Meredith Effect and the P-51

When the knowledgeable people from North American, like Atwood, Schmued and Horkey, have all stated that the radiator design (and not the laminar wing) was the key to the low drag of the P-51, that is good enough for me.

I have seen another Atwood's article where he mentions that the other aircraft that may have utilized the Meredith effect succesfully was the Mosquito.

So to compare the drag, the best way is to simply compare their Cd.
Allison Mustang vs. Bf109F vs. Spitfire V as contemporaries would make an interesting comparison.

Holtzauge said:

I remember reading about the P-51 duct problems and the "rumbling" sound somewhere else as well but do not remember the source right now. Interesting that they both solved the rumbling AND managed to lower the mass flow at the same time. Since the mass flow would be governed by the cooling need this would indicate that the necessary heat transfer could be done with less air flow which would directly translate into lower drag. Also interesting to see that the Mustang with the higher powered Merlin managed to make do with less mass flow than the Allison powered version.

H-the two best volumes are Gruenhagen's Mustang with an enormous body of technical details and Mustang Designer by Wagner. Wagner has significant recollections and narratives about the design issues encountered

The rumbling issues were caused by both boundary layer separation as well as the intlet geometry - with four different versions tested both for frontal geometry as well as vertical location of the upper 'lip' -

When they killed the 'rumbling, they next varied the internal geometry to obtain the final flow geometry and mass flow balance.

Another thing that that could go some way to explain why the P-51 had lower radiator drag than the Me109 is the very rapid expansion after the inlet in the Me109 compared to the more gradual compression in the P-51 duct.

I speculated on that geometry also - as there seems to be a large 'plenum' effect which would certainly slow the flow down but also seems to encourage boundary layer separation and increased pressure gradient? Would have to see the analysis to better understand 109 design approach

It's a well known fact in aerodynamics that the steeper the adverse pressure gardient (compression) is the higher the risk of a boundary layer separation right?

Yes - and it doesn't have to be 'compression' - i.e over an airfoil.

Over an airfoil the boundary layer grows with pressure gradient change - until (or near the point) where a positive pressure gradient occurs in the stream tube and it is at that point that the flow separates.

It is the latter condition that took some time to be able to model aircraft wing/bodies to obtain the impressive results that today's very fine 3-D Aero models like VSAERO achieve in calculating parasite drag using a potential flow model.


The same separation phonomena also occurs at a shock wave over an airfoil in transonic range where there is a discontinuity of pressure chord wise


Given the short distance from the inlet to the radiator, the Me109 engineers have no choise but the rapid expansion we see in the drawing provided by Kurfurst above. For the P-51 OTOH there is a more gradual compression which would lessen the risk of a separation in the duct.

That is an intuitive conclusion I also reached and did cause me to try to understand their philosophical approach to manage the bounday layer interaction under the wing upstream of the inlet

Finally a thought about radiator placement: If the wing position was a good way to go one has to wonder why Willy did not plan to place the radiators in the wing for the Me109 replacements under consideration: The Me309 placement was in the fuselage and Me209 had an annular radiator like on the Fw190D

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I tend to agree that the radiator design (significant) coupled with canopy windshield/carb intake/laminar flow wing (important) as keys to 51 low drag.

I know that the 51B and, to lesser extent 51H, NACA laminar airfoils did not quite achieve hoped for delay at ~40% but it did have a lower Cd0 than the conventional airfoils preceeding it on the P-40, P-38, P-47

The 51H wing seemed to have less drag than 51B/C/D/K but I haven't seen the wind tunnel tests or reports commenting on conclusions (.i.e. airfoil section (thinner) or wing geometry at inboard gun to fuselage).

Timppa said:

When the knowledgeable people from North American, like Atwood, Schmued and Horkey, have all stated that the radiator design (and not the laminar wing) was the key to the low drag of the P-51, that is good enough for me.

They did state that the radiator design was the key, but to state that the laminar flow wing was not important would be to take their comments out of context... simply that the laminar flow airfoil had more drag than the theoretical calculations suggested - but still less Cd0 than comparable fighter wings. It did contribute to less parasite drag for the entire airplane

I have seen another Atwood's article where he mentions that the other aircraft that may have utilized the Meredith effect succesfully was the Mosquito.

So to compare the drag, the best way is to simply compare their Cd.
Allison Mustang vs. Bf109F vs. Spitfire V as contemporaries would make an interesting comparison.

Click to expand...

We have had a very difficult time (on this forum) finding reliable source with reliable (and complete) breakdown from total airframe parasite drag in level flight to the components of parasite drag withing the airframe up to and including trim drag in sveral reynolds number ranges.

Do you hopefully have them?

Kurfürst said:

Do you have any figures for the respective radiator drags or its a guess/speculation?



Take for example the Me 209 and Me 309. Both designs opted for wide undercarriage, and a simple glace on their drawings reveal that there is simply no place in the wings to place the radiators there.

Me 309:

Taking a look at the Me 209 II,



again we see the wide undercarriage, and no space for installing of radiators or ducting in the wings. Given that the proto was merely a modified, stock Bf 109G-5, there would be no space in/under the fuselage either (the fuel tank is there, and taking the radiator even further is probably a not a good idea CoG-wise).

In addition the team was forced to use the Jumo 213 instead of the DB 603, and the Jumo already used the annular radiator in existing designs (Ju 88 etc). Why change that, esp. when there is nowhere else to put that radiator anyway? The reason for the choice of an annular radiator is rather obvious... besides if I would want to go down on the road of good old Holtzeuge-style rhetorics, I would ask, why was the Me 209 II and Me 309 eventually loose out against the old Me 109 design...? (for a myriad of other reasons than radiator arrangement, of course).

Click to expand...

Concerning radiator and landing gear placement: Which came first? The hen or the egg?

BTW, the simple reason the Me209 and 309 did not make it into production is obvious when you look at them. They are simply to ugly. No teutonic knight worth his salt would ever mount one. Infantry service in a penal battalion on the eastern front would be preferable. No wonder the RLM opted for the Ta152.....

Timppa, I think to credit the good performance of the P-51 to a single factor is a bit simplistic.

I agree with Drgondog: I think there is not a single factor but that the cooling system, wing profile, workmanship on joints and rivets and general clean lines of the P-51 all contributed. As I said before, even if the FULL potentail of the laminar flow was not realised in service, even a slight advantage in profile drag adds up with the other factors to produce a good design.

AFAIK there was only one concession made concerning drag and that is the bubble canopy which reduced the top speed of the P-51D (around 5 mph or thereabout?) and later models. However, this was a very good tradeoff IMHO.

Hi Timppa,

>I have seen another Atwood's article where he mentions that the other aircraft that may have utilized the Meredith effect succesfully was the Mosquito.

Hm, didn't the Mosquito normally have a two-position outlet flap like the Spitfire? I remember reading about the photo reconnaissance version requiring modification because the abrupt opening of the cowl flaps would slow down the machine during a photographic leg which required constant speed for the proper overlap of the shots.

>So to compare the drag, the best way is to simply compare their Cd.
Allison Mustang vs. Bf109F vs. Spitfire V as contemporaries would make an interesting comparison.

This might give you a measure of the total drag, but not of the radiator efficiency. Cd figures alone do not serve well as metrics for aerodynamic quality either ... add wing area to an aircraft without changing the fuselage, and top speed will drop while the aircraft assumes a lower Cd. If you compare Cd figures only, the slower aircraft will win out ...

Regards,

Henning (HoHun)

Holtzauge said:

Timppa, I think to credit the good performance of the P-51 to a single factor is a bit simplistic.

I agree with Drgondog: I think there is not a single factor but that the cooling system, wing profile, workmanship on joints and rivets and general clean lines of the P-51 all contributed. As I said before, even if the FULL potentail of the laminar flow was not realised in service, even a slight advantage in profile drag adds up with the other factors to produce a good design.

AFAIK there was only one concession made concerning drag and that is the bubble canopy which reduced the top speed of the P-51D (around 5 mph or thereabout?) and later models. However, this was a very good tradeoff IMHO.

Click to expand...

H~ contact me via PM and we can take this off line. Some comments to set the possible conversation up. The commonly referenced top airspeeds for both with 'full internal fuel/oil/coolant plus guns and ammo' is 437mph and 442mph (NAA).

http://www.ww2aircraft.net/forum/flight-test-data/p-51-performance-thread-12670.html#post457519


The Lednicer model actually implied a slight reduction in drag due to delay of separation over the bubble canopy when compared to birdcage. There is some reason to believe the Malcolm Hood may have been nearly as good as the Bubble... but one consistent difference is the angle of the windscreen from the B to a 3 degree lower angle for the D.)

The D was also heavier by nearly six hundred pounds. That should have Slightly lowered top speed and also impacted climb and range performance more proportionate to weight increase, moreso than parasite drag difference - which seems to be better for the D.

From all the flight test results I have seen in the past including those on Williams site, the reduction in airspeed is consistent with the increase (about 5% increase in weight, about 1.5% (5+mph) decrease in top speed - with the same 1650-7 engine.) This just has to be largely attributed to slight increase in the induced drag of the D over the B

Curiously the P-51H is about the same weight as the B but yet again significantly faster. It had three aerodynamic changes of importance - different radiator cowl (longer), different windscreen to spinner top line (steeper by dropping thrust line-giving better pilot visibility also), and longer re-designed canopy.... plus a more powerful engine

It is uncertain how much parasite drag was reduced by the bubble canopy, but VSAERO model implication is that it was reduced.

The Lednicer model I posted earlier for Colin is the only modern aero study I have ever seen for comparisons between Mustang versions (B/D) and Fw 190A/D and Spit IX so I can't say it is definitive, but it is provocative in countering some of my previous assumptions similar to yours.

HoHun said:

>So to compare the drag, the best way is to simply compare their Cd.
Allison Mustang vs. Bf109F vs. Spitfire V as contemporaries would make an interesting comparison.

This might give you a measure of the total drag, but not of the radiator efficiency. Cd figures alone do not serve well as metrics for aerodynamic quality either ... add wing area to an aircraft without changing the fuselage, and top speed will drop while the aircraft assumes a lower Cd. If you compare Cd figures only, the slower aircraft will win out ...

Not a good analogy to use to refute Timppa's point of suggesting Cd between the ships in question. True however, that no conclusions can be made regarding radiator design as Cd in his analogy is theroetically total Cd.

Not enough facts in evidence to make the last comment 'true' relative to 'quality' - If what I wanted to improve as my priority was range and climb and turn manueveribility at the expense of a very small reduction to top speed, then increasing the Mustang Wing span by two feet probably would have been advantageous at every flight profile except dash speed at optimum altitude! Might be a real 'win' at greater cost in labor $$ and use of raw materials

Additionally, If Thrust is equal between the airframes contrasted, the lower Cd will 'win' with respect to top speed.

Cd remains a very important metric for aerodynamic efficiency, but as you say- not the only one.

What is your definition of 'quality'. If it is 'efficiency' and 'efficiency' is translated to max speed to THp ratios, range to fuel consumption ratios for same engine/prop combo - then Cd is The crucial metric

Simply speaking, if Cd in Timppa's definition is meant to reference all components of drag, the two primary components are Induced Drag (Cdi) and Parasite Drag (CDp)- with the latter having many contributors, one of which for radiator contribution is the overall profile/flate plate drag equivalency.

One is surface friction, one is form drag, one is drag due to wheel wells or control surface gap drag, one is trim drag, others include contributions due to radio masts, turrets, open spaces on fuselage, etc.

Obtaining the Sum [{CD}for components a-z] is derived same as CL - namely dividing each component by Q*Reference Area for that component.

Reference area for CL is the Wing Area, for CDi is the Wing Area, for Cdp Friction is the wetted area of the airframe/wing/tail, Cdp for radiator is frontal surface/flat plate drag equivalency, etc., etc.

HH ~ for your analogy above, the Reference area increase of interest when you extend the wing span, keeping all else equal, is the surface wetted area of the wing. You add a little weight and little more friction surface along with a little wing area and an increase of Aspect ratio.

So, what increases slightly is total Parasite (friction component only) Drag on the wing surface. However, for the same ship and speed and new wing span, the denominator - q*RefArea - also increases.

For the extreme limit, such as zero friction airfoil surface, the increase in wing area/span will actually Reduce the CDp for the friction drag component of the wing because the surface/wetted area in the denominator Increased, does it not? However, taken to other extreme is infinite increase in span/weight which takes the result in the other direction

Taken to another extreme example, if you kept the same wing area but doubled the chord (same airfoil geometry and section characteristics) to cut span (and AR) in half, you theoretically keep Cdp for wing surface friction drag the same, keep the wetted reference area the same, but possibly double the Cdi depending on tip and taper geometry.



Regards,

Henning (HoHun)

Click to expand...

So, total Cd comparisons between the ships in question are very important, recognizing that nobody can change the characterisics of the airframe geometry... and no, nothing can be extracted from total Cd to lead to conclusions regarding radiator geometry or so called 'meredith effect'.

And last, total wind tunnel derived Cdp should be compared at same Reynolds number to have basis of comparitive relevance, or reduced to flat plate equivalence.

Kurfürst said:

Do you have any figures for the respective radiator drags or its a guess/speculation?



Take for example the Me 209 and Me 309. Both designs opted for wide undercarriage, and a simple glace on their drawings reveal that there is simply no place in the wings to place the radiators there.

Me 309:

Taking a look at the Me 209 II,



again we see the wide undercarriage, and no space for installing of radiators or ducting in the wings. Given that the proto was merely a modified, stock Bf 109G-5, there would be no space in/under the fuselage either (the fuel tank is there, and taking the radiator even further is probably a not a good idea CoG-wise).

In addition the team was forced to use the Jumo 213 instead of the DB 603, and the Jumo already used the annular radiator in existing designs (Ju 88 etc). Why change that, esp. when there is nowhere else to put that radiator anyway? The reason for the choice of an annular radiator is rather obvious... besides if I would want to go down on the road of good old Holtzeuge-style rhetorics, I would ask, why was the Me 209 II and Me 309 eventually loose out against the old Me 109 design...? (for a myriad of other reasons than radiator arrangement, of course).

Click to expand...

That's a better arrangement overall, especially the 309 I always like the tricycle landing gear. The 109 was a poor ground handling plane, fixing the close-spaced langing gear and, in my opinion, having a lower drag cooling system would have made a lot of small differences that would add up.

I am not a mechanical engineer, nor an aerodynamicist. Just FYI so I don't get bashed.

I would think putting a radiator on the wing would never be as good as on the fuselage. Let the wing do its thing with developing lift. Hypothetically, lets say the Bf 109 and P-51's two different systems gave the exact same amount of drag and cooling efficiency. Taking the radiator from the wing would either allow the interior volume of the wing to be used for something else, like fuel or ammo. And, because less of the wing is "disturbed", you would either gain some lift or could make the wing smaller in square footage and equal the bigger wing with disturbed airflow.

MikeGazdik said:

I am not a mechanical engineer, nor an aerodynamicist. Just FYI so I don't get bashed.

I would think putting a radiator on the wing would never be as good as on the fuselage. Let the wing do its thing with developing lift. Hypothetically, lets say the Bf 109 and P-51's two different systems gave the exact same amount of drag and cooling efficiency. Taking the radiator from the wing would either allow the interior volume of the wing to be used for something else, like fuel or ammo. And, because less of the wing is "disturbed", you would either gain some lift or could make the wing smaller in square footage and equal the bigger wing with disturbed airflow.

Click to expand...

Yes - and No. First the radiator 'cowl' for the 109 and Spit were not placed in the first 30% of the chord. Separation was already occurring in most cases aft of the 30-35% Chord point. Having the radiator cowl there may actually help by introducing a local area where the airflow could 're-attach' helping reduce profile drag somewhat.

Hard to make general judgments without looking at specifics. I have zero clue what the aero study of either the Spit or 109 analyzed or concluded so I am speculating also>

There is no real use for storage in the area behind the aft spar in most two spar wing design - a.) way behind cg, b.) any added storage there potentially introduces additional spar mass to take out the load.

That area is usually reserved for control linkages etc.

Ok, makes sense. I didn't realize these wing radiators were behind the 2nd spar. How would a leading edge duct affect the air over the wing? Such as the radiator for the P-39 or oil coolers on the F4U ? I would suspect the F4u design would cause some real disturbance. The P-39, if it is ducted well out the other side, may have a less affect.

MikeGazdik said:

Ok, makes sense. I didn't realize these wing radiators were behind the 2nd spar. How would a leading edge duct affect the air over the wing? Such as the radiator for the P-39 or oil coolers on the F4U ? I would suspect the F4u design would cause some real disturbance. The P-39, if it is ducted well out the other side, may have a less affect.

Click to expand...

Sorry to say "It Depends". Classic design sez- "don't disrupt flow over the leading edge of the wing' until forced to do so (like multi engine design requirements).

Any 'duct' on leading edge of the wing is in essence a large drag item until clever design reduces its effect. Both of the above examples were placed in the root area and may have been determined from wind tunnel tests as 'low lift' contribution locations for the wing and therefore least disruptive.

I haven't seen any design literature which discusses the approach and why that type of duct was selected over an underwing (Spit/109) approach

Going back to the Meredith Effect. If the Bf 109 and the P-51 both employed this technology, maybe the the P-51 was able to capture its effects better. The thin wing of the Messerschmitt (or any wing application) would allow much less volume of air as compared to the fuselage adaptation of that principle.

MikeGazdik said:

Going back to the Meredith Effect. If the Bf 109 and the P-51 both employed this technology, maybe the the P-51 was able to capture its effects better. The thin wing of the Messerschmitt (or any wing application) would allow much less volume of air as compared to the fuselage adaptation of that principle.

Click to expand...

Yes. Simply stated, the deeper 51 radiator forced a 'large volume of air' into it for cooling which by definition enabled the resulting heat transfer to the air flowing through the radiator to absorb energy.

The adjustment of the exhaust door was more akin to a jet or rocket nozzle in moving the heated air from the radiator at medium speed to higher velocity exhaust via the venturi effect of large area/Radiator closing to small area/Door.

I haven't seen any analytical heat and mass flow studies presented for either the 109 or the 51 to substantiate a significant 'Meridith Effect' thrust. All it means to me is that it is plausible - but I suspect Net Drag reduction is more plausible than achieving Net Thrust > Drag of lower Radiator system in it's entirety.

drgondog said:

...I suspect Net Drag reduction is more plausible than achieving Net Thrust > Drag of lower Radiator system in it's entirety.

Click to expand...

I agree 100% with all your points dr
I agree with your final point too, which I left above, the P-51 air pump was so effective that only around 40 horsepower was lost to cooling drag; it didn't overcome it but it reduced its effect to something like 3% of the total.

Shooter8 said:

As the speed increases, the inlet inertia is higher and the effect is more positive

Click to expand...

Air pump pressure increases as the square of the speed (so if you double your speed, you quadruple the pump pressure).

I'm not sure if laminar-flow contributed, either greatly or in small part, to the overall effectiveness of the P-51 design. The following points are excerpts from Pursue and Destroy by Leonard 'Kit' Carson and were given by Carson explaining why in real life laminar flow simply did not occur on the P-51's wing.

1. The effects of propeller slipstream: Airflow within the arc of the
prop is very turbulent, "the whole fuselage and inboard section of the
wing next to the fuselage operate in that turbulent stream. Tests in
the Langley wind tunnel revealed that airflow within the arc of the
prop (the prop was 11 feet in diameter which meant that turbulent air
was encountered all the way out to within 13 inches of the inner gun
position) was "90 to 95 percent turbulent" (in other words non laminar)

2. Vibration: Engine and propeller vibrations transmitted through the
structure will induce transition to turbulence. Tests indicated that
laminar flow on twin engine aircraft was greater with one engine
feathered than with both running. Engineers surmised that the lack of
engine/prop vibration on the dead engine side promoted laminar flow.
Honest, that's what the book said. Of course with both props turning,
more of the wing would be bathed in the prop slipstream which as has
been mentioned above, trips laminar flow to turbulent.

3. Airfoil surface condition: Mud, dirt, ice and frost will induce the
transition to turbulent conditions. Fuel truck hoses, ammo belts,
tools, guns and large feet in GI. shoes found the way to the tops of
wings the scrapes and dents this servicing caused had negative effects
on laminar flow.

4. Manufacturing tolerances: The Mustang was the smoothest airplane
around in 1940, but there is a practical limit in construction. We're
talking about surface roughness or waviness of .01 inches which will
cause transition to turbulence. (remember the afore mentioned dust
and scotch tape which was observed to trip airflow to turbulent). Some
aerodynamicists have stated that true laminar flow did not occur
outside the wind tunnel until the advent of Burt Rutan's Vary E-Z in
the early 70s with it's incredibly smooth fiberglass over carved foam
wing and aft mounted engine which of course kept the wing ahead of the
prop slipstream.

5. Wing Surface Distortion in Flight: Flight brings flight loads which
can and did distort the wing and cause ripples in the wing surface
which were fully capable of tripping the laminar flow to turbulent.

So if it wasn't the laminar flow wing that gave it it's high speed and
extensive range, what was it?

The most prominent speed secret was the dramatic reduction of cooling
drag. Placing the airscoop on the belly just in front of the rear edge
of the wing removed it as far as was practicable from the turbulence of
the prop and placed it in a high pressure zone which augmented air
inflow. Tests in the wind tunnel with the initial flush mounted scoop
were disappointing. There was so much turbulence that cooling was
inadequate and some doubted that the belly scoop would work. The
breakthrough was to space the scoop away from the surface of the belly
out of the turbulent boundary layer of the fuselage. Further testing
showed that spacing it further out would increase cooling but at a cost
to overall drag. Various wind tunnel tests established the spacing at
the current distance which represents the best compromise between
spacing out from the turbulent flow of the fuselage, drag and airflow.

With the flow into the scoop now smooth and relatively non-turbulent,
the duct leading to the radiator/oil cooler/intercooler was carefully
shaped to slow the air down (the duct shape moves from narrow to wide,
in other words a plenum chamber) enough from the high external speeds
to speeds through the heat exchangers that allowed the flow to extract
maximum heat from the coolant. As the air passed through the radiators
and became heated, it expanded. The duct shape aft of the radiator
forced this heated and expanded air into a narrow passage which gave it
considerable thrust as it exited the exhaust port. The exhaust port
incorporated a movable hinged door that opened automatically depending
on engine temperature to augment the airflow.
The thrust realised from this 'jet' of heated air was first postulated by a
British aerodynamicist in 1935.
The realization of thrust from suitably shaped air coolant passages is named
after him and called the Meredith Effect. Some have said that at
certain altitudes and at a particular power setting the Meredith Effect was
strong enough to actually overcome all cooling drag; this is not regarded as
being accurate by most aerodynamicists. It greatly contributed to overall
efficiency of the cooling system but never equaled or overcame cooling drag.

Could you expand a bit more upon this 'conic' design principle that you mentioned?

Shooter8 said:

At high speeds, the divergent inlet duct, radiator, convergent exhaust duct with variable exit area is certainly capable of generating positive thrust. As the speed increases, the inlet inertia is higher and the effect is more positive.

That IS the theory. My personal challenge in contemplation of the Meridith effect is how to assess the attendant drag caused by bringing the inlet airflow to near stagnation at the front surface of the radiator - and then estimate the emerging stream tube velocities Aft of the Radiator as it emerges with higher heat energy but lower velocity, then accelerates to convergent exhaust duct.

I have never seen any documentation of analysis where even a rudimentary sensor array was used to record both the inlet and the exit flow velocities and temperatures, nor a heat/Mass transport energy diagram.

If you have been following the dialogue you might see that nobody is disagreeing the physics behind the Meridith effect. The difficulty is that nobody seems to be able to point to an analysis that is both quantifiable and verifiable wrt result!

Like a ramjet, inlet air is decelerated by the divergent duct, heated by the radiator and then accelerated by the convergent duct. While such claims are easy to make, they are much harder to disprove. The main problem is that there are three possible answers as to why the P-51 was much faster than the lighter and smaller Spitfire with the same engine installed and no one can decide which is responsible for how much of that difference.

1. The "Laminar Flow" wing.
2. The fancy radiator and duct.
3. Profile form derived from the "Conic" design principle used.

Click to expand...

It is easy to postulate All Three... But, as you say - hard to prove.

Schmeud and Atwood stated 1st (Laminar Flow wing), 2nd (redesigned 51B/D radiator form and boundary layer control over inlet duct, 3rd (Meridith effect).

There is one other possible contribution, namely the windscreen/canopy design. Lednicer presented interesting potential flow model that showed the 51D design to be superior to Mk IX and FW190D canopy and slightly better than a P-51B.

The 51D design seemed to avoid flow stagnation at base of windscreen and therfore improved attached flow properties over top and rear surfaces of the canopy.

But the airframe is a wing/body and analysis on separate contributions are difficult (impossible). The Lednicer report I published was the soundest modelling approach I have seen so far.

Colin1 said:

The following points are excerpts from Pursue and Destroy by Leonard 'Kit' Carson and were given by Carson explaining why in real life laminar flow simply did not occur on the P-51's wing.

1. The effects of propeller slipstream: Airflow within the arc of the
prop is very turbulent, "the whole fuselage and inboard section of the
wing next to the fuselage operate in that turbulent stream. Tests in
the Langley wind tunnel revealed that airflow within the arc of the
prop (the prop was 11 feet in diameter which meant that turbulent air
was encountered all the way out to within 13 inches of the inner gun
position) was "90 to 95 percent turbulent" (in other words non laminar)

There is room to speculate that this statement may not be strictly 'true'. In fact, whether helicoptor rotor or conventional propeller system, the stream tube behind (downstream) a prop/Rotor is one that shrinks in diameter until some equilibrium point downstream where the tube remains somewhat constant. Think of this phenomena as the tip vortex effect of the prop tip - same basic function as induced drag on a wing. You will also get velocity vectors in the stream tube that are normal to the freestream. In a trailing wake behind a 'heavy' the stream tube behind the a/c is smaller in diameter than the wing span with huge downwash along centerline of a/c within the 'tube'

2. Vibration: Engine and propeller vibrations transmitted through the
structure will induce transition to turbulence. Tests indicated that
laminar flow on twin engine aircraft was greater with one engine
feathered than with both running. Engineers surmised that the lack of
engine/prop vibration on the dead engine side promoted laminar flow.
Honest, that's what the book said. Of course with both props turning,
more of the wing would be bathed in the prop slipstream which as has
been mentioned above, trips laminar flow to turbulent.

I would buy into this if the vibration frequency was close enough to natural frequency of the wing to induce amplification response in the wing itself. Would not suspect this to be true in P-51.

3. Airfoil surface condition: Mud, dirt, ice and frost will induce the
transition to turbulent conditions. Fuel truck hoses, ammo belts,
tools, guns and large feet in GI. shoes found the way to the tops of
wings the scrapes and dents this servicing caused had negative effects
on laminar flow.

All true as contributory to premature local BL separation. Practically speaking surface roughness is not a significant factor unless and until something else is introducing a positive pressure gradient downstream/chordwise direction. All wings near a stall are in the realm of Chaos theory with respect to boundary layer separation initiation and propagation.

4. Manufacturing tolerances: The Mustang was the smoothest airplane
around in 1940, but there is a practical limit in construction. We're
talking about surface roughness or waviness of .01 inches which will
cause transition to turbulence. (remember the afore mentioned dust
and scotch tape which was observed to trip airflow to turbulent). Some
aerodynamicists have stated that true laminar flow did not occur
outside the wind tunnel until the advent of Burt Rutan's Vary E-Z in
the early 70s with it's incredibly smooth fiberglass over carved foam
wing and aft mounted engine which of course kept the wing ahead of the
prop slipstream.

I have heard and read this also but have to caveat in this way. The first P-51 NACA wing (modified by NAA) was a laminar flow wing with max t at 40% chord and 2-D flow analysis predicted extremely low Cdo due to that approximate loss of laminar flow point.

Reality and different wind tunnel tests showed variations of between 33 and 37 % for a highly polished (3-D) wing. I have heard but not seen results, of discussions of separation over normal rough wing at locations from 25% to 30%. I don't know what a combat ready 51 should do but full scale wind tunnel tests showed lower Cd0 for this wing than comparable wings that were considered in case the NACA Laminar Flow airfoil failed its promise

5. Wing Surface Distortion in Flight: Flight brings flight loads which
can and did distort the wing and cause ripples in the wing surface
which were fully capable of tripping the laminar flow to turbulent.

Once again - true - but also local unless right on the ragged edge of a stall where the BL was already in near sepration positive pressure state

So if it wasn't the laminar flow wing that gave it it's high speed and
extensive range, what was it?

It was part of the equation. How much is debatable.

Could you expand a bit more upon this 'conic' design principle that you mentioned?

Click to expand...

The question was to Shooter8 but - The lines on the Mustang were the first mathmatical 2nd degree curve design that I am aware of. Ed Horkey was the initiator and developer of the second degree approach and all foils and fuselage were blended into one smooth shape that had desired minimum drag as boundary conditions

FYI - the original calculations for Meridith effect thrust was 300 pounds..

Have we overlooked the design of the prop? In comparing books I have on each, the Mustang prop looks to be different than the Mk IX Spit prop. So maybe the Mustang got a little more speed from the: Laminar wing, belly radiator, canopy, "conic" design, and maybe the propeller too.

If each contributed a few miles per hour airspeed, together they mean alot.