Pitch and power coupling is a type of aerodynamic coupling. When the power setting of an airplane is changed in flight, a change in pitching moment and longitudinal trim is induced. The magnitude of this can vary considerably with airplane design, but is most notable for causing a **nose-up pitching moment when power is increased**, in conventional single-engine designs. In a conventionally designed airplane, longitudinal (pitch) stability is achieved with a center of gravity ahead of the center of pressure (wing lift vector), and a balancing downward aerodynamic force generated by the horizontal tail surfaces. ![[PHAK pitch stability.png]] ## Accelerated Airflow Conventional tail designs place the horizontal tail surfaces in the accelerated airflow from the propeller, or "propwash". Increasing power increases this accelerated airflow, and produces a **nose-up pitching moment**. Likewise, decreasing power decreases this accelerated airflow, and produces a **nose-down pitching moment**. Other tail designs, such as **cruciform** or **T-tail** configurations may experience some or none of this phenomenon. Additionally, increases in wing lift due to accelerated airflow over the wings can contribute to a nose-up pitching moment. ![[A4NA propwash horizontal tail.png]] ## Flow Turning Like a wing, the propeller induces flow turning; it changes the speed and direction of the airflow around it and passing through it. As the airflow is accelerated and turned downward, an upward reaction force is experienced at the propeller. This produces a **nose-up pitching moment** in airplanes with the propeller ahead of the center of gravity. This flow turning will produce a maximum reaction force at **low airspeed, high power, and high angle of attack** conditions. ![[A4NA propeller normal force.png]] ## Vertical Location of Thrust Line The vertical location of the thrust line, with respect to the center of gravity, will also contribute to pitching moment and longitudinal trim. Airplanes with a thrust line **below** the center of gravity will experience a **nose-up pitching moment when power is increased**. Airplanes with a thrust line **above** the center of gravity will experience a **nose-down pitching moment when power is increased**. ![[A4NA thrust line vs pitch moment.png]] ## Factors Affecting Pitch and Power Coupling For a particular model of airplane, pitching moment and longitudinal trim changes resulting from power changes will be greatest in low airspeed, high angle of attack conditions. ### Aircraft Design - less longitudinal stability (ie shorter tail, smaller horizontal tail surfaces, etc) will resist pitching moment changes less - more available engine power can produce larger changes in pitching moment - propeller location ahead of the center of gravity will determine arm for flow turning effects - vertical location of thrust line may introduce additional pitching moments ### Power Setting - increasing power setting increases accelerated airflow, flow turning, and thrust line effects ### Angle of Attack - higher angle of attack moves the center of pressure closer to the center of gravity and reduces longitudinal stability - higher angle of attack increases flow turning effects ### Airspeed - lower airspeed decreases longitudinal stability - lower airspeed results in greater acceleration of airflow - lower airspeed is usually associated with high angle of attack conditions ### Configuration - depending on aircraft design, extension of high lift devices (ie flaps) may increase downwash on the horizontal tail surfaces, and contribute to a greater nose-up pitching moment; this is especially true for **high-wing, conventional tail designs** ## Additional Information For additional information, see *Longitudinal Stability (Pitching)* (pgs. 5-15 to 5-17) in the *Pilot's Handbook of Aeronautical Knowledge*, and *Power Effects* (pgs. 259 to 264) in *Aerodynamics for Naval Aviators* # References - [[faa.gov/regulations_policies/handbooks_manuals/aviation/faa-h-8083-25c.pdf](https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/faa-h-8083-25c.pdf)]() - [[00-80T-80.pdf](https://www.faa.gov/sites/faa.gov/files/regulations_policies/handbooks_manuals/aviation/00-80T-80.pdf)]() ### By Kevin Sakson