P-51 Airfoil and Turn Performance

[Krusty]

Originally posted by gripen
BTW here is something tested on the P-47 as well.

The P-47C compared to the P-47D11 is probably as different as the P47D11 is to the P-47D40.

The XP-51 is probably as different from the P-51B as the P-51B is from the P-51D. Keep in mind the early mustangs had different wing roots and other changes in their airframe.


It's nice, but I don't think it applies. Those versions are too early to compare to the later models.

EDIT: Maybe the 47 is pretty close, on second thought. The 51 wing changed between early models and later ones, though.

[gripen]

If you care to read the docs posted, you will find out that there is data on P-51B as well as on P-51D.

[Krusty]

Still reading it. Seems to mention XP-51, SB2C-1, and P-41C-1. Mentions an XF2 in passing, but no P-51Bs or Ds in the text

[gripen]

Take the second of the first three docs I posted.

[Krusty]

Ah, you misunderstand. I was referring only to the one I quoted.

Originally posted by gripen
BTW here is something tested on the P-47 as well.

That one.

[Widewing]

Here's the same chart with the P-47D-25 and P-51D tested without flaps for reference.



My regards,

Widewing

[Viking]

Originally posted by Benny Moore
While such things can easily be explained by differences in flaps, power, wing shape, and other factors, you're quite right; all of the tests I've seen say that the P-51 should definitely out-turn a P-47 and should turn almost as well as a Spitfire, though not as tight as a P-38 (provided that the P-38 is using the maneuver setting for its Fowler flaps).

BS!

[Widewing]

I tested the Bf 109K-4 with no flaps and full flaps. I have compared the results to the P-51D.

You will note that the turn radius clean is virtually identical, as is the turn rate. However, when flaps are deployed, the 109K-4 gains a large advantage in radius and turn rate. Inasmuch as these aircraft used a similar flap layout, I find it troubling that the Mustang is so badly hampered by what appears to be a unique issue with drag rise.



My regards,

Widewing

[dtango]

A correction on my part, HT talking about different sources of data was on an entirely different topic and not this one.

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Widewing:

I absolutely agree with you regarding the differences in wingloading if we were discussing instantaneous turn performance between the P-51 and P-47 for the weights you quoted.

We're talking about sustained turn performance however.  Here's the turn radius equation in terms of L/D, T/W, and V:



You can see that in a sustained turn Cl (and thus wingloading) is only part of the equation when considering radii for sustained turns.  Drag (Cd) and thrust factor into it as well, not just wingloading.

Pure speculation on my part but with flaps popped maybe the reason the P-47 has a smaller sustained turn radius compared to the Mustang is because of the better cl/cd ratio of the Jug because of it's slotted flaps vs. the Mustang's plain flaps and the 2300hp of the Jug vs. the 1720hp of the Pony.

The only turn radius data for the Mustang I've seen posted was the figure you have that shows the Mustang turn radius compared to the Meteor, etc.  Unfortunately I've never seen any other key details of that report (e.g. config of the aircraft, alt tested, weights, engine power output etc.) that tell me what they were doing that resulted in the data for the turn radii.

Tango, XO
412th FS Braunco Mustangs

[Widewing]

Originally posted by dtango
A correction on my part, HT talking about different sources of data was on an entirely different topic and not this one.

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Widewing:

I absolutely agree with you regarding the differences in wingloading if we were discussing instantaneous turn performance between the P-51 and P-47 for the weights you quoted.

We're talking about sustained turn performance however.  Here's the turn radius equation in terms of L/D, T/W, and V:



You can see that in a sustained turn Cl (and thus wingloading) is only part of the equation when considering radii for sustained turns.  Drag (Cd) and thrust factor into it as well, not just wingloading.

Pure speculation on my part but with flaps popped maybe the reason the P-47 has a smaller sustained turn radius compared to the Mustang is because of the better cl/cd ratio of the Jug because of it's slotted flaps vs. the Mustang's plain flaps and the 2300hp of the Jug vs. the 1720hp of the Pony.

The only turn radius data for the Mustang I've seen posted was the figure you have that shows the Mustang turn radius compared to the Meteor, etc.  Unfortunately I've never seen any other key details of that report (e.g. config of the aircraft, alt tested, weights, engine power output etc.) that tell me what they were doing that resulted in the data for the turn radii.

Tango, XO
412th FS Braunco Mustangs

Well, we can look at thrust. Typically, the P-51D generated about 30% less thrust than the P-47D-25. However, in clean condition, the P-51D has nearly half the drag of the P-47D. Weight as tested in the game was about 8,500 lb for the Mustang and 13,500 lb for the P-47. So, the P-47 is 37% heavier. I do not see anything in the flap drag that can offset the drag and weight advantages to such a huge effect to produce the results I have shown.

Moreover, the Bf 109 also employs a plain flap, but with only a fraction of the drag shown by the P-51s. Something is rotten in the drag modeling of the Mustangs. Test data shows it getting progressively worse with each tweak of the overall drag model.

My regards,

Widewing

[Stoney74]

Perhaps its not Clmax that's the issue, but the coefficient of drag.  The P-47 airfoil is basically a NACA 23012, which has a rather flat "laminar bucket", which results in a much lower Cd at a higher Cl than those associated with a 6 series, i.e. a NACA 64415 (what I thought would be a close approximation to the P-51 root shape shown in Gripen's sources).  Those airfoils have much higher Cd's than the 23XXX family at the same Cl.  So, theoretically, as the Pony turns tighter, it's Cd increases much faster than the P-47's Cd.  For example, at a Cl of 1.2, the 23012 section Cd is .012 (at Re of 3X10^6).  The 64415 has a Cd of almost .020 at the same reynolds number and Cl.  Is this significant?

[dtango]

Widewing:

Keep in mind that it's not only Cd increase but also Cl increase as well.  We know that Cl increase has the effect of reducing maneuvering velocity.  The reduction in velocity also means an increase in thrust as well.  We can see from the equation that the combination of increase in Cl, increase in thrust, and reduction of velocity all equate to smaller turn radius.

In the case of the 109, it also has leading edge slats so we can't quite compare apples to apples with the P-51 plain flaps.  Again, the case may be the differential in the change of cl/cd due to slotted flaps (P-47) and LE slats + plain flaps (109) vs. plain flaps (P-51), the resulting change in differentials in maneuvering velocities, and changing differentials in thrust give us these results for turn radius.

I readily acknowledge this is just speculation by looking at the equation vs. having data to actually crunch through the equation to check it.  Unfortunately I have no ready reference for cl/cd polars for the aircraft in question with and without flaps.

I also wholeheartedly acknowledge the degradation of turn performance of the P-51 compared to the other AH planes as you have eloquently pointed out.  However the physics suggests that it's imminently possible for the results that we have.  Thus, I can't conclude that the P-51 performance is out of line.  This is a certainly a possibility but without any other specific aircraft data to look I can't come to this conclusion.

Tango, XO
412th FS Braunco Mustangs

[dtango]

Stoney:



Yep, it would effect turn radius.  Cl/Cd is in the denominator.  The key is that turn radius is a function of all the variables above and not just cl/cd.  2ndly all those variables (except for gravity and in our case, weight) quite frankly vary!  I suppose that's why they call it aero-DYNAMICS.  We can't assume a linear relationship because of this.

That's why it could be perfectly feasible that in the case of the P-47D-25 vs. the P-51 that even though without flaps the P-47 has a worse turn radius compared to P-51 (even if the P-47's cl/cd is better as you suggest), the P-47 actually has a better turn radius with flaps popped vs. the P-51 because the differential in cl/cd between the 47 and 51 grows even larger which changes the result in turn radius.

Tango, XO
412th FS Braunco Mustangs

[dtango]

A quick correction:  The chief aerodynamicist for the Mustang was Ed Horkey (not Horker) that Lednicer and Gilchrist received the information from on the Mustang airfoil.

Tango, XO
412th FS Braunco Mustangs

[Widewing]

Originally posted by dtango


In the case of the 109, it also has leading edge slats so we can't quite compare apples to apples with the P-51 plain flaps.  Again, the case may be the differential in the change of cl/cd due to slotted flaps (P-47) and LE slats + plain flaps (109) vs. plain flaps (P-51), the resulting change in differentials in maneuvering velocities, and changing differentials in thrust give us these results for turn radius.

This doesn't stand up to the fact that the slats are out when tested with flaps up. The 109 doesn't turn better than the P-51D under those circumstances. Only when flaps are lowered does the 109 gain a huge edge. Again, that makes absolutely no sense.

According to the RAF, the Mustang III out-turned a 109G with relative ease.

By the way, did you read the P-51 test reports in Gripen's links? There is no reason to assume that the P-51's drag coefficient goes up excessively approaching CLmax. It doesn't.

What I find very confusing is that in level flight, the P-51s have less drag than the 109s, even with full flaps. I test this by cutting power and timing speed loss. the 109K-4 slows much faster that the P-51D. Yet, when turning, drag rises rapidly in the P-51s and lift degrades, but not in the 109s..

You gents are trying to use formulas to confirm the flight characteristics of code... Don't waste your time. You cannot assume that the dynamics match the real aircraft as you have no idea what data is used in the model. HTC isn't talking or telling us what their parameters are. Thus, you are guessing at best.

What we do know is obtained from flight testing. I have 3 different versions of Aces High installed and all three present different test results. In each case, the P-51s have deteriorated to the current mess.

Perhaps Pyro can offer us some insight.

My regards,

Widewing