Alan Arnold Griffith
Encyclopedia
Alan Arnold Griffith was an English
engineer, who, among many other contributions, is best known for his work on stress
and fracture
in metals that is now known as metal fatigue
, as well as being one of the first to develop a strong theoretical basis for the jet engine
.
(RAE).
Some of Griffith's earlier works remain in widespread use today. In 1917 he and G. I. Taylor
suggested the use of soap films as a way of studying stress problems. Using this method a soap bubble is stretched out between several strings representing the edges of the object under study, and the coloration of the film shows the patterns of stress. This method, and similar ones, were used well into the 1990s when computer power became generally available that could do the same experiment numerically.
for that material. However it was well known that those materials would often fail at 1000 times less than this predicted value. Griffith discovered that there were many microscopic cracks in every material, and hypothesized that these cracks lowered the overall strength of the material. This was because any void in a solid concentrates stress, a fact already well known to machinists at the time. This concentration would allow the stress to reach E/10 at the head of the crack long before it would seem to for the material as a whole.
From this work Griffith formulated his own theory of brittle fracture, using elastic strain energy
concepts. His theory described the behavior of crack propagation of an elliptical nature by considering the energy involved. The equation, Griffith's criterion, basically states that when a crack is able to propagate enough to fracture a material, that the gain in the surface energy is equal to the loss of strain energy, and is considered to be the primary equation to describe brittle fracture. Because the strain energy released is directly proportional to the square of the crack length, it is only when the crack is relatively short that its energy requirement for propagation exceeds the strain energy available to it. Beyond the critical Griffith crack length, the crack becomes dangerous.
The work, published in 1920 ("The phenomenon of rupture and flow in solids"), resulted in sweeping changes in many industries. Suddenly the "hardening" of materials due to processes such as cold rolling were no longer mysterious. Aircraft designers immediately understood why their designs had failed even though they were built much stronger than was thought necessary at the time, and soon turned to polishing their metals in order to remove cracks. The result was a series of particularly beautiful designs in the 1930s, such as the Boeing 247
. This work was later generalized by G. R. Irwin, in the 1950s, applying it to almost all materials, not just rigid ones.
shape for the blades that would dramatically improve their performance. The paper went on to describe an engine using an axial compressor
and two-stage turbine, the first stage driving the compressor, the second a power-take-off shaft that would be used to power a propeller. This early design was a forerunner of the turboprop
engine. As a result of the paper, the Aeronautical Research Committee
supported a small-scale experiment with a single-stage axial compressor and single-stage axial turbine. Work was completed in 1928 with a working testbed design, and from this a series of designs
was built to test various concepts.
At about this time Frank Whittle
wrote his thesis on turbine engines, using a centrifugal compressor
and single-stage turbine, the leftover power in the exhaust being used to power the aircraft directly. Whittle sent his paper to the Air Ministry
in 1930, who passed it on to Griffith for comment. After pointing out an error in Whittle's calculations, he stated that the large frontal size of the compressor would make it impractical for aircraft use, and that the exhaust itself would provide little thrust. The Air Ministry replied to Whittle saying they were not interested in the design. Whittle was crestfallen, but was convinced by friends in the RAF to pursue the idea anyway. Luckily for all involved, Whittle patented his design in 1930 and was able to start Power Jets
in 1935 to develop it.
Griffith went on to become the principal scientific officer in charge of the new Air Ministry Laboratory in South Kensington. There he invented the contraflow gas turbine, which used two sets of compressor disks rotating in opposite directions, one "inside" the other. This is as opposed to the more normal design in which the compressors blow the air against a stator, essentially a non-moving compressor disk. The effect on compression efficiency was noticeable, but so was the effect on complexity of the engine. In 1931 he returned to the RAE to take charge of engine research, but it was not until 1938, when he became head of the Engine Department, that work on developing an axial-flow engine actually started. Joined by Hayne Constant
, they started work on Griffith's original non-contraflow design, working with steam turbine manufacturer Metropolitan-Vickers
(Metrovick).
After a short period Whittle's work at Power Jets started to make major progress and Griffith was forced to re-evaluate his stance on using the jet directly for propulsion. A quick redesign in early 1940 resulted in the Metrovick F.2
, which ran for the first time later that year. The F.2 was ready for flight tests in 1943 with a thrust of 2,150 lbf, and flew as replacement engines on a Gloster Meteor
, the F.2/40 in November. The smaller engine resulted in a design that looked considerably more like the Me 262, and had improved performance. Nevertheless the engine was considered too complex, and not put into production.
Griffith's original rejection of Whittle's concepts has long been commented on. It certainly set back development of the jet engine in England. His motivations have long been the topic of curiosity, with many people suggesting that his endless quest for perfectionism was the main reason he didn't like Whittle's "ugly" little engine, or perhaps the belief that "his" design was innately superior.
Griffith joined Rolls-Royce
in 1939, working there until 1960. He designed the AJ.65 axial turbojet which led to the development of the Avon engine
, the company's first production axial turbojet. He then turned to the turbofan
(known as "bypass" in England) design and was instrumental in introducing the Rolls-Royce Conway
. Griffith carried out pioneering research into vertical take-off and landing (VTOL) technology, culminating in the development of the Rolls-Royce Thrust Measuring Rig
.
English people
The English are a nation and ethnic group native to England, who speak English. The English identity is of early mediaeval origin, when they were known in Old English as the Anglecynn. England is now a country of the United Kingdom, and the majority of English people in England are British Citizens...
engineer, who, among many other contributions, is best known for his work on stress
Stress (physics)
In continuum mechanics, stress is a measure of the internal forces acting within a deformable body. Quantitatively, it is a measure of the average force per unit area of a surface within the body on which internal forces act. These internal forces are a reaction to external forces applied on the body...
and fracture
Fracture
A fracture is the separation of an object or material into two, or more, pieces under the action of stress.The word fracture is often applied to bones of living creatures , or to crystals or crystalline materials, such as gemstones or metal...
in metals that is now known as metal fatigue
Metal Fatigue
Metal Fatigue , is a futuristic science fiction, real-time strategy computer game developed by Zono Incorporated and published by Psygnosis and TalonSoft .-Plot:...
, as well as being one of the first to develop a strong theoretical basis for the jet engine
Jet engine
A jet engine is a reaction engine that discharges a fast moving jet to generate thrust by jet propulsion and in accordance with Newton's laws of motion. This broad definition of jet engines includes turbojets, turbofans, rockets, ramjets, pulse jets...
.
Early work
A. A. Griffith took a first in mechanical engineering, followed by a Master’s Degree and a Doctorate from Liverpool University. In 1915 he was accepted by the Royal Aircraft Factory as a trainee, before joining the Physics and Instrument Department the following year in what was now been renamed as the Royal Aircraft EstablishmentRoyal Aircraft Establishment
The Royal Aircraft Establishment , was a British research establishment, known by several different names during its history, that eventually came under the aegis of the UK Ministry of Defence , before finally losing its identity in mergers with other institutions.The first site was at Farnborough...
(RAE).
Some of Griffith's earlier works remain in widespread use today. In 1917 he and G. I. Taylor
Geoffrey Ingram Taylor
Sir Geoffrey Ingram Taylor OM was a British physicist, mathematician and expert on fluid dynamics and wave theory. His biographer and one-time student, George Batchelor, described him as "one of the most notable scientists of this century".-Biography:Taylor was born in St. John's Wood, London...
suggested the use of soap films as a way of studying stress problems. Using this method a soap bubble is stretched out between several strings representing the edges of the object under study, and the coloration of the film shows the patterns of stress. This method, and similar ones, were used well into the 1990s when computer power became generally available that could do the same experiment numerically.
Metal fatigue
Griffith is more famous for a theoretical study on the nature of stress and failure in metals. At the time it was generally taken that the strength of a material was E/10, where E was the Young's modulusYoung's modulus
Young's modulus is a measure of the stiffness of an elastic material and is a quantity used to characterize materials. It is defined as the ratio of the uniaxial stress over the uniaxial strain in the range of stress in which Hooke's Law holds. In solid mechanics, the slope of the stress-strain...
for that material. However it was well known that those materials would often fail at 1000 times less than this predicted value. Griffith discovered that there were many microscopic cracks in every material, and hypothesized that these cracks lowered the overall strength of the material. This was because any void in a solid concentrates stress, a fact already well known to machinists at the time. This concentration would allow the stress to reach E/10 at the head of the crack long before it would seem to for the material as a whole.
From this work Griffith formulated his own theory of brittle fracture, using elastic strain energy
Energy principles in structural mechanics
Energy principles in structural mechanics express the relationships between stresses, strains or deformations, displacements, material properties, and external effects in the form of energy or work done by internal and external forces...
concepts. His theory described the behavior of crack propagation of an elliptical nature by considering the energy involved. The equation, Griffith's criterion, basically states that when a crack is able to propagate enough to fracture a material, that the gain in the surface energy is equal to the loss of strain energy, and is considered to be the primary equation to describe brittle fracture. Because the strain energy released is directly proportional to the square of the crack length, it is only when the crack is relatively short that its energy requirement for propagation exceeds the strain energy available to it. Beyond the critical Griffith crack length, the crack becomes dangerous.
The work, published in 1920 ("The phenomenon of rupture and flow in solids"), resulted in sweeping changes in many industries. Suddenly the "hardening" of materials due to processes such as cold rolling were no longer mysterious. Aircraft designers immediately understood why their designs had failed even though they were built much stronger than was thought necessary at the time, and soon turned to polishing their metals in order to remove cracks. The result was a series of particularly beautiful designs in the 1930s, such as the Boeing 247
Boeing 247
The Boeing Model 247 was an early United States airliner, considered the first such aircraft to fully incorporate advances such as all-metal semi-monocoque construction, a fully cantilevered wing and retractable landing gear...
. This work was later generalized by G. R. Irwin, in the 1950s, applying it to almost all materials, not just rigid ones.
Turbine engines
In 1926 he published a seminal paper, An Aerodynamic Theory of Turbine Design. He demonstrated that the woeful performance of existing turbines was due to a flaw in their design which meant the blades were "flying stalled", and proposed a modern airfoilAirfoil
An airfoil or aerofoil is the shape of a wing or blade or sail as seen in cross-section....
shape for the blades that would dramatically improve their performance. The paper went on to describe an engine using an axial compressor
Axial compressor
Axial compressors are rotating, airfoil-based compressors in which the working fluid principally flows parallel to the axis of rotation. This is in contrast with other rotating compressors such as centrifugal, axi-centrifugal and mixed-flow compressors where the air may enter axially but will have...
and two-stage turbine, the first stage driving the compressor, the second a power-take-off shaft that would be used to power a propeller. This early design was a forerunner of the turboprop
Turboprop
A turboprop engine is a type of turbine engine which drives an aircraft propeller using a reduction gear.The gas turbine is designed specifically for this application, with almost all of its output being used to drive the propeller...
engine. As a result of the paper, the Aeronautical Research Committee
Aeronautical Research Committee
The Aeronautical Research Committee was a UK government committee established in 1919 in order to coordinate aeronautical research and education following World War I...
supported a small-scale experiment with a single-stage axial compressor and single-stage axial turbine. Work was completed in 1928 with a working testbed design, and from this a series of designs
Turbojet development at the RAE
Between 1936 and 1940 Alan Arnold Griffith designed a series of turbine engines that were built under the direction of Hayne Constant at the Royal Aircraft Establishment . The designs were advanced for the era, typically featuring a "two-spool" layout with high- and low-pressure compressors that...
was built to test various concepts.
At about this time Frank Whittle
Frank Whittle
Air Commodore Sir Frank Whittle, OM, KBE, CB, FRS, Hon FRAeS was a British Royal Air Force engineer officer. He is credited with independently inventing the turbojet engine Air Commodore Sir Frank Whittle, OM, KBE, CB, FRS, Hon FRAeS (1 June 1907 – 9 August 1996) was a British Royal Air...
wrote his thesis on turbine engines, using a centrifugal compressor
Centrifugal compressor
Centrifugal compressors, sometimes termed radial compressors, are a sub-class of dynamic axisymmetric work-absorbing turbomachinery.The idealized compressive dynamic turbo-machine achieves a pressure rise by adding kinetic energy/velocity to a continuous flow of fluid through the rotor or impeller...
and single-stage turbine, the leftover power in the exhaust being used to power the aircraft directly. Whittle sent his paper to the Air Ministry
Air Ministry
The Air Ministry was a department of the British Government with the responsibility of managing the affairs of the Royal Air Force, that existed from 1918 to 1964...
in 1930, who passed it on to Griffith for comment. After pointing out an error in Whittle's calculations, he stated that the large frontal size of the compressor would make it impractical for aircraft use, and that the exhaust itself would provide little thrust. The Air Ministry replied to Whittle saying they were not interested in the design. Whittle was crestfallen, but was convinced by friends in the RAF to pursue the idea anyway. Luckily for all involved, Whittle patented his design in 1930 and was able to start Power Jets
Power Jets
Power Jets Ltd was a United Kingdom company set up by Frank Whittle for the purpose of designing and manufacturing jet engines.-History:Founded on January 27, 1936, the company consisted of Whittle, Rolf Dudley-Williams, James Collingwood Tinling, and Lancelot Law Whyte of investment bankers O T...
in 1935 to develop it.
Griffith went on to become the principal scientific officer in charge of the new Air Ministry Laboratory in South Kensington. There he invented the contraflow gas turbine, which used two sets of compressor disks rotating in opposite directions, one "inside" the other. This is as opposed to the more normal design in which the compressors blow the air against a stator, essentially a non-moving compressor disk. The effect on compression efficiency was noticeable, but so was the effect on complexity of the engine. In 1931 he returned to the RAE to take charge of engine research, but it was not until 1938, when he became head of the Engine Department, that work on developing an axial-flow engine actually started. Joined by Hayne Constant
Hayne Constant
Hayne Constant, C.B., C.B.E., F.R.S, F.R.Ae.S. was an English mechanical and aeronautical engineer who developed jet engines during World War II....
, they started work on Griffith's original non-contraflow design, working with steam turbine manufacturer Metropolitan-Vickers
Metropolitan-Vickers
Metropolitan-Vickers, Metrovick, or Metrovicks, was a British heavy electrical engineering company of the early-to-mid 20th century formerly known as British Westinghouse. Highly diversified, they were particularly well known for their industrial electrical equipment such as generators, steam...
(Metrovick).
After a short period Whittle's work at Power Jets started to make major progress and Griffith was forced to re-evaluate his stance on using the jet directly for propulsion. A quick redesign in early 1940 resulted in the Metrovick F.2
Metrovick F.2
The Metropolitan-Vickers F.2 was an early turbojet engine and the first British design to be based on an axial-flow compressor. It was considered too unreliable for use during the war, and never entered production...
, which ran for the first time later that year. The F.2 was ready for flight tests in 1943 with a thrust of 2,150 lbf, and flew as replacement engines on a Gloster Meteor
Gloster Meteor
The Gloster Meteor was the first British jet fighter and the Allies' first operational jet. It first flew in 1943 and commenced operations on 27 July 1944 with 616 Squadron of the Royal Air Force...
, the F.2/40 in November. The smaller engine resulted in a design that looked considerably more like the Me 262, and had improved performance. Nevertheless the engine was considered too complex, and not put into production.
Griffith's original rejection of Whittle's concepts has long been commented on. It certainly set back development of the jet engine in England. His motivations have long been the topic of curiosity, with many people suggesting that his endless quest for perfectionism was the main reason he didn't like Whittle's "ugly" little engine, or perhaps the belief that "his" design was innately superior.
Griffith joined Rolls-Royce
Rolls-Royce Limited
Rolls-Royce Limited was a renowned British car and, from 1914 on, aero-engine manufacturing company founded by Charles Stewart Rolls and Henry Royce on 15 March 1906 as the result of a partnership formed in 1904....
in 1939, working there until 1960. He designed the AJ.65 axial turbojet which led to the development of the Avon engine
Rolls-Royce Avon
|-See also:-Bibliography:* Gunston, Bill. World Encyclopaedia of Aero Engines. Cambridge, England. Patrick Stephens Limited, 1989. ISBN 1-85260-163-9-External links:**** a 1955 Flight article on the development of the Avon...
, the company's first production axial turbojet. He then turned to the turbofan
Turbofan
The turbofan is a type of airbreathing jet engine that is widely used for aircraft propulsion. A turbofan combines two types of engines, the turbo portion which is a conventional gas turbine engine, and the fan, a propeller-like ducted fan...
(known as "bypass" in England) design and was instrumental in introducing the Rolls-Royce Conway
Rolls-Royce Conway
The Rolls-Royce RB.80 Conway was the first by-pass engine in the world to enter service. Development started at Rolls-Royce in the 1940s, but it was used only briefly in the late 1950s and early 1960s before other turbofan designs were introduced that replaced it. The Conway powered versions of...
. Griffith carried out pioneering research into vertical take-off and landing (VTOL) technology, culminating in the development of the Rolls-Royce Thrust Measuring Rig
Rolls-Royce Thrust Measuring Rig
The Rolls-Royce Thrust Measuring Rig was a pioneering vertical take-off and landing aircraft developed by Rolls-Royce in the 1950s. The TMR used two Nene turbojet engines mounted back-to-back horizontally within a steel framework, raised upon four legs with castors for wheels...
.
