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Not all helicopters use hydraulic systems, particularly the (very)
light ones. In this configuration, all controls are manpowered.
When helicopters increase in size, however, the forces needed to control the rotorhead become too
great for a pilot to operate on his own. In these circumstances, an intermediate system
provides additional control power since the pilot controlled forces are not
fed directly into the rotorhead control system. One of two types of hydraulic system
are used: fully powered and power assisted. When using the former (large helicopters), considerable measures have to be taken to ensure that the system is extremely reliable
as a loss of power over the controls will almost certainly lead to disaster. Accordingly,
we often see completely duplicated hydraulic systems in large helicopters.
In smaller crafts, the controls are powered assisted, so, in the event of
a failure, they can
still be operated by the pilot, albeit requiring
a great deal of hard work.
Hydraulic systems also increase pilot comfort, since the 'feel' of a hydraulically controlled helicopter is less stressful. Vibrations and other mechanical forces
are less pronounced at the pilot’s controls than when they are mechanically
linked to the rotor head.
The workings of a hydraulically powered
system is analogous to a servo system. As in servo (or closed loop control) systems,
it all starts with the difference between the desired and the actual output position.
The desired position is the control input (operated by the pilot). When the input and output positions are in the desired place, no action is needed; it is said that the control error is zero.
However, when the input is moved and the output remains
in the old position, a difference between the desired and actual positions arises.
This error is measured by a hydraulic error valve, and is then transmitted to the actuator valve.
This is done with oil under high pressure, which leads to an amplified error. This, in turn,
leads to a much stronger control than input force, meaning
that the output moves to the desired position. When the input and output positions
meet again, the error is minimized, as is the correcting output force. Note that the amplification of the
error is known as gain in controlled loop servo systems. Hydraulic actuators tend
to be self damping, which in most cases leads to a stable control system without the need for any
other measure to be taken.
Next topic > Digital System
Cyclic & Collective
- The title of this book leads me to wonder what more it will teach me in addition to its content about these two, most frequently used, helicopter input controls. As it turns out, the answer is: a lot more. Of course, all of the obligatory subjects like basic physics, rotor aerodynamics and helicopter performance are dealt with as well, as are piston engine and basic helicopter manoeuvres. Yet the scope of this book is actually much wider than one might initially think.
Firstly, it is divided into a 'beginners’ and an 'advanced’ section. This means that the book can treat more complex concepts in depth, even though the focus in the first section is directed more towards newcomers to the field.
Secondly, subjects like turbine engines, multi-engine helicopters and autopilots are also examined. This is particularly helpful, since these topics are not usually covered in the majority of helicopter books aimed at this target audience.
Thirdly, the book deals with many things that you will not normally find in a text book: helicopter related experiences and a great deal of interesting detail. This is the sort of information that can only be provided if you have flown a lot of different helicopters and have been working in this industry for some time. What’s more, this tone is amplified by the consistently narrative style of the book.
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Art of the Helicopter (Hardback)
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theoretically very accurate, and rather than mathematics, its focus is always on enabling the reader to achieve an understanding of
helicopters from a technical or engineering point of view. The more technically orientated reader will love this work.
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Principles of Helicopter Flight (Paperback)
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aerodynamics. This means not only understanding which forces play a role and the laws of physics, but also being able to put it all
together and apply your knowledge to a rotating system consisting of a number of rotor blades. This is a demanding task and requires
some very hard work. It is, undoubtedly, worth the effort though, and will help you to become a better pilot. There are many books out
there to help you with this task of exploring the principles of helicopter flight, but they tend to fall into two camps:
populist and taking a rather simplistic approach, or highly technical and assuming the reader has a degree in mathematics.
This book is different, because it clearly explains the principles of flight in a step by step way that is easy for most
readers to follow. Further benefits are that a lot of attention is paid to flight manoeuvres and operations, and every chapter
concludes with questions as a study aid.
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