Allison T40

T40
T40-A-10 at the National Museum of the United States Air Force
Type	Turboprop
National origin	United States
Manufacturer	Allison Engine Company
Major applications	A2D Skyshark Convair R3Y Tradewind

The Allison T40, company designation Allison Model 500, was an early turboprop engine, composed of two Allison T38 power sections driving a contra-rotating propeller via a common gearbox.

Design and development

The T40 originated at Allison in 1944, where design studies were carried out on a 4,000 shp (2,982.80 kW) turboprop engine. Taking advantage of this effort the US Navy awarded Allison a contract for the design and development of a 4,100 shp (3,057.37 kW) turboprop engine. The resulting T40 combined two Allison T38 engines side-by-side with a common reduction gearbox powering a contra-rotating propeller. Similar in layout to the Armstrong Siddeley Double Mamba, the T40 differed in that each engine drove both the forward and the rear propellers, unlike the Double Mamba, where each Mamba power section drove either the front or the rear propeller separately.

T38

The power sections of the T40 consisted of T38A-1 engines, which was also developed separately as a stand-alone engine. The T38 started life with a 19-stage axial compressor, eight can type combustion chambers, a 4-stage turbine driving the compressor and the extension shaft to the reduction gearbox.

Initially rated at 2,000 shp (1,491.40 kW) the T38 first ran in 1947 and flew in the nose of a Boeing B-17 Flying Fortress test-bed on 19 April 1949, rated at 2,250 shp (1,677.82 kW). Problems with gearbox vibration and combustion were dealt with during the test programme and were mirrored by problems in the T40 programme. The engines fitted to the Convair CV-240-21 Turboliner were rated to 2,750shp.

The McDonnell XF-88B with a T38 turboprop - shown here with the propeller feathered - in the nose

Although the only aircraft slated to receive the T38 as a production engine, the Convair T-29E, was cancelled, the T38 did power a converted Convair CV-240, and was fitted in the nose of the McDonnell XF-88B to drive experimental supersonic propellers. Further development of the T38 provided the power sections for the T40 as well as forming the basis for the Allison T56 and the projected Allison T39.

T39

A projected 9,000 shp (6,711.30 kW) development of the T38 which was cancelled before hardware had been produced.

T40

The result of the US Navy's contract for the design and development of a 4,100shp turboprop engine; the T40 consisted of two T38-A-1 power sections, with 17-stage compressors giving a pressure ratio of 6.3:1, eight combustion chambers, 4-stage turbines and a combining reduction gearbox driven via extension shafts. Each transmission shaft incorporated a clutch to allow the power sections to be run independently.

Accessories included a Master Control Unit, mounted above the compressors and an air driven starter on the gearbox. The compound reduction gear had an overall reduction ratio of 15.75:1 and incorporated an air-screw brake to prevent wind-milling when the engines were not running. Anti-icing for the air intakes was supplied by compressor bleed air. The contra-rotating Propeller consisted of two 14 ft (4.27 m) three-bladed Aeroproducts propellers fitted to concentric shafts.

Testing of the XT40 began on 4 June 1948 revealing some major vibration problems from the reduction gearbox due to mis-matched noise frequencies caused by poorly designed gear teeth. The de-coupling system, designed to disconnect a power section in the event of failure. During a test run in a test cell one power section developed a major oil leak and an attempt to shut it down failed because the engine carried on running on the leaking oil. After many attempts to shut it down the offending power section could only be shut down by destroying the compressor with buckets of nuts and bolts thrown down the intake, proving the de-coupling system.

First flight of the T40 was in the Convair XP5Y flying boat on 18 April 1950 fitted with four T40-A-4 developing 5,250 shp (3,914.92 kW). Service use of the T40 revealed problems with integrity of the propellers and gearboxes with at least one instance each of a propeller breaking free and a gearbox separation. One unusual problem was discovered during ground running of the Republic XF-84H Thunderscreech turboprop powered fighter; the special transonic 3-bladed single propeller set up harmonics that proved to be harmful to humans within a certain distance from the aircraft.

The major problems of the T40 were its fragile gearbox, and the propeller control system which used 25 vacuum tubes, and was far from reliable. As the individual power plants were clutched into the gearbox, it was intended in most instances that the aircraft could cruise on one half of the engine and only engage the second power-section when there was need. In practice the system did not work so well. Failure to recognize that one of the T38s had failed, and its compressor was devouring power produced by the other section, led to the loss of the first prototype Douglas XA2D-1 and its pilot Lt. Cmdr. Hugh Wood on 14 December 1950.

T40 engines on the XP5Y-1 prototype in 1950

The overall failure of the T40 was a blow to US Naval aviation in the early 1950s, almost as severe as the Westinghouse J40 fiasco which affected diverse but important programs such as the Douglas F4D Skyray, Douglas A3D Skywarrior, and McDonnell F3H Demon.

Allison T44

The company proposed the Allison Model 503 (military designation T44), with three power sections, but none were built and the project was cancelled.

Allison T54

After the initial limited success of the T40 the US Navy sponsored development of a new turboprop about 28% larger than the T40, but this work was soon overtaken by the Allison T56 / Allison 501-D and development was cancelled.

Operational history

The first aircraft to fly with the T40 was the Convair XP5Y-1 which became redundant after the US Navy changed the role of the aircraft from Anti-Submarine Warfare to Transport. Modifications such as provision of air conditioning and pressurisation produced the Convair R3Y Tradewind, which was to become the only aircraft using the T40 to actually enter service. These large four-engined flying boats served primarily between NAS Alameda and Hawaii during the mid-1950s, (replacing the Martin Mars flying boats). Numerous problems with the T40s, ended in near disaster when an R3Y managed to land with a runaway engine, resulting in a collision with a seawall in 1958, giving the US Navy more reason to ground the R3Y, which it did soon after..

The only other aircraft to be produced in any quantity to be powered by the T40 was the Douglas A2D-1 Skyshark, of the sixteen examples built, the twelve used for evaluation suffered similar problems to the R3Y with propeller control problems and gearbox failures amongst others.

The T40 was also fitted to the North American A2J-1 Super Savage but the poor performance of the aircraft and continuing difficulties with the engines forced cancellation in favour of the Douglas A3D Skywarrior.

The most notable successes of the T40 were in the field of vertical take-off aircraft, being used to power three different types, the Convair XFY-1 Pogo, Lockheed XFV-1 and the Hiller X-18 tilt-wing research aircraft. Fitted with the more powerful 7,100shp YT40-A-6 the XFV-1 flew the first full flight from vertical take-off to horizontal flight and back to vertical flight for landing, of a fixed wing aircraft, in November 1954. Continuing concerns over propellers and the poor payload and performance precluded further development.

Some flying was carried out with the Hiller X-18 but most researchwas caried out with the aircraft attached firmly to a force measuring platform, that could be raised hydraulically, to gather data on ground cushion effects.

Limited flying was carried out by the two Republic XF-84H Thunderscreech turboprop fighter aircraft, but the continuing difficulties with the T40 engine, supersonic propeller, not to mention performance completely overshadowed by contemporary rivals led to cancellation of production plans.

Applications

Data from:

T38

Convair CV-240-21 Turboliner

McDonnell XF-88B

Convair T-29E cancelled

T40

A T40 of a Convair XP5Y-1.

Second Douglas A2D being prepared for its first flight in April 1952.

Douglas A2D Skyshark

(1 × XT40-A-2)

North American A2J Super Savage

(2 × XT40-A-6)

Convair P5Y

(4 × XT40-A-5)

Convair R3Y Tradewind

(4 × XT40-A-10)

Hiller X-18

(2 × T40-A-4)

Lockheed XFV

(1 × YT40-A-14)

Convair XFY

(1 × YT40-A-14)

Republic XF-84H Thunderscreech

(1 × XT40-A-1 driving a single 3-bladed transonic propeller)

Specifications (XT40)

Data from Turbojet History and Development 1930-1960 vol.2

General characteristics

• Type: Turboprop
• Length: 13ft 11in (4,620mm)
• Diameter: 3ft 3in (990mm)
• Dry weight: 2,500lb (1,134kg)

Components

• Compressor: 17-stage axial flow (2 of)
• Combustors: 8 can-type per power-section
• Turbine: 4-stage axial flow (2 of)
• Fuel type: Kerosene based jet fuel
• Oil system: pressure spray scavenged

Performance

• Maximum power output: 4,000shp
• Overall pressure ratio: 6.3:1
• Specific fuel consumption: 0.6
• Power-to-weight ratio:

See also

Related development

• Allison T38
• Allison T56
  

Comparable engines

• Armstrong Siddeley Double Mamba
• Pratt & Whitney Canada PT6T
• Teledyne CAE T67
• Kuznetsov TV-2
  

Related lists

• List of aircraft engines
  

References

• "Designations Of U.S. Military Aero Engines - Jet and Turbine Engines, 1946 - 1968". Andreas Parsch. http://www.designation-systems.net/usmilav/engines.html. Retrieved 2009-11-21. 

Kay, Anthony L. (2007). Turbojet History and Development 1930-1960 vol.2 (1st ed.). Ramsbury: The Crowood Press. ISBN 978 1 86126 939 3. 

Further reading

• Gunston, Bill (2006). The Development of Jet and Turbine Aero Engines, 4th Edition. Sparkford, Somerset, England, UK: Patrick Stephens, Haynes Publishing. ISBN 0-7509-4477-3. 
• Leyes II, Richard A.; William A. Fleming (1999). The History of North American Small Gas Turbine Aircraft Engines. Washington, DC: Smithsonian Institution. ISBN 1-56347-332-1. 

External Links

• Flight 1950 article
• Dual Turbine Produces 5,500 h.p. early article with good illustration showing how two engines are connected to one gear box