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 (http://en.wikipedia.org/wiki/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 (http://en.wikipedia.org/wiki/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://en.wikipedia.org/wiki/P-38_Lightning#cite_note-30)

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/344384-counter-rotating-propellers.html

and

http://www.pprune.org/aviation-history-nostalgia/344894-p-38-prop-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.

:ok:

[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]

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... :O

Keep it up. :ok:

[Brian Abraham]

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

http://i101.photobucket.com/albums/m56/babraham227/s0009.jpg

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

[AARON O'DICKYDIDO]

Just out of interest, the AIRBUS A400 has counter rotating props on each wing. But the engines do not counter rotate. Two of the engines have gearboxes fitted to enable the props to counter rotate. When looking at the aircraft head on the props 'go down between the engines'.

Aaron.

[IcePaq]

I thought the P38s ordered by Britain were not counter rotating but rather both were right handed.

Also............it's the camshaft and not the crankshaft that determines engine rotation direction.

Yeah, I know it's an old thread.

[megan]

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 directionI've heard some tales in my time, but that takes the cake. I'll assume you're a troll.

The P-38 props rotate in the direction they do for the simple reason it reduced the power on/off pitching moment, thus making for a more stable gun platform - Kelly Johnson, P-38 designer.it's the camshaft and not the crankshaft that determines engine rotation directionI have this from a shop that builds up/overhauls V-1710's.

To make a left hand engine from a right hand engine, you have to reverse the crankshaft, replace a regular gear with an "H" gear (this is a gear that skips over a gear that was used), and add a standard gear to reverse the prop rotation after you skip with the "H" gear. Every Allison engine has the ability to be reversed if you have an "H" gear and the added gear.

The right hand bank as viewed from the distributor end (rear of the engine) must have several spark plug leads interchanged because the cam lobes are backwards. It works out the same for the left-hand bank, somehow ... no left bank changes to the firing order

Basically, to make a left from a right, the engine must be almost disassembled because you must be able to get to the gear case in front and must split the case and disconnect all the rods to reverse the crankshaft.

However, if you are BUILDING a left or right from parts, the difference in build up is trivial, assuming you have an "H" gear, the new gear, and you know the plugs to interchange (this means rewiring the ignition harness on one side ... so it is MUCH easier to simply build a left or right wiring harness than it is to change one that is already wired).

The "H" gear:

A Standard gear is just a gear with a keyed center. An "H" gear looks like two standard gears joined by a small cylinder in the middle to skip over the gear that was formerly meshed by the standard gear. The new gear you add is to turn the skipped gear in the other direction. All gear cases have the ability to turn either way ... the gear bosses are in all of them, internally.

For both left and right engines, the cams turn the same direction and the crankshaft is reversed and turns backwards.

There is no other engine I know of from WWII that as so easy to make turn in either direction when being built up ... two gears, turn the crankshaft around, and change the right bank firing order ... that's it.

You need a starter that turns the other way, and you need an idler type gear to reverse the direction of the cam towers.

[DaveReidUK]

I got as far as "spinning the aircraft around the prop axis" before I gave up reading. :O

[IcePaq]

So the camshaft is turned the opposite direction by fiddling with the gear drive.

I knew it was far more than simply flipping the crankshaft as originally stated.

If you flip the crank, you have to change the rotation direction of the camshaft.

Now on a chevy v8, you usually only have to change the camshaft direction as I used to do on big block chevys (marine applications) by the same method as in the v1710 but the crankshaft can stay.

[megan]

The left engine was the one modified to reverse rotation. The firing order was changed thus.

1L 2R 5L 4R 3L 1R 6L 5R 2L 3R 4L 6R (Right engine)
1L 6R 5L 2R 3L 4R 6L 1R 2L 5R 4L 3R (Left engine - modified)

Easily seen the order on the left bank does not change.

[DaveReidUK]

Allison V-1710 Gear Train for Left-Hand Rotating Engines (http://www.enginehistory.org/Piston/Allison/V-1710Details/allisonv1710geartrainforlefthandrotatingengines.html)

Allison V-1710 Gear Train for Right-Hand Rotating Engines (http://www.enginehistory.org/Piston/Allison/V-1710Details/allisonv1710geartrainforrighthandrotatingengines.html)

[Rory57]

The left engine was the one modified to reverse rotation. The firing order was changed thus.

1L 2R 5L 4R 3L 1R 6L 5R 2L 3R 4L 6R (Right engine)
1L 6R 5L 2R 3L 4R 6L 1R 2L 5R 4L 3R (Left engine - modified)

Easily seen the order on the left bank does not change.

On neither bank does the firing order change. That would require a different camshaft.
The confusion comes from stating the firing order as beginning from one particular cylinder.
Of course an engine does not start to run from, ie cylinder 1 Left, it runs continuously: the left bank order being 60 degrees in advance of the right bank for one direction of rotation, and trailing 60 degrees behind the right in the other direction.
If you read through the above firing orders for either cylinder in modified and unmodified form you will see they are individually identical, just phase-shifted.

It is not obvious why these Allison engines required the flipping of the crankshaft to accommodate this reversal in running direction.
There is no a-priori reason for so doing but I feel sure it was not done for fun.

[Allan Lupton]

As this thread demonstrates, there are lots of ways by which the engine was said to be made to rotate widdershins, most of which are unlikely to have any foundation in fact.
e.g. "flipping" a six-throw crankshaft just puts the prop- and accessories-drives in the wrong place - it would make no difference to the firing order and would not itself cause reverse rotation.
The two diagrams of the timing gear linked by DaveReidUK show the main element of what was done, but everyone else just added an idler to the prop reduction gearbox. I can only think of one possible reason why Allison reversed crankshaft rotation which is that the gyroscopic effects of the two heavy crankshafts would cancel. Most of the rest of the engine (camshafts, supercharger, etc.) was not reverse rotating.

[megan]

On neither bank does the firing order changeThat is the case if you take each bank in isolation. The firing order is the order in which a spark is sent from the magneto to a particular cylinder, and is the reason the right hand bank had the ignition harness wiring changed, but the left bank did not. A phase shift yes.everyone else just added an idler to the prop reduction gearboxAs they did on the Merlin for the Hornet/Sea Hornet. One does wonder why Allison did not take a similar route. Perhaps Allan you may be correct re gyroscopic forces, though one wonders in what way that may have forced the solution they came to. Does seem an overly complex way of solving the problem.

[Old Nic]

I still can't see why the firing order would change, if the crank is rotating in same direction and output direction is changed in the gearbox??

[megan]

The crank is rotating in the opposite direction. See image of reduction gearbox below. DaveReid supplied image links above for where the idler mods took place at the rear of the engine for the ancillaries (to keep them rotating in the same direction as an unmodded engine).

[DaveReidUK]

DaveReid supplied image links above for where the idler mods took place at the rear of the engine for the ancillaries (to keep them rotating in the same direction as an unmodded engine).

And note that the camshafts, too, rotate in the same direction on right-hand and left-hand engines.