Aviation fascinates me. I've never had the opportunity to "pull a stick" and savor the essence of biting into the blankets of air with the right mix of throttle, science and adventure. To fly you need airspeed. And money. In my lifetime, I do hope I get a chance to experience those things.
It is truly remarkable that between the Wright brother's first flight in 1903 and the Luna 2 landing on the moon in 1959, there was essentially a span of just 60 years. How in the world did we learn so quickly how to construct an aircraft, lay down requirements on what constitutes a 'good flying machine', to go from simple piston engines to gas turbines to rocket engines and from sandy shores of North Carolina to the crusty surface of the moon, in just 60 years?
The rate of technological progress in aviation is something to be amazed at. Suppose we took airspeed as an attribute with which to study the rate of upward progress in this field; airspeed, because it is a genuine parameter that man felt strongly about improving. If we looked at the change of airspeed on the y axis with respect to time on the x axis, what would the slope look like?
Dr. John H. Lienhard, Professor Emeritus of Mechanical Engineering and History at the University of Houston, presented a slope of airspeed improvements in his paper 'Some Ideas About Growth and Quality in Technology'. In the plot that he presents, you can see that starting with a speed of a mere 8.5 mph in 1884, air travel had reached almost 5000 mph by 1975!
If 'Q' is the speed at a later time 't' and 'Qo' is the speed the technology started out from at 'to', and if L is the approximate lifetime of a technologist (an aeronautical engineer), then a constant n-folding rate of the progess can be defined as :
When computed for airspeed in this particular example, assuming L to be 30 years, the value of "n" comes out to be 10.1! Compare this to the slope for ground vehicle speed, which started and ended on different points within the bounds of the graph, indicating that ground travel had its own inherent set of limitations which determined the rate of increase. In the case of the automobile, "n" comes out to approximately 2.
What a difference!
Early aircrafts were backyard inventions of amateur technologists. When the need arose for air travel as well as air superiority in the great wars that were to come, groups of professionals took a hard look at aircraft design.
In the 1920's and 1930's. most aircraft designs had lateral symmetry, a straight wing and horizontal and vertical tail at the rear. People within the aeronautics field grappled with the problem of coming up with a proper set of design specifications and limits from "subjective" flying qualities as experienced by pilots.
Suppose one pilot said about a plane 'ah, the ailerons, rudder and elevators responded very well in climbing and banking' or that it's straight line or directional stability was "good", what constituted good? And how do you translate that to hard numbers that can be shared within the design community for the evolution in understanding and future design?
This learning process on what constituted good in stability and flying qualities was no easy matter, infact it took about 25-30 years to go from what was essentially an ill-defined problem of subjective pilot sensations to well defined objective requirements that could be validated and put into aeronautical practice.
A number of different elements within the aeronautical community bit into the problem. Some groups looked into what constituted stability criterion for an aircraft to have desirable flying qualities. Other groups wondered how the aircraft should be proportioned to fly well.
Early analytical thinkers developed the notion of negative pitching moment slope as a guide for longitudinal stability.This said that as an external disturbance forces a plane from one angle of attack to a higher angle of attack, the response of the plane should be such that the net moment at the center of gravity of the plane effects nose down orientation. This was 'inherently' stable.
There were disagreements as to the applicability of these rules. Others would define short and long mode oscillations for dynamic stability and stick force vs elevator angle as other control variables.
Concerted efforts went into defining what level of stability was required for a particular type of airplane from studying pilot reactions on instrumented planes. World War 1 would provide intensive experience as a basis for learning.
Data was gathered, put together and discussed and after much deliberations on the part of these engineers, someone came up with a idea that stick force vs g force yielded a practically significant parameter to base a general design guidelines upon.
This individual, Dr. Robert Gilruth of the NACA, would go on to state that in order to avoid "heavy controls" and pilot fatigue, a limit of 6 pounds of stick force per g was to be set for fighter aircraft. For bombers and transport airplanes which didn't need as much maneuverability, a value of 50 pounds of stick force could be set. The other overriding requirement was that an attainment of the maximum number of g's the structure was designed to withstand should call for a steady pull of not less than 30 pounds.
The aeronautics community coherently agreed to this. Existing specifications from a stability and control standpoint were rewritten to accept this new maneuverability criterion. It was a watershed moment in aeronautics to correlate human factors and aircraft design and codify practical limits into the body of aeronautic practice.
The epistomiological undertakings of the early aircraft design pioneers is beautifully summed up in the beginning chapeters of "What Engineers Know and How They Know It" by Walter Vincenti. I couldn't ever describe this detailed and complex process as well as Mr. Vincenti did.
I do want to point out that the interesting aspect of learning a new system and codifying how it should be made and what characteristics can be considered favorable is a 7 step process as defined by Mr. Vincenti.
He wrote (Page 102, Chapter 3) :
"In the detailed history [of flying quality specifications] we can distinguish at least seven interactive elements, all essential to the story : together they show the complex epistomological structure of the learning process. ....
1. Familiarization with vehicle and recognition of problem : The challenge and definition of the problem needs to become commonplace before a community can attack it.
2. Identification of basic variables and derivation of analytical concepts and criteria : The interaction of the elements that comes out of the intellectual attack of the problem lay down the predominance of criteria and concepts.
3. Development of instruments and piloting techniques for measurements in flight :
4. Growth and refinement of pilot opinion regarding desirable flying qualities : The growth of opinion occured cumulatively and in part subconsciously over 3-4 decades in the general flying community. Through these years, pilots became confident enough to define what they "liked" and what they were apprehensive about their machines.
5. Combination of partial results from 2, 3 and 4 into deliberate practical scheme for flying quality research.
6. Measurement of relevant flight characteristics for a cross section of aircraft : This became the de facto scheme for organized research into the problem. Fights were instrumented, test protocols were laid out and evaluations done of the data. Synthesis of these elements went into the body of data based knowledge of the problem.
7. Assessment of results and data on flight characteristics in light of pilot opinion to arrive at general specifications.
In the dark ages of our technological past, pioneers stepped forward to explore the unknown, define our problems and make the impossible possible. I truly marvel at those times when people didn't know what to expect of an airplane when they sat down to design it. And in 60 years hence, we were on the moon. We stand on the shoulders of giants, really.
Dr. Robert Gilruth, who is widely considered the "father" of America's human spaceflight program, died on August 17, 2000. His 1943 report titled "Requirements on the Satisfactory Flying Qualities of Airplanes" can now be read online from NACA's database.