Stability and Control characteristics of the Early Mark Spitfires

[Crumpp]

NACA shows progressive stick travel and elevator force over lift coefficient and normal load.

I have to figure the slope anyway for the bugtracker. Might as well do it now.

Yellow:

Acceleration over time 3.5G-(-.5G) divided by 4.5s-3.5s = m
m = 4

Stick force over time: (19lbs - 0lbs) divided 5lbs/G all divide by 4.5s-3.5s = m
m = 3.8

The slopes should match and they are close enough. However, our stick force grows at a slower rate than our acceleration. This is the initial input of the pilot.
Now let's see the instability.

Green:
Stick force over time 15lbs-15lbs divided by 5lbs/G all divided by 6.8s-5.5s = m
m = 0
Of course m = 0, our stick is held fixed by the force measurement equipment
Acceleration over time 4.2G-3.2G divided by 6.8s-5.5s = m
m = .76

So, while our stick remains fixed, the aircraft continues to accelerate on its own. As the nature of instability, there is no correlation stick force input and acceleration.

Now, our pilot in this case only input force to reach 3.5G. In a stable airplane, we should see the aircraft dampen all subsequent accelerations which means the aircraft would not exceed 3.5G without control input.
In this case, the instability or divergent oscillation a 4.2G acceleration with stick fixed slightly below the stick force required to produce a 3.5G acceleration.