Altitude: how it affects fuel/air ratio

[Robbins]

1.
Pressure differential across venturi will remain the same.

2.
Pressure available to push fuel through the needles is also reduced proportionally to the pressure (hence your column of air). However, if we assume flow of a liquid through an orifice, that is on the square root of the pressure change. Therefore there continues to be an increase (more fuel to air) in stoichiometric ratio, though much smaller.

3.
Another issue to consider - a portion of the fuel added is used for cooling. Going to higher altitude (less power) produces less heat. Thereby the small component of fuel used for cooling (a portion of which is not needed) adds to the "richness" experienced. Could one compensate for this by achieving better cooling to reduce the need to use excess fuel for cooling?

1: I agree, as it is dependent on only engine rpm

2: I agree under technicality. The fuel is pulled at the mixture needles by the negative pressure in the Venturi. The only push is the force delivered by power/transfer stroke applied to case side of diaphragm. Of course one could technically say that the needle spring pushes the diaphragm back to accept the next pulse, but atmospheric influence here is quite negligible.

3: I'm a little lost with you on this one. I do not disagree, however, I say a properly tuned engine will utilize oil, not gasoline to keep temps in check. Of course a sloppy rich engine will run cooler.

As far as your last statement... Of course! Enter baffling and water cooling

Hi guys
I'm flying here in Israel from about -1200' to +3000' and you can feel that there is a different...

what engines do you commonly run between those locations, Taz

 

[Naughty Monkey]

Monkey's Uncle?


BJ Swope

What about my uncle?

 

[TazmanianDevil]

what engines do you commonly run between those locations, Taz

DA 150. DA 50. MVVS 175.

 

[Naughty Monkey]

What's up Robbins?
At cornhuck, Joe and I were surprised at how little difference there was coming from 4500'. And yes, there is a reason it is called a metering diaphragm...

 

[Robbins]

What's up Robbins?
At cornhuck, Joe and I were surprised at how little difference there was coming from 4500'. And yes, there is a reason it is called a metering diaphragm...

Yo @Naughty Monkey wazzzzzza?????
Good to hear you chime in. Hope all is going well on your side of the force. The force is strong with naughty monkey!

I am not grasping the context of the comment you made on the metering diaphragm. Please elaborate so I can get on the same page, paragraph, and sentence. Hell, let's go same phrase.

 

[Pistolera]

Also, remember that density altitude greatly affects the pull your prop has as well as the lift generated by your wings. There are tons of DA calculators on-line, but as an example......at sea level on an 80 degree day in high humidity the plane and engine can be operating as though they're flying at around 1800 feet above sea level (MSL). Same conditions at 3000' MSL and it's flying in air equivalent to around 5400'.

So....your engine is not producing as much power (even if it is tuned properly), the prop generates less thrust, and the wings generate less lift!

 

[Robbins]

Also, remember that density altitude greatly affects the pull your prop has as well as the lift generated by your wings. There are tons of DA calculators on-line, but as an example......at sea level on an 80 degree day in high humidity the plane and engine can be operating as though they're flying at around 1800 feet above sea level (MSL). Same conditions at 3000' MSL and it's flying in air equivalent to around 5400'.

So....your engine is not producing as much power (even if it is tuned properly), the prop generates less thrust, and the wings generate less lift!

@Pistolera, I LOVE your avitar!

Absolutely why i converted alkyl's units to oz/in^2 , its just easier to understand because of our familiarity with wing loading

and to expand on the great points you presented, you can definitely feel the aircraft fly like it is Heavier. i dont know how else to explain it.

I dont often change props on a given airframe but i do prefer keep a spare prop with a lower PLF (and change to it) for those nasty days.

Pressure available to push fuel through the needles is also reduced proportionally to the pressure (hence your column of air).

I stand corrected as I was not considering pop off pressure.
I was interpreting your comment in relation to jetting.

where the heck is @49dimes

i think he could add some very good information to this discussion.

 

[Naughty Monkey]

Yo @Naughty Monkey wazzzzzza?????
Good to hear you chime in. Hope all is going well on your side of the force. The force is strong with naughty monkey!

I am not grasping the context of the comment you made on the metering diaphragm. Please elaborate so I can get on the same page, paragraph, and sentence. Hell, let's go same phrase.

It does exactly what it's name implies, meters the fuel in the carb. Now, how well it does this is up for debate. When you hear about an engine bogging down in knife edge, this is the culprit many times. If the air pressure over the cover plate changes, it changes the fuel rate in the carb. 3w had their snorkel that the port went into. Theory was then the metering diaphragm would always be seeing the air pressure at the carb opening. on one motor I was having problems with, for the heluvit I soldered on a tube up to the top of the carb. The engine did run and transition as good as any I have had.Might just be co-inkidink though.

 

[Alky6]

1: I agree, as it is dependent on only engine rpm

3: I'm a little lost with you on this one. I do not disagree, however, I say a properly tuned engine will utilize oil, not gasoline to keep temps in check. Of course a sloppy rich engine will run cooler.

You are right in line with this (sloppy rich, of sorts). IMHO and my observations:

I have concluded that when an engine is on the ragged-edge it will detonate or pre-ignite. Consider first an alky engine. Methanol has a very wide usable stoichiometric ratio (SR). Methanol also has a very high latent heat of vaporization. I assume we can agree that the higher the heat inside the engine cylinder, the more likely detonation, or pre-ignition (methanol has a high tendency to pre-ignite) the result is the engine fades, cuts out, or worse yet - burns a hole in the piston or sticks a piston. This high heat also leads to complete burning of the fuel and oil. Our old slimers would rarely burn or stick a piston.

Oil aids in cooling, but with methanol’s high latent heat of vaporization, the more fuel added, the cooler the engine’s cylinder becomes (and as a side note the more air that can be brought through the carb due to this cooling – hence an approx. 10% power increase simply by using methanol). There are other cool factors but not worth going into for this discussion. This extra fuel and cooling effect helps dampen the detonation factor, but is hot enough that you still get a complete burn even on the richer side of the SR. The problem is in the intermediate areas of the power curve where you don’t get complete burn and you are on the rich side. So, one leans the jets to help with the mid-range richness (where the cooling may not be needed) and bam, stuck piston. Some get stuck in this routine of constantly fiddling with their jets.

Now consider a de-tuned engine, not on the ragged edge of burning (from the inside) airplane engine, instead, it is on the ragged edge from not enough cooling from the outside. Further consider gas vs. methanol with a much smaller usable SR and lower latent heat of vaporization than methanol. You are now playing the same game by being on both sides of the of SR but it is much narrower and more temperamental band. You are really rich in the midrange and really “lean” (or threatening detonation) at the top. Again, due to heat problems. One small atmospheric change and you’re fiddling with the jets again.

From my personal observations with our airplane engines this occurs when you don’t have adequate cooling. The engine becomes very difficult to jet and run consistently throughout the rpm range and is usually exacerbated on hot days. One could conclude a properly tuned and cooled model airplane engine (not a full-race version) is one that would most likely require minimal adjustments, if any, due to atmospheric conditions.

 

[lowdown]

Anyone need a beer while Iam up?