Coming to Terms: Rotor Redux
By Daniel I. Newman
Vertiflite, Jan/Feb 2022
This series addresses the uses of terminology that drift toward becoming routine expressions or idioms — or already are — but are misleading or erroneous. This installment proposes a more rigorous approach to the terms for rotary-wing propulsors on vertical lift aircraft.
Last issue’s “Coming to Terms” column on the definition of the word “rotor” generated a significant amount of discussion. The article was featured on the VFS eVTOL.news website and e-newsletter, with several thoughtful comments posted the first day, followed by emails to the author and Vertiflite.
In a detailed comment, Prof. Manfred Hajek concurs that “rotor” be preserved for devices with flapping installed to respond to edgewise flight, as Dr. Anubhav Datta’s companion piece explains. And the professor points out that this was “the rotorcraft world for decades until” the recent explosion of interest in electric vertical takeoff and landing (eVTOL) aircraft.
Prof. Hajek then warns those not already aware that the asymmetric flow of flying a rigid propeller edgewise will incur oscillating bending moments and forces at the blade roots. Designing the “propeller” blades and the remainder of the system to handle these loads incurs weight — the price to pay to avoid the complexity of blade flap. It is a classic designer’s choice between including adequate structure to restrict motion and handle the resulting stress, strain and transmitted loads/vibration, or articulating to allow motion and avoid the loads. So, the reduction in maintenance of a simple flexure relative to a hinge or mechanism isn’t without consequences. Yet again in engineering as in life, TANSTAAFL (“there ain’t no such thing as a free lunch”) applies. It is just another trade best performed by accounting for all penalties along with the benefits.
Another contributor to the comments, eVTOL.news directory webmaster Mike Hampson, shared that he personally has described “rotors” by including their installation in the reference, as in “main rotor” and “tail rotor.” He went on to ask if there should be definitions for “tilt-rotor” or “tilt-prop.” The answer is “no” — according to the premise of the original column that the rotary-wing thrust device terminology should uniquely define the component, regardless of its installation on the aircraft. So, Hampson’s qualifiers of “main,” “tail” and “tilting” are not necessary to define the devices installed (propeller or rotor) and serve only to describe their installation on the aircraft configuration.
Hampson went on to ask about Joby Aviation’s differing installation of propellers on its S4 eVTOL passenger aircraft. Specifically, with two propellers tilting using a linkage and four tilting with their motor nacelles, he asks if there are “proper names for each of these types of tilting propellers.” And per the premise, there are: they are all propellers, as they do not have flapping provisions. The installation isn’t relevant to their nomenclature. The S4’s multi-propeller configuration has all the propellers tilt (a “vectored thrust” configuration), while other designs only tilt some of their propellers (e.g. “lift plus cruise”). Regardless, they are all propellers. The discussion of whether they tilt and how is at the aircraft configuration level. Stand by for a future “Coming to Terms” column on configuration terminology.
Hampson then suggests that descriptions of these components and aircraft be first in engineering terms and then again in layman’s terms. However, the premise here is that one set of clear and concise terms for all can provide clarity with brevity, something we all can appreciate.
In a direct email, Alan Lawless posed another great question: “What provision does a rigid rotor aircraft have to accommodate edgewise flow?” For rigid rotors with a root-end flexure that is softer than the blade itself is in the flapping direction, it is as easy as this is a dedicated flapping mechanism. It is not the mechanical device (a hinge) of an articulated rotor, but it is installed to accommodate flapping. There is an effective hinge offset and it is inboard of the blade.
However, it does get messy for hingeless rotors that do not have a soft inboard section or a specific feature for flapping, such as on the original Airbus BO105 (and all its successors) and the Boeing A160 Hummingbird. Here, the blade exhibits all flapping along its span. It is credible to claim that these have no specific flap “provisions” as the response is blade deformation as on a propeller. However, the blade design accounts for flapping and the associated loads, and has an effective hinge far inboard — e.g. nominally a 10% span — as opposed to an articulated rotor with hinges between 3% and 6% span. As of yet, no one has identified a specific definition. However, it may be necessary to specify a flapping stiffness cutoff for rotors — or a hinge offset cutoff — just as there is a cutoff for solidity (0.5) to distinguish between propellers and fans. But in short, a rotor is designed to accommodate bending in the flapwise direction whereas propellers must avoid it.
And finally, with this opportunity to follow-up the original column, it is appropriate to address a mild conflict that exists between the last line in the main body of the original column and the last line in Datta’s addendum. Some may have noticed this conflict, but no comments have specifically mentioned this discrepancy. The main column stated that the term “rotor” should be preserved for devices with cyclic blade control or flapping for edgewise flight, or both. In contrast, the addendum stated “a rotor is loosely an airscrew that generates lift, and strictly an airscrew that has flapping motion.” The similarity is the flapping to address edgewise flow, but the difference is whether a rotor generates lift or thrust. For a conventional helicopter main rotor or a tilting rotor, there is no difference, as they both provide lift. However, for a tail rotor this difference is not insignificant as its thrust is not strictly “lift,” as described in Datta’s addendum. But hopefully this nuance is forgivable.