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Bilbo_Baggins

Same engine for P51 and Spit- low alt speed difference?

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Posted (edited)

Gents, given these machines have the same engine, I'm interested to know what difference there will be in low altitude speed between these 2 airframes? What estimates do folks make for sea level max speeds vs the Spitfire? Our in-game Spit 9 has 542kmh max speed, which given it's turning authority and maneuverability I think is pretty formidable- what will the Mustang be able to get?

 

Regards

 

 

Edited by Bilbo_Baggins

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Posted (edited)

It should be around the 370-375 mph range iirc. 

 

Edited by Legioneod

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Posted (edited)

That 4-7km will be where 109s and 190s will try to fight it

 

Tempest should still be slightly faster down low even on +9lbs, and crazy fast on +11lbs, and P-51 should be king of the high alts

Edited by 77.CountZero

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Posted (edited)

That is a truly incredible difference given the same motor. 542kmh Spit airframe and 603-610kmh for mustang.

 

Didn’t know how amazing that airframe really was.

Edited by Bilbo_Baggins

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16 minutes ago, Bilbo_Baggins said:

That is a truly incredible difference given the same motor. 542kmh Spit airframe and 603-610kmh for mustang.

 

Didn’t know how amazing that airframe really was.

Yep. P-51 is a much slicker airframe.

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Posted (edited)

It’s funny though, the Spit has much softer lines. The Mustang is kind of angular overall. If they didn’t hang all those drag inducing paraphernalia on the Spit’s underside......

 

Yes, I know there is more too it than that.

Edited by II/JG17_HerrMurf

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Posted (edited)

It (P51 speed advantage)  is almost all due to the radiator design. See the Atwood article in this thread - 

 

Edited by unreasonable

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24 minutes ago, unreasonable said:

It (P51 speed advantage)  is almost all due to the radiator design. See the Atwood article in this thread - 

There are in fact many factors (as well as this one), see the Lednicer Article. The Mustang was neither the only aircraft using laminar wings or making use of the Meredith effect for radiators. But the engineers at NA did manage to produce a fighter airframe that had the least drag despite having one of the largest wetted areas. This also without a running engine.

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45 minutes ago, ZachariasX said:

There are in fact many factors (as well as this one), see the Lednicer Article. The Mustang was neither the only aircraft using laminar wings or making use of the Meredith effect for radiators. But the engineers at NA did manage to produce a fighter airframe that had the least drag despite having one of the largest wetted areas. This also without a running engine. 

 

But the NA engineers were just about the only ones to get Meredith to work (other than DH with the Mosquito) as noted in your source:  "This efficient cooling system arrangement is credited with much of the Mustang's superior performance over the Spitfire." and testing of an in service aircraft showed "... the aircraft was unlikely to have a substantial laminar flow on the wing and transition occurred in the first 15% of the chord."  The ideal being 47% chord. That's about all your source goes into on the Mustang.  Of note, both Messerschmitt and Supermarine failed to get Meredith to work and consequently spent much effort into getting Laminar flow to work without much success either.  The bottom line is the Mustang just didn't have any appreciable cooling drag which, by itself, is huge.

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47 minutes ago, chuter said:

The bottom line is the Mustang just didn't have any appreciable cooling drag which, by itself, is huge.

Sure. I‘m just saying even without that, the Mustang beats the competition in terms of aerodynamic efficiency. 

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Posted (edited)

One reason the Mustang is so much faster is that it features what's known as "Laminar Flow Airfoils"

 

This was somewhat revolutionary at the time, while nowadays all modern jetliners have airfoils like that.  

 

 

At a superficial glance, one might not even notice any difference, but it is quite an advancement from the conventional wing design, see:

 

Image result for laminar flow airfoil

 

Another thing that greatly helped the P51 achieve it's astounding speed and range, was that it made full use of the Meredith Effect in it's radiator design.

Related image

 

That means, the whole cooling structure underneath, not only negates its own parasitic drag, but in certain conditions actively contributes to net thrust.

The P51's radiator thus becomes a little ram jet, as energy is added to the air inside the duct in the form of (normally wasted) heat from the engine.

 

That's why the '51 is so fast.  So fast indeed, that it is faster even than it's modern private jet namesake, the Citation Mustang. Obviously also it is much faster than the sports car of that same name, and vastly beyond the top speed of the four legged mammal all of them are named after.

 

 

 

Edited by 19//Moach

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Except, Moach, that the laminar airflow was not particularly effective on operational planes: as both Atwood and Lednicer state.  Factory finish was simply not smooth enough. I expect you can get it on a polished racing Mustang.

 

I do not recall seeing a credible source that states that the cooling system could actually generate net thrust on the P-51: Atwood certainly does not claim this. Still, getting 80-90% of net drag reduction is a huge boost to speed and operational range.

 

Fun article on radiator/Meredith etc by a RC DIY chap here. I found it an interesting and relatively comprehensible discussion of the practical issues. I leave it to the engineers to point out any errors.  https://www.kitplanes.com/the-meredith-effect-fact-or-fiction/

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26 minutes ago, 19//Moach said:

At a superficial glance, one might not even notice any difference, but it is quite an advancement from the conventional wing design, see:

Yes, in theory. In practice it was common knowledge that in the real world you do not have this perfect laminar flow at all. A bug on your leading edge is enough to preclude laminar air flow. Same as someone walking around on your wing before take off. Not even talking about gun ports here. Besides, the P-63 had such a laminar profile as well and it’s performance was just adequate. Others openly decided against using laminar wing profiles that comes with downsides as well.

7 minutes ago, unreasonable said:

I do not recall seeing a credible source that states that the cooling system could actually generate net thrust on the P-51: Atwood certainly does not claim this. Still, getting 80-90% of net drag reduction is a huge boost to speed and operational range.

This.

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The majority of enclosed radiators with adjustable outlets (i.e. Hurricane, 109, Spitfire, Yak1/7/9 and LaGG3s)  have some amount of Meredith effect and the '51 certainly was not the only plane where the designers knew about and incorporated the effect into their designs.

As for laminar flow; it's better just to talk about how the 51 had thin wings.

 

The 51's performance is a result of all round good design choices, it's not down to any kind of 'magic bullet' like Meredith or laminar flow .

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31 minutes ago, [DBS]Browning said:

The majority of enclosed radiators with adjustable outlets (i.e. Hurricane, 109, Spitfire, Yak1/7/9 and LaGG3s)  have some amount of Meredith effect and the '51 certainly was not the only plane where the designers knew about and incorporated the effect into their designs.

 

They certainly didn't take the most advantage compared to the Mustang's design

Mustang's radiator intake has boundary layer control which makes the turbulent air attached to the fuselage not enter the radiator. Since only the high speed clean air enters the radiator, intake mass flow increases which allows the radiator intake to be smaller. Compare the frontal intake area of the Spitfire and Mustang which similar engine and hence need the similar cooling capacity for the coolant, intercooler and oil.

 

Aircraft like the Hurricane have the whole radiator exposed to the airflow and other aircraft have most of the radiator as such. The Mustang's coolant radiator is mostly inside the fuselage with only the intake scoop and oil radiator out of the fuselage and in the airflow.

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Posted (edited)

I'm sure that's all true, but it's also something we can't isolate the effects of.

Did such design features make better use of the radiators? Certainly to some extent!

Did such design features significantly add the the 51's speed compared to how similar features effected the speed of other planes? With so many other factors at work, it's impossible to say.

Edited by [DBS]Browning

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Posted (edited)

Just for fun, the chart for the P51 A with Allison engine...

 

P-51A-1-43-6007-Chart-1400.jpg

 

The Mustang is almost as fast at sea level as the P40 is at it's critical altitude.  That was one slick airframe.

Edited by BlitzPig_EL
auto correct strikes again

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4 hours ago, [DBS]Browning said:

I'm sure that's all true, but it's also something we can't isolate the effects of.

Did such design features make better use of the radiators? Certainly to some extent!

Did such design features significantly add the the 51's speed compared to how similar features effected the speed of other planes? With so many other factors at work, it's impossible to say.

 

No it is not.  Spitfires were tested with a flatter windscreen and retractable tail wheel. A few mph difference. There was no effective laminar flow advantage because there was no laminar flow. The vast majority of the speed difference is down to the radiator. 

 

The P-51 radiator design was capable of saving 80-90% of cooling drag at cruise speeds. This is huge. No other single seat fighter came remotely close to this efficiency.

 

But then again if you want to ignore the statements of the development team, ie people who actually know what they are talking about, go for it. 

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I read the title as Steam engine for P51 and thought nope that will never fly..

yeah just woke up.

 

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Well, a steam turbine could definitely make enough BHP, but carrying enough water and fuel for it would definitely not help the gross weight.

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I think it mostly just Shows that the Spitfire Airframe was just absolute Crap in Terms of Performance. It has extremely oversized Wings and the Shittiest Aspect Ratio of all WWII Fighters, and some of the Thickest Wings, due to the Spar Design and just sheer Size.

The Wings, as soon as Cannons were introduced grew Worse Blisters than the Bf109s, as did the Cowlings. The Radiators have comically large Intakes, probably the Largest of all WWII Fighters, yet still insufficient in their Effect.

I got to Compare the 109 and Spitfire Intakes in a Museum, and the Spitfire Pertrudes roughly my entire Hand, while the 109 is barely my Index Finger.

image.thumb.png.c5381a7ce4419267ec810d79973d4b85.pngimage.thumb.png.fa7223a82bcec4dffa1268f7ec100393.png

 

The Mustang has a High Aspect Ratio, very Thin, Highly Loaded and clean Wing with Flush Rivets on the Entire Plane, Retractable Gear and Covers etc. And the Higher Wing Loading definetly helps too. The Spitfire simple is underloaded, and the Wing Twist and so on just creates so much unnecessary Drag.

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Posted (edited)
4 hours ago, 6./ZG26_Klaus_Mann said:

I think it mostly just Shows that the Spitfire Airframe was just absolute Crap in Terms of Performance. It has extremely oversized Wings and the Shittiest Aspect Ratio of all WWII Fighters, and some of the Thickest Wings, due to the Spar Design and just sheer Size.

The Wings, as soon as Cannons were introduced grew Worse Blisters than the Bf109s, as did the Cowlings. The Radiators have comically large Intakes, probably the Largest of all WWII Fighters, yet still insufficient in their Effect.

I got to Compare the 109 and Spitfire Intakes in a Museum, and the Spitfire Pertrudes roughly my entire Hand, while the 109 is barely my Index Finger.

  Reveal hidden contents

 

image.thumb.png.c5381a7ce4419267ec810d79973d4b85.pngimage.thumb.png.fa7223a82bcec4dffa1268f7ec100393.png
 

 

 

The Mustang has a High Aspect Ratio, very Thin, Highly Loaded and clean Wing with Flush Rivets on the Entire Plane, Retractable Gear and Covers etc. And the Higher Wing Loading definetly helps too. The Spitfire simple is underloaded, and the Wing Twist and so on just creates so much unnecessary Drag.

 

Aspect ratios:

Bf 109:   6.14

FW 190: 6.02

P-51:      5.83

Spitfire: 5.60

P-47:     5.53

 

As for 'wing twist', the P-51, the Spitfire and the FW 190 all have much the same: 2 degrees, though there are slight differences in the way the twist is implemented.

http://www.wwiiaircraftperformance.org/mustang/Lednicer_Fighter_Aerodynamics.pdf

 

Edit: regarding flush riveting on the Spitfire, I've posted this before, but some may not have seen it:

Spit_Rivets.png

 

From The Aerodynamics of the Spitfire, J.A.D. Ackroyd, Journal of Aeronautical History, 2016/03

https://www.aerosociety.com/media/4953/the-aerodynamics-of-the-spitfire.pdf

 

Possibly as maximum speeds increased, there would have been greater benefits for flush riveting all over, though as always, performance increased had to be balanced against increased production costs. 

Edited by AndyJWest
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P-51D wing: Root  Max thickness 16.5% at 38.9% chord.  Tip Max thickness 11.4% at 46.3% chord. 

 

From J.A.D. Ackroyd paper:

 

1151131536_Wingthickness.thumb.JPG.775cb4d0e89686f426d5dd0dd4c1d5c5.JPG

 

P-51 wing chord at the root is 104 inches  so maximum thickness = 17.16 inches.  Spitfire wing chord at root is 100 inches, maximum thickness therefore 13 inches.

 

Taking average chord as wing area / wingspan and average % thickness we get:

 

P-51 avg chord 1.98 m, avg thickness 13.95%, avg thickness = 0.276m

Spitfire avg chord 2.00 m, avg thickness  9.5%, avg thickness = 0.19m 

 

Whatever the benefits the P-51 enjoyed relative to the Spitfire, they did not come about because the Spitfire had thick wings.   

 

 

 

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On 7/26/2019 at 10:09 PM, Legioneod said:

It should be around the 370-375 mph range iirc. 

 

one hopes the mustang is faster than that, or they tone down the K4. At 1.8ata the K4 is clocking in at 377mph, about 20mph faster than german estimates at sea level. Of course if we geta 150 octane pony it should be clocking in at 380-390 on the deck.

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9 hours ago, 6./ZG26_Klaus_Mann said:

It has extremely oversized Wings and the Shittiest Aspect Ratio of all WWII Fighters, and some of the Thickest Wings, due to the Spar Design and just sheer Size.

 

The oversized wing came in handy when the gross weight doubled during it's service life time.

Compare that to the miserable small-size wing of the 109.

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4 hours ago, Bremspropeller said:

 

The oversized wing came in handy when the gross weight doubled during it's service life time.

Compare that to the miserable small-size wing of the 109.

 

The standard Spitfire wing was 242.1 square feet. The P-51 was 235 square feet. A difference of only 3%.

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What the British analyzed:

 

bla017.thumb.jpg.3423d04b5cfabcc1bc438e26df1cb84a.jpg

 

Unfortunately, quite a bit of unknown on the Spitfire IX. Still, a point not mentioned by anyone so far is gun installation, which was significantly less draggy on the P-51.

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I think that the effect of the laminar flow wing is both under and overstated. While the factory finish might not have been great, it’s well documented that ground crews typically took very good care of their aircraft sometimes going above and beyond when it came to polishing surfaces and cleaning canopies. Further, a few bugs or chunks of mud against the leading edge of the wing will have an effect on the laminar flow of the wing, but localised to where and behind where the debris is stuck. Not the entirety of the wing.

 

Because of this, I feel that while the wing didn’t do as well as is often stated, I do believe that it was delivering some benefit.

 

 

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6 hours ago, AndyJWest said:

The standard Spitfire wing was 242.1 square feet. The P-51 was 235 square feet. A difference of only 3%.

 

The Spit started out much lighter than the P-51.

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15 minutes ago, Bremspropeller said:

 

The Spit started out much lighter than the P-51.

 

It did, though if you are comparing Spits with P-51s, it makes more sense to compare the heavier versions that were contemporaries. Though making comparisons between fighters designed to different criteria at different times is rather a pointless exercise anyway, so addressing the theme of this thread, I'd say that overall, the P-51 was a 'better' aircraft for the late-war scenario, but that isn't surprising. Aircraft designers should learn from experience - ideally someone else's.

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14 hours ago, Fumes said:

one hopes the mustang is faster than that, or they tone down the K4. At 1.8ata the K4 is clocking in at 377mph, about 20mph faster than german estimates at sea level. Of course if we geta 150 octane pony it should be clocking in at 380-390 on the deck.

Thats on the deck with 67" only. If we get 150 octane like you said it'd be faster.

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Posted (edited)
21 hours ago, 6./ZG26_Klaus_Mann said:

I think it mostly just Shows that the Spitfire Airframe was just absolute Crap in Terms of Performance.

Ah yes, but listen to the sound of victory ;)

 

I think the Spit, P-51 and the 109 are all fantastic aircraft in their own right but the Spit (for me)  wins the beauty contest (and I'm not even a Spitfire fanboi)

Edit: I prefer the Hurricane! 

 

Edited by 6./ZG26_Custard

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19 minutes ago, 6./ZG26_Custard said:

I think the Spit, P-51 and the 109 are all fantastic aircraft in their own right but the Spit (for me)  wins the beauty contest (and I'm not even a Spitfire fanboi)

 

Beauty lies in the eye of the beerholder.

 

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1 minute ago, Bremspropeller said:

Beauty lies in the eye of the beerholder.

 

Indeed, but even with my beer goggles on the spit still wins ;)

 

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The P-51 is simply an inherently slicker airframe, the "laminar flow" wing in practice infers little advantage over a conventional airfoil. Properly applied Meredith theory, universal flush-riveting, lower thrust line and generally more refined aerodynamics allowed it to be faster than the Mk IX. The Spitfire IX also has one of the slicker airframes of the period, but the radiators (which couldn't properly apply Meredith theory, despite the fact they were designed to) and projecting cannon barrels created parasitic drag which cut down top speed a fair bit. What should have been done was get rid of the cannon and go for a 4x .50cal setup, much like the P-51B/C. This would have eliminated much of the parasitic drag being produced by the compromises made to fit the Hispano and its feed motor properly.

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Posted (edited)

Regardless of degree of "laminarness" in various stages of flight, the P-51 wing is very slick and very efficient in high speed combat.   What the relatively symmetric wing form gives up at low speed to more conventional, button riveted, cambered airfoils, it makes up for with less drag at higher speeds, less airflow separation due to surface irregularities (more lift per square foot of wing)  at the  moderate angles of attack that occur when maneuvering.    Low speed loss of lift is compensated for with flaps, and a relatively aggressive high speed stall at crit angle of attack is handled easily with experience.   It's a good trade off that was a real advantage at late war combat speeds.

 

One example of the advantage of a 'clean' wing (flush rivet, smooth even skin surface) is the ability of the P47 to pull aggressively out of a high speed dive, where an aircraft like the FW 190 has a tendency to 'mush' in the recovery.    By 'mush' I mean excessive airflow separation occurs over the wing at relatively high alpha, and lift is lost.   It is also probably the reason the P47  is very effective at maneuvering while pulling 'g' either straight or during a roll...

 

Too add to the 'low drag' Mustang air frame concept...   extensive use of second degree conics were use on the curves of the air frame (-https://www.historynet.com/in-the-mustangs-wake.htm)

 

And details of wing shape were designed according to extensive pressure distribution calculations. 

 

Also, the radiator inlet area was fairly small - out the the boundary layer, and leading to a long, thoughtfully shaped duct that minimized internal turbulence. 

Edited by Barfly
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On 7/28/2019 at 9:04 PM, JtD said:

What the British analyzed:

 

bla017.thumb.jpg.3423d04b5cfabcc1bc438e26df1cb84a.jpg

 

Unfortunately, quite a bit of unknown on the Spitfire IX. Still, a point not mentioned by anyone so far is gun installation, which was significantly less draggy on the P-51.

 

Wow, that's a very interesting data sheet!  May I know its source document?

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