Kurt Tank FW 187/ Grumman XP5F 1935/1938 et al.

[Alpakinator]

The XP-39 is easy - take a look at pg. 84 to 85 of 'Vee's for victory' book, and especially at the graph at pg. 85 that gives 340 mph at 20000 ft for the 'original airplane [XP-39]'. No wonder, the XP-39 was draggy affair, with Cd0 = 0.0329.

The XP-50 - I have no firm data that it never did 425 mph, however there is also no firm data that it did. As a reality check, we can look at the P-38J, that bested 420 mph mark with 2 x 1600 HP (ie. 2 x 400 HP more than the XP-50 was supposed to have), while without the drag of the R-1820s to act as airbrakes.

added: the figures for the XF5F are even more funny, to be polite - it is/was trumpeted as capable of doing 383 mph at S/L (= faster than a P-47D with water injection and 150 grade fuel - 2800 HP - by about 40 mph), while the USN data sheet gives just above 310 mph at S/L.

Thanks! I think by comparing XP-50 to XF-5F we can see how 425 mph at 20000 ft is way too high of an estimate. Since XF-5F reaches 312 mph at sea level at 1000hp, we could assume similar for XP-50. There's no way 312 mph at sl (1000hp) and 425 mph at 20000 ft (1200 hp) can be true at the same time. It would be lower, maybe 380-400 mph?

 

[Shortround6]

Unfortunately there is a bit of confusion or potential confusion in that data sheet, and it is not in favor of the XP-50.

The box for "eng. rat's used for perf." is not filled out (no entry) so we don't know if they were using the "normal" power which was max continuous or if they were using take-off power rating, which for just about every other plane that used that model of the Cyclone engine was the same as "military" power. This was very common in US engines once you get out of the 1930s. Other countries did things differently and the take-off power was a special combination of rpm and boost that was not allowed anywhere else in the flight envelope. This does depend on engine/s so be careful. US engines from 1930s' to 1938(?) sometimes had a 1 minute rating for take-off, later changed to a 3 or 5 minute rating, check engine specs.

Back to the XF5F data sheet. We have to believe that they ran all those speed tests at the max continuous power setting (1 hour or more) and were not max speed, in other words they were max cruise speeds and they never listed the actual max speed OR we have to believe that all those speeds were the actual max speeds and were done at military power. As we know WEP/WER was a different power rating. The R-1820 G231 engine was never given a WEP/WER rating. The WEP/WER ratings for any engine did not show up until well after the XP-50 had crashed.

 

[SaparotRob]

The F5F gave stellar service for the Blackhawks.

 

[Shortround6]

The F5F gave stellar service for the Blackhawks.

we need pictures!!

 

[tomo pauk]

Thanks! I think by comparing XP-50 to XF-5F we can see how 425 mph at 20000 ft is way too high of an estimate. Since XF-5F reaches 312 mph at sea level at 1000hp, we could assume similar for XP-50. There's no way 312 mph at sl (1000hp) and 425 mph at 20000 ft (1200 hp) can be true at the same time. It would be lower, maybe 380-400 mph?

380 mph at 20000-25000 ft seems much more believable for a combat-worthy P-50 (ie. has guns and protection both for pilot and fuel).