Why do P-38s have counterrotating props?

[stepwilk]

To eliminate the problem of "the critical engine" in an engine-out situation, I'd always thought. But yesterday I saw a photo of a restored P-38 that made it obvious that the propellers counterrotate in the wrong direction for this to be true: seen from the cockpit, the left engine turns counterclockwise, the right engine clockwise, giving the P-38 _two_ "critical engines": regardless of which engine fails, the down-going blade of the other one is outboard of the nacelle, doing its best to roll the airplane, especially in a high-angle-of-attack situation.

So maybe it's to cancel out torque steer on takeoff? Nope. I've flown lots of fairly powerful twins, and all of them would roll straight on takeoff with your feet flat on the floor. Twins don't react to torque the way a single-engine airplane does.

Anybody know why they have two slightly different engines? Did the restorers I noted inadvertently reverse the mounting of their engines???

[ab33t]

That was my first thought , did not have the correct engines . This plane had severe roll on engine failure

[stepwilk]

Funny, I did a Google search right after posting this, "twin engine critical engine," and it took me straight to a Pprune thread some six or seven years ago, posing the exact same question. Nobody had a good answer, but apparently that indeed is the correct configuration for a P-38.

The most interesting thing was that it was posted in the flight instructors forum, and half of them had no idea what a P-38 was. Kids.

[handsfree]

From Wikipedia:

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.[

Doesn't answer the question of why the unique design feature though.

[oxenos]

The D.H. Hornet was originally fitted with contra rotating engines which rotated inward, i.e. left engine clockwise. They were swopped over when it was found that this had an adverse effect on the effectiveness of the rudder. ( source Wikipedia ) Given that the hornet had a single, central rudder,and the P-38 had two rudders,it is not easy to see how the same issue would have applied to the P-38.

[Jhieminga]

The reason is in someway connected to the airflow over the fuselage/tail of the airplane. I'd need to look it up (and the book is hidden in a box in the basement) but that is what my memory tells me.

Also, bear in mind that the term critical engine applies to one engine being more critical than the other. When you have two engines and the effects of one of them failing are the same then in effect you don't have a critical engine.

[Saab Dastard]

Two recent threads:

http://www.pprune.org/flight-testing...ropellers.html

and

http://www.pprune.org/aviation-histo...-rotation.html

SD

[Brian Abraham]

As has been noted in previous posts the direction of rotation on the production aircraft was the worst possible if "critical engine" was your consideration.

Following the crash of the prototype Lockheed embarked on detailed test (wind tunnel) and development while designing the YP-38. The tail buffeting problem was solved by fitting large leading edge fillets at the wing/fuselage interface. It was stressed to crews that proper fitment of unmodified and undamaged fillets were critical to safe flight.

The sole reason for choosing the production direction of rotation was that it reduced the power on/power off pitching moments, thus making the aircraft a better gun platform. The designer Kelly Johnson affirmed this reason in a speech. The direction chosen for the prototype was the worst possible in this respect, but the pitching moment issue only came to light during the wind tunnel tests following the crash of the XP-38.

[stepwilk]

"Also, bear in mind that the term critical engine applies to one engine being more critical than the other. When you have two engines and the effects of one of them failing are the same then in effect you don't have a critical engine."

Well, of course you're right, semantically...but it's sort of like saying "When you have two flat tires...you don't have 'a' flat tire." What I wrote in the post above was intended to be a simple way of saying that instead of having one safer and one more dangerous engine in terms of their effect on an engine-out situation, now you have two equally dangerous engines. But you knew that, right?

Of course, one could say that your description of critical engine "applies to one engine being more critical than the other" is endlessly arguable. Although maybe you saw it on the Internet, which must mean it's true.

[Phenom100]

There are two main reasons why the number 1 engine is the critical engine on a a/c with with props rotating in the same direction.

1. Slipstream effect:

If the props are rotating in the same direction i.e clockwise if viewed from behind, then only no 1 engine will produce a sideways slipstream force on the fin. This has the effect of assisting the rudder side force needed to counteract the yawing moment for the no 2 engine failure. However, for a no 1 engine failure the slipstream from the no 2 engine will produce a sideways force that aggrivates the yawing moment, resulting in a more critical engine situation.
Therefore a greater control force is required, resulting in a need for a higher critical speed to make the rudder more effective during the no 1 engine failure, which threfore determines that the no 1 engine is the critical engine.

2. Asymmetric Blade Effect:

Prop blades produce more thrust in the downward rotation than the upward rotation. Therefore, the point through which the thrust acts will be displaced toward the down going blade. Depending on the direction of rotation on the props, this either increases or decreases the thrust moment arm and in a/c with props rotating in the same direction, failure of the shortest moment arm will produce the greatest yawing moment from the other live engine.
Therefore the engine with the shortest thrust moment arm is the critical engine and for counterclockwise rotating props this makes the no 1 engine the critical engine.

[stepwilk]

All very nice, but actually the basic reason why a still-outputting-power engine can be critical is less such aerodynamic niceties than it is the fact the the polar moment between a thrust-producing prop blade that is (at its mid-chord point) say 36 inches from the aircraft centerline creates vastly less rolling force than does a thrust-producing prop blade 108 inches from the centerline. What you say is all true but verges on the academic in the face of this enormously effective piece of basic physics.

The engine with the longest moment arm to its downward-rotating blade is the critical engine.

I'm curious: are you writing as a multi-engine pilot--which you may very well be--or as a theoretician?

[Brian Abraham]

In the case of the production P-38 there was no "critical engine" as when we normally talk of asymmetric operations, that is, no one engine gave worse/better performance/handling difficulties than the other. In this case it only means that the Vmca is higher than it would otherwise be than if the props rotated in the opposite direction as on the XP-38. It should be noted the Vmca is the same as it would be if the props rotated in the same direction, and so have a "critical engine".

[Birthday Boy]

Actually stepwilk you will find that phenom100 has it correct. I think you should review your conclusion. Tell me that you have a multi-engine rating.

[Phenom100]

I'am a pilot, but currently been un-employed for 6 months.....like most of us l guess.

[Jhieminga]

Originally Posted by stepwilk

Of course, one could say that your description of critical engine "applies to one engine being more critical than the other" is endlessly arguable. Although maybe you saw it on the Internet, which must mean it's true.

Actually I was referring to the explanation given in paragraph 23.149 of the CS-23 AMC Flight Test Guide (Minimum Control Speed).

The regulation requires that VMC determination be made ‘when the critical engine is suddenly made inoperative’. The intent is to require an investigation to determine which engine is critical from the standpoint of producing a higher VMC speed. This is normally accomplished during static VMC tests.

What this says is that if one engine produces a higher VMC speed than the other, then that is the critical engine. The reason for the higher VMC speed can be slipstream effect, P-factor or a multitude of other options. From this I would conclude that if both engines produce the same VMC speed then - for the purpose of testing the aircraft - there is no critical engine.

J. (ME pilot and theoretician)

[stepwilk]

"Tell me that you have a multi-engine rating."

Since 1970. Mainly in Shrike Commznder, Aztec, Twin Comanche, Citation, Beagle 206.

[TURIN]

Time and again PPRUNE shows me that I really do know very little about aviation.
I had no idea about any of this...

Keep it up.

[Brian Abraham]

Posted at the request of barit1. The graph explaining why the props rotate in the direction they do.



As an aside, it may be of interest that most, but not all, P-38s only had one generator, that being mounted on the left engine. Remembering that the props were electrically operated, the CSU had to be turned off (to conserve the battery), so that you were then left with a fixed pitch prop.

The FAA defines "Critical engine" as " the engine whose failure would most adversely affect the performance or handling qualities of an aircraft." I guess the left engine could be deemed to be the critical engine, if one were to nit pick, on the 38 on that basis.

[Double Zero]

Just a stab in the dark compared to the fascinating info' above, and from a strictly armchair type, ( the point about only one generator struck me as indeed ' critical ' though ) - could the prop' arrangement have anything to do with flow into the turbocharger inlets as well as the aforementioned tail surfaces ?

[jsallington]

Okay, real quick. Looking at the engines from the front of the aircraft, the right hand engine rotated clockwise, the left hand engine rotated counter-clockwise. Yes, this was a safety feature, if one engine failed, the remaining engine would not attempt to roll the airplane quite a badly as it would have if they were rotating in the opposite direction.
How? Say you were looking at the airplane from the front. The left hand engine is out, the only one turning is the right hand engine. The right hand engine is now the only one keeping the aircraft aloft, and any rotational forces being applied to the aircraft are coming from that motor, (and propeller) The prop is attempting to twist, or rotate the entire aircraft around the axis of the centerline of the prop. If it could, it would cause the entire aircraft to rotate in a clockwise direction. But! In order to do this, it has to lift the entire left side of the aircraft. The same holds true if the right hand engine goes out, and the left one is the only one turning. In order to spin the ENTIRE AIRCRAFT around the axis of the propeller, it would have to lift the entire right hand side of the aircraft. Let's turn the props in the opposite direction. If the left hand motor quits, (and is no longer providing power to keep the left side of the aircraft up) and the right hand motor is spinning in a counter-clockwise direction, the remaining motor is attempting to spin the ENTIRE AIRCRAFT counter-clockwise around the axis of the prop. In other words, the aircraft is attempting to spin counter-clockwise, (because of the dead motor on the left hand side) and the prop is also attempting to spin the aircraft counter clockwise. Disaster can only ensue. As for the cranks and camshafts etc. etc. The Allison V-1710 aircraft engine was designed so that the crankshaft could be used on either side, simply by taking the crankshaft out, reversing it, and reinstalling it into the engine. It could then be run rotating in the opposite direction. Remember, IT WAS DESIGNED THAT WAY! Using a symmetrical firing order. (It would fire in the correct order, as long as the cams were reversed as well.) Genius. (p.s. here is a link to a youtube video showing a P-38 start-up, and then shut down. You can easily tell which direction the props are turning on start-up.)https://youtu.be/mBL6EcyZ9Xk