Aerodynamics

Study-aid disclaimer

Why aerodynamics matters (written / oral / ACS)

Aerodynamics is the shared language of the written exam, the oral, and the airplane. Examiners are not hunting for calculus—they want accurate mental models: what the wing is doing, when it stops doing it, and what you do first.

ACS framing (high level):

Instructor point: “Stall speed” on a POH placard is a 1G, specified-weight, specified-configuration number. Airspeed alone is a poor stall-margin meter once you add bank, pull-up, ice, or an aft CG. Think AOA and load factor.

Suggested internal links: [Aircraft Systems → /learn/aircraft-systems/] · [Emergency Procedures → /learn/emergency/] · [Night Flying → /learn/night-flying/] · [How to Become a Private Pilot → /learn/how-to-become-a-private-pilot/] · [Exam prep → /exam-prep/]

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Four forces & drag types

Primary source: PHAK FAA-H-8083-25C Ch. 5 (and Ch. 4 framing).

The four forces

In straight-and-level, unaccelerated flight, four forces act on the airplane:

Equilibrium nuance (PHAK): In steady flight the sum of opposing components balances. The classroom shorthand “lift = weight and thrust = drag” is the level, unaccelerated case—not a claim that all four force magnitudes are identical to each other in climbs and glides. In a steady glide, a component of weight acts along the flight path and “pays” for drag when thrust is zero or idle.

Parasite drag vs induced drag

Total drag = parasite drag + induced drag.

Parasite drag — does not help produce lift. PHAK groups it as:

• Form (pressure) drag — shape and frontal area; reduced by streamlining
• Skin friction — boundary layer rubbing the surface
• Interference drag — mixing airstreams (e.g., wing–fuselage junction); fairings help

Parasite drag rises with airspeed (roughly with the square of speed for a given configuration).

Induced drag — the aerodynamic price of lift. Wingtip vortices and downwash tilt the lift vector aft. Induced drag is highest at high AOA / low airspeed (and increases with weight and load factor for a given speed).

L/DMAX and best glide

PHAK Ch. 5: plot CL, CD, and L/D versus AOA. L/DMAX occurs at one specific AOA/CL. In steady flight at that condition, total drag is at a minimum. Any lower or higher AOA reduces L/D and raises total drag for that airplane.

Operational meaning for private pilots:

• The airspeed (for a given weight/configuration) that places the wing near L/DMAX is where the airplane glides farthest for altitude lost in still air—the idea behind the AFM/POH best-glide speed.
• PHAK Ch. 11 also associates L/DMAX with maximum range efficiency.
• Fly the number in your AFM/POH for best glide / emergency distance; do not invent a universal knot value.

Instructor point: Below L/DMAX you are on the “back side” of the power curve—more power (or a lower AOA) is needed to go slower without descending. That is why slow flight feels busy.

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Angle of attack, critical AOA, stalls

Primary sources: PHAK Ch. 5; AFH Ch. 5; ACS PA.VII.

AOA definition

Angle of attack (AOA) is the angle between the wing’s chord line and the relative wind. Pitch attitude is not AOA. You can stall nose-low in a pull-up; you can be unstalled nose-high in a zoom if AOA is low.

Critical AOA

A stall is caused by exceeding the critical AOA—airflow separates, CL peaks then drops sharply, and the wing cannot produce the lift demanded. PHAK: the wing does not stop producing all lift; it cannot produce enough lift for the flight condition.

For a given wing/configuration, the airplane always stalls at the same critical AOA, regardless of:

• Airspeed
• Weight
• Load factor
• Density altitude
• Bank angle
• CG (critical AOA itself; CG still changes control and indicated stall speed)

PHAK cites typical critical AOA values on the order of ~16°–20° depending on design—remember the concept, not a universal degree to recite as “your” stall angle.

What does change: stall speed

Published stall speeds (VS0, VS1, etc.) assume 1G, coordinated flight, and a stated weight/configuration. Stall speed rises when you need more lift (higher weight, higher load factor) or when the wing is contaminated (ice/frost). Stall speed is not a fixed redline for every maneuver.

Factors private pilots must be able to explain:

Recognition (AFH): impending stall — buffet, horn, stick shaker, mushy controls, high descent rate; full stall — uncommanded nose-down or roll that cannot be readily arrested. Practice both power-off (approach) and power-on (departure) scenarios at safe altitude.

Stall recovery — priority, not a universal checklist

AFH Ch. 5 is blunt: the most important action for an impending or full stall is to reduce AOA. Prioritizing power or altitude first has killed people.

Generic AFH stall-recovery template (adapt to the airplane; POH wins):

1. Disconnect autopilot / wing leveler if equipped
2. Pitch to reduce AOA (nose-down as required) until stall warning stops; trim as needed
3. Roll wings level with ailerons after AOA is reduced; cancel yaw with rudder
4. Add power as needed
5. Retract speedbrakes/spoilers if equipped
6. Return to the desired flightpath without a secondary stall

Many fixed-gear trainers collapse this to: reduce AOA → level wings → power as required → recover to climb/cruise. Still: memorize your POH, not a website.

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Load factor, turns, and stall speed

Primary sources: PHAK Ch. 5 (verified); AFH Ch. 5; ACS PA.V.A.K2.

What load factor is

Load factor (n) is the ratio of the aerodynamic load on the airplane to its weight—spoken in Gs. Straight-and-level, unaccelerated: about 1 G. Any curved flight path (level turn, pull-up, gust) demands lift > weight and n rises.

PHAK reasons pilots must care:

1. You can overload the structure.
2. Higher load factor raises stalling speed, so stalls become possible at “safe-looking” airspeeds.

Coordinated level turns

In a coordinated constant-altitude turn, lift must balance weight + centrifugal force. PHAK: load factor vs bank has a trigonometric lift/weight relationship. The standard expression of that relationship is:

[
n = \frac{1}{\cos\phi}
]

where (\phi) is bank angle.

PHAK explicitly: 60° bank → 2 Gs; load factor rises steeply past ~45–50°. AFH Ch. 5: same 2 G / ~41% stall-speed increase example for a level 60° turn.

Stall speed relationship (PHAK, verified):

[
V_{S_n} \approx V_{S_{1G}} \times \sqrt{n}
]

Example PHAK uses: 50 kt unaccelerated stall → 100 kt at 4 G (because (\sqrt{4} = 2)).

Accelerated stalls & VA

Stalls above 1G are accelerated stalls—common in steep turns, abrupt pull-ups, and botched base-to-final corrections. Design maneuvering speed (VA) (and any AFM VO where published) is the speed below which, in smooth air, a single full control deflection about one axis should stall the wing before exceeding limit load. VA decreases as weight decreases—check the AFM. ACS steep-turn skills: use manufacturer recommended speed, or not above VA.

Limit load factors (PHAK category examples): Normal ≈ +3.8 / −1.52; Utility ≈ +4.4 / −1.76; Acrobatic ≈ +6.0 / −3.0 (plus the required 1.5 ultimate factor of safety). Do not treat the reserve as a plaything.

Overbanking tendency: In steep banks the outside wing travels faster and produces more lift—the airplane wants to increase bank. Hold opposite aileron as needed; stay coordinated (PA.V.A.K2a–b).

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Spins (awareness; recovery = POH / AFH)

Primary sources: AFH Ch. 5; PHAK Ch. 5 (load factor / stall context); ACS PA.VII.D; AC 61-67. POH/AFM first.**

What a spin is

AFH: a spin is an aggravated stall with autorotation. Ingredients:

1. Wing(s) stalled (critical AOA exceeded)
2. Yaw / sideslip (rudder misuse, adverse yaw, engine/prop effects, etc.)

The airplane follows a downward corkscrew; at least one wing is stalled. Phases AFH teaches: entry → incipient → developed → recovery.

Private-pilot emphasis: prevent the spin. Uncoordinated slow flight and the classic base-to-final skidding turn (overshoot → bottom rudder + back elevator) are the accident pattern—not intentional aerobatic entries.

Incipient vs developed

• Incipient: from stall/rotation start until the spin is fully developed (often up to roughly a turn or two). Early stall recovery / opposite rudder to stop yaw can arrest many accidental entries—still follow the airplane’s procedure.
• Developed: rotation rate, airspeed, and vertical speed roughly stabilized. Recovery may take additional turns and substantial altitude.
• AFH altitude teaching point: intentional spin practice needs enough height that recovery completes at or above 1,500 ft AGL (and AFH notes large altitude loss per turn)—another reason accidental spins near the pattern are often unsurvivable.

Recovery — POH first; AFH template only if POH is silent

There is no universal spin checklist that is safe for every airplane. AFH: follow the manufacturer. Many normal-category trainers are placarded against intentional spins; certification spin testing for some designs only required recovery from a one-turn / three-second spin—not a fully developed spin. Assume a placarded airplane may not recover from a developed spin.

AFH generic spin-recovery template (use only when the AFM/POH does not provide procedures—and still treat as handbook guidance, not a license to spin a prohibited airplane):

1. Power (throttle) — idle (power can flatten/ aggravate the spin)
2. Ailerons — neutral
3. Rudder — full opposite to rotation; hold
4. Elevator — briskly forward (through neutral as required) to reduce AOA / break the stall — do not wait for rotation to stop before this step
5. Neutralize rudder when rotation stops
6. Recover from the dive smoothly; avoid secondary stall / overspeed / overstress

Training aid only — PARE-style memory aid

Labeled training aid; must remain consistent with AFH order; never overrides the POH/AFM.

If your POH uses a different sequence or control emphasis, the POH wins.

Weight & balance: aft CG and out-of-limit loading can make spin behavior and recovery worse. Stay in the AFM envelope—especially for any airplane approved for intentional spins (typically utility category with restrictions).

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Ground effect

Primary source: PHAK Ch. 5. AFH takeoff/landing technique discussions reinforce the hazards.

What it is

Near the surface, the ground restricts the wing’s three-dimensional flow—upwash, downwash, and wingtip vortices diminish. Induced drag falls; for the same AOA, CL rises (or the same CL needs less AOA). Thrust required at low speed decreases.

How close?

PHAK quantitative teaching points (constant CL):

PHAK: ground effect is most realized at altitudes less than the wingspan, and a large reduction occurs only when the wing is very close to the ground—liftoff and just before touchdown.

Takeoff risks

Leaving ground effect reverses the benefit: induced drag jumps back up. If you force liftoff below recommended speed, the airplane may fly in ground effect then sink or mush when you climb out—especially at high weight / high density altitude. Do not retract gear/flaps before a positive climb is established (as applicable). Fly the AFM takeoff speed.

Landing / flare risks

Entering ground effect at constant AOA: more CL, less drag → float. Excess speed in the flare = long float, long landing, possible runway overrun. Reduce power as needed so the airplane does not balloon above the glidepath. Expect possible airspeed/altitude indication quirks from static-source position error in ground effect (PHAK: often lower indicated airspeed/altitude).

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Tie-ins: systems, emergency, night, how-to-become

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Common pitfalls

1. Equating pitch attitude with AOA — stalls happen when critical AOA is exceeded, at any attitude.
2. Trusting published VS in a steep turn — load factor raises stall speed by (\sqrt{n}); 60° ≈ 2 G ≈ 41% faster.
3. Fixing a base-to-final overshoot with rudder — skid + back pressure is the textbook spin entry; go around.
4. Recovering a stall with power alone — AFH: reduce AOA first.
5. Using ailerons aggressively while fully stalled — may aggravate wingtip stall/roll; unload first, then roll.
6. Forcing takeoff in ground effect below book speed — flies until induced drag returns; then it doesn’t.
7. Floating in ground effect with excess speed — long landing, overrun risk.
8. Treating PARE (or any memory aid) as universal law — POH/AFM first; placarded airplanes may not recover from developed spins.
9. Ignoring aft CG — may stall at a lower airspeed and resist recovery.
10. Ice on the wing — critical AOA effectively reduced; expect early buffet/stall and climb failure.

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FAQ

Does the wing stop producing lift in a stall?
No. PHAK: lift drops sharply after critical AOA; the wing usually cannot support level flight, but it does not produce zero lift.

Can I stall at any airspeed?
Yes—any airspeed, attitude, or power setting if you exceed critical AOA (PHAK / AFH).

Why is 60° bank special on the knowledge test?
PHAK’s clean teaching number: coordinated level 60° → 2 G, and stall speed scales with √2 ≈ 1.41.

Is best glide the same as Vx or Vy?
No. Best glide is about maximum distance (L/DMAX idea). Vx / Vy are climb-angle / climb-rate speeds (PHAK Ch. 11). Use the AFM labels.

Should private pilots practice intentional spins?
Only in an airplane approved for spins, within weight/CG limits, with a qualified instructor, and per the AFM—and ACS private privileges emphasize spin awareness, not intentional spin proficiency. Placarded airplanes: do not.

What is the first control input in stall recovery?
Whatever your POH says—with the AFH priority that reducing AOA comes before chasing altitude or stuffing in power as the sole fix.

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Practice questions

1. In straight-and-level unaccelerated flight, which pairs of forces are in equilibrium?

Show answer

**Lift = weight** and **thrust = drag** (PHAK Ch. 5 level-flight case).

2. Which drag type predominates at high AOA / low airspeed?

Show answer

**Induced drag**.

3. At L/DMAX, what is true of total drag?

Show answer

**Total drag is at a minimum** (PHAK Ch. 5).

4. A given airplane’s critical AOA changes with which of the following: weight, bank, density altitude?

Show answer

**None of those**—critical AOA is unchanged by weight, bank, or density altitude (PHAK). Configuration/contamination is a different story.

5. Load factor in a coordinated level 60° bank is approximately?

Show answer

**2 G** (PHAK).

6. If 1G stall speed is 50 KIAS, approximate stall speed at 4 G?

Show answer

**100 KIAS** — stall speed scales with √n (PHAK example).

7. What two conditions are required to enter a spin?

Show answer

**Stall (critical AOA exceeded)** and **yaw** (AFH).

8. What is the first priority in stall recovery per AFH?

Show answer

**Reduce angle of attack** (then follow POH/AFM procedure).

9. At a height equal to one wingspan, PHAK’s approximate induced-drag reduction due to ground effect is?

Show answer

About **1.4%** — significant ground effect requires being **much closer** than one full span.

10. Why is forcing liftoff below recommended speed dangerous?

Show answer

Airplane may fly in **ground effect**, then lack performance when induced drag returns out of ground effect (PHAK).

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