Gaston,
You have been quoting extracts from a flight test comparison conducted by the Society of Experimental Test Pilots, entitled "Ending The Argument". However, that report is very clear and specific about several points on which you appear to be speculating. For example, you speculate about the corner speed of the P-51D in that report as follows:
This test here was limited to 6Gs... I asked an actual prop aircraft designer, and he did not find it implausible that the "Corner Speed" would be that high ("Very close" can only be above 300 MPH, and is likely as high as 330+)...
If you can pull high Gs (5 G +) below 300 MPH, it could simply mean the "Corner Speed" is a modest 6 G peak at 330-350...
Besides, it is obvious from the "Society of Experimental Test Pilots", since the Me-109G's elevator, with tail-heavy trim, could beat a WWII-vintage fabric-elevator P-51D Mustang's above 400 MPH, that the Me-109G's "Corner Speed" is more like a P-51D's than an A6M Zero's... Lowest speed to reach 6 Gs for a Me-109G was likely more in the (tested) Mustang neighborhood of 320-350 than the Zero's likely 200-250 MPH range...
But why are you guessing at the corner speed when the report you are quoting provides corner speed values for all of the aircraft tested? For example, if you check FIG 10 in that report it shows the corner speed for the P-51D to be at 237 KIAS. Then if you look at FIG 9 it shows the sustained turn curve for the P-51D and at 1G, the low end is the stall speed, and the high end is the top speed. The low end is at 83 KIAS, which agrees with the 1G power on stall speed quoted in Table II of that report. The top end of that curve is at 238 KIAS, (320mph) at 10,000 feet which is the top speed for their test. They ran the tests at max cruise settings not full military power, which explains why that top speed is lower than you would expect.
We therefore have a corner Velocity at 237 KIAS, and Top Speed of 238 KIAS.
So the report was right to state that the corner velocity was close to top speed, under the conditions used in the test, there was only a 1 KIAS difference.
However, A WWII P-51D was more likely to get closer to 400mph (300 KIAS) at 10,000 feet and at Military Power.
Not only that, but examination of FIG 10 shows that the 1G and 3G stall speeds quoted in the report and the 6G stall speed don't all occur at the maximum angle of attack. The 3G accelerated stall tests were conducted in a descending turn, which explains why that data point appears where it does, but the speed quoted for the 6G corner velocity is too high. If you examine the data points on the stall speed curve, only the first 3 points correspond to the stalling AoA and lift coefficient for the 1G power on stall speed. Those first 3 points go up to 3G, after 3G, the line indicates that the test pilot was pulling to a point below maximum AoA and was therefore at a lower lift coefficient than he was achieving at the lower G values.
It appears that only 3G accelerated stalls were conducted during those tests and there is a clue in the report as to why that might be. It says that the P-51D they tested had "High Stick Forces, Inadequate stall warning, and Vicious departure characteristics" If the pilot were to allow the stall to fully develop, there was a real risk of exceeding structural limits and spinning the aircraft. Power-on spins were prohibited, because they were (and still are) considered very high risk. Recovery could take as much as 10,000 feet, but these tests were carried out at 10,000 feet, so it is clear that while the pilot was reaching 6G, it was not at the 6G accelerated stall speed, because the resulting "vicious departure" may have overloaded the airframe and left the pilot without enough room for recovery. Not to mention that during departure the rapid onset of normal and lateral forces on the pilot would cause blackouts much more rapidly than the slow G onset normally commanded by the pilot in the approach to the stall, thereby increasing the risk. So the pilot who conducted this test was actually reaching 6G at a speed above the 6G accelerated stall speed for safety reasons, and this was therefore not the true corner velocity. This can also be confirmed by inspection of FIG 10 from this report, when superimposed on curves of what would have been aerodynamically possible. In each case, the pilot gave himself a safety margin and did not reach the 6G accelerated stall that is normally quoted as the Corner Velocity, and instead recorded a higher speed where the 6G limit was reached. That margin in the case of the P-51D test was 34 KIAS above the speed at which a departure would have occurred.
Given that the 1G stall speed and the Corner Speed occur at the maximum lift coefficient, the Corner Speed based on the 1G power-on stall speed of 83 KIAS as stated in Table II and Figure 9 of that report would actually result in a Corner Velocity of 203 KIAS, and a top speed of 300 KIAS for a WWII P-51D at 10k and full military power. That tells a very different story.
Yes, the report was right to state that the corner velocity was close to top speed, but only under the conditions used in the test. Those conditions were of a fighter being flown below full power and thus achieving a top speed well below what it was capable of, and being pulled to 6G at a speed above the 6G stall speed for safety reasons, and thus not achieving its true corner velocity at the 6G accelerated stall.
The difference is shown in the EM diagram below:
This diagram shows that given the more appropriate wartime conditions of a P-51D at 10,000ft and full military power, with a stall speed of 83 KIAS, a corresponding corner speed of 203 KIAS and a top speed of 300 KIAS, would place the corner speed much closer to the middle of the envelope and not near the top speed, as shown in the diagram above. Of course those values change with altitude and the weight of the aircraft.
The main conclusions are:
- The report and its conclusions are correct, but only under the stated conditions.
- The corner velocity for most WWII fighters did not occur close to top speed, but at a speed closer to the mid range of the envelope.
- For most WWII fighters the corner velocity does indeed approach and exceed top speed, but only at much higher altitudes.
- Corner speed occurs at the accelerated stall, and not at some point above it where the pilot feels safe. So the 6G corner velocity really does occur at 2.45 times the stall speed, even if a pilot would not actually risk pulling 6G at that speed for fear of departure.
Since you have been quoting this report, I assume you have studied the figures and tables I have been referring to, particularly with respect to the features I have described above. If you need any help understanding the technical aspects of that report, please let me know because I would be happy to clarify any of the issues for you.
Hope that helps…
Badboy
