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The Stability and control characteristics of the Early Mark Spitfires:
Now let's look at the Spitfire in an abrupt pull out as measured by the NACA.
First thing to notice is the stick forces. There are light but acceptable in abrupt pull outs. While very steep, the slope of the curve matches our acceleration curve and the controls float without overcoming the inherent stability of the design.
The steepness of the curve tells us the pilot is able to very rapidly load the airframe. In fact, the NACA had to make allowance in their stick fixed measurements to prevent damage to the aircraft from acceleration because of the rapid onset the controls allowed.
However, if we look at the acceleration curve we see an abrupt change and not the desirable smooth curve. This points to the stability characteristics contributing to the rapid fluctuations in acceleration that the aircraft exhibits under other conditions.
Next we will get into the unacceptable longitudinal stability characteristics of the design.
We will look at a condition of flight essential to a dogfighter. The ability to make abrupt turns.
The pilot must be able to precisely control the amount of acceleration he loads on the aircraft. All aircraft performance depends on velocity. In order to get maximum performance out of the aircraft above maneuvering speed, Va, he needs to be able to make a 6 G turn and not exceed that load factor to prevent damage to the airframe. Below Va, the pilot needs to control the acceleration so that he does not stall the aircraft making the abrupt maneuver as well being able to maintain a maximum performance turn.
Doing that in an early Mark Spitfire was difficult and something only a skillful pilot could perform.
First the NACA report. Abrupt 180 degree turns were conducted at various entry speeds to gauge the level of control the pilot had in maintaining steady accelerations. The turns were also done to the stall point in order to gauge the behavior and amount of control.
"In turns at speeds high enough to prevent reaching maximum lift co-efficient" means turns above Va.
"By careful flying" a pilot can hold a steady acceleration. That agrees with the Operating Notes warning for the pilot to brace himself against the cockpit to get better control when making turns.
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.
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.
Next let's look at the opinion of Stability and Control Engineers on the Early Mark Spitfires.
There is no doubt that the Air Ministry was aware of the longitudinal instability of the early mark Spitfires.
Just some of the many references to the Longitudinal instability found in all of the early Mark Spitfires.
Spitfire Mk I Operating Notes, July 1940:
