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.