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I understand the traditional reasons for cyclical control (with swash plates) on helicopters; ie, avoid gyro effect, shaft can be fixed. However, if the power plant was a modern EV motor, and collective pitch was achieved by servo motors connected to the blades, could a design that tilts the whole rotor have some advantages?

I am thinking of this design for light units, or drone types which would carry less than 1,500 kg of payload.

Would the removal of the swash plate and flapping blades provide tangible benefit; ie, reduced engineering complexity and manufacturing cost.

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  • $\begingroup$ Do you really mean 1,500 kg payload? That is quite a good sized helicopter. And how do you plan to engineer the electrical power connections to the motors? $\endgroup$ Jul 12, 2021 at 9:12
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    $\begingroup$ Related: why-does-a-helicopter-need-cyclic-control-rather-than-just-tilting-the-whole-mai $\endgroup$
    – Koyovis
    Jul 12, 2021 at 10:22
  • $\begingroup$ In the fully articulated rotor, the effect of the conventional cyclic control via swash plate is to tilt the entire rotor disk, as defined by the tip-path plane. $\endgroup$
    – xxavier
    Jul 12, 2021 at 11:55

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Tilting the rotor would not work!

The cyclic (and you could replace the swashplate with electric motors, but it would still be cyclic controls, just less reliable) works by shifting the centre of lift over the area covered by the rotor, and can shift it by significant fraction of the blade length. This is necessary to compensate for the effect of forward flight, which shifts the centre of lift significantly towards the advancing blade (which creates pitch up moment).

By tilting the shaft you could not create nearly as large moments as you can with cyclic. The centre of gravity isn't that far from the hinge, so you'd need larger angle than the geometry allows. With cyclic, the large moment is either created during rolling or pitching or for maintaining speed and the fuselage follows, so the tilt isn't large.

You would also get the full consequences of the gyroscopic effect. The helicopter would tilt to one side to accelerate and to the other side to decelerate! That is because the tilt just creates a moment on the rotor, but the rotor is still a gyroscope that still tilts with (a bit less then!) a 90° lag. The swash-plate deals with this simply by mounting the riders corresponding angle ahead of the blades, with the ultimate effect that the rotor is always trying to align itself with the swashplate (minus the effect of speed).

In fact, many helicopters (e.g. the UH-1 Iroquois (Huey) / Bell 204/205) have teetering rotor and in these the fuselage can't apply any moment on the rotor at all, because (as the name suggests) the pair of blades is allowed to freely teeter on the hub, and the fuselage is just freely hanging below it. Here the property that the rotor is aligning itself to the swashplate is crucial for preventing it from striking either the fuselage or the mechanical stops and getting damaged—when the pilot pushes the cyclic, the rotor starts to tilt, but as it aligns itself with the swashplate, the pitching moment will reduce until the fuselage catches up and the swashplate tilts with it some more.

In fact in all helicopters the rotor is free to tilt in response to the aerodynamic forces and the fuselage is mostly just hanging below it. When the blades have offset flapping hinges (fully articulated rotor) or rely on flex (hingeless rotor) some moment is transferred directly to the shaft, but it is always relatively small compared to just pulling the hub to the appropriate side. Beside dealing with the lift asymmetry due to forward speed, this free flapping of the blades is also necessary to deal with turbulence and similar disturbances.

So no, you can't tilt the rotor mechanically by adjusting the angle to the shaft, you have to do it aerodynamically, and the swash plate is the mechanically simplest mean to achieve that. Separate electric motors would just add more points of possible catastrophic failure.

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You are pretty much describing a fly-by-wire rotor control system.

You could certainly do that by using a slip-ring electrical connection at the base of the hub through which you power and control electrically powered servos that regulate blade pitch, with an elastomeric hub allowing for lead/lag and flapping as with most modern helicopter rotors.

With computers monitoring blade position, and controlling blade angle to control the disc plane's tilt in response to commands, there's no reason you can't achieve an "electronic swashplate" through sensors, servos, and software.

A much cruder version of such a thing was done with the Curtiss Electric Propeller in the 30s (an electric actuator driving collective blade angle through simple analog electrical commands in that case, powered through slip rings).

The challenge is certifying such a system in a helicopter that carries people. The amount of multi-channel passive redundancy required, in the control, sensing and servo elements, to reduce the risk of disaster to an acceptable level, makes it too expensive for the benefit achieved in a normal helicopter up to this point.

For a drone however, you don't have that as much, other than the risk of it falling on someone's head, so I would not be surprised at all to hear that somebody is developing such a system for an un-manned helicopter today, and systems like that in regular helicopters will certainly be commonplace is 20 or 30 years.

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  • $\begingroup$ Thanks John. I think you're describing where servos undertake the role of the cyclical pitch operation from a swashplate. I am thinking more about only varying pitch for collective lift. So for forward movement, the whole motor/shaft/hub are tilted in direction of flight. $\endgroup$ Jul 13, 2021 at 5:51
  • $\begingroup$ Even easier. You only need a single servo motor in the hub to operate the blades in pitch collectively. You're describing the Curtiss Electric Propeller from 1940, but with electronic digital control instead of analog. $\endgroup$
    – John K
    Jul 13, 2021 at 16:03
  • $\begingroup$ You need servo for each blade, or a swash plate. Otherwise as soon as you start moving, you run into trouble with lift differential between advancing and retreating blades... $\endgroup$
    – Jpe61
    Jul 13, 2021 at 20:29
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Tilting the whole rotor assembly, even with a small helicopter, the gyroscopic forces would be notable. In addition to that, the maneuverability would be quite limited, since achieving enough force to rotate and roll the helicopter, the rotor shaft would have to tilt quite a bit.

And: in flight you would still need to compensate for advancing and retreating blades lift effect, I do not know for sure, but I think you would run into all kinds of problems there if you omitted the swash plate arrangement and implemented rotor shaft tilt to handle this specific problem (if I understood your concept correctly).

If you plan on using single big rotor with servo motors at the root of each blade, why not simply (lol) program a control logic that takes on the task of the swash plate assembly and adjusts every blade independently. If reliable enough servo motors are available (I imagine they are), this arrangement would be quite simple and if more than three blades were used, also reliable and redundant. The program driving the servos might be a nightmare to develope though 😃

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  • $\begingroup$ Thanks for the answer. Would the gyroscopic forces affect arrangements like the Osprey tilt rotor? I know the blades are as big as a helicopter, but they seem significant. $\endgroup$ Jul 13, 2021 at 5:49
  • $\begingroup$ Yes @SteveClancy , but Osprey has two counter rotating rotors, so gyroscopic forces pretty much cancel out. $\endgroup$
    – Jpe61
    Jul 13, 2021 at 20:25

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