Greg of Auto and Airplanes has asked for a Debate

[drgondog]

What was the effect of the bomb racks? As I remember it was around 10 MPH maybe slightly less. 10MPH was around the difference made to a Spitfire by putting on a BP windscreen and two cannon. The notion that two filled 150 gal tanks suspended from the wings of a P-47 only cost 10MPH in top speed gets this years Balkenkreuz award for ignoring the rules of physics. If 300 gallons of external fuel just cost 10 MPH why bother putting any inside?

The reports stated that 'clean' condition was with bomb racks. That makes sense because the tests were focused on Long Range escort to Japan. The racks remained after tanks ejected. All P-51 range tests were conducted thusly.

 

[drgondog]

like 4mph with the later racks and 8mph with early ones. For the P-51.

The P-47 loses like 25mph with wing racks.

While I agree the pylon drag at cruise speed was higher than P-51B and much higher than D, do you have a source for 25mph?

 

[Low Battery]

While I agree the pylon drag at cruise speed was higher than P-51B and much higher than D, do you have a source for 25mph?

A long time ago I was looking at different speed tests on thunderbolt. Most listed no wing racks but one I found in with wing racks was about 20-25mph less. There was also a book I was reading I don't recall name of that stated this difference in speed which was why I was trying to find documentation on it in the first place. It's not impossible I made a mistake that's just what I recall.

 

[Low Battery]

Ok so looking back(in at work so I was skimming) it's the 150 octane test for p47. At 56 inches and with wing racks only hits 418mph. Compared to other p47 tests where it hits 435 at same Map but those tests iirc have no racks. The planning chart also stats data is without racks and quotes similar 435mph iirc. So 17mph my memory was a bit off

 

[pbehn]

The reports stated that 'clean' condition was with bomb racks. That makes sense because the tests were focused on Long Range escort to Japan. The racks remained after tanks ejected. All P-51 range tests were conducted thusly.

I just took it to be the standard fit out of planes on operations so as to be representative.

 

[drgondog]

Ok guys I did a brief check on pages 47/48 of the P-47D Pilot's Flight Instructions dated 25 Jan 1945 (-25 model and beyond) and this is what I found:

1) with 300 gallons fuel on board and no external tanks the optimal suggested best speed at 20,000 feet is 330 mph with a maximum range of 485 statute miles.

2) with 300 gallons of fuel on board AND two 165 US gallons drop tanks the optimal suggested best speed attainable at 20,000 feet is 291 mph which provided a maximum range of 455 statute miles.

To me it looks like that at the same altitude and taking into account the optimal range profile there was nearly a 40 mph reduction in maximum level speed and a loss in range of 30 statute miles when drop tanks were carried.

Big difference from the 10 mph Greg quoted in his video. And to lose 30 miles of range with the same fuel load is also a clear indication that those tanks created a lot more drag than what he wants everyone to believe.

By the way, the manual I quoted can be found on-line and can be downloaded for free.

Correct - also note that the range is straight line calculation, with no operational complications such as 48+ ship take off, circle to form assembly and climb in circular path over say, East Anglia to at least 15000 feet before continuing climb across Channel or N. Sea to avoid the majority of coastal flak batteries.

In fairness ti Greg, SOP for New Guinea would be far different as air bases and flak defenses were remote. That said, he projects all calcs as straight line Fighter Mission and conflates such to Close Escort to make his case. Nor does he emphasize true drop in airspeed while carrying tank.

 

[Holtzauge]

Ballpark estimate of the effects of two 150 gallon tanks on a P-47 D30's speed at 20000ft using this simulation tool. (Simulated the D30 since I had that modeled already).

Baseline: Assuming a cruising type power setting yielding 310 mph (499 km/h) at 20000 ft in clean configuration.

Then adding effects of drop tanks with drop tank parasitic drag estimated as Cd0=0.33*d/l as per formula 21 on page 6-16 in Hoerner's book Fluid Dynamic Drag.

Speed using same power setting as clean, but now with two 150 gallon drop tanks added at 6100 m this gives a 455.8 km/h TAS. This is taking both parasitic and induced drag due to added fuel weight into account.

So 500-455.8=44.2 km/h or 27.5 mph with both parasitic and induced drag accounted for (If only taking the parasitic drag into account then the speed loss is only 11.5 mph).

But this is only for the drop tank drag and does not including drag and weight of rack (Don't have that info).

Say that this deducts an additional 4 mph then the total speed loss at 20000 ft would then be around 27+4=31 mph.

Again, only a ballpark estimate, but seems reasonable if the P-51 (which is a much cleaner design with a lower span loading, i.e. lower induced drag) loses around 40 mph carrying a couple of 110 gallon wing tanks.

Anyway, in summary, I would say that this ballpark estimate shows that assuming a speed loss of only 10 mph when adding a couple of 150 gallon tanks to a P-47 would be way too optimistic.

 

[DarrenW]

Ballpark estimate of the effects of two 150 gallon tanks on a P-47 D30's speed at 20000ft using this simulation tool. (Simulated the D30 since I had that modeled already).

Baseline: Assuming a cruising type power setting yielding 310 mph (499 km/h) at 20000 ft in clean configuration.

Then adding effects of drop tanks with drop tank parasitic drag estimated as Cd0=0.33*d/l as per formula 21 on page 6-16 in Hoerner's book Fluid Dynamic Drag.

Speed using same power setting as clean, but now with two 150 gallon drop tanks added at 6100 m this gives a 455.8 km/h TAS. This is taking both parasitic and induced drag due to added fuel weight into account.

So 500-455.8=44.2 km/h or 27.5 mph with both parasitic and induced drag accounted for (If only taking the parasitic drag into account then the speed loss is only 11.5 mph).

But this is only for the drop tank drag and does not including drag and weight of rack (Don't have that info).

Say that this deducts an additional 4 mph then the total speed loss at 20000 ft would then be around 27+4=31 mph.

Again, only a ballpark estimate, but seems reasonable if the P-51 (which is a much cleaner design with a lower span loading, i.e. lower induced drag) loses around 40 mph carrying a couple of 110 gallon wing tanks.

Anyway, in summary, I would say that this ballpark estimate shows that assuming a speed loss of only 10 mph when adding a couple of 150 gallon tanks to a P-47 would be way too optimistic.

Interesting analysis.

Another comparison can be made to the speed loss experienced by two other large US fighters with the R-2800, the F4U-1D and F6F-5. I would say that aerodynamically the F4U-1D was probably a tad bit cleaner than the P-47D, with the F6F-5 being somewhat worse.

According to 1945 NAVAER ACPs, both the F6F-5 and F4U-1D lost 20 mph while carrying one full 150 gallon tank. Both aircraft were in a "normal" power setting and flying at a Critical altitude of 23,900 ft and 24,600 ft, respectively.

And while both naval fighters could carry two 150 gallon drop tanks, only the F4U-1D ACP has data listed for the aircraft in this configuration. It shows a loss of 41 mph at 24.200 ft when utilizing a "normal" power setting.

Edit: I forgot to mention that the speed loss was calculated between aircraft which were normally configured with wing racks so the loss in speed stated was due to drop tanks alone.

 

[Holtzauge]

Interesting analysis.

Another comparison can be made to the speed loss experienced by two other large US fighters with the R-2800, the F4U-1D and F6F-5. I would say that aerodynamically the F4U-1D was probably a tad bit cleaner than the P-47D, with the F6F-5 being somewhat worse.

According to 1945 NAVAER ACPs, both the F6F-5 and F4U-1D lost 20 mph while carrying one full 150 gallon tank. Both aircraft were in a "normal" power setting and flying at a Critical altitude of 23,900 ft and 24,600 ft, respectively.

And while both naval fighters could carry two 150 gallon drop tanks, only the F4U-1D ACP has data listed for the aircraft in this configuration. It shows a loss of 41 mph at 24.200 ft when utilizing a "normal" power setting.

Edit: I forgot to mention that the speed loss was calculated between aircraft which were normally configured with wing racks so the loss in speed stated was due to drop tanks alone.

OK, thanks for the info. Then it looks like I was a bit optimistic and the speed reduction is actually bigger than in my ballpark estimation. Came as a bit of a surprise to me that the induced drag was such a large part of the total drag though, looking at the calculations. Anyway, if the speed loss is as large as 41 mph for two 150 gallon tanks on the F4U, then something similar on the P-47 does not seem unreasonable then.

 

[drgondog]

Interesting analysis.

Another comparison can be made to the speed loss experienced by two other large US fighters with the R-2800, the F4U-1D and F6F-5. I would say that aerodynamically the F4U-1D was probably a tad bit cleaner than the P-47D, with the F6F-5 being somewhat worse.

Darren - the P-47 was actually very clean without racks - I don't have the report close at hand but IIRC, wind tunnel in standard service condition, at 100mph in full scale wind tunnel the P-47 CDp was 0.0265, F4U = 0.0284, F6F = 0.0293, P-51B (with racks) = 0.0209

The remarkable feature of the NACA Drag Tests was the jump from sealed to unsealed (take off condition) for the F4U and F6F compared to the P-51B. They both incremented ~ 0.007 and 0.008 respectively to 0.003 for the P-51B

According to 1945 NAVAER ACPs, both the F6F-5 and F4U-1D lost 20 mph while carrying one full 150 gallon tank. Both aircraft were in a "normal" power setting and flying at a Critical altitude of 23,900 ft and 24,600 ft, respectively.

And while both naval fighters could carry two 150 gallon drop tanks, only the F4U-1D ACP has data listed for the aircraft in this configuration. It shows a loss of 41 mph at 24.200 ft when utilizing a "normal" power setting.

Edit: I forgot to mention that the speed loss was calculated between aircraft which were normally configured with wing racks so the loss in speed stated was due to drop tanks alone.

The 150gal USN version had about the same drag as one 108gal tank on the Mustang based on those results. The Brisbane 200gal tank design for the P-47C installed with the equivalent of two dirty racks (vs single B-7) with probably a lot of additional turbulent pressure drag also between the tank and skid plate. I would suspect at least the equivalent drag as the 2x108 (47mph) penalty for the P-51D

 

[DarrenW]

Darren - the P-47 was actually very clean without racks - I don't have the report close at hand but IIRC, wind tunnel in standard service condition, at 100mph in full scale wind tunnel the P-47 CDp was 0.0265, F4U = 0.0284, F6F = 0.0293, P-51B (with racks) = 0.0209

Hi Bill...yeah I've seen those NACA figures for CDp and you're absolutely right that the F4U definitely has a higher number than the P-47. I was thinking because the aircraft tested was a very early birdcage version that there were improvements over time which made the F4U-1D a somewhat cleaner machine but this is only speculation.

FWIW wasn't the F4U considered the faster of the two at sea level, even when the P-47 wasn't carrying wing racks? This is with both aircraft producing 2,000 hp. To me this is an indication that it was the less "draggy" of the two but maybe I'm over-simplifying things too much here...

 

[Snautzer01]

Love that techie techie talk. Specially when you guys make it understandable.

 

[DarrenW]

The remarkable feature of the NACA Drag Tests was the jump from sealed to unsealed (take off condition) for the F4U and F6F compared to the P-51B. They both incremented ~ 0.007 and 0.008 respectively to 0.003 for the P-51B

That's a very good point. It truly shows how aerodynamically clean a service condition P-51 really was.