Mixture-Controls and Fuel/Air Ratio

This will probably sound stupid but, how did the mixture-control system maintain a given fuel/air ratio with changes in altitudes?

For your typical Allied Fighter, Bomber, etc. with a Pressure Carburetor , Pressure carburetor - Wikipedia

From a WW2 Vintage Comic book and some other sources..

See posts #3, #27 and #33

The last 3 pages of the MI flyers last article:


Edit: Some muppet (me) mis-attributed the preceding part of the article. Thanks for the correction MITasol.

I think you mean MI flyers article. I am flattered that you chose me but the glory is not mine
Mi

So, early systems did require mixture-ratio adjustments due to changes in altitude? Was that where the auto-rich/auto-lean matter came in?

Most aero engines have mixture adjustment to accommodate changes in mixture required due to change in altitude. The main issue is that engines require a range of ratios of mass of fuel to mass of air. Carb venturis work by feeding fuel into the air stream based on density and velocity. With changing altitude, both the mass of air and volume of air reduce however the rate of change is different between them. Thus the required mixture strength needs to change with altitude. [I'm not explaining this well, can someone jump in and help out.]

The auto-rich and auto-lean was implemented to provide a very simple method for the pilot to select an appropriate mixture for the current flight phase.

Auto-rich was for take-off, climb and combat. It provided a suitable mixture strength for high power operation and sufficient detonation margin. The downside was high fuel consumptions. Roughly best power on the figure below.

Auto-lean was for general cruising, as it gave a reduced bsfc for the power obtained. This resulted in a reasonable miles per gallon. The downside was high power settings were not available. Roughly best economy on the figure below.

The cams/capsules etc all worked to maintain the F/A ratio constant with altitude, so all the pilot had to do was select either auto-rich or auto-lean to suit the phase of flight.


From "The Aircraft Engine and its Operation" Pratt & Whitney.

The Lycoming document "Power from fuel flow" provide some insight into the relationship between F/A ratio and % power. Google will locate a copy.

Simon Thomas said:

Most aero engines have mixture adjustment to accommodate changes in mixture required due to change in altitude. The main issue is that engines require a range of ratios of mass of fuel to mass of air. Carb venturis work by feeding fuel into the air stream based on density and velocity. With changing altitude, both the mass of air and volume of air reduce however the rate of change is different between them. Thus the required mixture strength needs to change with altitude. [I'm not explaining this well, can someone jump in and help out.]

The auto-rich and auto-lean was implemented to provide a very simple method for the pilot to select an appropriate mixture for the current flight phase.

Auto-rich was for take-off, climb and combat. It provided a suitable mixture strength for high power operation and sufficient detonation margin. The downside was high fuel consumptions. Roughly best power on the figure below.

Click to expand...

So auto-rich/auto-lean wasn't about covering altitude but simplifying the amount of fuel/air ratio adjustments for the different conditions of flight?

Yes. Auto rich and lean worked with the altitude compensator to do that.

Auto rich and auto lean can be thought of typical average settings that the manufacturer decided were practical for all operations under a given set of circumstances. The advantage of them is that the pilot only has two options and can switch between them "instantly" rather than spending an extended period manually leaning and richening the mixture to get peak power or economy at altitude or the richest stable mixture during take-off and climb where the engine requires large volumes of fuel for cooling.

Altitude compensation was built in separately and operated in conjunction with auto rich and lean

Postwar, the appearance of torquemeters and fuel flowmeters allowed more accurate control of mixture. For instance, the 1962 C-131 flight manual says, "as automatic carburetor operation will not permit the attainment of maximum precision in the control of fuel flow and mixture strength, and since the AUTO LEAN mixture position is set to smooth engine operation rather than fuel economy, some method of manually leaning the cruise mixture is desirable."

At high power (1200+ hp from each R-2800) the procedure is to manually adjust the mixture lever to attain the fuel flow specified in the chart. At lower power you use the torquemeter. Slowly move the mixture lever, searching for the point of peak torque. That's "best power" mixture. Now lean the mixture until the torque reduces by the amount in the chart.

This method for attaining best economy was not restricted to transports and bombers. The Skyraider had a torquemeter (no flowmeter though) and you used a similar process to find best economy cruise.

Getting back to the C-131, that plane was a military version of the Convair 240 short haul airliner, one of which infamously crashed in 1977 with the band Lynyrd Skynyrd aboard after exhausting its fuel. It has been speculated that excessively rich fuel metering was a factor, though the investigation couldn't pin down anything definite. But if the plane had torquemeters and flowmeters like its military sibling, such a malfunction should have been evident to the pilots.