Flaps

Flaps

Flaps are moveable surfaces at the trailing edge of the wing. They increase the maximum lift coefficient CL_max for take-off and landing, without compromising cruise performance (flaps retracted).

Effect

Flap deflection alters the airfoil:

1. Camber increased → higher CL at the same α.
2. Effective chord enlarged (with Fowler flaps).
3. Wing area extended in some flap types (slot effect).

Results:

• CL_max rises (typ. 1.4 to 2.4 with full flaps).
• Stall speed Vs drops: Vs0 (full flap) < Vs1 (clean).
• CD rises too → steeper descent angle possible (good for steeper approach).
• Nose-down pitching moment (typical) — trim correction required.
• α_stall shifts to lower values (flap-out profile stalls earlier).

Flap types

1. Plain flap

• Simple rotation of trailing edge downward.
• Effect: CL_max +0.5; CD rises significantly.
• Examples: Cessna 152 (pre-1979), basic trainers.

2. Split flap

• Lower profile half hinges down, upper stays.
• Effect: CL_max +0.5; CD rises strongly — good for steep approaches.
• Examples: DC-3 (historical), little modern use.

3. Slotted flap

• Plain flap with a slot between flap and wing → flow from below blows through → energises boundary layer → later separation.
• Effect: CL_max +0.7; CD moderate.
• Very common in GA and airliners.

4. Fowler flap

• Moves rearward and downward — effectively enlarges wing area.
• Effect: CL_max +0.9 to +1.0; greatest effect of all flap types.
• Examples: Cessna 172 (post-1980), DA-40, B737, A320.

5. Double / triple slotted Fowler

• Multi-slot Fowler: even more effective boundary-layer energisation.
• Examples: B747, B777 (triple slotted), A380.
• Very complex mechanically.

Flap settings

Typical values (POH-specific):

Vfe — Maximum Flap Extended Speed

Vfe is the maximum speed at which flaps may be extended (POH). Exceeding → structural damage possible.

C172 Vfe (full flaps): 85 KIAS.

Flap-extension sequence

On approach:

1. Downwind, pattern altitude: possibly first flap setting (10°).
2. Base, reduction to Vapp: flaps 20° or 30°.
3. Final: full flaps or slightly less depending on crosswind / wind shear risk.

Flap-retraction sequence

On go-around:

1. Throttle FULL immediately.
2. Carb heat OFF.
3. Pitch to Vy.
4. Flaps retract gradually (10° at a time), after speed builds.
5. Consult POH — some aircraft require flaps directly retracted.

Pitching-moment effect

Flap extension produces nose-down pitching moment (typical). Pilot compensates with pitch-up trim.

Some aircraft (e.g. DA-40 with auto-trim) correct automatically.

Stalls with flaps

Stall speed lower with full flaps (Vs0 < Vs1), but:

• Stall AoA is lower (flap profiles stall at lower α).
• Recovery can be more complex due to extended flap moment arm.
• Therefore: never enter stall with full flaps without a plan, practice with instructor.

Construction aspects

• Flap drive: mechanical (hand crank C172) or electric (PA-28, DA-40).
• Switch: usually 3- or 4-step in cockpit.
• Indication: flap position indicator.

Efficient flap use

• Short-field take-off: 10° for lift, Vr at lower speed.
• Short-field landing: full for minimum Vs, steep approach.
• Crosswind landing: less flap (e.g. 20° instead of full) for better aileron effectiveness.
• Soft field: 10° + roll + lift-off in ground effect.