X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Received: from wx-out-0506.google.com ([66.249.82.228] verified) by logan.com (CommuniGate Pro SMTP 5.1c.3) with ESMTP id 1350421 for flyrotary@lancaironline.net; Sat, 26 Aug 2006 22:30:41 -0400 Received-SPF: pass receiver=logan.com; client-ip=66.249.82.228; envelope-from=barrygardner@gmail.com Received: by wx-out-0506.google.com with SMTP id i29so1251712wxd for ; Sat, 26 Aug 2006 19:30:02 -0700 (PDT) DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws; s=beta; d=gmail.com; h=received:message-id:from:to:subject:date:mime-version:content-type:x-priority:x-msmail-priority:x-mailer:x-mimeole; b=Wr6zuuEDW8n7iTx+OV5hjxYzUn5XRLjrgZJanLsXsnDUpK11gLf9KuADv4604vZbso/sPG3LS08o2Et7of5Gr2FYUHpRYHWxOm7jRQd5phxp40ZYFs2fx3MxWyB1agMt3P7ZQkV9gDIlkWvLReYvx93VuAe0gmevQzIK3R7ACtM= Received: by 10.70.32.13 with SMTP id f13mr7066316wxf; Sat, 26 Aug 2006 19:30:01 -0700 (PDT) Return-Path: Received: from dellc400 ( [75.4.105.130]) by mx.gmail.com with ESMTP id 3sm4175181wrh.2006.08.26.19.30.01; Sat, 26 Aug 2006 19:30:01 -0700 (PDT) Message-ID: <0e2701c6c980$ad505e00$800101df@dellc400> From: "Barry Gardner" To: "Rotary motors in aircraft" Subject: prop speed Date: Sat, 26 Aug 2006 21:29:51 -0500 MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_0E24_01C6C956.C3C2ECE0" 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_0E24_01C6C956.C3C2ECE0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Guys, I've been pondering the hoverhawk.com prop speed calculator (which in = turn is from the pponk.com aviation site).=20 While I think it's okay for hovercraft (hoverhawk's goal), I'm wondering = if it doesn't contain an error for our purposes. The hoverhawk calculator calculates the tip velocity of user-input prop = diameters, adjusted for temperature and rpm. They recommend tip = velocities of .88 to .92 mach, which really cranks up the RPMs if you = look at it.=20 However, isn't the tip velocity the sum of both the rotational tip speed = (i.e., the number they calculate) PLUS the forward vector? Don't you = need both to figure out how fast the tip is going?=20 You know that the forward velocity component is not in there because the = forward speed is not a user-input variable in the equation. So while the hoverhawk calculator would be perfectly applicable to a = relatively slow-moving hovercraft, wouldn't it substantially understate = the mach speed of an airplane's propellor carving a forward-moving helix = in the air at the rate of 150 to 200 mph? After all, that forward vector = itself could add 20-25% to the mach number if my theory is correct. That would mean that the rpm they calculate would be too high for an = aircraft (except for the first five seconds of takeoff) and the prop = length recommended would be longer than an aircraft could use because = the longer tip could go supersonic when the forward velocity is added = in. Anyone want to tackle this or straighten me out? Thanks. Barry Gardner Wheaton, IL ------=_NextPart_000_0E24_01C6C956.C3C2ECE0 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable
Guys,
 
I've been pondering the hoverhawk.com = prop speed=20 calculator (which in turn is from the pponk.com aviation site). =
 
While I think it's okay for hovercraft = (hoverhawk's=20 goal), I'm wondering if it doesn't contain an error for our=20 purposes.
 
The hoverhawk calculator calculates the = tip=20 velocity of user-input prop diameters, adjusted for = temperature and=20 rpm. They recommend tip velocities of .88 to .92 mach, which really = cranks up=20 the RPMs if you look at it.
 
However, isn't the tip velocity the sum = of both the=20 rotational tip speed (i.e., the number they calculate) PLUS the forward = vector?=20 Don't you need both to figure out how fast the tip is going? =
 
You know that the forward velocity = component=20 is not in there because the forward speed is not a user-input = variable in=20 the equation.
 
So while the hoverhawk calculator would = be=20 perfectly applicable to a relatively slow-moving hovercraft, wouldn't it = substantially understate the mach speed of an airplane's propellor = carving a=20 forward-moving helix in the air at the rate of 150 to 200 mph? After = all, that=20 forward vector itself could add 20-25% to the mach number if my = theory is=20 correct.
 
That would mean that the rpm they = calculate would=20 be too high for an aircraft (except for the first five seconds=20 of takeoff) and the prop length recommended would be longer than an = aircraft could use because the longer tip could go supersonic when the = forward=20 velocity is added in.
 
Anyone want to tackle this or = straighten me=20 out?
 
Thanks.
 
Barry Gardner
Wheaton, IL
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