P-38 vs P-51

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We ferried the plane 2 hrs back to homeplate and tried everything we could do to cause it to repeat. We took the DOM and the shift supervisor up and tried everything we could do to make it stutter. No joy. They swapped it out of our plane, and after it came back from P&W, put it on another plane where it lived out its scheduled life without a hiccup.
In ten years of operating PT6s with three different companies and 30+ planes, I'm only aware of one actual failure, and that was a doozy. A 1900 at MGTOW departing Watertown NY on a wet runway with 500 ft scud overhead and 3 Kts tailwind, blew its right engine at Vr. And I mean BLEW! Flames and engine parts out both stacks bouncing off the wing and rattling down the side of the fuselage. They got it around the pattern under the scud layer and back on the ground intact, but it wasn't pretty. The plane had to go back to the paint shop for a little cosmetic treatment.
That plane is out in CO or WY now hauling UPS packages for Ameriflite.
Cheers,
Wes
 
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P-39 Expert said:
Why didn't they just switch engines from one side to the other making both engines inward turning? Lose one on takeoff and the effects are not nearly as bad.
Click to expand...

Hello P-39 Expert,
Just about every other twin has the issue of a critical engine to deal with; The loss of one of the engines causes asymmetrical thrust effects much greater than the lateral offset would suggest. The issue is not new. It just happens that with the P-38, there is the same issue with both engines.


Hello XBe02Drvr,
I believe there is a little more to the issue than the AoA of the Wing Center section.
If that were the only issue, then changing the airfoil or the incidence of that section of the wing would have addressed the situation.
Most other twins have one propeller rotating inboard and one rotating outboard to deal with.
The P-38 already had different airfoils at the wing root and wing tip and early in the design, a wing root fillet was added to address turbulence problems from the nacelle to the stabilizer.
Note also that some export Lightnings sold to the British had both propellers rotating in the same direction and I believe their biggest issue was that the lack of a turbo gave up a lot of altitude performance.

It has been stated in various places that having outboard rotating propellers is aerodynamically more efficient.
I believe it might have something to do with the propeller slipstream offsetting some of the wingtip vortices, but perhaps someone with more knowledge of aerodynamics can sanity check this idea.

- Ivan.
 
Ivan1GFP said:
I believe there is a little more to the issue than the AoA of the Wing Center section.
If that were the only issue, then changing the airfoil or the incidence of that section of the wing would have addressed the situation.
Click to expand...
Whoa! I don't think it's that easy. Visualize the airflow as it spirals around the nacelle and strikes the wing. It's going to be striking the leading edge and upper curvature of the airfoil at an angle that defies smooth boundary layer flow. It will actually destroy what could be a smooth lifting airflow at very low power settings when you energize the slipstream with more power.
How are you going to fix this with airfoil or incidence changes that don't: A) screw up the internal spar structure, B) create high drag or negative lift at either end of the speed spectrum, or C) create these same vices at either end of the power spectrum?
As for the vortices issue, intuitively at least, outward rotation should accelerate spanwise flow which should energize, rather than inhibit, wingtip vortices, reducing L/D.
I think both of us may be stabbing in the dark here. Any bona fide aerodynamicists out there?
Cheers,
Wes
 

Hello XBe02Drvr,
I believe we have a slight mismatch in terminology. The "Nacelle" when used in the context of the P-38 refers to the center pod that the pilot sits in. At least that is how I have seen it described in documentation.

If we back up even further, consider that the spiral airflow from the propeller is going to hit one side of the wing on the top surface and the other side on the bottom surface. With your concern about what is happening on the wing root section, you might also want to note that the opposite is happening on the section outboard of each engine.
Which would cause a worse effect? The Leading Edge on the outboard section of the wing is a typically a bit further from the propeller, but this is not always the case. There are plenty of twins with a straight leading edge such as the A-20 Havoc.
Also, if this effect were so great, then most twins without propellers that rotated in opposite directions would have this kind of problem.

As stated before, I am certainly no aeronautical engineer, but as I see it, the smooth boundary layer flow is most important at high forward speeds during which the direction of the airflow is not greatly affected by the propeller. At very low speeds, you want the slight vortices to energize the airflow to prevent separation and actual drag isn't terribly important because those huge Fowler Flaps are hanging out in the airstream anyway.


First of all, I do not believe this would be a problem for reasons stated above.
It is an issue encountered by every twin with engines mounted out on the wing.
If it were a problem though:
A) I don't believe the differences in airfoil shape would require any serious structural changes. A slight change in the shape of the leading or trailing edges would not affect the location of wing spars.
I also don't believe that a couple degrees of incidence which is quite a lot would significantly affect structure either.
B) In reality, it is only the high end of the speed range that is of concern. Note that this is an interceptor aircraft (or at least this was the mission it was designed for). Perhaps cruise conditions might be of interest, but so many things change at the low end of the speed range that it should not be difficult to address.
If there is negative lift at the low end of the speed range, then the huge change in effective camber when the Fowler Flaps are deployed should address that issue.
C) As mentioned before, if the propeller creates one situation on the inboard side, it is doing just the opposite on the outboard side, so the situation will happen somewhere.

XBe02Drvr said:
As for the vortices issue, intuitively at least, outward rotation should accelerate spanwise flow which should energize, rather than inhibit, wingtip vortices, reducing L/D.
I think both of us may be stabbing in the dark here.
Click to expand...

Intuition is an interesting thing. I don't think mine agrees with yours, but that doesn't make yours any less valid. I will try to explain how I see things and maybe someone here can shoot a few holes in the "theory".

Assuming Outboard Rotation of propellers:
Airflow would be moved outboard on the upper surface of the wing and inboard on the lower surface of the wing.
Normal wing tip vortices happen because there is a higher pressure on the underside of the wing than on top of the wing and this pressure differential equalizes at the wing tip
The vortex is caused by air flowing outboard on the underside and inboard at the top side.
Note that this is opposite to the direction of air flow being created by the propeller.

Another effect would be that the two propellers are pushing air inboard between the booms on the underside of the wing center section on each side of the nacelle. This should create a higher pressure area and additional lift in that portion of the wing similar to what we might see in a hovercraft.

- Ivan.
 
The effect on the inboard section would be greater, as the wing is attached at the top of the fuselage, bringing the spiral flow from an inwardly rotating prop directly down on the top surface of the inboard wing section. On the outboard side, the fuselage protrudes significantly below the wing, somewhat breaking up the spiral flow and creating an area of turbulent flow and reduced pressure under the wing.
Ivan1GFP said:
I also don't believe that a couple degrees of incidence which is quite a lot would significantly affect structure either.
Click to expand...
I also don't believe a couple degrees of incidence would fix this problem either.
Cheers,
Wes
 
From an old post..

"Warren M. Bodie, in his book The Lockheed P-38 Lightning: The Definitive Story Of Lockheed's P-38 Fighter, states that, "Engine rotation was changed so that the propellers rotated outboard (at the top), thereby eliminating or at least reducing the downwash onto the wing centersection/fuselage juncture. There was, by then, no doubt that the disturbed airflow, trapped between the two booms, was having an adverse effect on the horizontal stabilizer. No problem was encountered in reversing propeller rotation direction; they merely had to interchange the left and right engines."
 

It is often claimed that the CCW Allison engines could not support generators and other accessories.

This is what is politely called a crock of shit.

In the Engine manuals folder there is a copy of the Allison Service School Handbook in post 2 at V-1710 Operators Manual. That document shows the details but not in a simple format.

The attachments below from the Allison Handbook of Operation and Maintenance describes the accessory drives far better. The starter and oil pump being "before" the idler gear are handed (have opposite DoR) but all other drives (supercharger, cam shafts, magneto, distributors, etc) and accessories turn the standard direction. Given the oil pump is internal this means the starter is the only accessory that differs between left and right engines.



 
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Excellent info! I thought this was accomplished by by a gear pad mounted at the accessory base, evidently I'm thinking of another engine. It took a while to figure it out but it's right there, item K. Thanks for posting this! Now the only question - was this configuration available during early P-38 production runs?
 
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Yes - the F2 (V-1710-27, -29) series engines the accessory case has the number 9 and number 10 locations needed for this change. I have never seen a manual for the F1 engine used on the P-38 prototype but I would be surprised if it did not because one of the original design features of the Allison was the ability to produce left and right engines with the minimum of different parts.
 
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Not true on p38 elevator. Most others damage to one side could survive. P38 hit in middle would break same at appendages. All axis pilots were trained to aim and damage that area. Rendering aggressive maneuvering problematic!
 
Dan Fahey said:
Not true on p38 elevator. Most others damage to one side could survive. P38 hit in middle would break same at appendages. All axis pilots were trained to aim and damage that area. Rendering aggressive maneuvering problematic!
Click to expand...

OMG!!! Do You have something to confirm this gibberish??? Do you have combat reports, references or other data to PROVE this?!?!? Like most of your posts I think this is another figment of your imagination!
 
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Didn't the same thing happen with the Twin Mustang? Wouldn't leave the ground until the engines were reversed or something like that?
 

Hello XBe02Drvr,
It took me a while to figure out what you meant. By "Fuselage" in this case, I presume you are referring to the boom behind each engine.
The condition that you are describing is what would happen with an inboard propeller rotation but note that the wing and boom are nearly symmetrical with the propeller, so if the propeller were rotating outboard, the same thing would be happening to the outer wing panels.
The only really significant difference as I see it is that there is a nacelle on the centerline which acts as a fence that does not exist on the outboard side.

XBe02Drvr said:
I also don't believe a couple degrees of incidence would fix this problem either.
Click to expand...

My belief is that the angle of relative airflow on the wing center section was not that significant from a performance point of view.
The reason for this belief is that the XP-38 had inboard rotating propellers and was not noted for particularly poor flying characteristics.
In fact, the reports were quite good and easily enough to justify building a YP-38 after the loss of the prototype.

The reduction of turbulence on the horizontal tail mentioned by others sounds like a pretty good explanation for changing the propeller rotation but obviously the amount of turbulence was not enough to be a show stopper if the XP-38 was successful and if the British export Lightning had one propeller rotating inboard and one rotating outboard.

Dan Fahey said:
Not true on p38 elevator. Most others damage to one side could survive. P38 hit in middle would break same at appendages. All axis pilots were trained to aim and damage that area. Rendering aggressive maneuvering problematic!
Click to expand...

Hello Dan Fahey,
The kindest way I can put this is that I believe you are making some assumptions that may not be true.
It is a pretty silly thing to claim anything for "All axis pilots....".
Keep in mind also that the size of the tail surfaces of an aircraft is not an arbitrary thing. The size is chosen to give enough longitudinal stability and control to fly the aircraft. Extra size isn't beneficial because it just causes unnecessary drag. Sometimes, losing some area from the horizontal stabilizer may not kill you but sometimes it will. It depends on how marginal the stability is under those flight conditions.

Note that the B-17 in the attached images did not survive. All 11 crew on board died.

- Ivan.





 
Ivan1GFP said:
Hello Dan Fahey,
The kindest way I can put this is that I believe you are making some assumptions that may not be true.
It is a pretty silly thing to claim anything for "All axis pilots....".
Click to expand...

Mr Ivan;

I commend your eloquence and tact with regards to that reply. If I had half of your smoothness I'd be making this year's commencement speech at Yale!
 
"Lieutenant Thomas Smith's P-38 smashed headlong into a disintegrating Bf 109 as the German fighter's spinning propeller tore gashes in the P-38 from engine to tail and severed the horizontal stabilizer, which extended between the Lockheed's twin booms. Worst of all, the P-38's right engine froze with the prop blades at a high-rpm setting, so they were set almost flat to the airstream.

Smith radioed that he was bailing out and jettisoned his canopy—an action he would regret in the cold hours that followed. As soon as he let go of the control yoke and took his feet from the left rudder pedal, the P-38 rolled violently into the dead engine. It would be nearly impossible for the aircraft to stay stable long enough for Smith to jump.

Smith made it back by flying a series of climbing orbits followed by a short dash toward home base. When he stumbled over the enemy-occupied town of Trieste, Italy, every flak burst sounded horrifyingly close as he flew with no canopy. Incredibly, Smith made a wheels-up landing. Besides being exhausted and half-frozen, he suffered his only injury upon landing—a lump on his head from hitting the gunsight."


Read more at Unbreakable | History | Air & Space Magazine

 

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Since the thread is involving a comparison between the '51 and the '38...what do you do in a similar situation in a '51 when you lose an engine?
 
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