blade pitch settings within which the flexible rotor could be operated.

Research at the Langley helicopter test tower showed that the rotor attained very high mean lift coefficients because of the large amount of aerodynamically induced camber. The investigation also revealed how luffing restricted the envelope of tip speed and collective pitch angles within which the rotor could be operated. In the rotor's operating range, variations in tip speed altered the blade camber and resulted in substantial tip speed variations in hovering performance, particularly at the higher thrust conditions. The results suggest that there may be optimum combinations of tip body mass, tip center of gravit and tip stabilizer incidence for each value of tip speed since these variables determine the amount and distribution of blade camber and twist.

Another type of research was concerned with instrument display requirements for VTOL aircraft, which may be more severe than those for STOL and CTOL (conventional takeoff and landing) aircraft because of differences in speed control and guidance in the final phase of the approach. In VTOL landings, the pilot must vary the speed precisely to bring the aircraft to a stop over a prescribed location on the ground; he must then execute a descent to touchdown at essentially zero speed. The instrument display, therefore, must present the ground speed and guidance information with sufficient accuracy and in such a form that the pilot will be able to maneuver with great precision. In addition, since the attitude and position of the aircraft can change rapidly and continuously during the final phase of the approach, guidance information must be presented so as to facilitate rapid assimilation by the pilot.

Attempts to reduce the difficulty of information assimilation have been made from two viewpoints:

• combining individual items of information in a computer and presenting the combined signal as a single indication; this method reduces the number of indications and also makes it possible for the indications to take the form of simple control commands;

• combining the items of information in the presentation itself in a realistic or pictorial form that permits rapid interpretation in terms of the pilot's real-world experience.

Eight instrument displays based on these two methods of combining information were assembled as representative examples, classified in terms of basic display concepts, and compared on

the basis of a common set of instrument display requirements. In tests of displays in simulators or in flight, the only display with which VTOL landings were achieved in flight (under simulated zero-zero conditions) was the "real-world" display of a closed-circuit television system. These results suggest that a televised display of a simulated real-world landing site may be the form for achieving VTOL landings under zero-zero conditions.

Supersonic Transport

At the request of the FAA, NASA assigned approximately twenty technical experts to assist in the technical validation of the proposed SST design. NASA personnel also participated in the FAA sponsored Government-industry review of tentative airwor thiness standards for the SST held in October.

Tests were made in the 40- by 80-foot wind tunnel of a 1/5 scale

model of an SST design based on SCAT-15F (Fig. 4-14). Initial results indicated signficiantly better low speed lift/drag values than had been predicted.

XB-70 Flight Research Program

The XB-70 Number 1 made seven flights during this period, to acquire data related to the design and operation of large supersonic aircraft. (Fig. 4-15) These flights, together with the six flights made during the first half of 1968, brought the total number of XB-70 flights to 128. Thirty three of these flights (all flights after November 3, 1966) were in direct support of the national supersonic transport program.

The 7 most recent flights were made to obtain data on the structural dynamics of the airplane and the effects of atmospheric turbulence; stability, control and handling qualities; overall airplane performance; engine inlet air duct performance; boundary-layer noise and skin-friction; and take-off and landing noise.

The structural dynamics tests used the recently-installed modal control (elastic mode control) system, which is designed to reduce the structural dynamic response of the airplane to atmospheric gusts. The preliminary tests indicated that the system should be

effective in reducing the structural response to turbulence. When fully developed, the system should improve the "ride" characteristics of large flexible airplanes in rough air, thereby extending the fatigue life of the aircraft. Data obtained from these flights were being reduced and analyzed at the end of 1968.

In December NASA and the Air Force decided to terminate the XB-70 flight program on or before January 31, 1969; one flight may be made in January. Upon termination, the XB-70 aircraft and all available equipment, material, and facilities loaned to NASA and the contractors will revert to the Air Force.

Military Aircraft

At the request of the Department of Defense, NASA conducted a fighter aircraft study to provide idealized fighter aircraft concepts embodying the upper level of current technology in external and internal aerodynamics, engine/airframe integration, and airplane stability and maneuverability throughout the speed range. Also, in response to an Air Force request to participate in the F-X source selection, NASA technical specialists were assigned to Air Force technical committees evaluating proposals submitted by prospective contractors.

X-15 Research Aircraft Program

The X-15-1 made four flights-on July 16, August 21, September 13, and October 24-to obtain data for several highpriority Air Force experiments.

The objectives of the July 16 flight were to evaluate the USAF Western Test Range (WTR) launch monitor experiment and to obtain data from the Air Force high-altitude sky-brightness experiment. A malfunctioning pressure trandsucer caused an erroneous indication of hydraulic pump failure during the flight, and the pilot elected to reduce maximum altitude as a precautionary measure. As a result, the WTR equipment was not activated and no data were obtained, but satisfactory data were obtained from the high-altitude sky-brightness experiment.

The August 21 flight continued checkout and evaluation of the WTR experiment and also sought data from the MIT/Project Apollo Simultaneous Photographic Horizon Scanner. The WTR experiment was activated but it again experienced component failures. Satisfactory data were obtained from all data channels of the Horizon Scanner experiment except the star magnitude channel. The edgetracker portion of the experiment functioned normally and data were obtained.

The September 13 flight was made to continue the development of the WTR experiment hardware and to maintain pilot proficiency for the scheduled high-altitude flights.

The flight on October 24 was made to obtain data for the WTR and the sky-brightness experiments, but equipment problems were encountered with both experiments. Data were obtained, however, from the fixed-sphere flow direction sensor and the fluidic temperature measurement equipment.

Following a joint USAF/NASA review of the current status of the X-15 flight program, it was decided to terminate the X-15 program no later than December 31, 1968.

The three X-15 aircraft (Fig. 4-16) made a total of 199 flights, from the first flight on June 8, 1959, to the last on October 24, 1968. Of these, 154 exceeded Mach 4, 109 exceeded Mach 5, and four exceeded Mach 6. The maximum speed and altitude attained during the program were 4,520 miles per hour (6,630 feet per second; Mach 6.70) and 354,200 feet, respectively. Thirteen flights, by eight different pilots, exceeded an