Stability and Control characteristics of the Early Mark Spitfires

[Crumpp]

The early mark Spitfire was a excellent fighter.

It was not without its issues like many aircraft. Raw performance numbers for speed and climb do not tell the whole story about any design. The pilots ability to precisely maneuver and use a fighter to the edge of its envelope as an effective gun platform is just as important to the fighting ability of the design as any other performance parameter.

Amoung the Western Front warring powers during World War II, only two nations had measurable and definable stability and control standards. Stability and control was a young science. Airplanes had simply been two slow and light previously. The forces were small enough such that there was little need. The two nation were the United States and Germany.

At the beginning of World War I, the United States found its aviation industry in a dismal state of affairs. Despite being the nation credited with the first manned, powered, controllable, and heavier than air flight, the US aviation industry had only 8 airplanes and 14 trained pilots in 1914. France on the other hand, had 260 aircraft and 171 trained pilots in 1914. This prompted a flurry of US Government action to bolster the aviation industry included the creation of the NACA in 1915. By wars end, the United States produced 7,000 airplanes and 20,000 engines. The surplus was such that post war, for ~$500 dollars cost for the pilot's license a successful solo resulted in the award of a JN-4 Jenny airplane to the new pilot. The emphasis on the aviation industry continued throughout the 1920's and 1930's.

Everyone is familiar I believe with the Nazi parties emphasis on aviation. Like the events in Russia, unfortunate circumstances would align to bolster the introduction of fresh ideas and innovation in the aviation sector.

Let's not be obtuse. None of this is to claim other nations did not progress in aviation or contribute. It is only to lay the historical foundation as to why these were the only Western Nations to adopt stability and control standards.

This thread is going to cover the definable and measure stability and control characteristics of the Spitfire. It is not going to cover opinion outside of stability and control engineers.

What this thread is not going to do:

1. Get into a debate about "easy to fly". It is not definable and has no bearing on the measured facts.

2. It is not going to discuss the sustained level turning ability of the aircraft. That is also measurable and definable. For Example, anyone who is capable of doing the math will see that the Spitfire outturns the Bf-109 is steady state constant altitude turns at low velocity.

Stability and control is the measurable science of flying qualities as they appear to the pilot. This means it is the science of creating a control system that the pilot can safely extract maximum performance of the aircraft. While there are some subjective areas because historical data is incomplete, It is not pilot opinion and while it is branch of aerodynamics, it does not tell us specific performance numbers of a solid body outside of the control system moments.

Moderators, I ask you to keep a close eye on this thread.

Let's lay some of the ground work for the discussion by first looking at what is acceptable and what is not acceptable for longitudinal stability.

The first condition we will discuss is the Longitudinal stability characteristics in an abrupt pull up and release of the stick.

The NACA found the Spitfire to be acceptable in this area. Why discuss it? It is harbringer of things to come in the Spitfires Longitudinal stability and I think the light bulb will come on for most readers as to the importance of stability and control. It will lay the foundation for more informed discussions of other designs included in the game and in the future as the game grows.

These first charts come from the USAAF and USN stability and control standards as adopted during World War II.

Acceptable Longitudinal stability characteristics in an abrupt pull out:



Things to notice....

1. The amount of force required is larger than one would think. A separate NACA study determined that the average pilot can easily apply 80lbs of pull force on the controls. In roughly .5 seconds the pilot is able to apply ~80lbs of force to precisely produce an acceleration of ~3.5 G's. Upon release of the stick, which is the top of the stick force curve at ~80lbs, the acceleration curve is nicely sloped without any wild fluctuations.

So even though the stick is moving around and we see the force move from push to pull, there is no change in the airframes acceleration. The controls are floating after release. The control system friction is sufficiently dampened by the inherent stability of the design.

In other words, the pilot can quickly and precise apply a specific amount of acceleration to the airframe and control it.

The high stick forces act as a solid foundation to resist pilot induced oscillations under aceleration. The pilot does not need to brace himself but can precisely control the aircraft from normal seating.

Now let's look at what is unacceptable longitudinal stability in abrupt pull outs:



First thing to notice is the small amount of stick force that produces a large acceleration. This means the pilot must brace himself if he is going to control the accelerations. It also means it is very easy to apply more accelerations than the airframe can handle. The pilot does not have a solid foundation to resist pilot induced oscillation.

The next thing is the slope of the curve of stick force application to release and the accelerations do not match. This means the acceleration increase is not proportional to the amount of stick travel. As the accelerations increase, less stick travel is required to increase them.

After the stick is released, accelerations continue to increase.

The controls do not float. Instead, they released control continues to produce accelerations as the inherent stability of the design cannot overcome the control system friction. Now that does not mean the control force friction is excessive. It could be that but it can also be the design does not have sufficient dampening on the longitudinal axis.

Our aircraft begins to porpoise and begins another acceleration cycle.

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.

The harbinger's of things to come:

1. The steepness of the curve tells us the pilot is able to very rapidly load the airframe.

2. The light stick forces does not provide a solid foundation for the pilot to resist oscillation.

Next we will get into the unacceptable longitudinal stability characteristics of the design.

to be Con't.......