Preliminaries

Garbage in — garbage out. So goes the adage. Some of this website may seem pedantic. That is deliberate in the interest of thoroughness. Without exploring and questioning the underlying assumptions and the unspoken premises of well-meant statements, one cannot discover their truth. Although well-intentioned, many statements of ‘fact’ are not well-formed and may yield, under scrutiny, a certain incompleteness or niavete. In the case of propeller development, much of what has gone before has been subject to a certain peculiarity of groundbreaking research in a new field of study. I shall call this distorting factor “concurrency” for want of a better word. Concurrency is a phenomenon of newness, wherein certain information of a broadly applicable nature has not had a chance to mature, to soak into the collective wisdom-pool shared by researchers as is commonly found in more established lines of inquiry. The development of high-speed airfoils is a good example of the problem of concurrency manifested in industry. The Davis airfoil eventually used on the B-25(?) bomber exhibited less drag than contemporary airfoils. There was no explanation for it, only experimental evidence. The design procedure used to create the airfoil had no basis in physical science, yet the result was remarkably and demonstrably better than existing contemporaries. Meanwhile, separate researchers working on the problem of drag reduction had found that the “direct” method of airfoil design could no longer point the way to better airfoils. The direct method consists of drawing an airfoil shape then experimentally and/or mathematically determining its aerodynamic performance. The mathematics had been honed sufficiently to predict the experimental results with good accuracy, the time was ripe to “turn the machine around” and generate airfoil shapes from the desired aerodynamic performance — to “invert” the mathematical process. The direct method had shown the way forward to improved airfoils by demonstrating that airfoils with a more uniform pressure distribution exhibited less drag. More correctly stated, the hypothesis had been formed as a result of searching for some common marker that would show the way forward, and the apparent uniformity of the pressure distribution had been identified as a candidate parameter. What was needed was an “indirect” method of airfoil design. Of course, there are many “indirect” methods by which to derive an airfoil shape, and the odd, purely geometric construction method that Davis had employed certainly was “indirect”. Then again, tracing the top view of a trout could be construed as an indirect method, via Nature’s evolutionary process to create a more efficient swimming machine. More specifically than just an “indirect” method, an “inverse” method was required, one in which the pressure distribution could be specified and the resultant airfoil shape thereby defined. So, the airplane design team at Consolidated grappled with the question of why the Davis airfoil performed so well and whether to commit to using it in the face of its uncertain heritage, while the airfoil design community at the N.A.C.A. wrestled with the mathematics to generate airfoils from what they were sure were the controlling parameters, namely, a uniform, gently progressive pressure distribution along the chord that would promote a laminar boundary layer flow on a greater proportion of the airfoil’s leading edge surface — precisely what the Davis airfoil was demonstrating in the wind tunnel. But neither set of researchers knew well enough of the others’ work to make the leap to collaboration. In retrospect, the Davis airfoil clearly exhibits a distinguishing characteristic of all modern low-drag airfoils: its point of maximum thickness is farther aft along its chord than that of it’s contemporaries. Today we recognize the high-drag airfoils with forward thickness as “turbulent” airfoils and the low-drag airfoils with maximum thickness further aft are collectively called “laminar” airfoils. Any student of airplane design will naturally gravitate toward the latter with good reason. But, at the time, a leap of faith was required to adopt the unpedigreed Davis airfoil in the face of so much known data on other airfoils, despite the “known” characteristics of low-drag airfoils being generated in the N.A.C.A., sometimes even in the same labs where the Davis airfoil was sent for testing. The history of propeller development is fraught with concurrency issues, many of which, unfortunately, have never been addressed. The airfoil problem, despite the knowledge gleaned in the seventy intervening years since the Davis airfoil story related above, continues to plague the propeller design world. Widely-held beliefs about airfoil performance in propeller applications are carried like the elixirs and potions of magicians, their mere existence in the arsenals of the established propeller shops serving as ample “proof” that they are correct and incontrovertible. So, nearly a century after they were first penned and with no more analytical basis than the pleasing of the eye of their respective designers, the Clark ‘Y’ and the RAF 6 airfoils continue to dominate the literature and the lore of propeller design. Endure the pedantry (if you can) and join in this exploration of the lost or unwritten assumptions and sometimes-unfounded premises on which propeller lore is based. Along the way, one may find a more enlightened and useful understanding of propeller design.