MOSQ can you confirm the configuration for your test with the Ki-84 is 25% fuel, full ammo on the deck?
Yes, at 500 ft.
First Notch- 250 mph
Second Notch- 225 mph
Third Notch- 200 mph
Fourth Notch- 175 mph
Fifth Notch- 150 mph
The F4U handbook says to never drop flaps faster than 200 Knots which equals 230.15 MPH. At that speed they "blow up" which we call auto-retract.
Comparing the in game to the F4U handbook:
IG: 250, Handbook 230 (200 Knots)
IG: 225, Handbook 230 (200 Knots)
IG: 200, Handbook 195 (170 knots)
IG: 175, Handbook: 166 (145 knots)
IG: 150, Handbook: 149 (130 knots)
IG weight by E6B, full fuel, full ammo, no external weapons vs Handbook
F4U1-C IG: 12,471 lbs, Handbook 12,063 lbs
F4U1-D IG: 12,176 lbs, Handbook 12,028 lbs.
This is assuming the handbook wasn't superseded later by an updated version, which it may have been.