ABSTRACThelicopters because of the limitations imposed by aerodynamic physics, there is hope that tiltrotor topend and The requirements for increased speed and productiv-cruise speeds may increase further with improved engiity for tiltrotors has spawned several investigations asso-i n current limitations on speed for the V-22 tiltrociated with proprotor aeroelastic stability augmentation tor are associated with control loads, control margins, and and aerodynamic performance enhancements. Included power, while the tiltrotor is power limited. The among these investigations is a focus on passive aeroe-aeroelastic stability of tiltrotor systems is also an imporlastic tailoring concepts which exploit the anisotropic ca-tant concern, as the stability margins associated with curpabilities of fiber composite materials. Researchers at rent tiltrotors are not far beyond the speed limitations set Langley Research Center and Helicopte~ have de-by loads and power today. It is anticipated that the u p voted considerable effort to assess the potential for using per velocity limit for future high-speed tiltrotors may be these materials to obtain aeroelastic responses which are set by both loads and aeroelastic stability considerations. beneficial to the important stability and performance con-To achieve higher speeds for tiltrotors, structural tailoring siderations of tiltrotors. Both experimental and analyt-of blades and wings using advanced composite ical studies have been completed to examine aeroelastic has been considered in several past investigations. tailoring concepts for the tiltrotor, applied either to the wing or to the rotor blades. This paper reviews some of the results obtained in these aeroelastic tailoring investigations and discusses the relative merits associated with Researchers at Lagley Research Center and Bell Helithese approaches.copter have devoted considerable effort to assess the potential for using composite materials to obtain aeroelastic responses which are beneficial to the important stability INTRODUCTION and performance considerations of tiltrotors. Both experimental and analytical studies have been completed qytrotor aircraft have advantages over conventional he-which examine aeroelastic tailoring concepts for the tiltrolicopters with respect to speed and range. While a heli-tor, applied either to the wing or to the rotor blades. copter is limited at high speeds by compressibility This paper reviews some of the results obtained in these on the rotor advancing side and stall on the rotor retreat-aer0elaStic tailoring investigations and discusses the reling side, a tiltrotor converts from a helicopter mode to ative merits associated with these approaches. While an airplane mode for high speed flight is less re-the material presented in this report focuses on activi-
Rofur-L)y,zamics U.S. Artfly Vehicle Sfrrrctares r)irecto,nte Bell Helicopter Texfro~r, hic., For1 W0rtI1, TX NASA Lar~gley Research Genre,; Hnnrprorr, VA A composite tailored tiltrotor wing was designed and tested on a US-scale semispan aeroelastic model to demonstrate that composite tailoring techniques can be used to improve pmprotor stability. Structural tailoring of the model-scale wing torque box is acconlplished by using unbalanced composite laminates to modify wing bending torsion coupling, which is shown analytically to improve proprotor stahility in high-speed airplane mode flight. The analytical methodologies used to develop the math models and predict the improved aeroelastic stahility characteristics of the conlposite tailored wing are discussed. The frequency and damping characteristics of the critical wing beam and wing chord modes were measured during wind tunnel tests performed in airplane mode. The structural modes mere excited a t their natural frequencies and the damping values were measured from the time history decays following the excitations. The composite tailored wing aeroelastic stahility boundary is compared to the stahility boundary of a baseline untailored wing configuration. Finally, the modal damping and measured instabilities are then compared to analytical predictions for both the baseline and tailored wing configurations.
A preliminary study of a soft-inplane gimballed tiltrotor model subject to ground resonance conditions in hover has been completed. Parametric variations of the rotor collective pitch and blade root damping, and their associated effects on the aeromechanical stability of both the isolated rotor system and the coupled wing/pylon/rotor system were examined. Results showed aeromechanical behavior which is significantly different from that associated with classical soft-inplane helicopter rotor systems. The unstable mode of the coupled aeromechanical system was a fixed system wing mode while in classical ground resonance the rotor lag mode becomes unstable. Damping of the isolated rotor lag mode was determined to be extremely sensitive to the collective pitch setting, as was damping and the associated stability of the critical wing mode for the coupled aeromechanical system.
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