PoF 18/08/25

Describe the geometry of a typical propeller blade element and define geometric pitch, effective pitch, and propeller slip.

A propeller blade element includes the chord line, rotational velocity, and true airspeed vectors. Geometric pitch is the theoretical distance a propeller would move forward in one full rotation. Effective pitch is the actual distance it travels. Propeller slip is the difference between geometric and effective pitch, representing efficiency loss.

In straight and level flight, what happens as angle of attack (alpha) increases?

As alpha increases, lift (CL) rises until the critical angle (alphaCRIT), after which stall occurs. The stagnation point moves, center of pressure (CP) shifts, drag (CD) increases, pitching moment becomes more nose-down, buffet may begin, pitch authority is reduced, and uncommanded roll can happen near the stall.

How does induced drag affect the CL–alpha and CL–CD graphs?

Induced drag increases rapidly with higher CL. It curves the CL–CD (polar) graph and limits further lift on the CL–alpha graph. A higher aspect ratio wing reduces this effect. The parabolic drag polar is expressed as CD = CPD + kCL².

What are the typical stall warning indications?

Stall warnings include aerodynamic buffet, reduced aileron or pitch authority, stick shaker activation, visual or aural alerts, and uncommanded nose drop or roll. These occur just before stall to warn the pilot.

What is the difference between angle of incidence and angle of attack?

Angle of incidence is fixed—it’s the angle between the wing chord line and aircraft longitudinal axis. Angle of attack is variable—it’s the angle between the chord line and relative airflow. Incidence is set during manufacturing, while alpha changes during flight.

What affects the gust-load factor on an aircraft?

Gust-load factor depends on wing loading, aspect ratio, sweep angle, airspeed, altitude, and vertical gust speed. Heavier aircraft, higher speeds, or stiffer wings can produce greater load spikes from gusts.

Why might buffet or stall occur in pilot-induced situations?

Poor configuration or aggressive maneuvers like steep turns, TOGA go-arounds, or forgetting to extend slats/flaps can raise alpha too quickly or reduce lift, triggering stall or buffet unexpectedly.

What affects the Cm–alpha graph and pitch stability?

Aft CG reduces stability (flatter slope), forward CG increases stability (steeper slope). Elevator movement or trim shifts the graph vertically. Wing adds destabilizing nose-up moments; tail adds stabilizing nose-down moments.

What are the risks of exceeding VMO or MMO, and what is coffin corner?

Exceeding VMO (max operating IAS) risks structural damage; exceeding MMO (max Mach) risks shock stall or loss of control. At high altitudes, stall speed increases and MMO decreases—creating a narrow speed margin called “coffin corner.”

What is Dutch roll and how is it controlled?

Dutch roll is an oscillation combining yaw and roll, caused by strong lateral but weak directional stability. It's damped automatically by the yaw damper. Without it, the motion can become unstable and dangerous.