Ed Anderson
Joined: 10 Jan 2006
Posts: 475
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 Posted: Mon Mar 26, 2007 11:30 am Post subject: A internet view of TAS and VNE |
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Here is an explanation that appears to make it fairly clear as to why TAS rather than IAS is the critical factor in flutter.
Ed
Ed Anderson
Rv-6A N494BW Rotary Powered
Matthews, NC
eanderson(at)carolina.rr.com (eanderson(at)carolina.rr.com)
http://members.cox.net/rogersda/rotary/configs.htm#N494BW
http://www.dmack.net/mazda/index.html
http://www.auf.asn.au/groundschool/flutter.html#flutter
Indicated airspeed really reflects dynamic pressure rather than airflow velocity so if the structural limitations which define Vne for an aircraft type are particularly associated with the distribution of forces associated with velocity then the specified indicated Vne has to be decreased as altitude is increased — to adjust for the increase in true airspeed which latter is about 1.5% greater than IAS/CAS for every 1000 feet of altitude, see rule of thumb #2.
13.3 Aerodynamic reactions to flight at excessive speed Flutter Wing structures are akin to a 'tuning fork' extending from the fuselage. When a tuning fork is tapped the fork vibrates at a particular frequency, the stiffer the structure the higher its 'natural' frequency. The natural frequency of a wing or tailplane structure may apply another limiting airspeed to flight operations – related to structural instabilities: flutter and wing divergence.
When the airflow around a wing or control surface is disturbed by aerodynamic reactions or pilot inputs, the structure's elastic reactions may combine as an oscillation or vibration of the structure (possibly evident as a buzz in the airframe) which will quickly damp itself out at normal cruise speeds. At some higher speed — the critical flutter speed — where the oscillations are in phase with the natural frequency of the structure the oscillations will not damp out but will resonate, rapidly increasing in amplitude. (Pushing a child on a swing is an example of phase relationships and amplification). This condition is flutter and, unless airspeed is very quickly reduced, the severe vibrations will cause control surface [or other] separation within a very few seconds.
The following paragraph is an extract from an article by William P. Rodden appearing in the McGraw-Hill Dictionary of Science and Technology; it provides a succinct description of flutter:
"Flutter (aeronautics) – An aeroelastic self-excited vibration with a sustained or divergent amplitude, which occurs when a structure is placed in a flow of sufficiently high velocity. Flutter is an instability that can be extremely violent. At low speeds, in the presence of an airstream, the vibration modes of an aircraft are stable; that is, if the aircraft is disturbed, the ensuing motion will be damped. At higher speeds, the effect of the airstream is to couple two or more vibration modes such that the vibrating structure will extract energy from the airstream. The coupled vibration modes will remain stable as long as the extracted energy is dissipated by the internal damping or friction of the structure. However a critical speed is reached when the extracted energy equals the amount of energy that the structure is capable of dissipating, and a neutrally stable vibration will persist. This is called the flutter speed. At a higher speed, the vibration amplitude will diverge, and a structural failure will result."
Inertia has a role in flutter development requiring that control surfaces – ailerons, elevators, rudder – be mass balanced (i.e. the centre of gravity of the control surface coincides with the hinge line) to limit the mass moment of inertia. It may be acceptable for the control surface to be over-balanced, i.e. the cg is slightly forward of the hinge line. Mass balancing of the control surfaces will prevent them fluttering but the possibility for wing [for example] flexing/twisting flutter may still exist.
The critical flutter airspeed [or something akin to it] may eventuate well below Vne if wear in control surface hinges, slop in actuating rods/cables/cranks/torque tubes, water or ice inside control surfaces or absorbed within a foam core, mud outside, faulty trim tabs, additional surface coatings applied after balancing or other system disturbances exist which alter the structure's reactions.
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_________________
Ed Anderson
Rv-6A N494BW Rotary Powered
Matthews, NC
eanderson@carolina.rr.com |
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