The effects of wing inertia variations on spin recovery characteristics of single-engine general-aviation aircraft

An experimental study of the effects of variations in rolling moments of inertia on spin recovery was conducted for single-engine general-aviation aircraft. The test method selected was flight test of a Froude number ]/6th dynamically scaled radio-controlled model, typical of a general-aviation aircraft. A model of a Piper PA-28-180 Cherokee of 1970' s vintage was constructed, instrumented with a flight data recorder and hobby-type sensors, and fitted with a spin recovery parachute system. Ground video recordings supplemented the on-board instrumentation to provide data to analyze spin and recovery behaviour. To replicate increased rolling moments of inertia for the full-scale airplane, the experimental approach was based on changes in rolling moments of inertia created by variations of fuel capacity in the wing of the full-scale airplane. A total of 11 flights and 38 spins were carried out over an IYMP range from -56 x 10-4 to 8 x 10-4. Angle of attack, spin rate about the spin axis, and turns to recovery were essential to analyze the spin behaviour. Premature recoveries prevented the stock and increased inertia spins from developing their potential energy level resulting in lower than expected angle of attack and spin rate. Generally, however, angle of attack and spin rate increased as the rolling moment of inertia was increased, but turns to recovery never exceeded 1 ¾ turns using the standard spin technique. Ailerons-against spin maneuvers caused turns to recovery to rapidly increase due to higher spin rates, with higher rolling moments of inertia worsening the recovery to the point of being unrecoverable. This occurred only for left spins with right spins being more benign. Spin rate and angle of attack were determined to be the key elements deciding whether or not a spin would be more severe from increasing the rolling moment of inertia. Increased rolling moments of inertia may not always cause spin recovery to degrade due to other predominant forces and moments. However, given enough spin turns, suitable additional wing inertia, and the right control deflections, there wi11 be a point where the spin wi11 become unrecoverable. lt was found that an increase of 20 gallons of fuel load in the wing at mid-span (IYMP of 8 x 10-4) caused unrecoverable spins in only the ailerons-against left spin. The Piper PA-28-180Cherokee and other similar general-aviation airplanes should exhibit worsening spin recovery characteristics as fuel capacity in the wing is increased to the point where at approximately 20 additional gallons of fuel may trigger unrecoverable spins. Overall, this and previous research from the 1940's to the 1970's concluded that increasing wing mass degrades spin recovery given the right circumstances. It was also found that hobby-type piezo-electric rate gyros are suitable for spin testing in radio-controlled models, however, they must be calibrated over their complete dynamic range to ensure the gain setting is correct and they must be corrected for temperature changes by pre-flight data or other means. Recommendations go to the general-aviation community. Pilots must avoid applying ailerons against the spin (either purposely or inadvertently) during entry or recovery for this Piper PA-28-180 Cherokee design, as recovery may never occur. Precautions must also be exercised when spin testing or certifying a general-aviation airplane that has fuel in the wings, or has been modified with additional fuel capacity in the wings, as unrecoverable spins may result.