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!
