Rotorheads and Freedom of Movement
The purpose of the rotorhead is to rotate the blades and control the tilting
direction of the rotordisc, which is the way in which the direction of thrust is managed.
It turns out that, to function properly, the rotorhead must cope with rotorblades that want to move and vibrate, meaning that it has to support blade movements
in several different directions! For this reason, understanding the design of a rotorhead
begins with an understanding of what
kind of blade movements must be supported. This section will summarize these for you. Note that these explanations are largely
contained in the section:
Aerodynamic Rotor Theory.
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The blades must rotate around a shaft (applying torque which leads to a circular
motion).
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Blades have to feather (pitch) around their feathering shaft, in order to implement
cyclic and collective control.
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When the rotordisc tilts
(as a result of cyclic control), the blades have to flap
up and down.
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The asymmetry of the lift of a
rotordisc in forward flight requires blades that can flap up
and down.
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By applying cyclic pitch, a tilted rotordisc, in combination with blade coning,
leads to the Hookes' joint effect: the blades must lead and lag in the plane of rotation.
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The flapping movement of the blades creates a change in their Centre
of Gravity (C of G). The law of the Preservation of Angular Momentum dictates
that the blade must lead and lag (the second reason for the lead / lag of rotor blades). An equivalent
explanation is provided by looking at the Coliorus forces at work in a rotating frame
of reference.
These are the most important reasons why blades must both flap up and down and lead
and lag (also called blade dragging).
As might be expected, there are several ways of designing a rotorhead. The required freedom of blade movement is realized by using hinges, and /or by applying flexible materials at strategic positions in the
rotorhead. Moreover, the blade can be designed
to provide some flexibility in itself (for example, in the root section).
A rotorhead which doesn’t support the required blade movements has to cope with
the
huge forces that work upon it, and the results thereof would be catastrophic mechanical
failure. Hinges and flexible materials not only give the blades their required freedom of movement, but also prevent the blade forces (more accurately,
moments) from being transmitted
to the rotorhead.
Next Topic > Rotorhead Types
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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)
- Well structured text that covers many technical aspects. It starts with an introduction to helicopters, followed by a treatment of
the technical background needed when studying them. Thereafter, dynamics, rotor systems, engines and transmission are explained in detail.
The book concludes with a section on performance and other types of rotorcraft. Its main asset is that the text is technically and
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)
- If you are wondering how a helicopter flies, and really want to comprehend the process, you have no choice but to delve into
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.
- 320 pages
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