Category Propellor Aerodynamics

High propeller tip speeds result in the separation of the air flow boundary layer over the blades, causing noise and a loss in efficiency due to compressibility problems. The problems of high tip speed were investigated as far back as 1949-58 in the USA and Europe, when research was conducted on propellers designed to operate in a supersonic air flow. Curtiss Electric built the first supersonic propeller to fly with the first test – flight taking place on the 14th April 1953. The four-blade, 10 feet (3.05 m) prop turned at supersonic speed powered by an Allison XT-38 turboprop engine mounted in the nose of a McDonnell XF-88B Voodoo prototype escort fighter (the forerunner of the F101). Research into supersonic propellers was continued at Edward Air Force Base, California, in 1955.

A modified Republic Thunderstreak, the XF-84H, was used to flight-test the three-blade Aero Products supersonic propeller with a first flight on 22 July 1955. An Allison XT-40-A-1, 5850 ESHP turboprop powered the supersonic prop. The prop was adorned with a very large spinner to mate with the aircraft’s nose, giving the propeller blades a short stubby appearance despite their 12 feet (3.64 m) diameter. The inaudible hypersonic sound waves emanating from the prop during ground running tests caused nausea to nearby personnel and combined with other problems with the prop and engine, the flight-test program was terminated. Many millions of dollars were spent on the project before supersonic props were deemed impractical due to their high noise levels and loss of efficiency at high operating speeds. The research was eventually brought to a close.

Record Breakers

The quest for high speed and improved aircraft performance has been present ever since the Wright brothers first flew. Speed records were being broken at a steady rate as improvements in aircraft, engines and propeller design allowed. The battle for the Schneider Trophy is a good example of the desire to fly faster. The trophy was captured by the British Supermarine

5.6- b floatplane on 13 September 1931, flown by Flt. J. N. Boothman at an average speed of 340 MPH (295 knots). The Supermarine S.6-B went on to attain 407.5 MPH (354 knots) two weeks after winning the Schneider Trophy The design experience gained on the S.6-B by R. J. Mitchell was to lead on to the design of the Spitfire of World War Two fame. The

5.6- B used a 2300 BHP Rolls Royce R engine, which became the forerunner of the famed Merlin engine, which powered the Spitfire, Hurricane and Lancaster bombers.

The Italian Macchi MC-72 Castoldi, flown by Francesco Agello in 1931, was built to compete for the Schneider trophy but failed to enter. However, it did gain the World’s speed record for floatplanes of 440.68 MPH (385 knots), which this author believes still stands in 2014. It is note worthy the Macchi’s engine was a Fiat 2850 BHP, which powered a contra­rotating propeller to combat prop torque; a very powerful engine for such a small plane! All speed records were held by floatplanes during the years 1928 through to 1939 due to the Schneider Trophy challenge, which was only open to floatplanes. The Macchi MC-72 held the overall world speed record for all classes of aircraft until 26 April 1939 when the German Messerschmitt Bf 109R raised the world speed record to 468.9 MPH (408 knots).

Other speed records of interest followed, taken by the Chance Vought F4-U Corsair, Grumman Bearcat and the TU 95/142. The F4-U Corsair fighter of World War Two fame had the largest propeller found on a fighter, radius 13 feet 2 inches

(4.01 m). The prop was driven by the P&W R-2800 Double Wasp of 2250 BHP, enabling the Corsair to be the first US fighter to exceed 400 MPH (350 knots) during a subsequent test flight on 10 October 1940.

Lyle Shelton’s Grumman F8F Bearcat Rare Bear holds the present world speed record for piston/prop aircraft. The record was gained on 21 August 1989 at 528.31 MPH (460 knots) at Las Vegas, Nevada. The record for the world’s fastest turboprop goes to the Russian TU 95/142 at 545.076 MPH (473 knots) on 9 April 1960. It is claimed by some authorities, the Republic Thunderchief, mentioned earlier with a supersonic prop achieved a speed of 670 MPH (580 knots) during flight test program to become the fastest propeller driven aircraft. However, the National Museum of the USAF in Dayton where the Thunderchief resides, records a lower speed. Did it reach Mach 1? This would have depended on the ambient temperature at the time (the speed of sound varies with temperature) and also, is the claimed speed of 670 MPH correct? No doubt, all these speeds records will be broken again some day in the future.

It is now common knowledge the Vickers Viscount V.630 was the World’s first turboprop transport, which first flew on 29 July 1948. But, what was the World’s first turboprop aircraft to fly? In 1930, Frank Whittle (1907-1996) (later Air Commodore, Sir Frank Whittle) patented his jet engine design with work beginning on the engine in 1937. However, the development of the turboprop engine dates back to 1925 through the work of a UK scientist Dr. Alan A. Griffiths (1895­1963) at the Royal Aircraft Establishment, Farnborough. His axial flow compressor and turbine, driving a propeller progressed as far as the wind tunnel testing stage in 1926; however, the research was brought to an end due to the ‘depression’ and lack of interest by the industry. It was to be another nineteen years before the turboprop idea was revised, following on after the development of the jet engine during the years of World War Two

So, to return to the question of what was the World’s first turboprop aircraft to fly. The aircraft was a converted Gloster Meteor F.1., jet fighter which first flew as a twin-engine turboprop aircraft on 20 September 1945. The Meteor’s twin 750 SHP (559 kW) Rolls Royce Derwent 2 turbojet engines were modified with reduction gear and drive shafts to turn 7 feet 11 inch (2.14 m) five-blade Rotol props. The props were

The Gloster Meteor F9-40 prototype, similar to the
Meteor that flight-tested the first turboprop engines. This
example is located in the RAF Museum Cosford, UK.

The Vickers Viscount was the World’s first turboprop
airliner. This example, a 744 model, is located in
the Pima Air & Space Museum, Tucson, AR.

relatively small to allow for clearance between the tips and the fuselage. With this conversion installed, the engines were known as Rolls Royce R. B. 50 Trents. [It was Rolls Royce policy to name their engines after British rivers].

The Vickers Viscount 630 was the World’s first transport turboprop aircraft, but it was not the first revenue-earning turboprop. This distinction was claimed by two converted Douglas DC-3 cargo planes, powered by Rolls Royce Dart R. Da 3/505 fixed-shaft turboprop engines. The two aircraft were placed in service by British European Airways in 1951-3 to become the World’s first revenue-earning turboprop aircraft, with the first revenue-earning flight from London to Hanover, Germany on 15 August 1951. The engines, more powerful than the prototype Viscount’s original engines, were on flight trial in preparation for use on the Viscount Series 500 production aircraft. The Viscount’s inaugural revenue-earning flight occurred on 18 April 1953 from London to Cyprus. Other Douglas DC-3’s have also been converted to turboprop configuration over the following years.

Early propellers were made from one piece of solid wood and later props were made from several laminations of a hardwood, usually spruce or mahogany and then hand carved. Shortly after World War I, the first successful metal props appeared; this was the ‘Reed’ type developed by Dr. S. A. Reed. The props were cut, shaped and twisted from one piece of Duralumin, an aluminium alloy. From 1926 onwards, metal props became more common on high-powered engines, but wood props are still used today, usually on low-powered home-built aircraft.

Ernest G. McCauley (c1885-c1930) founded the McCauley Propeller Company in 1938. Right from the inception of the company, all their propellers were made from metal, starting with a ground adjustable prop they invented and made from solid steel, in 1938. In 1946, they made the first metal props for civilian light aircraft, called a ‘Met-L-Prop’. Metal props are now made from either a single piece of forged Duralumin, hollow steel or a light alloy. Prior to 1930, most wooden propellers consisted of either two or four-blades. A three-blade prop was unusual due to the difficulty of attaching the blades to the prop hub, while still maintaining sufficient strength. With metal props this is of course, less of a problem. One notable exception was the Junkers JU-88 twin-engine bomber. Apart from being the most-produced German bomber of the Second World War, it also had three-blade, wooden, constant-speed props. The Cessna Aircraft Company purchased the McCauley Aviation Corporation in 1960 and operated it as a separate division. The same year saw the introduction of McCauley’s two-blade, constant-speed propeller with full feathering and alcohol de-ice system. Electric de-ice followed in 1967.

As the years progressed, further advancements saw the introduction of three-blade, full feathering props and reverse pitch props for turboprop aircraft in 1977. Next up was the four-blade prop in 1983 followed by the hugely successful

McCauley ‘Blackmack’ series of propellers. In 1992, the five – blade scimitar shaped prop was made for turboprop aircraft of up to 1650 shaft horsepower. The company received a name change in 1996 to McCauley Propeller Systems and it is now the largest propeller manufacturer with over 250,000 props made over the year, operating on aircraft worldwide.

In 1911, Robert Hartzell expanded his family’s Walnut Furniture manufacturing company to make aircraft propellers with the name changed to Hartzell Walnut Propeller Company of Ohio, USA. The name was later shortened to the Hartzell Propeller Company. They concentrated on the high-performance light aircraft and turboprop market; they no longer make fixed-pitch propellers. They were the first company to manufacture feathering propellers and pioneered the use of composites materials in 1945 in propeller manufacturing. Their first composite propeller (and the world’s first) was patented in 1949 and flight-tested on a Republic Seabee amphibian aircraft. In 1978, Hartzell installed the first production run prop made totally from composite materials on the Spanish CASA 212c Aviacar. Composite prop blades are 25-50% lighter than metal blades with the added advantage of higher strength, greater reliability and performance, fatigue resistance and better vibration damping, etc. The blades are made from polyester or epoxy-resin with a fibre to provide directional strength. The fibre may be glass, carbon or a synthetic Aramid fibre such as Kevlar, which is also found in bulletproof vests, amongst other uses. It is also lighter in weight and more expensive than ordinary fibreglass. In addition to using new materials, propellers are still being refined with new designs using ‘Q-tips’, sweptback tips and scimitar shaped blades

The Sensenich Propeller Company was founded in the USA by the Sensenich brothers in 1932. By 1942, it was the largest manufacturer of wooden propellers in the USA and commenced fixed-pitch metal propeller manufacturing in 1947 and later composite propellers. McCauley, Hartzell and Sensenich are the three leading propeller manufacturers in the USA for light to medium size aircraft. So, is it by chance or coincidence that two of the world’s major propeller manufacturing companies, McCauley and Hartzell are located within a few miles of each other in the same city of Dayton, Ohio, the Wright Brothers home town! It was on the request of Orville Wright to his friend Robert Hartzell to make propellers that saw the start­up of the Hartzell Propeller Company.

From the early days of flying, ice build-up on the wings and propeller blades has always been a serious problem. Not only is the extra weight a problem, but of greater consequence is the change in the leading edge profile, which alters the airflow over the wings and prop blades, having a detrimental effect on the aircraft’s performance characteristics beyond acceptable limits. To help alleviate wing icing, in 1934, the American company B. F. Goodrich pioneered the system of pulsating rubber boots on the wing’s leading edge to break off the ice. Ice protection for the prop blades followed later, in the form of de-ice boots, electrical heater mats and a chemical slinger system

It is interesting to note, the Lockheed YC-130A Hercules prototype and also the first ten production models were equipped with Curtiss-Wright, three-blade, electric propellers.

Powered by the Allison T56 turboprop engines, the props rotated at a constant 1108 RPM, the thrust was adjusted by the prop’s blade angle. However, a problem with the electrically operated governor caused the CSU to overcompensate or hunt in either direction. This resulted in uneven thrust being produced and caused the aircraft’s nose to yaw from side to side. On occasions the hunting was so bad the propeller’s reduction gearbox would overheat and cause sever damage requiring an in-flight engine shutdown and the propeller to be feathered. The problem with the Curtiss-Wright props was eventually corrected, but not before a change was made to hydraulic CSU’s made by Aero Products, Allison’s subsidiary company. Subsequent Hercules models from the C-130B onwards were fitted with Hamilton Standard three-blade, and later four-blade propellers. In addition, to follow the trend of modern turboprop aircraft, the latest C-130J models have a new type of turboprop engine, an Allison AE 3100 D3 flat rated to 4591 SHP, driving six-blade composite props.

On twin-engine aircraft, the propellers of each engine may rotate in opposite directions (counter-rotate) to enhance engine out handling. The idea of using counter or contra­rotating propellers is not a new one. The Wright brother’s Flyer I built in 1903 had counter-rotating propellers, chain – driven off one engine. The chains were not bicycle chains as some people believed, but were made especially for the Flyer I.

The 11-feet (3.35 m) Curtiss electric props on the Lockheed XP-38 Lightning (1939) experimental fighter rotated top blade in. On the YP-38 Lightning prototype, and all subsequent production models, the prop’s direction of rotation was reversed to rotate top blade out. The result was exceptional longitudinal stability for the P-38.

The Piper Twin Comanche and its bigger brother the Navajo Chieftain, both have counter-rotating props rotating top blade

in towards the fuselage, the conventional way for modern aircraft. An exception was made with the Piper Aerostar 700P; this aircraft has its counter-rotating props turning top blade away from the fuselage, the same direction as on the P-38 and also for stability purposes. Because the blades rotate top blade outwards, the centre of thrust is placed further outboard but the critical engine is still eliminated.

Another aircraft of historical significance with counter­rotating propeller is the Shorts S.39 Triple-Twin. The name sounds contradictory until one realizes the layout of the aircraft; the two seat aircraft looks similar in some respects to the Wright Flyer I, with a wing span of 34 feet (10.4 m) and a length of 45 feet (1.7 m). It had two Gnome rotary engines of 50 BHP (37 kW) each, driving three propellers, hence the name Triple-Twin. The engines were mounted in tandem in the fuselage; the front engine powered two tractor, counter­rotating propellers by chain drive, similar to the Flyer I, while the rear mounted engine powered a single pusher prop. The Triple-Twin first flew on 18 September 1911, to become the world’s first aircraft driven by three propellers, albeit with only two engines, the forerunner of the push/pull arrangement found on later aircraft. On 13 May 1913, the flight of the world’s first four-engine aircraft occurred; this was the Bolshoi Baltiski (Grand Baltic 1) developed by Igor Sikorski. First designed as a twin-engine biplane that was underpowered, two more engine and propellers were added in tandem with the original Argus engines, each of 100 BHP.

A contra-rotating propeller (or contra-prop for short) is one that consists of two co-axial mounted propellers and driven by the same engine, but rotating in opposite directions. With this arrangement, the total amount of power absorbed by the propeller can be greatly increased. Contra-props also date back to the early days of flying. At least as far back as 1909 when the Piggott brothers built their Piggott biplane with a contra – prop. [There could have been others before this one, but not

to this author’s knowledge]. The two props on the Piggott were well spaced being about 3 feet (1 m) apart. Conventional contra-props are placed adjacent to each other as shown in the photograph of the Avro Shackleton.

A Deperdussin with a contra-prop followed the Piggott’s biplane in 1912 . Moreover, 1944 saw the appearance of the experimental Boeing XF8-B1 carrier-based fighter/bomber powered by a contra-prop. It never passed the experimental stage and so, did not make it into military service.

In 1944-45, a Supermarine Spitfire XIV was flight tested for 230 hours with a six-blade contra-rotating propeller installed. The first flight was performed in August 1944 and was followed by the carrier based naval version of the Spitfire – the Seafire 47. A Rolls Royce Griffon engine of 2350 BHP driving a six-blade contra-prop powered the experimental Seafire. This was a substantial increase in power from the early ‘Mark’ of Spitfire’s engines of 1000 BHP driving a two-blade propeller.

Four years later, The UK’s Fleet Air Arm’s Fairey Gannett prototype became the world’s first contra-prop equipped aircraft to be powered by a turbine engine – the Armstrong Siddeley Double Mamba 2950 ESHP turboprop powered the eight-blade contra-rotating propeller. The Gannett’s first flight took place on 19 September 1949. It went on to make further aviation history on 19 June 1950 when it became the first turboprop aircraft to make a carrier landing; the ship was the HMS Illustrious.

The prototype Bristol Brabazon piston-engine airliner was another contra-rotating propeller equipped aircraft worthy of note. Its eight Bristol Centaurus engines each producing 3500 BHP, were coupled in pairs; each powered an eight – blade Rotol contra-prop. After 400 hours of flight-testing over a period of four years, the prototype was scrapped. Its

demise was initiated by the high cost of development and the fact the DeHavilland Comet 1 jet airliner made its maiden flight about six weeks before the Brabazon’s first flight, which was accomplished on 4 September 1949. The end of the large piston powered airliners had finally arrived; jets were here to stay!

Moving on to larger aircraft… The Russian built Antonov AN-22 Antheus (1965) was for a short time the world’s largest aircraft until the arrival of the Boeing 747 and the Lockheed C-5A Galaxy military transport. The An-22 is powered by the world’s largest turboprop engines; these are the Kuznetsov NK-12MA fixed-shaft engines developing an incredible 15,000 Shaft Horsepower (11,185 kW) each. The four engines each drive massive 20 feet 4 inch (6.2 m) eight-blade contra-props at a very low 750 RPM, and producing a very distinctive noise in the process. As a further point of interest, the AN-22’s props are very large but they are not the largest propellers to be used. Although it was not a contra-prop, the record for the world’s largest propeller goes to the 22 feet 8 inch (6.9 m) Garuda prop. The Garuda was mounted on the German designed, Polish built Linke-Hoffmann R-11 prototype aircraft, with a first flight on 19 January 1919. It looked like a single-engine aircraft on steroids, but had a wing span of 138 feet 4 inches (42.25 m) and a length of 66 feet 8 inches (20.2 m). Compare this to the wing span of an early Boeing 737 at 93 feet (28.35 m). The single Garuda prop was powered by four Mercedes D. IVa of 260 BHP engines each, giving a maximum speed of 81 MPH. Due to the propeller’s large diameter, it turned at a very low 545 RPM. Only the one prototype was ever completed.

As mentioned above, it became necessary over the years to increase the number of blades mounted on a prop to absorb the greater horsepower of newer types of engines. Four-blade props became more common and still are, and later five-blade props were produced. Due to interference caused as the air flow cascades over the following blades, it was found that five – blades were the maximum that could be used before propeller efficiency deteriorated. However, where the metal prop was limited to five-blades, props with six-blades are now powering modern turboprop transports and cargo planes. Composite materials enabled six-blades to be mounted on each propeller hub, generating greater thrust more efficiently than a metal prop could ever achieve. For example, the BAe ATP and later models of the Aerospatiale ATR and also the latest Lockheed Hercules C130J all have six-blade composite props, to name just a few.

To this author’s knowledge, the only six-blade metal prop to fly (not counting contra-rotating props) was a pusher prop mounted on the Japanese Kyushu J7W1 Shinden that was being developed at the end of the Second World War, but never saw combat service.

The counter-rotating propellers on this Lockheed P-38
Lightning are clearly visible. This aircraft is located in
the Udvar-Hazy Center, near Washington D. C.

The Supermarine Spitfire of Second World War fame first flew with a two-blade, fixed-pitch, wooden prop during 1938-9. By the early stages of the war in 1940, the two-blade propeller had been exchanged for a metal three-blade, constant-speed prop. In 1942, the Spitfire Mark IX was flown with a four – blade propeller to absorb the increased brake horsepower of the more powerful engines that followed. Late model Spitfires with the Griffon 65 engine or similar, were powered by a five – blade prop and later still, with a six-blade contra-rotating propeller

The Spitfire’s stable mate, the Hawker Hurricane was similarly equipped with the early models flying with a Watts, two-blade, fixed-pitch wooden prop. In 1939-40, the two – blade prop was swapped for a three-blade metal, two-position VP propeller. Canadian built Hurricanes with the Merlin III engine powered a DH-Hamilton Standard Hydromatic, three – blade propeller

The constant-speed propeller was patented in 1924, in the UK, by Gloster/Hele/Shaw/Beacham. Flight-testing was carried out on a Gloster Grebe in 1926-7. The propeller built of compressed wood was later produced by Rotol, now Dowty Rotol in the UK. Although the constant-speed prop was developed in the UK, the American built Grumman FF-1 is believed to be the first production aircraft to use a constant – speed prop. This was a single-engine biplane fighter, which entered service with the USAAF in late 1931.

It was to be another ten years before the Royal Air Force operated aircraft with constant-speed propellers. In 1929, another propeller manufacturer known then as Hamilton Aero, in the USA, merged with Standard Steel to form the now well-known Hamilton Standard company. On 10 June 1999,

Hamilton Standard’s parent company United Technologies, acquired the Sundstrand Corporation merging it with Hamilton Standard to form Hamilton Sundstrand. During the 1930s, Hamilton Standard designed a constant-speed, three-blade prop made from a light alloy, the pitch-change mechanism being hydraulically operated. This was first used on the Boeing 247D, to become the first modern airliner to use a constant-speed prop, in 1934. The three-blade Hamilton Standard prop was powered by a geared Pratt & Whitney Wasp radial engine of 550 BHP (410 kW). This prop replaced a direct drive Wasp turning either a two-blade or a three-blade variable-pitch propeller found on earlier models of the Boeing 247. The improved performance given by the constant-speed propeller enabled the Boeing 247D to become the first airliner capable of climbing on one engine.

A constant-speed propeller mounted on a Boeing B-17G Flying
Fortress. The chrome dome on the hub houses the constant-
speed unit piston. Pima Air & Space Museum, Tucson, Ar.

Hamilton Standard invented the ‘paddle blade prop’ circa 1940, to produce greater static thrust for better take-off performance. Paddle blade props have broad constant chords, usually with blunt tips on modern propellers. Hamilton Standard also introduced the ‘Hydromatic’ feathering propeller, which went into service in 1936, and during World War 2, Hamilton Standard went on to make propellers for three quarters of the allied aircraft.

The first company in the USA to manufacture propellers was the Regina Gibson Company in 1909, which was placed in the hands of the Canadian engineer Wallace R. Turnbull. An alternative to the hydraulic constant-speed unit is the electrically operated system, first designed by Turnbull in 1925 and followed by flight-testing in 1927. Curtiss-Wright in the USA licensed and improved on Turnbull’s design and made aircraft propellers with electrically operated pitch change mechanisms. The Curtiss Electric propeller was very popular during WW II, being used on many types of American built single-engine fighters and light twin-engine bombers. Because of necessity for high-performance aircraft, the constant-speed propeller is assured a long and very productive future.

Fixed-pitch props are quite suitable for low-speed light aircraft with low horsepower. However, as engines with greater horsepower were developed producing greater cruise speeds, the fixed-pitch props suffered in performance. Thus problem was resolved when the next important step in propeller history occurred in 1916 when a variable-pitch (VP) propeller with forward and reverse thrust was first used on an airship to help with manoeuvring. The idea of a VP prop was first mooted by the Frenchman Croce-Spinelli as early as 1871 without any progress in that area. The first variable-pitch propeller mounted on an aircraft was flight-tested on 23 October 1922 by Sandy Fairchild; this was a VP propeller made by the American Propeller Company, with two positions for forward and reverse flight. Nevertheless, the major part of the prop’s development was performed in 1926 in the United Kingdom at the Royal Aircraft Factory in Farnborough (now known as the Royal Aircraft Establishment or simply as Farnborough). A Royal Aircraft Factory RE-8 airplane was used to flight-test the four-blade, wooden VP propeller. The flight-testing of VP props was also progressing well in Germany, with the German R-36 bomber being the first multi-engine aircraft to fly with twin VP props. Variable-pitch props were used on many aircraft types between 1928 and 1940. The propeller manufacturer Standard Steel in the USA made the ground adjustable VP propeller for Charles Lindbergh’s Ryan NY-P ‘Sprit of St. Louis’ in which he achieved the first solo transatlantic flight on 20-21 May 1927.

The US Army aircraft Engineering Division at Wright Field, Dayton, Ohio, further developed VP props under the direction of Frank Caldwell. He later joined Hamilton Standard Division of the United Aircraft Company as their Chief Engineer, where he perfected controllable pitch propellers in 1933 for which he received the Collier Trophy.

Reverse thrust props were not operational until 1943 when they became popular for manoeuvring flying boats on the water. Two years later reverse thrust found its way onto conventional, wheeled aircraft to assist landing roll braking. One of the first aircraft to be so equipped was the Boeing B17 Flying Fortress bomber. Just about all modern turboprop aircraft are now equipped with reverse pitch propellers. It is rare to find a reverse pitch prop on a single-engine aircraft but not altogether unknown. Although VP props have remained in limited production over the year’s right up to the present day, its place has been taken over by the introduction of the constant-speed propeller for high-performance aircraft.