Principles of Flight

Lift, weight, thrust, and drag. The common teaching shortcut — "lift equals weight and thrust equals drag" — is only true in steady, unaccelerated, level flight, and even then only when you resolve the forces correctly relative to the flightpath. In a climb, a component of weight acts along the flightpath opposing thrust, which is why thrust must exceed drag to climb: a 10° climb requires thrust equal to drag plus the rearward component of weight (PHAK ch. 5).

Teaching point: correct the shortcut before the student builds an incorrect insight on it, because unlearning is expensive (AIH ch. 2, primacy).

The wing stalls when a sufficiently high angle of attack is imposed, the smooth flow over the airfoil breaks up and separates, and lift is abruptly lost — and any airplane, within the limits of its structure, may be stalled at any airspeed (PHAK ch. 5). That sentence is the whole lesson. The critical angle of attack is a property of the wing, not of the airspeed indicator; a wing always stalls at the same AOA regardless of airspeed, weight, load factor, or density altitude (PHAK ch. 5).

Teach the AOA-first framing from day one and the accelerated stall stops being a surprise.

Stalling speed increases in proportion to the square root of the load factor (PHAK ch. 5). In a coordinated, constant-altitude turn the load factor is 2 Gs at 60° of bank and 5.76 Gs at 80° — the curve rises steeply after about 45–50° of bank (PHAK ch. 5).

At 2 Gs the stall speed is about 1.41 times the 1-G value. For the average GA airplane, the approximate maximum bank for a coordinated constant-altitude turn is 60°; an additional 10° of bank adds roughly 1 G, bringing it close to the yield point (PHAK ch. 5).

• Normal — +3.8 to −1.52
• Utility (mild acrobatics, including spins) — +4.4 to −1.76
• Acrobatic — +6.0 to −3.00

For aircraft with gross weight over 4,000 pounds the normal-category limit load factor is reduced. A 50 percent safety factor is added to these limit loads — the structure must support one and one-half times the limit load factor without failure (PHAK ch. 5).

Static stability is the initial tendency after displacement — positive (returns toward equilibrium), neutral (stays displaced), or negative (departs further). Dynamic stability is the response over time — positive (motion damps out), neutral (oscillations neither damp nor grow), or negative (oscillations grow) (PHAK ch. 5).

The combination matters: an airplane can be positive static and negative dynamic, which means it starts back toward level and then diverges into progressively larger oscillations (PHAK ch. 5). Draw the three damping curves on the board; the picture does the teaching.

• Aileron — roll — longitudinal axis — lateral stability
• Elevator/stabilator — pitch — lateral axis — longitudinal stability
• Rudder — yaw — vertical axis — directional stability

(PHAK ch. 6.) Students reliably swap "lateral" and "longitudinal." Teach it as: the stability is named for the motion it resists, not the axis it rotates about.

• Stability — the inherent quality of the airplane to correct for conditions that disturb it and return to or continue on the original flightpath (PHAK ch. 5).
• Maneuverability — the quality that permits it to be maneuvered easily and to withstand the stresses of maneuvering.
• Controllability — the capability to respond to pilot control input, especially with regard to flightpath and attitude.

The trade is the teaching point: more stability means less maneuverability. A trainer is stable so the student can let go and think; an aerobatic airplane is not.

1. Torque reaction — Newton's third law. The engine and propeller turn clockwise as viewed from the pilot's seat, so the airframe is rolled to the left about the longitudinal axis; on the ground this puts more weight on the left main (PHAK ch. 5).
2. Corkscrew (spiraling) slipstream — at high propeller speed and low forward speed, as in takeoff, the spiraling slipstream strikes the vertical fin and produces a yaw to the left (PHAK ch. 5).
3. Gyroscopic precession — a force applied to deflect the propeller out of its plane of rotation produces a resultant force 90° ahead in the direction of rotation. Raising the tail applies a force to the top of the disc; the result is a yaw to the left. Most prominent in tailwheel airplanes during the takeoff roll (PHAK ch. 5).
4. Asymmetric loading (P-factor) — at high angles of attack the descending (right) blade meets a greater resultant velocity than the ascending blade, so it produces more thrust, moving the center of thrust right of the disc centerline and yawing the nose left (PHAK ch. 5).

The down-going aileron produces more lift, and therefore more drag, so that wing slows slightly and the airplane yaws opposite the direction of bank (PHAK ch. 6). It becomes more pronounced at low airspeeds because aerodynamic pressure on the control surfaces is low, larger aileron deflections are needed to maneuver, and the vertical stabilizer/rudder is simultaneously less effective — so the problem grows while the cure weakens (PHAK ch. 6). The correction is rudder; the amount required is greatest at low airspeed, high AOA, and large aileron deflection.

Within roughly a wingspan of the surface, the ground restricts the vertical component of airflow around the wing, altering upwash, downwash, and wingtip vortices. That reduces the induced angle of attack and induced drag, so the wing needs a lower AOA to produce the same lift coefficient (PHAK ch. 5).

The magnitude falls off fast with height:

• Wing at a height equal to its span — induced drag reduced only 1.4 percent
• At one-fourth the span — 23.5 percent
• At one-tenth the span — 47.6 percent

(PHAK ch. 5.) That is why it is only noticeable in the flare and just after liftoff.

It will:

• Require an increase in AOA to maintain the same lift coefficient
• Experience an increase in induced drag and thrust required
• Experience a decrease in stability and a nose-up change in moment
• Experience a reduction in static source pressure and an increase in indicated airspeed

(PHAK ch. 5.) Ground effect also usually increases local pressure at the static source, producing a lower indicated airspeed and altitude while in it — which is why an airplane can be airborne at an indicated airspeed lower than normally required, then settle back on when it climbs out of the cushion.

Vortices sink at several hundred feet per minute, and within 100 to 200 feet of the ground they move laterally at 2–3 knots. A light quartering tailwind is the worst case — the vortices can lie along a significant portion of the final approach and extended centerline, not just the touchdown zone (PHAK ch. 14).

• Landing behind a larger aircraft, same runway — stay at or above its approach path and land beyond its touchdown point
• Landing behind a larger aircraft on a parallel runway closer than 2,500 feet — consider drift; stay at or above its path
• Landing behind a departing aircraft, same runway — land prior to its rotation point
• Departing behind a large aircraft — rotate prior to its rotation point and climb above its climb path until clear
• After a large aircraft's low approach, missed approach, or touch-and-go — wait at least 2 minutes

Anchor on the two things that are always true and observable from the seat: lift depends on angle of attack and airspeed, and the wing works by accelerating a mass of air downward. Reduced pressure on top of the airfoil is essential to lift, but it is only one contributor to the overall effect of pushing an air mass downward (PHAK ch. 5).

Then make it operational immediately: at high AOA, induced drag is high, and since high AOA corresponds to low airspeed, induced drag predominates at low speed (PHAK ch. 5). Now the student has the tool to explain the region of reverse command, the go-around, and the short-field approach. Teach the concept in a form the student can use, and the definitions will follow.

The integrated (composite) method has the student use outside references and flight instruments to establish and maintain attitude and performance from the first lesson. Roughly 90 percent of the pilot's attention should be devoted to outside visual references and scanning for traffic; the instruments validate the attitude and confirm performance (AFH ch. 3).

Applied to aerodynamics: teach the student to set an attitude for the AOA they want, then confirm with the airspeed indicator — attitude is the input, airspeed is the result. That habit is what makes stall recovery instinctive later.

Because all four turning tendencies peak in the same regime. Torque reaction is proportional to engine power, so it is greatest at full throttle. The spiraling slipstream is strongest at high propeller speed and low forward speed — exactly the takeoff condition — and it strikes the vertical fin (PHAK ch. 5). P-factor requires a high angle of attack, which is what you have at low airspeed. Gyroscopic precession appears whenever the propeller is deflected out of its plane of rotation, which happens on the takeoff roll as the tail rises.

The instructor's payoff: this is why the student who nails coordination in cruise is all over the runway on takeoff, and why "step on the ball" is not a useful correction — teach the anticipation of right rudder, applied before the yaw develops.

Multiengine airplanes are subject to P-factor exactly as single-engine airplanes are. The descending blade of each engine produces greater thrust than the ascending blade at positive angles of attack — and the descending blade of the right engine sits farther from the center of gravity, giving it a longer moment arm. So failure of the left engine produces the most asymmetric thrust, making it the critical engine on a conventional twin (AFH ch. 13). The single-engine principle you teach on day one becomes the multiengine certification concept.

Longitudinal stability about the lateral axis is designed in, and the center of lift on most asymmetrical airfoils moves forward with an increase in AOA and aft with a decrease (PHAK ch. 5). Move the CG aft and you shorten the moment arm between the CG and the tail, reducing the tail's restoring authority — the airplane becomes less longitudinally stable, stall recovery is more difficult, and control forces lighten to the point of over-control.

The same principle shows up in the twin: VMC increases as the CG moves aft, because the moment arm of the rudder — and therefore its effectiveness — is reduced (AFH ch. 13). One idea, two ratings.

That the student holds a plausible-sounding but wrong model and flies it. Three specific ones to hunt for and correct on the spot:

• "The airplane stalls at 48 knots" — any airplane can stall at any airspeed within the limits of its structure; this one kills people in the base-to-final turn (PHAK ch. 5).
• "Steep turns are only about bank angle" — load factor rises steeply past 45–50° of bank, reaching 2 Gs at 60° with stall speed up about 41 percent (PHAK ch. 5).
• "Ground effect gives you extra lift you can climb on" — leaving ground effect increases induced drag and thrust required and can drop you back onto the runway; establish a positive rate of climb and a safe altitude first (PHAK ch. 5).

The correction technique is not the lecture — it is the question. Ask the student to predict, let them commit, then demonstrate.

1. Brief on the ground — the numbers first: 2 Gs at 60°, 5.76 Gs at 80°, stall speed as the square root of load factor, so the student knows what to expect (PHAK ch. 5).
2. Clear the area — establish an altitude floor and clear with clearing turns, having the learner verbalize "clear left, right, above, below" (PHAK ch. 14 lists clearing turns before all practice maneuvers).
3. Progressive bank — roll into a coordinated 45° bank first, then 60°, and have the student read the stall warning and control forces rather than watching the G meter.
4. Take the controls — when the bank steepens past the briefed value, the nose drops with back pressure still applied, or the student's attention locks inside; use the positive three-step exchange (AIH ch. 9).