Imagine you are the pilot of a light aircraft on a cross-country VFR flight. As you tick-off each landmark along your route, you watch as it disappears below the leading edge of the wing to reappear a few seconds later behind the trailing edge. Your thoughts turn to your ground studies and knowledge of basic classical aerodynamics. You understand how the airflow separates at the wing’s leading edge and flows over and under the wing to rejoin at the trailing edge. You also understand the forces of lift and drag produced by the aerodynamic reaction of the aircraft’s wing. Now, as you look ahead for the next approaching landmark, your view is through the almost invisible blur of the propeller disc. Because the prop is an almost continuous blur, we tend to ignore its presence and take it for granted.
Therefore, what about the propeller’s forces of thrust and torque, or the prop’s stress, tip speed, power absorption and it’s efficiency of operation, etc? Do you ever think about them? Where did propellers originate? They have been around a great deal longer than you may realise! How do you operate a constant speed or feathering propeller? How do props do their job of producing thrust? That is what this book is all about – propeller aerodynamics, covering the history of the propeller’s development, its operation and of course, the aerodynamics associated to the propeller
Most text books on general aerodynamics give only a brief mention to propeller aerodynamics. It has been this author’s intent to present this material in an easy to understand manner, suitable for study by the low time private pilot. Nevertheless, that’s not to say the more knowledgeable reader won’t benefit from this book. It has been assumed the reader has an understanding of basic aerodynamics to at least the private pilot’s level. The text on propeller aerodynamics will therefore, compliment his/her knowledge on this fascinating subject.
We start with a look at the propeller’s history of development and followed by the different aerodynamic theories put forward by William Rankine, Robert and William Froude and Stefan Drzeweicke, which in this book, concentrates mainly on the blade element theory and briefly, on the axial momentum theory. The text continues with different aspects of propeller pitch and the factors that affect the propeller’s efficiency. This is followed by the forces acting on the propeller during different operating conditions and is followed by a brief look at turboprops, Propulsor and Propfans. The book concludes with a chapter on propeller operation. A few simple formulas have been included along with several diagrams to help clarify the text. Note, all diagrams have been drawn free hand by this author and then computer scanned and do not represent any on particular propeller or airplane.
In writing this book, a choice had to be made on the use of either Imperial or Metric units. With my home country of New Zealand and many other overseas countries turning to the Metric system more and more, this was at first deemed to be the most appropriate system to use. However, in the aviation industry, Imperial units are still commonly used. For example propeller size and manifold pressure are still measured in inches, and piston-engine power mostly in brake horsepower, true air speed in Knots, prop thrust in pounds, and prop disc area in square feet. One notable exception being temperature measured in the Metric system of degrees Celsius. Young pilots may well be familiar with the Metric system while older generation pilots (myself included) will be more familiar with Imperial units, which I have chosen to use in this work with Metric equivalents in brackets.
A total of sixty-one photographs are included from this author’s collection with the exception of the MD-80 Propfan test plane photograph, which was freely donated by Hamilton
Standard of Connecticut, USA, to whom I am truly thankful. My thanks also go out to those pilots and ground engineers who were most helpful in supplying useful information and answering my numerous questions on aircraft propellers and arranging access to aircraft parked in the restricted operational areas of the airport I visited.
Wellington, New Zealand.
Front cover photo: Lockheed P3-C Orion propeller.
In the Beginning…
Before the advent of jet propulsion, a piston-engine driving a propeller to provide the necessary thrust or forward motion powered all aircraft. To this end, the propeller has always been an accepted part of an airplane. However, how many people realize that propellers were around long before the first airplane flew? So, where did propellers originate? Some authorities claim the propeller originated in China several centuries ago as a descendant from the windmill. In Europe, the windmill can be traced as far back as the 13th century AD, but windmills built in China before this time were of a different type, their axles being vertical It is believed the windmills of China have no apparent relationship with European windmills, however, from the principle of the windmill the idea of the propeller was born
The word ‘airscrew’ was introduced to aviation to distinguish between the aeronautical and marine type propellers (which were usually referred to as ‘screws’). The word airscrew was more commonly used in Europe than in the USA. Between the 1920s and 1950s, the name airscrew usually referred to a ‘tractor’ propeller (a propeller in front of the engine as opposed to pusher propeller behind the engine). It is now a virtually redundant term and has been replaced by the word propeller or prop for short, although the word airscrew is quite often used by writers of early aeronautical history when writing about propellers of that era. The term propeller was first used to describe any mechanical device used to propel
The term ‘airscrew’ is synonymous with early aircraft such as this example of a Sopwith F.1. Camel Scout.
a vehicle and it came into aviation terminology circa 1850 to have the same meaning as the word airscrew.