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 ; Convair XFY Pogo
Developed from	Allison T38

The Allison T40, company designation Allison Model 500, was an early American turboprop engine composed of two Allison T38 power sections driving a contra-rotating propeller via a common gearbox.^[1]

Design and development

The T40 concept originated at Allison in 1944, where design studies were carried out on a 4,000 shp (2,983 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 kW) turboprop engine. The resulting T40 combined two Allison T38-A-1 power sections side-by-side with a common reduction gearbox powering contra-rotating propellers. 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.^[1]

The T38-A-1 power sections each contained 17-stage compressors giving a pressure ratio of 6.3:1, eight combustion chambers, and four-stage turbines. A combining reduction gearbox was driven via extension shafts from the power sections. Each transmission shaft incorporated a clutch to allow the power sections to be run independently.^[1]

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 airscrew brake to prevent windmilling when the engine was stopped. Anti-icing for the air intakes was supplied by compressor bleed air. The contra-rotating propeller consisted of two 14 ft (4.3 m) three-bladed Aeroproducts propellers fitted to concentric shafts.^[1]

Testing of the XT40 began on 4 June 1948. There was a vibration problem in the reduction gearbox due to the coincidence of tooth-meshing frequencies which required a redesign. The de-coupling system, designed to disconnect a power section in the event of failure worked successfully. During a 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 debris thrown down the intake, proving the de-coupling system.^[1]

First flight test of the T40 was in the Convair XP5Y flying boat on 18 April 1950 fitted with four T40-A-4 developing 5,250 shp (3,915 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.^[1]

The major problems of the T40 included gearbox failures, and the propeller control system (all the engine accessories were powered by a vacuum system which used 25 hoses), and the engine 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 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 on 14 December 1950.^[1]

Operational history

The first aircraft to fly with the T40 was the Convair XP5Y-1 patrol aircraft prototype. As flying boat patrol aircraft became redundant, the US Navy changed the role of the aircraft from Anti-Submarine Warfare to Transport. Modifications to the XP5Y-1 (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). There were numerous problems with the T40s. One resulted in near disaster in 1956, when an R3Y managed to land with a runaway engine, resulting in a collision with a seawall. This event gave the US Navy one more reason to ground the R3Y, which it did soon after.^[1]

The only other aircraft to be produced in any quantity to be powered by the T40 was the Douglas A2D Skyshark. Of the sixteen examples built, the twelve used for evaluation suffered similar problems with the propellers and gearboxes.^[1]

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 A-3 Skywarrior.^[1]

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 Pogo, Lockheed XFV and the Hiller X-18 tilt-wing research aircraft. Fitted with the more powerful 7,100shp YT40-A-6 the XFY-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.^[1]

Some flying was carried out with the Hiller X-18 but most research was carried out with the aircraft attached firmly to a force measuring platform, that could be raised hydraulically, to gather data on ground cushion effects.^[1]

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.^[1]

Applications

Data from:^[1]

Variants

Model 500: Company designation for the T40
Model 501: Company designation for the T38 (half a T40, for development of the T40)^[2]
Model 501-D: Company designation for the T56
Model 503: Company designation for the T44
XT40-A-1
XT40-A-2
T40-A-4
XT40-A-5
XT40-A-6
XT40-A-10
YT40-A-14
T40-A-20
T40-A-22
T44: (Model 503), with three power sections; none were built and the project was cancelled.^[3]
T54-A-2: Re-designed T40 giving 7,500 shp (5,600 kW) equivalent. 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.^[1]^[4]
T56: (Model 501-D) Successful development of the T40 / T38 with a single power section.

Specifications (T40-A-6)

Data from Aircraft engines of the World 1953^[4] Turbojet History and Development 1930–1960 vol.2^[1]

General characteristics

Type: Turboprop
Length: 167 in (4,200 mm) with gearbox; 84 in (2,100 mm) (power section)
Diameter: 23.5 in (600 mm) (gearbox)
Width: 39 in (990 mm) (power section)
Height: 25 in (640 mm) (power section)
Dry weight: 2,500 lb (1,100 kg) (power section)

Components

Compressor: 17-stage axial flow (2 of)
Combustors: 8 can-type per power-section
Turbine: 4-stage axial flow (2 of)
Fuel type: 100/130 Gasoline or Kerosene based jet fuel
Oil system: dry sump, 65 psi (450 kPa) pressure spray, scavenged

Performance

Maximum power output: 5,100 shp (3,800 kW) + 830 lbf (3.7 kN) for take-off at 14,300 rpm at sea level; 5,500 shp (4,100 kW) equivalent
Overall pressure ratio: 6.3:1
Specific fuel consumption: 0.63 lb/(hp⋅h) (0.38 kg/kWh) (equivalent shp)
Power-to-weight ratio: 2.222 hp/lb (3.653 kW/kg)</li.
Gearbox: Combining gearbox to drive contra-rotating propellers at 0.0635:1 (908 rpm for take-off)

References

^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j ^k ^l ^m ⁿ ^o ^p Kay, Anthony L. (2007). Turbojet History and Development 1930–1960 vol.2 (1st ed.). Ramsbury: The Crowood Press. ISBN 978-1-86126-939-3.
^ Nolan, D. J. (8 August 1952). "TURBO-LINER : Development of the Allison T-38 Engine in a Convair 240" (pdf). Flight. LXII (2272): 157–159. Retrieved 5 January 2019.
^ "Designations Of U.S. Military Aero Engines – Jet and Turbine Engines, 1946–1968". Andreas Parsch. Retrieved 2009-11-21.
^ ^a ^b Wilkinson, Paul H. (1953). Aircraft engines of the World 1953 (11th ed.). London: Sir Isaac Pitman & Sons Ltd. pp. 68–69.

"Designations Of U.S. Military Aero Engines – Jet and Turbine Engines, 1946 – 1968". Andreas Parsch. 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.

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
"American Turboprops" a 1950 Flight article
"Tradewind Turboprops" a 1955 Flight article

[KayTHaD-1] ^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j ^k ^l ^m ⁿ ^o ^p Kay, Anthony L. (2007). Turbojet History and Development 1930–1960 vol.2 (1st ed.). Ramsbury: The Crowood Press. ISBN 978-1-86126-939-3.

[Flight080852-2] Nolan, D. J. (8 August 1952). "TURBO-LINER : Development of the Allison T-38 Engine in a Convair 240" (pdf). Flight. LXII (2272): 157–159. Retrieved 5 January 2019.

[Parsch-3] "Designations Of U.S. Military Aero Engines – Jet and Turbine Engines, 1946–1968". Andreas Parsch. Retrieved 2009-11-21.

[Wilkinson2-4] Wilkinson, Paul H. (1953). Aircraft engines of the World 1953 (11th ed.). London: Sir Isaac Pitman & Sons Ltd. pp. 68–69.

[1]

[2]

[3]

[4]

v t e Allison Engine Company aircraft engines
V engines	VG-1410 V-1710 V-3420
Turbojets	J33 J35 J71 J102
Turbofans	AE 3007 AR.168R† TF32 TF41 F136†
Turboprops/Turboshafts	250/T63 AE 2100 T38 T39 T40 T44 T54 T56 (variants) T61 T78 T406 T701 T800†
Propfan	578-DX†
† Joint development aeroengines

v t e United States military gas turbine aircraft engine designation system
Turbojets	J30 J31 J32 J33 J34 J35 J36 J37 XJ38 J39 J40 XJ41 J42 J43 J44 J45 J46 J47 J48 XJ49 J51 J52 J53 J54 J55 J56 J57 J58 J59 J60 J61 J63 J65 J67 J69 J71 J73 J75 J79 J81 J83 J85 J87 J89 J91 YJ93 J95 J97 J99 J100 YJ101 J102 J400 J401 J402 J403 J700
Turboprops/ Turboshafts	T30 T31 T33 T34 T35 T36 T37 T38 T39 T40 T41 T42 T43 T44 T45 T46 T47 T48 T49 T50 T51 T52 T53 T54 T55 T56 (variants) T57 T58 T60 T61 T62 T63 T64 T65 T66 T67 T68 T69 T70 T71 T72 T73 T74 T76 T78 T80 T100 T101 T400 T405 T406 T407 T408 T700 T701 T702 T703 T706 T708 LHTEC T800 APW T800 T900 T901
Turbofans	TF30 TF31 TF32 TF33 TF34 TF35 TF37 TF39 TF41 F100 F101 F102 F103 F104 F105 F106 F107 F108 F109 F110 F112 F113 F117 F118 F119 YF120 F121 F122 F124 F125 F126 F127 F128 F129 F130 F135 F136 F137 F138 F400 F401 F402 F404 F405 F408 F412 F414 F415
Adaptive cycle engines	XA100 XA101 XA102 XA103