What is the difference between buffeting and fluttering?

Question

As I understand, both are dynamic effects of aeroelasticity - however, what is the exact difference between buffeting and flutter?

Answer 1

The vernacular meaning is...

1) Flutter is un-commanded self perpetuating (positive feedback) destructive & adverse cyclic movement of any part of an aircraft. It most commonly occurs on a control surface but may be an entire wing, tail surface, or more rarely fuselage or other part or device. Flutter may be induced either mechanically or aerodynamically and may be sustained mechanically (i.e. engine or prop, landing gear or ground resonance of a helicopter skid) or aerodynamically by self perpetuating (positive feedback) force. Mechanical inception can be (but not limited to) brisk or pilot induced oscillation, weak, unsupported, or flimsy structures. Aerodynamic inception can be (but not limited to) undesirable cg, or aerodynamic forces on parts. Flutter is always undesirable, dangerous and a destructive force.

2) Buffeting is a turbulent stream of air striking any part of the airframe. The turbulent air may be completely external (weather) or created by the aircraft it self (prop, wing,etc). The most common form of buffeting is turbulent airflow over a wing (i.e. during a stall or wind gusts) striking the wing or horizontal tail. Buffeting is not usually considered self perpetuating with positive feedback, nor is it considered to be precisely cyclic or harmonic - though it may be repetitive. Buffeting is usually considered undesirable but benign and not a destructive force.

Answer 2

Flutter is characterized by a frequency neighbourhood of a structural eigenmode and a cyclic aerodynamic load. This causes their motions to be mutually reinforced.

Buffeting, on the other hand, is just the consequence of a cyclic aerodynamic load, such as an oscillating shock, or separated flow hitting another surface downstream.

Answer 3

Both buffet and flutter is a vibration of a surface of the aircraft, such as a wing or tail, due to the air flowing over it (especially if the air is turbulent). The difference, I would say, is;

• If the surface does not exhibit an increasing amplitude, as it is naturally damped at the frequency of the excitation force, it is called buffet.
• Conversely, if the surface is not naturally damped at the frequency of the excitation force, its amplitude of oscillation will continue to grow, and likely structurally fail, catastrophically, it is called flutter.

In flutter, there is a coupling between the aerodynamic forces and structural response, that needs to be taken account of, in analysis of flutter, which is not required in buffet. See the first paragraph of this technical paper.

The flutter analysis is generally conducted by complex eigenvalue analysis, whereas the buffeting response is typically estimated using a mode-by-mode approach that ignores the aerodynamic coupling among modes.

As examples of each, here is a video of flutter. As can be seen, the amplitude of vibration starts to grow, and it wouldn't appear surprising if the tail plane failed very quickly.

In comparision, the F-18 has experienced tail buffet from vortices from the wing leading edge extensions. As can be seen, the tails vibrate, but its a much smaller oscillation than the flutter example. While it could cause cracking in the long term, it would not be expected to fail instantly.