When flying Warbirds and pulling G's sometimes the screen will darken, and go black if enough G's are pulled. What is unknown is a) whether the G levels that this occurs at varies for different planes, and b) if the levels don't vary, what are they? This study attempts to determine the G level when the screen starts to darke, and the G level when the screen is fully black.
Robey Price (another WBer) and I exchanged email for a bit with some preliminary results (it was he who showed me how to determine G from the stall speed and the blackout speed). Turns out that if you take the square of the stall speed (at which wings have maximum Angle-of-Attack at 1g of lift), and divide the result by the square of another speed at the same angle-of-attack, you can determine the G force on the plane. I used this principal to determine G forces for both the point at which the screen starts to darken, and the point at which the screen is completely black.
My method was to load up 10% fuel, and start at 5,000ft. I would dive until at 350mph or more, level out, and execute a max-g left turn. I would then record the speed at which the screen became visible (taking care not to pull so many G's that the screen wouldn't clear if I let go of the stick). As speed slowed down, I then recorded the speed at which the screen was completely normal (not darkened). I repeated this test two more times picked the two closest to each other and averaged the results. For stall speed, I flew at 1,000ft and maintained level flight The point at which the plane could no longer stay flying level was called the stall speed and recorded. All tests were done with no flaps, and with the F6F 1024x768 art.
The results:
| Plane | Stall Speed | Blackout Ceases | Effective G | Screen clear | Effective G |
| F6F | 95mph | 280mph | 8.7g | 220mph | 5.3g |
| F4F | 85mph | 225mph | 7.0g | 190mph | 5.0g |
| FM2 | 85mph | 230mph | 7.3g | 195mph | 5.2g |
| F4U | 100mph | 270mph | 7.3g | 220mph | 4.8g |
| A6M2 | 65mph | 200mph | 9.5g | 170mph | 6.8g |
| A6M3 | 70mph | 205mph | 8.6g | 175mph | 6.3g |
| A6M5 | 70mph | 205mph | 8.6g | 175mph | 6.3g |
| Ki43 | 70mph | 205mph | 8.6g | 175mph | 6.3g |
| Ki84 | 90mph | 270mph | 9.0g | 220mph | 6.0g |
| Bf109F4 | 90mph | 265mph | 8.7g | 215mph | 5.7g |
| Bf109G6 | 95mph | 270mph | 8.1g | 220mph | 5.4g |
| Bf109G6R6 | 95mph | 275mph | 8.4g | 230mph | 5.9g |
| Bf109K4 | 100mph | 280mph | 7.8g | 240mph | 5.8g |
| Fw190A4 | 105mph | 305mph | 8.4g | 265mph | 6.3g |
| Fw190A8 | 105mph | 310mph | 8.7g | 270mph | 6.6g |
| Fw190D9 | 110mph | 325mph | 8.7g | 285mph | 6.7g |
| SpitV | 85mph | 265mph | 9.7g | 220mph | 6.7g |
| SpitIX | 90mph | 270mph | 9.0g | 230mph | 6.5g |
| P38F | 90mph | 260mph | 8.3g | 215mph | 5.7g |
| P38J | 95mph | 265mph | 7.8g | 220mph | 5.4g |
| P38L | 95mph | 270mph | 8.1g | 225mph | 5.6g |
| P39D | 105mph | 280mph | 7.1g | 240mph | 5.2g |
| P40E | 90mph | 260mph | 8.3g | 220mph | 6.0g |
| P47D | 110mph | 320mph | 8.5g | 270mph | 6.0g |
| P51D | 95mph | 270mph | 8.1g | 230mph | 5.9g |
| Average | 91mph | 263mph | 8.4g | 221mph | 6.0g |
As far as accuracy, stall speed is probably +/- 10mph, blackout cease speed +/-20mph, and Screen Clear speed +/-15mph. That gives a G error for Blackout Ceases of about 2g or so, and maybe 1.5g for the Screen Clear speed. Thus having such a wide range of planes with which to test allows a much more accurate reading. Unfortunately, this sort of error makes determining if different planes have different G limits almost impossible. Also, since I had to see the screen as clear or no longer completely black, all the speeds should be biased low, thus the effective G's should be higher. Based on the data I've collected, my money's on the G limit for visibility being 9g and if the screen darkens you are pulling more than 6 g's. Keep this in mind next time you go BnZing :)
On a related note, corner velocity (the lowest speed that maximum structural/human G tolerance allows), is around 400mph (9g)for an F16 (one of the most manoeverable fighters today). This would produce a stall speed of around 135mph. The average 9g speed for WB's planes is around 260mph. How times have changed! Btw, assuming a wing efficiency identical to a SpitV (28lb/sq ft wing loading), the F16 would have an effective wing loading of about 70lb/sq ft. I wonder how far off this number is :)