The tests went on for a number of years, as the MIT lab was constantly modifying the sight based on the test results. The sight was regularly maintained and modified by an MIT technician or technicians at Eglin, but the airplane had to be flown to Bedford whenever major modifications were to be made. There the gunsight would be removed and taken to Cambridge. All of the gun camera film shot during the test flights was analyzed frame by frame at MIT, and the targets were scored and hits charted by the MIT crew at Eglin.
The targets were flag targets, 6 by 30 feet, with radar corner reflectors. They were towed by the squadron's B-26 (later by Douglas A-26s), flown by the medium and light test squadron, at 200 mph at 8,000 to 10,000 feet a few miles offshore along the Gulf ranges. All firing passes were made toward the Gulf, but camera gunnery passes could be made in either direction.
The 20mm cannon and two of the .50-caliber machine guns had been removed from the nose to make room for the radar and sight. The two remaining .50s were fitted with hydraulic chargers, and the ammunition belts had dummy rounds after each ten live rounds, so only ten rounds could be fired per gun during a single pass. This precluded expending too many rounds on a single good pass and made the test more precise. The test ran for several years, and I was the test officer from early in 1947 until late in 1949, except for the period that I was at test pilot school. The last part of the test was flown in a Republic F-84 Thunderjet.
It is not especially difficult for a good fighter pilot to hit an enemy aircraft from within a narrow cone of about 40 degrees astern and a range of less than 200 yards. The problem becomes progressively more difficult as the range and angle off the target increase. (A fighter flying at a right angle to the path of the target is 90 degrees off the target; zero angle off the target is from dead astern.) The Davis-Draper sight was a radar-ranging, lead-computing sight designed to allow accurate firing from high angles off (more than 60 degrees) and long range (about 750 yards); therefore, the gunnery passes were set up to fire at 60 degrees angle off and 750 yards range. We did quite a bit of camera-only gunnery to test the tracking capability of the sight before the actual firing phase of the test. Two other pilots, Maj. Don Rodewald from the weapons test branch and Capt. Don Dessert from the fighter squadron, participated in the testing. Since all three of us were named Don, there could have been considerable confusion, but fortunately I answered only to "Lope" and Rodewald to "Rode," leaving "Don" exclusively to Dessert.
A typical firing mission went as follows: The fighter took off some twenty minutes after the plane with the target, to allow it time to get into position and trail the target. As soon as the fighter was airborne the pilot turned on the gunsight to warm up the sight and the radar. As he approached the tow plane he checked to see that the radar was locking onto the target, as indicated by a light on the side of the gunsight. When the tow pilot called that he was ready for firing passes, the fighter took a position flying parallel to the tow plane, about 1,000 feet higher and 1,500 to 2,000 yards away. If the fighter was on the right side of the tow plane, the pilot would depress the button on the control wheel that caged the gunsight, locking the pipper into a fixed position in the center of the windscreen, and then roll into a diving turn to the left, putting the pipper on the target as he rolled level before starting a tracking turn to the right. When the radar light indicated target lock-on, the pilot released the cage switch and held the pipper (which was now continuously indicating the proper, and constantly changing, distance to lead the target) as close to the center of the target as possible. At about 750 yards and 60 degrees angle off, which would occur simultaneously if the pilot had judged his approach correctly, the pilot pulled the trigger and fired a ten-round burst from both guns. He would then pull up and turn to the right to get into position for the next pass while activating the switch that recharged the guns and noting any comments about the pass on his knee board. (After the mission one of the most difficult tasks is trying to decipher these comments.)
Usually three or four passes were made in one direction, and then the tow plane reversed its course and passes were made in the opposite direction. If the tracking was not smooth or the range and angle off were not correct, the pilot did not fire. The tracking was recorded, however, since the camera that filmed the pilot's view through the gunsight was activated by the caging switch, not the gun trigger as is standard.
The P-38 was a good gun platform, and as my proficiency increased, I achieved some excellent results that were gratifying to the MIT team and to me. On some of the best passes my average tracking error was only half a mil (one mil is the angle subtended by a chord of 1 foot at a range of 1,000 feet). Since I was firing from 750 yards (2,250 feet), my pipper was within 1.125 feet of the target center throughout the tracking portion of the pass. On these passes 25 to 27 percent of the rounds fired were hits. A score of 30 percent was required to be rated an expert aerial gunner, but that was based on firing from a much shorter distance and lower angle off. The gunnery specialists at the Wright Field armament lab told me that with a six-by-thirty target mounted on a ground range, a .50-caliber machine gun firing from 750 yards would score only about 27 percent because of the muzzle waver while firing. In other words, the Davis-Draper sight was very effective in the hands of an experienced pilot.
The sight was designed to be a gun-bomb-rocket sight. We did not participate in the rocket-firing tests, but we did considerable dive-bombing with the P-38; later, with the P-84, we tested all three modes. The dive-bombing procedure was quite simple. The pilot climbed to 10,000 feet, set the sight to the bomb mode, held down the cage button, and rolled into a dive of about 70 degrees. When established in a steady dive with the pipper on the pyramid target (a four-sided wooden pyramid, about 20 feet on a side and about 10 feet high, painted to contrast with the ground), the pilot released the cage button, and the pipper was held on the target. This caused the plane to fly at slightly less than one g as the dive angle slowly increased. When the accelerometer-actuated computing mechanism in the sight calculated that the time was right, the bomb was released automatically, and a light on the sight signaled the pilot to pull out. The release occurred at about 4,000 feet, giving the pilot plenty of altitude to recover from the dive. The starting altitude and dive angle were varied throughout the testing, and the results were quite good, with many direct hits and near misses recorded. To eliminate as many variables as possible, it was necessary to enter each run at a precise altitude, airspeed, and dive angle.
During this period most of the pilots in the fighter squadron were flying on Project Highball, a series of tests to determine the most effective methods for intercepting incoming jet bomber raids with jet fighters using the P-80, the only jet fighter available in numbers at that time. It tested not only the aircraft but also the ground controlled intercept (GCI) system. Instead of jet bombers (none were available at that time), two P-80s with tip tanks were flown in formation to simulate the radar return from a jet bomber. They could not be detected by radar at long range, so they were equipped with AN-APN 19 radar beacons. During the first phase of this test we flew seven days a week, and on one of those days the squadron logged forty-two hours on Project Highball alone.