WI: NACA Modified P-38

It has always amazed me how often science-fiction writers wish to introduce fantastic changes to a timeline. More often than not the changes would require precursor techs that didn't exist, yet when it comes to aircraft simple changes, made just a few years earlier would have reaped incredible rewards. Hindsight is always more accurate than foresight.
EverKing, I am going to shamelessly steal your draft pick analogy for use on another site I frequent.
 
And yet the solution, so simple and discovered in March of 1942, wasn't implemented until early 1944. A P-38 with the dive recovery flaps installed could be dived at full power from any altitude. It would still enter compressibility once it accelerated past Mach.68 but with the DFRs extended the shock waves coming off the wing would pass just below the stabilizer and elevator. What the pilot would experience diving past 500 MPH was still having the full control of the plane and getting no more then moderate buffeting. Problem solved.
Well, to be fair the DRFs weren't actually invented until summer '42, irrc (August is sticking in my mind, but that may have the aileron boosters--I'd have to dig back through all of my notes to find the specifics again) and according to the 1947 report NACA RM-A7F09 they were first tested at the 16' Ames wind tunnel in October of '42. They were also only a partial solution, delaying the onset of Mach-tuck by 20-25 mph at altitude. True an earlier introduction would have been advantageous but it still left the OTL P-38 with a lower diving "speed limit" than many contemporary fighters. This is what is so intriguing to me about the full NACA solution, as applied ITTL--it actually increases the Mach limit of the airframe beyond the simple reshaping of the shockwave provided by the DRFs.
 
Well, to be fair the DRFs weren't actually invented until summer '42, irrc (August is sticking in my mind, but that may have the aileron boosters--I'd have to dig back through all of my notes to find the specifics again) and according to the 1947 report NACA RM-A7F09 they were first tested at the 16' Ames wind tunnel in October of '42. They were also only a partial solution, delaying the onset of Mach-tuck by 20-25 mph at altitude. True an earlier introduction would have been advantageous but it still left the OTL P-38 with a lower diving "speed limit" than many contemporary fighters. This is what is so intriguing to me about the full NACA solution, as applied ITTL--it actually increases the Mach limit of the airframe beyond the simple reshaping of the shockwave provided by the DRFs.

I think the DRFs would give an additional 50MPG before the compressibility shock waves started heavily impacting the stabilizer. Which by that point you are reaching close to the terminal velocity of the P-38 at high altitude. But exactly at what speed with DFRs deployed compressibility is encountered is unclear. Depending on density altitude and also that with the DRFs deployed they would prevent the sudden occurrence of the compressibility tuck. Instead it was a more gradual effect as the airspeed kept increasing in the dive. At least that would give the pilots time to react to it.

However I think that adopting the NACA modifications @EverKing describes in this thread would be a far more advantageous route to have taken. It provided solutions to other problems as well as increasing the critical Mach number for the P-38.
 
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I think the DRFs would give an additional 50MPG before the compressibility shock waves started heavily impacting the stabilizer. Which by that point you are reaching close to the terminal velocity of the P-38 at high altitude. But exactly at what speed with DFRs deployed compressibility is encountered is unclear. Depending on density altitude and also that with the DRFs deployed they would prevent the sudden occurrence of the compressibility tuck. Instead it was a more gradual effect as the airspeed kept increasing in the dive. At least that would give the pilots time to react to it.

However I think that adopting the NACA modifications @EverKing describes in this thread would be a far more advantageous route to have taken. It provided solutions to other problems as well as increasing the critical Mach number for the P-38.

DRF increased the P-38's limits by 20 mph IAS, and improved controlability in dives. Note the caption under the second graph.
 
DRF increased the P-38's limits by 20 mph IAS, and improved controlability in dives. Note the caption under the second graph.

A prudent warning to pilots not to push the plane beyond the recommended airspeed limits. Similar to G loading placards. That doesn't mean the airplane can't take a little bit more but any pilot exceeding the limits is pushing their luck. They'd better know how to handle what they're getting into. But, this wasn't unusual for wartime flying in combat. Fortunately Lockheed built the P-38 like the proverbial brick shithouse.
 
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It would have been fantastic if Lockheed had built Johnson his own wind tunnel, but that would also require a bit of a crystal ball on their part to see just how radical and influential Johnson's designs would be.
I'd say Lockheed builds the tunnel for corporate use, not just because they've hired Johnson. He might be the main beneficiary of it, but he won't be the only one.
Yes, I believe a model would reveal at least some of the solution. In fact in the article @Draconis previously shared, and supported by the reports I listed here, some of the aerodynamic flaws were discovered using a 1/6 scale model.
Some, yes, I take that as given. I meant, I don't see the tunnel as a panacea.

I also wonder if model testing might suggest the twin-boom design is sub-optimal & leads to it being rejected:eek: :eek::teary::teary: (even while other improvements, such as proposed throughout this thread, are adopted).
The compressibility problem was clearly the number one problem affecting the P-38. And the dive recovery flaps were an effective and simple solution. Why the DRF approach wasn't understood and introduced into P-38 production after March 1942 I don't know. Somebody at Lockheed didn't recognize the significance of the 1941 NACA testing?
AIUI, the interference with production schedules was considered too great. There was a conversion kit built, but the supply of those was interrupted when the delivering C-54 was shot down. (What happened to the next batch of kits, IDK.)

As for why the problem wasn't recognized & addressed sooner, Caidin blames it on the wreck of the prototype setting back the test program. How much of it was priority on the Hudson & not the Lightning, IDK.
 
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I also wonder if model testing might suggest the twin-boom design is sub-optimal & leads to it being rejected:eek: :eek::teary::teary: (even while other improvements, such as proposed throughout this thread, are adopted).
Sounds like a version of the Tigercat a couple of years early.
I'm... actually pretty fine with this, although it's at the cost of the Lightning's 'special something'.
 
Sounds like a version of the Tigercat a couple of years early.
I'm... actually pretty fine with this, although it's at the cost of the Lightning's 'special something'.
It does suggest something like the F7F, yeah. I'd be very unhappy indeed, even allowing the changes got it in service sooner.

That said, was there even a small chance of the Navy buying a folding-wing variant? Or the Corps getting a rebadged AAF model (in the fashion of the Army getting SBDs as A-24s)?
 
A folding wing Lightning? Not that I'm aware of, I'm just going off the top of my head but I'm pretty sure the Navy had a blanket requirement for all naval aircraft to have radials.
Not sure if Lockheed ever made a radial-engined Lightning, but I'd love to hear about it if they did.
 
I'm pretty sure the Navy had a blanket requirement for all naval aircraft to have radials.
Not sure if Lockheed ever made a radial-engined Lightning, but I'd love to hear about it if they did.
IIRC, you're right on the first. As for the second, not AFAIK, but there's a great-looking kitbash phantom out there I've seen pix of, so if we're going to posit big changes thanks to an early wind tunnel, why not go that way? ;)

Of course, a radial Lightning might more properly deserve her own thread...;)
 
IIRC, you're right on the first. As for the second, not AFAIK, but there's a great-looking kitbash phantom out there I've seen pix of, so if we're going to posit big changes thanks to an early wind tunnel, why not go that way? ;)

Of course, a radial Lightning might more properly deserve her own thread...;)
id be a reverse p-40
 
Pretty sure there's a thread for novel engine swaps out there, probably on the Secret Weapons forum. I've had my eye out for good renders of a radial Spitfire for a while now.
 
I've had my eye out for good renders of a radial Spitfire for a while now.
That makes me miss Just Leo. :'( He'd have been reading this thread, & he'd have whipped one up between replies, & it'd be gorgeous.

So let me ask: has anybody seen one of his? I haven't, but I do recall he did a few radial "conversions", including (IIRC) a Hurricane.

Edit:
We aren't the only ones who think the P-38 is cool. Olds used one in a '77 Toronado ad:
'77 oldsmobile toronado w p-38 ad (indieauto.org)-----.jpg
 
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McPherson

Banned
The compressibility problem was clearly the number one problem affecting the P-38. And the dive recovery flaps were an effective and simple solution. Why the DRF approach wasn't understood and introduced into P-38 production after March 1942 I don't know. Somebody at Lockheed didn't recognize the significance of the 1941 NACA testing?

Just to clarify the matter for all readers. The P-38 would encounter compressibility at a slightly lower Mach number then most other fighters. This didn't mean it couldn't dive faster then that speed. What it did mean was the plane needed the DRFs so the P-38 could exceed Mach .68 without having the transonic shock waves from the centre wing section violently hammering the stabilizer and elevator and also losing the stabilizer's effect on pitch stability causing the plane to pitch forward nosedown. This was described as compressibility tuck. It was a problem that could make the P-38 very difficult to recover from a power dive from high altitude or could also cause the tail to be torn away. Very bad indeed.

And yet the solution, so simple and discovered in March of 1942, wasn't implemented until early 1944. A P-38 with the dive recovery flaps installed could be dived at full power from any altitude. It would still enter compressibility once it accelerated past Mach.68 but with the DFRs extended the shock waves coming off the wing would pass just below the stabilizer and elevator. What the pilot would experience diving past 500 MPH was still having the full control of the plane and getting no more then moderate buffeting. Problem solved.
It is because a lot of so called aerodynamicists did not understand what those flaps actually DID.

They are not "brakes", they are wind flow disruptors that move the loading and pressure differential (compressibility stall function further aft from the leasing edge of the wing chord, which in the P-38 specifically was DEADLY when the wing fared into the pod as the pod's teardrop shape brought the unstuck flow into a vorticed swirl condition just aft of the pilot and forward of the horizontal stabilizer. The air flow unstick problem at the main wing and the tendency of the air disrupted roiled flow tended to increase left in the horizontal stabilizer and pitched the nose down into an uncontrolled yaw induced stall conditions and bring on tail control lock in pitch as an added fail mode.

John Stack diagnoses the P-38 Lightning problem correctly at NACA and passes it along. It probably would not have mattered if Lockheed had the wind tunnel. They did not have the right man.

How do we know this? Let us look at what Corky Meyer, test pilot for Grumman on the F7F and F8F, has to say about it:

The P-38L had two features that I wanted to check-The installation of the new Dive Recovery Flaps and the Mach meter/airspeed indicator gauge. The latter was an instrument that told the pilot when he was up against the new compressibility phenomenon, which had been first discovered in the Lightning. The Lightning ran into a strong buffeting that occurred at low dive angles long before it attained its limit speed. It was so strong that it tore the entire tail section off of P-38s in early test aircraft and killed 3 test pilots. In flight tests all 3 of Grumman’s new fighters had been found to be lacking in recovery control after entering compressibility. This was a very serious predicament for all fighters and test pilots.

The NACA researched the problem and determined that small, power operated dive recovery flaps should be installed on the P-38 wings just outboard of the fuselage. When extended to 20 degrees they would completely eliminate the frozen stick, nose down tendency and pitch the aircraft up to a 4-G recovery pullout, automatically pulling the aircraft out of the dive. Their drag would also rapidly slow the aircraft enough to back it out of the compressibility speed range.

These gadgets were a godsend to test pilots who had been previously probing the compressibility limit of fighter aircraft without any positive means of pulling out of the dive. I was now a convinced customer for dive recovery flaps. They were equally satisfying to military pilots who got so engrossed in tail-chasing combat that they couldn’t take their eyes from the enemy to monitor their speed restrictions and thus could easily run into this terrible buffeting condition with no pullout capability. I’m sure that a lot of military pilots gave their lives needlessly before dive recovery flaps were discovered and installed on fighters.

I was most eager to try out the dive recovery flaps and the new Mach-meter airspeed system so pilots could accurately gauge when they were about to enter this transonic region.
Note that he is working on Grumman planes in 1942 and 1943?
Attaining the safe altitude of 30,000ft I entered a 30 degree dive and extended and retracted these new flaps. As I increased my airspeed to its critical Mach number I found that actuating the dive recovery flaps gave a constant 2.5 to 3G pullout without any effort on my part. Although I knew that this model P-38 also had a much stronger structure than the one I had flown previously (a P-38E in another joint program in 1943) I still nudged it into the buffeting and compressibility regime very carefully. I noted that the aircraft had slowed down about 25mph before I retracted the dive recovery flaps. I decided then and there that Grumman was going to install this beautiful device on all of our dive test aircraft before any further investigation into compressibility. I had not seen the Mach limit instrument before and I fell love with it immediately.
Note that Meyer was testing everything in a joint test program operated among the aircraft companies during this period?
While I hastened down I tested the effectiveness of the P-47M dive recovery flaps and I found that they performed as well into the compressibility areas as the P-38L- an additional argument for their installation in all Grumman fighters.

My source?

Now about "Kelly" Johnson; HE had seen this problem before, so he knew exactly what Stack analyzed. There was the Lockheed Electra in which Johnson, a junior engineer at the time, and Lockheed's chief engineer, Hibbard had a disagreement about...

Johnson, during 1933—a year prior to the initiation of the Model 10 tunnel tests—had applied to Lockheed for an engineering job. He had not been turned down, but instead had concurred when von Hake recommended that he first return to Michigan and complete his master of science degree in aeronautical engineering. Von Hake also told Johnson that though Lockheed still was in the throes of its post-bankruptcy start-up, there might be an open engineering position if and when forthcoming aircraft programs began to gather momentum. Johnson took von Hake at his word; shortly after the conclusion of Stalker's Model 10 tunnel study, he returned to Lockheed's Burbank, California offices... master's degree in hand...and again asked for employment. Cyril Chappellet (one of the major shareholders in the new company) and Hibbard were the people who actually hired Johnson—as Lockheed's 36th employee. It would be looked back upon as perhaps the single most important personnel transaction in the aircraft manufacturer's history. "Kelly's" debut shortly after his arrival was anything but auspicious. In a meeting with Chappellet and Hibbard during the course of one of his first days at work, he told the engineering group their new Model 10 was only marginally stable. Bluntly, he told the group he did not agree with Prof. Stalker's conclusions. He felt a better and safer aircraft could be designed. These comments came as somewhat of a shock to Hibbard. A day after listening to Johnson's matter-of-fact declaration, he cornered the neophyte engineer and orally reviewed his academic credentials in a face-toface confrontation. He felt certain he could repudiate the Model 10 disparities that had been brought to light. Hibbard would recall, years later, he came close to firing Johnson on the spot. Several months after the encounter, and after concluding Johnson's claims might have some merit, he told the young engineer to return to Michigan and solve the Model 10's stability shortcomings. Johnson drove to Michigan with the large Model 10 wind tunnel model awkwardly squeezed into the back passenger compartment of his car. Some 73 tunnel runs followed, this number being necessary to narrow the sources of difficulty. The idea of attaching "controllable plates" to the horizontal tail tips surfaced while run number 72 was underway. These auxiliary surfaces, tested during run number 73, "worked very well", particularly after the destabilizng wing root fillets were removed. The next step improved control even further. Full, twin vertical tail surfaces now replaced the "plates", supplementing the original fuselage-mounted vertical tail. Superb directional control resulted. It was concluded later the original vertical tail was unnecessary...and it was removed. Johnson, who until the Model 10 assignment had been working as a tool designer at Lockheed, returned from Michigan somewhat a hero and most importantly a "full-fledged member of the engineering staff". In solving the Model 10's problems, he had set the company and its product line on the path to financial success. The Model 10 would give birth to several perturbations including the Model 12 (a scaled-down Model 10) and the highly successful Model 14. Consequently Johnson would be partially responsible for the financial latitude implied in the prototyping of the Model 22...which later would become worldrenowned as the P-38 Lightning. Even in consideration of the several shortterm successes realized during the period from 1934 to 1937, Lockheed's future, by 1938, again had begun to look bleak. Placing most of its eggs in a single basket, a concerted effort under the aegis of corporate representative Kenneth Smith resulted, on June 23, 1938, in the consummation of a contract for between 200 and 250 Model B14L (bomber version of the Model 14) Hudsons for the British Royal Air Force.
Now to be sure, the TAIL CONTROL issues that Johnson identified in the Electra/Hudson were solved in yaw (rudders at the horizontal stabilizer end tip wind vortices, but when the P38 came along, Johnson understood Stack's diagnosis as a similar TAIL CONTROL problem for the Lightning in pitch. The DRF was his spoiler solution to smooth out disrupted air in the Lightning. I do not believe the universal applicability was understood across the entire US fighter flight line until the composite test program was run, so that may have been a reason for the general delay of its application for all American fighters then under development. That might include Lockheed as part of the test program. MOO.
 
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It is because a lot of so called aerodynamicists did not understand what those flaps actually DID.

They are not "brakes", they are wind flow disruptors that move the loading and pressure differential (compressibility stall function further aft from the leasing edge of the wing chord, which in the P-38 specifically was DEADLY when the wing fared into the pod as the pod's teardrop shape brought the unstuck flow into a vorticed swirl condition just aft of the pilot and forward of the horizontal stabilizer. The air flow unstick problem at the main wing and the tendency of the air disrupted roiled flow tended to increase left in the horizontal stabilizer and pitched the nose down into an uncontrolled yaw induced stall conditions and bring on tail control lock in pitch as an added fail mode.

John Stack diagnoses the P-38 Lightning problem correctly at NACA and passes it along. It probably would not have mattered if Lockheed had the wind tunnel. They did not have the right man.

Thanks McP. For including those two links to more papers on the subject. And yes, the DRFs where definitely not air brakes. The rest of the paragraph you wrote I'm having trouble visualizing. Think I'll re-read it tomorrow morning.
 

McPherson

Banned
Thanks McP. For including those two links to more papers on the subject. And yes, the DRFs where definitely not air brakes. The rest of the paragraph you wrote I'm having trouble visualizing. Think I'll re-read it tomorrow morning.
I added some details @Draconis. To properly understand the compressibility problem in the P-38, one has to understand that the lockup was primarily elevators and not ailerons in that aircraft. This was another problem that aerodynamicists had trouble visualizing. There was AIRLERON lockup, but this was not a main wing problem as it was a HAMMER coming off the pilot pod on the horizontal stabilizer problem, and by hammer I mean a harmonic effect that forcefully oscillated the elevators (flutter) as the shock waves hit until pressure equalized and the control surfaces exceeded both the mechanical control forces available to move them and the tensile strength of the aircraft structure. The tail tore off and that indicated to "Kelly" Johnson, that he had the "tail control" issue. It was in the main wing spar that the "hammer" was set up, but it was definitely a tail control issue. He had to stop that hammer from forming in the first place. It was the era of "intuition", and he mentally visualized the wind flow pattern and had NACA and test pilots test for it to determine if his hunches were correct; to put numbers to the concept he imagined. My guess is that Stack beat him to the root cause, but it was Johnson who figured out where the "unstick at the upper boundary layer occurred. What I do not understand, is why Johnson missed the wing sweepback solution, which was the other way to deform the shock wave?
 

McPherson

Banned
Thanks McP. For including those two links to more papers on the subject. And yes, the DRFs where definitely not air brakes. The rest of the paragraph you wrote I'm having trouble visualizing. Think I'll re-read it tomorrow morning.
"Left" should be "loft". That is what I get for not proofing my copy.
 
First, I want to thank the General:openedeyewink: for his links & quoted material. It's been very interesting reading. 😎
What I do not understand, is why Johnson missed the wing sweepback solution, which was the other way to deform the shock wave?
Even more mystifying, when the Germans had figured it out in 1935, before the L.22 was even conceived. AIUI how this problem worked, it wouldn't even have needed wing sweep as such: that is, the spar could still attach perpendicular. It just needed leading edge sweep. Or have I missed something?

Two things puzzle me (& both probably reflect my lack of understanding ).

First, when hitting control freezing due to compressibility, why not just pull the throttle back (60% power?): speed reduces, flow over wings slows, tail unfreezes. I recognize, doing this in combat was not a desirable approach...but it would've saved engineering test pilots & allowed them to report what the aircraft was doing, & so get to an answer sooner.

Second, was it impossible, when faced with an inability to pull out, to push through & outside loop? Was the tail so blanked that was impossible? (That's my guess.) Was it too heavy a strain on the aircraft? (That seems possible.)
 
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