Can turbulence cause flutter?

[hellkitty]

Hi,

I'm looking for an explanation of how turbulence affects on aeroelasticy (especially I'm interested in flutter & wing divergence)?
Is it possibile that during the turbulence flutter could appear below critical flutter speed?

Regards

[John Farley]

I would suggest an airframe is either free from flutter inside its approved envelope or it is not.

I don't see that the nature of the excitation (flight test equipment or turbulence) matters.

[Another St Ivian]

As per JF's message, in short...the answer is no, there should be no flutter.

Consider the traditional velocity-damping graph:

http://i.imgur.com/2BNtM4m.png

Provided that the airspeed is kept below V_flutter (in this example), then the structure will provide positive damping regardless of the excitation source. You may find that turbulence could excite a mode, but as per the definition of maximum operating speeds, it will be damped.

Possibly interesting side note: It's not uncommon to specifically go and look for a little light chop during flight testing to get some airframe excitation and help the engineers get some modal data :8

[FCeng84]

If flutter is defined as an unstable oscillation due to aero-elastic coupling, I agree with the notion that an airplane is either flutter prone or flutter free at a given flight condition regardless of the turbulence level. Due to non-linearities in structure, aerodynamics, and/or flight controls there can be situations where a tendency for undamped oscillation (i.e., flutter) is greater at elevated deflections of a control surface or structure. In that event, turbulence could act to trigger a flutter oscillation where flight through calm air with small control surface deflections would not.

If flutter is defined as an airplane oscillatory response that risks exceeding structural limits (but is not necessarily unstable), the level of turbulence can play a major role. If the airplane response is lightly damped, the energy imparted via turbulence might be sufficient to cause a large enough response to put the integrity of the structure at risk.

[hellkitty]

Thank you All for answering my question :ok:Since yesterday I thought that turbulence has a lot in common with flutter. I've been told that it could act as a part of the excitation forces. Now I know it's generally false idea.

@ FCeng84, I'll be very grateful If you write something more about those situations when bigger deflections of control surfaces may cause flutter during the turbulence. Are there any publications available on the Internet concerning that matter? Currently I'm writting a short disseration trying to answer a question how turbulence can affect the flight of an a/c. Such kind of information would be very useful.

[Another St Ivian]

I've been told that it could act as a part of the excitation forces. Now I know it's generally false idea.

Just picking up on a minor point there - Turbulence can excite certain modes, however in normal operations the aircraft is operated at speeds where those modes are positively damped.

FCeng84 may be able to offer you some further thoughts however in aircraft in 'transient' conditions, i.e. control surfaces deflected.

[FCeng84]

Prior to the advent of highly augmented flight control systems flutter analysis did not need to consider the role of the control system outside of control surface balance and stiffness characteristics. With a control system that includes feedback loops that can act at flutter frequencies the augmentation becomes an important player in flutter analysis.

A prime example is a control system that provides longitudinal stability augmentation via pitch rate feedback to the elevator. Augmentation designed to improve short period handling qualities in the pilot's maneuver frequency range of up to 1 Hz may inadvertently provide structural mode destabilizing feedback at higher frequencies associated with flultter. Flutter analysis must be done with simulations that model both the aero-elastic structure and the flight control system with sufficient fidelity over the frequency range of concern with respect to flutter.

As to the point I was making above about turbulence triggering a flutter issue, consider the roll axis of a transport aircraft. Several pairs of control surfaces found on the wing are typically used for roll control, but often in a non-linear fashion. For instance, a configuration with more than one set of ailerons (inboard and outboard for example) may use one pair of surfaces exclusively for small control inputs but bring in other surfaces for large commands. The closed loop (that is with consideration of the control system) stability may be quite different for small inputs that use only one set of surfaces vs. larger inputs that bring additional surfaces into play. In this scenrio, evaluation with small disturbances that only employ one set of surfaces might yield a much larger flutter-free flight envelope than evaluation with larger disturbances that get more surfaces involved.

Prudent flutter evaluation must consider the control system and take into consideration control system non-linearities that could yield different results for small disturbances vs. large ones.