Subject: Number Eighty-Eight in the Series--McDonnell XF-88 Voodoo

From: [email-masked]
Date: 16 Sep 93 16:23:50 GMT

>From [email-masked]

The USAAF had found by painful experience in World War II that fighter 
escort was absolutely vital for the survival of bombers in enemy 
airspace.  Unfortunately, the first jet aircraft were notorious fuel 
hogs and lacked the range and endurance of their piston-engined 
counterparts, and would be unable to escort long-range bombers such as 
the B-29, B-50, and B-36 all the way to their targets.  In an attempt 
to solve this problem, the USAF considered all sorts of proposals for 
markedly increasing the range of jet fighter escorts, some of which 
bordered on the bizarre.  One proposal for the solution to the escort 
fighter range problem was for the bombers to tow their escorting 
fighters into the combat zone and release them when their protection 
was needed.  Several experiments were made with B-29s or B-36s towing 
P-80 or P-84 jet fighters, none of them being very successful and some 
being downright dangerous.  Other proposals revived the parasite 
fighter concept of the 1930s, this time with jet fighters being 
launched from platforms suspended from the bellies of large bombers.  
The best known example of this idea was the McDonnell XF-85 Goblin.  
Other ideas included the use of mixed power concepts such as that 
which produced the Convair XP-81.  Others involved the construction of 
large, bulky fighters that were virtually flying fuel tanks, e.g., the 
Bell XP-83.  

Initial attempts to produce jet-powered fighters with the endurance of 
piston-engined aircraft (e. g. the Bell XP-83 and the Convair XP-81) 
were disappointing, and in early 1946, the USAAF informally requested 
proposals for a "penetration fighter" with a combat radius of at least 
900 miles and a performance capable of meeting all opposing fighters 
on more than equal terms.  In addition, the USAAF wanted to keep the 
gross weight of the aircraft below 15,000 pounds.  They didn't ask for 
much, did they? :-) 

Spurred on by the USAAF request, the McDonnell Aircraft Corporation of 
St. Louis, Missouri initiated work on the Model 36 on April 1, 1946.  
The Model 36 project called for a large twin-jet aircraft powered by a 
pair of 3000 lb. st. Westinghouse J34 engines mounted side by side in 
the lower part of the central fuselage.  The engines were fed by 
straight-through air intakes mounted in the wing roots.  The jet 
exhausts were underneath the rear fuselage.  This configuration, it 
was hoped, would leave enough space in the fuselage for the fuel 
needed for the long-range penetration mission.  A 35-degree sweptback 
wing was fitted, and a V-tail was to be used.  A set of dive brakes 
was mounted on the rear fuselage, hinged at the rear.  The pilot's 
cockpit was situated well forward of the wing.  The armament was to be 
six 20-mm cannon.  

The USAAF was sufficiently interested in the proposal that a Letter of 
Intent was awarded to McDonnell on May 7, 1946.  A mockup was ready by 
the summer of 1946.  Following inspection of the mockup in August, 
some changes were made.  The wing root intakes were given 40 degrees 
of sweep and a boundary layer ramp was added on the intakes to improve 
pressure recovery.  The V-tail was replaced by conventional swept 
surfaces to prevent a predicted adverse rolling moment and 
insufficient longitudinal stability near the stall.  

The USAAF was satisfied with the changes, and ordered two prototypes 
on February 14, 1947 under the designation XP-88.  The serials were 
46-525 and 46-526.  

On June 11, 1948, the designation XP-88 was changed to XF-88 when the 
P designation was replaced by F.  

The first prototype (46-525) was taken out to Muroc Dry Lake for 
testing.  It made its maiden flight on October 20, 1948, piloted by 
Robert M.  Edholm.  The powerplants were a pair of 3000 lb.st.  
Westinghouse XJ34-WE-13 axial-flow turbojets.  No armament was fitted.  

Some minor problems were encountered during the test flight program.  
Some loss of thrust was encountered during takeoff due to choking in 
the S-shaped air ducts.  This was solved by fitting spring-loaded 
blow-in doors in the wheel well section of the ducts.  The rolling 
rate was found to be insufficient.  It was improved to a certain 
extent by increasing the aileron chord by 26 percent, but the full 
cure for this problem required an increase in the torsional rigidity 
of the wing.  When the dive brakes were fully open, there was 
excessive buffeting.  This problem was eliminated by perforating the 
brakes and restricting their opening angle to 45 degrees.  In order to 
correct problems with inadequately damped directional oscillations and 
objectionable roll coupling, artificial means of stablization had to 
be devised, and a yaw damper system was adopted using a yaw rate gyro 
and controlling the rudder.  

With these changes, handling characteristics were generally 
satisfactory.  However, performance was rather disappointing, due 
primarily to the demanding range requirements and to the weight 
increases which had taken place since the initial design phase.  
Maximum speed at sea level was only 641 mph, slower than that of the 
F-86 Sabre which was already in production.  It took approximately six 
minutes for the XF-88 to reach an altitude of 30,000 feet.  Initial 
estimates of a combat weight of only 16,500 pounds had proven to be 
overly optimistic, and the installation of additional equipment as 
requested by the USAF and the various modifications required as a 
result of flight testing were estimated to push the combat weight to 
over 20,000 pounds, making matters even worse.  In order to satisfy 
the range requirements, the 734 US gallon internal fuel capacity was 
to be supplemented by 350-gallon wingtip tanks.  However, wind tunnel 
testing indicated that serious stalling problems would be encountered 
when these tip tanks were mounted.  

It would appear that the XF-88 needed more power.  McDonnell proposed 
to fit afterburners to the J34 engines that powered the second 
prototype, and the production F-88s were to be powered by 6000 lb.st.  
Westinghouse J46 afterburning turbojets.  The USAF did not choose to 
fund the J46 installation, but they did approve the adaptation of the 
second prototype to the afterburning J34 engines.  However, this 
adaptation proved more difficult than expected.  Westinghouse found it 
difficult to achieve the additional thrust that was required with an 
afterburner only 52 inches long (the maximum which could be allowed 
due to ground clearance considerations).  McDonnell was forced to 
develop the afterburner on its own.  The length of the 
McDonnell-devised afterburner turned out to be only 30 inches and 
weighed only 218 pounds.  It boosted the thrust of each Westinghouse 
XJ34-WE-15 engine from 3600 to 4825 pounds.  

The second prototype (46-526) was redesignated XF-88A, and was fitted 
with the afterburning XJ34-WE-15 engines.  It was also fitted with 
bladder fuel cells in the wings to increase internal fuel capacity to 
834 gallons.  The XF-88A made its first flight on April 26, 1949.  The 
performance improvement was apparent--maximum speed at sea level was 
almost 700 mph, time to climb to 30,000 feet was cut to 4 minutes, and 
takeoff run was reduced by 20 percent.  

The XF-88A was faced with some stiff competition for the USAF 
penetration fighter order.  The Lockheed XF-90 and the North American 
XF-93A had also been entered as contenders for the USAF penetration 
fighter order.  The XF-90 was a twin-jet design which first flew in 
June of 1949.  The XF-93A was a beefed-up derivative of the F-86 
Sabre, initially ordered under the designation of F-86C.  In the 
penetration fighter competition, the USAF initially favored the North 
American design because of its commonality with other Sabre variants, 
and in June of 1948 they supplemented the contract for the two F-86Cs 
with a contract for 118 production aircraft.  In December of 1948, 
McDonnell was instructed to stop design and development work on the 
F-88, but was permitted to continue flight testing on the two 
prototypes.  

It would seem that the curtain had been brought down on the XF-88.  
However, the F-93A production contract contract was suddenly cancelled 
in February of 1949.  Several reasons were given.  Perhaps the most 
important reason was a severe reduction in the military budget for 
that year.  With limited funds available, it was decided to give 
priority to interceptors and to strategic bombers.  In addition, a 
Senior Officers' Board felt that no production order for any 
penetrator fighters should be awarded until a competitive flyoff 
between the three contenders could be carried out.  

The flyoff between the Lockheed XF-90, the McDonnell XF-88, and the 
North American YF-93A took place between June 30 and July 8 of 1950.  
On August 15, 1950, the Evaluation Board declared the McDonnell XF-88A 
to be the winner of the contest.  However, McDonnell's victory was 
rather hollow since no penetration fighters were every actually 
manufactured or placed in service because wartime pressures mandated 
that priority be given to the procurement of existing types for use in 
Korea.  In addition, the development of long-range, high-speed jet 
bombers such as the B-47 and the B-52 eliminated any real need for 
penetration fighters.  

At the request of the Air materiel Command (AMC), in October 1949 
after completing its Phase I tests at Edward AFB, the first XF-88 
(46-525) was returned to McDonnell for modification as the XF-88B, a 
propeller research vehicle.  Provisions were to be made for testing 27 
combinations of propellers driven by an Allison T38 turbine.  The 
XF-88 was to be fitted with a 2750 shp Allison XT38-A-5 turboprop 
offset to port in the nose, and the nosewheel was to be moved to 
starboard.  The non-afterburning XJ34-WE-13s were to be replaced by 
afterburning XJ34-WE-15s, and a fuel cell was to be installed in the 
wing as was done for the XF-88A.  The fuselage fuel capacity was to be 
reduced to 543 gallons to provide space for flight test equipment.  
240 pounds of ballast was to be placed in the rear fuselage to balance 
out the weight of the turboprop in the nose.  

Modifications began with the installation of the afterburning 
turbojets.  However, before the turboprop engine could be installed, 
46-525 had to be returned to the USAF to replace the XF-88A (46-526) 
which had been damaged on June 16, 1950.  The turboprop installation 
was not completed until early 1952.  It was first tested in flight on 
April 24, 1953.  The XF-88B spent most of its time at the Langley 
Memorial Aeronautical Laboratory.  It was joined by the damaged XF-88A 
in 1955, which served as a source of spare parts for the XF-88B.  
Testing continued until 1956.  It had the distinction of being the 
final propeller-driven fighter in the USAF designation series.  

Although the F-88 never attained production status, it was the 
inspiration for the later supersonic F-101 Voodoo, which performed 
successfully in any number of roles, including penetration, escort, 
tactical strike, nuclear strike, photographic reconnaissance, 
interceptor, and conversion training.  

Following the completion of its testing, the XF-88A sat in a junkyard 
at Langley AFB in Virginia for several years.  Both the XF-88A and the 
XF-88B were eventually scrapped.  

Specification of the XF-88: 

Two 3000 lb.st. Westinghouse XJ34-WE-13 axial-flow turbojets.  
Dimensions: wingspan 39 feet 8 inches, length 54 feet 1 1/2 inches, 
height 17 feet 3 inches, wing area 350 square feet.  Weights: 12,140 
pounds empty, 18,500 pounds loaded, 23,100 pounds maximum.  Maximum 
speed: 641 mph at sea level.  Climb to 35,000 feet in 14.5 minutes, 
service ceiling 36,000 feet, range 1737 miles.  The planned armament 
of 6 20-mm cannon was not installed.  

Specification of the XF-88A: 

Two Westinghouse J34-WE-15 turbojets, 3600 lb.st. dry, 4825 lb.st 
with afterburning.  Dimensions: wingspan 39 feet 8 inches, length 54 
feet 1 1/2 inches, height 17 feet 3 inches, wing area 350 square feet.  
Weights: 12,140 pounds empty, 18,500 pounds loaded, 23,100 pounds 
maximum.  Maximum speed: 641 mph at sea level.  706 mph at 20,000 
feet.  Initial climb rate 8000 feet per minute.  climb to 35,000 feet 
in 14.5 minutes, service ceiling 39,400 feet, range 1737 miles.  
Armament consisted of six 20-mm cannon in the nose.  

Sources: 

  McDonnell Douglas Aircraft Since 1920: Volume II, Rene J. Francillon, 
  Naval Institute Press, 1990.  

  Fighters of the United States Air Force, Robert F. Dorr and David 
  Donald, Temple Press Aerospace, 1990.  

  The American Fighter, Enzo Angelucci and Peter Bowers, Orion, 1987.  

Joe Baugher		AT&T Bell Laboratories   	
2000 North Naperville Road      Naperville, Illinois 60566-7033