Document
N94- 27917
WING DESIGN FOR A CIVIL TILTROTOR TRANSPORT AIRCRAFT: A PRELIMINARY STUDY by Professor Masoud Rais-Rohani Aerospace Engineering Department Mississippi State University Mississippi State, MS 39762 Abstract A preliminary study was conducted on the design of the wing-box structure for a civil tiltrotor transport aircraft. The wing structural weight is to be minimized subject to structural and aeroelastic constraints. The composite wing-box structure is composed of skin, stringers, ribs and spars. The design variables include skin ply thicknesses and orientations, and spar cap and stringer cross-sectional areas. With the total task defined, an initial study was conducted to learn more about the intricate dynamic and aeroelastic characteristics of the tiltrotor aircraft and their roles in the wing design. Also, some work was done on the wing finite-element modeling (via PATRAN) which would be used in structural analysis and optimization. Initial studies indicate that in order to limit the wing/rotor aeroelastic and dynamic interactions in the preliminary design, the cruise speed, rotor system and wing geometric attributes must all be held fixed.
Introduction The tiltrotor aircraft is a flight vehicle which combines the efficient take-off, landing, hover and low speed characteristics of a helicopter with the efficient high-speed cruise characteristics of an airplane. 1 With the success of Bell XV-15 program and its derivative the Bell-Boeing V- 22, the tiltrotor concept has been seriously considered for civilian applications. 2 The civil tiltrotor transport aircraft is required to carry 40 passengers (8,000 lb) and cruise at 375 Knots with a range of 600 N. Miles. The tiltrotor aircraft is among many V/STOL configurations (e.g., tiltwing, variable-diameter rotor, etc.) that have been considered for civil transport application in the past few years; however, in terms of rapid payload delivery and fuel consumption versus the disk loading and in terms of manufacturability, the tiltrotor is judged as being the most efficient design. 3 The preliminary efforts in this study have been focused on two tasks: (1) To perform a literature review to learn more about the unique dynamic and aeroelastic characteristics of the tiltrotor configuration and the procedures to analyze them; and (2) To work on the structural modeling and analysis techniques necessary in the tiltrotor wing design optimization. This abstract highlights the important aspects of each task performed.
Wing Design Problem The design objective is to determine the optimum set of structural parameters that minimize the wing structural weight while satisfying all structural and aeroelastic constraints. The wing geometry is to be the same as that defined in a recent Bell Helicopter Textron study. 4 The wing box structural components (i.e., skin, stringers, spars, and fibs) are all made of graphite- epoxy composites. The wing design variables include skin ply thicknesses and orientations, and stringer and spar cap areas. These design variables would allow the tailoring of the composite materials to meet the design requirements most efficiently.
Since the civil tiltrotor aircraft must fly much faster than its military counterpart (i.e., V-22), total drag in general and compressibility drag in particular become important design drivers.
The wing airfoil section on V-22 has a thickness to chord ratio of 23%. The structural design requirements on the civil tiltrotor are less stringent than those for the V-22; hence, a thinner airfoil would satisfythe structuraldesignrequirements while reducingthecompressibilitydrag.
The goal is to usean 18%thick supercriticalairfoil for the civil tiltrotor wing to increasethe
drag-divergence Mach numberandlower the compressibilitydrag. Thewing structuraldesign
is basedon the limit load factors in helicopter and airplanemodesstatedin FAR: PartXX
(Interim AirworthinessCriteria:PoweredLift TransportCategoryAircraft). The 2.0-gvertical
jump take-off loads createthe highest wing root bending momentsas shown in the figure
below--setting the requirements for wing strengthin the form of maximumstressconstraints
on the stringersandthe sparcaps,andmaximumstrainconstraints on the skinplies.
The large proprotor at eachwing tip, resemblingmore like a helicopterrotor than an airplane
propeller, produceshigh dynamicandaerodynamic loads. Furthermore,rotor hub andblade
motionsalong with wing flexibility producedynamicand aeroelastic couplingsthat may lead
to several instabilities. The instabilities associatedwith the tiltrotor configuration may be
classified as:(1) Mechanicalinstabilities;and(2) Aeroelasticinstabilities. The heavymasses
placedat the wing tips reducethewing bendingandtorsionfrequencies.The wing motionsin
bendingand torsion(symmetricor antisymmetric)setoff motionsin the rotor hub. The rotor
hub motions cancausethe bladesto depatternwhich causefurther vibration of the rotor hub
leadingto mechanicalinstability andaneventualdestructivefailure. To eliminatethis form of
instability, the blade natural frequenciesin the plane of rotation must be greaterthan rotor
speed. 3 In the airplanemode,theoscillatoryaerodynamic anddynamicforcesgenerated by the
rotorscombinedwith the flexibility of thewing may tilt the axisof rotationcausingtherotor to
whirl. This whirling motion changesthe fixed-wing aeroelasticflutter to what is known as
whirl flutter. To eliminate this form of instability, the wing natural frequencies(mainly
torsion) must be kept awayfrom the rotor natural frequencies. Also, more importantly, the
wing beamwise bending and torsional frequencies must be kept separated. 5 In this study, in order to limit the wing/rotor dynamic and aeroelastic interactions, the cruise speed, rotor system and wing geometry are all held fixed. Hence, the wing box stiffness is dictated by the aeroelastic instability boundary. The guidelines established in Ref. 4 will be used to create proper design constraints for aeroelastic stability and natural frequency placements.
Some work has also been done on the generation of the finite-element model of the wing/pylon using PATRAN. This model along with material property information will be used in MSC/NASTRAN for the static and dynamic structural analyses of the wing model. Following the completion of this task, and the proper formulation of aeroelastic and structural constraints, the design optimization will proceed.
References 1. Mark, H. and Lynn R.R., "Aircraft Without Airports-Changing the Way Men Fly," VERTIFL1TE, Vol. 34, No. 3, 1988.
2. "Civil Tiltrotor Missions and Applications: A Research Study," Boeing Commercial Aircraft Company, Renton, WA, NASA-CR-177452, Summary Final Report, July 1987.
3. Loewy, R.G., "Aeroelasticity and the Tiltrotor VTOL Aircraft," VERTIFLITE, Vol. 38, No. 3, 1992.
4. Rogers, C. and Reisdorfer, D., "Advanced Material and Structural Concepts, Civil Tiltrotor Point Design-Model 940A," NASA-CR-191446, 1992.
5. Nixon, M.W., "Parametric Studies for Tiltrotor Aeroelastic Stability in High-Speed Flight," Proceedings of the 33rd AIAAIASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference, Dallas, Texas, April 13-15, 1992, Part 4, pp. 2027-2037.
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