Counter vs Co-Rotating Props

[Mongoose]

  Ok, my two cents worth.  In the P38, the counter-rotating props would cancel out the counter torque from the two engines. 

I remember reading in a book, a long time ago (and no, I don't still have the book), that both engines were the same, but the left hand engine had a gear system which reversed the rotation of the left hand prop.

As for the direction of the props, when I was younger, I always thought they rotated that way so that if you crashed, and the props broke off, they would roll away from the plane, not toward the cockpit. 
Turns out it was actually because it made for better airflow.  Go figure.

[WWhiskey]

  Ok, my two cents worth.  In the P38, the counter-rotating props would cancel out the counter torque from the two engines. 

I remember reading in a book, a long time ago (and no, I don't still have the book), that both engines were the same, but the left hand engine had a gear system which reversed the rotation of the left hand prop.

As for the direction of the props, when I was younger, I always thought they rotated that way so that if you crashed, and the props broke off, they would roll away from the plane, not toward the cockpit. 
Turns out it was actually because it made for better airflow.  Go figure.

in the 38 the motors did not rotate in the same direction!

[smoe]

I can't find the web page that explained why the P-38's props turned as they are. All I remember is the P-38 props were tried with the propeller tops rotating both towards and both away from the pilot's point of view. With the propellers tops rotating towards the pilot's cockpit (left prop=clockwise, right prop counter-clockwise) allowed a better torque advantage should one engine fail. The reference article said the main decision to make the propellers tops rotating away from the pilot's cockpit (as they were all built; left prop=counter-clockwise, right prop=clockwise) was to prevent rain water from flying off the props and onto the pilot's windshield, thereby, possibly blinding the pilot's frontal view. I can't remember if the article said the water splashing (left prop=clockwise, right prop counter-clockwise) was a problem on the ground, in the air, or both.

A couple of interesting quotes:

http://bbs.hitechcreations.com/wiki/index.php/P-38_Lightning :

Another issue with the P-38 arose from its unique design feature of outwardly rotating counter-rotating propellers. Losing one of two engines in any twin engine non-centerline thrust aircraft on takeoff creates sudden drag, yawing the nose toward the dead engine and rolling the wingtip down on the side of the dead engine. Normal training in flying twin-engine aircraft when losing an engine on takeoff would be to push the remaining engine to full throttle; if a pilot did that in the P-38, regardless of which engine had failed, the resulting engine torque and p-factor force produced a sudden uncontrollable yawing roll and the aircraft would flip over and slam into the ground. Eventually, procedures were taught to allow a pilot to deal with the situation by reducing power on the running engine, feathering the prop on the dead engine, and then increasing power gradually until the aircraft was in stable flight. Single-engine takeoffs were possible, though not with a maximum combat load.

http://www.answers.com/topic/counter-rotating-propellers

Counter-rotating propellers, are found on twin-, and multi-engine, propeller-driven aircraft and have propellers that spin in opposite directions.

Generally, the propellers on both engines of most conventional twin-engined aircraft spin clockwise (as viewed from the rear of the aircraft). Counter-rotating propellers generally spin clockwise on the left engine, and counter-clockwise on the right. The advantage of counter-rotating propellers is to balance out the effects of torque and p-factor, eliminating the problem of the critical engine.

In designing the Lockheed P-38, the decision was made to reverse the counter-rotation and having the "tops" of the propeller arcs rotating outwards, away from each other. Tests on the initial XP-38 prototype demonstrated greater accuracy in gunnery with the unusual configuration. The German World War II Henschel Hs 129 ground attack aircraft, Heinkel He 177 heavy bomber and Messerschmitt Me 323 transport's counter-rotating powerplants used the same rotational "sense" as the production P-38 did.

Drawbacks of counter-rotating propellers come from the fact that, in order to reverse sense of rotation of one propeller, a gearbox needs to be used or the engine or engine installation must be different. This may increase weight (gearbox), or maintenance and spare parts costs for the engines and propellers, as different spare parts need to be produced in lower numbers, compared to a conventional installation.

[Plazus]

I can't find the web page that explained why the P-38's props turned as they are. All I remember is the P-38 props were tried with the propeller tops rotating both towards and both away from the pilot's point of view. With the propellers tops rotating towards the pilot's cockpit (left prop=clockwise, right prop counter-clockwise) allowed a better torque advantage should one engine fail. The reference article said the main decision to make the propellers tops rotating away from the pilot's cockpit (as they were all built; left prop=counter-clockwise, right prop=clockwise) was to prevent rain water from flying off the props and onto the pilot's windshield, thereby, possibly blinding the pilot's frontal view. I can't remember if the article said the water splashing (left prop=clockwise, right prop counter-clockwise) was a problem on the ground, in the air, or both.

During prototype developements of the P38, the propellers actually spun with the props bottom arc away from eachother; with the top of the prop arc turning inwards. Even though there was still counter torque effects, the nose had the tendency to pitch downwards.

The operational configuration of the P38 was different, and as you correctly stated, the top of the props arcs turned away from eachother; with the bottom of the prop arc turning inwards. While I dont have the sources on hand to back this up, one of the main reasons why they configured the engines like they did is so that the gun platform would be more stable. Another reason for the engine configuration is because the counter-torque would create a nose-up attitude and thus helped improve the P38s turning ability.

[thorsim]

in the 38 the motors did not rotate in the same direction!

the allison was designed with a "flip-able" err reversible crankshaft ...

>>>
Another feature of the V-1710 design was its ability to turn the output shaft either clockwise or counter-clockwise by assembling the engine with the crankshaft turned end-for-end, by installing an idler gear in the drive train to the supercharger and accessories and by installing a starter turning the proper direction. So, there was no need to re-arrange the ignition wiring and firing order, nor the oil and Glycol circuits to accommodate the direction of rotation.
<<<

http://en.wikipedia.org/wiki/Allison_V-1710

[CAP1]

The main reason for counter-rotation of props is to make the airplane more manageable with an engine inop (it's a p-factor thing), however in the case of the P-38 they turn the props "out" (done in search of a cure to the compressibility thing) so in effect have made both engines the critical engine.

i was wondering why they rotated the engines out, rather than in......

[smoe]

i was wondering why they rotated the engines out, rather than in......

When stating if the p-38 engines are critical or not may be a moot point. The defintion of a critical engine is very arguable for some planes. However, in the p-38 (w/props outward) neither one of the engines could be considered a "critical engine," they may only be considered "equally critical." See link below for reference.

http://en.wikipedia.org/wiki/Talk:Critical_engine

Another reason for the engine configuration is because the counter-torque would create a nose-up attitude and thus helped improve the P38s turning ability.

From a pure mechanical perspective, I wonder if the nose-up attitude was a direct affect of the wing and/or elevator flexing do to the large engine torque placed on the twin booms. For example, I would believe the under full power an engine on the p-38 could twist the entire engine boom structure and change the aerodynamics of the wing and/or elevator which produced a nose-up or nose-down attitude. Depending on how the elevator was connected to the rear tail boom, the elevator could experience a bowing up or down. This bowing effect could change the lift on the elevator same as if the elevator trim were adjusted by the pilot. Remember, I’m only speculating based on a purely mechanical point-of-view.

Also, mechanically speaking, assuming the p-38 props could produce enough torque to flex the wing and/or elevator and change the aerodynamics. I could see the advantage of counter-rotating props for the p-38. If the p-38 didn’t have counter-rotating props the wings and/or elevator could experience a wicked change in the lift under full engine power, mainly in the wings. If the p-38 had props that spun the same direction the plane may require a stronger and heavier airframe.

[colmbo]

Any structure that would be deformed by torque would most likely come apart with any inflight loads.

[W7LPNRICK]

Not meaning to be argumentative, but the flex of the P-38 fuselage/tail boom is probably so small, it would be difficult to measure, unless you did it with lasers during flight. It certainly wouldn't change flight characteristics or cause a nose up attitude. It's simply the force of torque in 3D space. If suspended in a vacuum it would do back flips "'till the cows came home". anyone using high speed power tools, wheel grinders etc., has felt it. 

[2ADoc]

The main reason to use counter rotating pros is to eliminate the torque.  Your over haul costs double or triple on the counter rotating side, due to the fact that the prop on the counter rotating side are different, as are many of the acessories, such as the vacum pump, Magnetos, and the like.  Counter rotating props do not have that much effect in cruise, the big difference between the 2 setups, is in take-off and landing.  The Piper Seminle has counter rotating props it is a great plane, and easier to land, and take-off than an Apache, or Aztec, and a Beech 18 is a handful either way.  Also there is a BIG difference in "engine out performance".  In referance to Fliping the engine over, in a push pull configuration there are a few planes that use that set-up.  The Donier 335 was one of them, and the Cessna 337 is a more modern one but the most up to date example is the Adams A-300.  For this it is called a center line thrust, if you get your multi engine rating in one of these configurations, your certificate states it and you cant leagaly carry passengers in a conventional multi-engine aircraft.  For the simple reason that P-factor and other characteristics of multi engine aircraft are either non existant or nominal.  One Problem with a Center line thrust aircraft is that since the thrust line of both engines in in the center line, you do not get the adverse yaw when you loose an engine like you do in a conventional twin.  It is harder to determine when you loose an engine on take-off.  You still have a "Blue Line" in a push pull but not for the same reasons as in a conventional twin.  In a conventional twin the "Blue line" is the minimal single engine controlable airspeed at gross weight.  Below the Blue line at gross weight if you loose an engine there is a good chance that you will end up in an aluminum ball somewhere near one end of the ends runway. 

[colmbo]

You still have a "Blue Line" in a push pull but not for the same reasons as in a conventional twin.  In a conventional twin the "Blue line" is the minimal single engine controlable airspeed at gross weight.  Below the Blue line at gross weight if you loose an engine there is a good chance that you will end up in an aluminum ball somewhere near one end of the ends runway. 

Blue line is Vyse....best rate of climb, single engine.

Vmc is marked with a red line.

[2ADoc]

Blue line is Vyse....best rate of climb, single engine.

Vmc is marked with a red line.

My Bad your right. 

[CAP1]

I can't find the web page that explained why the P-38's props turned as they are. All I remember is the P-38 props were tried with the propeller tops rotating both towards and both away from the pilot's point of view. With the propellers tops rotating towards the pilot's cockpit (left prop=clockwise, right prop counter-clockwise) allowed a better torque advantage should one engine fail. The reference article said the main decision to make the propellers tops rotating away from the pilot's cockpit (as they were all built; left prop=counter-clockwise, right prop=clockwise) was to prevent rain water from flying off the props and onto the pilot's windshield, thereby, possibly blinding the pilot's frontal view. I can't remember if the article said the water splashing (left prop=clockwise, right prop counter-clockwise) was a problem on the ground, in the air, or both.

A couple of interesting quotes:

http://bbs.hitechcreations.com/wiki/index.php/P-38_Lightning :

http://www.answers.com/topic/counter-rotating-propellers

dunno the truth to this, but here's somethign i just found.......

http://www.examiner.com/x-21467-Cleveland-Private-Aircraft-Examiner~y2009m11d28-A-Modern-Marvel-Counter-Rotating-PropellersThe-P38-which-way-do-they-Rotate

[Ack-Ack]

If the p-38 had props that spun the same direction the plane may require a stronger and heavier airframe.

It didn't.  Model 322's were the "Castrated P-38s" that didn't have counter-rotating propellors, both spun to the right.  There wasn't a need to strengthen or otherwise modify the airframe to compensate for the torque.


ack-ack

[Angus]

Just more trimming....