X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml@lancaironline.net Date: Mon, 11 Sep 2006 23:59:48 -0400 Message-ID: X-Original-Return-Path: Received: from web34909.mail.mud.yahoo.com ([209.191.68.188] verified) by logan.com (CommuniGate Pro SMTP 5.1c.3) with SMTP id 1375129 for lml@lancaironline.net; Mon, 11 Sep 2006 19:11:55 -0400 Received-SPF: none receiver=logan.com; client-ip=209.191.68.188; envelope-from=n103md@yahoo.com Received: (qmail 275 invoked by uid 60001); 11 Sep 2006 23:11:13 -0000 DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws; s=s1024; d=yahoo.com; h=Message-ID:Received:Date:From:Subject:To:In-Reply-To:MIME-Version:Content-Type:Content-Transfer-Encoding; b=teuAstSpj5Z75F0WDmrk4FB0hzcI7Y6yD53hRouRNxdxY+zdiw1c06bLQWFbQ0ZBx5F+mNG8vDJa+eUquTSL04y2B7nJuv0BkhmZnhNxgapxqt2a84e7yNaqQ+dNSkIRh/e9x0eElEOanqgjphfMxn0lnegKgeAQrDzoin6Pl00= ; X-Original-Message-ID: <20060911231113.273.qmail@web34909.mail.mud.yahoo.com> Received: from [12.146.139.19] by web34909.mail.mud.yahoo.com via HTTP; Mon, 11 Sep 2006 16:11:13 PDT X-Original-Date: Mon, 11 Sep 2006 16:11:13 -0700 (PDT) From: bob mackey Subject: [LML] Do the Dew Point affect the "Do" point - Like "Do" it go slower? Faster? X-Original-To: Lancair Mailing List In-Reply-To: MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="0-1861185767-1158016273=:89058" Content-Transfer-Encoding: 8bit --0-1861185767-1158016273=:89058 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: 8bit Humidity in the air has a small effect on the oxygen content, when then has an effect on horsepower output from the engine. A much larger effect is the bouyancy of humid air relative to dry air. Because humid air is less dense than dry air, it tends to rise. A small vertical velocity of the air mass will give a large change in the cruise speed of slippery little lancairs. 100 fpm lift or sink is a hardly noticeable pitch change if you push or pull to maintain altitude. But that amount of lift or sink could easily add or subtract a few kts from cruise speed. How much speed change do you see if you push the nose over to give a 100 fpm descent? To take an extreme case, 1000 fpm lift under a line of cumulus is enough to allow some sailplanes to maintain well over 100 kts with zero engine horsepower. In still air, the same aircraft cannot maintain altitude at any speed. So... if you measure a speed difference that is correlated with humidity, the speed difference is probably caused by vertical motion in the airmass, rather than horsepower changes. -bob mackey (no engine for my first 2000 hours :-) --------------------------------- Do you Yahoo!? Get on board. You're invited to try the new Yahoo! Mail. --0-1861185767-1158016273=:89058 Content-Type: text/html; charset=iso-8859-1 Content-Transfer-Encoding: 8bit Humidity in the air has a small effect on the oxygen content,
when then has an effect on horsepower output from the engine.
A much larger effect is the bouyancy of humid air relative to dry air.
Because humid air is less dense than dry air, it tends to rise.

A small vertical velocity of the air mass will give a large change in
the cruise speed of slippery little lancairs. 100 fpm lift or sink is
a hardly noticeable pitch change if you push or pull to maintain altitude.
But that amount of lift or sink could easily add or subtract a few kts
from cruise speed. How much speed change do you see if you
push the nose over to give a 100 fpm descent?

To take an extreme case, 1000 fpm lift under a line of cumulus
is enough to allow some sailplanes to maintain well over 100 kts
with zero engine horsepower. In still air, the same aircraft cannot
maintain altitude at any speed.

So... if you measure a speed difference that is correlated with humidity,
the speed difference is probably caused by vertical motion in the airmass,
rather than horsepower changes.

-bob mackey
 (no engine for my first 2000 hours :-)

Do you Yahoo!?
Get on board. You're invited to try the new Yahoo! Mail. --0-1861185767-1158016273=:89058--