From: email@example.com (Jack Dunn) Organization: AAOB/SySD LaRC NASA Date: 14 Apr 94 12:33:56 PDT References: 1 2
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In article <1126@orchard.Chicago.COM>, firstname.lastname@example.org (Richard G. Estock) writes: >I would appreciate an e-mail reply to the following query: > >I enjoyed reading and recommend to all Clive Irving's "Widebody: The >Triumph of the 747". The author mentions the problem of flutter with >all of the swept wing aircraft that Boeing (and presumably all other >manufacturers as well) developed. > >I am confused about what this problem actually is, and how it >manifests itself. The term "flutter problem" covers a large set of dynamic load limits that elastic aircraft are designed. Classical flutter involves the exchange of energy between the pitch and plung motion of the surface and the air. The system can be modeled as two uncoupled second order systems coupled by the aerodynamics. The magitude of the aerodynamic energy is proportional to the dynamic pressure, qbar, of the flow, (high speed low level flight == high qbar). There exist a value of qbar that this system will become unstable, that is the motion of the wing cause forces to be applied to the system that cause the motion to increase. The load limit of the wing is quickly exceeded, (5 cycles). There are other forms of flutter such as stall flutter which involves the stalling of the aerodynamic surface. Transsonic flutter can be influenced by the nonlinear characters of the aerodynamics at transonic speeds. Panel flutter can cause problems with skin panels. Each of the different types of flutter have its own analysis method. Some of the methods are better than others. Transonic flutter is so bad that dynamicly scaled models are used in wind tunnels to verify that the analysis methods will work on the full scale aircraft. > >What, then, is flutter? What is the cause? What happens to an >aircraft that is experiencing flutter? If an observer were in a >chase plane, what motion would the observer see with an aircraft >ahead that is undergoing flutter? The observer whould only see the wing breaking apart. >What corrects flutter? The basic cure for flutter is to make the wing stiffer. Some solutions try to seperate the natural frequencies of the two modes that are involved with limited success. Some designs try to place the cg of the structure forward by placing the engins out on pylons. > >As I understand it, on a 7X7 wing there is in succession an: > inboard flap > inboard spoiler > tabs (2) > aileron > outboard flap > outboard spoiler > >>From one source, flutter was described as an oscillation >at some critical speed of the aileron. Why just the aileron? Or in >this case are the flaps, spoilers, et al, collectively considered an >aileron? All vibrating surfaces will flutter. The question is at what speed and altitude. > >Is the problem of flutter more pronounced with swept wings than >straight wings? Or is it more a function of speeds approaching Mach >1 regardless of wing geometry? Sweep by itself does affect flutter. However, swept wings are stiffer. But, aerodynamic effects lead the designer to use more sweep for higher speed aricraft. There is no simple answer. The major design parameters that have an effect on flutter, are 1) mass ratio (mass of wing to the mass of the air); 2) sweep angle; 3) taper ratio; 4) aspect ratio; 5) chordwise position of cg; 6) elastic axis position; 7) pitch radius of gyration; and 8) bending torsion frequency ratio.