Wednesday, October 09, 2013

Weather, navigation, and maneuvers. Take a deep breath.

Weather review was pretty straight-forward.  I may not remember the name of exactly which graphical weather product to go to for any specific item, but I'm confident I can find it during preflight in ForeFlight or online.

Navigation was easier than expected.  I actually remembered VOR navigation, which is the only type I was a little anxious about.  Other than GPS and pilotage, that's what I'd use.  I didn't bother reviewing the NDB nav process, and hopefully that won't come back to bite me in the keister.

Maneuvers made me a little anxious.  Mentally, mostly, I've got it.  When you're in the plane, though, it has to be(come) natural with muscle-memory and innate feel and reactions.  It's just a lot to try to visualize and anticipate all at once.  The first time around, this stuff was all spread out over a few months.  I'm interested to see how much feeling comes back in that first flight.  It helps to have flown with Jas during the past years, but being PIC will be a different story.

I have my first flight scheduled for Friday morning.  It's a 1982 Cessna 172P (just downloaded the POH) with at least one Garmin 430.  The instructor scheduled us in the plane for 2 hours and another hour on the ground.  I'm hoping we can do the BFR in this time, but it all depends on the return of the feeling and competence, at least enough to continue solo PIC for practice, calling in Jas or the instructor as needed.

I'm also happy to be doing it during autumn -- beautiful, crisp days with less turbulence, so less distraction from learning!

Monday, October 07, 2013

Charts, airspaces, communications, information...

This stuff is coming back pretty quickly...  yay!  I do have to say, though, that a lot has changed since I first became a certificated private pilot.  Yes, reading paper charts is a necessary fundamental for safe flying and navigation.  But in the interceding years since last I sat left seat, I've be right seat, an armchair pilot, and a developer for ForeFlight.  Digital tools like ForeFlight make it so easy to find out what you need to know.

For instance, one of the things I didn't remember was that the ticks coming out from an airport icon on a chart mean that during business hours, the airport offers services and fuel.  Nowadays, I wouldn't look at a sectional to find that out; I'd see the airport on the sectional in ForeFlight and tap it to find out its details, including when it operates, whether it offers fuel (self-serve or by lineman), and so many other things from current METAR to frequencies to airport elevation and pattern altitude and so forth.

I'm not starting a discussion about digital v. paper.  All I'm saying is that the information is easily and quickly available at a tap on the iPad.  Barring device failure.  And backup device failure.  :)

Another thing to say about my approach to flying is that I'm a planner.  I like to thoroughly debrief every spot along the projected path, all airports along the way, and really try to minimize surprises.  That's probably the way of most student and low-time pilots like myself; but the killing zone is on the horizon, and that, I imagine, comes partly from complacency about these kinds of details.

Moving on...  Airspaces.  Almost everywhere I've ever flown has been Class E, like my primary training homebase of KJGG and now KUZA, or Class D (towered with no radar services), like KPHF.  KUZA is a little more interesting since it's under one of Charlotte's Class B shelves.  This means from the surface up to 3600' MSL we're in Class E and can fly under VFR rules and choose our own destinies.  Once we go above 3600', or head into an inner ring of Charlotte airspace where the floors of the Class B shelves are lower, we must already be in contact with ATC, must have a Mode C transponder (reporting altitude and assigned code), and must follow their directions.

The main thing that's important to VFR pilots are the environmental rules.  To participate in a VFR flight, you have to be able to see, and the minimum requirements are 3 statute miles of visibility (think low haze) and the ability to stay clear of clouds by at least 500 feet below, 1000 feet above, and 2000 feet horizontally.  For safety.

Class B airspace is reserved for mega busy airports, like Chicago and Atlanta.  Class A is used between 18000' MSL up to 60000' MSL (FL180-FL600) and requires an IFR flight plan.  Above that, it goes back to Class E, but you usually only find space-faring vehicles there....

There's also a Class G airspace, but that's rare, except to bush pilots in Alaska.  The rules there are fewer yet, and almost boil down to common sense.  (Update: I think I'm wrong here; abundance of airports just makes it more practical to treat non-controlled airspace on the east coast as all Class E.)

Special airspaces, MOAs, restricted airspaces, ADIZs, ...  all on the charts.  TFRs change airspaces periodically and must be verified before takeoff.  NOTAMs should also be consulted before takeoff, but usually pertain to non-standard airport operations (equipment that's offline, change in traffic pattern, scheduled event altering landing availability, etc).

Moving on....  Communicating.  The transponder is what allows radars to find an aircraft.  The radar pings, and the transponder responds.  Mode C transponders report both the squawk code and altitude, and are required to interact with ATC.  When flying VFR (without flight following), the transponder is set to the VFR code of 1200.  The transponder will still respond to pings, but ATC will only know that there's somebody out there at that location and altitude; this is helpful for advising any pilot of traffic (you!).  Though transponders are nearly ubiquitous, they are not required for VFR operations and so traffic may be out there that ATC can't see and that your in-cockpit traffic advisor (traffic scope, ADS-B) can't alert about.  That alone underscores the importance of a VFR pilot maintaining situational awareness and keeping a good scan going.

Other important squawk codes are 7500 (hijack), 7600 (lost communications), and 7700 (mayday).  When dialing in a code, it's important to be mindful that these aren't entered accidentally, even for a moment.

Radio communications should be brief, concise and professional.  CTAF (common traffic advisory frequency) is published for each non-towered airport, or towered airports when unattended, and is how aircraft in the area coordinate and avoid each other; it's also usually the frequency the pilot would use to activate pilot-controlled lighting for night operations.  UNICOM, sometimes the same as CTAF, allows the pilot to talk to someone at the airport for advisories, to request fuel, etc.

Lots of times, the same frequency is used at multiple airports and you'll hear transmissions that are irrelevant.  It's important to (1) listen for the location of the other transmissions and (2) remember to include yours.  The standard flow for self-announcing via CTAF is "Audience, identification, message (frequently location and intention), audience."  For instance, "Rock Hill traffic, Cessna 4321A, 10 miles southwest of the airport inbound for landing runway 20, other traffic please advise, Rock Hill."  Proper radio usage also means not "stepping on" other transmissions; only one person should be speaking at a time, so wait until the freq is clear before starting your transmission.

When talking with ATC, you typically hail them and state your identification, then wait for them to get back to you.  There's a good chance they're managing other aircraft and may be busy at the moment.  "Charlotte approach, Cessna 4321A."  This applies at towered airports as well.  You need to engage the controller before starting the conversation.  This gets a lot more important when flying IFR, so I'm not going to dwell on it here.  Also, the flying I expect to do in the near term will not use this, so I'll get more detailed when the time comes.

In case of lost communications, there are some basic troubleshooting steps to take, like verifying the frequency, checking that the headset is plugged in, and trying the alternate transceiver (radio).  If all else fails, squawk 7600 and be extra vigilant.  For landing in Class D airspace, you'll need to watch the tower for light signals -- this is a good time to reference the signal legend you have on your kneeboard or in ForeFlight.

7700 on the transponder usually goes with 121.5 on the radio (although if already interacting with ATC you'll probably keep these comm settings as they are unless instructed otherwise).  121.5 is the mayday frequency.  Distress signals are started with "MAYDAY, MAYDAY, MAYDAY."  Urgent situations start with "PAN-PAN, PAN-PAN, PAN-PAN."  The aircraft's ELT (emergency locator transmitter) also broadcasts on 121.5 automatically upon impact.  The freq should be checked periodically to make sure your ELT isn't sending false alarms, and there are procedures governing testing the ELT.

Moving on...  Information.  A/FD.  FAR/AIM.  NOTAMs.  ACs.  If not getting it from ForeFlight, would be my next resource, especially for NOTAMs and TFRs, and in the air an FSS or UNICOM can be consulted for up-to-date advisories.

Next:  Weather!  (Thanks to intermittent work with ForeFlight, this part should be quick.)  Aircraft performance and weight and balance.  Navigation.

Wednesday, October 02, 2013

Approaching a stall, maneuvering in flight

The next few textbook sections covered basic forces of flight, control surfaces, center of gravity, aircraft stability, and here we are at stalls.

A stall happens when the wing can no longer produce enough lift to support the aircraft.  The reference point for this condition is called the critical angle of attack.  It can happen in various situations, but the clearest to consider is the climb, when the wings are inclined and the air flowing over the wings is less.  You can easily envision a breaking point when the airflow around the wing is just messy, and that's the stall point.  We talk about it as stall speed, because the cockpit instrument that measures the airflow over the wing is the airspeed indicator, and the airflow needs to be at least VS1 (or VS0 with flaps out) to keep from stalling the wing.

The stall speed can change.  More weight, loading the airplane with a CG too far forward, and the presence of ice or other irregularities on the wing can increase the stall speed.  Use of flaps decreases the stall speed, allowing slower controlled flight.

Two main types of stalls are practiced during flight training:  power-on and power-off.  A power-on stall happens when you typically have the throttle in, such as during take-off or a climb.  Stalls during this phase of flight when lift is disrupted due to a too-high angle of attack (nose too high) or retracting the flaps too early.  Power-off stalls happen when the throttle is out, such as during landing, and are actually desired to be the last thing to happen as you touch down -- you've "bled off" all the speed you can, stall and settle the last inch onto the runway.

No matter the type or reason for the stall, the recovery process is the same:  nose down and power in.  As the airflow over the control surfaces is quickly restored, return to straight-and-level flight and adjust the throttle to an appropriate setting.

Stalls usually give me sweaty palms.  I can totally deal with the concepts involved, and in practice I have recovered them and used them upon landing to my advantage.  However, it's the potential for an unrecovered stall to progress into a spin that freaks me out.  So I suppose that spins give me the sweaty palms, but stalling is the first step in spinning!  General recovery process (check POH for detailed recovery):  power out, neutral ailerons, opposite rudder, return to straight-and-level flight.  At a typical loss of 500 ft per turn, and a turn happening in just 3 seconds, there's no time to consult an emergency checklist.

Okay, moving on to maneuvers.  Climb, descend, turn.  Points to remember:

When climbing, the aircraft tends to turn left slightly due to things like engine torque and asymmetrical thrust produced by the twist of the propellor blades meeting the angle of attack.  Slight right rudder is used to maintain a straight flight path.

When descending without power, glide speed and angle are preeeeeetty important.  The POH will indicate the best glide speed for the aircraft.  Upon engine out, the first thing on the checklist is to trim for best glide speed (then troubleshoot); this will keep you in the air the longest while you attempt a restart or select a landing site.  Best glide speed can be affected by wind, so for once you'd be looking to land with the wind.

When turning, pay attention to load factor.  The increased Gs on the wings decreases lift and increases stall speed.  To maintain altitude, some back pressure will be needed.  Load factor that goes too high can damage the structure; for the normal category aircraft we fly, they're limited to 3.8 positive Gs and 1.52 negative.  Also relevant here is the maximum maneuvering speed (VA) published in the POH; it's the max speed at which abrupt control inputs or turbulence can be tolerated by the airplane.

That wraps up the fundamentals of flight.  Next up are the practical matters of reading charts and understanding airspaces, followed by radio communications, weather, navigation and flight planning.  These next parts should go quickly, thanks to continue to fly with Jas and being involved with ForeFlight....

Gyroscopic instruments

The turn coordinator, attitude indicator (artificial horizon) and heading indicator are based on gyroscopes.  Gyroscopes have spinning wheels that maintain their position as their anchoring hardware moves around them.  These wheels require some sort of power to spin; the turn coordinator is typically electric, while the other two are vacuum powered.  I don't think I ever wondered "why vacuum?" before, but you can probably guess that I did today!

There's either a vacuum force created by design of the system, or a vacuum pump within the system, for these instruments.  It sucks air from the intake, through a filter, through tubes and instruments, through a pressure release valve, and then exhausts it.  The book doesn't have a diagram, but I'm imagining fins on the gyro wheel to catch the air, like on a water wheel for catching water.  As long as the air is flowing, the wheels will be spinning.

Because of precession, or the introduction of error in gyro-based readings due to friction, gyro instruments must be periodically cross-checked and/or recalibrated.  Most notorious is the heading indicator, also called the directional gyro or DG, which should be compared to the whiskey compass (magnetic) every 15 minutes or so.  The whiskey compass, however, gives temporarily inaccurate readings while turning, accelerating or decelerating and is affected by turbulence as well.

Another direction-relevant complication is variance, or the difference between true north and magnetic north.  Instruments are set relative to a magnetic reading, yet charts and publications use a true north reference.  Since the magnetic field of the Earth changes from place to place, it's important to know the variance at your location.  To give you an idea, the variance along the east coast ranges from roughly 0 degrees west to around 20!

Monday, September 30, 2013

Pitot-static system

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.

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!

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....

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?