Into the early 1950s, both the Air Force and the
NACA had maintained a continuing interest in high-performance turboprop
aircraft. NACA was interested in flight testing
new supersonic propeller designs that would overcome the thrust limitations of
conventional propellers at high subsonic speeds while the Air Force, still
faced with the problems of high fuel efficiency and range for both future
intercontinental bombers and potential escort fighters, made investments into
parallel programs exploring and developing the technology. Up to that point, conventional propeller
designs were restricted to a maximum flight speed of about Mach 0.725. The goal was to develop an engine-propeller
combination that would be capable of sustained speeds of Mach 0.95.
By late 1951, the Boeing XB-52 was preparing for its maiden flight under the
power of eight of the new J57 power plants.
Although offering a clear performance advantage over the turboprop
engines that had originally been envisioned, many in the Air Force remained
skeptical that the turbojet-powered B-52 would meet its very ambitious cruise
performance figures. Meanwhile, the
hybrid power plant arrangement of the Consolidated-Vultee XP-81 remained attractive for both escort fighters
and strike fighters. During 1951, the
Air Force signed a contract with Republic Aviation for what would become the
XF-84H. In the meantime, both the Air Force and
contractors would need flight performance data to work with to pursue any
future developments.
On 25
July 1949, McDonnell began detail design of a turboprop research airplane to
be converted from XF-88 No. 1 under Contract AF33 (038)-7442. The airplane would be modified with the
installation of a 2,500-horsepower Allison T-38 Model 501 F-1
engine in the nose
as the McDonnell Model 36J. The modification
program was delayed with the crash of the XF-88A and the need for the first aircraft to stand
in during the penetration fighter evaluation.
Restricted funding meant that both XF-88s remained tied down and in
storage from the summer of 1950. Funding
for the conversion was finally released in February 1952 to begin installation
of the Allison XT-38A-5 turboprop into 46-525 as the XF-88B. The XF-88 was removed from storage at the end of March
and readied for conversion by the beginning of June. Conversion work commenced in August 1952 and
continued into the winter of 1953.
 |
Modified
nose structure of XF-88 46-525 prior to installation of the Allison XT-38-A-5
turboprop engine. Gerald Balzer
Collection, Greater St. Louis Air & Space Museum.
|
 |
| The Allison XT38-A-5 turbobprop engine during
NACA flight testing. The “rake” arms
projecting from either side were not present in the original engine
installation. Jack Reeder Archive, NASA. |
 |
| 1953 photo of original Curtiss Phase I 4-bladed
propeller mounted on the XF-88B. Gerald
Balzer Collection, Greater St. Louis Air & Space Museum. |
By
February 1953, the conversion had been completed. In addition to the
new Allison turboprop, the XF-88B was now
equipped with J34-WE-34 engines, retaining the McDonnell afterburners of the
earlier J34-WE-15 engines. With the new nose fuselage, the XF-88B was 58 feet long. A forerunner of the later T-56 turboprop, the
Allison XT-38A-5 turboprop engine could produce propeller rotational speeds of
1,700 rpm, 3,600 rpm, and 6,000 rpm using three interchangeable gearboxes. Curtiss would provide
the twelve different research propellers that had been envisioned for flight
testing, although as it would turn out only three other propeller designs would
be tested over the course of the NACA evaluation; the Phase Va, Phase Vb, and
Phase VIIb variants. The maximum
propeller diameter that could be accommodated was 10 feet, consistent with the Phase Vb design. As built, the XF-88B had a long, conical spinner and a four-bladed Curtiss Phase I propeller. On 16 February, the XT-38A-5 engine was run
up for the first time during a 29-minute ground run. Two months later, on 14 April 1953, the
XF-88B made its first flight from Lambert Field under
the power of its J34 turbojets with McDonnell test pilot Philip Houghton at the controls. The
XT-38 engine was not started in flight, with the first air-start occurring on
the fourth flight. An abbreviated series
of shakedown flights were flown by McDonnell prior to a brief evaluation by
USAF and NACA pilots from 23 June 1953. Capt. John
Fitzpatrick flew the USAF acceptance flights and would ferry the aircraft to
Langley via Wright Field, while John P. “Jack” Reeder, one of the two NACA
project pilots, flew three familiarization flights on 30 June and 1 July 1953. After the conclusion
of this initial evaluation, the XF-88B was ferried
to the NACA facility at Langley, Virginia for further evaluation and
development work, arriving at Langley AFB on 13 July 1953. By this point, 46-525 had conducted 188 flights for a total of 142.5
hours. The first public announcement of
the XF-88B supersonic propeller test bed was made on 4
July 1953.
 |
| Ground run of XT-38A engine in XF-88B. Art
Davies, Jr. Collection, Greater St. Louis Air & Space Museum. |
 |
| XF-88B in the air with feathered propeller, during
an early McDonnell test flight. Gerald
Balzer Collection, Greater St. Louis Air & Space Museum. |
 |
| NACA Project Pilot Jack Reeder (L) and McDonnell
pilot Phil Houghton (R) during acceptance testing of the XF-88B at Lambert
Field in late June 1953. Jack Reeder Archive, NASA. |
 |
| Handover of XF-88B to NACA personnel at Lambert
Field. From left to right: Project
Engineer Jerry Hammack NACA, Art Vogeley NACA, Capt. John Fitzpatrick USAF, Jack
King USAF, Project Pilot John P. Reeder
NACA, Project Pilot William L. Alford NACA and Gene Smith NACA. Gerald Balzer Collection, Greater St. Louis
Air & Space Museum. |
Once the XF-88B arrived at
NACA Langley, the aircraft was statically tested before undergoing further
modification. With the original Curtiss
Phase I propeller still installed, sound levels were checked and recorded to
provide a baseline to compare future supersonic propeller designs with. NACA test pilots Jack Reeder and William
L. “Bill” Alford were assigned as the project pilots for the XF-88B. The first NACA
research flight of the XF-88B took place on 20 November 1953. In the first series of flights, the XF-88B
tested non-rotating “dummy” spinner configurations without blades in order to
determine their true aerodynamic characteristics. The original conical spinner was compared
against two “spherical” spinner designs, one with a 45-degree cone and
the other with a 60-degree cone. The
60-degree spinner was selected for subsequent testing
by the NACA. The 60-degree design
appeared rather ungainly-looking, but provided suitable flow conditions for the
thin, three-bladed, low-aspect ratio Phase Va supersonic
propeller to produce section Mach numbers in the region of Mach 1.5. In this configuration, the spinner contour
was formed by a 17.5-inch sphere faired into a 60-degree included angle conical
nose section. The propeller blades were
constructed of SAE 4340 steel using a NACA 16-series section varying from 8-percent to 2-percent thickness. Each blade measured
11.1 inches in chord at the spinner surface and 8.4 inches at the tip. The entire propeller was 7.2 feet in
diameter. Used
with the 3,600 rpm gearbox, the Phase Va propeller produced a tip speed of
Mach. 1.7 and a design flight speed of Mach 0.95. Speeds of up to Mach 1.01 were
attained by the XF-88B during the course of
testing, although during acceptance evaluation at
McDonnell the previous June, Capt. Fitzpatrick reached speeds of up to Mach 1.2
which resulted in considerable complaint from residents in and around St. Louis. While the propeller itself performed well, on
the ground it produced noise levels in excess of 130 decibels, measured at 100
feet from the aircraft during static tests. Flight testing at NACA Langley continued
through 1957.
 |
| Test instrumentation for the XF-88B during NACA
flight testing. Jack Reeder Archive, NASA. |
 |
| The XF-88B after modification with the new
60-degree spherical spinner and Curtiss Phase Va blades. Used with the 3600 rpm gearbox for the
Allison XT38-A-5 turboprop, the combination produced propeller tip speeds of
Mach 1.7. Jack Reeder Archive, NASA. |
 |
| NACA pilot Jack Reeder climbing into the cockpit
of the Phase Va-equipped XF-88B, possibly on the occasion of its first NACA
research flight on 20 November 1953. Jack Reeder Archive, NASA. |
In an attempt to attain the desired Mach 0.95 design point and
reduce noise levels to a range comparable with that of conventional propellers, the XF-88B was tested with a new supersonic propeller
design mated with the conical spinner, the Phase Vb. A four-blade design that would have been ten
feet in diameter, it was tested in a three-blade configuration that was 9.8
feet in diameter and optimized for the same
flight speed of Mach 0.95 as the preceding Phase Va
blade design. Also using a NACA
16-series section and SAE 4340 steel construction, the new blades varied in
section thickness from 8-percent at the root to
2-percent at the tip. Blade chord was
16.1 inches at the spinner surface and 11.6 inches at the tip. The conical spinner had a base radius of 17.5
inches and had an included nose angle of 41 degrees. In this configuration, the propeller operated
at 1,710 rpm. Despite
the greater length and similar overall configuration to the preceding Phase Va
blade design, the slower rotational rate of the 1,700 rpm gearbox produced a
tip speed of Mach 1.3 for the Phase Vb blades.
This reduced rotational speed greatly aided noise reduction
efforts. These tests continued into the
early summer of 1957. Final tests
with the Phase Va blades used a new non-rotating elliptical spinner16.57 inches in
diameter and 55.1 inches long to evaluate flow conditions and result in less
interference with the propeller. As with
the other configurations tested, both the propeller designs and the XF-88B gave good results. However, the noise and vibration produced by
the supersonic propellers was often difficult for ground personnel to endure.
An
attempt to overcome the very high noise levels of previous propeller designs
was tested in the XF-88B with the elliptical spinner matched to a new
transonic blade design, the Phase VIIb, which was
6.85 feet in diameter and optimized for a lower flight speed of Mach 0.82. As with the Phase Vb
propeller, the Phase VIIb used the 1,700 rpm gearbox for the Allison turboprop
engine. With a tip speed of Mach 1.1,
the new design provided a modest decrease in noise levels but also exhibited reduced efficiency.
In a 6 January 1981 letter to author Richard Koehnen, Jack Reeder
offered his recollection of the XF-88B program:
The supersonic propeller program was a joint effort of
the Air Force, Navy, and NACA to explore the design and practicability of
propellers for economic propulsion of aircraft to cruise at Mach numbers of up
to 0.95. The NACA considered both the
B-45 and B-47 as candidate test beds, but the Air Force volunteered the XF-88
number 46-525, which was excess to their needs, and this was modified to become
the XF-88B. The Air Force contributed
the required modifications to the aircraft (a 62-gallon fuselage fuel tank was
removed for installation of the T-38 power plant), a conventional 4-blade
propeller (Curtiss) and the research propellers (built by Curtiss to the
requirements of the NACA research program), the propeller gearbox and a spare, which
could accommodate three propeller rotational speeds through internal gear
changes, whirl tests of the research propellers at WPAFB and acceptance tests
and reports of the modifications accomplished.
The Air Force Project manager at WPAFB was Charles Beinnaman.
The Navy contributed the engines to the program. These included two Allison XT38-A-5 turboprop
engines (one spare) to drive the propellers, and four Westinghouse J34-WE-34
jet engines (two of these were in the XF-88A [sic?] airframe. The afterburners were of McDonnell design.
The NACA was responsible for the research instrumentation, the
program and its conduct, the data and reporting of research results.
Acceptance tests were conducted by the Air Force after aircraft
modifications at McDonnell in June and early July of 1953. NACA personnel were in attendance to take
part and for familiarization with the final product including flight and
operating characteristics of the aircraft….
The XT-38 drive shaft and gear box had torque and thrust sensors,
respectively. The thrust meter did not
function well and was not used. The
torque meter strain gauges were on the shaft between engine and gear box. The actual torque absorbed by the propeller
was, however, determined from the wake survey rake as was the thrust. Although provisions for the research
instrumentation were made during the conversion work on the aircraft, the
actual installation (including the momentum survey rakes) was completed after
delivery to NACA Langley.
The XF-88B was ferried, after acceptance, by Capt. Fitzpatrick to
Langley with a stopover at WPAFB where it was flown by other Air Force
personnel. It arrived at Langley on July
13, 1953. The XF-88A, number 46-526, was
later delivered to NACA Langley, also, where it served for spare engine and
parts support….
In research like this a great deal of work goes on between flights
to reduce and examine data so as to decide what should be done on the next
flight, and what changes and corrections must be made to the instrumentation,
propellers, and propeller governing, gear boxes, etc., as well as to perform
required maintenance on the one-of-a-kind equipment from airplane to
propellers. The research proceeds
slowly, but not because of a lack of effort.
The XF-88A, which had an all-moving horizontal tail, in contrast to
the fixed stabilizer and elevator of the XF-88B, was not flown by NACA because
of lack of resources for operating both aircraft. It was used for spare engines and airframe
spares, primarily. However, the aircraft
was used briefly for evaluating a takeoff performance meter concept, but no
lift offs were made. Taking account of
weight, temperature, and runway length the takeoff meter indicated whether,
upon brake release, the longitudinal acceleration was adequate for safe takeoff
under the prevailing conditions.
When acquired, the XT-38’s were limited to 25 hours before scheduled
overhaul, but before such time was acquired it had been extended to 50
hours. However, at between 6-7 hours of
operation foreign object damage was suffered which required compressor blade
replacements.... Of course, the spare
engine was installed while repairs were made on the first engine. It’s not surprising that foreign object
damage did occur because of the numerous and lengthy ground runs for noise
measurements and the vulnerable position of the XT-38 engine air intake.
The XT-38 propeller gear box ratios for 1700 and 3600 rpm were used
in the research, but not that for the 6,000 rpm. A disintegration of the propeller brake
(for-feathered operation) required rebuilding of one gearbox. Only three propeller blade designs were flown
and reported on, the phases Va, Vb and VIIb….
The program was discontinued when it was felt that maximum payoff had
been achieved. An important factor was
the rapid swing to jet power (with its speed and productivity potential) for
military and commercial use. Actually,
the program was a little late, for the times, in implementation.
The XF-88B aircraft performance for this testbed role was
marginal. Thrust-to-weight ratio for
takeoff was 0.3 or less. Takeoff run,
with afterburning, to 155 mph was generally 5000 feet or more (8000-foot runway
at Langley at that time). Fuel limitations
with the performance prevailing made the whole operation time critical. Flight time could not exceed 40 minutes, and
usually only one high speed run at altitude could be achieved. As soon as familiarity was achieved the XT-38
was started at 5000 feet (blade stall flutter was a limitation for takeoff) and
the test propeller configuration was used for climb. Engine nozzles in non-afterburner operation
were manually adjusted by rheostats in the cockpit for maximum temperature to
obtain military thrust, which was the power used in climb. Sometimes at full throttle the nozzles would
not close in response to rheostat adjustment until the throttles were
retarded. Then they could be advanced
again.
The afterburners did not perform satisfactorily. The thrust augmentation with afterburning was
estimated to be about 1.41 in design.
However, ground static tests at Langley assured only 1.12 to 1.19 in
actuality. Furthermore, the tailpipe
nozzles, automatically controlled by temperature in afterburning operation,
would frequently hunt, causing large fluctuations in thrust which damped poorly
or not at all. The problem lay in the
friction in the Arends’ controls (flexible wire enclosed in flexible, anchored
guide tubes) used to move the nozzles and did not seem to be amenable, for any
length of time, to maintenance steps or lubrication. Afterburning light off could not reliably be
achieved above 20,000 feet, if used.
However, blowout of one or both afterburners frequently occurred above
20,000 to 25,000 feet. This was hard to
detect at times, and a rapid loss of fuel would occur before recognition. Re-lights were not generally possible without
descending. We eventually did away with the
afterburners and replaced them with straight tailpipes.
The aircraft was usually towed to the head of the runway where
assurance of prompt takeoff clearance was obtained before engines were
started. Engines were run up with the
brakes on and turbine outlet temperature limits set with the cockpit
rheostats. Generally, both afterburners
were lit, one at a time, and checked for temperature and steady operation
before brake release. However, one day I
was startled by the aircraft sliding down the runway. I thought the brakes weren't holding. Actually, the afterburners were putting out
their best. After takeoff, the
afterburners were shut down as soon as 230 mph in the clean configuration was
obtained. The XT-38 was started at 5,000
feet and 250 mph and used in climb.
During all operation with the propellers the cockpit noise was similar
to that with a reciprocating engine, such as the Merlin or the Allison. Climb was made with the aircraft position
with respect to the airbase in mind so that, when the fuel state dictated, the
data run could be started with the aircraft heading toward home.
For a speed over about Mach 0.85, the climb was continued to as high
an altitude as the fuel state would permit, generally 30-35,000 feet. The highest altitude obtained during the NACA
program was 39,000 feet over Wallops Flight Center during an airspeed static
source calibration using ground radar.
At the highest altitude achieved for a test run the aircraft was
accelerated in level flight, data system running, as long as practical. It was then dived to obtain the speed desired
by 30,000 feet, if possible, but not so steeply that the rate of speed change
might lead to large off-speed conditions of the propeller or oscillations in
rpm. The governor time constant was
adjustable to obtain to obtain good performance, but its setting was only an
estimate for each propeller configuration.
The governor never did cause a problem but such dives to obtain the
high speeds were not made often enough with a given propeller configuration to
become thoroughly familiar with governing limitations.
On one flight, following the installation of a more elaborate XT-38
fire detection and warning system, which sensed temperature rate as well as
temperature, the fire warning light came on during a dive. The throttle of the XT-38 was pulled back,
but to no avail. Finally, the propeller
was feathered in the dive at a Mach number of 0.95 at 30,000 feet…. No major adverse effects were noted (or
remembered) in this case. Also, the fire
warning proved to be false and adjustment of the warning system sensitivity
took care of the problem.
The elevator control of the aircraft was adequate to achieve the
limit load factor or maximum usable lift coefficient throughout the subsonic
operational envelope. At a Mach greater
than 1.0, however, full back stick could develop only 0.75 “g” increment, or a
load factor of 1.75 “g” for recovery, an indication of the increased degree of
longitudinal stability and the loss of elevator effectiveness caused by Mach
number effects. This was of no
consequence operationally in this case, however.
The general handling characteristics were adequate, and the drag
rise and trim changes (tuck) in achieving supersonic speeds were mild and posed
no difficulties. Buffet at 1 “g” in
transonic flight was non-existent, to the best of my memory. The highest Mach reached was 1.2 by
Fitzpatrick in the acceptance tests at St. Louis. Many public complaints were received from the
resulting sonic boom, however, which caused McDonnell authorities to prohibit
further supersonic tests. Therefore,
this kind of operation was minimized at Langley.
The control forces were manageable with boost off in up-and-away
flight. However, one day after I had
just made a high-speed, high-propeller-power pass over some high-ranking
visiting observers at Langley the hydraulic control system failed. Although lateral control could be handled
below 200 mph with one hand without major problems away from the ground, I
found it a different story on the approach.
I carried more speed (about 180 mph) to avoid the lateral trim changes
during landing…and also to avoid a “settling” situation if forced to use two
hands on the stick, leaving none for throttle operation. Actually, two hands were required on the stick
continually to keep the the aircraft lined up with the runway (it was a real
sweat). I was beginning to be deeply
concerned about flaring while keeping the aircraft aligned when, just about at
flare height, the boost came back in, allowing me to land decently. The nose-down attitude during the approach
apparently allowed remaining fluid into the system.
By
late 1957, it was clear that turbojet and newer
turbofan engines would represent the wave of the future in both the military
and civilian spheres, and the NACA supersonic propeller research program was concluded
with its final research flight on 17 January 1958. Over the course of its research career with
NACA, the XF-88B accrued 28 hours of flight time with 43 research flights and
three familiarization flights over a period nearly 50 months at Langley. Jack Reeder had conducted 30 research
flights, while Bill Alford had 13 XF-88B flights in his logbook. NACA pilot Robert Champine had two
familiarization flights for a total of 0.6 hours in the aircraft, while J. B.
Whitten had one. While Jack Reeder continued
in a long career with NASA, Bill Alford was killed the following year in
England during a pitch-up event while landing a prototype Blackburn NA.39
Buccaneer. With the end of the supersonic propeller evaluation, the
XF-88B was turned over the
base salvage at Langley Field for disposition on 16 September 1958. The XF-88B was then transferred to Eglin AFB, Florida for
ordnance testing. The XF-88B was tested to destruction at the proving
grounds and removed from the USAF inventory in February 1959. Thus was the end to the distinguished career
of the first Voodoo.
