X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml@lancaironline.net Date: Thu, 14 Jun 2007 22:54:09 -0400 Message-ID: X-Original-Return-Path: Received: from web53709.mail.re2.yahoo.com ([206.190.37.30] verified) by logan.com (CommuniGate Pro SMTP 5.1.9) with SMTP id 2103495 for lml@lancaironline.net; Thu, 14 Jun 2007 22:15:57 -0400 Received-SPF: none receiver=logan.com; client-ip=206.190.37.30; envelope-from=kyrilian_av@yahoo.com Received: (qmail 78973 invoked by uid 60001); 15 Jun 2007 02:15:21 -0000 DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws; s=s1024; d=yahoo.com; h=X-YMail-OSG:Received:Date:From:Subject:To:In-Reply-To:MIME-Version:Content-Type:Content-Transfer-Encoding:Message-ID; b=Y4JlupbDbepSpG/7z3DXa1HzIQvdT4ByiQzORLbvEW3y1iNNUIUQvsmTM2vKG0u6oKbGO+q/G2Murl3A5yYGh162xL+vHxGiTfC+/yF4G1Yl+En+ZuTczd0wb9eQ8hNLzZhy7FI5yAHBuQZ9u4X9Jhi0EHPx3XuUlmutsIJ3dig=; X-YMail-OSG: uI7rf70VM1k6LEXsxsoAi4mqNQQp49lpiaEN_sq1VSCoQNmjwYBoMoRF.Z_jPjHnnw-- Received: from [66.32.85.171] by web53709.mail.re2.yahoo.com via HTTP; Thu, 14 Jun 2007 19:15:21 PDT X-Original-Date: Thu, 14 Jun 2007 19:15:21 -0700 (PDT) From: Kyrilian Dyer Subject: Re: [LML] prop clock X-Original-To: Lancair Mailing List , Paul Lipps In-Reply-To: MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="0-1195783347-1181873721=:78415" Content-Transfer-Encoding: 8bit X-Original-Message-ID: <427057.78415.qm@web53709.mail.re2.yahoo.com> --0-1195783347-1181873721=:78415 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: 8bit I know you said no surmises, but I'm gonna give you one anyway. The engine and the prop both have the potential to be mass imbalanced. In a perfect world, the engine and prop would have minimal imbalance individually, but what matters more is whether the combination is balanced. My understanding is that airplanes are balanced with the prop mounted. If the higher mass location on the prop and engine (crank, cylinders, etc) are aligned, the masses are added and vibration results. If the locations are opposite one another, they offset one another and vibration only results from the difference (just as we do with the red and yellow dots on wheels and tires). This may not yield a perfect solution but it may get pretty close. Further vibe reduction would require balancing with an accelerometer/phase pickup/analyzer. My expertise is in helicopter dynamics flight test. We balance the blades and then dynamically balance the rotor once the blades are installed. Just as with prop balancing, this entails looking at the vibration amplitude (through one or more accelerometers) and phase (from an optical or magnetic pickup). These numbers are then plotted on a chart (or input to a computer) and mass is applied appropriately. Drive shafts are balanced similarly. Balance weights (set screws or washers) are used when possible. However, in some cases weights are not used and the shaft is simply indexed to the next bolt. In some cases this is one 12th of a turn; in others it is a whole third of a rotation. Perfect balance isn't too likely using this method, but it's usually close enough. Single plane balance is simple, and we often treat helicopter vibrations as such. I imagine a perfect engine balance would require slightly more complex multiple plane balancing. Piston firing-induced vibrations also play a role, I imagine. I don't know much about that though... Hope this helps, - Kyrilian Paul Lipps wrote: Is there anyone who knows the actual reason why changing the position of the prop on the crank flange may make a rough running engine-prop combo become smooth or vice versa? An acquaintenance of mine told me that when he moved his prop around one mounting hole it became very smooth. I have read the same on various sites, but the reason why that would be truly escapes me. No guesses or surmises; just the facts, mam! --------------------------------- Yahoo! oneSearch: Finally, mobile search that gives answers, not web links. --0-1195783347-1181873721=:78415 Content-Type: text/html; charset=iso-8859-1 Content-Transfer-Encoding: 8bit
I know you said no surmises, but I'm gonna give you one anyway.
 
The engine and the prop both have the potential to be mass imbalanced.  In a perfect world, the engine and prop would have minimal imbalance individually, but what matters more is whether the combination is balanced.  My understanding is that airplanes are balanced with the prop mounted.  If the higher mass location on the prop and engine (crank, cylinders, etc) are aligned, the masses are added and vibration results.  If the locations are opposite one another, they offset one another and vibration only results from the difference (just as we do with the red and yellow dots on wheels and tires).  This may not yield a perfect solution but it may get pretty close.  Further vibe reduction would require balancing with an accelerometer/phase pickup/analyzer.
 
My expertise is in helicopter dynamics flight test.  We balance the blades and then dynamically balance the rotor once the blades are installed.  Just as with prop balancing, this entails looking at the vibration amplitude (through one or more accelerometers) and phase (from an optical or magnetic pickup).  These numbers are then plotted on a chart (or input to a computer) and mass is applied appropriately.  Drive shafts are balanced similarly.  Balance weights (set screws or washers) are used when possible.  However, in some cases weights are not used and the shaft is simply indexed to the next bolt.  In some cases this is one 12th of a turn; in others it is a whole third of a rotation.  Perfect balance isn't too likely using this method, but it's usually close enough.
 
Single plane balance is simple, and we often treat helicopter vibrations as such.  I imagine a perfect engine balance would require slightly more complex multiple plane balancing.  Piston firing-induced vibrations also play a role, I imagine.  I don't know much about that though...
 
Hope this helps,
- Kyrilian
 

Paul Lipps <elippse@sbcglobal.net> wrote:
Is there anyone who knows the actual reason why changing the position of the prop on the crank flange may make a rough running engine-prop combo become smooth or vice versa? An acquaintenance of mine told me that when he moved his prop around one mounting hole it became very smooth. I have read the same on various sites, but the reason why that would be truly escapes me. No guesses or surmises; just the facts, mam!

Yahoo! oneSearch: Finally, mobile search that gives answers, not web links. --0-1195783347-1181873721=:78415--