X-Virus-Scanned: clean according to Sophos on Logan.com X-SpamCatcher-Score: 1 [X] Return-Path: Sender: To: lml Date: Sat, 02 Dec 2006 19:19:51 -0500 Message-ID: X-Original-Return-Path: Received: from smtp109.sbc.mail.mud.yahoo.com ([68.142.198.208] verified) by logan.com (CommuniGate Pro SMTP 5.1.3) with SMTP id 1633061 for lml@lancaironline.net; Sat, 02 Dec 2006 16:29:31 -0500 Received-SPF: none receiver=logan.com; client-ip=68.142.198.208; envelope-from=elippse@sbcglobal.net Received: (qmail 17306 invoked from network); 2 Dec 2006 21:29:11 -0000 DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws; s=s1024; d=sbcglobal.net; h=Received:X-YMail-OSG:Message-ID:From:To:Subject:Date:MIME-Version:Content-Type:X-Priority:X-MSMail-Priority:X-Mailer:X-MimeOLE; b=u8FyJjuPRTy6RGcu4FL+FdCfh/x/tgwkYbbcJW08scCS+sc651PO8xJmuhJjkH1aEQg/zfvRSPkrqKif1ZpJAWFdgJerEt7sGwK8JIIlcwk4mvDLFA9Xs1I3oNHVpAlRqPfWOXbXrsX3QUhPeogw/wS6Olj3uZAL/29kusamHQY= ; Received: from unknown (HELO Computerroom) (elippse@sbcglobal.net@69.227.129.253 with login) by smtp109.sbc.mail.mud.yahoo.com with SMTP; 2 Dec 2006 21:29:11 -0000 X-YMail-OSG: 9TAbRmMVM1mfA7Aee9ygn.FJ_5f5DjqJwQwqWrCF6GJS4YUNj.1BsXgJY.D3y68aK5Mo46afjbML8aWWmN7NMEuldmVwFPw1uR.JZqXyYNkZGl8deMhtaQ-- X-Original-Message-ID: <000b01c71658$e8346730$fd81e345@Computerroom> From: "Paul Lipps" X-Original-To: "Marv Kaye" Subject: nosewheel vs prop dia. X-Original-Date: Sat, 2 Dec 2006 13:29:06 -0800 MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_0008_01C71615.D7817BA0" X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 6.00.2900.2869 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.2962 This is a multi-part message in MIME format. ------=_NextPart_000_0008_01C71615.D7817BA0 Content-Type: text/plain; charset="Windows-1252" Content-Transfer-Encoding: quoted-printable The hp and thrust of a prop is based on the amount of air which EACH = blade sweeps out in one revolution. Since that is a function of disc = area, which is proportional to the square of the diameter, if you change = from a two-blade prop to a three-blade prop, you can reduce the diameter = to 82%, or to 71% with a four-blade, and still have the same swept area. = So if you have a 72" 2-blade, you should be able to get away with a 59" = three-blade, or even a 51" four blade, and still have the same or better = efficiency IF the prop blade has a streamlined shape where it enters the = spinner. The reason that the myth about multi-blade props which give = better take-off and climb but are not as good in cruise, is that they = had such terribly klunky shapes in the root sections, which gives a lot = of drag. Tom Aberle's Phantom biplane racer had a 64" diameter two-blade = prop in 2003 which gave him 221 mph. In 2004 he had my three-blade = design of 59" diameter which gave him 241 mph at 250 rpm less. This year = he did 252 mph with my 59" four-blade design with the same rpm he had = with the two-blade. The advantage of a shorter multi-blade prop, = spinning at the same rpm, is that it will have less noise from less tip = loss which adds to its efficiency. So if you want more ground clearance, = reduced noise, more efficiency for better cruise, and better takeoff and = climb, go find a prop manufacturer who understands this and buy a three = or four-blade prop from them! And no, scimitar tips are in the same = league as the swept vertical stab on the Cessnas. If your prop has any = chord whatsoever at the tip, it just generates drag proportional to the = chord and absolutely no thrust. If it's a thin or symmetrical airfoil, = its parasite drag will be much greater than a 12% or even 15% thick = airfoil. And the hp loss is greater the farther out on the blade you go = since hp is the product of torque and rpm, and the greater the radius = where you have drag, the greater the multiplier of that drag through the = radius to get torque. One pound of drag at 6" radius is 0.5 lb-ft, at = 12" radius it's 1 lb-ft, at 24" radius it's 2 lb-ft, and at 36" radius = it's 3 lb-ft. Now that drag can be either parasite or induced, so if = your planform generates higher lift with the squared dynamic pressure at = the outboard radii, the rearward lift vector, the so-called induced = drag, uses much more power to generate thrust than do the inboard = sections. p.s. Fan jet engines have multi-blade, fixed-pitch props at the front = end, which generates the majority of the thrust! ------=_NextPart_000_0008_01C71615.D7817BA0 Content-Type: text/html; charset="Windows-1252" Content-Transfer-Encoding: quoted-printable
The hp and thrust of a prop is based on the = amount of air=20 which EACH blade sweeps out in one revolution. Since that is a function=20 of disc area, which is proportional to the square of the=20 diameter, if you change from a two-blade prop to a three-blade prop, you = can=20 reduce the diameter to 82%, or to 71% with a four-blade, and still have = the same=20 swept area. So if you have a 72" 2-blade, you should be able to get away = with a=20 59" three-blade, or even a 51" four blade, and still have the same or = better=20 efficiency IF the prop blade has a streamlined shape where it enters the = spinner. The reason that the myth about multi-blade props which give = better=20 take-off and climb but are not as good in cruise, is that they had = such=20 terribly klunky shapes in the root sections, which gives a lot = of drag. Tom=20 Aberle's Phantom biplane racer had a 64" diameter two-blade prop in = 2003=20 which gave him 221 mph. In 2004 he had my three-blade design of 59" = diameter=20 which gave him 241 mph at 250 rpm less. This year he did 252 mph with my = 59"=20 four-blade design with the same rpm he had with the two-blade.=20 The advantage of a shorter multi-blade prop, spinning at the same = rpm, is=20 that it will have less noise from less tip loss which adds to = its=20 efficiency. So if you want more ground clearance, reduced noise, more = efficiency=20 for better cruise, and better takeoff and climb, go find a prop = manufacturer who=20 understands this and buy a three or four-blade prop from them! And = no,=20 scimitar tips are in the same league as the swept vertical stab on the = Cessnas.=20 If your prop has any chord whatsoever at the tip, it just generates drag = proportional to the chord and absolutely no thrust. If it's a thin or=20 symmetrical airfoil, its parasite drag will be much greater than a 12% = or even=20 15% thick airfoil. And the hp loss is greater the farther out on the = blade you=20 go since hp is the product of torque and rpm, and the greater the radius = where=20 you have drag, the greater the multiplier of that drag through the = radius to get=20 torque. One pound of drag at 6" radius is 0.5 lb-ft, at 12" radius it's = 1 lb-ft,=20 at 24" radius it's 2 lb-ft, and at 36" radius it's 3 lb-ft. Now that = drag can be=20 either parasite or induced, so if your planform generates higher lift = with the=20 squared dynamic pressure at the outboard radii, the rearward lift = vector, the=20 so-called induced drag, uses much more power to generate thrust = than do the=20 inboard sections.
 
p.s. Fan jet engines have multi-blade, = fixed-pitch props=20 at the front end, which generates the majority of the thrust!=20
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