Introduction

A propeller is a surface rotated by a powerplant operating in an atmosphere (or fluid) in proximity to a fuselage or hull for the intent of producing thrust to propel the fuselage or hull through the atmosphere. Thus, there are apparently four variables involved: The propeller, the atmosphere, the fuselage or hull, and the powerplant that provides the rotative force.

Certain metrics apply by which the comparative usefulness of propellers can be determined. In comparisons, the usual relation is the “efficiency,” though sometimes the raw thrust or thrust-horsepower becomes a more significant figure of merit. Sometimes, the durabilty or robustness of a design wins over efficiency; Like a drag-racer’s engine, what good is a highly efficient propeller if it self-destructs before the race is won?

The fundamentals of all this were laid out between 1868 and 1929, with some minor tweaks in the late 1940’s and 1950’s. This site attempts to explore and, at times, explain about those four variables, measures of efficiency and usefulness, and the fundamentals outlined in the writings of Helmholtz, Prandtl, Betz, Goldstein, Theodorsen, Lerbs, Kerwin, et al.

The first variable to examine is the atmosphere. How boring! The atmosphere is well studied and documented. One would think that its effect on propeller performance and design would be straight-forward. But, unfortunately, the effect of the atmosphere on engine performance is not well documented, leading to difficulties in estimating the actual rotative power delivered to the propeller, compromising propeller design methods and the accuracy of performance predictions. “Garbage in – garbage out.” Without a good estimate of power we lose the ability to estimate efficiency of existing propellers and to predict performance of proposed propeller designs. Without understanding the atmosphere, we may not be able to accurately estimate engine power.

The next variable to examine, then, is the engine. Besides the effects of the atmosphere, there are important considerations about the engine itself that bear scrutiny. A two-stroke-per-cycle (“two-stroke”) engine not only makes power differently than a four-stroke-per-cycle (“four-stroke”) engine, it also has different operating parameters and limits, and very different torque and power characteristics throughout its operating range. Further, differences in the way the engine an installed may result in severely limited power output or altered operating limitations, such as heat management issues or power losses due to a non-optimum exhaust system.

The fuselage or hull has a tremendous, yet routinely overlooked influence on the fluid flow in the immediate vicinity of the propeller. This influence is discounted by most and outright summarily dismissed as “too difficult” by some who should know better. Yet others acknowledge it, but only look at its most superficial aspects. We’ll examine this “complex” topic and find that there are, indeed, ways to address it that are not “too difficult” or obscure.

The final variable, the propeller itself, is our main object of investigation. This deceptively simple device has mysteries that confound and defy most analysts. The list of great men given above — Helmholtz, Prandtl, Betz, Goldstein, Theodorsen, Lerbs, Kerwin, etc. — though not all-inclusive, is still short for good reason. The understanding that these men gleaned came at great price of personal effort, dedication, and sacrifice. To read their works is to understand the level of genius that has been applied to the problem of the propeller; to understand at the level they understood is surely difficult, for none of them fully grasped all aspects of the problem with which they grappled. Instead, they each contributed increments to the collective knowledge wrought. Yet, the final product is conceptually simple, almost to the point of being magical. Make no mistake, though, that the application of that concept is easy — furtunately we live in an age where computers are ubiquitous. We will make ample use thereof in investigating the propeller.

Lastly, we cannot investigate design methods without having some idea of what our design objectives might be. The measure of our success may be difined as an efficiency, or it may be defined as a comparative value of some absolute measurement, or an objective or subjective compromise between several such variables. No one can fault the choice of an automobile on the basis of its frugality, yet everyone “knows” that the red one will be faster. Thus, the objective merits of the engineering task must be tempered with some acknowledgement of certain subjective merits. Likewise, some objectively “superior” designs may be found wanting in particular attributes such as little details that make it manifestly impossible to actually build such a propeller:

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Such “trivial” details aside, the practical application of various propeller design methods is not intractably arcane. Several viable methods have been presented over the years, requiring only some modest effort to generate reasonable results. Nothing to be found on this journey is “new” or “novel.” The references are all available, and ideas presented are taken from the literature, not invented here. Yet, it is hoped that readers will find newness here. As mentioned above, so much of propeller theory was not fully grasped by any individual researcher; to tie together prior works may constitute a fresh insight even absent fresh discovery. The unfortunate fact is that much of currently accepted propeller wisdom is only repetition of what once was new discovery, but is, today, only an intermediate partial truth. Such is the nature of progress.

Now let us move on to investigate tools, conditions, environment, and technique in the Propeller Design Workshop.