Greetings For those who are unable to access the EAA site; I took Larry's email content and created a PDF for my personal library and uploaded for anyone who would like a copy:
https://aviation.griffin.co.za/docs/tony_bingelis-control_surface_flutter_problems-1979_july.pdf I have had way too many great web resources disappear over the years and have started copying the content of sites I find valuable to documents :) Regards שָׁלוֹם עֲלֵיכֶם (Shalom aleichem) Michael L Griffin mileg...@gmail.com Fax : 086 733 0650 Cel : +27 83 462 0462 Please consider the environment before printing this email He who play in root, eventually kill tree. Linode Private Server :: http://goo.gl/OnpGMS EvE Online 21day trial :: http://goo.gl/TAZSJ8 On Sun, 5 Jun 2022 at 19:32, Flesner via KRnet <krnet@list.krnet.org> wrote: > On 6/5/2022 12:28 PM, Flesner via KRnet wrote: > > > The EAA has an easy to read article on flutter and gives you the basics in > the first page or two. I'm assuming even non-[eaa members can load the > site. If not, advise, and I'll cut and past. > > > https://www.eaa.org/eaa/aircraft-building/builderresources/next-steps-after-your-airplane-is-built/operating-articles/general-operation/control-surface-flutter-problems > > Larry Flesner > > > ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ > > It's short. Here it is.............., every builder should have a set of > Tony's books............... > Control Surface Flutter Problems By Tony Bingelis (originally published > in EAA Sport Aviation, July 1979) > > December 01, 1993 > > July 01, 1993 > > FLUTTER? WELL, SIR, flutter is what a flag does wildly on the flag pole on > those days when it is too windy for you to fly. But to better relate it to > our subject, it should be described as a potentially destructive vibration > or buffeting of an aircraft due to an out-of-balance condition of one or > more of its control surfaces. > > Now, imagine one of your control surfaces acting like a flag in the breeze > . . . in flight at 100 or 200 mph. How long do you think it would stay with > the airplane? Not for long, I'll wager! > > Most of us are aware that the flutter problem is a complex one and it has > been around aviation for a long time. So long that flutter specialists must > be wallowing around knee deep in the accumulation of flutter fodder > generated from years of research and testing. Fortunately, there are a few > useful assumptions and certain recognized `good practices' which have been > sifted out and any builder willing to apply the guidelines can do much to > avoid having a flutter problem. But before I continue, let me discuss a few > terms and phrases. > > *Static Balance* > Static balance - A condition that exists when an object (wheel, propeller, > control surface, anything) remains stationary while supported on, or > suspended from its own center of gravity. Relating this more specifically > to our subject, it also means balancing a control surface while it is at > rest (not in flight). > > Automobile wheels, as you know, can be balanced statically (while at > rest). A more effective way, however, is dynamic balancing (spin > balancing). The dynamic balance of aircraft surfaces is similarly effective > but homebuilders really have no practical way of working out the dynamic > balance of a control surface subjected to the stresses of flight. For this > reason, they must fall back on what might be called a `good practices > concept' and assume that, for all practical purposes, when a control > surface is properly mass balanced in its static condition, it should also > be in dynamic balance. If, in principle, the main objective of dynamic > balance is to prevent or minimize torsional stress in flight, we can > accomplish this adequately by evenly distributing the mass balance weight > along the span of the control surface. > > Broadly speaking, to attain a static balance state in a control surface, > we add lead weight to the nose until the center of gravity falls on the > hinge axis. Let me expand on this a bit. > > For example, if you were to suspend a control surface from its hinge axis, > one of three static balance conditions would become immediately apparent to > you. > > 1. If the control surface assumes a trailing-edge-low attitude, it is > statically underbalanced and a tail-heavy condition exists. See *Figure 1* > . > > [image: Operating] > > 2. If the control surface remains in a level (horizontal) attitude, it is > said to be statically 100% balanced and its center of gravity (cg) is > co-located with the hinge axis. > > 3. Should the trailing edge of the control surface rise some position > above a horizontal plane, an overbalance condition is apparent. > > Two of the three conditions described above result in a control surface > that will have a fairly predictable flutter-free flight performance. The > one that is 100% balanced to a level attitude should consistently give good > results. The other surface having a slight nose down attitude is a typical > overbalance condition essential for good results in high-performance > aircraft. Conversely, the static underbalance, or tail heavy condition > first described, is the least desirable as it may result in unpredictable > flight performance. > > The conventional flap type (aileron, elevator, rudder) control surface, as > constructed, is typically tail-heavy. That is to say, most of its structure > is distributed behind the hinge axis. It is this sort of tail-heavy > out-of-balance condition that is generally considered to be the major cause > of control surface flutter and buffeting incidents. True, speed through the > air is also a factor and there is no doubt that flutter is a more frequent > occurence in high performance aircraft than it is with the slower > varieties. However, it would be dangerous to assume that slower homebuilts > are immune from such a propensity. I'll bet you have heard many times that > homebuilts having cruising speeds under 150 mph were exempt from the > flutter problem. Don't you believe it! Any airplane can experience flutter > . . . even your light and slow VW-powered job under certain conditions. > > Does this mean that you must balance the control surfaces of your project > even though the plans don't call for it? Not at all. Undoubtedly the > prototype of the airplane you are building was built and flown without > having exhibited flutter tendencies and the designer, therefore, found no > need to require static balancing of the control surfaces. However, you > should understand that, although many other examples of this same design > may have been built and flown, there is no assurance that yours will > likewise be free of flutter problems. > > The only way you can prove your airplane to be free of flutter tendencies > is to flight test it with that purpose in mind. This is a potentially > dangerous adventure and must be done only under carefully controlled > conditions. You must prove that your airplane is controllable, free from > flutter, and will be safe to fly. No amount of reassurance derived from > theoretical calculations can substitute for this requirement. > > *Other Flutter Provoking Conditions* > Although there is less risk of encountering flutter in slower aircraft > than in high performance types, individual builders can cause changes, > inadvertently, which could introduce flutter tendencies. For example, a > wing lacking torsional rigidity could induce a bad case of aileron flutter > even at the relatively low airspeeds generally associated with low and > medium-performance aircraft. > > A newly constructed aileron or elevator that is excessively heavy (due to > the use of heavier substitute materials or uncalled for reinforcements) can > be flutter-prone. Flutter is most difficult to suppress in very large or > heavy control surfaces and the balance weight requirement becomes excessive. > > Would it surprise you to learn that even time-tested production-line > aircraft are not immune to the flutter phenomenon? True! The reason being > that anytime anything changes the balance of the control surfaces it may > induce flutter in an aircraft that has had no history of such tendencies. > For example, there have been instances where flutter developed simply > because mud adhered to the control surfaces following muddy field operation. > > In an incident reported by the FAA, moisture had collected inside the > ailerons during winter operations and had frozen (seems to happen every > winter) thereby causing an unbalanced condition that was not detected > during the preflight . . . result? In-flight flutter and an accident. > > During the long days and nights in the life of an aircraft many changes > take place. Dirt accumulates inside the control surfaces, patches are added > to repair dings and tears, and in time, the surfaces are repainted. All of > these things cause a cumulative change in the mass balance of the control > surface. At some point, the amount of change becomes just too much . . . > and increases the risk of flutter if no steps are taken to rebalance the > reworked surface. > > Loose balance weights, water absorption in foam structures, improperly > located or clogged drain holes are all elements which could contribute to > an aerodynamic imbalance situation and result in flutter. > > Avoid free play or slack in the control cables. Stiffness in the control > system does have a useful damping effect on the control surfaces further > inhibiting flutter tendencies. However, this should not be completely > relied upon as later, in service, the wear and occasional lubrication could > free the system of much of its original friction and result in an increased > risk of flutter. > > Adding a fixed trim tab to an aileron can further upset a marginal balance > condition. > > Controllable trim tabs, too, can be a problem. Trim tab control linkage > failures and trim tabs with loose or improperly installed and adjusted > linkages have caused a considerable number of accidents and near accidents > by exciting flutter in the control surfaces to which they are attached. A > recent incident of that nature has just come to my attention. Involved was > a widely built and proven design . . . the staid ol’ Emeraude. Here's how > it happened. > > *Flutter . . . A First Hand Account* > "I knew the trim wasn't working - but who needs trim for a ten minute > every-which-way hop! I'm not one for flying level long, so we went into a > turning dive -somewhere over 140. All hell broke loose and I about lost > control . . . elevator flutter - it was violent! Honest, each wing and the > whole tail was shuddering. > > I came off the power . . . leveled my wing . . . very hard to do, and > started looking for a place to dump her. About 90, the flutter slowed but > still bad - at 80 it quit! > > I kept my head, let her glide for a few seconds and then added power - > kept my nose high and flew back to the airport but slow - making shallow > turns and a long straight in to a God awful landing - but safe! > > Once on the ground I found the problem - the trim tab. Suddenly I > remembered. Earlier a boy and his dad were visiting with me while I was > working on my brakes. The boy was in back playing with the elevator. He > must have bent the tab control wire - leaving the tab to start fluttering > at high speed - thus causing the elevator to flutter. > > It took two minutes to fix the cable. I checked for other damage - none. > She is an awful strong design, that Emeraude. > > It took a little longer to get up enough guts to fly her again, but I did > the same day. > > She's fine now but I have a little more respect for small items. I also > preflight a little more carefully now. I don't know if you have ever > experienced flutter or really know what it's like - I'm afraid of it now." > > There are not too many folks around who can tell you, first hand, how > sudden and destructive control flutter can be. We do know it can happen and > does happen all too often. This gent was lucky. He had a good stout > airplane and did just the right thing. > > -- > KRnet mailing list > KRnet@list.krnet.org > https://list.krnet.org/mailman/listinfo/krnet >
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