X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Received: from mail-iw0-f180.google.com ([209.85.214.180] verified) by logan.com (CommuniGate Pro SMTP 5.4.0) with ESMTPS id 5042305 for flyrotary@lancaironline.net; Tue, 05 Jul 2011 01:36:37 -0400 Received-SPF: pass receiver=logan.com; client-ip=209.85.214.180; envelope-from=fluffysheap@gmail.com Received: by iwn9 with SMTP id 9so7584248iwn.25 for ; Mon, 04 Jul 2011 22:36:01 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=gamma; h=mime-version:in-reply-to:references:date:message-id:subject:from:to :content-type; bh=cx6NJrwDsEc2pea8z00hhHIDztMqoCkeC4z1syJ7Qws=; b=oh32labr+sSPKwbB7grKSyBEUU8jERdgVgSjJmmOZPSqxafIupuVuvAavitMsfgaWv yBBpk9DKOn8SRISx1gYT4+UBUYhh0v2SMN0P7GHS2ibUeBkGR8OzDd5szR9ilBwKZFvh /1FriCVVg8iryGxLVOBJT3uk/Bnuhh6rCQf3c= MIME-Version: 1.0 Received: by 10.231.63.81 with SMTP id a17mr6373671ibi.66.1309844161318; Mon, 04 Jul 2011 22:36:01 -0700 (PDT) Received: by 10.231.154.17 with HTTP; Mon, 4 Jul 2011 22:36:00 -0700 (PDT) In-Reply-To: References: Date: Mon, 4 Jul 2011 23:36:00 -0600 Message-ID: Subject: Re: [FlyRotary] Re: VNE / Flutter Testing From: William Wilson To: Rotary motors in aircraft Content-Type: multipart/alternative; boundary=000e0cd56bb0f2507f04a74bdbcd --000e0cd56bb0f2507f04a74bdbcd Content-Type: text/plain; charset=ISO-8859-1 This is at best an oversimplification. Flutter is a function of the amount of energy that the moving air can impart on the flexible parts of the aircraft. It occurs when the energy going into the fluttering component exceeds the ability of the structure to absorb it. To make a counter-oversimplification, if you take the amount of energy available from the relatively moving air, it is proportional to the square of the velocity, but it is also proportional to the mass of the air. Velocity increases with higher TAS, but mass decreases with increased altitude, where IAS and TAS differ. (This is an oversimplification because a lot more goes into the initiation of flutter than just the energy available in the air!) Some aircraft are now built with Vne published at different values at different altitudes. Others have a cruise speed (published in TAS) at high altitude which exceeds their Vne (published in IAS)! Saying that "Vne is actually a function of TAS" is at least equally wrong as saying "it is simply a function of IAS." The truth is somewhere in between, and varies from one aircraft design to another. Flutter is a relatively rare occurrence, and in-flight breakups as a result rarer still. I don't think a flutter-induced breakup has ever happened to a Rutan-designed aircraft, and if it's happened to RVs, it certainly hasn't happened often. The published Vne numbers have a safety factor, and the designers should have taken flight typical altitudes into consideration when calculating them. There are a lot of ways to get yourself killed in an experimental aircraft, and a few extra if you fly with an experimental engine. The potential difference in Vne from altitude is not high on the list, and probably not on the list at all if your airplane is not substantially overpowered. On Sun, Jul 3, 2011 at 3:14 PM, Ed Anderson wrote: > > Also worthy of note, a lot of debate raged about the following viewpoint - > a case was made in one of Van's Rvatiors that flutter was a function of *True > Airspeed* rather than *indicated Airspeed*. The point was that while all > other airspeed restrictions could safely be related to indicate airspeed, > Vne could not. > > The argument was that flutter was a function of the velocity of the * > moving* air molecules flowing across the control surface/wing. Further > while the dynamic pressure encountered varied with both the density and > velocity of the air molecules, the excitation force (causing the flutter) > was a function of velocity of the molecules and not their density. Well, > not being an aerodynamic engineer - it got beyond me. > --000e0cd56bb0f2507f04a74bdbcd Content-Type: text/html; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable This is at best an oversimplification.=A0 Flutter is a function of the amou= nt of energy that the moving air can impart on the flexible parts of the ai= rcraft.=A0 It occurs when the energy going into the fluttering component ex= ceeds the ability of the structure to absorb it.

To make a counter-oversimplification, if you take the amount of energy = available from the relatively moving air, it is proportional to the square = of the velocity, but it is also proportional to the mass of the air.=A0 Vel= ocity increases with higher TAS, but mass decreases with increased altitude= , where IAS and TAS differ.=A0 (This is an oversimplification because a lot= more goes into the initiation of flutter than just the energy available in= the air!)

Some aircraft are now built with Vne published at different values at d= ifferent altitudes.=A0 Others have a cruise speed (published in TAS) at hig= h altitude which exceeds their Vne (published in IAS)!

Saying that &= quot;Vne is actually a function of TAS" is at least equally wrong as s= aying "it is simply a function of IAS."=A0 The truth is somewhere= in between, and varies from one aircraft design to another.

Flutter is a relatively rare occurrence, and in-flight breakups as a re= sult rarer still.=A0 I don't think a flutter-induced breakup has ever h= appened to a Rutan-designed aircraft, and if it's happened to RVs, it c= ertainly hasn't happened often.=A0 The published Vne numbers have a saf= ety factor, and the designers should have taken flight typical altitudes in= to consideration when calculating them.=A0 There are a lot of ways to get y= ourself killed in an experimental=20 aircraft, and a few extra if you fly with an experimental engine.=A0 The po= tential difference in Vne from altitude is not high on the list, and probab= ly not on the list at all if your airplane is not substantially overpowered= .

On Sun, Jul 3, 2011 at 3:14 PM, Ed Anderson = <eanderso= n@carolina.rr.com> wrote:
Also worthy of note,=A0= a=A0lot of debate raged about=20 the following viewpoint - a case was made in one of Van's Rvatiors =A0t= hat=20 flutter was a function of True Airspeed rather than=20 indicated Airspeed.=A0 The point was that while all other=20 airspeed restrictions could safely be related to indicate airspeed, Vne cou= ld=20 not.=A0
=A0
The argument was that=A0flutter=A0=A0was a=20 function of =A0the velocity of the moving air molecules=20 flowing across the control surface/wing.=A0 Further while the dynamic=20 pressure encountered varied with both the density and velocity of the air= =20 molecules, the excitation force (causing the flutter) was a function of vel= ocity=20 of the molecules and not their density.=A0 Well, not being an aerodynamic= =20 engineer - it got beyond me.

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