Effectiveness of the P-38

[Reluctant Poster]

Easily the best twin-prop fighter of WWII.

Day fighter. The Mosquito was a far superior night fighter.
Actually it's an artificial category. A day fighter is a day fighter. The design choices made to accomplish the goal are irrelevant. The competition is single engine fighters. The Albacore was the best biplane torpedo bomber of WWII but so what.

 

[Thumpalumpacus]

Day fighter. The Mosquito was a far superior night fighter.

You're right, I wasn't thinking of night-fighters.

 

[Holtzauge]

I should have seen that due to the Mach No. columns. That said I interpret the CL vs M plot as indeterminate with respect to 'first' reaction, without a plot of CMac as a function of time when dive flap deployed, overlayed with CL change as function of time.

Well in report WR A 66 this is not indeterminate and NACA is pretty clear on the flaps function (to increase the lift) and what is the 'first' reaction: The flap increases the lift and the pitch up moment follows from the increased down wash on the stabilizer:

 

[drgondog]

Well in report WR A 66 this is not indeterminate and NACA is pretty clear on the flaps function (to increase the lift) and what is the 'first' reaction: The flap increases the lift and the pitch up moment follows from the increased down wash on the stabilizer:

View attachment 785513

the words 'nearly' and probably are not great to boost confidence in explaining cause and effect.

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.

We'll have to agree to disagree.

 

[Holtzauge]

the words 'nearly' and probably are not great to boost confidence in explaining cause and effect.

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.

Well if NACA says something in a published report that weighs more heavily with me than personal opinions. Especially if there is a missing understanding of basic subsonic aerodynamic theory which tells us that if something is changed on the "bottom surface" then this will change the flow conditions on the upper surface as long as you don't have a supersonic leading edge, which occurs at significantly higher velocities than we are dealing with here. And all of this is irrespective of if there is a shock wave on the upper wing surface or not. So I do not agree with your idea that there must be some other (as yet unexplained by you) "immediate phenomena" that changes the downwash.

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

Yes, and this solution (pulling out by increasing the angle of attack) would be possible with an adjustable stabilizer. In this case the adjustable stabilizer has the control authority that the elevator lacks and can overcome the situation without the dive brake. And this is covered in the NACA report WR A 66 as well but is a completely different solution to the problem.

We'll have to agree to disagree.

Good point. I think we have to agree on having totally different ideas about how the dive flaps worked on the P-38 and leave it at that.

 

[drgondog]

Well if NACA says something in a published report that weighs more heavily with me than personal opinions. Especially if there is a missing understanding of basic subsonic aerodynamic theory which tells us that if something is changed on the "bottom surface" then this will change the flow conditions on the upper surface as long as you don't have a supersonic leading edge, which occurs at significantly higher velocities than we are dealing with here. And all of this is irrespective of if there is a shock wave on the upper wing surface or not. So I do not agree with your idea that there must be some other (as yet unexplained by you) "immediate phenomena" that changes the downwash.

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

Take what you want and leave the rest.

Yes, and this solution (pulling out by increasing the angle of attack) would be possible with an adjustable stabilizer. In this case the adjustable stabilizer has the control authority that the elevator lacks and can overcome the situation without the dive brake. And this is covered in the NACA report WR A 66 as well but is a completely different solution to the problem.

And therefore irrelevant to this discussion.

Good point. I think we have to agree on having totally different ideas about how the dive flaps worked on the P-38 and leave it at that.

agreed

 

[drgondog]

The converted P40s were restricted to training only. That freed up Merlins for front line duty.
Bill Marshall can correct me, but the only P51s with RR built Merlins were conversions by RR used in testing programs.

AFAIK, you are correct. That category includes Mustang I original conversions as well as the few Mustang III modified with Merlin 100 for V-1 chasing as well as Mustangs built in Australia.

I suppose possible that other replacements occurred at RAF bases, but I have no idea on specifics.

 

[Holtzauge]

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.

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

 

[Peter Gunn]

I don't watch this guy's videos. I must be old. His clickbait titles piss me off.

You are.

 

[Peter Gunn]

I was always under the impression that more speed wasn't a real possibility with the P-38, aerodynamically speaking I thought it was at about its limit.

 

[drgondog]

I was always under the impression that more speed wasn't a real possibility with the P-38, aerodynamically speaking I thought it was at about its limit.

Power soothes many drag ills, but basically the wing and the drag rise at 0.6 Mach for the inner P-38 wing was buzz kill.

 

[drgondog]

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

Your opinion is forever treasured. What is gobbledygook about a resultant force imposed on the lower wing due to change in momentum? Or restoring normal flow and pressure distribution of local subsonic flow from Mcr, and the subsequent restoration of a normal trailing wake with designed downflow at the elevator?,

 

[GregP]

How about this?

When you push the mass of the air on the underside of the wing downward, Newton's Laws dictate a resulting push upward on the wing where dive flap is located. The old "equal and opposite forces: thing. Since the dive flap is located in front of the center of pressure, the result is a nose-up pitching moment on the aircraft.

The dive flaps were experimentally located and sized in the wind tunnel so the pitching moment was enough to nose up, but not enough to overstress the airplane. Coincidentally, it was also not enough to injure the pilot, which was likely a requirement, too.

Saving the airplane while simultaneously killing the pilot is not a good solution to supersonic dive recovery unless you REALLY don't like him. Say, to the tune of the cost of a P-38 dislike. If you hate him that badly, don't let him fly it. It's way cheaper.

 

[pbehn]

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

In my experience here it is a mistake to believe that an explanation of something in aerodynamics is the full complete explanation. Frequently it is an abbreviation to make things understandable to reasonably intelligent, educated lay persons. The science is fluid mechanics and its sub sciences of fluid dynamics of which aerodynamics is part. Below is a link from wiki about fluid dynamics which is full of stuff I barely understand the principles of and it references other stuff like quantum mechanics, Newtonian and non Newtonian fluids which is all gobbledygook to me, but not the people who study and make their living at it. Fluid mechanics - Wikipedia

 

[drgondog]

In my experience here it is a mistake to believe that an explanation of something in aerodynamics is the full complete explanation. Frequently it is an abbreviation to make things understandable to reasonably intelligent, educated lay persons. The science is fluid mechanics and its sub sciences of fluid dynamics of which aerodynamics is part. Below is a link from wiki about fluid dynamics which is full of stuff I barely understand the principles of and it references other stuff like quantum mechanics, Newtonian and non Newtonian fluids which is all gobbledygook to me, but not the people who study and make their living at it. Fluid mechanics - Wikipedia

I was surprised that Wiki actually got this right.

 

[pbehn]

I was surprised that Wiki actually got this right.

A stopped clock? The wiki article on Chaos Theory includes an example on turbulence, but states that this was from mathematics and ideas starting around 1944-7, so even the top guys in NACA still had stuff to find out about Chaos theory - Wikipedia

 

[drgondog]

A stopped clock? The wiki article on Chaos Theory includes an example on turbulence, but states that this was from mathematics and ideas starting around 1944-7, so even the top guys in NACA still had stuff to find out about Chaos theory - Wikipedia

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.

 

[Holtzauge]

In my experience here it is a mistake to believe that an explanation of something in aerodynamics is the full complete explanation. Frequently it is an abbreviation to make things understandable to reasonably intelligent, educated lay persons. The science is fluid mechanics and its sub sciences of fluid dynamics of which aerodynamics is part. Below is a link from wiki about fluid dynamics which is full of stuff I barely understand the principles of and it references other stuff like quantum mechanics, Newtonian and non Newtonian fluids which is all gobbledygook to me, but not the people who study and make their living at it. Fluid mechanics - Wikipedia

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.

 

[SaparotRob]

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.

You're not drinking enough.

 

[pbehn]

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 remember you mentioned chaos theory before, you will neither be shocked nor surprised to find that is one thing we have in common.