X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml@lancaironline.net Date: Fri, 15 Feb 2008 11:16:32 -0500 Message-ID: X-Original-Return-Path: Received: from mail13.syd.optusnet.com.au ([211.29.132.194] verified) by logan.com (CommuniGate Pro SMTP 5.2.0) with ESMTPS id 2730950 for lml@lancaironline.net; Fri, 15 Feb 2008 09:31:31 -0500 Received-SPF: pass receiver=logan.com; client-ip=211.29.132.194; envelope-from=fredmoreno@optusnet.com.au Received: from fred ([202.139.5.198]) (authenticated sender fredmoreno) by mail13.syd.optusnet.com.au (8.13.1/8.13.1) with ESMTP id m1FEUSqK019480 for ; Sat, 16 Feb 2008 01:30:37 +1100 From: "Fred Moreno" X-Original-To: "Lancair Mail" Subject: Cold Induction, Power, and Speed X-Original-Date: Fri, 15 Feb 2008 23:30:28 +0900 X-Original-Message-ID: <00e301c86fdf$56b594f0$c6058bca@fred> MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_00E4_01C8702A.C69D3CF0" X-Priority: 3 (Normal) X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook, Build 10.0.6822 Importance: Normal X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.3198 Thread-Index: Achv309tju7EC8g+TG+uYDvNK6RwHw== This is a multi-part message in MIME format. ------=_NextPart_000_00E4_01C8702A.C69D3CF0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: quoted-printable "Increase the power, reduce the drag and the limit is raised but it is = in no way linear past 200 knots . It is very exponential." =20 Not linear at all, in fact. At these speeds in the thick air we = customarily fly in with aspirated airplanes and with the weight and aspect ratio of = the wings, the total drag is almost entirely parasitic drag. Only a vanishingly small amount of induced drag (drag arising from lift) occurs = at the speeds discussed. =20 =20 This means that to a very good approximation, with a propeller airplane, power (shaft horsepower) goes as the cube of speed.=20 =20 So it is cubic curve.=20 =20 However.. One can linearize the curve for small deviations without = incurring much of an error (mathematically throw away the third order and higher terms) to arrive at a simplification for constant conditions (same drag coefficient, same flight conditions) -=20 =20 Which is:=20 =20 For a 1% increase in speed, you need 3% more power. For a 2% increase in speed, you need 6% more power For n% more speed, you need 3n% more power for small n (say less than = 10%) =20 Test - for 10% more speed, the simplification yields 30% more power. Compare this to 1.1 cubed which is 1.331, or 33.1% more power. So the simple approximation is not bad.=20 =20 It is MUCH BETTER to reduce drag than increase power. Unfortunately, = for constant power, to get a 1% speed increase requires a 3% drag = coefficient decrease, and 2% speed increase requires 6% drag coefficient decrease = etc. etc. (another linear simplification for small changes). =20 No free lunch.=20 =20 Fred Moreno AKA Captain Tuna, Chicken of the Skies =20 =20 =20 ------=_NextPart_000_00E4_01C8702A.C69D3CF0 Content-Type: text/html; charset="us-ascii" Content-Transfer-Encoding: quoted-printable

“Increase the power, reduce the drag and the limit is raised but it is in no way = linear past 200 knots . It is very = exponential.”

 

Not linear at all, in fact.  = At these speeds in the thick air we customarily fly in with aspirated airplanes = and with the weight and aspect ratio of the wings, the total drag is almost = entirely parasitic drag.   Only a vanishingly small amount of induced = drag (drag arising from lift) occurs at the speeds discussed.  =

 

This means that to a very good approximation, with a propeller airplane, power (shaft horsepower) goes = as the cube of speed.

 

So it is cubic curve. =

 

However…. One can linearize = the curve for small deviations without incurring much of an error (mathematically = throw away the third order and higher terms) to arrive at a simplification for constant conditions (same drag coefficient, same flight conditions) = –

 

Which is: =

 

For a 1% increase in speed, you = need 3% more power.

For a 2% increase in speed, you = need 6% more power

For n% more speed, you need 3n% = more power for small n (say less than 10%)

 

Test - for 10% more speed, the simplification yields 30% more power.  Compare this to 1.1 cubed = which is 1.331, or 33.1% more power.  So the simple approximation is not bad. =

 

It is MUCH BETTER to reduce drag = than increase power.  Unfortunately, for constant power, to get a 1% = speed increase requires a 3% drag coefficient decrease, and 2% speed increase = requires 6% drag coefficient decrease etc. etc.  (another linear = simplification for small changes).

 

No free lunch. =

 

Fred = Moreno

AKA Captain Tuna, Chicken of the = Skies

 

 

 

------=_NextPart_000_00E4_01C8702A.C69D3CF0--