When learning to fly, you get a brief physics lesson about the four forces involved in flight: weight, lift, drag and thrust. Three of the four of these are pretty easy to grasp just through common sense and everyday experiences. The one that isn't as obvious is lift, and that's where I started dissecting my understanding of how air density affects both lift and engine performance.
In 1738, Daniel Bernoulli wrote a manuscript called Hydrodynamica to describe the motion of fluids. Air behaves like a fluid, and if you ask a pilot why an aircraft flies, you're likely to hear "Bernoulli's principle" as part of the answer. The principle boils down to this: as the velocity of a liquid increases, its internal pressure decreases. Suppose air is moving through a tube, like the hose attachment of a vacuum machine. Air is being sucked through that hose at a certain velocity. I'm starting with the assumption that the air is moving at the same velocity when it enters the hose from the floor as when it exits the hose into the vacuum machine, which is valid for a steady-state fluid dynamics discussion: the principle of continuity demands that the rate at which mass enters the system equal the rate at which mass exits the system. Bernoulli's principle says that if you compress the middle of the hose, creating a venturi, the air will move faster through that section; for the same number of molecules to move out the other side in a given time frame when fewer can pass through the venturi at a time, they'll have to move faster to maintain overall throughput. (More questions? Check out the venturi effect on Wikipedia. For curious housewives, this might get you excited about your Dyson, which exploits venturis and the Bernoulli principle!)
Ok, good, I can accept that. But how does that lead to pressure decreasing? The law of conservation of mechanical energy says you can't create or destroy energy, so if you're increasing kinetic energy by speeding up the molecules, something else has to give, and in this case it's the pressure within the venturi. That's the book reason, but it kinda makes my brain spin. Once I see that the velocity changes, I can reason that there will be fewer molecules per unit of volume and so therefore there's less pressure in that area of the system. I can reason that this creates a pressure gradient where molecules from the higher pressure (lower velocity) area want to flow into the lower pressure (higher velocity) area. I can reason that this results in the vacuum process of sucking matter from one area to another. This last bit will help later...
So, air enters the tube at a certain rate and pressure, encounters a venturi and speeds up while decreasing pressure, then exits the venturi into an area that matches the initial rate and pressure.
From experience, I know that if the vacuum's hose is narrowed too much, mostly or fully blocked by something a young boy dropped at breakfast, the machine makes an awful noise and the suction stops. At this point, the system is no longer in a steady state and all bets are off. Fluid dynamics calls this the choke point. Theory aside, that suggests there's a practical limit to the narrowing in the venturi to maintain effective flow.
Let's try to tie this in to air density. The area of lower pressure inside the venturi is an area of lower density, right? If my thinking above is correct, then this must be correct, too. Well, it seems that for incompressible fluids (like water), this is not true, but that as long as the air is moving slower than the speed of sound, we're all good!
How does this apply to the force of lift? Lift is essentially the airplane being lifted into an area of lower air density, which is created by moving air faster over the wing than under. You can get A LOT more complicated in exploring all of the various aspects of air flow over a wing, but the bottom line is that when the wing slices through the air, the windstream above the wing has a higher velocity than the windstream below the wing. You might achieve this higher velocity by changing the cross-section of the wing or by changing the angle of attack of the wing. Air moving farther over the wing means it moves faster over the wing, which means there's lower pressure over the wing, which makes it easier for the wing to enter that space. I don't know if it's accurate, but that whole bit about the vacuum process from matter wanting to migrate to an area of lower pressure gives me an image of the wing being sucked upwards.
Another big part of the discussion is the fact that lower pressure means lower temperature. Carbureted engines that use a venturi-type carburetor are subject to carb icing and will have a carb heat switch in the cockpit. To oversimplify, the fuel is drawn into the low-pressure flow through the venturi, resulting in the air-fuel mixture that will be sent to the cylinders for combustion. The venturi is at a lower temperature on account of the lower pressure*. Even when the air temperature is significantly above freezing, the venturi could become lined and eventually blocked by ice that forms when humid air enters the venturi and experiences a sudden temperature drop. The expected observations are a drop in RPMs, roughness, amd eventually fuel starvation. Why? Lower RPMs means less fuel is being delivered. Is less fuel delivered because less is being sucked into the venturi because air flow has been reduced and pressure is higher? Also, the throttle valve may become iced in place. The fix to all of these problems seems simple: Use carb heat to draw air heated by the engine exhaust through the carburetor instead of from outside, warming the carburetor, melting the ice and reopening the venturi and valve.
When you turn on carb heat, air density resurfaces for examination: Hot air is less dense, so now less dense air is being mixed with same-density fuel in the carburetor, meaning -- if fuel flow is the same -- a richer mixture going to the engine. An overly rich mixture can cause sparkplug fouling and carbon buildup in the cylinders. However, does the less dense air (lower pressure) also lead to higher fuel flow into the venturi, meaning an even richer mixture? Or perhaps the path for warm air to enter the carburetor is set up for slower intake, so higher pressure maintains more consistent fuel flow? Or what about the old chemistry triangle demonstrating that constant volume with higher temperature yields higher pressure? Perhaps the more energetic hot-air molecules are providing higher pressure, keeping the fuel flow in check. Oh, and the process of vaporizing the fuel (*) as it enters the carburetor results in a further temperature drop (which is exploited for the power of good when the engine is overheating), exacerbating the icing situation. Brain in loops.
That's where I stand right now with processing air density, how to manipulate it, and what its effects on lift and engine performance are.
Let's introduce altitude into the mix!
Preflighting the aircraft includes setting the altimeter, which means checking the current air pressure at your location. On the ground here at KUZA (667 MSL) the pressure is currently 29.94" Hg. KJGG is 29.98" Hg. KDEN is 29.80" Hg. KDET is 30.02" Hg. Standard air pressure is 29.92" Hg. Why? Why for any of that? Let's reason it out. Sea level is the closest thing we can have to a constant altitude around the globe. To ignore tides and such, we talk about mean sea level (MSL) as the zero-point for the altitude scale. We know that the air gets thinner as your altitude gets higher; like when coastal athletes struggle in high-altitude competitions, or when mountain climbers get altitude sickness. So air density (air pressure) is highest at sea level and decreases with altitude. Other factors, like weather, can affect air pressure resulting in airports at the same altitude having different barometric readings.
What happens if you're at high altitude where the air density is lower to start with? For engine performance, we already know (I think) that less dense air gives us a richer fuel mixture, so that's why aircraft manuals and instructors tell us to lean the mixture after climbing to our cruise altitude. This means for the same throttle setting, we're providing less combustible fuel to the engine. At cruise, throttle is how we maintain altitude; more throttle = climb, less throttle = descend. If throttle is constant and the mixture is leaned, shouldn't that cause RPMs to decrease? If they do, but altitude stays the same, does that mean that less lift is needed to maintain altitude when air density is lower?
Thinking it through another way.... At constant mixture, more throttle = more fuel to engine = higher RPMs = more air moving over wing = more lift. Full throttle for takeoff. At constant throttle, leaner mixture = less fuel to engine = lower RPMs = less air moving over wing = less lift. Lean/cutoff to stop the prop. I just can't remember or quite get through the logic to get the how air density fits into this. At sea level, you have lots of air molecules to move over the wing, so you gotta move them fast to create the area of low pressure that gives you lift. At high altitude, you have fewer air molecules, so, what? The whole point is the pressure differential between the airstreams above and below the wing, and now I'm lost.
To think about later: when you hop in the airplane and dial in the current barometer, the altimeter is adjusted. Ideally, the correct current baro yields the actual altitude for your current location! And if you fiddle with the dial until your current altitude is shown, then the baro ought to match the current pressure observation. But why? Let's say the altimeter manufacturer calibrates the altimeter for 0 MSL at 29.92" Hg. I hop into the plane at KUZA and dial in 29.94" Hg today and 30.01" Hg tomorrow. How does the altimeter come up with 667' in both conditions? How does the static port play into this?
