X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml@lancaironline.net Date: Tue, 08 Jul 2014 07:54:51 -0400 Message-ID: X-Original-Return-Path: Received: from nm15-vm5.bullet.mail.gq1.yahoo.com ([98.137.176.77] verified) by logan.com (CommuniGate Pro SMTP 6.0.9e) with ESMTPS id 6962112 for lml@lancaironline.net; Thu, 03 Jul 2014 07:57:22 -0400 Received-SPF: pass receiver=logan.com; client-ip=98.137.176.77; envelope-from=casey.gary@yahoo.com Received: from [98.137.12.191] by nm15.bullet.mail.gq1.yahoo.com with NNFMP; 03 Jul 2014 11:56:46 -0000 Received: from [216.39.60.198] by tm12.bullet.mail.gq1.yahoo.com with NNFMP; 03 Jul 2014 11:56:46 -0000 Received: from [127.0.0.1] by omp1085.mail.gq1.yahoo.com with NNFMP; 03 Jul 2014 11:56:46 -0000 X-Yahoo-Newman-Property: ymail-3 X-Yahoo-Newman-Id: 120805.65659.bm@omp1085.mail.gq1.yahoo.com Received: (qmail 75540 invoked by uid 60001); 3 Jul 2014 11:56:45 -0000 X-YMail-OSG: rOXcjPwVM1lSF5pH8vRb_FvhxIT6YMhMMTe94nF_BZYaoTu up_i6OQ8NGrpcn97_WJo2wCG6e9HwvnwAB8sP5QXXnOpHEVbZPXUXMD4Vl.. a7RETPuNsFkR4uFA9YyOpnI8A05Jyakv7F8UEE4VPBrL.g1CnHeXRLBpSMcC DbfgIQ2ZUEnaYzC6iW4FGzjr4rWPi1nPuCS6E1STfyJbUxmfIpLe.ujadVuT xleBTeU7jNnXk8T6LOTmZb12RwpDYnbx3ya5QX4m0oYKOOFP_6KC0uxRdwN9 53ae77P0XxcuNM.gTLNogOHzyYwpqoUN4mQJD_vxzcE_E8CWqc4xDQz4jEnd xqxL.ViRWZm0CTZ71aEo7V0jjUsurB5gaLWyRJuxitADKmuHCPBhFxWsDNta cDmWzLX1pcGD25m6x.RfM9SyRLYinP8D5KM8gGHU1ScwmR_zsm_HTQwoUEnp vmua4IRP8BO.Xq5o1o4KOoBSl3Echaqq6FerXK2kcIWnOwkwcT9jipCjvD8z CnHKvshEiaaZ2w5SmdZcJleE8IGfkQea_8MfqsWCKeqOgSMH7hl5XNPs.MQ- - Received: from [24.247.14.116] by web163501.mail.gq1.yahoo.com via HTTP; Thu, 03 Jul 2014 04:56:45 PDT X-Rocket-MIMEInfo: 002.001,SSBnb3QgY3VyaW91cyBhYm91dCB0aGUgbWF0aCBzbyBJIGRpZCBzb21lIGNhbGN1bGF0aW9ucyBvZiB0aGUgdmVjdG9ycyBpbiBhIGNsaW1iLiDCoEFzc3VtaW5nIHRoZSB3aW5nIGFuZ2xlIG9mIGF0dGFjayBkdXJpbmcgY2xpbWIgaXMgMyBkZWdyZWVzIGhpZ2hlciB0aGFuIGF0IGNydWlzZSwgwqBUaGUgImRvd24iIGJsYWRlIChyaWdodCBzaWRlZCkgaGFzIGEgMC41IGRlZ3JlZSBoaWdoZXIgYW5nbGUgb2YgYXR0YWNrIHRoYW4gdGhlIGxlZnQgc2lkZSBibGFkZS4gwqBGdXJ0aGVyLCB0aGUgcmlnaHQgYmwBMAEBAQE- X-Mailer: YahooMailWebService/0.8.191.1 References: X-Original-Message-ID: <1404388605.7692.YahooMailNeo@web163501.mail.gq1.yahoo.com> X-Original-Date: Thu, 3 Jul 2014 04:56:45 -0700 From: Gary Casey Reply-To: Gary Casey Subject: Re: Two-blade or three-blade prop for I-550 X-Original-To: Lancair Mailing List In-Reply-To: MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="633453505-552521117-1404388605=:7692" --633453505-552521117-1404388605=:7692 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: quoted-printable I got curious about the math so I did some calculations of the vectors in a= climb. =A0Assuming the wing angle of attack during climb is 3 degrees high= er than at cruise, =A0The "down" blade (right sided) has a 0.5 degree highe= r angle of attack than the left side blade. =A0Further, the right blade see= s a 3% higher apparent wind. =A0Both effects will make the right blade prod= uce more thrust. =A0So it is probably significant. =A0The total thrust is s= omething less than 700 pounds at that (130 mph) speed, so what would the th= rust difference be? =A0At the highest I would guess the difference is less = than 10% and that calculates to a torque of 280 ft-lb trying to turn the ai= rplane to the left. =A0But to some extent the prop "makes" its own wind and= that reduces the effects. =A0But there is also the sprial slipstream that = is acting on the tail and turning the plane left. =A0Also, the engine torqu= e itself is trying to roll the plane left (noticeable if you have a P-51). =A0But the question was, is there a difference between 2 and 3-blade prope= llers? =A0I say the difference is very, very minor. =A0If the 3-blade has a= smaller diameter, the effects of asymmetrical thrust is a tiny bit less th= an with a 2-blade. =A0Other things being equal (they never are), the 3-blad= e will produce more static thrust because it has more blade area, but the s= maller diameter is working against it - a rule of thumb is that the static = thrust is proportional to the square of the diameter. =A0But the greater bl= ade area of the 3-blade has necessarily higher drag, which makes itself fel= t at cruise. =A0And then if the diameter (of the 2-blade) is such that the = tips go transonic, the drag can go up a lot. =A0Thousands of detail comprom= ises have to be made. =A0Each blade "flies" in the wake of the previous bla= de, so the closer it is the poorer the effiicency. =A0The thrust, and conse= quently the amplitude of the noise pulse of each blade is less with more blades, but the frequency is higher. =A0So you pays your money and you tak= es your choice. =A0To me, a lightweight, slippery plane tends to favor the = 2-blade. =A0But most everyone puts a 3-blade prop on them.=0A=0AHopefully r= eading this dissertation didn't cost you more than it was worth :-)=0A=0A= =0A=0AOops...=A0 It is the ascending blade that has a lower AOA.=0A=3D20=0A= Grayhawk --633453505-552521117-1404388605=:7692 Content-Type: text/html; charset=iso-8859-1 Content-Transfer-Encoding: quoted-printable
I got = curious about the math so I did some calculations of the vectors in a climb= .  Assuming the wing angle of attack during climb is 3 degrees higher = than at cruise,  The "down" blade (right sided) has a 0.5 degree highe= r angle of attack than the left side blade.  Further, the right blade = sees a 3% higher apparent wind.  Both effects will make the right blad= e produce more thrust.  So it is probably significant.  The total= thrust is something less than 700 pounds at that (130 mph) speed, so what = would the thrust difference be?  At the highest I would guess the diff= erence is less than 10% and that calculates to a torque of 280 ft-lb trying= to turn the airplane to the left.  But to some extent the prop "makes" its own wind and that reduces the effects.  But there is also= the sprial slipstream that is acting on the tail and turning the plane lef= t.  Also, the engine torque itself is trying to roll the plane left (n= oticeable if you have a P-51).  But the question was, is there a diffe= rence between 2 and 3-blade propellers?  I say the difference is very,= very minor.  If the 3-blade has a smaller diameter, the effects of as= ymmetrical thrust is a tiny bit less than with a 2-blade.  Other thing= s being equal (they never are), the 3-blade will produce more static thrust= because it has more blade area, but the smaller diameter is working agains= t it - a rule of thumb is that the static thrust is proportional to the squ= are of the diameter.  But the greater blade area of the 3-blade has ne= cessarily higher drag, which makes itself felt at cruise.  And then if= the diameter (of the 2-blade) is such that the tips go transonic, the drag can go up a lot.  Thousands of detail compromises have to be= made.  Each blade "flies" in the wake of the previous blade, so the c= loser it is the poorer the effiicency.  The thrust, and consequently t= he amplitude of the noise pulse of each blade is less with more blades, but= the frequency is higher.  So you pays your money and you takes your c= hoice.  To me, a lightweight, slippery plane tends to favor the 2-blad= e.  But most everyone puts a 3-blade prop on them.

Hopefully reading this dissertation did= n't cost you more than it was worth :-)



Oops...  It is the ascending blade that= has a lower AOA.
=3D20
Grayhawk

<= /body> --633453505-552521117-1404388605=:7692--