# Category Propellor Aerodynamics

## Aerodynamic Theories

Several theories on propeller aerodynamics have been put forward by different authors. The theories include the vortex, general momentum, axial momentum and the blade element theory. The blade element is the most widely accepted theory, along with parts of the axial momentum theory, leaving the vortex and general momentum theories to fall by the wayside.

In 1865, the Scottish engineer and scientist William George Rankine (1820-1872) founded the axial momentum theory while working on the theory of ship’s propellers. At a later date further work and development on the axial momentum theory, was covered by Robert Edmund Froude (1846­1924) also an engineer. The blade element theory was first introduced by William Froude (1810-1879) the father of R. E. Froude, an engineer and naval architect in 1878, when he also was working on ship’s propeller theories. Note the theory of ship and aircraft propellers is virtually the same, because air at subsonic speed behaves very similar to flowing water.

Stefan Drzeweicke (1844-1938) further developed the blade element theory from 1892 onwards and he has been credited with the majority of the research work. The axial momentum theory, also known as the Rankine-Froude theory after its two authors, deals with the energy change given to the air mass after it passes through the propeller disc. It also includes the effect of the rotational propwash, the friction drag of the propeller blades and the loss of energy in the propwash caused by the interference of the engine nacelle or the fuselage, amongst other factors The blade element theory deals with the forces acting on the propeller as it moves through the air at a uniform velocity. It also includes the blade’s shape and number of blades and assumes the propeller blade to be made from an infinite number of blade elements, hence the name blade element theory. Theodore Theodorsen (1897-1978) of NACA also performed aircraft propeller research, circa 1930s. The diagrams in this book represent the blade element theory

After the Wright brothers’ initial success, further testing and advances in prop design by other engineers led to the first generation of propellers. Lucian Chauviere (1876-1966), a French aeronautical engineer, is noted for introducing his Integrale aircraft propellers of advanced design, (he introduced the laminated propeller mentioned above) the forerunner of all propellers to follow. Louise Bleriot (1872-1936) had a Chauviere propeller mounted on his Bleriot XI monoplane for his cross-channel flight in 1909. During the First World War, Chauviere’s company produced about 25% of the propellers manufactured for allied aircraft.

## The Wright Brothers

It is now a well-known fact the Wright brothers are credited with being the first to achieve sustained, powered flight in an airplane using their Wright Flyer I, on 17 December 1903. The flight took place at Kittyhawk, North Carolina, USA, with Orville Wright (1871-1948) at the controls. Several other pilots from different parts of the world also claim to be the first to fly, before the Wright brother’s famous first flight. However, the Wright’s first flight has long been recognized as the first successful ‘controlled and sustained flight’ in heavier than air aircraft, which the other pilots failed to achieve The Wright’s success was achieved through having a suitable engine and propeller combination. Both were of course, designed and made by them with the assistance of their mechanic Charles Edward Taylor (1868-1956. Their propeller design calculations were remarkably accurate, with the prop twist being correct for the speed ratio of their props. When they designed their props, they had to rely on their own calculations because not many (if any at all) calculations were available on aircraft propellers back in those days of early flight. As we shall see shortly, various theories were available for ship’s propellers which have since been applied to aircraft propellers. Each prop at 350 RPM produced sixty – seven pounds of thrust. The propellers were hand carved from three laminations of spruce wood and painted with aluminium to prevent their work from being copied by any competitors. It is not known which of the two Wright brothers carved the props; the design was never patented. Because the props were contra-rotating, it was not a simple matter of making the second prop a copy of the first one; it had to be a mirror image due to rotating in the opposite direction. Their mechanic Charles Taylor built the 12 horsepower (9 kW) four – cylinder engine, which turned by train drive the two 8.5 feet (2.59 m) propellers. One chain was twisted into a figure of eight loop to drive the second prop in the opposite direction to the first. Tests conducted at the NASA Langley Full Scale Wind Tunnel, revealed an efficiency of 81.5% on the Wrights’ 1911 propellers, a remarkable achievement for that era. Their efforts were a great contribution to the development of aircraft and propellers that have since followed.

Normal stress forces acting on the propeller are to be expected and are allowed for in the modern propeller’s design but undue stress can lead to disaster. In fact, the first person to be killed in an aircraft accident was due to propeller failure. Lt Thomas Selfridge was killed while flying with Orville Wright in 17 September 1908. Orville was severely injured but survived.

## The First Airborne Props

At the end of the 15th century, circa 1490, the artist and inventor Leonardo Da Vinci (1452-1519) designed a crude form of airscrew (or prop) for his helicopter design. The design never left the drawing board but the word helicopter has been with us ever since. The French mathematician, J. P. Paucton introduced the idea of using two propellers on airships, one to propel the craft forwards and the other to lift it upwards; it was not successful! It was to be nearly three hundred years until 16 October 1784, when Jeanne-Pierre Blanchard (1753­1809) used the first airborne propeller on his hot air balloon. The propeller consisted of three metal plates attached to the end of poles, which were rotated by hand and again, were not successful. On 24 September 1852, Henri Gifford (1825-1882) a French engineer, used a three horsepower (2.24 kW) steam engine on his dirigible (steerable) air ship driving a three-blade 11 feet (3.35 m) propeller to achieve the first powered flight covering a distance of 18 miles (30 km) from Paris to Trappes. On 9 August 1884, Captains Charles Renard and Arthur Krebbs of the French Corps of Engineers completed a circular coarse of five miles (8 km) in their airship La France. The 23 feet (7.01 m) four-blade wooden propeller was powered by a nine horsepower (6.7 kW) Gramme electric motor. The prop turned at a very slow 50 RPM. Around this time, other pioneers had varying amounts of success, propelling their airships using propellers turned by hand, or powered by electric or petrol driven engines

## – The History of Aircraft Propellers

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.

Frank Hitchens,

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

Airscrews

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.