Basic Helicopter Aerodynamics

John M. Seddon, Simon Newman, Peter Belobaba, Jonathan Cooper, Allan Seabridge, Peter Belobaba, Jonathan Cooper, Allan Seabridge

• Publisher: Wiley
• Year: 2011
• Print ISBN: 9780470665015
• eBook ISBN: 9781119972723
• Edition: 3
• Topic: Technology & Engineering
• Subtopic: Aeronautic & Astronautic Engineering
• Index: Technology & Engineering

Basic Helicopter Aerodynamics is widely appreciated as an easily accessible, rounded introduction to the first principles of the aerodynamics of helicopter flight. Simon Newman has brought this third edition completely up to date with a full new set of illustrations and imagery. An accompanying website www.wiley.com/go/seddon contains all the calculation files used in the book, problems, solutions, PPT slides and supporting MATLAB® code.

Simon Newman addresses the unique considerations applicable to rotor UAVs and MAVs, and coverage of blade dynamics is expanded to include both flapping, lagging and ground resonance. New material is included on blade tip design, flow characteristics surrounding the rotor in forward flight, tail rotors, brown-out, blade sailing and shipborne operations.

Concentrating on the well-known Sikorsky configuration of single main rotor with tail rotor, early chapters deal with the aerodynamics of the rotor in hover, vertical flight, forward flight and climb. Analysis of these motions is developed to the stage of obtaining the principal results for thrust, power and associated quantities. Later chapters turn to the characteristics of the overall helicopter, its performance, stability and control, and the important field of aerodynamic research is discussed, with some reference also to aerodynamic design practice.

This introductory level treatment to the aerodynamics of helicopter flight will appeal to aircraft design engineers and undergraduate and graduate students in aircraft design, as well as practising engineers looking for an introduction to or refresher course on the subject.

Chapter 1

Introduction

1.1 Looking Back

1.1.1 Early Years

The first foray into rotary-winged flight occurred around 400 BC with a toy known as the Chinese top. It was constructed with a central shaft to which was attached wings consisting of feathers or flat blades inclined to the rotation plane normal to the shaft. This was spun between the hands and by generating thrust flew for a short period of time. Jumping forwards to AD 1325, a Flemish manuscript contained the first known illustration of a helicopter rotor which was operated by the pulling of a string.

Just over 150 years later we encounter probably one of the most influential milestones in the work of Leonardo da Vinci. His famous design of a rotary-wing vehicle, see Figure 1.1, forms an ideal illustration of the origin of the term ‘helicopter’. It is commonly considered to be the combination of the words ‘helix’ and ‘pteron’ giving the concept of the ‘helical wing’.

Figure 1.1 Leonardo da Vinci's helicopter concept

In 1784, Launoy and Bienvenu built a mechanical model with two rotors. It was effectively the Chinese top but extended to a pair of rotors on the same axle but rotating in opposite directions. It was powered by a leaf spring and strings. It is similar to the coaxial configuration of more modern times.

In 1810, Sir George Cayley wrote an aeronautical paper preparing the ground for future helicopter development. He designed an air vehicle consisting of two pairs of contra-rotating rotors on either side of a canvas-covered central fuselage structure to generate lifting thrust. The rotor blades were inclined sectors with no noticeable aerofoil. Forward propulsion was provided by a pair of pusher propellers mounted at the rear. This design was the forerunner of many features of modern rotary-winged vehicles. The absence of a sufficiently powerful engine prevented the project from leaving the drawing board – let alone the ground.

The year 1878 saw Forlanini build a model powered by steam. It flew to a height of 40 feet for a period of 20 seconds. It was impossible to achieve this by carrying a steam engine – because of the weight – so Forlanini devised a method whereby a sphere was positioned underneath the model and provided a storage vessel for the steam. This was then slowly tapped to obtain the required propulsive torque.

The lack of suitable power also hampered the work of Thomas Alva Edison. He initially conducted tests on rotors to examine the thrust v power performance. Finding this to be unfavourable he then focused on the engine and wrote of his attempts to use gun cotton in the cylinder of an engine fired with a spark. He obtained good results but injured himself and one of his colleagues by singeing! Edison was not daunted and later work provided estimates of the required power/weight ratio that would allow a flyable helicopter to be achieved.

The absence of a powerful enough engine dogged the early 1900s; however, in September 1907 Louis Breguet achieved a milestone of the first man lifted with a tethered rotary-winged aircraft. The altitude was a mere 2 ft, but a 1 minute flight was the result. However, stability was obtained by four assistants actually holding the aircraft. He later built two other air vehicles but suffered from lack of sufficient power and lack of control both in flight and in landing.

November 1907 saw Paul Cornu achieving what is often considered to be the first ‘true flight’ of a helicopter. The vehicle rose to an altitude varying between 1 and 5 feet for a period of 20 seconds. It was fitted with two rotors of approximately 20 feet diameter mounted in a tandem layout – see Figure 1.2. Forward propulsion was provided by vanes positioned underneath the rotor discs which deflected the rotor downwash backwards and downwards. The efforts of Breguet and Cornu highlighted the importance of stability and control in flight.

Figure 1.2 Cornu helicopter

In 1909, a pioneering figure appeared who was to become very influential in the history of the helicopter. He was Igor Sikorsky, who initially built two helicopters which proved unsuccessful. This was the short-term result, but it taught the Russian about what to do and what not to do. In the interim period he moved to fixed-wing aircraft but 20 years later he was to bring the lessons of the past into focus and become a dominant figure in helicopter technology.

The year of 1912 saw the Danish aviator Ellehammer use cyclic pitch, successfully giving stable and controllable flight for the first time. The vehicle was lifted by two contra-rotating rotors on the same axle. The construction of both was of a ring to which was fitted the blades. The lower rotor was fitted with fabric to increase the lift force. Forward flight was aided by a conventional propeller.

1.1.2 First World War Era

In the First World War, Petroczy built a vertically lifting machine hoping to replace captive observation balloons, which were very vulnerable. The technical and experimental development was conducted by the illustrious Theodore von Karman. It consisted of two contra-rotating rotors positioned within a framework which also contained three radial engines and the undercarriage was pressurized bags placed in the centre and at the end of three legs. An observation basket was placed above the rotors. Wilhelm Zurovec is often neglected in any description of this air vehicle except that the designation is PKZ (Petroczy Karman Zurovec).

In May 1918, the PKZ2 (see Figure 1.3), Hungary's first military helicopter, flew tethered to a height of 50 m with new 31 hp Le Rhone engines.

Figure 1.3 PKZ 2

1.1.3 Inter-war Years

In 1921 George de Bothezat directed the first US Army programme into helicopters. After working in secrecy at Dayton, the de Bothezat helicopter (see Figure 1.4), made its first flight in December 1922.

Figure 1.4 The de Bothezat Quadrotor

The rotor layout was to have one rotor on each of four arms with the pilot positioned in the centre. Numerous flights were made in 1923 carrying up to four people. The US Army did not pursue the design but acknowledged its contribution to helicopter technology generally.

In the 1920s Emile Berliner developed a helicopter with the usual, at that time, layout of contra-rotating rotors with vertical and horizontal vanes positioned under the rotor aiming, unsuccessfully, to give control. In 1922, the design moved into placing rotors on wing tips fitted to what was essentially a fixed-wing fuselage based on a Nieuport 23. Pitch control was achieved by a small propeller located just forward of the fin. The problems faced were those of not sufficient rotor size to generate sufficient thrust and blockage of the rotor downwash by the wing surfaces.

The French engineer Etienne Oehmichen began helicopter experiments by using balloons to assist the rotors in lifting the aircraft. In 1922, he was able to discard the balloon and use a four-rotor configuration. There were also five small, horizontal, variable pitch propellers to control the aircraft's attitude in flight. Many flights were made lasting several minutes and in April 1923 the record distance of 358 m was achieved. In April 1924 Oehmichen raised the straight line distance record to 525 m only to have it broken the following day by the Spaniard, Marquis Pateras Pescara. In the following May he succeeded in flying a 1 km closed circuit course winning a 90 000 franc prize from the Service Technique de L'Aeronautique.

Pescara flew for 736 m with a design of four biplane rotor blades on each of two rotors. These were placed on the same vertical axle and rotated in opposite directions. The pilot had control over the blade pitch and this helicopter was important in this respect as it heralded the use of autorotation for safe landing in the event of engine failure. Pescara attempted the closed course flight but was unsuccessful and it was left to Oehmichen to achieve this the following year.

In England Louis Brennan was developing his helicopter at the Royal Aircraft Establishment in Farnborough – see Figure 1.5. It was a two-bladed rotor powered by propellers fitted to the blade tips.

Figure 1.5 Brennan helicopter

Thus external propulsion did not need torque reaction of a second rotor. Tethered flights inside a hangar were undertaken in 1924 with free flights beginning in 1925. They were halted in 1926 after an accident.

In 1925, the Dutch engineer Baumhauer developed a single two-bladed main rotor helicopter which had the beginnings of blade control via a swash plate system. It also had torque control provided by a separate vertical rotor, which was the forerunner of the modern day tail rotor. The flight of the helicopter was not altogether successful and chains were hung from the corners of the airframe to improve the stability. While ultimately unsuccessful, this design provided important technological pointers for the future.

The Hungarian engineer Oscar von Asboth developed a sequence of designs which comprised two contra-rotating main rotors and control was achieved using vanes placed in the downwash. The fourth variant was the AH4 and in 1930 it climbed to a height of 31 m, flying a distance of 2800 m. Also in 1930 the Italian Co...

Table of contents

• Cover
• Aerospace Series List
• Title Page
• Copyright
• Dedication
• About the Authors
• Series Preface
• Preface to First Edition
• Preface to Second Edition
• Preface to Third Edition
• Notation
• Units
• Abbreviations
• Chapter 1: Introduction
• Chapter 2: Rotor in Vertical Flight: Momentum Theory and Wake Analysis
• Chapter 3: Rotor in Vertical Flight: Blade Element Theory
• Chapter 4: Rotor Mechanisms for Forward Flight
• Chapter 5: Rotor Aerodynamics in Forward Flight
• Chapter 6: Aerodynamic Design
• Chapter 7: Performance
• Chapter 8: Trim, Stability and Control
• Chapter 9: A Personal Look at the Future
• Appendix: Performance and Mission Calculation
• Index