Judging from the NTSBs files, more than a few pilots have had the misfortune of encountering flutter. Whether induced by the pilots actions or by improper maintenance procedures, flutter is a very serious problem that requires instant corrective action in flight. The stakes are high. Flutter can disfigure your airframe or even rip it apart.
I personally encountered flutter in flight a few years ago – and at a speed that greatly surprised me because it was relatively low.
I was in a sailplane working an afternoon thermal with my wife in the front seat. I was circling in the thermal at about 55 knots when the wing started to vibrate. It was enough that we definitely felt it in the cockpit. We flew back to the airport at a slower speed and landed.
I checked the aircraft for any looseness, but could find nothing suspect. We took the wing off the aircraft and had a mechanic inspect the wing, but he was unable to find anything, either. The wing was remounted and the aircraft test flown, but the flutter never returned and we never determined its source or cause. The only guess I can make is that somewhere in the process of taking the wing off the aircraft and mounting it back on the aircraft, we might have tightened some bolt that had previously worked itself loose.
Luckily my encounter with flutter was at such a slow speed that it ended uneventfully. Many other pilots have not been so fortunate.
Flutter can quickly cause substantial and possibly catastrophic damage to an aircraft, so preventing flutter is very important. Engineers, pilots and maintenance technicians all play a part in preventing flutter, which describes a range of aeroelastic problems.
Engineers try to prevent aeroelastic problems in the design stage. Design is a three-way battle, with structural engineers, aerodynamicists and performance engineers duking it out.
It would be much easier for the structural engineer to use an I-beam and call it a wing, but I-beams have terrible aerodynamic characteristics. The aerodynamicist is happy with long, thin high-aspect wings. Unfortunately, long thin wings are more subject to aeroelastic problems.
Aeroelasticity is a term for the interaction between aerodynamic bending loads on the wings and the internal stiffness of the wing that resists the bending. As the wing flows through the air, it produces a small bending moment due to the uneven pressure distribution over the airfoil section.
This bending moment at small angles of attack attempts to bend the leading edge down. This is a desirable stable condition.
At slower airspeeds, the aerodynamic forces are very small, so the bending moments are quite small and aeroelastic effects are small. At higher airspeeds this becomes a factor. As the deflections go back and forth, it may reach the point where an oscillation begins.
Further complicating the analysis of aeroelasticity is the fact that the airfoil changes shape slightly as it is twisted. The angle of attack changes, which changes the aerodynamic pressures over the wing and thereby changes the twisting motion further. Additionally, as the structures stiffness attempts to twist the structure back to its original position, there is some inertia involved and the structure can slightly overshoot the initial position. This oscillation is flutter.
Many objects in nature oscillate at select frequencies. A flag in a breeze is a good example. At slow airspeeds, the flag droops. As the wind increases, the flag starts to vibrate back and forth.
Vibrations can become so severe that the structure fails, which in fact happened at the infamous Tacoma Narrows Bridge. The winds started blowing at a certain strength, which bent the bridge a little. Additionally, a unique aerodynamic airflow around the structure produced forces that acted to twist the bridge.
The internal stiffness of the bridge resisted this twisting and applied an opposite twisting moment. This reversed the twisting motion and produced a motion back toward the equilibrium position. This motion did not stop at the neutral position but rather overshot and twisted the bridge a little too much in the opposite direction.
In this case, the wind was a driving force that continued to twist the bridge. The bridge reached a point at which it twisted violently back and forth at its natural frequency until the bridge collapsed. In aircraft, the two structures most likely to flutter are the wings and the control surfaces, with empennage flutter being particularly predominant in some models of aircraft.
Flutter, Design and Stick Forces
Flutter is primarily a function of the stiffness of the structure, its mass and the aerodynamic force. If you change any of those factors, you can change the flutter frequency. As an aeronautical engineer, I have used some of the equations and models to predict the airspeed for the onset of flutter, but the equations and models cannot adequately model the complexity of a structure.
Engineers have to assume fairly small deflections and reduce the math models complexity in order for the math to work. The engineers also have to make simplifications about the aerodynamics to reduce computational complexity. For all of these reasons, aerodynamic models and equations cannot adequately predict flutter in an aircraft, which must be flight-tested to prove that the design is free from the effects of flutter.
The flight test aircraft are thoroughly inspected for airworthiness before the flight test, with particular attention paid to the joints, control rods, assemblies and control surfaces. If flight testing discovers flutter within the aircrafts envelope, then post-design modifications to stiffen the surface are often made. Several high performance sailplanes have required structural stiffness to be added even after initial deployment of the design. The Learjet required stiffeners to be added to the trailing edges of the ailerons as a preventive measure against flutter.
The next time you practice stalls, turn around and look at the tail surfaces. You will be shocked to see how much they vibrate.
Generally speaking, flight testing has established that the prototype aircraft is free of flutter throughout the aircrafts flight envelope by the time the design receives its type certificate. One notorious exception was the V-tailed Bonanza. A 1986 AD required a beef-up of the ruddervator after the design had been in production for some 40 years.
The case of the Bonanza is a rather interesting one. The empennage surfaces on any aircraft take a real subtle beating from the propeller slipstream, which is turbulence flow. The turbulent airflow of the propwash induces small vibrations in the surfaces of the empennage. Not only does this create a prime breeding ground for fretting fatigue, but it also loosens joints and control assemblies.
Many of the most recent examples of empennage flutter can be found among Beech 35s, despite the earlier empennage beef-up. The FAA issued Airworthiness Directive 98-13-02 in 1998, restricting the aircraft to 144 mph on Beech 35, A35, and B35 airplanes, which had been exempt from the earlier AD. Beech then issued Service Bulletin 27-3358 in February 2000, which required an extensive inspection of empennage components. The service bulletin also changed range of acceptable ruddervator underbalance from 16.8-19.8 inch-pounds to 16.8-18.0 inch-pounds in order to lift the speed restriction. It is very important to comply with these directives.
When an aircrafts flight control system is designed, the engineers try to strike a compromise so that the stick displacement produces an acceptable rate of pitch and roll change in the aircraft while using an acceptable amount of stick force.
The leverage of the stick (or yoke) is one feature affecting this equation. Another feature is the hinge moment on the control surface. If you have a large surface area concentrated far away from the hinge for the control, the aerodynamic forces on the hinge can be very pronounced.
Engineers are able to tweak the design with two tools, mass balancing and aerodynamic balancing, to help lessen or increase the control forces as necessary.
Time for Mass
Mass balancers are used on many kinds of airplanes. Counterweights on the underside of the ailerons are mass balancers, as are the protrusions on the outboard leading edges of many elevators. In the case of the elevators, there is a weight in the front of that protrusion to help provide mass balance. The rudders of some aircraft have similar protrusions.
These balancers are there for aerodynamic balance, which must be kept within certain ranges. This wouldnt be much of a problem if you always kept your aircraft inside and never painted it. However, sometimes water or melting snow can seep into these areas of the control surface and then freeze. This will drastically change the mass and the balance of the flight control.
Most control surfaces are designed with weep holes to allow water to leak out, but you should still avoid situations where moisture can collect within your control surfaces and then quickly freeze. Aircraft that are left outside in the elements are more prone to this.
Something as simple as repainting your airplane can cause flutter at slower speeds. Believe it or not, the added weight of paint on the relatively light control surface can substantially change the mass of the overall control surface, and this change can lead to premature flutter. Mass balancing should be part of every paint job, though some people continue to regard it as an unnecessary extra.
The NTSB records include the last flight of a Grob 115-D whose rudder was repainted without being rebalanced. Hunters reported seeing pieces of airplane falling from the sky. The top portion of the rudder assembly was found about 2,400 feet from the main wreckage.
Balance checks of the left aileron showed that its residual hinge moment exceeded the manufacturers specifications. A review of the aircraft maintenance logs revealed the airplane had been repainted, but the flight control surfaces had not been rebalanced. The NTSB determined that the rudder surface fluttered due to improper balance. The elevator attachment delaminated, causing an empennage overload that led to wing overload and separation.
It doesnt take much to make your aircraft different than the flight test aircraft. A slight amount of play in the controls cables can cause flutter at much slower speeds than the speeds checked by the flight test teams. Poor maintenance, cursory preflight inspections, aging, and loosening of the connections through normal use will increase the probability of flutter occurring at airspeeds you may think too low for flutter. Even changes in temperature can affect cable tension, particularly at very cold or very hot temperatures.
If It Happens to You
You will definitely feel the onset of flutter. The aircraft will begin to produce a noticeable shake and high frequency, low amplitude vibration. Flutter will grow dramatically with just a small increase in velocity, which will lead to the amplitudes of the vibrations growing larger. Flutter is normally encountered well before divergence in straight subsonic wings, but a small increase in speed leads to a greater possibility of divergence. Atmospheric turbulence, even slight bumps, can bring on flutter when the aircraft is flying at higher speeds.
Once flutter develops, there is nothing the pilot can do except to slow down – and do it gently.
The accident records demonstrate that flutter can quickly cause substantial damage to the aircraft, which in turn can lead to a fatal loss of control as the airplanes control surfaces fail. Its important to keep your aircraft comfortably within the aircrafts flight envelope, as the risk of flutter greatly increases with higher airspeeds.
The maximum structural cruising speed, Vno, is the bottom of the yellow arc. The aircraft is designed to fly only in smooth air conditions in the yellow arc. Remember, it is painted yellow for a reason, to warn you that you are approaching a danger zone.
Some designs are more prone to flutter than others. Homebuilts, notably, are more prone to flutter because of the vagaries introduced during the airplanes construction. Although the designer attempts to account for differences in construction skill, there are some problems it does not anticipate.
For most pilots, however, it doesnt matter whether you are flying a production aircraft or a homebuilt, all of the parameters for control surface attachment need to be quantified, built to specifications, tested to ensure quality control and then maintained to within acceptable standards.
Storage, proper maintenance and thorough preflight actions are necessary to prevent flutter in your aircraft.
Whenever the flight controls undergo periodic maintenance, it is critical that the proper procedures be followed. However, because a loose cable or joint can be slightly loose and therefore undetectable, flutter does occasionally occur in flight. You need to slow down and try to avoid bumpy air until you can get the aircraft on the ground.
By all means, do not fly the aircraft until the airplane gets a thorough checkup. You dont want to fool around with flutter.
-by Pat Veillette
Pat Veillette is an aviation safety researcher, engineer and B-727 pilot.