On a commercial aeroplane like a Qantas plane or a Boeing 747 etc, do the propellers or blades in the engine turn the same direction on both sides of the plane?

Or is it the case the engines on the left hand side turn in a different direction to those on the right.

From : Don Stackhouse

In most cases they turn the same direction, but it depends on the plane.

Propellers have what's called "P-factor" when the airflow into them is not parallel to the propshaft, such as when the airplane is at a high angle of attack during climb. The skewed airflow relative to the prop disk causes the blades on one side of the disk (the downward-moving blades in the case of a nose-up climb) to see a higher angle of attack and airspeed than the blades on the other side. This causes them to make more thrust than the blades on the other half of the disk. The unequal thrust tries to yaw the plane towards the side of the disk that has less thrust, typically to the left for one of the right-handed props we typically use in the USA. The pilot has to hold some right rudder during climb to compensate for this.

Slipstream effects, due to the swirl the prop imparts to the air striking different parts of the airframe behind the prop unequally, can add to this effect. Powered free-flight models often use the positioning of the motor, wing pylon and fin to help compensate for this.

Some airplanes may use some right thrust and/or have some offset in the vertical fin to help compensate. For example, the Aeronca 7AC "Champion" has the leading edge of the fin offset to the left about 1/2", which acts like built-in right rudder. This is about the right amount to make the plane fly straight in cruise, and also reduces the amount of rudder the pilot has to hold during climb. Unfortunately it doesn't go away in a power-off glide, so the pilot has to hold some left rudder to keep the plane properly coordinated in a glide.

On multi-engined planes, especially those with low power loadings (i.e.: lots of power in comparison to their weight), this can become a major issue in the case of an engine failure. If you already have P-factor trying to yaw the airplane to one side, and then add the effects of an engine failure on that same side (the "critical" engine), the combined yawing effects can be difficult for the rudder to overcome. Such airplanes have a minimum control airspeed, or "Vmc". At speeds below Vmc, the combined effects of P-factor and the asymmetric thrust due to an engine failure could cause the plane to lose control and roll over. VMC is an even more important factor than stall speed in the design and operation of most twin-engined airplanes.

Note that for one engine of a twin, a failure of that engine results in the asymmetric thrust tending to cancel the P-factor and torque effect, while for the other engine (the "critical engine") these effects add to each other. By using counter-rotating props, you can eliminate this critical engine, creating a case where a failure of either engine results in the effects tending to cancel each other. This can dramatically improve the plane's single-engine low speed handling. The P-38 Lightning and the Piper Twin Comanche CR both used this setup.

That swirl in the slipstream also represents some lost energy and therefore some efficiency loss. Using counter-rotating props to cause the swirl to be opposite the direction of the wing-tip vortices (so that the two tend to cancel each other) can recover some of this energy for a possible performance improvement. Unfortunately, the direction that results in this benefit is normally the opposite direction from the one that helps the critical engine problem. The P-38 tried it both ways, eventually settling on the direction that improved performance. It's tough to get excited about previously having great low-speed handling if the Zeros have just shot your tail off!

The Fairchild "Merlin" corporate turboprops and "Metro" commuter airliners use counter-rotating Garrett turboprop engines,as do a number of other planes with this engine (such as the OV-10 Bronco). The final stage of this engine's planetary gearbox allows for a very simple modification that can reverse the direction of rotation of the propshaft without a lot of new engine parts.

The down side of counter-rotating props is that engines cost money, and props cost money. The folks who build the airplane and the folks who operate and maintain the airplane have to pay twice as much for parts, because they now need to stock left and right-handed versions of everything. They also have to be very careful not to get them mixed up and accidentally install a right-handed something on the side of the plane that's supposed to be left-handed, ore vice-versa. This expense and risk can be much bigger than you might expect, and as a result, the vast majority of twins use props that all rotate in the same direction. With few exceptions, contra-rotating props just aren't enough benefit to be worth the trouble.

Turbojet and turbofan aircraft such as the B-747 all turn in the same direction, without any exceptions that I'm aware of. The inlet duct of the engine tends to straighten out the flow (which reduces or eliminates P-factor), and the stator vanes in the engine are carefully designed to maximize thrust, which also means reducing or eliminating any swirl in the exhaust flow. Thus, there is little or no benefit to counter-rotation. Meanwhile, then cost of making left and right-handed versions of all the turbine blades, stator vanes, disks to mount them on, accessory drives, etc., would just about double the tooling costs and drive the already sky-high manufacturing costs up to interstellar levels, for no measurable benefit. About the only noticeable effect would be a cancelling of gyroscopic effects when changing attitude, such as during the rotation on takeoff. These are usually pretty minor, and in any case very brief, so they are essentially a non-problem in almost all cases.

In the case of very high power engines, we sometimes see "contra-rotating" props (i.e.: two props turning in opposite directions on concentric shafts on one powerplant). The GE and Hamilton-Standard Propfans used this to eliminate swirl and maximize efficiency, as do some of the big Soviet turboprops such as the "Bear", with its 14,000 horsepower Kuznetsov turbine engines. At very high power loadings, the swirl can become great enough to represent as much as a 10-15% efficiency loss, making it worth the trouble to add a second prop and its drivetrain to take that swirl out. Also, in the case of single-engine airplanes with extremely large engines, the effects of massive P-factor in a tiny airframe can put practical limits on just how much power the plane can handle. They ran into just this problem in the development of the later versions of the Spitfire with the more powerful RR Griffon engines, and resorted to the use of contra-rotating props to solve it.

The pre-war Napier-Heston Racer, with its race version 5000 hp Napier "Sabre" H-24 engine, should have had a contra-rotating prop system. Instead they used a 16 ft. single prop. The pilot was cautioned to be careful with the throttle at low speeds, and took off carefully with reduced throttle. Once safely off the ground, he opened the throttle wide open while still at too low of an airspeed, and the airplane rolled inexorably onto its back and crashed.

Counter-rotation is expensive, and for jets there really isn't any significant need for it. Even for propeller-driven airplanes it's usually just not worth the trouble. There are exceptions such as the ones I've mentioned above, but the vast majority of planes use props and engines that all turn in the same direction.

Don Stackhouse
DJ Aerotech