The pitot tube generally hangs from the wing with the opening forward and positioned forward of the area where air becomes displaced by the wing. This device senses pitot (or ram) pressure and is used by the airspeed indicator. The pitot tube has a drain at the back to release any moisture from the air that may otherwise accumulate and affect the readings. Where there's moisture, there's the potential for ice, and so the pitot tube is equipped with a heater that can be activated from the cockpit. How do you know when there's ice (or another blockage), though? That depends...
If the pitot intake is blocked, but the drain is still open, then the air already in the system will have a path to escape, resulting in a gradual reduction in pressure. Lower pressure is usually a sign of less air being forced into the pitot tube, so the airspeed indicator will show a slower speed.
If the pitot intake and drain are both blocked, or if there's a block between the pitot tube and the instruments, then the air in the system is trapped and the pressure will remain constant. With no changes in altitude or atmospheric pressure, the airspeed indicator would remain unchanged, and so the pilot may not suspect a problem.
A problem will become evident, however, as altitude changes. Generally, airspeed decreases during a climb and increases during descent. The airspeed indicator uses not only the ram pressure but also the static (ambient) pressure to determine airspeed; basically, the static pressure fills an area of the instrument, and the ram pressure tries to inflate something within the area -- less inflation means slower speed. (Incidentally, an airplane with higher groundspeed at a higher altitude can show the same indicated airspeed as an airplane with lower groundspeed at lower altitude, thanks to air density.) So imagine that the airplane is indicating 140 knots when the static pressure increases, say upon descent. Increased static pressure with constant ram pressure will cause the device to deflate a bit, which will result in lower indicated airspeed, which is typical of a climb. This observation should raise a red flag.
The static pressure is sampled by a static port, mounted usually along the fuselage and in such a way that it just senses the ambient air pressure. Air pressure is measured in inches of mercury (" Hg), standard of 29.92" Hg at 0 MSL, and broadcast as part of the METAR for local airfields. As part of preflight, you hop in the plane, tune in the local AWOS or ATIS, and dial the current barometric reading into the altimeter. In fact, they announce it as "altimeter 3-0-0-1." If all goes well, adjusting the barometer will result in an altimeter reading that matches the airport's altitude above sea level.
When the static port gets clogged, that affects the airspeed indicator, the altimeter and the vertical speed indicator (VSI). The altimeter will just get stuck since the pressure is trapped inside. The VSI will get stuck at 0 since it registers changes, and no changes will occur. The airspeed indicator, though, will give incorrect measurements. Typically the static pressure and ram pressure are "in sync," so to speak -- high pressure means high air density in the chamber and lots of air molecules funneling through the pitot tube into the inflatable portion of the airspeed indicator; low pressure means low air density in the chamber and fewer air molecules to be rammed. When the static port is blocked, the static pressure is held constant regardless of outside air pressure, so it's no longer coordinated with the ram pressure. Say it gets blocked at 3000' (that is, at a specific air pressure). On a normal climb from here, the ram pressure would be decreasing and so would the static pressure, but now the ram pressure will decrease but the static pressure will not, resulting in slower than usual indicated airspeed readings. On a normal descent from 3000', the ram pressure would increase and so would the static pressure, but in this case only the ram pressure will increase and the airspeed indicator will read faster than expected.
Those are considerations when there's a malfunction of some sort. Pilots also need to consider what happens just when moving from region to region when the air pressure changes. The saying goes "High to low, look out below." Let's dissect that. Your altimeter is set for a barometer of 30.01" Hg as Charlotte Approach just gave you. In the vicinity of Charlotte, you'd expect your altimeter to be fairly accurate. Flying at 2600', you head west toward the mountains, where the pressure is dropping. The standard conversion is 1" of mercury for every 1000' of altitude. If you don't adjust your altimeter and try to maintain your 2600' of indicated altitude as you fly into an area with a pressure of just 29.01" Hg, your true altitude is now just 1600' -- look out below! As the terrain comes up, you really don't want to be going down. Lower pressure means less dense, which under standard conditions means higher altitude; under non-standard conditions it means update your altimeter!
That's it for today. I started off today with Rod Machado's Private Pilot Handbook. It is so dreadfully corny, and way too distracting for me to use as a serious refresher aid. Sigh. I like my old Guided Flight Discovery textbook, but in places it's just so flat. I also did an experiment today and decided to study from home instead of at the library. That just doesn't work! My focus was definitely less honed. The dishwasher needs unloading, the laundry needs folding, there's a bug on the window, a lizard just crawled up the deck railing, the cat wants to be petted, there's that new "green monster" smoothie recipe I was thinking of trying, the Keurig needs more water, .... Back to the library on Wednesday. With one of these tasty new smoothies.
Monday, September 30, 2013
Friday, September 27, 2013
Moving on to practical matters...
Okay, so I want to understand precisely how things work. I want the theory. I want the knowledge. Now is not the time for that, however.
Now is the time for a practical understanding that will facilitate safe flight. Now is the time for wisdom and explanations that are good enough to build upon with experience.
There will be time for study of complex circulation and academic examination of all things later.
Right now I want to become proficient again, and working through the flight manuals is the path.
Okay. Whew.
Moving right along into engine and electrical systems this morning....
The typical normally aspirated general aviation airplane uses a four-stroke combustion engine. Mixture is pulled into the cylinder on the intake stroke (1), it's compressed (2), it's ignited by the sparkplug and expands rapidly (combustion - 3), and the burned gases are released (exhaust - 4). The throttle controls the amount of the mixture that is sent to the engine; more mixture means more fuel means more combustion means faster strokes means faster turns of the crankshaft (higher RPMs) means faster spinning propeller.
Briefly tying the engine in to general flight with the lift discussion from Wednesday, the airplane is configured for different types of flight using two main criteria: Attitude (angle of attack) and power (throttle). The angle of attack is the angle at which the wing meets the air, and this is the criteria that determines the area of the wing that is producing lift; higher angle of attack means more lift (until you reach the stall point). When climbing during and after takeoff, the nose is high, the angle of attack is high, the throttle is full forward for maximum power, and lots of lift is being generated. At level flight, the angle of attack varies with the power setting. In slow level flight, the angle of attack is very high, and the attitude is nose-high ("hanging from the prop"); at a fast cruise, the angle of attack is low and the attitude is closer to nose-level ("slicing through the air"). When descending to land, power is typically lowered, attitude can be nose-level or -low, and the angle of attack is very low. Gliders are non-powered, yet manage their entire flight based on attention to and planning for angle of attack in various phases.
When the plane is set in a landing configuration, the flaps are extended to change the wing's angle of attack, allowing a shallower attitude at low power. We'll save that discussion for later.
The propeller's blades are little wings, basically, that have their own angle of attack relative to the air as they spin. This "pulls" the airplane through the air, or produces thrust.
Engines need fuel (100LL typically for us GA folks). Fuel tanks in a 172, the classic training platform, are mounted in the high wings and use a gravity to feed the engine, compared to a low-wing aircraft like a Cirrus that requires a fuel pump to deliver fuel up to the engine. There may be a switch in the cockpit for left, right, both or neither fuel tank.
Let's think about the engine in a practical sense. When everything is working properly and the aircraft is being flown in a nice, safe, "standard" environment, everything is just chipper. But life isn't like that, right? We have to be on the lookout for unusual symptoms so we can cut off any developing trouble before it becomes critical.
The three main problems that crop up with an engine are loss of power, roughness or overheating. It is important to know why these might happen, what the consequences are, and, most importantly, what to do to fix them.
Loss of power means the RPMs have dropped, which means the propellor has slowed, which means thrust and lift have dropped as well. Engine roughness means the RPMs may be inconsistent, there may be "backfires," the vibrations might feel unusual. Why might the power drop or vary without a change in throttle? The four-stroke cycle isn't working properly. But why? Perhaps...
- the mixture is too lean (not enough fuel to burn). Adjust the mixture to be richer. Apply carb heat if carb ice is suspected. Switch tanks in case of a blocked fuel line. Turn on boost pump in case of a failed fuel pump.
- moisture in the fuel. Hopefully a thorough preflight included testing the fuel at all sump points and draining of any evident moisture, and so there's a very limited bit of moisture to endure; stay sharp for further problems and land if it doesn't resolve quickly.
- ignition isn't working right (fuel isn't getting burned completely). Check the ammeter for load. Land and check for fouled sparkplugs or loose wires. With great caution, a magneto test could be performed in flight to confirm.
- detonation may be occurring (fuel is being burned at the wrong part of the cycle). This happens when junk has built up within the cylinder that ignites before the spark is provided. It can be a sign of engine wear or flight under taxing conditions (too long at too rich a mixture).
- engine wear (insufficient lubrication). Check oil pressure and temperature.
These are situations, explanations and corrective measures discussed in the book for general background. The POH and aircraft manual will provide checklists for abnormal and emergency procedures that should be used when symptoms manifest.
Overheating leads to engine damage. The engine is cooled by air (externally) and by oil (internally). A nose-high attitude typically limits the airflow through the cowling and baffles and over the engine. Lowering the attitude (climbing more slowly, or descending) will help by increasing airflow. High power settings lead to higher oil temperatures The oil system is diagnosed using two measures: temperature and pressure. Pressure will rise to normal operating range soon after startup, though may take a little longer in cold weather. Pressure that's too low may indicate a leak; pressure that's too high may indicate a blockage. Temperature rises more gradually, and typically too-high temperature is the only concern, which could indicate low oil levels or an overheating engine. Part of the oil circulation includes routing through an area that allows heat to bleed off before re-entering the engine. And of course during preflight you checked to see that the air intakes and exhaust were clear, that the oil level was good, and that there were no obvious leaks.
Back momentarily to the electrical system. Electricity is supplied from the battery and from the alternator (when the engine is running). The magnetos use the battery to provide the initial spark when starting the engine, then the alternator is used. The ammeter measures the power level on the battery, and often there's a second ammeter to measure the alternator output. In either case, the needle swinging left means there's load; swinging far left should be cause to lessen the load by turning some things off. The airplane will have a circuit breaker panel; if something electrical isn't working, check there and pop the breaker back in. If it fails again, leave it off and assess the safety of continuing the flight without that equipment. The other big problem might be an electrical fire. Turn everything off and land. You could potentially isolate the problem area and leave that off so as to make use of the remaining equipment, but you'd have to weigh the time and safety of this procedure against the big picture (I need to check a POH to refresh myself on the emergency checklist for engine fire).
That's good for today. Next up, flight instruments and the pitot-static system! I'm eager for that one!
Now is the time for a practical understanding that will facilitate safe flight. Now is the time for wisdom and explanations that are good enough to build upon with experience.
There will be time for study of complex circulation and academic examination of all things later.
Right now I want to become proficient again, and working through the flight manuals is the path.
Okay. Whew.
Moving right along into engine and electrical systems this morning....
The typical normally aspirated general aviation airplane uses a four-stroke combustion engine. Mixture is pulled into the cylinder on the intake stroke (1), it's compressed (2), it's ignited by the sparkplug and expands rapidly (combustion - 3), and the burned gases are released (exhaust - 4). The throttle controls the amount of the mixture that is sent to the engine; more mixture means more fuel means more combustion means faster strokes means faster turns of the crankshaft (higher RPMs) means faster spinning propeller.
Briefly tying the engine in to general flight with the lift discussion from Wednesday, the airplane is configured for different types of flight using two main criteria: Attitude (angle of attack) and power (throttle). The angle of attack is the angle at which the wing meets the air, and this is the criteria that determines the area of the wing that is producing lift; higher angle of attack means more lift (until you reach the stall point). When climbing during and after takeoff, the nose is high, the angle of attack is high, the throttle is full forward for maximum power, and lots of lift is being generated. At level flight, the angle of attack varies with the power setting. In slow level flight, the angle of attack is very high, and the attitude is nose-high ("hanging from the prop"); at a fast cruise, the angle of attack is low and the attitude is closer to nose-level ("slicing through the air"). When descending to land, power is typically lowered, attitude can be nose-level or -low, and the angle of attack is very low. Gliders are non-powered, yet manage their entire flight based on attention to and planning for angle of attack in various phases.
When the plane is set in a landing configuration, the flaps are extended to change the wing's angle of attack, allowing a shallower attitude at low power. We'll save that discussion for later.
The propeller's blades are little wings, basically, that have their own angle of attack relative to the air as they spin. This "pulls" the airplane through the air, or produces thrust.
Engines need fuel (100LL typically for us GA folks). Fuel tanks in a 172, the classic training platform, are mounted in the high wings and use a gravity to feed the engine, compared to a low-wing aircraft like a Cirrus that requires a fuel pump to deliver fuel up to the engine. There may be a switch in the cockpit for left, right, both or neither fuel tank.
Let's think about the engine in a practical sense. When everything is working properly and the aircraft is being flown in a nice, safe, "standard" environment, everything is just chipper. But life isn't like that, right? We have to be on the lookout for unusual symptoms so we can cut off any developing trouble before it becomes critical.
The three main problems that crop up with an engine are loss of power, roughness or overheating. It is important to know why these might happen, what the consequences are, and, most importantly, what to do to fix them.
Loss of power means the RPMs have dropped, which means the propellor has slowed, which means thrust and lift have dropped as well. Engine roughness means the RPMs may be inconsistent, there may be "backfires," the vibrations might feel unusual. Why might the power drop or vary without a change in throttle? The four-stroke cycle isn't working properly. But why? Perhaps...
- the mixture is too lean (not enough fuel to burn). Adjust the mixture to be richer. Apply carb heat if carb ice is suspected. Switch tanks in case of a blocked fuel line. Turn on boost pump in case of a failed fuel pump.
- moisture in the fuel. Hopefully a thorough preflight included testing the fuel at all sump points and draining of any evident moisture, and so there's a very limited bit of moisture to endure; stay sharp for further problems and land if it doesn't resolve quickly.
- ignition isn't working right (fuel isn't getting burned completely). Check the ammeter for load. Land and check for fouled sparkplugs or loose wires. With great caution, a magneto test could be performed in flight to confirm.
- detonation may be occurring (fuel is being burned at the wrong part of the cycle). This happens when junk has built up within the cylinder that ignites before the spark is provided. It can be a sign of engine wear or flight under taxing conditions (too long at too rich a mixture).
- engine wear (insufficient lubrication). Check oil pressure and temperature.
These are situations, explanations and corrective measures discussed in the book for general background. The POH and aircraft manual will provide checklists for abnormal and emergency procedures that should be used when symptoms manifest.
Overheating leads to engine damage. The engine is cooled by air (externally) and by oil (internally). A nose-high attitude typically limits the airflow through the cowling and baffles and over the engine. Lowering the attitude (climbing more slowly, or descending) will help by increasing airflow. High power settings lead to higher oil temperatures The oil system is diagnosed using two measures: temperature and pressure. Pressure will rise to normal operating range soon after startup, though may take a little longer in cold weather. Pressure that's too low may indicate a leak; pressure that's too high may indicate a blockage. Temperature rises more gradually, and typically too-high temperature is the only concern, which could indicate low oil levels or an overheating engine. Part of the oil circulation includes routing through an area that allows heat to bleed off before re-entering the engine. And of course during preflight you checked to see that the air intakes and exhaust were clear, that the oil level was good, and that there were no obvious leaks.
Back momentarily to the electrical system. Electricity is supplied from the battery and from the alternator (when the engine is running). The magnetos use the battery to provide the initial spark when starting the engine, then the alternator is used. The ammeter measures the power level on the battery, and often there's a second ammeter to measure the alternator output. In either case, the needle swinging left means there's load; swinging far left should be cause to lessen the load by turning some things off. The airplane will have a circuit breaker panel; if something electrical isn't working, check there and pop the breaker back in. If it fails again, leave it off and assess the safety of continuing the flight without that equipment. The other big problem might be an electrical fire. Turn everything off and land. You could potentially isolate the problem area and leave that off so as to make use of the remaining equipment, but you'd have to weigh the time and safety of this procedure against the big picture (I need to check a POH to refresh myself on the emergency checklist for engine fire).
That's good for today. Next up, flight instruments and the pitot-static system! I'm eager for that one!
Thursday, September 26, 2013
The next book...
Are you a reader? A book reader? There's always a list of books to be read when time allows, or when this book is closed, or when the kids eventually go off to college.
When I made the announcement to Jason that I was going to start flying again, I was between books. It seemed like a good time to consider ye old private pilot texts and have a crack at them. The problem was that I was in the mood for fiction, and, well, flying is anything but. Even more of a problem was that I have been doing a lot of thinking about rebellion lately, with all the junk in Egypt and Syria, with stories of parents refusing immunizations, with listening to NIN's Year Zero while cutting the grass, and what I really wanted was a gritty and visceral work of fiction about rebellion. That's not the kind of stuff you'll find when reviewing requirements for FAA exams; they kinda frown on rebellion in that arena. (It wouldn't be rebellion if they smiled on it, though, now would it?)
So Jas handed me his copy of Stick and Rudder, the 1944 Wolfgang Langewiesche explanation of flying. I think I made it part way through the first page before I sighed and put it down. I just wasn't going to be able to get into it with my brain in fight-the-power mode.
Long story short, I'm a third of the way through Atlas Shrugged, which I've been wanting to read for a long time and though it is bringing the gritty (yet?), it has the potential to bring the antiestablishmentarianism. (Score 50 for using all my letters!) This is a long dang book. I'm satisfying the fiction craving by night and whipping myself into study mode during the three hours of "free" time while the kids are both in school three mornings each week. You'll notice my meandering thoughts on air density were on a Wednesday morning, and most posts are likely to come during those timeslots.
Anywho, here I sit, typing this post with Stick and Rudder by my side. I'm going to forego Dagny Taggart and Hank Rearden tonight to see whether Mr. Langewiesche can help me uncock my eyebrows with regards to certain altitude-related concerns.
**Update: five minutes later and I'm on page 7 where he says the best thing you can do to understand how a wing flies is to forget Bernoulli. Ha! He doesn't know me at all! But, on the other hand, I do agree with his metaphor that for practical matters, you don't have to understand how rubber molecules move and change in a tennis ball in order to observe and predict its bounce....
When I made the announcement to Jason that I was going to start flying again, I was between books. It seemed like a good time to consider ye old private pilot texts and have a crack at them. The problem was that I was in the mood for fiction, and, well, flying is anything but. Even more of a problem was that I have been doing a lot of thinking about rebellion lately, with all the junk in Egypt and Syria, with stories of parents refusing immunizations, with listening to NIN's Year Zero while cutting the grass, and what I really wanted was a gritty and visceral work of fiction about rebellion. That's not the kind of stuff you'll find when reviewing requirements for FAA exams; they kinda frown on rebellion in that arena. (It wouldn't be rebellion if they smiled on it, though, now would it?)
So Jas handed me his copy of Stick and Rudder, the 1944 Wolfgang Langewiesche explanation of flying. I think I made it part way through the first page before I sighed and put it down. I just wasn't going to be able to get into it with my brain in fight-the-power mode.
Long story short, I'm a third of the way through Atlas Shrugged, which I've been wanting to read for a long time and though it is bringing the gritty (yet?), it has the potential to bring the antiestablishmentarianism. (Score 50 for using all my letters!) This is a long dang book. I'm satisfying the fiction craving by night and whipping myself into study mode during the three hours of "free" time while the kids are both in school three mornings each week. You'll notice my meandering thoughts on air density were on a Wednesday morning, and most posts are likely to come during those timeslots.
Anywho, here I sit, typing this post with Stick and Rudder by my side. I'm going to forego Dagny Taggart and Hank Rearden tonight to see whether Mr. Langewiesche can help me uncock my eyebrows with regards to certain altitude-related concerns.
**Update: five minutes later and I'm on page 7 where he says the best thing you can do to understand how a wing flies is to forget Bernoulli. Ha! He doesn't know me at all! But, on the other hand, I do agree with his metaphor that for practical matters, you don't have to understand how rubber molecules move and change in a tennis ball in order to observe and predict its bounce....
Wednesday, September 25, 2013
Thinking about air density
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?
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?
Wednesday, September 11, 2013
Ah, memories
It's so neat to pull out the logbook and see that training history!
First training flight: 9/20/2006
First solo: 10/21/2006
Dual cross-country: 12/5/2006
First solo landing at towered airport: 12/18/2006
First solo cross-country: 12/20/2006
First night cross-country: 1/3/2007
Checkride: 2/24/2007 (46.9 hrs)
Last PIC: 5/14/2007
Total hours: 68.6
First training flight: 9/20/2006
First solo: 10/21/2006
Dual cross-country: 12/5/2006
First solo landing at towered airport: 12/18/2006
First solo cross-country: 12/20/2006
First night cross-country: 1/3/2007
Checkride: 2/24/2007 (46.9 hrs)
Last PIC: 5/14/2007
Total hours: 68.6
Testing, testing.... Is this thing on?
Guess what!?!?!!!!! The little one just started preschool, which gives me three mornings each week of free time! You know what I'm going to do?
Massages every day.
Ha! That would be the life! But no, I have loftier goals indeed. I have already contacted the local flight school and am awaiting a callback from an instructor. I spent this morning at the library poring over my old "Guided Flight Discovery" books to refresh my brain before wasting the instructor's time. I got my third-class medical a year ago or so in preparation for the eventuality of training again.
AWAY WE GO!
I did pause to think that my books are outdated -- no mentions of ForeFlight! :) Well, they do predate the iPad... Hopefully the basics like carburetor icing and Vx and Vy and coordinated turns and radio etiquette haven't changed too much!
Why the break, you ask? I took my private pilot checkride in 2007 when I was pregnant with my first son. Adjusting to parenthood was pretty exhausting, then we moved, then we had our second son. However, now that we have a first-grader and a preschooler, my stay-at-home-mom hours are dwindling and it's time to shake the dust off my wings!
The next few weeks of blog posts will probably be a crash-course in aviating as I dig my memories from the juice-stained and toy-cluttered caverns of my mind.
Want to come along for the ride?
Massages every day.
Ha! That would be the life! But no, I have loftier goals indeed. I have already contacted the local flight school and am awaiting a callback from an instructor. I spent this morning at the library poring over my old "Guided Flight Discovery" books to refresh my brain before wasting the instructor's time. I got my third-class medical a year ago or so in preparation for the eventuality of training again.
AWAY WE GO!
I did pause to think that my books are outdated -- no mentions of ForeFlight! :) Well, they do predate the iPad... Hopefully the basics like carburetor icing and Vx and Vy and coordinated turns and radio etiquette haven't changed too much!
Why the break, you ask? I took my private pilot checkride in 2007 when I was pregnant with my first son. Adjusting to parenthood was pretty exhausting, then we moved, then we had our second son. However, now that we have a first-grader and a preschooler, my stay-at-home-mom hours are dwindling and it's time to shake the dust off my wings!
The next few weeks of blog posts will probably be a crash-course in aviating as I dig my memories from the juice-stained and toy-cluttered caverns of my mind.
Want to come along for the ride?
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