A (probably) daft question

[IO540]

Wow, what a complicated way of saying the oxygen levels decreases the higher you go

No, you are wrong.

Oxygen "levels" stay the same no matter how high you go. The % of o2 in the air remains more or less constant.

The need for leaning as one climbs is nothing to do with the atmosphere. It is (as I explained) due to the primitive way fuel and air are metered by current systems.

[S-Works]

No they don't. The partial pressure of oxygen reduces as the overall ambient pressure reduces. This gives an effective reduction in the percentage of oxygen available to burn.

Sea level 1bar 21% O2 79% Nitrogen Po2 is .21bar
18,000ft ambient pressure is .5 bar
Po2 is .1bar or 10%

[FullyFlapped]

No they don't. The partial pressure of oxygen reduces as the overall ambient pressure reduces. This gives an effective reduction in the percentage of oxygen available to burn.

Sea level 1bar 21% O2 79% Nitrogen Po2 is .21bar
18,000ft ambient pressure is .5 bar
Po2 is .1bar or 10%

Don't understand this, probably because I just don't understand the topic, but you seem to be saying that

0.21bar is 21% of 1bar, but 0.1bar is only 10% of 0.5bar

Do I have that right ?

FF

[BackPacker]

0.21bar is 21% of 1bar, but 0.1bar is only 10% of 0.5bar

0.1 bar (rounded) is 21% of 0.5 bar, I would say.

[IO540]

The law of partial gas pressures is nothing to do with why one needs to lean as one climbs.

[S-Works]

Really, pray to tell us why?

[IO540]

I have already explained why the carburretor / fuel servo does not automatically maintain the air/fuel ratio, thereby necessitating additional leaning, as one climbs.

I can't write it all in words of one syllable.

An engine doesn't give a damn about "partial pressure". All it cares about is how much mass of fuel and oxygen is gets fed to it.

[S-Works]

and you don't think that the gas laws have any effect on this?

if there was no effect from the gas laws then there would be no need to lean!

Partial pressure has a direct effect on mass.

[IO540]

You are getting an engine mixed up with a scuba diver, bose x

[S-Works]

god only knows what you have it mixed up with.......

[A and C]

Bose-x

According to your numbers the climb to FL120 costs you an extra 6 min flight time and saves 10lts of fuel over a 2 hour trip.
Using a typical UK fuel price (£1.40/ltr) the climb saves you £7/hour, this has to be balanced with the cost of 3 min of airframe & engine costs.

My guess is that if you are paying the UK price for fuel you will be better of by about £ 4/hour at FL120 but as soon as you uplift duty free fuel you would be better off at FL80.

This is of course all "still air" talk and a good look at the wind charts could change the whole picture.

[S-Works]

Well thats an interesting take on it A&C!!! But it does make my point that there is really quite a finite difference in this in reality. I guess for the big boys the numbers can start to add up.

You are quite right about the winds, the reality is that I choose the FL that gives me the most favorable routing based on the winds etc.

[frontlefthamster]

The Gypsy Queen engines in the DH Dove take care of all this for you...

Why Lycoming can't sort something similar out fifty years later is something only they can explain...

[llanfairpg]

Oxygen "levels" stay the same no matter how high you go. The % of o2 in the air remains more or less constant.

Yes but the weight of that same percentage is less, which reduces volumetric efficiency--try climbing Everest without oxygen