Propeller Torque and Slipstream Effects

A propeller does more than pull. It reacts against the airframe that turns it, it throws a twisting column of air back over the fuselage and tail, it loads its blades unevenly when the aeroplane flies nose-high, and as a spinning mass it resists changes in the direction of its axis. Together these effects make a single-engine propeller aeroplane roll and yaw whenever power, airspeed or attitude changes.

For a propeller that turns clockwise as seen from the cockpit, the arrangement assumed throughout FAA and EASA training material, every one of them turns the aeroplane to the left, and FAA texts group them as the left-turning tendencies. They are strongest at high power, low airspeed and high angle of attack: on the take-off roll, in the climb, in the go-around and in a power-on stall. That is why the pilot's right foot works hardest just after take-off. For a propeller turning the other way, every direction below is reversed.

Left-turning tendencies

Exam questions describe the rotation in two ways. FAA material says "clockwise as seen from the flight deck"; many European texts say "clockwise viewed from the rear". Both describe the same propeller, because the pilot looks forward from behind it.

The expressions propeller torque effect and torque effect are often used loosely for the whole left-turning tendency. Strictly, torque reaction is only the rolling moment described next.

Torque reaction

By Newton's third law, an engine that turns its propeller clockwise pushes the airframe anticlockwise with an equal and opposite moment. The result is torque reaction, a rolling moment to the left about the longitudinal axis. It is the counterpart of the propeller torque, the aerodynamic resistance in the plane of rotation that the engine must overcome to turn the propeller (see propeller aerodynamics). It grows with engine power and matters most at low airspeed, when the ailerons have little authority to oppose it.

In flight, manufacturers compensate with aileron trim or by giving the left wing a slightly greater angle of incidence, so that it produces a little more lift. On the ground the aeroplane cannot roll, so the reaction presses the left main wheel harder onto the runway. The extra rolling friction on that wheel helps swing the aeroplane to the left during the take-off roll.

Spiralling slipstream

The propeller gives the air behind it a rotation as well as a rearward velocity. This spiralling slipstream, spelt spiraling slipstream in the United States and also called the spiral slipstream effect or corkscrew effect, wraps round the fuselage in the direction of the propeller's rotation. With a clockwise propeller it strikes the left side of the fin and rudder. The side force pushes the tail to the right and yaws the nose to the left.

The effect is greatest when the spiral is tightly wound: at high power and low forward speed, as in the initial climb. At higher speeds the spiral stretches out, less of it meets the fin at an angle, and its influence fades. Designers counter it with an offset fin, a fixed tab on the rudder or a thrust line angled slightly to the right.

P-factor

P-factor, the asymmetric blade effect of European texts, appears when the propeller disc is inclined to the oncoming air. At a high angle of attack the propeller shaft points above the flight path, so the air arrives from slightly below the disc. The descending blade, on the right side of a clockwise propeller, meets that flow at a larger angle of attack and a higher relative speed than the ascending blade on the left. It produces more thrust, the centre of thrust moves to the right of the propeller shaft, and the nose yaws to the left.

P-factor depends on angle of attack, so it is negligible in level cruise, where the disc is nearly perpendicular to the airflow, and greatest in a slow, full-power climb or a power-on stall. It is the most pronounced of the four tendencies in those conditions, which is why a power-on stall needs substantial right rudder to keep the aeroplane in balance.

On a twin, the same shift of the thrust line decides which engine is critical. With both propellers turning clockwise, the right engine's thrust acts further from the centreline, so the loss of the left engine is the worse case (see asymmetric flight).

Gyroscopic precession

A spinning propeller is a gyroscope. When a force tries to tilt its axis, the effect appears 90° later in the direction of rotation: this is gyroscopic precession. For a propeller turning clockwise as seen from behind:

• pitching nose-up yaws the aeroplane to the right;
• pitching nose-down yaws it to the left;
• yawing to the left pitches it nose-up;
• yawing to the right pitches it nose-down.

Precession acts only while the attitude is changing, so it is a transient effect rather than a steady one. It is most noticeable in a tailwheel aeroplane when the pilot raises the tail on the take-off roll. Lowering the nose acts like a forward force on the top of the propeller disc; precession moves it to the right side of the disc, pushing the nose to the left just as torque and slipstream are also at work. The pilot anticipates it with right rudder. The same principle underlies the gyroscopic flight instruments (see gyroscopic principles).

Accelerated slipstream over the tail

The slipstream is faster than the free stream as well as rotating. Wherever it flows, over the inner wing, the fuselage and a conventional tail, the dynamic pressure is higher than the aeroplane's airspeed alone would give. This accelerated slipstream has several consequences:

• Control power. A tailplane and fin in the slipstream give more elevator and rudder authority at high power and low airspeed. At idle power, as in a glide or the flare, the same surfaces feel less effective. A T-tail sits above the slipstream in normal flight, which gives smoother elevator response but less pitch authority at low speed and high power (see tail configurations).
• Trim changes. Because the slipstream and the thrust line both change with power, a power change usually needs a pitch retrim, and often a change of rudder too.
• Lift. The slipstream raises the local dynamic pressure over the parts of the wing behind the propeller, and with the nose high part of the thrust supports the weight. The power-on stall speed is therefore lower than the power-off figure (see stall). The reverse matters in the flare: a sudden power reduction at low speed removes the slipstream lift and can cause a heavy landing.
• Stability. A propeller ahead of the centre of gravity, and the slipstream's effect on the downwash at the tail, are destabilising in pitch, most of all at high power and low airspeed (see longitudinal stability).

Contra-rotating propellers

Contra-rotating propellers are two propellers mounted one behind the other on the same axis and turning in opposite directions. Their torque reactions cancel, so the torque effect is eliminated, and the rotation of the slipstream, with its effect on the fin, is reduced as well. Because the pair has a high solidity, the arrangement is used where a very high power must be absorbed without increasing diameter (see propeller aerodynamics).

Counter-rotating propellers are a twin-engine arrangement: the two engines turn their propellers in opposite directions, usually with the descending blades on the inboard side. Exam texts distinguish the two terms. Counter-rotation makes the effects of the two engines symmetrical: it reduces the torque effect of the pair and leaves neither engine critical. Each engine on its own still produces a torque reaction and an off-centre thrust line, so rudder is still needed after an engine failure.

Correcting on take-off and climb

Designers remove as much of the tendency as they can for the condition in which the aeroplane spends most time. The fin may be offset, the engine canted slightly to the right, a fixed tab fitted to the rudder, and the wing incidence or aileron trim adjusted, so that the aeroplane flies in balance at cruise power and speed. Away from that condition the forces no longer cancel, and the pilot supplies the difference:

• Take-off roll. Apply power smoothly and keep straight with rudder, usually right rudder, increasing as power comes up. In a tailwheel aeroplane expect an extra swing to the left as the tail is raised.
• Lift-off and climb. The nose-high attitude adds P-factor while the airspeed is still low. Hold right rudder to keep the slip ball centred, and use rudder trim if fitted.
• Go-around. Full power at low speed brings all the effects back at once, and the power change also changes the pitch trim. Anticipate with rudder and hold the attitude.
• Power-on stall and slow flight. Keep the ball centred with rudder. An uncorrected yaw at the stall is the classic entry to a wing drop and a spin (see spins and spiral dives).

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.