Operating Conditions

Understanding the propeller in a clinical, theoretical, laboratory type of “understanding” may be sufficient for some students of the art, but the design of useful, efficient, practical propellers demands more. To go beyond the pristine, idealized, perfectly helical propeller operating in pure, uniform flow demands examination of the messiness of real-world operating conditions. Immediately, the problematic operating conditions separate themselves into two varieties. The first is that associated with the production of power and mechanical transmission of that power into the propeller. The second is the non-uniformity of the inflow to the whirling propeller. This latter is not as intractable as it may at first seem, whereas the former can be more formidable and insidious than any theory can anticipate. The flow problem can be adequately and quite accurately approximated for tractor airplanes and, with appropriate software, pusher airplane flow fields are only slightly more difficult to model. The highly non-uniform inflow into the pusher propellers under the sterns of ships we leave to the marine engineering community with this thought: You’ve correctly formulated the problem (Lerbs, Kerwin, et. seq.) but you’ve failed to correctly formulate the solution to that problem. Back to aerial propellers: Inflow modeling has been largely neglected in the development of propeller theories. Those theories that have included inflow modeling have not incorporated it as a core requisite but have, instead, “tacked” it on as an afterthought. The present work will not only include inflow modeling, but embrace it as fundamental and foundational to all that follows. This is not a bold, revolutionary development. Rather, it is the logical synthesis of all that has come before. The work of giants is not to be sneered at or discarded, but admired, corrected in its shortcomings, and completed. Concurrency can blind even the most astute researcher to that which hindsight illuminates even for dullards; the resuting synthesis should be apparent to even casual followers of the art. Likewise, the use of theoretical methods has historically led to solutions that only treat the “potential flow” that is so convenient to mathematics but which loses even the most fundamental connection to real flow, namely, the part of parasite drag called skin friction drag. This disparity is so profound that it prompted D’Alembert’s so called “paradox” which eventually led to the near-abandonment of theoretical methods in favor of wind-tunnel research and “theoretical” development through dimensional anaysis reduction alone. The disdain for theory, especially in American circles, became so pervasive that it persists to this day, despite D’Almbert’s challenge being answered over a century ago. That said, this work will also include the skin friction drag as fundamental to the development of a sound computational method for propeller design. Along the way, the computational solutions developed will be shown to be consistent with both theoretical predecessors and experimental evidence developed in wind tunnels. (draft)