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All the twins I have checked have counter-rotating blades, so I was wondering if this was true for all twins of if there are any twins that have both blades turning in the same direction.

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    $\begingroup$ Almost all WW2 twin engine aircraft use standard props and therefore do not have counter rotating props. $\endgroup$ – slebetman Oct 17 '14 at 4:54
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Twin enigned aircrafts without counter-rotating propellers

A great many twins don't have counter-rotating props:

One such aircraft is the BeechCraft Baron 58, as seen in this YouTube video.

Another aircraft I can think of is the King Air 350.

It is not any more or less common to have one or the other.

Contra-rotating propellers

Another interesting solution to the P-factor (prop pulling towards one direction), especially on single engine aircraft is contra-rotating props, two propellers on the same shaft that spin in opposite directions.

This is mainly for single engine racer aircraft, but there are multi-engine aircraft known to have it, such as the Soviet Tu-95 Bear bomber.

The pic shown here is a Spitfire Mk. XIV:

Spitfire Mk. XIV with contra-rotating props

A better explanation of the P-factor

P-factor is the term for asymmetric propeller loading, that causes the airplane to yaw to the left when at high angles of attack.

Assuming a clockwise rotating propeller it is caused by the descending right side of the propeller (as seen from the rear) having a higher angle of attack relative to the oncoming air, and thus generating a higher air flow and thrust than the ascending blade on the left side, which at the other hand will generate less airflow and thrust. This will move the propellers aerodynamic centre to the right of the planes centreline, thus inducing an increasing yaw moment to the left with increasing angle of attack or increasing power. With increasing airspeed and decreasing angle of attack less right rudder will be required to maintain coordinated flight.

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  • $\begingroup$ ah, you're right. I edited my post ;) $\endgroup$ – flyingfisch Oct 16 '14 at 19:16
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    $\begingroup$ DA42 also has both engines turning in the same direction $\endgroup$ – Federico Oct 17 '14 at 10:49
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Another interesting issue that effects aircraft with non counter rotating props is that they would have a Critical Engine. This in basic terms means that it's worse if a particular engine fails. The Diamond DA42 which is a popular modem trainer has this issue if the left engine fails.

The following from Wikipedia explains further.

Critical Engine

The critical engine of a multi-engine, fixed-wing aircraft is the one whose failure would result in the most adverse effects on the aircraft's handling and performance. On propeller aircraft, there is a difference in the remaining yawing moments after failure of the left or the right (outboard) engine when all propellers rotate in the same direction due to the P-factor. On turbojet/ turbofan aircraft, there usually is no difference between the yawing moments after failure of a left or right (outboard) engine. An engine can also be called critical when it is the only engine that drives a hydraulic pump for augmenting/ boosting flight controls.

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    $\begingroup$ An interesting thing about the P-38 Lightning is that it had TWO critical engines. The initial prototype had no critical engine, but later the prop rotations were reversed to improve airflow over the tail. $\endgroup$ – Fred Larson Oct 16 '14 at 19:48
  • $\begingroup$ Yeah. It mentions that in the article linked above. Apparently to make it a better gunnery platform. $\endgroup$ – vectorVictor Oct 16 '14 at 19:52
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Piper Twin Comanche was available in both configuration, but the most popular one had the engines turning in the same direction. Aztec and Apache are also models with engines spinning the same way. My experience is that most light twins have the same direction and that counter rotating is the exception. It makes sense that way as they share engines with singles, so one particular spinning direction is going to be much more common and spare parts will be easier to get...

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