Stability and Control characteristics of the Early Mark Spitfires

[DC338]

Originally Posted by Crumpp

ok

Now let's look at the measured results.



Here we see in a rapid left turn performed at 223 mph the test pilot is unable to hold constant acceleration on the airframe. Very small variations in stick movement and stick force changes of 1-3lbs results in large fluctuations in acceleration.

Taking two point we can compare the slope of the curves of stick input to acceleration over time.

For the intital pull up:

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.

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.

Now I understand that Figure 15 does hint at what you are getting at yet I see no such problem in figures 16, 17 & 18 of the same report and you don't seem to analyse them in your argument? Odd as they are essentially they are same test as figure 15 but in the opposite direction (16) and at higher speeds (17 & 1. Looks like a relatively constant G was held throughout by the pilot. Or am I missing something?

The report alludes to "careful" flying. Does that mean "not careful" flying in the other charts.

Next let's look at the pilots ability to control the accelerations in the pre-stall buffet.



Here we see the pilot was able to load the airframe to 5G's in 1 second to reach the pre-stall buffet 3 times. The smooth positive sloped portion of the curve represents the aircraft flying while accelerations are increasing. The top of the acceleration curve represents the pre-stall buffet. The bottom of the curve represents the stall point.

The amount of stick travel as measured by the NACA was not acceptable.

Yet it does not say that is was dangerous flying quality. It just did not meet the Requirements laid out in report 755. It was not built to that standard so should it surprise that it doesn't meet all of them?

Now on the Spit V they did use a inertia weight to combat over sensitive elevators on that Mark. Why did they not demand a retro fit of inertia weights to the MK I & II that would have been in the OTU squadrons at the time if it was such a problem?