person could add loads here.

and there are other techniques used in industrial robots like continuous strain and torque sensors on actuating rods. there are plenty of those in many craft... (non engine components I am referring to)

anyway, plenty of ideas. My journey in aviation will be about personally developing myself in my own technical field, as much as the actual flying and all the techniques, experience and judgement that goes with it.

Sounds like I should have a sandwich with Mark (Kyle) . Oh and if anyone feels I am really f***ing mad, please let me know, I take on all opinions. .

Is anyone doing advanced engine instrumentation ?

I am inexperienced (and new) with aviation engine mechanics. In that I don't know details about pistons designs, crankshaft loads on direct drive props etc

But from my signal processing and instrumentation life, there is alot of scope for real time and off line diagnostics of aviation engines to improve reliability and safety. It's something I thought I can bring to the RA community. Of course it might be useless, but I have always found in my industrial work that instrumentation that detects problems before they really are is oh so worthwhile.

And it need not cost alot.

Aviation engines in GA seems to be back in the 19th century compared to some other industries that have devices with high stress loads.

I'm in the process of getting into RA, will buy a old Jab and get new Gen4 engine in it, and instrumentate the hell out of it. Time line is 6-12 months. If anyone is interested in discussing this here, I'll be happy to write down a few of my ideas, hoping that seasoned engine (re)builders and designers can put the 5$ opinion in on what is worth while, what is not, what would be useful etc.

and I am not talking about a tacho. I mean real time all engine vibration/mode analysis, (so you might see a change in the structure of the metal in the cylinder jacket before it is a problem) , real time gas & oil & exhaust spectral analysis, real time thermal imaging analysis of cylinder heads, so comparing the thermal imagery to a template , all by machine learning which puts the workshop expert into the chip . etc etc etc

glen

I am a industry user (non aviation) of batteries. Comms, remote sites, high temp environments.

The biggest industry problem is people confusing LiFePO4 (3.25 terminal voltage, 3.65 max) with "lithium poly and lithium ion' (3.7V terminal, 4.1 / 4.2V max) .

They have very different behaviours, they are very different batteries.

I would have a LiFePO4 battery in my plane any day. They are essentially unconditionally stable and safe. I would rather not have a Lead Acid because of the rather nasty failure modes. I would NOT permit a 'Lithium ion' in my plane. No way. Not ever.

pronounced - LiFePO4 "Life - poh" and Lithium Ion often referred as " lie-po" or "lie poly"

Hi Jerry

thanks for the information. Yeah so grass is 3K for rec, 5K for commercial light- includes "all you can eat" in terms of charges. The GA might be moved down the track to the other side of the airport.

* Again highlights the need for a recreational / GA strip for Canberra region*

I don't want the stress of driving two hours to a field before I even get in a plane for a 4 hour flight somewhere.

mutter mutter. Closest other fields are 1 hour drive. (instead of 15 min) .

cheers.

Hi, My name is Glen, just joined. I live in Canberra these days.. I am 49 yo, a self employed electronics engineer with a youngish family..

My plan is to get my RAAus, RPL+endorsements, buy a LSA etc. I'll join RAAus as soon as the go button is pushed.

My industry background is high end micro and industrial electronics, radio, - aircraft avionics are a doddle ( for me) .

I have a good working day to day background in things mechanical , structural, so doing the maintenance on my LSA will be an enjoyable and satisfying task. I like things that are detailed and serious.

I've done a fair bit of right hand seated GA fixed flying for work and pleasure, and a fair bit of rotary time in G and C airspace.. Familiar with radio procedures . I know you don't want to p1ss-off the ATC op.

A PPL (1500h)+VH registered owner I used to fly with alot has recently stopped flying, so time to do it myself.

I think also I can provide some give-back to the organisation and associates in an aero club on my electronics caps from time to time..

Have been reading and interpreting CAO 95.55.....

As far as I can tell, I can, (but this is only my interpretation! I may be wrong) with a RAaus+RPL + specific endorsements, with a RAaus registered , factory built non modified (or having approved modifications) an LSA with radio and transponder, I can fly in and out of canberra YSCB. ... The thing is, I have never heard or seen a RAaus reg at the airport. Maybe needs to be VH, but according to my interpretations of 95.55, an RAaus factory built 23.xxxx or 24.xxx is OK.

-glen