Effectiveness of the P-38 (1 Viewer)

[pinehilljoe]

You're not drinking enough.

Fluids 301 was my not my favorite class in college, but I believe the effect may be caused by modial interaction of magneto-reluctance and capacitive diractance.

 

[pbehn]

Absolutely: Aerodynamics is a very complicated subject and which is why you should always be extremely vary when someone makes very detailed, complicated and assertive claims about why things "must" be in a certain way. So when you hear something like that and it makes no sense to you, you have to ask yourself does it sound like gobbledygook just because you don't understand it or simply because it actually IS gobbledygook? Personally, I find having an MSc in aerodynamics and structural engineering plus having worked in the aerospace defense industry for many years of great assistance in making such calls, and which was why I pulled the plug on the P-38 dive flap discussion.

I have been reading Bills posts for years and he is also a qualified expert in aerodynamics. With his posts, including the one in question, I read it and then read it again and then finally read it as a series of statements which eventually I understood. Personally, I worked surrounded by many very bright people, almost all graduates, some were on the genius level and some were complete chancers. Anyone waving an MSc in my face at the first instance comes across as a chancer, it has happened a few times on these boards. Now, which part do you consider "gobbledygook" maybe I can boil it down to something someone with only an MSc can understand.

 

[pbehn]

You're not drinking enough.

That is where I went wrong in fluid dynamics.

 

[drgondog]

Absolutely: Aerodynamics is a very complicated subject and which is why you should always be extremely vary when someone makes very detailed, complicated and assertive claims about why things "must" be in a certain way. So when you hear something like that and it makes no sense to you, you have to ask yourself does it sound like gobbledygook just because you don't understand it or simply because it actually IS gobbledygook? Personally, I find having an MSc in aerodynamics and structural engineering plus having worked in the aerospace defense industry for many years of great assistance in making such calls, and which was why I pulled the plug on the P-38 dive flap discussion.

Welcome to my world.

 

[MIflyer]

When I was working on overhaul of the F-106A automatic fuel transfer switch I wondered how I would be able to calibrate it for different altitudes. I proceeded to dig out one of my college textbooks, went into the boss's office to use the whiteboard, and performed a derivation that we had never done in college - a derivation that showed the Mach number pressure ratio was independent of altitude. Then I realized that HAD to be true or else the Shock tables would never work.

I was so pleased until I felt stupid. We left the calculations up on the board anyway, just to impress people.

 

[GregP]

Fluids 301 was my not my favorite class in college, but I believe the effect may be caused by modial interaction of magneto-reluctance and capacitive diractance.

Must have been made out of pre-famulated amulite.


View: https://youtu.be/Ac7G7xOG2Ag

 

[Holtzauge]

I was not planning to post anything more here but since this thread has gone so far south with ridicule and claims that there is no gobbledygoock to be found, here are a couple of examples:

I don't personally subscribe to the theory that the deflection of a flat plate (bottom surface) in the freestream immediately improves the lift distribution along the top of the wing- at least not until after the pitch up moment is generated. Some immediate phenomena must disrupt the shock wave before the downwash behind the wing stabilizes to re-engage elevator authority. Merely increasing local CL while the flow velocity > M=1 doesn't reduce the physics of the shock wave or flow properties aft of the shock wave.

An abrupt pitch up/increase drag/increase AoA Would affect the velocity (lower it) along the wing surface.

The explainable immediate phenomena is an instantaneous change in momentum (and force vector) of the flow from the LE along the bottom surface when the flap is deployed - causing a pitch up reaction to the change in momentum at 30% chord. The subsequent re-establishment of subsonic velocity on the top surface eliminates the shock wave and the blanketing of the horizontal stabilizer. Recall that dives are in 1G range and zero lift angle of attack, when the shock wave forms. Any pitch up 'disturbance' results in positive angle of attack, increased lift coefficient and higher drag. I'm not disputing the NACA language, just posing an incomplete explanation for the P-38 specifically.

The pressure distribution along the top surface to restore normal center of pressure is not attained, until the shock wave is eliminated. That occurred naturally as lower dive altitude increased air density and temperature to increase Mcr beyond the airflow velocity over the airfoil

So here we have claims that increasing the angle of attack lowers the velocity on the upper surface and re-establishes subsonic flow on the upper surface. But this is a fundamentally wrong statement. This is not how it works: The speed actually increases on the upper surface of the wing and the shock wave gets even bigger if the angle of attack is increased at a given Mach number. So totally different from what the citations above claim. And even if these ideas are cloaked in an impressive sounding aerodynamic word salad it's still fundamentally wrong: If you have a shock wave formation on the upper surface of the wing then increasing the angle of attack does NOT make the shock wave go away. Instead it's exactly the opposite: The shock wave problem actually INCREASES. For those still in doubt, the Schlieren phots below clearly shows this. So the idea that in order to get out of the dive, the P-38 has to pitch up in order to get rid of the shock wave is wrong. However, you can of course still do this by using an adjustable stabilizer to pitch up. And this will increase the Cl and will get you out of the dive. But this is NOT because you have eliminated the shock wave, it's because you are using brute force. This means you have to pitch up to a slightly higher angle of attack than if no shock wave was present, because the shock wave has lowered the lift slope and you need to generate more angle of attack in order to get enough Cl to get you out of the dive.

And with that my participation in this thread is done. I have no doubt that this will be met with yet another dose of impressive sounding aerodynamic word salad but I have no intention of wasting any more time on this.

 

[drgondog]

Well in my opinion this is just aerodynamic gobbledygook with no foundation in science so this means we are done here.

I thought more about this and confess curiosity to your specific aerodynamic 'non-gobbledygook' and detailed explanation is. In detail please educate us on the following:
1. Shock wave formation and movement exacerbated by the venturi effect of the fuselage centerbody/wing between the two engine nacelles
2. Wake turbulence formation and degradation of the wake flow downwash over the elevator
3. Wake behavior as shock wave migrates past max T/C and the influence of reduced CL on the wake
4. Immediate flowstream behavior, for top and bottom airfoil surface and detailed explanation for restoration of pre-shock pressure distribution on upper airfoil surface - and detailed explanation for the contribution made by the dive flap to restore top and bottom surface pressure distribution to restore wake to pre-shock behavior Prior to Pitch Up Moment.
5. Your technical explanation why the flap deployment did NOT cause the pitch up - only the restoration of normal wave flow behavior permitting restoration of elevator authority?

Anybody with a Bsc will be able to draw us a well educated synopsis based on the physics of the model described above. I personally am truly intrigued by your thoughts regarding 'restoring CL' when CL is near zero in a controlled dive.

Equally curious regarding your 'non gobbledygook' analysis why you don't believe that the alteration of the pressure distribution aft of the shock wave, and thereby the Center of Pressure of the wing, moved forward causing the Moment about the A/c to cause pitch down aerodynamic forces.

 

[Peter Gunn]

Dammit, I wish you guys would speak English...

 

[pbehn]

And with that my participation in this thread is done. I have no doubt that this will be met with yet another dose of impressive sounding aerodynamic word salad but I have no intention of wasting any more time on this.

View attachment 785806

As someone without an MSc it appears gobbledyook has been replaced with a picture. Is that a P-38 aerofoil and if so where is the section taken. It appears to be a 2 dimensional representation but as I understand it it was a three dimensional problem. That is, whatever the issue of compressibility was, it was made worse in the inner wing sections between the engine/boom sections and the pilots gondola.

 

[drgondog]

I was not planning to post anything more here but since this thread has gone so far south with ridicule and claims that there is no gobbledygoock to be found, here are a couple of examples:





So here we have claims that increasing the angle of attack lowers the velocity on the upper surface and re-establishes subsonic flow on the upper surface. But this is a fundamentally wrong statement. This is not how it works: The speed actually increases on the upper surface of the wing and the shock wave gets even bigger if the angle of attack is increased at a given Mach number. So totally different from what the citations above claim. And even if these ideas are cloaked in an impressive sounding aerodynamic word salad it's still fundamentally wrong: If you have a shock wave formation on the upper surface of the wing then increasing the angle of attack does NOT make the shock wave go away. Instead it's exactly the opposite: The shock wave problem actually INCREASES.

IF the drag did not increase significantly with instant increased angle of attack you would have a point. But..
The minimum pressure of the NACA 2015 airfoil is ahead of the T/C max. The velocity is highest at T/C max where the shock wave forms. The pitch up causing higher angle of attack increases lift distribution ahead of the shock wave (hence increased CL)

For those still in doubt, the Schlieren phots below clearly shows this. So the idea that in order to get out of the dive, the P-38 has to pitch up in order to get rid of the shock wave is wrong.

The pitch up causes the angle of attack to generate the lift/pressure distribution across the wing in front of the shock wave to a.) increase Lift, b.) negate the negative CMac

However, you can of course still do this by using an adjustable stabilizer to pitch up. And this will increase the Cl and will get you out of the dive. But this is NOT because you have eliminated the shock wave, it's because you are using brute force. This means you have to pitch up to a slightly higher angle of attack than if no shock wave was present, because the shock wave has lowered the lift slope and you need to generate more angle of attack in order to get enough Cl to get you out of the dive.

And the 'adjustable stabillzer' was not present for the P-38, and there was no other input to pitch up - due to the blanking of the elevator

And with that my participation in this thread is done. I have no doubt that this will be met with yet another dose of impressive sounding aerodynamic word salad but I have no intention of wasting any more time on this.

View attachment 785806

Well, Bye then.

 

[GregP]

The P-38's dive flaps worked in practical use. That is, they DID pull the P-38 out of a fast dive when deployed, which is good since that was the idea.

That shouldn't make Holtzauge get, as Tiny Tim once put it in his wedding vows, "all puffed up."

C'mon guy, the very nature of discussion is dissenting opinions. That doesn't make you enemies. It only means you disagree with one another. That's sort of the way discussions go in many cases. Lack of agreement shouldn't mean, "let's declare war!"

 

[drgondog]

Chaos Theory was my watershed moment in my MS degree. I got through Calculus of Variations and Control Theory, but the Chaos course earned me my only B (there were no As) and the fool teaching the course flunked half the class. He shall remain nameless but forever cursed.

I still have no clue regarding how to apply the theory to the practical prediction of the formation of turbulent eddies in the transient turbulent boundary layer.

I should have mentioned that Chaos Theory was a guiding barrier for me to not pursue a PhD.

 

[ThomasP]

I do not know if this would be of any help in this discussion.

This is the hand-written/drawn draft of the summary of results of Lockheed's (and NACA's?) dive tests on the YP-38 prior to July 1942, put together by Clarence 'Kelly' Johnson and R.L.Thoren. A lot of it is just numbers and math, but the flow chart is interesting in and of itself.

 

[PAT303]

Dammit, I wish you guys would speak English...

I'd even tried googling some of the wake turbulence whatever exacerbated by the venturi effect something or other and even it told me to step away from the keyboard and go play outside.

 

[GregP]

'Twas brillig, and the slithy toves
Did gyre and gimble in the wabe;
All mimsy were the borogoves,
And the mome raths outgrabe.

"Beware the Jabberwock, my son
The jaws that bite, the claws that catch!
Beware the Jubjub bird, and shun
The frumious Bandersnatch!"

He took his vorpal sword in hand;
Long time the manxome foe he sought—
So rested he by the Tumtum tree,
And stood awhile in thought.

And, as in uffish thought he stood,
The Jabberwock, with eyes of flame,
Came whiffling through the tulgey wood,
And burbled as it came!

One, two! One, two! And through and through
The vorpal blade went snicker-snack!
He left it dead, and with its head
He went galumphing back.

"And hast thou slain the Jabberwock?
Come to my arms, my beamish boy!
O frabjous day! Callooh! Callay!"
He chortled in his joy.

'Twas brillig, and the slithy toves
Did gyre and gimble in the wabe;
All mimsy were the borogoves,
And the mome raths outgrabe.

 

[SaparotRob]

'Twas brillig, and the slithy toves
Did gyre and gimble in the wabe;
All mimsy were the borogoves,
And the mome raths outgrabe.

"Beware the Jabberwock, my son
The jaws that bite, the claws that catch!
Beware the Jubjub bird, and shun
The frumious Bandersnatch!"

He took his vorpal sword in hand;
Long time the manxome foe he sought—
So rested he by the Tumtum tree,
And stood awhile in thought.

And, as in uffish thought he stood,
The Jabberwock, with eyes of flame,
Came whiffling through the tulgey wood,
And burbled as it came!

One, two! One, two! And through and through
The vorpal blade went snicker-snack!
He left it dead, and with its head
He went galumphing back.

"And hast thou slain the Jabberwock?
Come to my arms, my beamish boy!
O frabjous day! Callooh! Callay!"
He chortled in his joy.

'Twas brillig, and the slithy toves
Did gyre and gimble in the wabe;
All mimsy were the borogoves,
And the mome raths outgrabe.

First time he made sense.

 

[GregP]

To make it applicable to aerodymics, replace Jabberwok with Jabberflow and Bandersnatch with Bernoulasnatch.