This article, written by Daniel I. Newman and first published in Vertical Flight Society’s Vertiflite Magazine, addresses the uses of terminology that drift toward becoming routine expressions or idioms — or already are — but are misleading or erroneous.
This instalment proposes a more rigorous approach to the terms for rotary-wing propulsors on vertical lift aircraft.
The term “rotor” is currently being used very loosely for any open (unducted) rotating vertical thrust device. Although most working in the rotorcraft industry would state confidently that “rotors have cyclic and collective pitch control,” there are exceptions that invalidate most definitions.
There does not appear to be any authoritative reference for the terminology for rotating thrust devices for aviation or ‘propulsors’. Historically, this lack of a standard has not been a major issue for aviation, but the spate of new vertical flight configurations makes it necessary to bring consistency to the discussion.
Proper labelling can immediately define the approach, neither misleading nor requiring additional text or queries. There is an appetite in the vertical flight community to have and enforce definitions, so while some may not comply, this article sets out the logic for use of the term ‘rotor’.
This discussion of open/unducted rotary-wing thrust devices differentiates between the terms rotor, fan, propeller and proprotor. All convert rotational energy (typically shaft torque) to fluid momentum as thrust or vice versa (e.g. windmills, gyroplanes and helicopters in autorotation). All use rotating wings or “blades” to generate a pressure change across their disk area for the conversion to/from thrust.
The terminology is not rigorous for fixed devices with a single dedicated function, and gets more confusing when they articulate and perform multiple functions (e.g. tilt for both lift and thrust). The nomenclature, though, should describe the uninstalled device design, and be independent of the purpose and installation to avoid renaming a device when used for differing purposes (despite using the same part number to reduce costs).
So, whether tilting or fixed, for lift or thrust, the term for a device should be constant. And the naming should be based on the hardware and not the design intent. For naming an open rotary-wing device, the differentiating features are: the capability for off-axis control, provisions for edgewise flight and solidity (ratio of total blade area to disk area).
What’s a Rotor?
Historically — before the so-called ‘Electric VTOL Revolution’, when helicopters were the dominant configuration — all literature about vertical flight aircraft used the term “rotor” almost exclusively to refer to all open/unducted, fixed, horizontal, rotary-wing lift devices, such as helicopter and autogyro main rotors.
Conventional helicopter main rotors are used for vertical thrust, control and forward thrust. They use blade pitch-angle control to provide both collective blade variation (to vary thrust) and cyclic blade pitch variation (to control flapping and hub moments). The rotors generally operate at a relatively constant rotation speed (measured in revolutions per minute, rpm), as they are large and thus difficult to accelerate quickly, and have rpm resonance frequencies that must be avoided.
Propellers, on the other hand, are devices that have historically provided forward/horizontal thrust in axial-only flow along the aircraft longitudinal axis. As they are smaller and often directly driven by a power plant, they may use variable rpm for thrust control as an alternative to blade pitch. They only provide axial thrust (along the device rotation axis). Propellers have no accommodations for edgewise flight.
Similarly, “fans” are also axial-only, thrust-only propulsors, differing from propellers in having higher solidity (the ratio of total blade area to disk area) with a cross-over of 0.5 commonly used. Examples are the fans in turbofan engines and lift fans, such as used on the Ryan XV-5 and the Lockheed Martin F-35B.
But, as stated, the orientation and purpose shouldn’t matter, so the axial-only, thrust-only should govern for naming propellers and fans, whether controlling through variable blade pitch or rpm.
Helicopter main “rotors” have typically been thrust and moment devices, using blade pitch variation, fixed on a vertical axis. However, the rotorcraft community and literature also refer to fixed, open anti-torque devices as tail “rotors.” As typically tied to the main rotor drive, they also operate at constant rpm with collective pitch to vary thrust. Unlike propellers and fans, they are configured for high-speed, edgewise flight, often with hinges for blade flapping.
So, what warrants the term “rotor?” Is it the cyclic control? Or the vertical orientation? Based on the helicopter tail “rotor,” it is neither. The distinguishing feature is that tail rotors have blade flapping to facilitate edgewise flight. Otherwise, it would be a tail propeller.
With the success of the tiltrotor, the combination “proprotor” became common parlance for these tilting, open thrust devices. They are sized as a compromise, balancing the high thrust (>1g) for vertical lift and control when pointed up, and the lower thrust (0.2g) for fixed-wing cruise when pointed forward. The “proprotor” for a tiltrotor (or a tail-sitter, etc.) is really just a special case of a “rotor.”
Vertical Lift Propellers Are Not Rotors
Now, the recent outbreak of advanced air mobility (AAM) aircraft configurations with multiple thrusters warrants rigour as the terminology used by the new members of the vertical flight community has not been consistent.
Multiple devices enable attitude control of the vehicle using only independent control of the thrust, avoiding the need for the complexities of cyclic blade control — swashplates, pitch links, scissors, bearings, etc. — and obviating the need for blade flapping.
The lower inertia of these smaller devices can use rpm variation to avoid all rotating blade pitch controls. So, what shall we call the devices on these new multi-thruster aircraft? Shall we use the same name for fixed-pitch, rpm-controlled devices as we do for constant-rpm, pitch-controlled thrusters?
With the development of the quadcopter drone as a consumer product, the terms “quadrotor” and “multi-rotor” have gained popularity in the public eye, and have now begun to bleed into the manned aircraft vernacular, primarily by companies and individuals from outside the traditional rotorcraft industry.
However, the simple, variable-rpm thrusters and the constant-rpm, pitch-controlled thrusters devoid of provisions for edgewise flight are mechanically no different than airplane propellers, so they should be called propellers — regardless of installation.
The term “rotor” should be preserved for devices with cyclic blade control or flapping — or both — for edgewise flight.