Builder's Manual Points to Ponder
The builder's manual (Section 15) has a flutter discussion as pertains to RV's. In the RV training materials, over on the safety page?Vne discussion begins on page 343. The link to the current draft is in the sticky at the top of the page. You can download a PDF version of the training materials if you'd like. The table of contents is hyperlinked to help with navigation. If you?d like a Word version (or PPT version of the RV aerodynamics briefing), drop me a PM or e-mail. Treating Vne as TAS is a simple technique that will ensure design limits are not exceeded at any altitude. If you haven?t already done so, on the factory web site, there is a good article called ?Flying High and Fast? that has an excellent flight envelope discussion as applies to RV?s. It can be accessed via this link:
http://vansaircraft.com/pdf/hp_limts.pdf. If you don?t have a copy of the builder?s manual, it can be obtained with a set of preview plans from Van?s. This is an excellent resource for folks that may not have built their ?new to them? RV. If you are considering the necessity for flutter testing, please read the last paragraph in the builder?s manual carefully. I?ve highlighted it in bold below. One other point to ponder is the effect of aircraft aging and wear and tear (e.g., control bearing wear). Even if accurate flutter testing is properly conducted for a particular airplane, as the airplane ages, flutter characteristics may change, and actual flutter margin may be reduced. Therefore, for airplanes built in accordance with the plans and instructions, respecting the design margins and using the TAS Vne technique are recommended. This is experimental aviation after all, and if the airplane has been modified, then proper testing may be warranted.
For folks that don't have access to the builder's manual, here's what Section 15, Revision 8, published 06-08 says (please check with the factory to determine if there is a subsequent revision to this section):
?Flutter in an aircraft structure results from the interaction of aerodynamic inputs, the elastic properties of the structure the mass or weight distribution of the various elements, and airspeed. The word ?flutter? suggests to most people a flag?s movement as the wind blows across it. In a light breeze the flag waves gently but, as the wind speed increases, the flag?s motion becomes more and more excited. It is easy to see that if something similar happened to an aircraft?s structure the effects would be catastrophic. In fact, the parallel to a flag is quite close.
?Think of a primary surface with a control hinged to it (e.g., aileron). Imagine that the aircraft hits a thermal. The initial response of the wing is to bend upwards relative to the fuselage. If the center of mass of the aileron is not exactly on the hinge line, it will tend to lag behind the wing as it bends upwards.
?In a simple, unbalanced, flap-type hinged aileron, the center of mass will be downward. This will result in the wing momentarily generating more lift, which will increase its upward bending moment and its velocity relative to the fuselage. The inertia of the wing will carry it upwards beyond its equilibrium position to a point where more energy is stored in the deformed structure than can be opposed by the aerodynamic forces acting on it.
?The ?wing bounces back? and starts to move downward but, as before, the aileron lags behind and is deflected upwards this time. This adds to the aerodynamic down force on the wing, once more driving it beyond its equilibrium position and the cycle repeats.
?At low airspeeds, structural and aerodynamic damping quickly suppresses the motion but, as the airspeed increases, so do the aerodynamic driving forces generated by the aileron. When they are large enough to cancel the damping, the motion becomes continuous. Further small increases in airspeed will produce a divergent, or increasing, oscillation, which can quickly exceed the structural limits of the airframe. Even when flutter is on the verge of becoming catastrophic, it can still be very hard to detect. What makes this so is the high frequency of the oscillation which is typically between 5 and 20 Hz (cycles per second). It will take only a very small increase in speed to remove what little damping remains and the motion will become divergent rapidly.
?Flutter testing of factory prototypes has resulted in establishing a NEVER EXCEED SPEED (Vne) of 210 statute mph for the RV-3,4 and RV-6/6A, 230 statute mph for the RV-7/7A/8/8A and 210 statute mph for the RV-9A. This speed was determined through flutter testing at a speed of 20 mph above Vne. (FAA certification criteria require flutter testing up to Vne plus 10% or about 20 mph) The flutter testing performed consisted of exciting the controls by sharply slapping the control stick at various speed increments up to this level. Under all conditions, the controls immediately returned to equilibrium with no indication of divergent oscillation s indicative of flutter. This testing was performed on factory prototype aircraft, and the flutter free flight operation of subsequent amateur built RV?s has substantiated published Vne.
?The ?slap-the-stick? method of exciting the controls for flutter testing is potentially dangerous and requires a very skilled pilot trained to recognize the subtle control responses which indicate the onset of flutter. For this reason, it is suggested that amateur builders do not perform flutter testing of their RV?s [italics added]. Rather, the airplane should be constructed in strict conformity to the plans with particular attention paid to the control system?trailing edge radii, skin stiffness, control linkage free-play and static balance in particular. Maintaining conformity with the prototype (plans) will provide and adequate level of assurance against control surface flutter. Any design changes to the control surfaces, control system, or primary structure could invalidate the testing which has been done, and require that testing be re-accomplished.?
Fly safe!
Vac