Each flight mission usually lasted a little over an hour, but we would spend ten or more hours after the flight correcting the data for instrument error, temperature, and altitude and converting the readings to Standard Day conditions as set by the National Advisory Committee for Aeronautics (NACA): atmospheric pressure 29.92 inches of mercury, temperature 15 degrees centigrade (59 degrees Fahrenheit). Temperature and atmospheric pressure affect flight performance significantly; therefore, all figures must be converted to a standard so that data obtained under varying conditions may be compared. The resulting figures are then used to plot parameters such as speed versus altitude and horsepower versus altitude.
The next task was to determine the best climbing speeds at various altitudes using what are called sawtooth climbs. The climbs covered 1,000 feet, starting at 8,000 feet and at 16,000 feet. Power was set at 2,000 rpm and full throttle, since both tests were done above the critical altitude and 30 inches of mercury could not be obtained even with full throttle. A series of six climbs were made starting at 130 mph and repeated at 10-mph decrements through 80 mph. A seventh climb was made at 130 mph to allow a correction to be calculated for the decrease in weight due to fuel consumption during the climbs. For the low sawtooth a steady climb at the selected airspeed was established at 6,500 feet and continued to 9,500 feet. The time to climb from 8,000 feet to 9,000 feet was measured with a stopwatch and entered on the knee board. After diving back to 6,500 feet, the pilot made the next climb at 120 mph, and so on.
The high sawtooth was flown the same way, but the time to climb was measured from 16,000 to 17,000 feet. The high climb took about two and a half times as long as the low climb, since 17,000 feet is close to the ceiling of the AT-6 at the power settings used.
The term ''sawtooth climb" comes from the tracing on the barograph (carried on all test flights), which records on a rotating drum the altitude on the vertical scale and the time on the horizontal scale. The climb portion of each run appears as an upward-sloping line, and the dive back to the lower altitude makes an almost vertical line downward. The tracing of the series of climbs and dives is serrated like the teeth of a saw. After the flight the barograph tracing is carefully measured to determine the time to climb the 1,000 feet, thus providing a second source of data.
The apparent rate of climb, in feet per minute, was easily calculated from both the stopwatch readings and the barograph data and then plotted against the corresponding airspeed. After applying the weight correction at each airspeed, the student drew a curve through the points. The high point on the curve was the best climbing speed for that altitude. The best climbing speed is constant up to the critical altitude, 6,500 feet for this test, and is a straight line above the critical altitude. The student used the two speeds from the low and high sawtooths to plot altitude versus airspeed, creating a graph of the best climbing speeds for all altitudes up to the ceiling.
The final performance missions for the AT-6 were check climbs to the aircraft ceiling using the climbing speed from the previous plot. Two climbs were made from 2,000 feet to 20,000 feet, recording the stopwatch time every 1,000 feet. Using both stopwatch and barograph data, students plotted the rate of climb versus altitude and time to climb versus altitude.
In the flight testing of new aircraft, tests such as these produced the data needed to prepare the performance charts used in the military pilot's operating instructions and in civilian pilot's handbooks. Without this information, accurate flight planning would be impossible.
The next phase of the testing was more subjective; the pilot had to assess the handling of the AT-6 including stalling speeds, control response, stability, and spin characteristics, as well as cockpit layout. No special mission was scheduled to evaluate the cockpit layout. Instead the pilot based his opinion on observations made during all the tests. The arrangement of the instruments, their readability, the accessibility of the controls and switches, and the range of adjustment for the seat and rudder pedals (to accommodate pilots of various sizes) were among the subjects covered.
At that time about the only thing that was standard about aircraft cockpits was that the pilot faced forward. Over the intervening years a great deal of research has been accomplished in this area, and standardization has been greatly improved. For example, the six primary flight instruments — the altimeter, airspeed indicator, vertical speed indicator, flight indicator (artificial horizon), directional gyro, and turn-and-bank indicator — have a standard layout in all aircraft equipped for instrument flight, from a Cessna 150 to a Boeing 747. More significant than the instrument panel layout have been the ergonomic, or human engineering, studies that determine where controls and instruments should be located, how they should be shaped, and how they should be operated for ease of use and to avoid confusion, especially in emergencies. For instance, the grip on the handle that controls the retractable landing gear is shaped like a wheel, and the flap control is shaped like a section of flap. Those two changes alone have reduced the incidence of accidents caused by retracting the wheels instead of the flaps on the ground after landing.
We flew all of our trim and stability checks with the aircraft loaded so as to move the center of gravity (c.g.) slightly aft of the aft limit. In the cruise configuration the plane was trimmed for 105 mph and checked at three airspeeds: 95 mph, 105 mph, and 125 mph. Aileron, elevator, and rudder response were noted as we performed simple maneuvers. At the off-trim airspeeds the control forces necessary to hold the target airspeed were recorded. The stability was checked by displacing and releasing each flight control separately to see if and how the motions caused by the control movements would be damped out. The AT-6 was quite stable; lateral and longitudinal oscillations (resulting from aileron and elevator deflection) were damped out immediately, and directional oscillations (rudder) were damped out in one cycle.
The tests were repeated in the approach configuration, with gear and flaps down and 50 percent of normal-rated power. From 1.4 times the stalling speed (75.6 mph) down to the stalling speed (54 mph), the airplane vibrated and porpoised badly, making it difficult to hold a precise altitude and airspeed. There was no warning before the stall, which was sudden and sharp with a pronounced right wing drop. In the landing configuration with power at idle, the aircraft handled well and was controllable down to the stall.
Following the handling tests, we performed six stalls, three with power on and three with power off, in the clean configuration (with gear and flaps retracted), with gear down and half flaps, and with gear down and full flaps. Stalling speeds, controllability, type and amount of stall warning, and the effectiveness of the standard recovery procedure were evaluated and recorded.
Next came one of the most interesting parts of the test program, the spin testing. We made six 5-turn spins, three to the left and three to the right, with ailerons neutral, ailerons with the spin, and ailerons against the spin. All spins were entered from a straight-ahead, power-off stall at 10,000 feet in the clean configuration with the propeller set at 1,500 rpm and the mixture at idle cutoff to prevent carburetor flooding. The standard NACA recovery — applying opposite rudder, waiting a half turn, and then applying hard forward stick (down elevator)was used in all spins. The type of spin (steady or oscillatory), the number of turns required to recover, and the altitude at which level flight was resumed were recorded.
The steepest spins were encountered when the ailerons were held with the direction of the spin, and the most altitude (4,000 feet) was lost in these spins. The recovery from the spin to the right in this configuration required two and a half turns. It was difficult to enter a spin to the left with the ailerons against the spin, requiring about five seconds after the stall, since the AT-6 drops the right wing in all stalls.