X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Received: from cdptpa-omtalb.mail.rr.com ([75.180.132.120] verified) by logan.com (CommuniGate Pro SMTP 5.2.14) with ESMTP id 3636675 for flyrotary@lancaironline.net; Fri, 08 May 2009 20:03:33 -0400 Received-SPF: pass receiver=logan.com; client-ip=75.180.132.120; envelope-from=eanderson@carolina.rr.com Received: from computername ([75.191.186.236]) by cdptpa-omta04.mail.rr.com with ESMTP id <20090509000257555.GZUD13493@cdptpa-omta04.mail.rr.com> for ; Sat, 9 May 2009 00:02:57 +0000 From: "Ed Anderson" To: "'Rotary motors in aircraft'" Subject: Tachs, Tachs Tachs Date: Fri, 8 May 2009 20:19:37 -0400 MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_0000_01C9D01A.4F7FEA40" X-Mailer: Microsoft Office Outlook, Build 11.0.5510 Thread-Index: AcnQO9Y27qFa5j/7TfGllt/P8Sq2gQ== X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.5579 Message-Id: <20090509000257555.GZUD13493@cdptpa-omta04.mail.rr.com> This is a multi-part message in MIME format. ------=_NextPart_000_0000_01C9D01A.4F7FEA40 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Since there is some interest in Tachs at the moment , I thought I might mention some information that some on the list may not be aware of and perhaps save TACFRUSTATION. There are three common methods of deriving rotational information (RPM) from an internal combustion engine. 1.Frequency to DC conversion - This is probably the simplest (and cheapest) method in which you simply use a capacitor and resistor of the right values to integrate (sum) the voltages of a series of pulses. The faster the pulses arrive (higher rpm of the engine) the less time the RC circuit has to discharge and a voltmeter is used to show this rising voltage calibrated as your engine RPM. The down side is Frequency to DC conversion is probably the least accurate although for many uses acceptable and it is slower to react to changes. 2. Pulse counting - as it sounds, you simply count the pulses per unit time and if for example your had 1 pulse per 360 deg revolution and counted 100 pulses in one second you would know your rpm was 60(sec/min) /( 1/100 sec/rev) = 6000 rev/min or 6000 rpm. The disadvantage of pulse counting are measurement speed and resolution at low rpm. Very low rpm could take several minutes to count - not really a problem at our engine speeds {:>) 3. Period Measurement - offers the ultimate in measurement speed and resolution. With microchip clocks able to measure millionths of a second, the accuracy possible is far greater than any real use to our application. The downside is increased complexity and cost - but there is more flexibility (if designed in) to handle a greater diversity of types of inputs. So all tachs are not created equal and it can be frustrating not having the documentation of what your particular tach needs to register properly. Ed Anderson Rv-6A N494BW Rotary Powered Matthews, NC eanderson@carolina.rr.com http://www.andersonee.com http://www.dmack.net/mazda/index.html http://www.flyrotary.com/ http://members.cox.net/rogersda/rotary/configs.htm#N494BW http://www.rotaryaviation.com/Rotorhead%20Truth.htm ------=_NextPart_000_0000_01C9D01A.4F7FEA40 Content-Type: text/html; charset="us-ascii" Content-Transfer-Encoding: quoted-printable

Since there is some interest in Tachs at the moment , = I thought I might mention some information  that some on the list may = not be aware of and perhaps save TACFRUSTATION.

 

There are three common methods of deriving rotational information (RPM) from an internal combustion = engine.

 

1.Frequency to DC conversion – This is probably = the simplest (and cheapest) method in which you simply use a capacitor and = resistor of the right values to integrate (sum) the voltages of a series of pulses.  The faster the pulses arrive (higher rpm of the engine) = the less time the RC circuit has to discharge and a voltmeter is used to show = this rising voltage calibrated as your engine RPM.  The down side is = Frequency to DC conversion is probably the least accurate although for many uses =  acceptable and it is slower to react to changes.

 

2.  Pulse counting -  as it sounds, you = simply count the pulses per unit time and if for example your had 1 pulse per = 360 deg revolution and counted 100 pulses in one second you would know your rpm = was 60(sec/min) /( 1/100 sec/rev)      =3D 6000 rev/min or 6000 = rpm.  The disadvantage of  pulse counting are measurement speed and = resolution at low rpm.  Very low rpm could take several minutes to count = – not really a problem at our engine speeds {:>)

 

3.  Period Measurement – offers the = ultimate in measurement speed and resolution.  With microchip clocks able to measure = millionths of a second, the accuracy possible is far greater than any real use to our application.  The downside is increased complexity and cost – = but there is more flexibility (if designed in) to handle a greater diversity of = types of inputs.

So all tachs are not created equal and it can be frustrating not = having the  documentation of what your particular tach needs to register = properly.

Ed Anderson

Rv-6A N494BW Rotary Powered

Matthews, NC

eanderson@carolina.rr.com

http://www.andersonee.com

http://www.dmack.net/mazda/index.html

http://www.flyrotary.com/

http://members.cox.net/rogersda/rotary/configs.htm#N494BW

http://www.r= otaryaviation.com/Rotorhead%20Truth.htm

 



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