ASCI 309 — Chapter 4: Angle of Attack, the Boundary Layer, Stall, and High-Lift Devices

info. ## What this chapter is really about

Air has viscosity, so a very thin layer of it sticks to the wing and gets dragged along. That layer — the boundary layer — is fragile. Push the wing to a high enough angle of attack and the boundary layer runs out of energy, peels off the upper surface, and the lift collapses. That is a stall.

This chapter gives you the tools to predict all of it: Reynolds number tells you what kind of boundary layer you have; adverse pressure gradient tells you where it will separate; the stall speed equation tells you how slow you can safely fly; and high-lift devices are the engineering answer to making that number smaller.

The single sentence to carry out of here: a wing always stalls at the same angle of attack, never at the same airspeed.

1. Angle of attack: the master variable

Before the boundary layer, fix the logic of the whole chapter in place.

      α  (angle of attack)
       │
       ├──► sets the pressure distribution over the airfoil
       │        │
       │        ├──► sets C_L  ────► sets lift, for a given q and S
       │        ├──► sets C_D  ────► sets drag
       │        └──► sets C_M  ────► sets the pitching moment
       │
       └──► sets the strength of the ADVERSE PRESSURE GRADIENT
                on the upper surface
                     │
                     └──► past a critical value, the boundary layer
                          SEPARATES  ────►  STALL

Everything in this chapter hangs off that diagram. Angle of attack is the cause; lift, drag, moment and stall are the effects. Weight, altitude, bank angle and airspeed change what α you need, but they never change the α at which the wing stalls.

2. Viscosity and the boundary layer

2.1 Why there is a boundary layer at all

Air has viscosity — internal friction. Because of it, the air molecules touching the surface of a wing are brought essentially to zero relative velocity (the “no-slip condition”). Just above that, the air is moving a little; a bit higher, more; and at some small height above the surface the air is moving at the full local free-stream speed.

The boundary layer is that thin layer of air over the surface in which the local velocity is retarded by viscosity, from zero at the wall up to the free-stream value.

It is thin — millimetres to a few centimetres on a real wing — but it is where all the skin-friction drag comes from, and it is what decides whether the wing stalls.

2.2 Laminar → transition → turbulent

Follow the flow back from the leading edge of a smooth flat plate (Hurt Fig. 1.24; Anderson Fig. 4.53):

Laminar region. The flow near the leading edge moves in smooth layers (“laminations”) sliding over one another, with no mixing between elevations. The layer is very thin. Skin friction is low.
Transition. Friction keeps draining energy from the layer, and it thickens. At some distance back, a small oscillatory disturbance appears, grows into a waviness, becomes unstable, and destroys the smooth flow.
Turbulent region. The layer becomes chaotic, thickens rapidly, and air particles now travel between layers, exchanging energy. Skin friction is high.

Hurt’s picture for this: cigarette smoke in still air. First a smooth laminar ribbon, then a definite waviness, then a random turbulent pattern. Same physics.

Buried at the very bottom of a turbulent boundary layer there is still a paper-thin laminar sub-layer in contact with the surface.

(Two ways to find the transition point experimentally, both in Hurt: a turbulent layer transfers heat better, so frost/water/oil films clear faster aft of transition; and a probe on a stethoscope hears a low hiss in laminar flow and a sharp crackle in turbulent flow.)

2.3 The comparison you must be able to write

The velocity profile — how velocity varies with height above the surface — is what distinguishes the two.

	Laminar boundary layer	Turbulent boundary layer
Thickness	Thin	Thicker, and grows faster
Velocity next to the surface	Low	Higher (“fuller” profile)
Velocity gradient at the wall	Gradual	Sharp
Kinetic energy next to the surface	Low	High
Skin-friction drag	Low — about ⅓ of turbulent	High — roughly 3× laminar
Resistance to separation	Poor — separates early	Good — stays attached much longer

important. > Laminar gives you less drag. Turbulent gives you later separation and more maximum lift. You cannot have both, and every design decision about the boundary layer is a choice between them.

Why? Shearing stress is proportional to the velocity gradient at the wall. The laminar profile has a gentler gradient → less friction drag. But the turbulent profile has higher velocity and more kinetic energy right next to the surface → it can fight its way further into an adverse pressure gradient before it stagnates.

3. Reynolds number

3.1 What it is

Whether you get a laminar or a turbulent boundary layer depends on the combined effect of velocity, viscosity, distance from the leading edge and density. All of them are gathered into one dimensionless number.

Reynolds number is a dimensionless ratio that portrays the relative magnitude of dynamic (inertia) forces to viscous (friction) forces in the flow.

Two equivalent forms — know both, because both books use them:

Hurt's form:       RN = V x / ν

Anderson's form:   Re = ρ V L / μ

Symbol	Meaning	English units	SI units
RN, Re	Reynolds number	dimensionless	dimensionless
V	velocity	ft/s	m/s
x, L	characteristic length: distance from the leading edge, or the chord, or a sphere’s diameter — whatever the number is “based on”	ft	m
ν (nu)	kinematic viscosity = μ/ρ	ft²/s	m²/s
ρ	density	slug/ft³	kg/m³
μ (mu)	absolute (dynamic) viscosity	slug/(ft·s)	kg/(m·s)

Standard sea-level values (memorise or have them handy):

	English	SI
ρ₀	0.002377 slug/ft³	1.225 kg/m³
μ₀	3.7373 × 10⁻⁷ slug/(ft·s)	1.789 × 10⁻⁵ kg/(m·s)
ν₀ = μ/ρ	1.572 × 10⁻⁴ ft²/s	1.460 × 10⁻⁵ m²/s

warning. ## 3.2 Reading the number

Low RN ⇒ viscous/friction forces predominate. High RN ⇒ dynamic/inertia forces predominate.

From the definition, RN varies directly with velocity and with distance from the leading edge, and inversely with kinematic viscosity:

High RN ← large chords, high speeds, low altitude.
Low RN ← small chords, low speeds, high altitude (high altitude gives high kinematic viscosity, because ν = μ/ρ and ρ is small).

What RN predicts on a smooth flat plate (Hurt):

Reynolds number	Boundary layer
below ~500,000	entirely laminar (unless the surface is very rough or the stream is turbulent)
~530,000	transition begins on a smooth plate in a smooth stream
1 – 5 million	partly laminar, partly turbulent
above 10 million	predominantly turbulent — the transition point sits very close to the leading edge

Anderson uses Re_cr ≈ 5 × 10⁵ as the typical critical (transition) Reynolds number for low-speed flow over a surface. For a sphere, the critical Reynolds number is 3 × 10⁵ — this is the golf-ball number (see §4.5).

3.3 Scale effect

Because boundary-layer friction and kinetic energy depend on Reynolds number, the aerodynamic characteristics themselves vary with Reynolds number. That variation is called scale effect, and it is exactly why you cannot trust a small wind-tunnel model at low speed to tell you about a full-size aircraft in flight without correction.

The two section characteristics most affected: maximum lift and drag (pitching moment is usually negligibly affected).

As Reynolds number increases: c_l,max INCREASES (the higher-energy turbulent boundary layer stays attached longer) and c_d DECREASES.

This is also why the C_L curves in Anderson’s Appendix D are printed for three Reynolds numbers (3.1 × 10⁶ to 8.9 × 10⁶). Look at those curves and you will see: in the linear region they lie on top of one another (Re barely matters when the flow is attached), but they separate near C_L,max (because separation is a viscous phenomenon).

4. Pressure gradients and flow separation

4.1 Favourable versus adverse

Follow a fluid particle along the upper surface of an airfoil.

From the leading-edge stagnation point back to the minimum-pressure point, pressure is falling in the direction of flow. This is a favourable pressure gradient (high → low). It assists the boundary layer — it is being pushed downhill.
Aft of the minimum-pressure point, pressure must rise back toward free-stream/trailing-edge conditions. This is an adverse pressure gradient (low → high). It impedes the boundary layer — it is now being asked to climb uphill.

Meanwhile skin friction has been continuously draining energy out of the boundary layer all the way back from the leading edge. So the layer arrives at the uphill section already tired.

4.2 What separation actually is

Airflow separation results when the lower levels of the boundary layer do not have sufficient kinetic energy in the presence of an adverse pressure gradient. (Hurt — this is a definition worth memorising verbatim.)

Mechanically: the slowest air, right at the wall, is decelerated by the rising pressure until it stagnates prematurely — it stops. Once that happens, everything downstream has to overrun that stationary point, so the flow lifts off the surface. Behind the separation point the surface flow actually runs backwards, forward toward the separation point — a flow reversal.

How you can see it: tufts of string taped to the surface lie streamlined where the flow is attached, and flip forward in the reversed-flow region behind the separation point.

What separation costs you: the positive pressures are never recovered on the aft surface, so a large form (pressure) drag appears — and if it happens on the upper surface near the leading edge, the suction peak that was carrying most of the lift disappears, and the wing stalls.

4.3 Why turbulent boundary layers resist separation

Now the payoff from the table in §2.3. The turbulent velocity profile has higher local velocities and more kinetic energy immediately next to the surface. That extra energy is exactly what is needed to push through the adverse pressure gradient without stagnating.

If you have a choice, a turbulent boundary layer is preferable to a laminar one for resisting separation.

This one fact explains a startling number of things:

Every effective high-lift device — slots, slotted flaps, boundary-layer control — works by increasing the kinetic energy of the upper-surface boundary layer so it can survive the severe pressure gradients that come with high lift coefficients.
Vortex generators on jet wings create strong vortices that drag high-energy air down to the surface, delaying shock-induced separation and compressibility buffet.
Golf-ball dimples. A smooth sphere at Re below 3 × 10⁵ has a laminar boundary layer that separates early, giving a huge wake and high drag (C_D ≈ 0.4). The dimples trip the boundary layer turbulent, the separation point moves rearward, the wake shrinks and C_D drops to about 0.1 — a four-fold drag reduction. That’s why a golf ball flies so much further than a smooth ball.
Roughness, frost and ice do the opposite. Extreme surface roughness causes higher skin friction and greater energy loss in the boundary layer. A lower-energy boundary layer separates earlier ⇒ noticeably reduced C_L,max and increased stall speed. This is why frost on a wing is lethal even when it looks like nothing.

4.4 The three classic separation cases

Airfoil at high angle of attack. The pressure gradient on the upper surface becomes too severe for the boundary layer. The flow fails to stay attached near the leading edge, the high suction pressures are lost, and stall occurs.
Blunt bodies (cylinders, spheres). In ideal frictionless flow the pressure distribution would be symmetric front-to-back and there would be no drag at all — d’Alembert’s paradox. In real flow, the adverse gradient on the rear face separates the flow, the rear pressures are never recovered, and a large pressure drag appears. Friction resolves the paradox.
Shock-induced separation. At high subsonic speeds a shock wave forms on the upper surface. The abrupt static-pressure rise through the shock is an extremely strong obstacle for the boundary layer; if the shock is strong enough, separation follows and the turbulent wake produces compressibility buffet.

5. Stall

5.1 The C_L curve, now with the physics in it

Return to the lift curve from Chapter 3, and now attach the boundary-layer story to it (Anderson Fig. 5.8):

Point 1, on the linear part: the flow is fully attached over the whole airfoil. C_L rises about 0.1 for every degree of α.
Increasing α: the adverse pressure gradient on the upper surface becomes progressively stronger.
Point 2, the stalling angle of attack: the boundary layer can no longer withstand it. The flow separates from the top surface. Lift decreases drastically and drag increases suddenly.

Wind-tunnel oil-flow photographs (Anderson Fig. 5.9, a Grumman American Yankee model) show it happening: at α = 4° the flow is fully attached; at α = 11°, just past the stall, a large mushroom-shaped separated region has developed with low-energy recirculating flow; at α = 24° almost the entire wing has separated.

5.2 The two sentences that matter most

1. A given wing configuration reaches its maximum lift coefficient at ONE angle of attack and ONE pressure distribution. Weight, bank angle, load factor, density altitude and airspeed have NO direct effect on the stalling angle of attack.

2. Therefore recovery from a stall is always the same: REDUCE THE ANGLE OF ATTACK. It is precipitated by an excessive α, so α must be decreased. This is a fundamental principle common to any airplane.

Everything else follows:

Stall warning devices must sense angle of attack or pressure distribution — never airspeed. That is why angle-of-attack indicators exist.
The airspeed at which the stall occurs is determined by weight, load factor and altitude, but the angle never changes.
An aircraft can be made “stall-proof” by making the elevator too weak to hold a high α — but that would cripple its manoeuvrability and raise its landing and takeoff speeds, so it is not done on tactical aircraft.

5.3 Effect of Reynolds number on C_L,max

Because separation is a viscous phenomenon, higher Reynolds number ⇒ higher-energy turbulent boundary layer ⇒ separation delayed ⇒ higher c_l,max. This is why:

A model in a wind tunnel usually shows a lower C_L,max than the full-size aircraft it represents (scale effect).
Above about 20,000 ft, the change in compressibility and viscosity effects generally causes the indicated stall speed to increase. Below that it is a negligible effect.

6. Stall speed

Start with the lift equation and impose the two conditions that define a stall in level flight: lift equals weight, and the wing is at C_L,max.

L = C_L · q · S            (lift equation)
L = W                      (steady, level flight)
C_L = C_L,max              (stall, by definition)

Substituting q = ½ρV² and solving for V:

W = ½ ρ V² S C_L,max     →     V_stall = √[ 2W / (ρ S C_L,max) ]

That is Anderson’s Eq. (5.71). In aviation units, substituting q = σV²/295 instead gives Hurt’s form:

V_s = 17.2 · √[ W / (σ · C_L,max · S) ]

Symbol	Meaning	Anderson’s form	Hurt’s form
V_stall / V_s	stalling speed	ft/s or m/s	knots TAS
W	gross weight (= lift in level flight)	lb or N	lb
ρ	air density	slug/ft³ or kg/m³	—
σ	density ratio ρ/ρ₀ (1.000 at sea level)	—	dimensionless
S	wing area	ft² or m²	ft²
C_L,max	maximum lift coefficient of the whole configuration	dimensionless	dimensionless

If you want the answer in EAS instead of TAS, use σ = 1.000. Equivalent airspeed is defined at sea-level density, so the stall speed in EAS/IAS doesn’t change with altitude — only the TAS does.

6.2 What the equation tells you before you plug in numbers

Everything is under a square root, which is the key to every trap in this section.

V_s  ∝  √W          ∝  1/√σ          ∝  1/√S          ∝  1/√C_L,max

Heavier ⇒ higher stall speed, but only as the square root. A 2% change in weight causes about a 1% change in stall speed (Hurt’s rule of thumb).
Higher altitude (smaller σ) ⇒ higher TAS at stall, but the same indicated stall speed.
Bigger wing ⇒ lower stall speed. But a bigger wing is heavier and draggier — hence the appeal of high-lift devices instead.
Higher C_L,max ⇒ lower stall speed — and this is the term high-lift devices attack.

warning. > | % increase in C_L,max | 2% | 10% | 50% | 100% | 300% | |—|—|—|—|—|—| | % reduction in stall speed | 1% | 5% | 18% | 29% | 50% |

Doubling C_L,max only buys you a 29% lower stall speed. To halve the stall speed you would have to quadruple C_L,max — which needs “the most elaborate combination of flaps, slots, slats and boundary layer control throughout the span.”

7. Stall patterns

A stall does not happen all at once across the span. Where it starts is set by the spanwise lift distribution, which is set by the planform.

7.1 What a good stall pattern looks like

The desirable stall pattern is one that begins at the wing ROOT first. Three reasons: 1. The ailerons remain effective at high angle of attack, so you keep roll control. 2. The separated wake from the root washes over the empennage and aft fuselage, giving buffet — a natural stall warning. 3. The loss of downwash behind the root gives a stable nose-down pitching moment, which helps the aircraft recover on its own.

A tip stall gives you the opposite of all three: aileron buffet and wing drop, no empennage buffet, no nose-down moment, and little or no aileron effectiveness — exactly when you need roll control most.

7.2 Planform by planform

Planform	Spanwise lift distribution	Stall pattern	Verdict
Elliptical (taper ratio matched to an ellipse)	Local c_l is constant root to tip	The whole span stalls at once	Aerodynamically most efficient (lowest induced drag for a given aspect ratio), but poor stall behaviour: little warning, and ailerons may lack effectiveness. Also very hard to build.
Rectangular (λ = 1.0)	Low c_l at the tip, high c_l at the root (root c_l can be ~20% above the wing average)	Strong root-stall tendency	Best stall behaviour: buffet warning, aileron effectiveness, stable nose-down moment. But aerodynamically and structurally inefficient — an AR = 6 rectangular wing has ~16% higher induced angle of attack and ~5% more induced drag than an ellipse. Found on low-cost light aircraft.
Moderate taper (λ = 0.5)	Very close to elliptical	Similar to elliptical	The practical sweet spot: only ~3% higher induced angle of attack and ~1% more induced drag than an ellipse, with a far easier structure.
High taper (λ = 0.25)	Distinct peaks just inboard of the tip	Strong TIP-STALL tendency (starts inboard of the tip, where the local c_l peaks)	Dangerous if left alone — aileron buffet and wing drop with no warning. Must be tailored (see §7.3).
Pointed tip (λ = 0)	Low c_l at the root, extremely high c_l at the tip	Tip is stalled at essentially any lift condition	No practical subsonic application. Also inefficient: ~17% higher induced angle of attack, ~13% more induced drag than an ellipse.
Sweptback	Alters the distribution like reducing taper ratio; plus a strong spanwise crossflow of the boundary layer toward the tip	Tip stall, made worse by the thickened, low-energy boundary layer piling up at the tips	Worst of all: tip stall on a swept wing shifts the centre of pressure forward, producing an unstable nose-up pitching moment (“pitch-up”). Sweep also reduces lift-curve slope and C_L,max, so C_L,max can occur at enormous angles of attack (one low-AR swept aircraft peaks at ~43° α).

7.3 How designers fix a bad stall pattern

All of these are ways to make the root stall before the tip:

Washout (geometric twist) — build the wing with the tip at a lower incidence than the root, so the tip runs at a lower local α and stalls later.
Aerodynamic twist / section variation — put thicker, more cambered sections (with higher c_l,max) in the regions of highest local c_l.
Leading-edge slots or slats toward the tip — raise the local c_l,max and stall angle out there, preserving aileron effectiveness.
Stall strips / sharp leading edges at the root — deliberately reduce the section c_l,max at the root to force it to stall first. Crude but powerful.
Wing fences and vortilons on swept wings — block the spanwise boundary-layer crossflow.

7.4 Two things that change the stall pattern in flight

Flaps. Deflecting trailing-edge flaps raises the local c_l in the flapped area and lowers the stall angle there, so the initial stall usually begins in the flapped (inboard) area — which is convenient, because that is the root. Extending slats does the opposite locally: it lets the slatted area go to higher c_l and α, delaying stall there.
Power (propeller aircraft). At high power and low speed, the propeller slipstream induces flow over the wing root that can substantially delay root stall. So a propeller aircraft may have its worst stall characteristics power-ON, not power-off — the root is being artificially kept flying while the tips stall.

8. High-lift devices

8.1 The purpose

The primary purpose of high-lift devices (flaps, slots, slats, boundary-layer control) is to increase C_L,max and thereby reduce the stall speed — which reduces takeoff and landing speeds and distances.

Look back at V_stall = √[2W/(ρSC_L,max)]. For an aircraft of a given weight and size, at a given altitude, the only recourse to minimise stall speed is to maximise C_L,max. For a fixed airfoil shape, C_L,max is fixed by nature. High-lift devices are how we cheat.

8.2 Trailing-edge flaps

A trailing-edge flap is typically 15–25% of the chord. Deflecting it produces the effect of a large amount of camber added well aft on the chord.

Anderson’s two-part explanation of why the lift coefficient goes up: 1. The effective camber of the section is increased — more camber at a given α means a higher lift coefficient. 2. A “virtual” increase in angle of attack. Draw a line from the leading edge of the airfoil to the trailing edge of the deflected flap: that virtual chord line is rotated, so the section effectively sees a higher α.

The four types (Hurt Fig. 1.17 — a 25%-chord flap deflected 30°):

Flap type	What it is	Effect on c_l,max	Drag	Nose-down twisting moment
Plain	A simple hinged portion of the trailing edge	Significant increase	Increases greatly	Moderate
Split	A plate deflected from the lower surface only	Slightly greater than plain	Much larger than plain — huge turbulent wake	Least of the four
Slotted	Like a plain flap, but the gap between the wing and the flap is contoured so high-energy air from the lower surface is ducted onto the flap’s upper surface, re-energising the boundary layer and delaying separation	Much greater than plain or split	Much lower than plain or split	Large
Fowler	Like a slotted flap, but the flap also translates aft on tracks, increasing the chord and hence the wing area	Largest	Minimum change in drag	Greatest

Anderson’s ranking of C_L,max, worst to best (Fig. 5.69): airfoil alone → plain flap → split flap → leading-edge slat → single-slotted flap → double-slotted flap → double-slotted flap with a leading-edge slat → plus boundary-layer suction.

The extra drag of a split flap is not always a disadvantage — it lets you fly a steeper approach over obstacles, and it forces a higher power setting on approach, which shortens engine spool-up time if you have to go around.

Effectiveness depends on: how much of the span is flapped (some span is reserved for ailerons, so a real wing never reaches the flapped 2-D section’s C_L,max); wing thickness (any flap is less effective on a thin wing); and sweepback, which causes a significant further reduction.

8.3 The critical exam point: what flaps do to the LIFT CURVE

  C_L │            ┌── flaps down: HIGHER C_L,max,
      │           ╱╲     reached at a LOWER α
      │          ╱  ╲
      │         ╱     ╲___
      │        ╱        ╭──╮  ← flaps up (clean)
      │       ╱      ╱      ╲
      │      ╱     ╱          ╲
      │     ╱    ╱
      │    ╱   ╱      SAME SLOPE — the two curves are parallel
   0 ─┼───╱──╱─────────────────────────── α
      │  ╱ ╱
   flaps-down α_L=0    clean α_L=0
   (more negative)

danger. > This is the most-missed question on this topic. Deflecting flaps: - shifts the whole lift curve to the LEFT and UP (α_L=0 becomes more negative) - increases C_L,max - DECREASES the angle of attack at which C_L,max occurs - leaves the lift-curve slope dC_L/dα UNCHANGED — the curves are parallel - increases drag and produces a large nose-down pitching moment

The reason is intuitive once you have the picture: the flap has already spent some of the section’s “camber budget,” so the section reaches its separation-limited pressure distribution at a lower geometric angle.

8.4 Leading-edge devices — slots and slats

A fixed slot is a permanent gap near the leading edge that conducts high-energy air from the lower surface into the boundary layer on the upper surface, delaying separation to a higher angle of attack and lift coefficient.

A slot effects no change in camber. Therefore the higher C_L,max is obtained at a HIGHER angle of attack — the slot simply delays the stall to a higher α. This is the opposite of a flap.

An automatic slat is a leading-edge segment free to move on tracks. At low α the high positive local pressures at the leading edge hold it flush. At high α the strong local suction at the leading edge creates a forward chordwise force that pulls the slat out, opening the slot.

Advantages of slots/slats: - Only a negligible change in the pitching moment — so they are usable on tailless/flying-wing designs and at high speed where trim changes are unwelcome. - No significant increase in section drag at low α; near the stall the slotted section actually has less drag than the basic section. - More effective than trailing-edge flaps on highly swept wings, because slats are powerful at controlling the flow pattern.

Disadvantage: the higher stalling angle itself. If slots were the only high-lift device fitted, the takeoff and landing attitudes would be so nose-high that the landing gear would have to be absurdly long. That is why slats and flaps are used together — the slat raises C_L,max at a high α, and the flap pulls the maximum-lift angle back down to something usable.

8.5 Boundary layer control (BLC)

Directly attacks the underlying problem: keep the upper-surface boundary layer energised so it does not stagnate. Two methods:

Suction BLC — draw off the low-energy boundary-layer air through ports on the upper surface, replacing it with high-velocity air from outside the layer. Increasing suction gives greater C_L,max at higher angles of attack. Requires a dedicated pump.
Blowing BLC — inject a high-speed jet of air into the boundary layer. Same result, and far more practical, because a jet engine’s compressor is already a high-pressure air source. Typically used to augment a deflected flap.

Because BLC (like a slot) tends to increase the angle of attack for maximum lift, it is normally combined with flaps, which reduce it.

8.6 Summary table — the one to memorise

Device	Effect on C_L,max	Effect on stall ANGLE	Effect on drag	Effect on pitching moment
Trailing-edge flap (plain, split, slotted, Fowler)	Increase	DECREASE	Increase (large for plain/split, small for Fowler)	Large nose-down
Leading-edge slot / slat	Increase	INCREASE	Negligible at low α; reduced near stall	Negligible
Leading-edge flap / local LE camber	Increase	Increase	Increase	Small
Boundary layer control (suction or blowing)	Increase	INCREASE	—	Small
Flaps + slats together	Largest increase	Roughly unchanged (they cancel)	Increase	Nose-down

8.7 Managing flaps in flight

Retracting flaps after takeoff (three changes, all of which the pilot must handle): 1. Reduced camber changes the wing pitching moment — for most aircraft this is a nose-up moment change, requiring retrimming. 2. Drag coefficient at that lift coefficient decreases, improving acceleration. 3. Angle of attack must be increased to hold the same lift coefficient — so if acceleration is poor (heavy, hot, high), you must raise the nose or the aircraft will sink.

Never retract the flaps prematurely at insufficient airspeed: the clean configuration’s C_L,max may not be able to support the aircraft, and it will sink or stall.

Extending flaps for landing: 1. Nose-down moment change — retrim. 2. Increased drag — needs a higher power setting to hold airspeed and altitude. 3. Less α is needed for the same lift coefficient, so the aircraft tends to “balloon.” 4. Respect the flap limit speed (V_FE) — excessive airspeed with flaps down can cause structural damage.

The intermediate-deflection rule of thumb: for most aircraft, the first 50% of flap deflection gives more than half the total change in C_L,max, while the last 50% gives more than half the total change in C_D. Beyond about 30–35° of deflection you are buying mostly drag, not lift. That is exactly why takeoff flap settings are small and landing settings are large.

9. Common traps

“Flaps increase the stalling angle of attack.” ❌ Trailing-edge flaps DECREASE it. It is leading-edge slots and slats (and BLC) that increase it. This is the single most-missed exam point in the chapter.
“Flaps make the lift curve steeper.” ❌ The slope dC_L/dα is unchanged. The curve shifts left and up; it does not rotate.
“The aircraft stalls at 60 knots.” ❌ An aircraft stalls at an angle of attack, not a speed. Change the weight, bank angle or load factor and the stall speed changes; the stall angle does not.
Forgetting the square root. V_s ∝ √W and V_s ∝ 1/√C_L,max. Doubling C_L,max does not halve the stall speed — it reduces it by 29%.
Confusing turbulence in the boundary layer with separated flow. These are different scales. A turbulent boundary layer is small-scale mixing within a layer that is still attached — and it is good, because it resists separation. Separation is the large-scale breakaway of the whole flow.
“Turbulent is bad, laminar is good.” ❌ Laminar has lower friction drag; turbulent gives later separation and higher C_L,max. Which one you want depends on what you are optimising.
Using a Reynolds number without saying what length it is based on. Chord? Distance from the leading edge? Sphere diameter? The number is meaningless otherwise.
Putting knots into Re = ρVL/μ or q = ½ρV². Those need ft/s or m/s. Convert: 1 knot = 1.6878 ft/s = 0.5144 m/s.
Mixing up μ and ν. Re = ρVL/μ (absolute) = VL/ν (kinematic), where ν = μ/ρ. Using the wrong one is off by a factor of ρ.
Assuming stall speed rises with altitude on the airspeed indicator. The indicated/equivalent stall speed is essentially unchanged; the true airspeed at stall increases as 1/√σ. (Above ~20,000 ft the indicated stall speed does creep up, from compressibility and viscosity effects.)
Thinking a tip stall is merely inconvenient. On a swept wing, tip stall shifts the centre of pressure forward and produces an unstable nose-up pitching moment — the aircraft pitches further into the stall.
Ignoring frost, ice, dents or bugs. They roughen the surface, drain energy out of the boundary layer, cause earlier separation, and reduce C_L,max — which raises the stall speed with no warning on any instrument.

10. Can you do this?

Previous: ASCI 309 Ch 3 — Airfoils, Lift and the Aerodynamic Center · Self-test: ASCI 309 Ch 3-4 Cards

ASCI 309 Hub