Return-Path: Sender: (Marvin Kaye) To: lml Date: Sun, 08 Dec 2002 22:44:58 -0500 Message-ID: X-Original-Return-Path: Received: from [32.97.166.32] (HELO prserv.net) by logan.com (CommuniGate Pro SMTP 4.0.2) with ESMTP id 1897548 for lml@lancaironline.net; Sun, 08 Dec 2002 20:23:55 -0500 Received: from oemcomputer (slip-12-64-222-155.mis.prserv.net[12.64.222.155]) by prserv.net (out2) with SMTP id <2002120901234920205jfodhe>; Mon, 9 Dec 2002 01:23:50 +0000 From: "Larry Henney" X-Original-To: "'Lancair Mailing List'" Subject: RE: [LML] Re: AILERON WEIGHT X-Original-Date: Sun, 8 Dec 2002 19:23:54 -0600 X-Original-Message-ID: <000001c29f21$a5809600$9bde400c@oemcomputer> MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit X-Priority: 3 (Normal) X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook, Build 10.0.2627 Importance: Normal In-Reply-To: X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1106 >"Overbalanced is OK but underbalanced is not"? Please explain. I don't >understand. Jeff, Having understood this at one time in my past I thought I'd take a crack at it. In articulating my thougths I'm gaining a better understanding but have also come to the realization that I no longer have this topic nailed. Stop reading now if you need all the details. Or stop reading now and "Nike" so you'll be flying sooner. (Nike = Just Do It!) As you will see gray matter runs dry in about the fifth paragraph. Perhaps one of our JPL friends can clarify where my blather comes up short. My laymans understanding has to due with dynamic instability of the control surface, ie flutter. We are mass balancing to negate the possibility of flutter. My recollection all revolves around a momentary upset of the airframe such as a schwack from turbulence. The control surface reaction compared to the airframe action is where the potential for problems developes. The ideal case is when the aileron is 100% mass balanced for and aft of the hinge line. In this example the momentary upset will have a "balanced" reaction from the aileron. That is there is an equal and opposite moment on both sides of the hinge line. These moments cancel each other. The resultant reaction from the control surface is to remain neutral. The opposite and negative affect is when the aft portion of the aileron is heavier than the forward portion (less than 100% balanced-underbalanced). In this situation consider the airframe recieving a momentary upset, Down for example. The opposite upward reactive forces for and aft of the hinge line are unequal. The heavier aft portion having more momentum will tend to rise relative to the descending airframe. The airstream will then try to neutralize the control surface. As the heavier side moves back towards neutral it's greater moment may take it through the neutral position swinging into the airstream on the other side. This asymetric balance on the aileron can then possibly cause an excitation and subsequent harmonic vibration. That is, it can increase in vibration amplitude until it shakes itself and your plane apart. Finally, the third situation is greater than 100% mass balanced (overbalanced). This case is where excess lead is added to the leading edge. Taking the same example of a momentary downward force on the airframe causes the leading edge of the aileron to rise slightly. Again the aileron is forced towards neutral from the airstream. However, in this example the greater moment in front of the hinge line has a dampening affect. So, even if the aileron goes slightly through neutral as in the above example, it will tend to decrease in amplitude. Perhaps there will be a few dampening oscillations and then back to neutral. Therefore, overbalanced is safer than underbalanced because it will not lead to flutter. In the words of Forest Gump, "That's all I have to say about that". Larry Henney N360LH LNC2 IO-360 B1 16XS Ft Worth, TX PS: Now you aero guys help me. Am I correct in talking moments compared to weight? Is the blend of static stability correctly stated compared to dynamic stability? Have I lead this gentleman astray? How would one assess both ailerons and elevators mathmatically? My understanding is that 100% was always the known gaurantee of no flutter. This however may also be true at some smaller mass balance. Lance even made mention of this when they were flutter testing the L360 with German firm but made no comment after the testing. Could we mathematically consider an 80% mass balance? Or perhaps 70%? On elevators? On ailerons? Would empirical data be useful to provide a mathematical model to reduce the weight? Is Finite element analysis small enough to run on the home computer. When I studied it we had to use the CRAY which was big (interplanetary like big!) at the time. All this to save an ounce and gain perhaps a quarter knot. Hmmmmm? You bet it's worth it.