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Control Power

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When the rotordisc changes attitude, the fuselage will follow this to some degree. The extent to which this happens is known as control power. Control power depends upon the rotorhead (type), and works in 2 ways, both of which are reliant on transferring moments from the rotordisc to the shaft. The first transfers the moments that are involved with blade flapping to the shaft. This is only the case with hingeless ('rigid') rotorheads. The second mechanism transfers moments because of a tilted rotordisc. In these circumstances, the disc imposes a moment over the arm that exists if the flapping hinge offset is not at zero. So, this mechanism is not effective with a zero offset rotorhead. Note that in such a case, the first mechanism is also not in effect.

When rotorhead types are related to control power, we can state that a zero offset head doesn't possess any control power, whereas an articulated head possesses an intermediate level thereof, and a hingeless system has strong control power.

The effect of control power is a fuselage that (tries to) follow the disc attitude. For example, a forward tilted disc will lead to a helicopter with a fuselage that is also tilted forwards. A consequence of this rotordisc following behaviour is that the rotors stay clear of the fuselage, and the blades always have sufficient room to flap up and down. Therefore, a rotorhead with a lot of control power can be equipped with a short shaft (see picture, the Westland Lynx on the left). A zero offset rotorhead (for example, a teetering rotorhead), which possesses no control power whatsoever, will need a long shaft to provide sufficient blade room for the blades to flap down (the red Robinson R44).

Helicopter with short shaft
Helicopter with long shaft

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.
  • 536 pages

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.
  • 416 pages

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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1  David Wagner

Saturday, June 11, 2016 4:03 PM

I'm putting together a presentation on control power, and how it effects taxiing of wheel equipped helicopters. I have been able to find little to nothing (to support what I've had a good understanding of for years now) until I found this page! Thanks. Do you have anything more on the subject, or could you point me in the right direction to find additional info on control power?
Thanks again,
Dave Wagner
2  helistartgz

Sunday, June 19, 2016 12:12 PM

Hi David,

In general, I would recommend "The Art of Helicopter" as a good starting point (available at Amamzon).

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