Which One to Use: Volumetric Content of Oxygen or Density Altitude?

[Kafin Noe’man]

So I Just downloaded this app called AirLab which is an app that help you to calculate the air density as well as density altitude.
Other than that, we can also get the volumetric content of oxygen.

I’m familiar with the concept (and have collected the data) that, in a clear & normal sunny day, as the day progresses:
- the temperature gets hotter ;
- air gets less dense ;
- humidity goes lower ;
- density altitude rises ;
- and the volumetric content of oxygen gets lower.
That’s why, often times, at noon we have to add nitro and/or turn needle in, more revs needed in-flight.

The data from my flying field is:
7 AM : 82 F ; Density Altitude 2150 ft ; 
12 PM : 94 F ; Density Altitude 2915 ft


However, I have a another case when I was flying during a cloudy and a damp day:
- the temperatue gets colder ;
- air gets more dense ;
- humidity rises,
- density altitude goes lower,
BUT the volumetric content of oxygen still goes down, so I had to turn needle in, more revs needed in-flight.

So, the question is, which parameter should we use to decide the mixture/needle setting as well as the nitro percentage?
I've read a lot about density altitude is the most used parameter, I think it is mostly true if we have a clear & normal sunny day. But if you have a damp day, I think that parameter won't work.

Since the mixture/needle setting and nitro percentage have to do with the air and its content of oxygen, why don't we use the volumetric content of oxygen as the main parameter?

Looking forward to learning more about it from you.


Best,
Kafin

[katana]

Drag racers use 'adjusted altitude' for all at track tuning decisions and have done for probably last 50 years - works for normal aspiration or forced. I would be surprised if the relatively course adjustment on a model engine could benefit knowing this information?

[spare_parts]

Ideally, you compensate for everything.

Drag racers also look at water in the air. Serious racers use sensors to measure actual oxygen percentage vs. density and humidity.

[Brett Buck]

However, I have a another case when I was flying during a cloudy and a damp day:
- the temperatue gets colder ;
- air gets more dense ;
- humidity rises,
- density altitude goes lower,
BUT the volumetric content of oxygen still goes down, so I had to turn needle in, more revs needed in-flight.

So, the question is, which parameter should we use to decide the mixture/needle setting as well as the nitro percentage?
I've read a lot about density altitude is the most used parameter, I think it is mostly true if we have a clear & normal sunny day. But if you have a damp day, I think that parameter won't work.

Since the mixture/needle setting and nitro percentage have to do with the air and its content of oxygen, why don't we use the volumetric content of oxygen as the main parameter?

  I would use volumetric oxygen. Most people are "converting" that to "density altitude" which is sort of a full-scale pilots hand-wavy approximation of a variety of things that I can assure you is insufficient for our purposes, if that was all you had. Humidity, as you noticed, has a wildly disproportionate effect that is not at all reflected in the "density altitude".

    But, I caution you that spending a lot of time measuring things is going to tend to get in the way of learning how to set the needle and interpret results. No matter what you use as a atmospheric measurement, you still need to know how to do it by ear and by feel, because no matter what it says, you have to use your own judgement. Under no circumstances should you be attempting to dial in the engine setting by strictly doing measurements and calculations and the dialing the needle in an out looking at a tach.

    I use an air density meter, pressure altitude measurements, etc, not to decide what to do, but to interpret the results. That's why I suggested the "one click at a time" test the other day, so you can begin to tell what effect it has on the line tension, the control response, etc.

     Brett

[Ken Culbertson]

But, I caution you that spending a lot of time measuring things is going to tend to get in the way of learning how to set the needle and interpret results.
     Brett

This entire sport at the flying level is giving the plane what it wants to let it do what you want.  Lap times are a measure of speed but the plane will tell you what they should be.  RPM's are just a measure but the plane will tell you what they should be.  Instead of using formula and equations to predict, take notes of how everything is set and remember how it sounds when it is right.  Don't try and deal in inches when the sport is measured in feet.

Ken

[M Spencer]

's a $ 95 one .

[Brett Buck]

This entire sport at the flying level is giving the plane what it wants to let it do what you want.  Lap times are a measure of speed but the plane will tell you what they should be.  RPM's are just a measure but the plane will tell you what they should be.  Instead of using formula and equations to predict, take notes of how everything is set and remember how it sounds when it is right.  Don't try and deal in inches when the sport is measured in feet.

     I am not quite on board with that, the sort of precision you need to successfuly compete at the highest levels is very exacting, and understanding what is going on is very important. But you generally need to develop these skills progressively from the simplest to the most sophisticated and you can't just jump to the end or bypass the process. We are about 2 weeks away from having a lengthy thread about how to set a needle *which was only 2 clicks away from ideal* when we started. I still have no idea how that turned out, that sort of thing needs to be mastered before you have a chance of understanding anything more sophisticated.

    Brett

[Ken Culbertson]

     I am not quite on board with that, the sort of precision you need to successfully compete at the highest levels is very exacting

Not quite on board is enough for me.   I agree with your conclusion but not necessarily with the path to get there.  There are, several separate skills needed to succeed in the sport.  I only wish I were expert in any of them, you have been successful in all of them.  I personally think you need to be advanced enough with your flying skills to be able to recognize the effect of the small and subtle changes that make the difference at the highest levels.  Mastery of the power you have is certainly one of the more important skills but is it the first one you should develop?  Personally, I think learning the pattern and how to fly in various conditions is first.  I am saying that after flying two patterns today that were about 3's on a declining scale of 1-10 where 10 was pure crap but I knew everything I did wrong, why and how to fix it.  If I were in Kafin's shoes I would be using the most well-behaved engine I could find, LA46 for example and flying something SV like that is properly trimmed till the wings fell off.  When I could fly a respectable intermediate pattern then, and only then, start fine tuning.  I have seen too many good fliers get sidetracked by messing with engines and never learning to fly a plane when the engine run is not perfect. 

"Just me - and I could be wrong"

 Ken

[Steve Dwyer]

An informative article explaining density to oxygen relationship. Certainly, confirms our thinking that RH does affect oxygen content and engine performance.

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwiyovjwluOIAxXMF1kFHVXRMvwQFnoECBoQAw&url=https%3A%2F%2Fiflycoast.com%2Funderstanding-air-density-and-its-effects%2F%23%3A~%3Atext%3DAs%2520air%2520pressure%2520decreases%2520oxygen%2Clower%2520than%2520at%2520sea%2520level.&usg=AOvVaw1gVHQ00IMWjlQR4lUNcLol&opi=89978449

[Colin McRae]

This thread started out specific to an OS 46LA engine.

For relative newcomers to this hobby (like me), I first started out reading the OS owner's manual for the LA-s series engines. Does anyone even read manuals these days?

The OS manual has great instructions on the recommended method to set the needle for a good engine run on the stock engine. Very simple. Find peak rpm, then initially open the needle 20-30 degrees or so just until you notice the rpm drop off a bit. Then fly the model. Once I have had the model in the air, as Brett has sad, I may have tweaked the needle a click or 2 either way to find my desired optimum setting. Then I document the approximate needle setting and ground rpm for future reference.

I have used this method on 15 thru 46 LA engines (both silver and blue) and all my LA engines have run great and consistent. And I have run muffler pressure w/ uniflow tanks in all cases. But a good engine run is always predicated by having proper fuel, a good fuel delivery system, a healthy glow plug, no fuel system leaks, etc.

[Brett Buck]

This thread started out specific to an OS 46LA engine.

For relative newcomers to this hobby (like me), I first started out reading the OS owner's manual for the LA-s series engines. Does anyone even read manuals these days?

   Part of the issue is that if they set it that way, in many cases, they end up far too fast. That's because they pick the wrong size engine and the wrong prop. If you use the recommended 46LA prop, I think it is a 10-6 or 11-6, and you put it in a Nobler and set it per the instructions, you are going 3.8 second laps. This is the fallacy of "you can never have too much power", the power a function of the cube of the speed, period, by the definition of power (the *actual* definition, not the stunt definition of "power") These engines are FAR MORE POWERFUL for their displacement than vintage engines, and they run completely differently.

     These engines are generally used on airplanes far too small to absorb this power, so they end up absurdly too fast. The immediate reaction is to try to slow it down with the needle, so there goes your theory, they deviate from the instructions in the first 30 seconds. OR, they have already pre-destroyed the engine by modifying it for an erzatz spraybar and also made it nearly impossible to restore, with unknown results. Enter the "reworker", that is, every other intermediate flier with a Dremel tool and an article from 1979 about "how to tame schneurle engines for stunt". 

You saw the results over the weekend.

    The bottom line is that these engines work very well *if you leave them alone* and you do not put them on airplanes *that are too small for the enormous power". Unfortunately that is a rare convergence of coincidences.

    Brett

[Steve Helmick]

The data from my flying field is:
7 AM : 82 F ; Density Altitude 2150 ft ; 
12 PM : 94 F ; Density Altitude 2915 ft

Best,
Kafin

What's the actual altitude of your flying site?

This is a bit off topic, but still related. Recently had a conversation with an Expert flier who normally flies at almost 3,000 feet in a hot & dry location. When he came to a contest here, where we're flying at maybe 50' feet above sea level, cool and fairly humid, his pullouts were naturally too high and he had to work at getting them down to 5'. I found the same thing years ago, when I flew on "Little (dry) Lake" at 3,130' altitude, just off Hwy. 395, only my pullouts tended to be really good, while they were usually a bit high at home at 50'. Similar heat, low humidity and altitude to where our guest Expert flier lives. In my case, I narrowed the handle spacing 1/4". I'm wondering if our Expert guest widened the handle spacing. I'll ask.  Steve

[Kafin Noe’man]

Well I started this thread because I felt something was off when I went flying earlier this week without making any change from my last good session a week before. Everything was in the same setting

On my first couple of flights I felt my engine ‘lost’ its power and lap times got slower than the one I had had before.

Just before my third flight, I tached my engine and the RPM was about the same to the ones from my last good session, so then I decided to  try turning the needle in and everything seemed to get better.

I had only known that colder temperature / morning = more oxygen = move needle out ; hotter temperature = less oxygen = turn needle in. So my action that day, turned the needle in, on a colder temperature, got me thinking about someting else beyond the temprature alone.

That’s when I finally read and learn more about density altitude. At glance the basic idea is lower altitude = move needle out ; higher altitude = turn needle in. But this concept didn’t tally with my experience during that cloudy and damp day.

Thus, I started looking for another parameter that matches to and can explain what I had experienced which is the volumetric content of oxygen. The calculation shows when the humidity rises, the volumetric content of oxygen drops pretty significantly compared to when the day is clear, bright, and sunny. This parameter can also tell me why I actually don’t have to set the needle every single time before flying on a clear day at my flying field. The volumetric oxygen is pretty much consistent throughout the day.

And yes, I have learned my lesson, don’t get too obsessed with calculating & specific target number. But these experiment & calculation I did really help me to understand more about the basic concept of it so I can do troubleshooting faster and make more progress ahead.


Best,
Kafin

[Paul Taylor]

This is an app I used when I ran nitro. I’m mostly electric now. It’s just a simple air density app in the iTunes Store. Called Air Density &RAD meter.
It will show HP with the air density so you will kind of have an idea if you need to adjust the needle. But like others suggested, get use to tuning by how the engine sounds, feels on the lines and keep it simple.