Would it have been cheaper to....

NACA studies said the paint for the national insignia disrupted the laminar flow.

Going back to the P-40, lets not forget that the original XP-40 tried several different locations, starting with under the fuselage behind the cockpit and several versions of the under nose arrangement were tried before production started (planed picked up 40-50mph along the way) and several different radiator locations were tried on later models (not including the Q). It might not be as bad as we think.

Carb intakes get tricky, how much external drag? How much internal drag? How much RAM effect can they provide vs how much drag?

Notice now the carb intake changed positions as well as the inlet having various diameters and shapes before they finally settled with what was most efficient?

You can see the various changes made on the XP's cowlings...this was the case even with the P-51's development.

Hate to jump in here but OldSkeptic has a point. NASA came to the same conclusion about laminar flow as he did on several occasions. They even tried sucking in the boundary layer with the X-21. When it worked, like maybe once, they achieved something like 95% laminar flow. The rest of the time it didn't work.

The theory said if 80% of the wing had laminar flow then a 25% drag reduction could be achieved.

This was a real attempt at laminar flow and only worked a couple of times, so they really didn't GET the drag reduction they spent a huge sum of money to achieve. If they didn't, then the P-51 surely didn't since there was no boundary layer sucking to help it along.

In fact I've never heard anyone, including aerodynamics professors at Purdue University, say that laminar flow drag reduction approached the theoretical limits in the field. That didn't stop them from trying and the results DID lower drag a bit, but never to anywhwere near the theoretical limits, except in wind tunnels where there is no dust, no insects, no floating debris, etc.

Still, lower drag IS lower drag.

But the P-51 CANNOT promote its performance solely to a laminar flow wing. All you needs do is put one in a wind tunnel with typical foreign debris on it, like dirt, mud, incests, dents, scratches, peeling paint, etc. It will be good, but not much better than any other decent fighter wing. The Spitfire had a very high critical mach number, higher than the P-51, and didn't have a laminar flow wing. The Spiteful did, but not the Spitfire.

Laminar flow has been WAY overblown as to it's performance enhancement potential in aircraft that live out in the weather in the field.

GregP said:

Hate to jump in here but OldSkeptic has a point. NASA came to the asme conclusion about laminar flow as he did on several occasions. They even tried sucking in the boundary layer with the X-21. When it worked, like maybe once, they achieved something like 95% laminar flow. The rest of the time it didn't work.

Let's revisit what was said
"Not really. There is a mythology about the Mustang's low drag being because of its laminar flow wing, which is simply not true.
The slightest manufacturing defect, dents from use, or dirt or even just a insect splat on the leading edge destroys the laminar flow

Followed by
The Mustang's low drag came from it's advanced radiator design, which offset 90% of radiator drag, plus careful attention to the design and manufacturing of the all the leading edge surfaces.

The point I made about the 66 series airfoil that was second gen P-51H in contrast to the NACA 23015 is that despite not achieving predicted laminar flow goals for NAA, both the NACA/NAA 45-100 and the 66-(1.8) 015.5 was still far superior, in practice, to the alternative airfoil NACA 23015 positioned to replace the 45-100. It never did for any of the P-51A/B/C/D/K versions despite having higher CL and more benevolent high angle of attack characteristics.. and the reason for that is that the 45-100 showed the drag reduction in wind tunnel and metered dive tests without prop which validated the wind tunnel results.

Simple answer - a Great deal of the drag reduction was due to the 'laminar Flow Shape' of the 45-100, not because of achieving Laminar Flow. The thrust achieved by the Meridith Flow effect of the radiator design accounted for an increment approximately +30% of the exhaust thrust and was significant but not as significant as the wing.



The theory said if 80% of the wing had laminar flow then a 25% drag reduction could be achieved.

This was a real attempt at liminar flow and only worked a couple of times, so they really didn't GET the drag reduction they spent a huge sum of money to achieve. If they didn't, then the P-51 surely didn't since there was no boundary layer sucking to help it along.

True that 'true laminar flow' was not achieved. True that the section drag of the 45-100 was 15% lower than the corresponding 23015, True that NAA never deviated toward picking the conventional 23015 type airfoil with max T/C at ~ 25% and Higher CLmax because they were satisfied with the 45-100 and even more pleased with the 66-(1.8) 15.5 that they put on the P-51F/G/H/J.

If the phrase Low Drag replaced Laminar Flow would folks understand that is what it was?

In fact I've never heard anyone, including aerodynamics professors at Purdue University, say that laminar flow drag reduction approached the theoretical limits in the field. That didn't stop them from trying and the results DID lower drag a bit, but never to anywhwere near the theoretical limits, except in wind tunnels where there is no dust, no insects, no floating debris, etc.

I haven't either. It is also noteworthy that NAA covered the P-51 wings used in the dive tests to simulate the field conditions expected for the normal operational conditions as they plowed through full scale/no prop dive tests to .75 Mach.

Still, lower drag IS lower drag.

I believe this concurs with the statement I made above

But the P-51 CANNOT promote its performance solely to a laminar flow wing. All you needs do is put one in a wind tunnel with typical foreign debris on it, like dirt, mud, incests, dents, scratches, peeling paint, etc. It will be good, but not much better than any other decent fighter wing. The Spitfire had a very high critical mach number, higher than the P-51, and didn't have a laminar flow wing. The Spiteful did, but not the Spitfire.

Umm, no but the Spit with the same engine and the same boost at the same altitude was 30+ mph slower - with the thinner wing and associated field conditions of the test facilities at RAE for all models - in level flight.

Laminar flow has been WAY overblown as to it's performance enhancement potential in aircraft that live out in the weather in the field.

Click to expand...

Low Drag is hardly overstated however in contrast to the 23015 and say Clark Y of same and thinner wings..

Aozora said:

Can anyone point to a report on the effects of removing the O/D - Neutral Gray camouflage from the P-51? I would assume that removing any extra layers of paint from laminar flow wings would have been of some benefit.

Conversely, it would be interesting to know whether the addition of the D-Day stripes, field applied camouflage, or other identification markings to the P-51's wings also affected performance.

Click to expand...

I think the overall and overarching point is two fold. First, Laminar Flow in itself it not precisely defined as "delayed boundary layer separation from 'X' to 'Y' point on the airfoil at 'Z' Reynolds Number". What excited engineers in the late 30's was a demonstrated delay in separation for airfoils with T/Cmax > .25 to .28 Chord. What was at that time Laminar Flow, was more properly described as Low Drag airfoils later. One common characteristic of the low drag airfoils was a T/C closer to .35 to .5 which also has a less pronounced curvature of the airfoil from the LE to the Mx T/C. What this means is that the adverse pressure gradient tends to form further downstream - with attendant separation - than a conventional airfoil.

The Low Drag airfoils also typically have a lower CLmax and higher Profile Drag at High Angle of attacks.

Friction Drag increases in turbulent flow. If a 'laminar flow' separates earlier due to the higher velocity gradient over the more pronounced t=F(chord) to the a/c of ~ 25% C it tends to separate and STAY separated sooner, with an attendant greater friction drag - all things being equal such as paint grain size, gaps, rivet heads, gaps in flush rivet holes, depressions in flush rivet holes, etc.

Many Flight test reports will discuss surface prep. Few discuss comparing pre-improve vs post improve steps with respect to speed runs. Candidly 3mph on 400 is a nice result. 10 mph means that they scarped of a lot of mud, sanded the wing smooth and polished it to get that type of boost.

If you have Abbott and Doenhoff, there is a nice discussion on page 120 of the evolution to the 6 series as designers tried (and succeeded) getting better high critical mach numbers and improving the earlier attempts at laminar flow wings to improve CL at higher AoA.

Laminar flow did work on the P-51: laminarity did persist slightly further up the chord, however the bulk of drag reduction came from the higher critical Mach (low compressibility) of the wing. This property is a favourable outcome of the gradual pressure gradient across the surface of the wing that it was deliberately designed for. Remember the P-51 was flying at close to Mach 0.66

Most of the wing profiles used in WW2 were designed by NACA engineer Eastman Jacobs. He developed the NACA 4 5 digit series by systematic variations in a variable density wind tunnel to achieve good lift, drag and excellent pitch characteristics.

In 1935 he spoke to B Melville Jones (Cambridge's first aeronautics chair) and Geoffrey Taylor (Britain leading fluid dynamists) at Mussolini's 5th Volta conference and found out that a decreasing pressure gradient along the flow direction could maintain linearity. (tested on a Hawker Hart).

Back in the USA at the NACA Jacobs had a professional adversary; Theodore Theodosen, (the men apparently didn't get on) who being of Norweigen Extration had a European engineering education heavy in mathematics. They disagreed about how wings should be developed. Theodresen favouring a mathematical approach.

Theodresen had developed a way of mathematically estimating the pressure gradient around an arbitrary wing shape. A phenomenal achievement.

Jacobs wondered if he could reverse the process and specify the pressure (to get the gradient Melvil-Jones and Taylor recommended) and develop the shape out of it. Theodorsen(the theoretician) actually nonsensed Jacobs's(the experimentalist's) idea. Thus challenged Jacobs aggressively studied for a few days at home and succeded.

From that then P-51 wing was born. It looks like a fish when viewed from above with a distinct fish tail for pressure recovery. There were Laminar flow wings in development everywhere eg Germany (Me 309) and Japan (several aircraft) but Jacobs's seemed to have better pitch and stall characteristics. Moreover unlike the Davis wing it had a high critical mach number.

The other thing about the wing is that it was thick thus allowing strong spars and room for fuel.

These wings were standardised as the 6 digit series and broke the sound barrier with straight wings on the Bell X1.


True Laminarity can be maintained by fibre glass or other composites which have the required smoothness. The seem to be used on some very pricey German sail planes which have wipers on rails to clean of bugs from the leading edge in flight. Other ways include the secretion of liquid/detergents or slats that fold out from underneath the wing to protect the leading edge from bug strike.

Indirectly you can thank Mussolini for the Mustang.

Excellent summary Koopernic.

The Theodorsen Transformation that I was first exposed to in Engineering school was to map the pressure distribution around a rotating cyclinder in 2-D using Complex Variable math. The methodology was further developed to specify the pressure distribution and drive a mathematically 'suitable shape... which was good first step. I suspect students will still touch on this subject.

With the advent of really powerful computational methods, the use of Calculus of Variations was used to specify the pressure distribution required and develop the wing/airfoil combination in 3-D, then combine with powerful Navier Stokes models to further develop all the attributes of the 3 dimensional shapes of the wing body pressure distributions...

I was out of the airframe business before the latter methodology was practical.

drgondog said:

Excellent summary Koopernic.

The Theodorsen Transformation that I was first exposed to in Engineering school was to map the pressure distribution around a rotating cyclinder in 2-D using Complex Variable math. The methodology was further developed to specify the pressure distribution and drive a mathematically 'suitable shape... which was good first step. I suspect students will still touch on this subject.

With the advent of really powerful computational methods, the use of Calculus of Variations was used to specify the pressure distribution required and develop the wing/airfoil combination in 3-D, then combine with powerful Navier Stokes models to further develop all the attributes of the 3 dimensional shapes of the wing body pressure distributions...

I was out of the airframe business before the latter methodology was practical.

Click to expand...

I checked out Theodorsens work at the NACA report server. The man was amazingly active, likewise with Jacobs. In electrical engineering, my background, these methods were used to estimate charge distribution on electrical surfaces (high voltage insulators, capacitors etc).


Jacobs was aware as early as 1939 that the so called "Laminar Flow Wing" achieve a speed increase through lower shock drag and that laminar flow had little to do with it. Laminar flow was the marketing buzz word it seems.


NASA Technical Reports Server (NTRS) - Preliminary Investigation of Certain Laminar-Flow Airfoils for Application at High Speeds and Reynolds Numbers


Author and Affiliation:
Jacobs, E.N. (National Advisory Committee for Aeronautics, Washington, DC, United States);

Abbott, Ira H. (National Advisory Committee for Aeronautics, Washington, DC, United States);

von Doenhoff, A.E. (National Advisory Committee for Aeronautics, Washington, DC, United States)

Abstract: In order to extend the useful range of Reynolds numbers of airfoils designed to take advantage of the extensive laminar boundary layers possible in an air stream of low turbulence, tests were made of the NACA 2412-34 and 1412-34 sections in the NACA low-turbulence tunnel. Although the possible extent of the laminar boundary layer on these airfoils is not so great as for specially designed laminar-flow airfoils, it is greater than that for conventional airfoils, and is sufficiently extensive so that at Reynolds numbers above 11,000,000 the laminar region is expected to be limited by the permissible 'Reynolds number run' and not by laminar separation as is the case with conventional airfoils. Drag measurements by the wake-survey method and pressure-distribution measurements were made at several lift coefficients through a range of Reynolds numbers up to 11,400,000. The drag scale-effect curve for the NACA 1412-34 is extrapolated to a Reynolds number of 30,000,000 on the basis of theoretical calculations of the skin friction. Comparable skin-friction calculations were made for the NACA 23012. The results indicate that, for certain applications at moderate values of the Reynolds number, the NACA 1412-34 and 2412-34 airfoils offer some advantages over such conventional airfoils as the NACA 23012. The possibility of maintaining a more extensive laminar boundary layer on these airfoils should result in a small drag reduction, and the absence of pressure peaks allows higher speeds to be reached before the compressibility burble is encountered. At lower Reynold numbers, below about 10,000,000, these airfoils have higher drags than airfoils designed to operate with very extensive laminar boundary layers.
Publication Date: Aug 01, 1939
Document ID:
20090015023
(Acquired Apr 17, 2009)

Note this abstract (there is portably one on the wing itself) isn't about a specialised laminar flow wing, only about a modified NACA 4 digit wing that is semi laminar, but it shows that the deliberate gentle pressure gradients engineered into the laminar flow style wing did reduce compressibility drag since there were no 'pressure peaks' and that the engineers were aware of that factor.

Obviously a range of factors helped the P-51 achieve its exceptional speed with a relatively large airframe: tolerances and the way the 'radiator' was efficiently placed. The Spitfire and Me 109 had one oil/air cooler and two radiators disturbing a large area under the wing.

This is Jacobs Preliminary note on laminar air foils of August 1939:
NASA Technical Reports Server (NTRS) - Preliminary Investigation of Certain Laminar-Flow Airfoils for Application at High Speeds and Reynolds Numbers

Comparison of the calculated drags for the N.A.C.A. 23012 and 1412-34 airfoils indicates that the drag of the 1412-34 should be about 5 percent less than that of the 23012 in the Reynolds Number range from 20,000,000 to 30,000,000. Although direct extrapolation of variable- density-tunnel drag r e s u l t s indicates t h a t the drag of the N.A.C.A. 23012 mag be slightly lover than that of the N.A.C.A. 1412-34 in this range of Reynolds Numbers, it is felt that the skin-friction calculations give a more reliable estimate of the relative drag of the two air foils.

At any rate, it appears that the drag difference bbetween the N.A.C.A. 1412-34 and 23012 airfoils will be small at Reynolds Numbers above about 20,000,000. If the drag of the aerofoil section is the primary consideration, the N.A. C .A, 1412-34 should probably be selected since this airfoil does allow a possible drag reduction from the existence of a more extensive laminar boundary layer. More- over, there is always the possibility of more extensive laminar boundary layers being obtained in flight than in the present tests. For high-speed applications, the N.A. C .A. 1412-34 and 2412-34 air foils have the additional advantage of higher compressibility-burble speeds than conventional airfoils because of the absence of pressure peaks. For instance, the theoretical values of Mc (the ratio of the critical speed to tho speed of sound, reference 10) for the N.A.C,A, 1412-34 and 24J.2-34 airfoils aro 0.74 and 0.70,
respectively, at the ideal lift coefficients as compared with 0.61 for tho N.A.C.A. 23012 air foil.

The maximum lift coefficients for the N.A.C.A. 1412-34 and 2412-34 air foils = 1.12 and 1.22, respectively CL max are much lower than for air foils such as the N.A.C.A. 23012 (clmax = 1.74). In cases where the maximum lift I coefficient is important, the reduced maximum lift coefficients for these sections will severely limit their application. On the other hand, the advantage of the N.A.C.A. 23012 air foil in this respect is not as great as would appear, ear because the 1if% curve for this air foil breaks sharply from its maximum to a value of about 1.32. The extent to which values of the lift coefficient for this air foil higher than 1.32 can be used with safety is doubtful.

nuuumannn said:

The Mustang was designed for the British, not the Americans for starters. How could the Americans ignore the Mustang even with a P-40 derivative? Even if Curtiss could have gotten the P-60 into production, it's pretty likely the P-51 as it became with the USAAF i.e. a long range escort fighter, would have come about at any rate. It was a part of NAA's requirement for building the Mustang for the Brits that airframes were made available for evaluation by the USAAC.

Click to expand...

The arrival of the Mustang timed perfectly just after an exhaustive set of specs from the Army Air Force Pursuit Board had been received (NAA was not on the fighter 'list') and deliveries made to Wright Pat for testing. As such, and as a Lend Lease fighter, it had zero priority over the other types for flight test. So, the AAF can be given a little bit of a pass on overlooking it in the Fall to Spring of 1942.

Having said this I could argue that the delay was not much in the context of the first flight of a Production P-51B in May 1943.

The second pass that should be given to the AAF is that when it Was tested and great reviews were produced in late Spring 1942, the Mustang was nearing combat ops, Harker at Rolls had really advanced the ball with a proposal to RAE to stuff a Merlin it. The 8th AF was experiencing increased resistance and losses in the ETO and the only two fighters seemingly capable of longer range escort AND high altitude performance were the P-38 and P-47 - but the Mustang 'conversion activities' at RR were unknown to all in the AAF except Major Tommy Hitchcock. So, a lot of data points arrived on Arnold's desk in April/May 1942

Net - the Mustang still wasn't visible, nor capable of high altitude escort in the ETO and remained so until mid 1942 when the design potential performance calculations emerged from RR/RAE to Hitchcock at US Embassy in London - and from there to Arnold. It was in July 1942 that two Mustangs were pulled from the P-51 line and development work on the P-51B started at NAA - both for design and production planning. So the 'slap my head moment' that Arnold said he experienced regarding the slow acceptance of the Mustang's potential wasn't as bad as it seems to some historians... further, based on intel from RAF/RR/RAE from Hitchcock, as well as NAA nearing the end of the Brit order and subsequent drying up of funds (and skilled personnel) for the Mustang, Arnold in very quick order, utilizing unused funds for a Dive Bomber, authorized on his own the contract for the A-36 to keep the lines and development continuous, then released a contract for the P-51A because it WAS better than the P-40 and P-39. The AAF inserted a stipulation that the P-51A remaining blocks could be inserted into the forthcoming P-51B pending results of NAA and RR/RAE.

The A-36 contract and development introduced a HUGE feature that provided the last key feature necessary for exploiting the long range potential of the airframe aerodynamics, namely the combination bomb rack, fuel tank and feed capability to the P-51 wing.

First - at best the P-51 (pre P-51B/Mark X) was a superior fighter to the P-40 and P-39 but it didn't project well at high altitude combat in ETO versus the 109 but perhaps still 'equal' to the 190 as a result of improved performance of the Allison V-1710-89.

Second - given the time from Harker's flight test report on April 29, 1942 and the subsequent proposal to install a series 60 Merlin, the elapsed time for AAF to react and engage with NAA to keep the P-51 line open was almost instantaneous. Historically it is difficult to extract the contact dialogue between Flight Test at Wright Pat, Army Procurement and communications from US Embassy holistically to figure out an exact cause and effect. My belief is that the continued need for a dive bomber, and residual funds available, was the lucky trigger to shorten the cycle by ordering the A-36.

Whatever the reason, the time cycle to produce the P-51B was almost optimal as the ability of RR to produce enough engines for reverse Lend Lease, or ability to tool up in England to produce the airframe to combine with additional manufacturing capacity at Rolls Royce - and beat NAA with delivery of quantity in Spring 1943 is questionable. Additionally Packard took until October to deliver the first 1650-3 which failed on bench test..delaying the first flight of the XP-51B until November 30. But the RR/RAE had already achieved spectacular results in early October.

The argument that the P-51B would have been ready much earlier 'if only the AAF didn't have their heads up their ass' really doesn't hold much water because, primarily, the Packard merlin 1650-3 could not have rolled out any faster than September/October 1942 and both the Mark X had flown, and the XP-51B was ready to fly when it was available.

My only point is that it took an amazing assembly of actions and decisions to transform a "better P-39 and P-40" to the terror that the P-51B became in the ETO and MTO in late 1943, early 1944. The slack time between what was possible, and what actually happened wasn't very great.

Koopernic said:

Jacobs was aware as early as 1939 that the so called "Laminar Flow Wing" achieve a speed increase through lower shock drag and that laminar flow had little to do with it. Laminar flow was the marketing buzz word it seems.

Click to expand...

It seems that present-day armchair researchers can be thrown off by antiquated terminology, or terminology that was created for non-scientific marketing purposes.
For example, a turbocharger or turbo supercharger back in the day was often called a supercharger.
Likewise, the term "laminar flow wing" seems to be taken and used literally by some today causing great confusion.

But it does make great fodder for what-if scenarios

It does seem that even if Packard merlin 1650-3's could not have been available earlier, the Allison powered planes could have/should have begun replacing P-40's.

drgondog said:

The arrival of the Mustang timed perfectly just after an exhaustive set of specs from the Army Air Force Pursuit Board had been received (NAA was not on the fighter 'list') and deliveries made to Wright Pat for testing. As such, and as a Lend Lease fighter, it had zero priority over the other types for flight test. So, the AAF can be given a little bit of a pass on overlooking it in the Fall to Spring of 1942.

Having said this I could argue that the delay was not much in the context of the first flight of a Production P-51B in May 1943.

The second pass that should be given to the AAF is that when it Was tested and great reviews were produced in late Spring 1942, the Mustang was nearing combat ops, Harker at Rolls had really advanced the ball with a proposal to RAE to stuff a Merlin it. The 8th AF was experiencing increased resistance and losses in the ETO and the only two fighters seemingly capable of longer range escort AND high altitude performance were the P-38 and P-47 - but the Mustang 'conversion activities' at RR were unknown to all in the AAF except Major Tommy Hitchcock. So, a lot of data points arrived on Arnold's desk in April/May 1942

Net - the Mustang still wasn't visible, nor capable of high altitude escort in the ETO and remained so until mid 1942 when the design potential performance calculations emerged from RR/RAE to Hitchcock at US Embassy in London - and from there to Arnold. It was in July 1942 that two Mustangs were pulled from the P-51 line and development work on the P-51B started at NAA - both for design and production planning. So the 'slap my head moment' that Arnold said he experienced regarding the slow acceptance of the Mustang's potential wasn't as bad as it seems to some historians... further, based on intel from RAF/RR/RAE from Hitchcock, as well as NAA nearing the end of the Brit order and subsequent drying up of funds (and skilled personnel) for the Mustang, Arnold in very quick order, utilizing unused funds for a Dive Bomber, authorized on his own the contract for the A-36 to keep the lines and development continuous, then released a contract for the P-51A because it WAS better than the P-40 and P-39. The AAF inserted a stipulation that the P-51A remaining blocks could be inserted into the forthcoming P-51B pending results of NAA and RR/RAE.

The A-36 contract and development introduced a HUGE feature that provided the last key feature necessary for exploiting the long range potential of the airframe aerodynamics, namely the combination bomb rack, fuel tank and feed capability to the P-51 wing.

First - at best the P-51 (pre P-51B/Mark X) was a superior fighter to the P-40 and P-39 but it didn't project well at high altitude combat in ETO versus the 109 but perhaps still 'equal' to the 190 as a result of improved performance of the Allison V-1710-89.

Second - given the time from Harker's flight test report on April 29, 1942 and the subsequent proposal to install a series 60 Merlin, the elapsed time for AAF to react and engage with NAA to keep the P-51 line open was almost instantaneous. Historically it is difficult to extract the contact dialogue between Flight Test at Wright Pat, Army Procurement and communications from US Embassy holistically to figure out an exact cause and effect. My belief is that the continued need for a dive bomber, and residual funds available, was the lucky trigger to shorten the cycle by ordering the A-36.

Whatever the reason, the time cycle to produce the P-51B was almost optimal as the ability of RR to produce enough engines for reverse Lend Lease, or ability to tool up in England to produce the airframe to combine with additional manufacturing capacity at Rolls Royce - and beat NAA with delivery of quantity in Spring 1943 is questionable. Additionally Packard took until October to deliver the first 1650-3 which failed on bench test..delaying the first flight of the XP-51B until November 30. But the RR/RAE had already achieved spectacular results in early October.

The argument that the P-51B would have been ready much earlier 'if only the AAF didn't have their heads up their ass' really doesn't hold much water because, primarily, the Packard merlin 1650-3 could not have rolled out any faster than September/October 1942 and both the Mark X had flown, and the XP-51B was ready to fly when it was available.

My only point is that it took an amazing assembly of actions and decisions to transform a "better P-39 and P-40" to the terror that the P-51B became in the ETO and MTO in late 1943, early 1944. The slack time between what was possible, and what actually happened wasn't very great.

Click to expand...

gjs238 said:

But it does make great fodder for what-if scenarios

It does seem that even if Packard merlin 1650-3's could not have been available earlier, the Allison powered planes could have/should have begun replacing P-40's.

Click to expand...

Actually there was a great missed opportunity in not putting Merlin XX series (single stage, two speed) engines into a Mustang, a much easier conversion than a 60 series Merlin and there were plenty of them. That would it made it very competitive with the FW-190 (at least) in the 20,000-25,000ft region though maybe not the 109 at very high altitudes (30,000+ft).

OldSkeptic said:

Actually there was a great missed opportunity in not putting Merlin XX series (single stage, two speed) engines into a Mustang, a much easier conversion than a 60 series Merlin and there were plenty of them. That would it made it very competitive with the FW-190 (at least) in the 20,000-25,000ft region though maybe not the 109 at very high altitudes (30,000+ft).

Click to expand...

A Merlin XX powered P-51 would have made an interesting interim model. Considering that the cruising fuel consumption of the Merlin XX was lower than that of the 60 series, a Merlin XX P-51 could have had greater endurance than the P-51B/C/D

From Hurricane II Pilot's Notes:



Mustang III PNs


View attachment Rolls-Royce Merlin XX 1942.pdf

Remember that the important cruise performance was 300mph +- at 25000 feet where the Merlin 60 series were superb.
Having said that the conversion from Merlin XX to Merlin 60 would have been shorter and the performance of the Mark I and IA would have been even more impressive

Just so you know what the heck we are talking about with laminar flow, here is a demostration:

View: https://www.youtube.com/watch?v=_dbnH-BBSNo

It really makes me wonder .... maybe the P-51 should have been yellow. What it really shows is that the flow is predictable.

All it really means is the dye is in contact with the boundary layer by design (diameters of outside and inside cylinders are inside the boundary layer). The notion that the field P-51's had laminar flow over 80%+ of the surface is ludicrous. Don't take MY word for it, go ask any reputable aeronautical college about it on their fourums and then come back and post. Try Purdue, Embry-Riddle, or take your pick.

Did it have lower drag? Yes, really. As much as predicted? No. Did it help? Affirmative.

There are no wings flying today on plane in large-scale production with laminar flow over more than 60 - 70% of the wing, which doesn't satisfy the drag reduction theory for the laminar flow wing, but it helps. The Boeing 787, a paragon of efficiency, doesn't use a laminar flow wing; it uses a supercritical wing. Big difference.

Hi OldSkeptic

Actually there was a great missed opportunity in not putting Merlin XX series (single stage, two speed) engines into a Mustang, a much easier conversion than a 60 series Merlin and there were plenty of them. That would it made it very competitive with the FW-190 (at least) in the 20,000-25,000ft region though maybe not the 109 at very high altitudes (30,000+ft).


From the very excellent book Rolls Royce and the Mustang by David Birch.

Neil.

Neil Stirling said:

Hi OldSkeptic

Actually there was a great missed opportunity in not putting Merlin XX series (single stage, two speed) engines into a Mustang, a much easier conversion than a 60 series Merlin and there were plenty of them. That would it made it very competitive with the FW-190 (at least) in the 20,000-25,000ft region though maybe not the 109 at very high altitudes (30,000+ft).


From the very excellent book Rolls Royce and the Mustang by David Birch.

Neil.

Click to expand...

That book is a Must Have for the library as it details not only what you presented Neil, per the RR performance estimates, but also the thought processes and decisions made in a chronological order for RR, AAF at Wright Pat, NAA, Tommy Hitchcock, etc.

GregP said:

Just so you know what the heck we are talking about with laminar flow, here is a demostration:

It really makes me wonder .... maybe the P-51 should have been yellow. What it really shows is that the flow is predictable.

Hmm, scratching my head wondering how you are applying low RN viscous flow to RN>3-11x10^^6 air flow?

All it really means is the dye is in contact with the boundary layer by design (diameters of outside and inside cylinders are inside the boundary layer). The notion that the field P-51's had laminar flow over 80%+ of the surface is ludicrous. Don't take MY word for it, go ask any reputable aeronautical college about it on their fourums and then come back and post. Try Purdue, Embry-Riddle, or take your pick.

So, who has postulated 80%, or 50%, or 30% in your experience on this subject? That would be ludicrous

Did it have lower drag? Yes, really. As much as predicted? No. Did it help? Affirmative.

That does seem to be a concensus.

There are no wings flying today on plane in large-scale production with laminar flow over more than 60 - 70% of the wing,

Strictly speaking, just achieving 'attached flow' intermittently until reaching fully detached turbulent flow, to max T/C, would have been the Holy Grail... which for the example of the P-51 is near 40% IIRC. I may have stated 50% in one of the posts above for the 45-100, but that would have been wrong.

which doesn't satisfy the drag reduction theory for the laminar flow wing, but it helps. The Boeing 787, a paragon of efficiency, doesn't use a laminar flow wing; it uses a supercritical wing. Big difference.

Click to expand...

As do most of the high subsonic cruise aircraft.. very high Mcr and gentle transonic characteristics with respect to change of center of Pressure and severe turbulence.

Curiosity compels me to ask what airfoils you have in mind that have even 30% "Laminar Flow", much less the "60-70%" you cited above?

30% laminar flow I'd say might be the Cirrus SR22 and the Cessan Corvalis 350. Perhaps a carafully-built Lancair and Glassair ... simply due to the smoothness of the gel coat on the fiberglass. Several months down the road or a flight through bugs at low altitude ... maybe no. A LOT depends on the finish and how clean the thing is kept.

Planes that are stored in the open in field condition, like WWII machines, no.

It would be very nice to get something like an electronic field to repel dust and debris, but that's a bit beyond what we can do at this time, and I doubt something like a 5 - 8% thick wing would produce good lift characteristics acrosss a good angle of attack range compared with good lifting airfoils.

As the wing gets thinner it approaches a flat plate, which isn't all that good. Someplace is the optimum for the missions anticipated.In that category the P-51 did very well and still does.

Natural laminar flow airfoils (NLF) is an active area of research. Various turbulence-reducing technologies have been tried, usually on experimental aircraft and in wind tunnels, including riblets (Drag reduction by riblets), boundary layer suction (X-21), and careful shaping (NACA's 6X-XXX airfoils). With the last, the increase in critical Mach number is because the airflow is accelerated as far along the airfoil as practical, to reduce turbulence: decelerating airflow makes it more susceptible to transition to turbulence. It's also considered normal practice to assume that the laminar flow properties -- the "drag bucket" one sees in cl​ vs α plots -- doesn't happen: one uses the plot for the airfoil with standard roughness (which is actually pretty severe), not for a smooth airfoil. One category of human-scale aircraft can pretty reliably manage large regions of laminar flow in service: sailplanes. Of course, they don't have engines causing vibrations which can trip the boundary layer, propellers in front of the wing which can (and probably will) trip the boundary layer, gunports, which certainly will, leading edge devices, which probably will, and so forth. For very small aircraft, like model airplanes, large areas of laminar flow are normal -- the Reynolds Number never gets high enough for turbulence to persist.

The big advantage of the P-51 was that its designers probably did the best design of an aircraft cooling system that was practical for a combat aircraft of the era. They also did a their other detail design very well: most single-engined, piston-powered (radial or V-12) fighters had zero-lift drag coefficients of about 0.02 to 0.023. The P-51 was an outlier at about 0.017 (the Bf109 was, at least from some reports I've seen, an outlier in the other direction: 0.029).