Performance Analysis: P-47

[HoHun]

Hi everyone,

Here the results of my performance analysis of the P-47D.

As an introduction, let me point out that it's only possible to calculate the performance of a specific aircraft type when you have data on the power curve on the engine that powers this aircraft. In the case of the P-47, this was made difficult because the US system of rating engines pretends that the R-2800 yields constant power from sea level to critical altitude, while in combat use, the aircraft is flown with constant boost level up to critical altitude, which yields a not-so-straight power curve that shows a power maximum at medium altitude, with less power at the top and bottom end than you'd believe from the rating.

Since Mike Williams has provided some very good reports on the P-47 that were prepared by measuring the actual engine power in flight with a torquemeter (neat bit of technology!), I was able to approximate the power curve for static conditions (since at top speed in level flight, ram effect increases the intake pressure) and make a better analysis than my previous "constant rated power" attempts with Mike's data from P-47 Performance Tests

Here is a comparison of real-world data for the P-47D-10 equipped with a torquemeter and the data I calculated from the reverse-engineered static power graph, plus real-world data from the Tactical Chart for the P-47D-25RE and how it compares to the calculation I prepared using the same reverse-engineered power chart.

The fit of the P-47D-10 curves is fairly good. (In fact, I'm very happy with it.) The remaining difference could be due to two or three factors I consciously neglected while reverse-engineering the power chart to keep the complexity at a managable level.

Applied to the P-47D-25-RE, the same power chart yields a very satisfying fit. (In fact, I was surprised when it came out so well.)

Note the wavy form of the speed and climb calculation above critical altitude - this is the result of applying the measured torquemeter values. The charts that come with the reports on Mike's site don't show this wavy form, but how you get a smooth graph from a wavy power curve ... well, I assume they just smoothed it over.

The fit of the climb curves is more interesting. The P-47D-10 curve is correct in magnitude but doesn't have the correct shape. (Minor differences are to be expected as the torquemeter didn't yield exactly the same power curve in climb as in level flight, but I don't think these are responsible for the whole difference.) The P-47D-25-RE curve is great in its shape (though I changed one digit of a single figure from the Tactical Chart because it was an obvious typo), but about 1 m/s too low. Funny, but that both differences go the opposite way is somewhat encouraging ... more in my next post.

(The P-47D-10 is a razorback with the small-diameter "toothpick" propeller, while the P-47D-25-RE has a bubble canopy and the the large-diameter "paddle blade" propeller.)

Regards,

Henning (HoHun)

 

[HoHun]

Hi again,

Here is the same comparison again, now including P-47D #42-26147, a razorback with the paddle blade propeller. (I've smoothed the wavy part now, too.)

The fit of the speedcurve is worse than with the two P-47's analyzed earlier, but the measured curve of #42-26147 also deviates from the real-world P-47D-25-RE curve from the Tactical Chart. As the two aircraft are equipped with identical propellers and identical engines, such is deviation is not to be expected and can most likely be explained by the engine of #42-26147 not yielding exactly the same power as that of the other two aircraft.

This assumption can be verified by checking the climb chart, where #42-26147 displays a worse climb rate than the P-47D-10 tested with a torquemeter. As #42-26147 is a razorback with paddle-blade propeller and the P-47D-10 a razorback with "toothpicks", and as both aircraft are flying at the same weight, this is not what we'd expect. My calculation shows both aircraft at a very similar climb rate - we might expect the paddle-blade aircraft to have an advantage here, but the P-47D-10 has a bit less overall drag (as evident from its higher top speed) ... but it might be that my calculation slightly underestimates the difference between the propeller types, too.

In any case, we can already conclude that my calculations have the same degree of accuracy as the USAAF tests and show the performance of a P-47D with an engine performing according to its specifications fairly (though not perfectly) accurately.

Regards,

Henning (HoHun)

 

[HoHun]

Hi again,

To get a better impression of the combat capabilities of a late-war P-47D, I have proceeded from the reverse-engineered 56" Hg power curve and generated a generic 65" Hg power curve (without water injection, to keep it simple - this would have added perhaps 6 % additional power, but also changed the shape of the curve a bit).

As our #42-26147 was a test ship for different power settings, it's now possible to compare the measured and calculated data for different powers used by the same aircraft.

The fit of the speed curves is pretty good, allowing for the slightly sub-standard performance of #42-26147 at low altitude we have noticed earlier.

The fit of the power curves is not quite as good. That the calculated climb rate would be higher at low altitude was to be expected as the "on-spec" engine as measured by the torquemeter in the P-47D-10 obviously delivers more low altitude power than the engine in #42-26147. However, that the difference between measured and calculated climb rates at 56" Hg and 65" Hg boost pressure is almost twice as big for the calculation than for the test data is somewhat surprising. Possible explanations might be for example different cowl flap settings, a non-linear performance increase of the engine in the region of near overheating (which the test aircraft entered at 65" Hg). This difference is not so large that I'm concerned about the accuracy of my calculation, though.

Regards,

Henning (HoHun)

 

[HoHun]

Hi again,

And here is a diagram showing the performance of the P-47D-25-RE bubble top with paddle-blade propeller at combat weight with 56" Hg boost as well as with 65" Hg boost as made possible by the introduction of the "44-1 fuel" in mid-1944.

(The 56" Hg data still contains the "wavy" section for consistency while the 65" Hg data is smoothed in that section.)

Regards,

Henning (HoHun)

 

[syscom3]

Was this compared to Republic Aircraft flight test data?

 

[HoHun]

Hi Syscom,

>Was this compared to Republic Aircraft flight test data?

No. "AAF MC test" stands for "Army Air Forces Material Command test", "Tac. Chart" for the "Tactical Planning Performance Characteristics Chart", and "AAF FTEB test" for "Army Air Forces Flight Test Engineering Branch".

All of the test data I used can be found under the link I gave in my first post: P-47 Performance Tests

If you don't know Mike's site yet, you should really check it out - it's a treasure grove of flight test data.

Regards,

Henning (HoHun)

 

[timshatz]

Nice work HoHun. Interesting stuff.

Heard from a pilot that flew P47s during the war that you couldn't get a formation of P47s intact over 28K. Mustangs would get higher, but not the 47s. Some airplanes could get higher individually, but not as a group.

Just thought I'd toss that in there.

 

[HoHun]

Hi Tim,

>Heard from a pilot that flew P47s during the war that you couldn't get a formation of P47s intact over 28K. Mustangs would get higher, but not the 47s. Some airplanes could get higher individually, but not as a group.

Highly interesting! I wonder if this might be a result of the turbo-supercharger making the kind of subtle throttle handling impossible that is required for formation flying.

The P-47 Specific Engine Flight Chart supplies some instructions for throttle handling above full throttle height, which take the requirement to avoid turbine overspeeding into account. (And as it's for the R-2800-21, it's interesting to notice that turbine rpm is limited to 18250 on this chart, probably the value for early superchargers as my above analysis is based on a 22000 rpm limit.)

I haven't really thought through all the implications of having a turbo-supercharger in the loop at high altitude, but I remember reading about the bypass valve being too coarse for fine control in a auto racing context, and maybe a fully-loaded P-47 didn't actually have the excess power to allow the formation leader to throttle back a little at altitude so that the formation members have a little power margin to hold position.

Pretty complex question now that I think about it

(It's worth noting that for the sake of consistency, my charts are for a fixed weight, and in reality a P-47D-25-RE would no longer be at its combat take-off weight of 14600 lbs after having climbed to its ceiling. It would be even lighter over the target area if it flew for example an escort mission from England to Germany.)

Regards,

Henning (HoHun)

 

[vanir]

Hi HoHun, I'm doing a computer remodelling of the Ta152 series for an il2 mod and one of my team was particularly concerned about comparative performance of the Ta152H (Jumo motor) versus the P-47 at altitude. Keeping in mind detailed research suggests the TA came out a little under its calculated projections for performance particularly above 12km but did seem to perform fairly strongly up to 10km (circa 1800PS using MW50 at 8km in the high gear), what is your opinion of the comparative performance of the P-47 to this aircraft in combat at altitudes ranging from 8-10km?

I mean one of the things I noted researching the TA is that data sheets don't often reflect the actual flying qualities of the aircraft in use, a point reinforced by your comment about the operation of the R2800 turbocharger at altitude.