In our latest article in a series on the Bell D188A (a.k.a. XF-109 / XF3L) supersonic VTOL fighter of the late 1950s, we present more primary documents concerning this aircraft in a chronological sequence, specifically Bell progress reports and correspondence in the April—August 1958 period.
First we have a Bell Progress Summary from April 11. Since the inspection of the preliminary configuration mock-up, Bell redirected effort to the development of a production tactical airplane of Mach 2.5 capability. The initial flight and static test airplanes were to have been versions of the tactical airplane and have a Mach 2.0 flight limit.
During this reporting period, agreement with BuAer and ARDC had been achieved in regard to the tactical airplane avionics equipment. Specifications for development of this equipment were being prepared at the time of writing.
As a result of the configuration mock-up inspection and wind tunnel test experiences, certain changes had been made in the airplane configuration. These changes involved primarily an increase in vertical distance between the wing and horizontal tail; elimination of the cathedral and an increase in area for the horizontal tail; relocation of the nacelles with respect to the wing; reduction in fuselage area ruling and an increase in main landing gear tread.
The experimental programs had progressed to the extent of continued testing of the original supersonic model, the transonic-supersonic model, inlet models and jet impingement model. A hovering and transition model had been built and initial testing performed during this reporting period. Planning for other models and tests had also been accomplished.
Next is an internal BuAer memorandum regarding joint Air Force-Navy funding for the Bell VTOL fighter program. This is followed by the May Progress Report, No. 2000-933002. The report number is evidence that Bell had started using the Model 2000 designation around this time, superseding the original D188A model number. In the abstract of the report, Bell noted that vibration testing of the 1/3 scale wing deformation model with simulated nacelles and ailerons had been completed. Construction of a wing transonic model was to start in June. VTOL engine performance and data requirements were compiled and would be published in June. A decision was made to use integral fuselage tanks instead of bladder type tanks. The .048 model was tested through Mach 2.87 in the unitary plan wind tunnel at NACA-LAL Virginia.
Fabrication of the 1/40 scale model to fit the Naval Air Missile Test Center facility would start in June. Fabrication of the 1/7 scale semi-span model was continuing at the David Taylor Model Basin and testing of this model would start in June. Testing of a jet impingement model continued with an expansion in the number of test points covered including measurements on simulated landing gear tires. A “Structural Criteria and Design for Aerodynamic Heating” report was completed and submitted to BuAer. Studies of thrust modulating to provide pitch and lateral control during take-off and landing indicated certain advantages; these studies were continuing. Flight tests to study the pilot’s field of vision were initiated, employing a helicopter with cockpit enclosure masked to simulate the Model 2000 airplane.
In the June Progress Report, Bell stated that it had forwarded and coordinated its comments on BuAer Detail Specification SD-535-1 with the Navy Project office. The Model 2000 configuration was modified by enlarging the vertical fin and the substitution of two fixed ventral fins for the previous foldable ventral fin. Revised airplane basic data, based on wind tunnel test results, were being prepared. Bell predicted that the revised drag level would be essentially the same as that estimated.
Tests were continuing on the .048 scale transonic-supersonic and the 1/10 scale jet impingement models. Testing of the following models was scheduled to be started or resumed during July: the 1/40 scale six-engine model, 1/8 scale hovering and transition dynamic stability model, 1/6 scale half-nacelle model, 1/10 scale fuselage inlet model, and the 1/5 scale cold flow diverter model.
Models under construction or authorized for construction were: 1/40 scale eight-engine, 1/7.5 scale subsonic reflection plane, transonic full-span wing flutter, low speed nacelle and fuselage inlets, 1/8 scale fuselage inlet, and 1/4 scale nacelle inlet models.
Drawings, defining the following areas, were completed in initial form during the reporting period: general arrangement, canopy and forward fuselage areas, nacelle basic geometry and inlet, fuselage engine basic arrangements, the majority of the wing components, hydraulic system motor-pump units, armament system suspension mechanisms, and the central computer block diagram.
In the July Progress Report, a new set of airplane basic data, based on wind tunnel test results, was planned for completion during August. Drag levels were expected to be essentially the same as those previously estimated. Continuing investigations of the spoiler-slot-deflector lateral control arrangement had shown that this system was satisfactory over the entire conventional flight regime.
Tests were performed on the following models during July: .025 scale 6 engine configuration, 1/8 scale hovering and transition dynamic stability, 1/10 scale jet impingement, 1/6 scale half nacelle, and 1/5 scale cold flow diverter. Testing of the 1/10 scale fuselage inlet model was planned during August 1958.
Models under construction or authorized for construction were: .025 scale 8 engine configuration, 1/7.5 scale semi-span reflection plane, transonic full-span wing flutter, 1/3 scale wing elastic deformation, 1/6 scale subsonic flutter, semi-span hinge moments and loads, 1/7.5 scale low speed nacelle inlet, 1/7.5 scale low speed fuselage inlet, 1/8 scale fuselage inlet, and 1/4 scale nacelle inlet.
Drawings, defining the following areas, were completed in initial form during July: empennage components, nine fuselage frames, fuel system schematics, door-mounted antennas, escape system schematics, and liquid oxygen converter installation.
Finally, we have the August Progress Report of 1958. By this time, the final draft of the BuAer Detail Specification SD-535-1 was completed for BuAer approval. A decision was made regarding the low speed control system to effect pitch control by fuselage engine thrust undulation, while yaw and roll control would be achieved by reaction control nozzles supplied by engine bleed.
The .048 scale model and the hovering and transition dynamic stability model were planned to be modified to incorporate the spoiler-slot-deflector lateral control arrangement. Preliminary static and dynamic tests of the wing transonic flutter model had shown that the stiffnesses and frequencies were within the required tolerance.
A total of 20 drawings were completed during the month of August. Airplane weights previously reported were still applicable. All proposals from prospective avionics equipment subcontractors had been received and were being evaluated. A total of 38 proposals had been received on the five major groups of avionics equipment, representing 21 different possible suppliers.
Two alternate proposals for a full scale jet impingement program, using either J33 or J47 engines, were submitted to BuAer. A proposal covering estimated costs for the Phase II program through contractor’s flight test had also been submitted to BuAer. Mock-up construction was in progress on the horizontal and vertical tail surfaces, cockpit sills, frames, floor, radome, and plaster forms for nacelle skins.
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All images from NARA Archives II, College Park, MD, RG 72