Engine/Airframe Mods

Douglas A/B-26 Invader

Engine/Airframe Mods

Engine test-bed 44-34586 - General Electric GE-J31

In 1940, the United States, although neutral, was beginning to support the Allies. GE started expanding to meet their defense needs and built two new plants for turbo production. By mid-1941, GE turbos were in mass production in four states and were seeing combat service with Allied Air Forces under the Lend-Lease program.

Moss also led GE in developing its early gas turbine engine, which in America of the late 1930s, was still experimental and confined to the laboratory. Britain and Germany, on the other hand, had made steady progress in use of the turbine as a primary source of propulsion. Both Germany's Hans von Ohain and Britain's Frank Whittle had independently invented the turbojet engine in the mid 1930s.

Finally in 1941, GE received its first contract from the U.S. Army Air Corps to build a gas turbine engine based on Frank Whittle's design. Six months later, on April 18, 1942, GE's engineers successfully ran their I-A engine—the first jet engine to operate in the United States. On October 1, 1942, a Bell P-59 powered by General Electric I-16 turbojet engines made its first flight at California's Muroc Army Air Field. The jet age had come to America. The company followed shortly with the J-31, the first turbojet produced in quantity in the United States.

The XA-26F was a prototype for a high-speed version of the Invader. A single A-26B-61-DL (44-34586) was modified as the XA-26F prototype. It was fitted with a pair of 2100 hp R-2800-83 engines initially driving three-bladed propellers but later fitted with four-bladed units with large spinners. In addition, a 1600 lb.s.t. General Electric J31 turbojet was installed in the rear fuselage, with the exhaust pipe in the tail and fed by an intake above the central portion of the fuselage in place of the usual dorsal turret and gunner position. The eight-gun nose and the six wing 0.50-inch machine guns were retained. With all three engines running at full power, the aircraft achieved a maximum speed of 435 mph at 15,000 feet. However, this was deemed to be an insufficient performance improvement over the proposed A-26D to warrant any production.

Specifications GE-J31

Dry weight: 850 lb (386 kg)

Compressor: Single stage centrifugal

Turbine: Single stage

Thrust: 1,650 lbf (7.33 kN)

Power-to-weight ratio:

Serial #: 44-34586
Construction #: 27865
Civil Registration:
N66368
Model(s):
A-26B
XB-26F
Name: None
Status: Unknown
Last info: 1972

History:
Lindsay Hopkins Vocational School, Miami Airport, FL, 1964-1972.
- Registered as N66368

Built as DOUGLAS XA-26F - Prototype for a high-speed version of the Invader

Started out as an A-26B but modified in late 1945 as XA-26F with J31 turbojet aft of bomb bay.

During the years 1950 and 1951 flew for Shell Oil Co. on a bailment contract for the USAF doing fuel research.

Donated to Lindsay Hopkins Vocational
School in the 1950s, and was on the civil registry at least
1964-1969 as N66368. used for ground instruction at the George T. Baker aviation school in Miami Airport and reportedly scrapped in 1972.

XA-26F - Chief test pilot - Gene May

Serial no. 44-34586 prototype for a high-speed A-26F powered by two 2,100 hp R-2800-83 engines driving four-bladed propellers with a 1,600 lb.s.t. General Electric J31 turbojet installed in the rear fuselage. The prototype reached a top speed of 435 mph but the series was cancelled as performance gains were not sufficient.

On June 1946, the XA-26F covered a 621-mile (1000 kilometer) course with a 1000 kilogram load at an average air speed of 413 mph. The aircraft was being flown by Lt. Col. T.P. Gerrity and Capt. W.K. Rickert (Pictured above) With all three engines operating, the XA-26F reached a top speed of 435 mph at 15,000 feet."

Records set by the XA-26F

Speed over 1000 km with 1000 kg payload : 660.53 km/h

Date of flight: 20/06/1946

Pilot: T. P. GERRITY (USA)

Crew: W.K. Rickert

Course/place: Dayton, OH (USA)

Date of flight: 20/06/1946

Pilot: T. P. GERRITY (USA)

Crew: W.K. Rickert
Course/place: Dayton, OH (USA)

XA-26F - Prototype for a high-speed flight

Serial no. 44-34586 prototype for a high-speed A-26F

Powered by two 2,100 hp R-2800-83 engines driving four-bladed propellers with a 1,600 lb.s.t.

General Electric J31 turbojet installed in the rear fuselage.

The prototype reached a top speed of 435 mph but the series was cancelled as performance gains were not sufficient.

To aviation historians, the end of the Second World War in Europe was loaded with what-ifs? The majority of these speculations resided with the Luftwaffe and Nazi Germany's attempt to get various advanced aerial weapons into production and, more importantly, into combat. However, the Allies had smashed the Nazis and their very limited production capability so these "what-ifs" are mainly idle speculation - especially in the light that the Luftwaffe's new state-of-the-art warplanes such as the Me 262 were negated by poor production quality, lack of adequate fuel and supplies, and misguided higher command decisions on how they should be produced and deployed. Also, Allied air superiority just about negated any use of advanced warplanes for, once airborne, they would be swarmed by hundreds of fighters.

The newly-emerging jet engine technology was not lost upon Douglas who was busy developing the very advanced XB-42 (piston) and XB-43 (jet) series of bombers. However, they also knew that the A-26 Invader had very high performance and speculated how that performance could be enhanced if jet power was somehow incorporated into the airframe. At the same time, propeller technology was advancing so Douglas approached the USAAF with a proposal for a mixed power Invader and a contract was issued to convert A-26B-61-DL s/n 44-34586 to an experimental mixed-power platform.

Given the designation XA-26F, the modifications were fairly straightforward. The extra powerplant was a General Electric Model 7E-116-4 gas turbine and to install the unit in the rear fuselage, the gunner's sighting station and all related equipment was removed. The upper and lower turrets were also removed along with the Station 0 armor plate. The electrical equipment in the former gunner's compartment was relocated along with the radio compass. The SCR-695 (IFF) radio and radio compass loop antenna were also relocated while the aft portion of the flight control cables had to be rerouted.

A large air scoop for the jet was added atop the fuselage while the tail cone was refashioned into a tail pipe. A long exhaust pipe and shroud assembly ran from the engine to the tail cone. Under where the top turret would have been, a 125-gallon fuel cell was installed to hold the jet's Spec. AN-F-32 Grade K JP-1 (kerosene) fuel along with an eight-quart tank for the AAF Spec. 3580D medium grade oil. The fuel system was controlled by the operation of a master switch and the throttle. Fuel pressure ranged from 20 psi at engine idling speed to 380 psi maximum engine operating speed. The turbine would act as an assist to improve combat performance and make takeoffs possible from short runways or with extra heavy loads.

All flight controls, their maintenance and operation, remained unchanged except for cable routing in the vicinity of the aft engine installation. The throttle for the aft engine was isodraulically operated. This unit was self-contained and was in no way connected to the airplane hydraulic system.

The XA-26F was not to be a stripped-out test vehicle for it carried an eight-gun nose and a six-gun wing. Also, large four-blade paddle-style propellers had been added along with a set of spinners that had been made for the prototypes and early production aircraft. The engines were P&W R-2800-83s capable of 2100-hp each.

Before serious testing could really get underway, the war was over. However, the Air Force considered the XA-26F an important test vehicle and continued flying the aircraft in different configurations. On June 1946, the XA-26F covered a 621-mile (1000 kilometer) course with a 1000 kilogram load at an average air speed of 413 mph. The aircraft was being flown by Lt. Col. T.P. Gerrity and Capt. W.K. Rickert. With all three engines operating, the XA-26F reached a top speed of 435 mph at 15,000 feet.

With a whole new generation of jet warplanes on the horizon, it was obvious that it would not make practical sense to convert operational Invaders to the A-26F configuration. However, this did not mean the prototype's career was over.

Fitted with standard propellers and minus the spinners, the XA-26F was assigned to the Shell Oil Company in late 1949 for flight test work. Two oil company engineers were positioned in the cockpit while another two were crammed into the rear fuselage along with the jet. Under the direction of D.N. Harris, Shell's Project Engineer of Flight Research, the XA-26F was operated on numerous flights between Los Angeles and Oakland, California, to obtain experimental data on aviation fuels.

As one flight test engineer stated in a period publication, when both P&Ws were running at full power and the jet cut in it was like, "a kick in the butt."

When this valuable research was concluded in the 1950s, the XA-26F was stored for a period and then transferred to a technical school in Florida where it survived until the early 1970s when it was scrapped.

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Under wing cargo or personnel carrier

One unusual use of the B-26 was for use in a program designed to test the feasibility of droppable cargo containers. At least two B-26s, B-26B 44-34606 ( Which was accidently shot down by 64-17668 in 1966 and crashed into the sea ) and B-26C 44-35678 were used for this program and modified to carry the container under its right wing. A suspension apparatus was attached to the underside of the wing and a remote release mechanism was installed. The idea was to resupply forward operating bases and other inaccessible or hazardous areas without access to normal airlift, for example, frontline troops engaged in battle or aircrew members after an emergency/crash landing.

The container itself was a streamlined box with access panels built into the side for loading and unloading supplies. The "landing gear" consisted of a pair of skids in the center and a small wheel and tire at the nose. Various configurations were tested for stability in flight (vertical stabilizer added) and ruggedness on the ground.

The test program was marginally successful; however, advances in precision and low level air drops by more conventional cargo/transport aircraft made the use of combat aircraft to deliver supplies impractical for operational service.

*Type*	Number built/ converted	*Remarks*
B-26	At least 2	Special Project test aircraft

Below shows 44-35678 being used as a test bed for a carrying system to supply troops in forward combat areas.

Each wooden pallet was fitted with landing skids and a nose wheel, allowing it to be dropped at low altitude into small field and open areas.

The above shot shows "606" ptior to her accident with 64-17668 in 1966

Drogue chute test aircraft

One B-26B was used for testing drag and braking chutes and designated EB-26B (exempt). This aircraft was extensively modified for this test program and had a very unusual configuration. For a typical test, the EB-26B would accelerate up to landing speeds and deploy the test chute to determine its effectiveness. Since the aircraft didn't need to actually fly, major modifications were done to make the aircraft as light as possible and therefore decrease the amount of time and runway needed to get up to the necessary test speed. The most obvious modification was the removal of the wings outboard of the engine nacelles. For this reason the aircraft was nicknamed "Wingless Wonder." Further weight savings were gained by removing all armament and even the landing gear doors.

A test apparatus was fitted to the tail section and varied based on the type of chute or deployment mechanism being tested. For example, if a new aircraft design called for a downward opening drag chute compartment, a mockup was built and fitted to the EB-26B for testing. Other tests compared different types and sizes of chutes for stability and effectiveness. The EB-26B was used as a test aircraft throughout the early 1950s.

*Type*	Number built/ converted	*Remarks*
EB-26B	1	"Wingless Wonder" test bed

TECHNICAL NOTES:
Armament: None
Engines: Two Pratt & Whitney R-2800-27 radials of 2,000 hp each
Maximum speed: Approx. 150 mph (aircraft could not fly)
Span: N/A (outer wings were removed)
Length: 50 ft. 8 in.
Height: 18 ft. 6 in.
Serial number: 44-34137 (originally A-26B-45-DL)

The A-26B pictured is s/n 44-34137 which was used in the forties to conduct parachute braking tests. The outer wings, landing gear doors and other equipment was removed to lighten the aircraft so it could reach speeds simulating an aircraft that had just landed. After the trials ended in 1948, this Invader was scrapped. Several types of parachutes were tested, not only the type shown on the photo. I think the Air Force itself conducted these tests.

Photo -Recon with targeted flash bombing system

The B-26K was designed with an interchangeable nose section, which could changed from the eight .50-cal. machine gun solid nose to a bombardier's nose with clear sections. A reconnaissance "pallet" was designed to be fitted into the bomb bay as a unit consisting of five reconnaissance cameras, including a panoramic camera with in-flight photo processing capability. The bomb bay package also included photo flash bomb ejector racks for night reconnaissance missions. Special bomb bay doors with openings for the cameras and photo flash bomb ejectors allowed the bomb bay doors to remain closed during reconnaissance missions. The clear nose of the RB-26K had an optically flat panel for use with a forward oblique reconnaissance camera. The tail housed a vertical camera.

The RB-26K was capable of flying armed reconnaissance missions. The forward guns and internal bomb bay were replaced with cameras, but the eight wing pylons could still carry up to 8,000 pounds of mixed ordnance depending on mission requirements. Typically, wing ordnance was not carried to increase the speed of the aircraft.

*Type*	Number built/ converted	*Remarks*
RB-26K	-	Reconnaissance B-26K

TECHNICAL NOTES:
Armament: Eight wing pylons capable of carrying 8,000 lbs. of mixed ordnance
Engines: Two Pratt & Whitney R-2800-52Ws of 2,500 hp (maximum with water injection)
Maximum speed: 323 mph/281 knots
Cruising speed: 310 mph/270 knots
Range: 2,700 statute miles/2,346 nautical miles
Service ceiling: 30,000 ft.
Span: 71 ft. 6 in.
Length: 51 ft. 7 in.
Height: 19 ft.
Weight: 38,314 lbs. maximum
Crew: Two
Serial numbers: 64-17640 to 64-17679

The 6' Square windows were for the KA-56A panoramic mirror camera system which was equipped with an automatic in-flight processing system. Similar cameras were in service in WW2. The later F-492 camera system required 6" diameter windows in the bay doors.
A K-38 forward obligue recon camera was mounted looking through the bombardier's optically flat panel. In addition there was a P-2 vertical camera in the tail.
Flash bombs were not dropped from the forward part of the bomb bay as previously stated as the bomb bay doors could not be opened in flight wothout disrupting the camera mountings by the air stream regardless of the three air flow disrupters in front of the bomb bay.
Flash bombs were carried on the underwing hard points.
Flares were dropped from the standard flare tube system mounted in the bomb bay.

RB-26C Tactical Recon Squadron, special mods

The FA-26C was a reconnaissance conversion of the A-26C. Between 1945 and 1947, the F (secondary) prefix letter was assigned to aircraft when the primary mission was changed to reconnaissance. In this case, an A-26C modified for the reconnaissance role was re-designated FA-26C. Beginning in 1930 and prior to 1945, the F designator was used as a primary prefix for reconnaissance aircraft. For example, a P-51, built as a photo recon aircraft, was designated F-6 (not FP-51).

Only a few A-26Cs were modified for reconnaissance duties. Modifications typically involved removing all guns and installing cameras throughout the aircraft. Additionally, an aircraft intended for night reconnaissance was equipped with photo flash bombs. Some aircraft were also modified for electronic reconnaissance with the installation of radar and signal intelligence gathering equipment.

In 1948, the USAF dropped the F designator for reconnaissance aircraft and replaced it with the R designator. The F designator was reassigned to fighter aircraft. At the same time, the A (attack) designator was dropped and the A-26 was reclassified as a bomber. Thus the FA-26C was re-designated RB-26C.

With the start of the Korean War in June 1950, the USAF had very little tactical reconnaissance capability. As a result, the USAF ordered more RB-26C conversions for night reconnaissance duty.

The RB-26B, or simply RB-26, was a reconnaissance conversion of the Douglas B-26. When the Korean War began in June 1950, the 3rd Bomb Group (Light) was equipped with B-26Bs and stationed in Japan. This group had only two bomb squadrons initially (8th and 13th), but was pressed into immediate service, first covering the evacuation of U.S. citizens from South Korea in the first few days of the war and later in the interdiction mission. The B-26Bs were first used to attack North Korean ground targets (truck convoys, trains, rail lines, bridges, etc.) in daylight and achieved significant results. However, as the losses to B-26 interdiction raids became unacceptable, North Korea started moving supplies primarily at night.

In early July, the 162nd Tactical Reconnaissance Squadron (Night Photo) was told to move from Langley Air Force Base, Va., to Itazuke Air Base, Japan to begin flying reconnaissance missions as soon as possible. Supply delays with a new photo flash bomb system prevented the 162nd from beginning operations until August 1950. In September, the 162nd was assigned to the 543rd Tactical Support Group and in October the group moved from Japan to Taegu Air Base, South Korea.

The 162nd TRS was equipped with 16 RB-26s initially, but total combat-ready strength never rose above 20 aircraft. The mission of the squadron was to take night reconnaissance and damage assessment photos. However, the new photo flash bomb system was prone to failure and the flash bombs had a relatively high dud rate.

Because there were no preexisting tactics for night intruder (interdiction) missions, various techniques were tried, some involving RB-26s and B-26Bs in Hunter-Killer pairs. The reconnaissance aircraft would seek targets and when found, drop parachute flares to mark the location and call the B-26B to attack. This technique didn't work very well since the mobile ground targets usually moved away from the area illuminated by the flare or rapidly dispersed off the roadway.

Other techniques tried included adding a seven million candlepower searchlight to the aircraft. The aircraft with the light was accompanied by one or more attack aircraft and would light a target area for a short time allowing a more effective attack. There were two major drawbacks to this plan: first, the searchlight made an excellent target for enemy gunners and second, the light had a very limited time of operation (less than a minute).

Eventually, night interdiction tactics evolved so the RB-26 was not needed. Improved night photography aircraft were available early in the war and the 162nd TRS was deactivated on Feb. 25, 1951.

The RB-26 designation was used beginning in 1948 after the USAF dropped both the attack (A) and reconnaissance (F) designations. All FA-26s still in service were re-designated as RB-26s. Limited numbers of B-26Bs and B-26Cs were converted for use as reconnaissance aircraft. The gun turrets were removed and the bomb bay adapted to carry additional equipment depending on the intended mission. For example, night reconnaissance aircraft were fitted with a photo flash "bombing" system. The converted aircraft generally used the simple RB-26 designation, although aircraft fitted with a bombardier's (clear) nose could be RB-26C, while the solid nose type was the RB-26B.

When the Korean War began in June 1950, the Air Force had an urgent need for tactical night reconnaissance aircraft. The 162nd Tactical Reconnaissance Squadron was moved from Langley Air Force Base, Va., to Japan and began flying missions in August 1950. During the first year of the war, RB-26s flew 2,305 effective sorties. The RB-26 retained a limited offensive weapons capability when rockets or bombs were loaded on wing racks and during the first year of the war, 342 tons of bombs were dropped and 120 rockets were fired.

General Electric J31 and the XA-26F

General characteristics

Components

Performance


				Douglas A/B-26 Invader Engine/Airframe Mods



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