Increasing the number of propeller blades mainly serves to increase the power absorption capability by increasing the solidity of the propeller disk. Although adding blades can cause more aerodynamic interference and slightly reduce efficiency, it prevents noise increases associated with longer blades. It does not reduce noise nor significantly affect the constant speed unit’s effectiveness, but it allows the propeller to absorb more engine power safely.

Gyroscopic precession depends on the mass of the propeller, its rotational speed (RPM), and any force tilting the plane of rotation. Increasing RPM directly increases the magnitude of the precession effect, while changes in angle of attack or TAS affect RPM and thus reduce precession. Pitching affects precession, but rolling does not.

When the propeller lever is retarded in a constant speed propeller aircraft descending with throttle closed and RPM set at 2000, the constant speed unit increases blade angle toward feather to prevent overspeed. This reduces RPM and windmilling drag, resulting in both a lower RPM and a decreased rate of descent if the aircraft attitude is held constant.

The torque reaction from a right-handed tractor propeller causes the engine to roll the aircraft to the left, which increases weight on the left wheel and decreases weight on the right wheel during the take-off roll. This is a direct effect of the propeller’s torque on the airframe.

The propeller slipstream from a right-handed tractor propeller spirals around the fuselage in the same direction as the propeller rotation, striking the left side of the vertical fin. This creates an angle of attack on the fin, generating a force that pushes the tail to the right, causing the aircraft to yaw left during takeoff.

Geometric pitch defines the theoretical forward distance a propeller would travel in one full revolution assuming no slip—meaning perfect efficiency without any loss due to drag or airflow disturbances. This ideal measure helps understand propeller performance but differs from actual forward motion because some energy is lost as slip, which reduces the effective distance moved.

When a propeller is feathered, its blades move to the maximum coarse pitch, aligning near the zero lift angle of attack. This stops the blades from producing lift and windmilling, which causes high drag. As a result, the propeller experiences minimum aerodynamic drag, reducing engine windmilling torque and improving aircraft performance during engine failure or shutdown.

The angle of attack of a propeller blade is defined as the angle between the chord line of the blade and the relative airflow. This airflow results from the combined effect of the rotational velocity of the blade (due to RPM) and the forward velocity of the aircraft. It is a critical factor affecting thrust production and aerodynamic performance.

Critical tip speed refers to the point where the combined airflow—from blade rotation (RPM) and forward inflow (TAS + acceleration)—over the blade tips reaches the local speed of sound. This causes shock waves and marks the onset of compressibility effects, leading to a drop in propeller efficiency and potential structural concerns.

Propeller blade angle is defined as the angle between the chord line of the blade and the plane of rotation, which is perpendicular to the propeller shaft. This is a geometric angle, not to be confused with the angle of attack, which involves the relative airflow. Blade angle influences thrust generation and propeller performance across different speeds.

As the propeller rotates, the rotational speed increases from root to tip, causing variations in relative airflow along the blade. Without varying the blade angle, the angle of attack would become too high at the tip, risking stall, and too low at the root, reducing efficiency. A twist in blade design maintains an optimal angle throughout.

In a constant speed propeller, the RPM is maintained constant by automatically adjusting the blade angle. When TAS increases, the relative airflow over the blades increases, which reduces the angle of attack. This causes engine torque to exceed propeller torque, resulting in a tendency for RPM to increase. The constant speed unit responds by increasing the blade angle, which restores the original angle of attack, increases propeller torque, and brings the RPM back to the set value. This mechanism ensures efficient operation across a range of airspeeds by modulating blade pitch.

The angle of attack of a fixed pitch propeller is determined by the angle between the chord line and the relative airflow, which is a combination of rotational airflow (from RPM) and inflow (from TAS and air acceleration). Increasing RPM boosts the rotational component, increasing the angle of attack. Decreasing TAS reduces the forward component of the airflow, also increasing the angle. Thus, the angle of attack increases when RPM is increased and TAS is decreased, enhancing thrust production but also potentially risking blade stall if limits are exceeded.

Pulling into a climb without increasing power causes a decrease in TAS, as some thrust is redirected to oppose gravity. With RPM initially constant, the rotational component of airflow forms a larger proportion of the relative airflow, increasing the angle of attack. This results in greater propeller torque, eventually reducing RPM, but angle of attack increases first.

Propeller blades, shaped like aerofoils, move through the air at a positive angle of attack, generating a total aerodynamic reaction. This reaction is angled backward and not directly forward. The forward-facing component of this reaction is called thrust, which propels the aircraft. The opposing component creates propeller torque, balanced by engine torque to maintain RPM.

Propeller efficiency is maximized when the propeller wash speed is only slightly higher than the aircraft’s TAS. This minimizes the wasted kinetic energy imparted to the air, improving propulsive efficiency. According to Newton’s second law, thrust results from accelerating air mass, so handling a larger mass of air with minimal acceleration yields the most efficient propulsion.

A sudden increase in air density raises the dynamic pressure on the propeller blades, increasing the total aerodynamic reaction, which includes both thrust and propeller torque. Since engine torque initially remains unchanged, the increased propeller torque causes a temporary imbalance, leading to a reduction in RPM. Although the engine may eventually respond by producing more power, the initial effect is a drop in RPM.

A windmilling propeller experiences a higher drag than a stationary one because it operates at a negative angle of attack, generating a large total aerodynamic reaction. This includes both profile drag and induced drag, the latter being increased due to the propeller extracting energy from the airflow to turn the engine. In contrast, a stationary feathered propeller aligns with the airflow and produces only profile drag, making its drag significantly lower.

In a glide, reducing propeller pitch moves the blades away from the feathered (zero-lift) position, causing them to windmill and experience a negative angle of attack. This results in increased induced and profile drag, as the propeller now extracts energy from the airflow. The increased drag leads to a higher rate of descent and a lower lift-to-drag (L:D) ratio, reducing glide efficiency.

Propeller efficiency is defined as the ratio of Thrust Horsepower (THP) to Shaft Horsepower (SHP). It measures how effectively the mechanical power delivered by the engine shaft is converted into useful thrust by the propeller. Like all efficiency metrics, it is calculated as output over input, so:
Propeller Efficiency = THP / SHP.

In a piston engine aircraft, the engine produces Brake Horsepower (BHP), which is then converted into Thrust Horsepower (THP) by the propeller. Propeller efficiency measures how effectively this conversion happens. Like any efficiency formula, it is expressed as output divided by input, so:
Propeller Efficiency = THP / BHP.

In a glide, the propeller produces no thrust, but contributes to aerodynamic drag. Selecting a higher propeller RPM causes the blades to move to a finer pitch, increasing the windmilling effect and thereby increasing drag. This elevated drag reduces glide efficiency, leading to a decrease in both glide range and Lift-to-Drag (L:D) ratio, which directly shortens the aircraft’s potential travel distance during a glide.

During take-off with a right-handed tractor propeller (rotating clockwise when viewed from the cockpit), the engine applies torque to the propeller, and by Newton’s third law, an equal and opposite torque acts on the aircraft. This causes the aircraft to roll and yaw to the left—the roll due to torque reaction and the yaw due to uneven wheel loading before lift-off.

During the take-off run with a right-handed tractor propeller (clockwise rotation when viewed from behind), the torque reaction acts in the opposite direction, attempting to roll the aircraft to the left. Since the aircraft is on the ground, this roll is resisted by the undercarriage, leading to more load on the left wheel and less on the right.

During take-off with a right-handed tractor propeller, the slipstream spirals around the fuselage in a clockwise direction (viewed from behind). This spiral flow strikes the left side of the vertical fin, generating a side force that pushes the tail right and causes left yaw. The initial effect may include a right roll, but the left yaw induces a dissymmetry of lift, eventually leading to a left roll as well. Thus, the overall effect is left roll and yaw.

During take-off with a tailwheel aircraft and a right-handed tractor propeller, the aircraft is in a nose-high attitude, causing the propeller disc to be tilted relative to the flight path. This tilt results in the down-going blade (on the right side) experiencing a higher relative airspeed and greater angle of attack, producing more thrust than the up-going blade. This asymmetric thrust, known as asymmetric blade effect or P-factor, causes the thrust line to shift right, creating a yawing moment to the left. Hence, the aircraft tends to yaw left during take-off.

During take-off with a nose-wheel aircraft and a right-handed tractor propeller, asymmetric thrust effect (also called P-factor) becomes noticeable after rotation, when the aircraft adopts a nose-up attitude. This increases the angle between the propeller disc and relative airflow, causing the right (down-going) blade to produce more thrust than the left (up-going) blade. The lateral thrust shift causes the aircraft to yaw left.

During take-off in a tailwheel aircraft with a right-handed tractor propeller, the nose initially pitches down as the tail rises. Due to gyroscopic precession, this nose-down pitching moment causes a yawing force 90° ahead in the direction of propeller rotation. For a right-handed propeller, this results in a left yaw during the early part of the take-off roll.

Later, when the aircraft rotates nose-up for liftoff, gyroscopic precession would then cause a right yaw.

So, initial effect = left yaw

In a twin-engine aircraft with right-handed tractor propellers (both rotating clockwise when viewed from behind), the left engine is the critical engine. This is because if the left engine fails, the right engine’s thrust (acting farther from the centerline) creates a greater yawing moment, making directional control more difficult.

Gyroscopic precession depends on the rotational speed (RPM) and mass of the rotating object. Since the propeller’s mass is constant, RPM changes directly affect the magnitude of gyroscopic precession. While pitch or yaw can initiate precession, RPM determines its strength.

Gyroscopic precession is induced by any motion that tilts the plane of rotation, which occurs during pitching and yawing. These movements apply a force to the axis of the rotating propeller, leading to a precession force that acts 90° ahead in the direction of rotation. Roll does not affect the propeller’s rotational plane, so it doesn’t induce precession.

Geometric pitch is the theoretical forward distance a propeller would travel in one full revolution assuming no air slip and zero angle of attack. It’s based purely on blade geometry (chord line and blade angle). In reality, due to slip and angle of attack, the actual distance covered (effective pitch) is less than the geometric pitch.

Blade angle is the angle between the chord line of a propeller blade and the plane of rotation. The helix angle is the angle between the relative airflow and the plane of rotation. The angle of attack is the difference between the blade chord and the relative airflow. So, blade angle = helix angle + angle of attack.

A feathered propeller sets its blades at the zero-lift angle of attack, producing no lift and hence no induced drag, only minimal profile drag. This greatly reduces total drag, preventing the propeller from windmilling and thereby minimizing drag. This configuration is used after engine failure to maximize glide range, not to affect glide speed.

In a fixed pitch propeller, the blade angle is constant and cannot change. As TAS increases, the relative airflow approaches more from the front, reducing the angle of attack on the blades. Therefore, angle of attack decreases with increasing TAS. Options mentioning blade angle changes are invalid for fixed pitch propellers.

In a fixed pitch propeller, the blade angle is fixed and cannot change. Increasing RPM increases the rotational airflow component, which approaches from beneath the blade. This increases the angle of attack since the relative airflow vector shifts. Thus, at a constant TAS, angle of attack increases with increasing RPM, enhancing lift and torque.

In a constant speed propeller, the blade angle is automatically adjusted to maintain a constant RPM. If RPM increases unexpectedly, the constant speed unit increases the blade angle to create more drag, reducing RPM back to the selected value. Thus, blade angle increases with increasing RPM to stabilize rotational speed, keeping angle of attack nearly constant.

In a constant speed propeller, the system maintains a constant RPM by adjusting the blade angle. When TAS increases, it tends to reduce the angle of attack. To maintain performance, the constant speed unit increases the blade angle, thereby restoring the desired angle of attack and maintaining the selected RPM.

Hence, blade angle increases with increasing TAS.

A coarse blade angle is more efficient at high TAS because it maintains a more optimal angle of attack as airspeed increases. A fine pitch blade experiences a lower angle of attack at higher TAS and thus becomes less efficient. Since propeller efficiency peaks around a specific angle of attack (typically ~4°), coarse pitch better matches this at higher speeds, ensuring effective thrust generation.

Increasing power in a single right-handed tractor propeller aircraft during a stall will increase the torque reaction, which causes a left roll tendency. This increases the load on the left wing, making it stall more deeply, while unloading the right wing, potentially helping it recover. As a result, the left wing drop becomes worse.

Increasing the number of propeller blades increases the total blade area, allowing the propeller to absorb more engine power without stalling, thereby increasing its power absorption capability. However, it may reduce efficiency due to disturbed airflow and increase torque and noise.

Increasing propeller blade length increases the blade area, which enhances the power absorption capability. The higher aspect ratio also reduces induced drag by minimizing tip vortices, thereby improving efficiency. However, it will also increase torque and tip speeds, not reduce them.

If the propeller RPM lever is pushed forward during a glide, the constant speed unit decreases the blade angle, shifting the blades away from the feathered (coarse) position into a finer pitch. This causes the propeller to windmill more easily, which increases both profile and induced drag. Since more energy is extracted from the airflow, the drag increases significantly, reducing glide efficiency and range. The engine being idle or shut down means cooling is unaffected.

If propeller pitch is increased during a glide, the blades move closer to the feathered position, where they produce minimal drag. This reduces windmilling, resulting in a decrease in RPM and a reduction in total drag. As a result, the aircraft experiences an increase in glide range due to improved aerodynamic efficiency.

A constant speed propeller is designed to automatically vary blade pitch to maintain a constant RPM, thereby achieving optimum blade angle and maintaining maximum efficiency across a wide range of true airspeeds (TAS). This makes it more efficient than a fixed pitch propeller, which operates optimally only at a specific TAS. However, constant speed propellers are generally heavier, more complex, and not stronger than fixed pitch types.

The maximum propulsive efficiency of a coarse pitch propeller is lower than that of a fine pitch propeller. While a coarse pitch blade is more efficient at high TAS, its increased blade angle causes more of the total aerodynamic reaction to act in the direction of propeller torque rather than producing useful thrust. This reduces its maximum efficiency, even though it performs better at high speeds.

Propeller propulsive efficiency primarily depends on how effectively the engine’s rotational power is converted into useful thrust. While adding blades increases the propeller’s ability to absorb power, it also increases aerodynamic interference between blades. This disturbed airflow leads to higher drag and reduces efficiency. Therefore, although more blades improve power absorption, they come at the cost of lower propulsive efficiency.

Propeller blade twist reduces blade angle from root to tip to maintain a nearly constant angle of attack along the blade span. As rotational speed increases from root to tip, without twist, the outer blade sections would operate at inefficiently high angles. Twist compensates for this, ensuring optimal efficiency throughout the blade.

Centrifugal twisting moment always drives propeller blades towards fine pitch as it acts outward from the center of rotation. During windmilling, the aerodynamic twisting moment also drives blades toward fine pitch, so the centrifugal twisting moment assists it. In other cases, they usually oppose each other.

In reverse thrust, propeller blades are set at a negative blade angle. Due to blade twist, this results in the (negative) angle of attack increasing from root to tip. Hence, the greatest angle of attack is found at the blade tips, and most reverse thrust is generated there.

Propeller torque is the component of the total reaction that acts parallel to the plane of rotation. In powered flight, it acts opposite to engine torque, resisting rotation. However, in windmilling flight, the propeller is driven by airflow, reversing the direction of torque. Thus, in this condition, propeller torque assists engine torque.

Angle of attack on a propeller blade is influenced by the vector sum of airflows due to rotation, propeller slip, and TAS. Since rotational airflow increases from root to tip, angle of attack would also increase toward the tips. However, to prevent excessive loading at the blade tips, blades are twisted so that the angle of attack is lowest at the tips, where airflow is highest.

When a mass rotates, it exhibits gyroscopic rigidity and precession. Gyroscopic precession means that any force trying to change the rotation axis acts 90° ahead in the rotation direction. For a single left handed tractor propeller, pitching the nose up at VR induces a left yaw due to this precession effect.

When a rotating mass like a propeller experiences a change in its axis (such as pitching the nose up at VR), it exhibits gyroscopic precession, where the reaction force acts 90° ahead in the direction of rotation. For a single right handed tractor propeller (rotating clockwise when viewed from behind), this precession causes the aircraft to yaw right during nose-up rotation at VR.

The weathercock effect occurs because the propeller slipstream spirals aft in the direction of propeller rotation. For a right handed tractor propeller (rotating clockwise viewed from behind), the spiral slipstream hits the vertical fin on one side, producing a sideways lift force. Since the fin’s center of pressure is behind and above the aircraft’s center of gravity, this force creates both yawing and rolling moments.

At lift-off, this effect causes an initial right roll. Although the yawing moment tends to create a left yaw and subsequent left roll through dissymmetry of lift, the immediate effect during lift-off is a right roll due to the slipstream impact on the fin.

The asymmetric blade effect occurs when the propeller disc is tilted, causing the down-going blades to have a higher angle of attack and airspeed than the up-going blades. This produces more thrust on the down-going side, shifting the thrust line laterally and causing a yaw and roll to the left in a right-handed tractor propeller aircraft during climb-out.

In a twin-engine propeller aircraft, the high velocity propeller wash increases lift over the wings. If the right engine fails, the right wing loses this additional lift, causing the aircraft to roll right towards the dead engine. Simultaneously, the loss of thrust on the right side causes the aircraft to yaw right. Therefore, right roll and yaw during climb out are typically caused by right engine failure.

Propeller asymmetric thrust occurs when the plane of rotation is not perpendicular to the flight direction, especially at low speeds and high angles of attack. The down-going blades experience higher angle of attack and airspeed, producing more thrust on that side. This shifts the thrust line laterally towards the down-going side, creating a yaw moment towards the up-going side. This yaw then causes a roll in the same direction. For a left handed tractor propeller, this results in right yaw and roll during climb-out.

Propeller asymmetric thrust happens when the plane of rotation is not perpendicular to flight direction, most noticeable at low speeds and high angles of attack. The down-going blades have higher angle of attack and airspeed, producing more thrust on that side and shifting the thrust line laterally towards the down-going side. This causes a yawing moment toward the up-going side of the propeller disc, which for a left handed tractor propeller results in right yaw near VR. At speeds below VR, the ground prevents roll caused by this yaw.

In a twin-engine propeller aircraft, the lift from propeller wash over the wings increases total lift. If the right engine fails, that wing loses this extra lift, causing the aircraft to roll right toward the dead engine. The loss of thrust on the right side also causes a yaw to the right. This effect is sudden and prominent above VR.

Asymmetric blade effect is unlikely to cause sudden yaw above VR as it develops gradually. Torque reaction in contra rotating systems might cause issues only if the left engine fails.

As a propeller rotates, centrifugal force acts on each particle of the blade, pulling it outward. Due to the blade’s flattened cross-section, this force creates a twisting moment that aligns the blade more flatly with the rotation plane. This drives the blade towards fine pitch, which is the lowest blade angle, increasing RPM.