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P-factor

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P‑factor , also known as asymmetric blade effect and asymmetric disc effect, is an aerodynamic phenomenon experienced by a moving propeller , wherein the propeller's center of thrust moves off-center when the aircraft is at a high angle of attack . This shift in the location of the center of thrust will exert a yawing moment on the aircraft, causing it to yaw slightly to one side. A rudder input is required to counteract the yawing tendency.

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42-431: When a propeller aircraft is flying at cruise speed in level flight, the propeller disc is perpendicular to the relative airflow through the propeller. Each of the propeller blades contacts the air at the same angle and speed, and thus the thrust produced is evenly distributed across the propeller. However, at lower speeds, the aircraft will typically be in a nose-high attitude, with the propeller disc rotated slightly toward

84-451: A V MCA (often called V MCA1 under these circumstances), where the critical engine alone is inoperative, but also a V MCA2 that applies when the engine inboard of the critical engine, on the same wing, is also inoperative. Civil aviation regulations (FAR, CS and equivalent) no longer require a V MCA2 to be determined, although it is still required for military aircraft with four or more engines. On turbojet and turbofan aircraft,

126-408: A helicopter will be chosen in part to ensure that the backwards-moving blade does not stall. Slipstream#Spiral slipstream A slipstream is a region behind a moving object in which a wake of fluid (typically air or water) is moving at velocities comparable to that of the moving object, relative to the ambient fluid through which the object is moving. The term slipstream also applies to

168-500: A multi-engine aircraft plays a crucial role in maintaining directional control while an engine fails or is inoperative. The larger the tail, the more capable it will be of providing the required force to counteract the asymmetrical thrust yawing moment. This means that the smaller the tail is, the higher the V MCA will be. However, a larger tail is more costly and harder to accommodate, and comes with other aerodynamic issues such as increased prevalence of slipstreams . Engineers designing

210-432: A yaw. As with single-engine aircraft, this effect is greatest in situations where the aircraft is at high power and has a high angle of attack (such as the climb). The engine with the down-moving blades towards the wingtip produces more yaw and roll than the other engine, because the moment (arm) of that engine's center of thrust about the aircraft center of gravity is greater. Thus, the engine with down-moving blades closer to

252-515: Is defined for both part 23 <FAR 23.149 (c)> and part 25 aircraft in civil aviation regulations. However, when maximum thrust is selected for a go-around , the flaps will be selected up from the landing position, and V MCL no longer applies, but V MCA does. Due to the inherent risks of operating at or close to V MCA with asymmetric thrust, and the desire to simulate and practice these manoeuvres in pilot training and certification V SSE may be defined. V SSE safe single-engine speed

294-406: Is extremely significant for helicopters in forward flight, because the propeller disc is almost horizontal. The forward-going blade has a higher airspeed than the backward-going blade, so it produces more lift, known as dissymmetry of lift . Helicopters can control each blade's angle of attack independently (decreasing the angle of attack on the advancing blade, while increasing the angle of attack on

336-442: Is following another object, moving at the same speed, the rear object will require less power to maintain its speed than if it were moving independently. This technique, also called drafting , can be used by bicyclists. Spiral slipstream , also known as propwash , prop wash , or spiraling slipstream , is a spiral -shaped slipstream formed behind a rotating propeller on an aircraft . The most noticeable effect resulting from

378-440: Is greatest at high angles of attack and high power, for example during take-off or in slow flight. If using a clockwise turning propeller (as viewed by the pilot) the aircraft has a tendency to yaw to the left when climbing and right when descending. This must be countered with opposite rudder. The clockwise-turning propeller is by far the most common. The yaw is noticeable when adding power, though it has additional causes including

420-416: The spiral slipstream effect. In a fixed-wing aircraft, there is usually no way to adjust the angle of attack of the individual blades of the propellers, therefore the pilot must contend with P-factor and use the rudder to counteract the shift of thrust. When the airplane is descending, these forces are reversed. The descending right side of the prop is now moving slightly rearward with less angle of attack and

462-423: The V MCA as well: the further from the tail it is, the lower the minimum control speed, because the rudder will be able to provide a larger yawing moment, and so it is easier to counteract the imbalance in thrust. The lateral centre of gravity also has an effect: the nearer the inoperative engine it is, the larger the moment of the working engine, and so the more force the rudder has to apply. This means that if

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504-465: The V MCA will be lower. The bank angle also influences the minimum control speed. A small bank angle away from the inoperative engine is required for smallest possible sideslip and therefore lower V MCA . Finally, if the P-factor of the working engine increases, then its yawing moment increases, and the aircraft's V MCA increases as a result. Aircraft with four or more engines have not only

546-493: The air (V MCA ) is the most important minimum control speed of a multi-engine aircraft, which is why V MCA is simply listed as V MC in many aviation regulations and aircraft flight manuals . On the airspeed indicator of a twin-engine aircraft of less than 6000 lbs (2722 kg), the V MCA is indicated by a red radial line, as standardised by FAR 23 . Most test pilot schools use multiple, more specific minimum control speeds, as V MC will change depending on

588-413: The airspeed over part of the wings. It also reduces the stall speed of the aircraft by energizing the flow over the wings. Minimum control speeds The minimum control speed ( V MC ) of a multi-engine aircraft (specifically an airplane ) is a V-speed that specifies the calibrated airspeed below which directional or lateral control of the aircraft can no longer be maintained, after

630-421: The ascending left side of the prop moves slightly forward with greater angle of attack. This asymmetric thrust causes the airplane to pull to the right and the pilot uses left rudder to compensate. The fact that the left-right pull tendency reverses when descending, shows that differences in angle of attack on the left and right sides of the prop overwhelm other effects like the spiral slipstream. Put differently, if

672-522: The balance of forces and on the yawing and rolling moments after engine failure might also affect V MC s. When the vertical tail is designed and the V MCA is measured, the worst-case scenario for all factors is taken into account. This ensures that the V MC s published in the AFMs guaranteed to be safe. Heavier aircraft are more stable and more resistant to yawing moments, and therefore have lower V MCA s. The longitudinal centre of gravity affects

714-411: The direction of the failed engine. A sideslip develops, causing the total drag of the aircraft to increase considerably, resulting in a drop in the aircraft's rate of climb . The rudder , and to a certain extent the ailerons via the use of bank angle, are the only aerodynamic controls available to the pilot to counteract the asymmetrical thrust yawing moment . The higher the speed of the aircraft,

756-505: The easier it is to counteract the yawing moment using the aircraft's controls. The minimum control speed is the airspeed below which the force the rudder or ailerons can apply to the aircraft is not large enough to counteract the asymmetrical thrust at a maximum power setting. Above this speed it should be possible to maintain control of the aircraft and maintain straight flight with asymmetrical thrust. Loss of engine power of wing-mounted-propeller aircraft and blown lift aircraft affects

798-433: The effect is. In general, the more aerodynamic an object is, the smaller and weaker its slipstream will be. For example, a box-like front (relative to the object's motion) will collide with the medium's particles at a high rate, transferring more momentum from the object to the fluid than a more aerodynamic object. A bullet-like profile will cause less turbulence and create a more laminar flow . A tapered rear will permit

840-413: The effect this will have on the airplane's minimum control speeds. Minimum control speeds are typically established by flight tests as part of an aircraft certification process. They provide a guide to the pilot in the safe operation of the aircraft. When an engine on a multi-engine aircraft fails, the thrust distribution on the aircraft becomes asymmetrical , resulting in a yawing moment in

882-512: The failure of one or more engines. The V MC only applies if at least one engine is still operative, and will depend on the stage of flight. Indeed, multiple V MC s have to be calculated for landing, air travel, and ground travel, and there are more still for aircraft with four or more engines. These are all included in the aircraft flight manual of all multi-engine aircraft . When design engineers are sizing an airplane's vertical tail and flight control surfaces , they have to take into account

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924-410: The formation of a spiral slipstream is the tendency to yaw nose-left at low speed and full throttle (in centerline tractor aircraft with a clockwise-rotating propeller.) This effect is caused by the slipstream acting upon the tail fin of the aircraft: The slipstream causes the air to rotate around the longitudinal axis of the aircraft, and this air flow exerts a force on the tail fin, pushing it to

966-478: The fuselage will be the " critical engine ", because its failure and the associated reliance on the other engine will require a significantly larger rudder deflection by the pilot to maintain straight flight than if the other engine had failed. P-Factor therefore determines which engine is critical engine. For most aircraft (which have clockwise rotating propellers), the left engine is the critical engine. For aircraft with counter-rotating propellers (i.e. not rotating in

1008-423: The greater angle of the propeller disc to the vertical. P-factor is insignificant during the initial ground roll, but will give a pronounced nose-left tendency during the later stages of the ground roll as forward speed increases, particularly if the thrust axis is kept inclined to the flight path vector (e.g. tail-wheel in contact with runway). The effect is not so apparent during the landing, flare and rollout, given

1050-479: The horizontal. This has two effects. Firstly, propeller blades will be more forward when in the down position, and more backwards when in the up position. The propeller blade moving down and forward (for clockwise rotation, from the one o'clock to the six o'clock position when viewed from the cockpit) will have a greater forward speed. This will increase the airspeed of the blade, so the down-going blade will produce more thrust. The propeller blade moving up and back (from

1092-437: The lateral centre of gravity shifts towards the inoperative engine, the aircraft's V MCA will increase. The thrust of most engines depends on altitude and temperature; increasing altitude and higher temperatures decrease thrust. This means that if the air temperature is higher and the aircraft has a higher altitude, the force of the operative engine will be lower, the rudder will have to provide less counteractive force, and so

1134-440: The lift distribution over the wing, causing a roll toward the inoperative engine. In some aircraft roll authority is more limiting than rudder authority in determining V MC s. Aviation regulations (such as FAR and EASA ) define several different V MC s and require design engineers to size the vertical tail and the aerodynamic flight controls of the aircraft to comply with these regulations. The minimum control speed in

1176-409: The minimum safe control speeds remain the same as they would be for an aircraft being flown at 50% thrust on all four engines. Failure of a single inboard engine, from a set of four, has a much smaller effect on controllability. This is because an inboard engine is closer to the aircraft's centre of gravity, so the lack of yawing moment is decreased. In this situation, if speed is maintained at or above

1218-523: The outboard engines are usually equally critical. Three-engine aircraft such as the MD-11 and BN-2 Trislander do not have a V MCA2 ; a failed centerline engine has no effect on V MC . When two opposing engines of aircraft with four or more engines are inoperative, there is no thrust asymmetry, hence there is no rudder requirement for maintaining steady straight flight; V MCA s play no role. There may be less power available to maintain flight overall, but

1260-418: The particles of the medium to rejoin more easily and quickly than a truncated rear. This reduces lower-pressure effect in the slipstream, but also increases skin friction (in engineering designs, these effects must be balanced). The term "slipstreaming" describes an object travelling inside the slipstream of another object (most often objects moving through the air though not necessarily flying). If an object

1302-399: The published V MCA , as determined for the critical engine, safe control can be maintained. If an engine fails during taxiing or takeoff , the thrust yawing moment will force the aircraft to one side on the runway. If the airspeed is not high enough and hence, the rudder-generated side force is not powerful enough, the aircraft will deviate from the runway centerline and may even veer off

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1344-471: The relatively low power setting (propeller RPM). However, should the throttle be suddenly advanced with the tail-wheel in contact with the runway, then anticipation of this nose-left tendency is prudent. For multi-engine aircraft with counter-rotating propellers , the P-factors of both engines will cancel out. However, if both engines rotate in the same direction, or if one engine fails, P-factor will cause

1386-472: The retreating blade) in order to keep the lift of the rotor disc balanced. If the blades of the rotor were unable to independently change their angle of attack, a helicopter with counterclockwise-rotating rotor blades would roll to the left when in forward flight, due to the increased lift on the side of the rotor disc with the advancing blade. Gyroscopic precession converts this into a backwards pitch known as " flap back ". The never-exceed speed ( V NE ) of

1428-466: The right. To counteract this, some aircraft have the front of the fin (vertical stabilizer) slightly offset from the centreline so as to provide an opposing force that cancels out the one produced by the slipstream, albeit only at one particular (usually cruising) speed, an example being the Hawker Hurricane fighter from World War II . Propeller slipstream causes increased lift by increasing

1470-411: The runway. The airspeed at which the aircraft, after engine failure, deviates 9.1 m from the runway centerline, despite using maximum rudder but without the use of nose wheel steering, is the minimum control speed on the ground (V MCG ). The minimum control speed during approach and landing (V MCL ) is similar to V MCA , but the aircraft configuration is the landing configuration. V MCL

1512-494: The same direction) the P-factor moments are equal and both engines are considered equally critical. With engines rotating in the same direction, P-factor will affect the minimum control speeds ( V MC ) of the aircraft in asymmetric powered flight. The published speeds are determined based on the failure of the critical engine. The actual minimum control speeds after failure of any other engine will be lower (safer). P-factor

1554-405: The seven o'clock to the 12 o'clock position) will have a decreased forward speed, therefore a lower airspeed than the down-going blade and lower thrust. This asymmetry displaces the center of thrust of the propeller disc towards the blade with increased thrust. Secondly, the angle of attack of the down-going blade will increase, and the angle of attack of the up-going blade will decrease, because of

1596-429: The similar region adjacent to an object with a fluid moving around it. "Slipstreaming" or " drafting " works because of the relative motion of the fluid in the slipstream. A slipstream created by turbulent flow has a slightly lower pressure than the ambient fluid around the object. When the flow is laminar , the pressure behind the object is higher than the surrounding fluid. The shape of an object determines how strong

1638-418: The spiral slipstream were the dominant factor, the airplane would always pull to the left and would not pull right when descending. Pilots anticipate the need for rudder when changing engine power or pitch angle (angle of attack), and compensate by applying left or right rudder as required. Tail-wheel aircraft exhibit more P-factor during the ground-roll than aircraft with tricycle landing gear , because of

1680-483: The stage of flight. Other defined V MC s include minimum control speed on the ground (V MCG ) and minimum control speed during approach and landing (V MCL ). In addition, with aircraft with four or more engines, V MC s exist for cases with either one or two engines inoperative on the same wing. Figure 1 illustrates the V MC s that are defined in the relevant civil aviation regulations and in military specifications. The vertical tail or vertical stabilizer of

1722-404: The tilt of the propeller disc. The greater angle of attack of the down-going blade will produce more thrust. Note that the increased forward speed of the down-going blade actually reduces its angle of attack, but this is overcome by the increase in angle of attack caused by the tilt of the propeller disc. Overall, the down-going blade has a greater airspeed and a greater angle of attack. P-factor

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1764-456: The vertical tail must make a decision based on, amongst other factors, their budget, the weight of the aircraft, and the maximum bank angle of 5° (away from the inoperative engine), as stated by FAR . V MCA is also used to calculate the minimum takeoff safety speed . A high V MCA therefore results in higher takeoff speeds, and so longer runways are required, which is undesirable for airport operators. Any factor that has influence on

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