pinehilljoe
Senior Airman
- 732
- May 1, 2016
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.
Effectiveness of the P-38
- Thread starter tomo pauk
- Start date
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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.
That is where I went wrong in fluid dynamics.
drgondog
Major
Welcome to my world.
MIflyer
1st Lieutenant
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.
I was so pleased until I felt stupid. We left the calculations up on the board anyway, just to impress people.
GregP
Major
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Holtzauge
Airman 1st Class
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. 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.
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
Major
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
Master Sergeant
Dammit, I wish you guys would speak English...
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
Major
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)
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
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
Well, Bye then.
GregP
Major
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!"
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
Major
I should have mentioned that Chaos Theory was a guiding barrier for me to not pursue a PhD.
ThomasP
Senior Master Sergeant
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.
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.
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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
Major
'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.
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
Unter Gemeine Geschwader Murmeltier XIII
First time he made sense.
GregP
Major
To make it applicable to aerodymics, replace Jabberwok with Jabberflow and Bandersnatch with Bernoulasnatch.
Peter Gunn
Master Sergeant
GregP
Major
Hi Bill,
I've always thought the center of pressure moves aft when you go supersonic while the center of gravity doesn't move, thereby causing a nose-down pitching moment.
If the tail gets blanked by the shock wave, it becomes impossible to pull up until the airflow reattaches.
If you aren't supersonic, and you speed up, a stable aircraft will nose up in order to maintained trim airspeed.
