In case some folks can't be bothered to read the NACA tests, I'll post a part to put the "unacceptable longitudinal dynamic stability" into proper perspective.
Quote:
STALLING CHARACTERISTICS IN MANOEUVRES
The stall warning possessed by the Spitfire was especially beneficial in allowing the Pilot to reach maximum lift coefficients in accelerated maneuvers. Because of the neutral static stability of this airplane, the pilot obtained no indication of the lift coefficient from the motion of the control stick, nevertheless, he was able to pull rapidly to maximum lift coefficient in a turn without danger of inadvertent stalling....
With gun ports closed, the pilot was able to pull the stick far back without losing control or interrupting the turn. The airplane tended to pitch down when stalled and to recover by itself if the stick were not pulled back. It would be possible for a pilot pursuing an enemy in a turn to bring his sights on him momentarily by pitching beyond the stall without fear of rolling instability.
With gun ports open, a right roll occurred if more than about 10° up elevator were applied. This reaction caused the airplane to roll out of a left run and into a right turn. ... In spite of the lateral instability that occurred in turns with gun ports open, the pilot was able to approach maximum lift coefficient closely because of the desirable stall warning. The maximum lift coefficient reached in turns from level flight with flaps up was 1.22. The airplane could be flown beyond the stall at even lower lift coefficients.
CONCLUSIONS
The Supermarine Spitfire airplane possessed stalling chareteristics essentially in compliance with the requirements for satisfactory stalling characteristics given in reference 1. These characteristics may be summarised as follows:
1. Warning of the comlete stall was provided by the occurrence of buffeting that set in at speeds several miles per hour above the minimum speed and by the rearward movement that could be made with the stick after the start of the stall flow breakdown without causing violent motions of the airplane.
2. Stall recovery could be made by application of down elevator, although the recovery from a roll was somewhat slower than has been measured on some previously tested airplanes.
3. The airplane exhibited no dangerous ground-looping tendencies in landing. Tail-first landings could be readily made without the occurrence of either lateral or directional instability due to stalling.
The airplane possessed some unusual characteristics in stalls that are not required in reference 1. The motion beyond the stall was not violent and an unusual amount of lateral control was available in many flight conditions, even when full up elevator was applied. The good stalling characteristics allowed the airplane to be pulled rapidly to maximum lift coefficient in accelerated maneuvers in spite of its neutral static longitudinal
stability.
|
Eventually, the longitudinal stability was not unacceptable. The Spitfire was accepted into service with about 20+ military air forces, was built in 20000 examples, and is still being flown today. Unacceptable longitudinal stability would mean acceptance into 0 air forces, and a production of a handful of examples, and none would be cleared for flying today.
However, it is true that the Spitfire did not meet all the requirements set by NACA in "Requirements for Satisfactory Flying Qualities of Airplanes". Other planes that failed to meet all the requirements were for instance the P-39 or the XP-51.