X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml@lancaironline.net Date: Wed, 02 Jul 2014 08:19:26 -0400 Message-ID: X-Original-Return-Path: Received: from nm19-vm2.access.bullet.mail.bf1.yahoo.com ([216.109.115.97] verified) by logan.com (CommuniGate Pro SMTP 6.0.9e) with ESMTPS id 6960182 for lml@lancaironline.net; Tue, 01 Jul 2014 22:40:21 -0400 Received-SPF: neutral receiver=logan.com; client-ip=216.109.115.97; envelope-from=browncc1@verizon.net DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws; s=s2048; d=yahoo.com; b=ADol7SpbOVcn6Nr/tZwjfGyb5WiTGBsS4/NoLEQhwnSQoL/Gf27tp6QEM3KFb8a0ddef41hjt4lvB1bVrEx2bvwk29N9WVjwKWkdfWxgIwW9J4OvFBnaqGSB10NtQz2ZEYRqTlvoO7RxoiQzUIPqmFEIDj5HuAOVC+L5h62oEwR2OnrOhYPDogyLQBtKCa6kyB3baePUgk0p6oJAgKEQVt5/B/Mi9PXMj3a9S474xY02atczyF3qqxUMqfJbY4tH9SWRBgU5P+6tpuIQfoyjzUOig+w+T67GnCIp0kGXa+STdxvK34yXQuopRAP0O+Sja/Hb9eotjtbiSVfcrQBAmg==; Received: from [66.196.81.155] by nm19.access.bullet.mail.bf1.yahoo.com with NNFMP; 02 Jul 2014 02:39:47 -0000 Received: from [98.139.221.159] by tm1.access.bullet.mail.bf1.yahoo.com with NNFMP; 02 Jul 2014 02:39:47 -0000 Received: from [127.0.0.1] by smtp119.sbc.mail.bf1.yahoo.com with NNFMP; 02 Jul 2014 02:39:47 -0000 X-Yahoo-Newman-Id: 408165.59810.bm@smtp119.sbc.mail.bf1.yahoo.com X-Yahoo-Newman-Property: ymail-3 X-YMail-OSG: duMKYy8VM1kYqmIHvLrDlle_xG3GSgllphp1md9BVAexvOc hL7mhz54C7AND9rjn0WwlJVLP1OucFMN9HbPDUWpHeu8Cf0lRvFGAD35nyx8 czXuaimE8HO20rIOybe2kgJSgbZOjLiZAza27ejD2fWsp5K9qermBf41OIlh lQOTgKZf6Ti2s5ut0j0YlAOOR6e4S.m8SvOrIxLDyjC2d30pR8ZLm95op8fz FjP5iVRUbHDt.09YNgn2ZMfTsDffPjSpbKb6hJKugxN7a9iRB5gvHtZnIwz7 Z_9CipOEx4mt8sdBON4mB9A3wQgmas_Epfcg1EEyTiQ71V5V_sUj3gZwa0Km aq3TLG0knEoA8CQnKv84EuaWjLnkwEKMNJA5WprISYdQ_wpD7pvX0zZXSSPK deraDhFUthYMEKstYx86eZCg6SoaIcfmt84MzUPuZx1gXXn1SFVkaI11bIZw TYxk3bXDfNZa8uGk3LB3WWBXzxEen7k6WrABlSRgX_0rJ2UalofvzZN0gmxt 3U6T52RF6ufSe6M0yKd7saxVgJWd.mA_Y7QE- X-Yahoo-SMTP: F49l9g6swBC0R9n8vJIbm7Tf3P8Xlmia8rHIwTlO__Ml X-Rocket-Received: from chass-imac-2.home (browncc1@72.64.95.96 with plain [98.139.221.191]) by smtp119.sbc.mail.bf1.yahoo.com with SMTP; 02 Jul 2014 02:39:47 +0000 UTC From: Charles Brown Mime-Version: 1.0 (Apple Message framework v1085) Content-Type: multipart/alternative; boundary=Apple-Mail-3-759823981 Subject: Re: Two-blade or three-blade prop for I-550 X-Original-Date: Tue, 1 Jul 2014 21:39:46 -0500 In-Reply-To: X-Original-To: Lancair Mailing List References: X-Original-Message-Id: <05AD48D8-AE1A-4F51-8C9F-21F5D570579F@verizon.net> X-Mailer: Apple Mail (2.1085) --Apple-Mail-3-759823981 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=windows-1252 Les Doud asserted that there was a crossover around 300hp where you want = to go with 3 blades -- but without offering a really good rationale. = I'll make the following observations: 1. Tip Mach is probably only an issue at top speed. A quick hack shows = that at 2700 rpm and 250ktas forward speed (basically Legacy max = airspeed), the 70-inch 3-blade has a tip Mach of 0.84, vs the 72-inch = 2-blade at 0.86 Mach. That difference between .84 and .86 undoubtedly = costs wave drag penalties -- at max speed. At 2300rpm and 210ktas, the = tip Machs are 0.72 and 0.73 -- negligible diff in wave drag. =20 2. 3 tip vortices vs 2 --- is somewhat offset by decreased blade angle = of attack. The 3blade has more area, and operates at lower angle of = attack for a given load and speed. So the vortex shed by each 2-blade = tip will be stronger than at each 3-blade tip; assuming similar blade = aspect ratios, twist, etc. =20 3. Better takeoff and climb -- if you're operating at high horsepower = and slow speed, the reduced angle of attack of the 3-blade makes operate = at a better L/D, so it's a better "low gear". I integrated Les Doud's = static thrust for the two props from 0 to 75ktas, at 2100 lb gross = weight, and came up with a roll of 400ft for the 2-blade, 342 ft for the = 3-blade. The 3-blade is also a better airbrake on approach, if you cut = back to idle power. =20 4. The 3-blade is 20 lb heavier. Assuming that cruise L/D is around = 8:1, that 20 lb weight translates into about 2lb of drag, just offseting = Les Doud's claimed 2lb cruise thrust increment. (purists note: I've = assumed that the CGs are in the same place for 2- and 3-blades, so trim = drag is identical.) 5. Noise -- is probably better with 3-blade even if the noise energy is = identical -- because it's shifted to a 50% higher frequency that's = probably better damped by the airframe. =20 6. Looks -- no contest. I think the most significant differences between the two props are = purchase price, overhaul cost, takeoff roll, and looks. Also -- you = might choose the prop that puts the CG where you want it, if everything = else is done and you don't want to move anything. That 20 lb way out = front translates to something like a full inch of CG position at = middling aircraft weight. =20 Charley From: "John Barrett" Date: June 30, 2014 3:10:22 PM CDT Subject: RE: [LML] Re: Two-blade or three-blade prop for I-550 I understood from conversations with some prop designers a few years ago = (can=92t recall who) that the fewest number of blades to get the job = done is most efficient. One blade would be best except for obvious = balance issues. So the remaining factors include how much HP one blade = can absorb. I seem to remember that three was the minimum for an engine = that goes much over 300 HP at least for the hubs and props we were = looking at for the IVP. This info may have come from MT because I was = looking at the five blade MT vs four. My understanding at that time was = that more blades result in quieter smoother ops but give up small = amounts of efficiency. =20 Don=92t believe anything I say here. The memories are about 10 years = old. =20 John Barrett =20= --Apple-Mail-3-759823981 Content-Transfer-Encoding: quoted-printable Content-Type: text/html; charset=windows-1252

4.  The 3-blade is 20 lb heavier. =  Assuming that cruise L/D is around 8:1, that 20 lb weight = translates into about 2lb of drag, just offseting Les Doud's claimed 2lb = cruise thrust increment.  (purists note:  I've assumed that = the CGs are in the same place for 2- and 3-blades, so trim drag is = identical.)

5.  Noise -- is probably = better with 3-blade even if the noise energy is identical -- because = it's shifted to a 50% higher frequency that's probably better damped by = the airframe.  

6.  Looks -- no = contest.

I think the most significant = differences between the two props are purchase price, overhaul cost, = takeoff roll, and looks.  Also -- you might choose the prop that = puts the CG where you want it, if everything else is done and you don't = want to move anything.  That 20 lb way out front translates to = something like a full inch of CG position at middling aircraft weight. =  

Charley



<= br>
From: = "John Barrett" <jbarrett@carbinge.com>
Date: June 30, 2014 3:10:22 PM CDT
Subject: = RE: [LML] Re: Two-blade or three-blade prop for = I-550


I understood from conversations with some prop = designers a few years ago (can=92t recall who) that the fewest number of = blades to get the job done is most efficient.  One blade would be = best except for obvious balance issues.  So the remaining factors = include how much HP one blade can absorb.  I seem to remember that = three was the minimum for an engine that goes much over 300 HP at least = for the hubs and props we were looking at for the IVP.  This info = may have come from MT because I was looking at the five blade MT vs = four.  My understanding at that time was that more blades result = in  quieter smoother ops but give up small amounts of efficiency. =

 

Don=92t believe anything I say here.  The = memories are about 10 years old.

 

John Barrett

 

= --Apple-Mail-3-759823981--