 analyt treatment aircraft propel precess instabl analyt treatment aircraft propel precess instabl analyt investig precess-type instabl occur flexibl support aircraft-engin-propel combin ideal mathemat model compris rigid power-plant system flexibl mount pitch yaw fix backup structur condit requir neutral stabil determin paper examin sensit stabil boundari chang paramet stiff damp asymmetri engin mount propel speed airspe mach number propel thrust locat pitch yaw ax stabil depend damp stiff system nondimension chart theoret stabil boundari compar experiment wind-tunnel test aeroelast airplan model gener theoret account wing respons trend observ experiment set condit calcul airspe neutral stabil consist lower measur valu evid result fact wing respons add damp system 