Flight Testing of the Piper J4A Cub Coupe
Connick, Kenneth Burton
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Flight testing is an important part of in the airplane design process. This testing is performed to ensure that the airplane complies with the established requirements and regulations for flight safety, as well as to ensure it meets its mission objectives. The Federal Aviation Administration (FAA) currently governs these requirements and regulations for civilian aircraft and provides airworthiness certifications. The FAA regulations were introduced in 1965. Prior to that, civilian aircraft were certified through the Civil Aeronautics Administration using Civil Air Regulations (CARs). The Piper J4A Cub Coupe was originally certified under CAR 3 – Airplane Worthiness. This CAR was used to evaluate the airplane for its current level of both performance, and stability and control. To evaluate the airplane per CAR 3 requirements, it was put through a series of nine flight tests, 5 for performance, and 4 for stability and control. The results of the performance tests are described first. These tests showed that the position error fell within the 5mph error maximum as required by regulations. The stalling speed for the airplane was calculated to be 35.5 mph, which was below the allowable 70mph maximum. Level flight performance is not a requirement per CAR 3, but the test was performed in order to produce a chart of true airspeed vs density altitude, which is useful to the pilot. This chart was successfully produced, except for the full throttle line, which was due to a data collection error. The climb rate of the airplane was calculated to be 322 ft/min, which was above the 300 ft/min specified in CAR 3. Finally, the best rate of climb and aircraft ceiling were calculated to be 43.4mph and 4510 feet, respectively. Both of these numbers seem to be on the low side, because the best rate of climb is just above stall speed, and the ceiling obtained in the climb performance flight test was 11,485 ft. Parasitic drag may have played a part in the low numbers, as it is not accounted for in the calculations. The results from the stability and control flights showed that most of the maneuver points were forward of the aft C.G. limit, which indicates that the aircraft is unstable. The data used to generate the maneuver points is suspect, as we know that the aircraft is stable. The main cause of this is likely due to an insufficient spread in the center of gravity between the test flights. The lower C.G. was 16.06 inches, and the upper C.G. was 16.70 inches for the longitudinal static stability flight. The differences in the collected data from these two C.G. points was not great enough to show meaningful plots upon data reduction. The results of the longitudinal maneuvering stability flight produced better results, because the C.G. spread was greater. The maneuver points were still forward of the aft C.G. limit, which leads to a possible conclusion that the published C.G. range needs to be adjusted. Also, based on conversations with Dr. Isaac Silver, who piloted this aircraft on many occasions, a C.G. of more than 17 inches reduces the handling of the airplane, and is not recommended. By simply moving the aft C.G. limit closer to 17 inches, it would move most of the maneuver points aft of the upper C.G. limit, and indicate that the airplane is stable. Meaningful results were obtained for longitudinal dynamic stability, and showed that the phugoid motion was sufficiently damped. Finally, a partial test for static and dynamic lateral-directional stability was performed and showed that the Dutch Roll oscillation was heavily damped as required.