HI All, Finding your flutter speed sounds pretty important and I may try - carefully.
But we must not forget that flutter can happen at any speed - and can change. Flutter inducing dynamics increases with speed, but flutter speed can decrease dramatically with changes in rigging, weight, stiffness (including cracks or even temperature changes), wear in hinges and bearings, and even rain. Just two weeks ago I experienced flutter for the first (and hopefully last) time - at ~80mph! I have been fixing up a buddy’s Kitfox Model III. Its a fun airplane, but has a remarkably complex control linkage system for its flapperons. Every linkage has inherent slop and the the Model III has something like _eleven_ before reaching the flapperon. Later models reduced the complexity a bit. Compounding this, the wing design is strong but inherently flexible and the flapperons are attached on long trailing edge extension of the ribs. I’ve since removed a remarkable amount of slop from the control system - but as you can imagine, with a 11 control linkage system there will always be some slop. Haven’t flown it again yet - but soon perhaps. How does this translate to the KR? First of all is respect for the build and maintenance of your airplane. Ensure the control system and hinges are in good nick with as little slop as possible. Better to error toward friction (in the linkages) than looseness. That means that the cables are taught, the pulleys, bell crank and rod linkages are taught and not worn, and importantly they do not flex. If they flex, they will eventually crack and fail. Many experimentals were originally designed with too light bell crank brackets that later had to be modified to reduce flex. Also, if your hinges are worn, fix or replace. Balance and rebalance with any mods. And get familiar with the flex of your ailerons and wing - so than on preflight you can stress them (a bit) and feel for changes. Additionally, I will second the depower yank and bank suggestion. But not yank. As with all, firm but gentle. You may already be close to stress limits, so easy but firm and decisive to quickly reduce speed. Perhaps this is all a bit obvious or obsessive - but after a bit of sphincter tightening flutter, understandable. from: https://www.faa.gov/documentlibrary/media/advisory_circular/ac%2090-89a.pdf <https://www.faa.gov/documentlibrary/media/advisory_circular/ac%2090-89a.pdf> 1. OBJECTIVE. To understand the causes and cures of the condition known as flutter. 2. DESCRIPTION. Flutter in an aircraft struc- ture is the result of an interaction between aero- dynamic inputs, the elastic properties of the structure, the mass or weight distribution of the various ele- ments, and airspeed. a. To most people, the word ‘‘flutter’’ suggests 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 flags motion becomes more and more excited. It takes little imagination to realize if something similar happened to an aircraft struc- ture, the effects would be catastrophic. The parallel to a flag is appropriate. b. Think of a primary surface with a control hinged to it (e.g., an aileron). Imagine that the air- plane hits a thermal. The initial response of the wing is to bend upwards relative to the fuselage. c. 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. d. In a simple, unbalanced, flap-type hinged control, the center of mass will be behind the hinge line and the inertial lag will result in the aileron being deflected downwards. This will result in the wing momentarily generating more lift, increasing 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 struc- ture than can be opposed by the aerodynamic forces acting on it. e. 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. f. Flutter can happen at any speed, including take-off speed. At low airspeeds, however, structural 52 5/24/95 AC 90-89A and aerodynamic damping quickly suppress the flut- ter 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. g. Further SMALL INCREASES will produce a divergent, or increasing oscillation, which can quickly exceed the structural limits of the air- frame. Even when flutter is on the verge of becoming catastrophic it can still be very hard to detect. What causes this is the high frequency of the oscillation, typically between 5 and 20 Hz (cycles per second). It will take but a small increase in speed (1⁄4 knot or less) to remove what little damping remains and the motion will become divergent rapidly. h. Flutter also can occur on a smaller scale if the main control surface has a control tab on it. The mechanics are the same with the tab taking the place of the aileron and the aileron taking the place of the wing. The biggest difference are the masses involved are much smaller, the frequencies much higher, and there is less feed-back through the con- trol system. This makes tab flutter more difficult to detect. The phenomenon known as ‘‘buzz’’ is often caused by tab flutter. Since flutter is more prevalent at higher speeds, it is not recommended that the flight test plan call for high speed runs within 10 percent of red line. i. What can be done about it? Having described how flutter happens, the following sugges- tions should help reduce the possibility of it happen- ing to the amateur-builder’s aircraft: (1) Perform a mass balance of all flight controls in accordance with the designer/kit manu- facturer’s instructions. (2) Eliminate all control ‘‘free play’’ by reducing slop in rod end bearings, hinges, and every nut and bolt used in attaching flight controls. (3) Ensure that all rigging and cable ten- sion is set accurately to the design specifications using a calibrated cable tensiometer. (4) Re-balance any flight control if it has been repaired, repainted, or modified in any way. NOTE: If the pilot experiences flutter, or believes he did, reduce power immediately and land as soon as possible. Do not attempt further flight until the aircraft has been thoroughly inspected for flutter induced damage. This inspection should include all wing/tail attach points, flight controls, their attach points/hinges, hardware, control rods, and control rod bearings for elongated bolt/rivet holes, cracks, (especially rod end bearings) and sheared rivets. _______________________________________________ Search the KRnet Archives at https://www.mail-archive.com/krnet@list.krnet.org/. Please see LIST RULES and KRnet info at http://www.krnet.org/info.html. see http://list.krnet.org/mailman/listinfo/krnet_list.krnet.org to change options. To UNsubscribe from KRnet, send a message to krnet-le...@list.krnet.org
