X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml@lancaironline.net Date: Sun, 20 Apr 2008 22:37:07 -0400 Message-ID: X-Original-Return-Path: Received: from smtp116.sbc.mail.sp1.yahoo.com ([69.147.64.89] verified) by logan.com (CommuniGate Pro SMTP 5.2.2) with SMTP id 2864465 for lml@lancaironline.net; Sun, 20 Apr 2008 17:29:20 -0400 Received: (qmail 72874 invoked from network); 20 Apr 2008 21:29:04 -0000 DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws; s=s1024; d=sbcglobal.net; h=Received:X-YMail-OSG:X-Yahoo-Newman-Property:Message-ID:From:To:Subject:Date:MIME-Version:Content-Type:X-Priority:X-MSMail-Priority:X-Mailer:X-MimeOLE; b=YBGxl96hwaecUBdmtq2eZi7/DSjAYQAHpSbGiFB/l/t6pMEn3au0ciVrIG6ftvRveu9IwXpXrSedL5/JgKIhta7Q51oZzqr9dnekxncYUq2V0vaHziXZvxwtM28p/xaQTJG5IDiHx6DNHz/urTnSd5/QpiP0GPL4BFdG7VFcxpg= ; Received: from unknown (HELO Computerroom) (elippse@sbcglobal.net@75.15.114.247 with login) by smtp116.sbc.mail.sp1.yahoo.com with SMTP; 20 Apr 2008 21:29:04 -0000 X-YMail-OSG: vA6WZ8sVM1mnMtXzUGHrRf1KdMTzH2gcDXCEaIgv8_qkeRjZRwDD2DmFsA3QoW_NfTacmRcHgg-- X-Yahoo-Newman-Property: ymail-3 X-Original-Message-ID: <000801c8a32d$933db610$f7720f4b@Computerroom> From: "Paul Lipps" X-Original-To: "Marv Kaye" Subject: prop diameter X-Original-Date: Sun, 20 Apr 2008 14:29:02 -0700 MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_0005_01C8A2F2.E1F1BA80" X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 6.00.2900.3138 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.3198 This is a multi-part message in MIME format. ------=_NextPart_000_0005_01C8A2F2.E1F1BA80 Content-Type: text/plain; charset="Windows-1252" Content-Transfer-Encoding: quoted-printable Peter: You say "As every inch counts for efficiency,..." That's true for = low speed applications, but not necessarily true for high speed props. = Consider: thrust is a product of mass flow and velocity. At high speeds, = each blade of the propeller intercepts a volume of air that is a tube a = little larger that the prop diameter and as long as the forward speed. = So by keeping the diameter large, you intercept a greater volume of air = with the square of the increase in diameter. However, as you increase = diameter, for a given engine rpm, the tip Mach and resulting tip drag = increase at a much higher rate. Going from M0.65 to M0.85 can increase = the parasite drag CD by six to eight times. So to keep M low, you must = decrease rpm. When you do that, you decrease available power, which is = basically linear to rpm over a wide range on our engines. So for good = efficiency, you want to keep m-dot high and v low. Most pilots are not = aware that v on our high speed planes is very low; it's not the 40 mph = people think it is. On my little 235, at 200 mph TAS, 2800 rpm, 63" = three-blade prop, it's 4-6 fps! As I previously stated, each blade of = the prop intercepts that tube volume. So by going to more blades, you = can increase the mass flow while at the same time reducing the diameter = to keep the tip M low. The two biggest sources of efficiency loss on = props are the tip radius and chord, and the draggy, klunky shape of the = roots. A tip has no lift, only drag, and that drag is proportional to = area which is a function of chord. More chord, more drag. Then that drag = force is multiplied by the tip radius to give torque. The more tip = chord, the more drag, and the more radius, the more torque acting = against the engine. Nothing but pure loss! A three-blade, 60" diameter = prop with the tip chord narrowing to a point and streamlined blade roots = would give the most efficiency. And wide, swept tips are as functional = as the swept v-stab of a 172! ------=_NextPart_000_0005_01C8A2F2.E1F1BA80 Content-Type: text/html; charset="Windows-1252" Content-Transfer-Encoding: quoted-printable
Peter:
    You say  "As every inch counts for=20 efficiency,..." That's = true for low=20 speed applications, but not necessarily true for high speed props. = Consider:=20 thrust is a product of mass flow and velocity. At high speeds, each = blade of the=20 propeller intercepts a volume of air that is a tube a little = larger=20 that the prop diameter and as long as the forward speed. So by = keeping the=20 diameter large, you intercept a greater volume of air with the square of = the=20 increase in diameter. However, as you increase diameter, for a given = engine rpm,=20 the tip Mach and resulting tip drag increase at a much higher rate. = Going from=20 M0.65 to M0.85 can increase the parasite drag CD = by six to=20 eight times. So to keep M low, you must decrease rpm. When you do that, = you=20 decrease available power, which is basically linear to rpm over a wide = range on=20 our engines. So for good efficiency, you want to keep m-dot high and v = low. Most=20 pilots are not aware that v on our high speed planes is very low; it's = not the=20 40 mph people think it is. On my little 235, at 200 mph TAS, 2800 rpm, = 63"=20 three-blade prop, it's 4-6 fps! As I previously stated, = each=20 blade of the prop intercepts that tube volume. So by going = to more=20 blades, you can increase the mass flow while at the same time reducing = the=20 diameter to keep the tip M low. The two biggest sources of efficiency = loss on=20 props are the tip radius and chord, and the draggy, klunky shape of the = roots. A=20 tip has no lift, only drag, and that drag is proportional to area which = is a=20 function of chord. More chord, more drag. Then that drag force is = multiplied by=20 the tip radius to give torque. The more tip chord, the more drag, and = the more=20 radius, the more torque acting against the engine. Nothing but pure = loss! A=20 three-blade, 60" diameter prop with the tip chord narrowing to a point = and=20 streamlined blade roots would give the most efficiency. And wide, = swept=20 tips are as functional as the swept v-stab of a = 172!
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