When flying a helicopter, the pilot is continuously controlling 2 important parameters:
the amount of thrust, which is achieved by changing the pitch of all of the blades
simultaneously, also known as collective pitch control; and secondly, the direction
in which the rotordisc is tilted. This is achieved
by applying cyclic pitch, which influences the blade's angular position within
its circle of rotation. We will look at how these pitch changing mechanisms work with a rotorhead in this section.
Both collective and cyclic control are achieved with the swashplate system, which
is comprised of 2 rotating discs, and transfers the control inputs into the blade angles. The control inputs are connected to the lower disc, while the upper disc
is connected through the pitch links and horns to the rotating blades. The lower
disc is also called the stationary disc, because it does not rotate. The upper disc,
on the other hand, rotates along with the blades. The 2 discs are connected via a bearing. In
this way, control input is transferred into a change in blade pitch (angle).
The lower swashplate can also tilt in any direction by means of a spherical bearing.
A pair of jointed scissor links is included in the system. One of them is used to prevent
the stationary disc from rotating with the upper one, while the other connects
the upper disc to the rotating shaft. In this way, it (the upper disc) will rotate
at the same angular speed as the blades. Scissor links
are used because the swashplate assembly must be able to slide along the shaft.
Collective & Cyclic Pitch
The collective control works by sliding the swashplate up and down. This movement
will change the pitch of all of the blades simultaneously and by the same amount.
Cyclic control tilts the swashplate, thereby changing the pitch of each blade continuously
through each blade revolution. The control inputs are connected via a mechanical mixer
to the swashplate. This mixer allows the swashplate to rise and fall (collective
control) whilst tilting with the cyclic input.
Cyclic pitch in more detail
Some hard thinking is required to fully understand how cyclic control functions.
It can be very instructive to look at a real swashplate when someone is moving the
collective and the cyclic controls, and there are some very good animations of this
on YouTube. For example, take a look at the S-61 Sea King Rotor Head Animation,
in which the working of the scissor links (they are red) becomes pretty
clear. The video also shows how moving the swashplate up and
down changes collective
pitch. The real strength of this animation comes from its
demonstration of how cyclic pitch works, using
different camera positions to illustrate
the effect. This first camera looks at the swashplate
system from a static position,
whereas the second travels with a blade!
Note that the viewpoint is now from a rotating frame of reference, meaning that the
blade and control links and rods (yellow) appear to be static (non rotating). It
can now easily be seen that, with each revolution,
the blade pitch changes up and down. Note that with this camera position, the lower disc suddenly appears to rotate in the
opposite direction, because it is the upper
disc which is travelling. The third camera position also rotates with a blade, but it is now looking at the blade tip. This animation should show you how the swashplate functions
and effects blade pitch.
Cyclic pitch leads to a blade that is travelling up and down with each revolution. So, in
a rotorsystem which rotates at 450 RPM, each blade is driven by a cyclic pitch
with a frequency
of 7.5 Hz, when it is applied. Most swashplate systems are designed to drive the cyclic blade angle with a sinusoidal function.
Effect of Cyclic Pitch on Rotor Disc Attitude
When the swash plate tilts in a certain direction, cyclic pitch is introduced. The question
now is: how does tilting the swashplate in a particular direction influence rotor disc attitude
(and its thrust vector)? We will, therefore, now examine the
circumstances in which the swashplate is tilted forwards, and will assume that there
is a counter clockwise rotating system (viewed
from above). To assist our line of reasoning, we will first focus on 4 strategic points
on the circle of rotation, and will examine the blade angles at these points. The points
(A to D) are each 90 degrees apart, and the figure below sets out their
locations. To understand what the blade angle is at these points, there is something else we need
to know: the configuration of the control rod and links. In this example, we will assume that these are attached to the blades’ advancing site.
In these circumstances, the blade angles will be:
- A : Negative (max amplitude)
- B : zero
- C : Positive (max amplitude)
- D : zero
Note that the maximum amplitude is related to the angle at which the swashplate
The figure shows the quadrants in which the blade angle (pitch) is positive and negative, and we will now look at the behaviour of the blades in each of them. If the angle is positive,
it will rise. If the angle is negative, it will fall. Following
this line of reasoning, it is clear that the blades will be at their highest position at point D, because they have travelled for 180 degrees in an upwards direction.
blades will be at their lowest position at B. The altitude of the disc is, thus, sidewards, meaning that this helicopter would move to the left if the swashplate is tilted forwards. This probably isn’t what you expected! Clearly, there is a 90 degree difference in the tilting direction between the swashplate and the rotordisc. This is known as the phase lag.
Note that there is another way of explaining this phenomenon, namely in terms of
Both have the same result, and so we won’t consider this
If it is not compensated for, the phase lag between the swashplate and rotordisc would
make the life of the pilot very difficult. The way to overcome this problem is to
connect the control inputs to the swashplate 90 degrees in advance of the
blade to which they relate.
Next topic > Feathering and Blade Flapping
Cyclic & Collective
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