History.

Going back to basics;

The original concept of a propeller came from the simple screw form used in Egypt and Greece around 400 BC for the purpose of extracting water from wells to replace hand scoops.

It was Leonardo da Vinci of the Italian Renaissance who famously sketched a flying machine design in the 1400’s which was based on a rotating screw design as shown below.  Arguably the British engineer Sir George Cayley is one of the most important people in the history of aeronautics as he established the basic principles of flight and built the first helicopter in 1796 which had contra rotating screw propellers.

WRIGHT BROTHERS

In 1903 after their pioneering manned flight with gliders the Wright Brothers built and flew in America a powered glider with a petrol engine with two wooden airscrews.  Their assumption was that an airscrew is a wing rotating in the vertical plane. The ’Wright Flyer’ used two 8 feet diameter airscrews with some twist which were manufactured from lamination's of spruce.  Orville and Wilbur Wright reasoned that their airscrews would produce sufficient forward thrust for controlled flight of their flying machine.

Wright Flyer, 1903

Today, some 2400 years after the first concept, the science behind propeller design has not fundamentally changed as its purpose remains as to convert rotary motion from a power source to provide propulsive force.   The aerofoil design, or shape, of a propeller blade is now determined by complex aerodynamic theory and propellers are carefully selected for the particular application and its flight regime.

RAF B.E.2, 1915

Aerodynamic Force

The propulsive force, or lift, which is perpendicular to the air flow, is a result of the combination of the aerofoil shape and its angle of attack to the air flow.  Clark Y is the name of an aerofoil profile which is widely used in aviation and its aerodynamic properties are well studied and documented.  With a propeller the blade pitch may be fixed, manually variable with set positions, or an automatically-variable constant-speed configuration depending on the type of construction and the control system.

Bernoulli's equation helps explain that an aerofoil section can achieve lift because of its shape. They are concave shaped so that that air flows faster over the top of the wing and slower underneath. Fast moving air equals low air pressure while slow moving air equals high air pressure.

CONSTRUCTION

A fixed pitch propeller is the simplest form of propeller design. The angle of attack is set during manufacture and cannot be changed during flight.  This type of propeller introduced on the first monoplanes and biplanes, during the early 1900’s, were laminated from carefully selected planks of hardwood and considering the performance of the aircraft at that time were very efficient.

Fixed pitch propeller workshop, 1930’s

The variable pitch propeller concept has been around for a long time with the early types being ground adjustable to change the flight characteristics.  Today there are many types of controllable variable pitch propeller where the blade angle can be altered in flight including by mechanical, electrical and centrifugal force mechanisms.  Hoffmann Propeller developed a 3-position variable pitch propeller in the 1950’s which became the de facto standard for motor gliders allowing climb, cruise and feathered settings.

A constant speed propeller automatically changes the blade pitch so that a chosen rotational speed is maintained.  The power delivered is the product of propeller speed and engine torque, and a constant speed propeller has a control system which ensures that it operates with blade angles which have the most efficient orientation to the surrounding airflow.  In this way a constant speed propeller balances the trade-off that a fixed pitch propeller must make between climb and cruise performance.  All high performance propeller driven aircraft today utilise constant speed propellers.

GEOFFREY DE HAVILLAND

Four blade propeller patterns were used on Royal Flying Corps aircraft such as the early reconnaissance aeroplanes, for example the Royal Aircraft Factory B.E.2.   These were pioneering times and as Geoffrey de Havilland developed new aeroplanes for civilian use which were generally smaller, lighter and more streamlined; two blade fixed pitch patterns were the popular choice . 

Today De Havilland and other classic aeroplanes have strong associations of owners & operators who devote their attention to safeguarding the types and ensuring continued airworthiness.  The first DH.60 Moth flew in 1925 and it is unsurprising that replica propellers manufactured by Hoffmann Propeller who has 60 years of experience are the best.

ROTOL AIRSCREWS

Spitfire Mark I’s were fitted with a Watts fixed pitch two blade propeller and later an American Hamilton Standard variable pitch three blade propeller manufactured under license by the de Havilland Aircraft Company.  Rotol Airscrews Limited was formed after the British Government Ministry of Supply, conscious of the threat of war with Germany, realised it would be in the national interest to concentrate the talent of Rolls-Royce and the Bristol Aeroplane Company in a single organisation. The merger, whose name was a contraction of “ROlls-Royce and BrisTOL”, established an organisation capable of technical development of the Hele-Shaw & Beacham concepts for variable pitch airscrews with hydraulic control which could match the rapid development of aircraft engines.

Rotol airscrews were given designations which in part defined the SBAC (Society of British Aircraft Constructors) splined shafts to which they were installed and one of the first production propellers; Rotol R4/1 (No 4 spline); was for a three blade propeller for the Bristol Blenheim (of which 500 were subsequently ordered by the Air Ministry). 

The first Rotol airscrew for a Spitfire was RX5/1 for a Mark II aircraft with a Rolls-Royce Merlin XII engine, and this utilised magnesium alloy as opposed to wood blades because initially difficultly was experienced attaching wood blades to metal ferrules for fitment to the propeller hub.  Rotol gave attention to resolving the blade attachment problem with the advent of compressed resin-bonded composite wood, so that the advantages of weight, battle damage repair, flexible manufacturing etc. could be taken advantage of.

It was known that Germany was more advanced in the manufacture of blades for high performance aircraft using composite wood, and through pre-war co-operation experimental blades were made by Rotol under license from the German Heine and Schwartz propeller manufacturers.

With war imminent and rapid advancement in blade design and production, the Air Ministry placed an order with Rotol for 5000 airscrews at the beginning of 1939.  As the Spitfire developed and production increased, the Rotol three blade propeller became the standard fitment.  Rotol used three blade designs involving different layouts of the British manufactured densified woods; Jablo, Jicwood and Hydulignum.  Production of blades was undertaken at the dispersed sites of The Airscrew Company, Hordern-Richmond Aircraft Ltd and F Hills & Sons Ltd, in addition to the Rotol site.  By the close of the war, Rotol had produced over 100,000 airscrews in varying configurations for over 60 different training, fighter and bomber aircraft.