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Exian

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Joined
Jul 26, 2018
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52
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BORDEAUX
Some news about my planes :

First EXIA
My wankel engine AIXRO XF40 has now passed over 100h.
No particular maintenance performed expect changing the spark plug. Still good compression.

My only problem is that I kill one lead-acid battery per year without fully understanding why...
Electronic ignition doesn't engage with to low voltage, even though the starter turn the engine without problems... so lots of canceled fly at the end of each year because the engine does not start...

So next battery will be the highest grade I can find (I heard about Odyssey batteries used on Vans...)
 

Exian

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Joined
Jul 26, 2018
Messages
52
Location
BORDEAUX
Second, KYRIOS

I have the electric motor fitted in the nose, but I need to flip it by 30° so that the prop stops at an horizontal position.
IMG_20201031_133326.jpg

The back-plate holding the motor will be redone in CFRP with high temperature resin.
Silent blocs are made of TPU 3D printed to the suitable dimensions.
IMG_20201004_161359.jpg
Still many things to do before starting with the wings.
Fitting back the nose cone on the motor will be a long task, I have to figure how to open and close the front air vent in something that turns at 4500rpm...
 

berridos

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Oct 10, 2009
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madrid
Very impressive design and execution. Inspiring and lots of interesting details
Did you cfd the horizontal stabiliser?
 

rv7charlie

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Nov 17, 2014
Messages
987
Location
Jackson
I don't know what battery you're using, but the Odyssey (or any other brand) is not likely to solve the problem. I've flown RVs since 1994, almost from the beginning I've used whatever SLA battery was cheapest with enough CCA. Except for one that had a pinhole in the case (undetected shipping damage), I've always gotten 4-6 years life out of them. And there have been many years where I flew less than 15 hours a year, which is not very healthy for a battery.

edit: If you search the VAF forum for recent posts about Odysseys, you'll see that the newer ones are seeing a pretty high failure rate within the 1st year or so of use.

Can you give us some more info on battery brand/model, its AH rating, and more critically, your alternator's current rating and voltage output at cruise.

If THIS is your engine, and you have the stock alternator, that may well be your problem. Specs say,
alternator 75W/6A from 5000rpm
(180W/15A optional)


With only 6A output, if you're running any electrical stuff at all the output voltage may be pulled too low to charge the battery. An SLA battery needs between 14.3 & 14.7 volts to fully charge in a reasonable length of time. If your voltage is sagging below around 13.5V, it may not be charging much at all.

BTW, really cool designs!

Charlie
 

Exian

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Jul 26, 2018
Messages
52
Location
BORDEAUX
If THIS is your engine, and you have the stock alternator, that may well be your problem. Specs say,
alternator 75W/6A from 5000rpm
(180W/15A optional)


With only 6A output, if you're running any electrical stuff at all the output voltage may be pulled too low to charge the battery. An SLA battery needs between 14.3 & 14.7 volts to fully charge in a reasonable length of time. If your voltage is sagging below around 13.5V, it may not be charging much at all.

BTW, really cool designs!

Charlie
Hello RV7charlie!

This is indeed my engine. Alternator should be stock.
I will check how many amps my instruments are drawing (radio, EFIS, EMS) in their data sheet, but there is something about this alternator that I noticed :
The "from" 5000rpm is important, in fact, there is almost no power produced at all under these 5000rpm.
My current battery is a EXIDE YT9B-BS of 8Ah (before that a YUASA YTZ6V of 5,6Ah)

For a typical flight, I charge it up to 14v with a RC-charger during pre-light checks.
After start, as long as I am not at full power during take-off, tension is around 12,2V.
It takes around 30 minutes to 1h of cruise flight at 5500rpm to rise back to 14V.
I was told that going over 14V is not good, but if I do not cut the power supply by a switch, tension can keep rising during long cruise. This happened several times with max of 14,5V if I recall correctly.
I tried to do some economy cruise at only 4500rpm, but I can see voltage dropping, and I am close to 12V after 1 hour at this setting, forcing me to push the throttle to increase tension before landing where I am at idle during several minutes.
If I stay at the airfield after I land, I usually charge the battery again with my RC charger.

Clearly, if I only fly around the pattern, I will have less charge in the batterie after the flight than before...

This batterie is rated for 115A but I don't know what is really draw during start. Some people told me that my battery could be a little to small, but some people are using batteries of similar size for rotax 912S! (power safe SBS8 of 8ah that I am considering next).

What do you think?
 

rv7charlie

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Nov 17, 2014
Messages
987
Location
Jackson
I think that you've found your problem. :) As I said earlier, I never pay the extra money for Odyssey brand SLA batteries, but their technical info is high quality and is applicable to most other brands of SLA batteries. I've attached their manual in PDF form. I'd suggest reading through the whole thing; it has a lot of useful info. One important point is on pg 13; a graph showing the charge voltage profile to fully charge an SLA battery. That section is talking about a stand-alone 'smart' charger, but as you can see, the initial charge voltage is 14.7 volts. That's why I mentioned the '14.3--14.7 volt' range to recharge your battery. Any voltage under around 14 volts will take a *very* long time to recharge the battery, and if it gets under ~13.5V, it may never get fully recharged.

You said that you charge with an RC charger to 14V. If it truly measures 14V after disconnecting the charger and waiting a half hour or so to measure voltage, you're likely overcharging the battery. An SLA battery measures around 12.7-12.8 volts when fully charged, measured at least an hour or so after removing any charger. If your RC charger means 'remote control' and is made for lithium ion type batteries, you're almost certainly overcharging it. SLA batteries are roughly the same voltage per cell as older wet-cell lead/acid batteries, while most lithium tech batteries are much higher voltage per cell. A charger made for lithium will try to drive cell voltage much higher than it should be for an SLA battery.

If your alternator never gets above 14.7V, then you shouldn't be hurting the battery by leaving it on. The engine should have come with a rectifier/regulator module to ensure that the voltage never gets above 14.7. On the other hand, if your measured voltage in the plane (while the engine is running above minimum spec rpm for the alternator) gets below around 14V, the battery is either charging very slowly, or not being charged at all. The alternator on that engine is almost certainly a 'dynamo' or Permanent Magnet style alternator (different terms for the same type alternator). One characteristic of that style alternator is little-to-no output, below a certain rpm, whit output going up continuously as rpm rises. The regulator module should prevent voltage from getting too high when the engine is running at high rpm. Voltage dropping below 14V while at cruise rpm could indicate that your total electrical loads (charge current plus all the electrical gadgets in the plane) are greater than the 6A output of the alternator. If that happens, voltage starts to drop.

On the subject of battery size, Aixro should be able to tell you the proper size battery for their engine, but if yours spins the engine fast enough, with enough power left to run the ignition system, it's probably big enough. If you determine that your alternator isn't able to maintain at least 14V at all times (except at idle power), then going to a larger battery might improve its life expectancy. With a larger battery, you're taking out a smaller percentage of its total energy to start the plane, and you can recharge on the ground with the proper smart charger designed for SLA lead-acid batteries. Lower percentage of discharge means the battery will last through more charge/discharge cycles.

Hope that helps,

Charlie
 

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Exian

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Jul 26, 2018
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BORDEAUX
I think that you've found your problem. :) As I said earlier, I never pay the extra money for Odyssey brand SLA batteries...
...Hope that helps,

Charlie
Very interesting!
By RC charger, I mean your typical charger for the batteries of remote controled models planes, that use your car 12V battery as a supply source.
I carry with me a big lead-acid battery to able to charge my plane in the hangar that has no suitable 220v close by...

I just give to my charger the type of battery (lead), the max charging current and it does the rest by itself :
- deducing the number of cells, which will limit the max charging voltage, 14V in my case for lead, I cannot change this value.
- Regulating the input current : first constant current at programmed value, then when reaching 14v, decreasing it slowly while keeping 14v. Charging stops when the current drops to 1/10 of the programmed value.

From what your said, this is quite not enough to reach complete charge.
I can reach higher voltage during long flights, but the current is not strong enough to really fill it very well, and I don't make lots of thoses long flights right now!
So my problem is "undercharging" of the battery (by charger or alternator), that gets damaged by drawing the high start current while not in a state (full charge) that can really sustain it?
Solution would be to always charge the battery with a more suitable charger?

Overall electric consumption on my plane should be under 2Ah (no transponder yet...)

The AIXRO manual only recommends the battery to be at least 8Ah and to charge it completely before any use of the Engine.
This is no problem for the original use of this engine on go-karts that you can charge at home during the night before...
But it is really unpractical for an airplane in a remote hangar, and charging during the night would not make the airfield manager happy!
Definitely a great engine, but another detail showing that "real airplanes" constraints were not really taken into account!
 

proppastie

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Beautiful work and engineering. ...I am sure you will figure out a fix. Quick remove battery to charge at home is one option.
 

Scheny

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Feb 26, 2019
Messages
232
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Vienna, Austria
12 volt batteries have a nominal voltage of 12.5V. In order to load them, the voltage has to be higher. The charging is based upon the voltage difference and the internal resistance of the battery. So in case you charge it with 14.5V and the internal resistance is a fictional 1Ohm, this means you have a current of (14.5V-12.5V)/1Ohm = 2A. While the battery gets full, the voltage rises and charging current decreases.

This means, that the generator voltage is providing on how fast you are able to charge your battery. In cars, the nominal generator voltage is 13.8V standard. As modern cars have automatic motor stop, it needs to recharge much faster, as every stop needs lot of energy (for the starter). Because of this, starters have gotten rising voltages up to 14.5 volts. In some rare cases, the voltage is even 16.5V, but then the complete equipment must be specially designed for this voltage. My car has a variable voltage up to 28V, which is fed to a capacitor and then converted to 13.8V before being fed to the battery.

Some batteries like NiMh are (quick)charged with constant current. In this case, the voltage is rising until nominal current is reached. In order to check the voltage, it stops charging every ten or so seconds.
 

rv7charlie

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Nov 17, 2014
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987
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Jackson
It sounds like Aixro is 'telling' you that they know their alternator doesn't have the capacity to recharge the battery completely. If your charger has specific settings for lead-acid batteries, then it should be capable of properly charging the battery, but you may not be giving it enough time if you're doing it before each flight. Lead-acid batteries charge at a slower rate than lithium. There's a website with a lot of great info on managing various battery types. Here are a couple of links, showing the different charge profiles of lead vs lithium ion:
Charging Information For Lead Acid Batteries – Battery University
https://batteryuniversity.com/learn/article/charging_lithium_ion_batteries
If the battery never gets fully recharged, that may be shortening its life. Proppastie may have the right idea; a high current quick disconnect and a latch type mount for the battery would allow bringing it home each night so it can get a full recharge with the slow profile that lead-acid needs. Might be worth it, even if you have to move the battery. It *might* be worth trying a larger capacity SLA, that is designed for 'deep discharge' (typically found in powered wheelchairs, Uninterruptible Power Supplies, etc) . They typically have a somewhat higher internal resistance, meaning that their 'CCA' rating will be lower in relation to their AH capacity, but they tolerate greater levels of discharge without damage. I've used the 18AH -- 22AH versions for at least 20 years to start Lycomings, and they last 4-5 years on average.

But I'll bet your best solution is the 'remove & charge overnight' idea, along with ensuring that the alternator voltage stays above 'float charge' level, which will be around 13.2V. It's not actually charging the battery at that voltage, but it will prevent any further discharge.

Beautiful planes. Please keep us updated with progress on the electric version. I'd love to replace the 2 stroke 503 from my Kolb Twinstar with an electric; it'll never get flown for more than 15 minutes at a time, anyway. :)
Charlie
 

Exian

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Joined
Jul 26, 2018
Messages
52
Location
BORDEAUX
Wonder if that elevator is all flyig type ?
HS and VS are indeed all flying type, much simplier to build and to connect to the fuselage.

Some people really like the shape of these HS and VS, and some other really hate it (telling me I designed a ugly tail!).
If I build a second version of my EXIA, tail will be conventionnal...
 

proppastie

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HS and VS are indeed all flying type, much simplier to build and to connect to the fuselage.
can all flying surface be smaller?.....as yours seem to be.....seems like if that is the case simpler and lighter is better......without any stabilizer might it handle different?.....more sensitive ?, do you have an anti-servo tab?....is the aircraft "positive stable" ? Do you use springs for trim?
 

Exian

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Jul 26, 2018
Messages
52
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BORDEAUX
can all flying surface be smaller?.....as yours seem to be.....seems like if that is the case simpler and lighter is better......without any stabilizer might it handle different?.....more sensitive ?, do you have an anti-servo tab?....is the aircraft "positive stable" ? Do you use springs for trim?
I basically followed what worked for the MC30 Luciole, using little higher values of HS and VS volume coefficient because of longer nose. I do not remember the exact values, but I recall that they were over average (HS way over average, close to 1 fo my plane...).
I like and am used to sensitive planes, so I find my EXIA really pleasant to fly. It is not over sensitive to my taste and you still need to pull seriously at touch-down with full flaps. I will definitelly try a smaller HS one day, but nothing too radical.
No anti-servo tab, springs like in Luciole to get trim and feel in the stick.
Stability is marginal, but doesn't bother me for the time being. Maybe I will be less happy with it after a +3h flight...
 
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