Hmm well I understand Burniloi's principle, the four forces (well eight if you include that parody chart Autothrust Blue posted). It's just understanding the technical details that gets me. I tried reading Aerodynamics for Naval Aviators but I found it way too dry.
What about the PHAK? That's what you want to be reading.
Read the PHAK! Since I'm not doing anything at the moment, though, I might as well type this out.
My recommendation is to forget "what causes lift". Really.. just forget it. You can work on it later if you want, but in the end the absolute mechanics of lift is not a subject fundamental to controlling and understanding its production, and while we've managed to model it reasonably well, we haven't really reduced it to a "easy answer". There are many models for describing the generation of lift, from the "easy and simple" to the mathematically complex.
The key points of Aerodynamics According To Fox:
- Lift is created by altering the direction of airflow, resulting in a force.
- You can increase absolute lift by increasing the speed of the airflow, providing alteration is taking place.
- You can also increase lift by increasing the amount of directional alteration—AoA (angle of attack).
- Increasing the AoA also always increases drag.
- In a 2000lb airplane, you need to produce 2000lbs of lift (Plus a little for the tail-down force) to stay level.
Lift required:
At cruise speed, and 1° AoA, perhaps you're generating 2000lbs of lift in straight and level flight. Decrease the speed of the airflow, and you need to do what to stay level? Increase the AoA to compensate, of course. How do you increase the AoA? You produce additional tail-down force. How do you do that? By increasing the negative AoA of the horizontal stabilizer (or stabilator)... by pulling back on the yoke. What else does that do? Increases drag. What happens as you slow further? You need to progressively increase the AoA to maintain that 2000lbs of lift to stay level, which further increases drag.
What happens if we produce 1500lbs of lift? We descend. 2400lbs of lift? We climb.
Flight controls:
How do all the flight controls work? They increase the effective angle of attack of an airfoil, resulting in an increase or decrease in lift from that control surface. Rudder, aileron, elevator. What happens if you alter lift asymmetrically, ergo using the ailerons to enter a bank? You increase drag on the wing with the section of increased angle of attack and decrease drag on the wing with the section of decreased angle of attack. This causes the airplane to yaw opposite the direction of the bank, as the wing with the 'increased' AoA is generating more drag.
What happens if we pull back on the yoke? We deflect the elevator upward, increasing the negative AoA of the horizontal stabilizer, resulting in an increase in the downward force produced by the horizontal stabilizer. When the fulcrum (balance point) is the CG, this results in a rotation around the CG, an increase in pitch, and consequently an increase in angle of attack.
How do we fix the tendency of the airplane to yaw in the opposite direction of bank? Rudder.
Angle of Attack:
Everything can be tied to AoA. Controlling AoA controls lift. You will stall at a specific AoA, period. A symmetrical airfoil produces its lift by changing angle of attack only, and thus will stall at (approximately) the same AoA positive or negative. A cambered (asymmetrical) airfoil is equivalent to having a symmetrical airfoil with a slight net positive AoA at all times (With some small caveats).
If you're flying an airplane with a symmetrical airfoil with atmosphere but no gravity, what AoA will ensure level flight? Well, you need zero pounds of lift, so you need to ensure that you're not redirecting air, so you need to fly at 0° AoA. What if it's a cambered airfoil? You'll need a negative AoA.
With gravity, suddenly you need to produce significantly more lift. The symmetrical airfoil will be between ~2°-16° AoA to produce enough lift to maintain level flight, and the cambered airfoil will be between ~0°-15° AoA to produce enough lift to maintain level flight ... depending on the speed of airflow over the airfoil.
There are aerodynamic factors built into some aircraft that cause them to be more controllable at high angles of attack, such as wing twist/washout, where the wing root is at a higher angle of attack than the wing tip, thus allowing the stall to progress from root to tip, allowing aileron effectiveness deeper into the stall. That said, if the section where the aileron is right at the critical angle of attack, what happens when you try to 'lift' that wing with aileron? You increase the effective AoA of that section of airfoil and... yep, increase it right past the critical AoA.
All that clear as mud? Good, let's see if I can confuse you further
(Or, hint, hint: You can just read the PHAK, which has pretty pictures and nice, easy to follow diagrams)
Stability and controllability:
You can picture the airplane as a balance scale ala "the scales of justice". The airplane is suspended from its center of lift somewhere in the front-middle of the wing, and the weight on both ends must be equal—that is, the weight of everything forward of the Center of Lift must be offset by an equal force in the tail... the tail down force produced by the horizontal stabilizer*. If you increase the weight on the nose... say, by shifting weight forward... you increase the force that must be produced by the horizontal stabilizer. That increases the weight that must be supported by the wing, which increases... you guessed it, the angle of attack needed for level flight... which increases... drag. Which decreases... cruise speed. It also decreases the amount of remaining up elevator, since increased up elevator is required for level flight, which is why it can mean insufficient ability to pitch up in the landing flare. Of course, moving the center of gravity forward also increases the effective arm of the controls... and as we know, a force applied at a longer arm (distance from the fulcrum) has a stronger resultant effect than the same force applied at a shorter arm... so you'll have more rudder and elevator authority, within the remaining range.
Increase the weight in the tail, say by shifting weight aft, and you decrease the amount of tail down force required, decrease the weight the wing has to carry ... but you also decrease the "arm" between the CG and the tailplane control surfaces, which decreases their effectiveness.
You can also look at this relationship a different way, by considering the relationship of the "Center of Gravity" to the "Center of Lift". The center of lift is the string from which the airplane is suspended, mind you. With no tail-down force the nose will always drop in almost all airplanes, because the center of gravity is forward of the center of lift. This provides an important stabilizing effect. As the center of gravity moves aft, closer to the center of lift, the stability is decreased in the same way as if you move the legs of a tripod together.
There are other design criteria built into airplanes to effect stability around their axes, such as dihedral, which contributes to positive dynamic lateral stability. (As a CFI applicant, and thus a commercial pilot, I'll assume you understand the stabilities well, as I assume you understand W&B well.)
Left turning tendencies:
The airplane (in US-built aircraft) wants to torque roll to the left. All else being equal, it would. There are some design considerations applied to compensate for this trend.
When the airplane is at a high angle of attack, the oncoming air strikes the propeller disc at an angle. The descending blade has a "headwind", and the ascending blade has a "tailwind". The descending blade produces more lift, creating a yaw to the left at high power and high angles of attack.
How do we fix left turning tendencies? Rudder.
Lessee... did I miss anything (Aside from pictures and diagrams

? Too simple? Not simple enough?
That's most of the aerodynamics you need to know, as a CFI. Not all, but most. Knowing that trim tabs "fly" their control surfaces, that the balance tabs on a stabilator resist the motion of the control surfaces to prevent overcontrolling, etc.. all useful, depending on the airplanes you're flying. But for aerodynamics, that's pretty much the core of it. Unlike naval aviators, we don't yet need to really worry about compressible flow, what happens to swept wings, mach anything, etc. Just basic low-speed aerodynamics.
If any of this is useful and you want any clarification, I'd be delighted to try... just let me know.
If none of this is useful... well ... sorry. I tried! ^.^
~Fox