[Crumpp]

Originally Posted by JtD

Stick fixed is just one version of static stability, you're keeping the elevator angle constant over a speed range and check how the aircraft responds. In case of the Spitfire, no trim change occurred, so the plane would keep the same AoA over the entire speed range at the same elevator deflection. That's neutral stability.

First of all, stick fixed is not a version of static stability. Stick fixed is a control term and just means you are using the maneuver point that the pilot, controls, and the mechanical linkage has friction and mass. Static stability is the aircrafts initial reaction to displacement.

It has absolutely nothing to do with keeping the elevator constant. It is about the oscillations.

The other version NACA was looking at is stick force stability, if you want to call it that, where you are keeping the stick force constant through the speed range and check how the aircraft responds. In case of the Spitfire, stick force increased with the speed, which in turn leads to smaller elevator deflections which means some sort of positive stability.

They are looking for a stable gradiant. They are not keeping the force constant, they are looking for a slope as it moves away from trim speed. They are looking for a smooth increase in stick forces. The stick forces will change as they are based on velocity.

The Spitfire was dynamically stable.

The longitudinal dynamic stability (Long Period Oscillations) was neutral or negative as recorded by the NACA.
All for longitudinal stability.