JETFOIL TESTING PROGRAM (BOEING MODEL 929)

 

From July 1974 to February 1975, JETFOIL 001 underwent an extensive testing program to verify, evaluate and improve performance and to obtain vehicle class certification.

 

The ship was initially configured in a "Lead Ship" test arrangement. With the exception of the ACS (Automatic Control System), all basic ship systems were in a production configuration. The production ACS was installed about half way through the testing program. The passenger interiors were not installed, although the seats were available and used occasionally to accommodate observers, special test support personnel and to permit demonstrations to be carried out. A ballast barrel system, borrowed from Flight Test, was used to control weight and balance and to simulate passenger-loading conditions. The craft configuration was rigidly controlled throughout the testing program.

 

A data system provided for 150 measurements, 60 channels of FM and 90 of PCM. Real time monitoring of 16 selectable channels was available at all times. A secondary data system was also installed for use in "special" performance tests. It provided 30 additional channels of which 16 could be displayed in real time. This permitted us to double our on the spot monitoring capability when doing exploratory or diagnostic type testing.

 

Initial Tests

 

The initial scope of tests carried out in a new hydrofoil are intended to determine how the vehicle behaves relative to its design objectives and to establish when it is ready to proceed with the more critical testing phases of rough water trials, certification trials and customer demonstrations and builders trials. These tests are carried out in relatively ca]m water and light winds and the entire envelopes of performance and behavior are expanded in a gradual manner in the same way that an airplane program is carried forward. The first few days are spent evaluating hullborne handling characteristics - the taxi tests. Since the thrust vectoring system on the 929 was a new design, extra time was allowed for familiarization and evaluation. Hullborne maneuverability was good to excellent depending on the position of the foils. Maximum control was achieved with the aft struts up and the forward strut down with strut steering engaged.

 

 

Following the hullborne tests, speed was gradually increased to accomplish first takeoff and straight away flight. At this time, the first performance anomaly showed up when it became apparent that the thrust available was insufficient to achieve takeoff. The design thrust-drag curve for the 929 is presented below.

 

The curve demonstrates the classical "hump drag" associated with high speed marine vehicles. This arises because hull drag increases as the cube of the velocity while foil drag increases as the square. As lift develops on the foils, the hull drag builds up rapidly. When the hull finally breaks free, the drag drops and then follows the square law as does an airplane wing. The takeoff problem is one of setting the "hump drag" in a proper relationship with the peak of the thrust curve to obtain maximum acceleration force to get through the hump region. This can be affected by: the propulsion system, the takeoff controller program and the incidence setting of the foils. As a result of past hydrofoil work, it was anticipated that such a situation might occur. A modification was made to the waterjet exit nozzles and takeoff was successfully accomplished. Subsequent tests resulted in changes to both the takeoff control program and to the foil incidence angle setting, which permitted the original nozzles to be reinstalled.

 

Initial calm water tests continued through August in order to fully characterize takeoff performance, low speed trim and turning, low speed control dynamics and medium speed trim and maneuvering. Specific tests were run to verify various automatic control aspects including the takeoff con­troller, dynamic responses to step inputs, responses to sinusoidal inputs, behavior in low and high speed turns and high rate turns, shallow foil depth operations and simulated failures. Ship performance throughout this entire series was very close to predicted with two notable exceptions. The maximum speed attained was 43 knots, which was below the expected maximum speed. During shallow depth turning tests and during simulated broaching tests, turbine shut downs occurred. These were caused by the inlet un-wetting which in turn un-loaded the free turbine. A protective over-speed shutdown device in the engine automatically shut down the turbine when this condition occurred. It was apparent that both of these situations required additional testing in order to diagnose what was actually happening and to develop suitable fixes.

 

Engineering Trials

 

Although special tests to develop engineering data were run during the entire program the period from the end of August until mid-December was devoted almost exclusively to this effort. In all, 144 trials were run during this period for this purpose. Sixty-two were devoted to propulsion system testing in order to try to determine whether the inability to achieve expected maximum speeds was due to a thrust deficiency or to an excess of drag. In all, three different size nozzles, four different inlet designs and three different pump stators were evaluated, providing 12 basic combinations of real interest. At the same time, twenty-eight trials were carried out to evaluate the hydrodynamics, particularly drag. Tests were conducted at various foil incidence angles to determine optimum settings. Flutter tests were run and various fairing and pod revisions were made to assess their contribution to drag. A series of drogue tests and engine shut down tests were also run in an attempt to positively identify whether thrust was low or drag was high, with the results indicating low thrust to be the difficulty. By December, it had been possible to reduce the required power by 26% to produce the same speed or conversely to increase speed between four and five knots with the major contributions coming from a stator change (10%) and the incidence change (9%}.

 

Forty trials were run for foilborne control testing, twenty on the test ACS and twenty more when the production ACS was installed. These tests were aimed at optimizing the takeoff controller, closing the lateral acceleration to rudder stability loop at design gain, fully evaluating directional stability both open and closed loop and with and without the tip fins and to eliminate any height and speed transients during turns. In addition, a series of simu­lated failures were run to establish that the resulting craft motions were not hazardous to the passengers.

 

Fourteen trials were run to acquire data on the various subsystems, par­ticularly steering and reversing, fuel, engine space cooling, lubrication and sea water. During these tests, both interior and external acoustic tests were also run to verify that the design levels had been met.

 

Rough Water Tests

 

The period from mid-December to the end of Ship 001 testing on 1 February 1975, was primarily devoted to rough water testing and to certification trials. Generally, the seastate trials conditions were Seastate 4 (significant wave heights to eight feet); however, testing was also accomplished in large swell type seas {equivalent to Seastate 5) with measured waves to 24 feet high encountered.

 

Normally the JETFOIL platforms the waves, that is, the ACS attempts to keep the deck as level as possible at a pre-selected height above the mean surface of the sea. For seas with significant wave heights of eight to ten feet -- or about equal to the effective strut length -- the resulting vertical accelerations were within predicted values and the ride quality is excellent on all headings to the seaway. Pitch and roll motions are quite safe.

 

As the waves increase in size, hull cresting through the tops of the waves becomes a normal occurrence. The hull shape was designed to keep the resulting forces and accelerations to a minimum when this happens. In very large waves, on some headings, it is possible for the forward foil to come near the surface, or to completely fly out of the water. This is called "foil broaching". On other occasions, the water inlet would come to the surface and produce the turbine shutdown situation discussed earlier. Two modifications were developed which have been effective in solving this problem. A "contouring" mode of control was provided which would reduce both foil and inlet broaching, but at some degradation in ride quality. To further protect the engines from shutdown, a pressure sensing system was installed at the inlet, which automatically throttles the turbines back when the water pressure drops. By combining contouring with the sensing system and using good handling techniques, the shutdowns have been eliminated in all but extreme sea operations.

 

Directional stability in all seas and for all conditions tested was excellent. Lateral accelerations are low and were as predicted. Maneuverability was good with full rate turns possible in all seas. Hullborne operations, takeoffs and normal and rapid landings were made on all major headings bm the seas and craft performance was quite good.

 

Certification Trials

 

Certification testing consists of both dockside and underway tests and must be run on every vehicle to verify that it can be operated in a safe manner. Generally this involves simulating electrical failures, hydraulic failures and power failures - but the 929, being an automatically controlled hydro­foil, presented a new problem for the agencies. To fully prove out the vehicle, a new approach was developed.

 

A complete safety analysis was carried out in which all possible failures were simulated and the resulting craft motions and forces on passengers, crew or structure were predicted. The agencies then reviewed the analysis and selected those failures that they wished to have demonstrated (Table I). In addition to the usual ones, they also selected those that produced the largest motions and/or forces (*). Since it was impossible to simulate the latter without a considerable amount of special equipment, it was agreed that they would only need to be demonstrated on 001. The test results showed that the ship responses and the forces .were more benign than had been predicted by the analysis and they fully substantiated the structural integrity and safety of the design.

 

                                                           

REGULATORY AGENCY SIMULATED FAILURE TRIALS   FOILBORNE - RATED POWER - STRAIGHT RUNNING FORWARD FLAPS FULL UP AND FULL DOWN * FORWARD STRUT HARD OVER * AFT OUTBOARD FLAP FULL DOWN * AFT INBOARD FLAP FULL DOWN * SINGLE HYDRAULIC SYSTEM FAILURE SINGLE AND DUAL HEIGHT SENSOR FAILURES GYRO SYNCHRO FAILURE * ACS PRIMARY POWER FAILURE ACS TOTAL POWER FAILURE     FOILBORNE - RATED POWER - MAX. RATE TURN AFT OUTBOARD FLAP FULL UP * FORWARD FLAPS FULL DOWN *     HULLBORNE - NORMAL RUNNING SINGLE   ENGINE OPERATION SINGLE   HYDRAULIC SYSTEM OPERATION                                                                                   TABLE I

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

002 and 003 Tests

 

By December, 002 had entered into test. It was a slightly different con­figuration with the main performance difference being that it had much shorter struts than 001. In 25 testing days, 002 accumulated 62 flight hours and in the process completed builders trials and certification trials. The owner, Far East Hydrofoil Company, Ltd., accepted the ship in February and as the "Madeira" it entered scheduled service between Hong Kong and Macao in April, 1975.

 

003 began testing in March and completed 101 flight hours in 28 test days. The main highlights of its program was the completion of all U. S. Coast Guard certification testing and the completion of acceptance tests for the customer, Pacific Sea Transportation Company, Ltd. of Hawaii. 003, christened "Kamehameha", began inter island service in Hawaii in June, 1975.

 

SUMMARY

 

The testing program carried out on the Boeing 929 was the most extensive test program ever run on a hydrofoil. From a testers point of view, it was very successful. It demonstrated what the 929 could do, what its limitations were and where improvements had to be made. It provided the information that was needed to train the operating crews and main­tenance personnel.

 

From first flight to introduction of the first Jetfoil into scheduled service took ten months. It is interesting to note that the average time from first flight to first service for the 707, 720, 727, 737 and 747 was also ten months.

 

 

 

 

 

 

 

 

 

 

 

 

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