A36 Apache

[Stoney]

Well, I'm going to spit-ball for a second.  Perhaps, given the aerodynamic efficiencies of the P-51D are mainly efficiencies created by lower parasitic drag, that at high altitudes, where parasitic drag makes up less of the overall drag, and where induced drag makes up more of the overall drag, the Pony will enjoy a smaller speed advantage.  At sea level, where more of the drag is parasitic, we see a larger discrepancies in the two top speeds.  Assuming both the Spit 9 and Pony are making the same amount of thrust at all altitudes, the lower parasitic drag of the Pony would enable it to be even faster at sea level than it would at altitude.

This is a simplified example, but I believe it is probably what a more detailed analysis would reveal.  Ultimately, its the ability for the Pony to make full rated horsepower up to a very high critical altitude that gives it its excellent high altitude performance, not unlike its peer the P-47, which certainly did not earn its high-altitude performance from being dainty.

[Chalenge]

Not necessarily.  The Spit 9 versus P-51 is a good example.  Even though they are both powered by the same engine, and the P-51 is larger and heavier (I double checked this time ), the P-51 is still faster.  The Spit airfoil is also thinner at the root than the P-51, but still generates more drag.  My original point is the same though--the aerodynamic design of the P-51 was more efficient at all altitudes, not just at high altitude.

I dont believe thats accurate. The P-51D and Spitfire IXc have nearly identical wing root thicknesses (and other dimensions). The Spit is lighter and has maybe 50hp more. Also the induced drag of the Spitfires airfoil will always be more severe than the induced drag of the P-51. Thats the whole reason for the airfoil chosen for the Mustang. Also the relationship of the two forms of drag are not a function of altitude but of airspeed (parasitic increases dramatically with greater airspeed and the reverse being true of induced drag). Probably your opinion of the root sections is greatly biased by the fillets which are added to reduce interference drag (one component of parasitic drag) but interference drag was never a severe problem with the P-51. It may have been an issue with maintaining pure laminar flow.

Looking at those tables its hard to believe laminar flow was considered with AH at all or that the Spitfire was given the same advantage but I am not an aerodynamicist just a hobbyist glider pilot.

[Stoney]

I dont believe thats accurate. The P-51D and Spitfire IXc have nearly identical wing root thicknesses (and other dimensions). The Spit is lighter and has maybe 50hp more. Also the induced drag of the Spitfires airfoil will always be more severe than the induced drag of the P-51. Thats the whole reason for the airfoil chosen for the Mustang. Also the relationship of the two forms of drag are not a function of altitude but of airspeed (parasitic increases dramatically with greater airspeed and the reverse being true of induced drag). Probably your opinion of the root sections is greatly biased by the fillets which are added to reduce interference drag (one component of parasitic drag) but interference drag was never a severe problem with the P-51. It may have been an issue with maintaining pure laminar flow.

Looking at those tables its hard to believe laminar flow was considered with AH at all or that the Spitfire was given the same advantage but I am not an aerodynamicist just a hobbyist glider pilot.

Not counting the wing root fillets, the NACA 45-100 used on the P-51 had ~15% root thickness.  The Spitfire, until the late models where they completely redesigned the wings, had a NACA 2412 root which is 12% thick.  At higher angles of attack, the Spitfire airfoil will create lower profile drag than the P-51 airfoil.  The higher the altitude, the higher the required angle of attack to achieve the same lift coefficient (remember like I said, dynamic pressure is lower, therefore airfoils produce less lift).  So, at high enough altitudes, if say the Spit and Pony have to pull 6 degrees of AoA to fly level, the Spit could create lower amounts of profile drag. 

Now, considering the differences in dynamic pressure, altitude changes the influence levels of the two main types of drag, parasitic and induced, just as speed does.  The higher you fly, parasitic drag decreases and induced drag increases.  The lower you fly, induced drag decreases and parasitic drag increases.  The differences aren't as drastic as when you compare the differences created by speed, but the relationship is the same, and I believe probably accounts for the difference in speed at altitude, if we assume both aircraft create the same thrust on the same powerplant.  I'm glossing over some details here, but I think the theory is sound from an aerodynamic perspective...Maybe one of the other guys can jump in and either validate or correct me.  Regardless, a stimulating discussion.

EDIT:  Removed a section that could have been confusing... 

[Chalenge]

Stoney the definition of a laminar flow airfoil is one designed for minimum drag and uninterrupted flow of the boundary layer. Yes if both the Spit and Mustang had non-laminar flow design then your description would apply but not as the aircraft really were designed.

In real life the Spitfires drag at high speed would increase at a much greater rate than on the Mustang both in level flight and while maneuvering. To some degree this is represented in the game by aileron responses that stiffen at higher speeds but the more I look at it and think about it the less realistic it seems because a maneuvering P-51 should not hit the same drag that a Spitfire does. There is a point at which the P-51 will experience more severe drag in hard maneuvers that would approach the drag of the Spitfire but also in standard turns it would not.

No I think you got the effects of dynamic pressure on these examples incorrect. I think in fact that the P-51 has to experience an AOA in excess of 15 degrees before it experiences turbulent flow.

[Stoney]

I think in fact that the P-51 has to experience an AOA in excess of 15 degrees before it experiences turbulent flow.

Most laminar flow airfoils have extremely low profile drag at their design lift coefficient angle of attack.  Typically, anywhere from -2 to +2 degrees AoA off the design lift coefficient angle of attack makes up the "bucket" as its called, where laminar flow will exist.  As this chart shows, the design lift coefficient of the Mustang is around 1, as indicated by the point of lowest drag.

(Ignore the NACA 23015 curve here)



Inside of that bucket, and around the design lift coefficient, the Mustang will display much lower profile drag than the Spitfire.  I haven't done a drag polar on the 2412 airfoil for display, but at some point, probably around 6-8 degrees AoA, I'm guessing the Spit will show lower profile drag than the Pony, not unlike the polar for the 23000 airfoil shown in the chart.  Basically, laminar flow exists inside the "bucket" with increasingly turbulent flow outside of it.

[Chalenge]

Stoney what does that chart look like at 450 mph?

[Stoney]

Stoney what does that chart look like at 450 mph?

Probably close to the same, except that the overall drag numbers will be lower.  I'll see if I can't get some polars together at that speed, but XFOIL doesn't like high Reynolds numbers, so they could get a little goofy.  While I'm doing that, I'll also do some 2412 plots.

[jollyFE]

I thought I had heard somewhere that the radiator outlet (under the cockpit) actually produced a small amount of thrust which negated (slightly) drag of the propeller.  Can't remember where I heard this of if it's even correct.

[Chalenge]

I looked into the Spitfire to see if I could compare airfoil data from P-51D to Spitfire (any model) and discovered that at no time did the Spitfire have a NACA 2412 airfoil.

[Stoney]

You're right.  I mispoke earlier and called it a 2412, when in fact it was a 2213.