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C172p Mixture control is erratic

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C172p Mixture control is erratic

Postby kjd.nc » Sat Dec 12, 2015 3:18 pm

- Hover the mouse over the mixture knob
- Key 'm' repeatedly (don't hold it down) and watch the mixture % increase

Expected Result: for each keystroke, I expected the mixture to increase by 1% (controls.adjMixture(1)).
Actual Result: somewhere between 12 and 25%, it jumps suddenly to, usually 38%. There are several non-incremental jumps.

Same thing happens with 'M' to decrease the mixture.
Same kind of thing happens with my custom keyboard binding in effect to controls.adjMixture(5) and -5 instead of 1 and -1.
Same thing happens on FG3.6 and on FG3.7, DMG dated 5 December.
Same thing happens whether I'm using the laptop keyboard or an external bt keyboard.
MacOS 10.11.2, MacBook Pro.

I haven't found any other aircraft whose mixture control advertised the current setting. If someone would like me to try one in particular, I'll be happy to download that model.
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Re: C172p Mixture control is erratic

Postby sanhozay » Sat Dec 12, 2015 3:42 pm

What are you looking at to determine how much the mixture changes? The EGT instrument in the cockpit or the property on the property tree (controls/engines/engine/mixture)? Or something else?
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Re: C172p Mixture control is erratic

Postby gsagostinho » Sat Dec 12, 2015 4:21 pm

@kjd.nc Thanks for the report, I opened a ticket for it here: https://github.com/Juanvvc/c172p-detailed/issues/623

In reality, pressing either m or M modifies the mixture by 0.825% instead of 1%. Since the tooltip rounds it to the nearest integer, sometimes you see some numbers two while others may be skipped. So the increase isn't erratic but the value chosen was not a good one. We will take a look and change it to 1%, so then one keystroke equals to 1% of increase/decrease.

Cheers,
Gilberto

PS: in general it's better to report bugs about the c172p on its thread, since all of us from the team receive notifications when someone post there, but new posts may be overlooked.
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Re: C172p Mixture control is erratic

Postby gsagostinho » Sat Dec 12, 2015 8:23 pm

@kjd.nc So we investigate the problem. Onox wrote:

Here's the truth: Nasal/controls.nas in FGDATA uses THROTTLE_RATE and /sim/time/delta-realtime-sec for adjMixture. That file also defines adjThrottle, but in keyboard.xml it actually uses incThrottle, which does not use that /sim/time/delta-realtime-sec property, but instead always increments/decrements by 0.01.


We concluded:

- there shouldn't be anything unique to our plane regarding this issue. If you do see this inconsistent behaviour in the c172p, you should see with other planes as well. Can you try testing it again in several situations?
- in planes with propeller pitch, n/N keys should display the same behaviour as well because the code is similar, can you trythat?
- the fact that each keystroke may output slightly variable amounts of mixture has its advantages (see the issue in GitHub for the whole discussion), but the difference should be minimal. Currently, each keystroke should result in approx 1% of change. What is strange in your report is the spikes from around 12% to 38%, and we can't reproduce this at all. In which situations this happen? Can you consistently reproduce it?
- and just out of curiosity, how is the performance of our plane in your computer? What is the average fps you get?

Cheers,
Gilberto
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Re: C172p Mixture control is erratic

Postby kjd.nc » Sat Dec 12, 2015 11:18 pm

Thank you for the response. I didn't know if it was particular to the C172 or not - I tried all the other planes I have downloaded, but none of them displays the mixture setting. I'll see if I can watch the property with some of the debug tools. I had considered that it may have been a case of rounding; if it had occasionally jumped by 2% instead of 1%, I probably would not have even reported it. It was the big jumps that made me concerned.
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Re: C172p Mixture control is erratic

Postby gsagostinho » Sat Dec 12, 2015 11:22 pm

Indeed, our conclusion is that there is a reason behind the irregular small changes (onox explained well in that issue in our repository if you want to see the arguments) but you shouldn't see any large jumps at all. Here if I hold the key or press it a couple of times, the increments are tiny as expected. If you can find a way of reproducing it or narrowing down which situations make it happens, and also if it is related to our plane alone it or happens to others, then we can try to find out what is going on. And thanks for the report.
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Re: C172p Mixture control is erratic

Postby gsagostinho » Sat Dec 12, 2015 11:23 pm

By the way, you can watch the property "controls/engines/current-engine/mixture" (if you shift click on it on the list of properties, it will be shown in the screen, else just leave the property list dialog opened).
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Re: C172p Mixture control is erratic

Postby kjd.nc » Sun Dec 13, 2015 7:59 pm

Thanks, that helps!
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Re: C172p Mixture control is erratic

Postby someguy » Sun Dec 13, 2015 11:26 pm

Personally, I've never given the least bit of thought to the granularity of the 172 mixture control. I found the sweet spot on the EGT gauge, set the telltail there, and simply adjust to keep the needle on the telltail.
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Re: C172p Mixture control is erratic

Postby gsagostinho » Sun Dec 13, 2015 11:32 pm

Yes, we all agree that the issue is not any kind of small variation: in RL, you can't change your mixture from 78% to 79% either just by looking, this kind of precision does not exist, and there are advantages as we mention in the repository for the current behaviour. The problem is the large skips the OP reported, 10% skips are not normal and should be investigated.
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Re: C172p Mixture control is erratic

Postby kjd.nc » Mon Dec 14, 2015 1:06 am

I don't recall exactly what I was doing when I observed this big jump. I certainly was not following a script to "set the mixture to 28%" - as you described, when in flight, I lean it out until EGT peaks, then one click back to the rich side.

What I was probably doing was testing my keyboard binding. I changed the mM keys to increment by 5 instead of by 1 to speed the startup process. I was probably checking that the 5s worked, noticed that it had wound up set for some value that was not a multiple of 5, then started watching it one keystroke at a time.

I'm OK with the explanation that the code is optimized for realtime simulation performance, in which my machine can lag, therefore each keystroke isn't equal to every other. It doesn't change how I tune the mixture -- and even if I did, I've never had the sim complain to me when I've just left the engine on full rich. Overhauling the sim is a lot cheaper than overhauling a real engine. :-)
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Re: C172p Mixture control is erratic

Postby wkitty42 » Mon Dec 14, 2015 2:56 am

kjd.nc wrote in Mon Dec 14, 2015 1:06 am:I've never had the sim complain to me when I've just left the engine on full rich. Overhauling the sim is a lot cheaper than overhauling a real engine. :-)

while the sim may not complain, a good FDM will exhibit problems in such a state... take off from KSFO... set the heading bug to 90 once you're up... turn on the AP and set your climb rate to 700 with a destination altitude of 13000... adjust your climb rate as needed to avoid stalling... can you make it over 7000 feet without adjusting the mixture? ;)

NOTE: i've been flying with the 180HP engine and standard landing gear... i was just testing this and the sim crashed with a segmentation fault... looks like i need to start running my builds in the debugger again so i can get full backtraces to post to the code devs...
"You get more air close to the ground," said Angalo. "I read that in a book. You get lots of air low down, and not much when you go up."
"Why not?" said Gurder.
"Dunno. It's frightened of heights, I guess."
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Re: C172p Mixture control is erratic

Postby gsagostinho » Mon Dec 14, 2015 12:09 pm

kjd.nc wrote in Mon Dec 14, 2015 1:06 am:I don't recall exactly what I was doing when I observed this big jump.
[...]
What I was probably doing was testing my keyboard binding. I changed the mM keys to increment by 5 instead of by 1 to speed the startup process. I was probably checking that the 5s worked, noticed that it had wound up set for some value that was not a multiple of 5, then started watching it one keystroke at a time.


So in that case I will close the issue on our repository as invalid (also given that no one else could reproduce it), but if you do observe it again with the default values then please let us know and we will reopen it, ok? Thanks again for the help.
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