Return-Path: Received: from relay03.roc.ny.frontiernet.net ([66.133.131.36] verified) by logan.com (CommuniGate Pro SMTP 4.1.8) with ESMTP id 3051908 for flyrotary@lancaironline.net; Mon, 01 Mar 2004 00:44:47 -0500 Received: (qmail 10354 invoked from network); 1 Mar 2004 05:44:46 -0000 Received: from unknown (HELO frontiernet.net) ([170.215.99.215]) (envelope-sender ) by relay03.roc.ny.frontiernet.net (FrontierMTA 2.3.6) with SMTP for ; 1 Mar 2004 05:44:46 -0000 Message-ID: <4042CDC6.F4883EAF@frontiernet.net> Date: Sun, 29 Feb 2004 23:44:38 -0600 From: Jim Sower X-Mailer: Mozilla 4.77 [en] (Win98; U) X-Accept-Language: en MIME-Version: 1.0 To: Rotary motors in aircraft Subject: Re: [FlyRotary] Re: PROP Free Spinning or WindMilling??? References: Content-Type: multipart/alternative; boundary="------------8E55513BDE0B8AC6FF2BBA59" --------------8E55513BDE0B8AC6FF2BBA59 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit <... that's faster than the engine can turn it - me thinks that if this is true, we are getting something for nothing ...> 2400 was a nice round number that fit 80%. Actually, I get 80 kts at about 2000 RPM which divided by .8 is 2500. In any case, you're not getting something for nothing. You're getting nothing and not paying very much for it. The prop is "in trail". It's "feathered" in the sense that it's at nearly zero AoA. It's just that, being a fixed pitch prop, it has to spin pretty fast to arrive at zero AoA. Bob Darrah wrote: > > > > Jim Sower wrote (snip)>uite often stalled, so it > produces more drag and less lift). At cruise > >airspeed, the forward velocity of the airplane > becomes a component of the >AoA and effectively > reduces it making for smaller Cl and Cd, but the > >"airspeed" of the airfoil now has the airspeed of > the airplane added to >rotational speed, so this > higher speed increases total drag >(geometrically). > High Cd at no airspeed and lower Cd at higher > airspeed >tend to wash out and we end up with cruise > RPM not too far removed from >static RPM. > > >Power off: Let's assume a given airspeed (say 80 > kias) on our fixed pitch >prop. > > Engine "seized": Our prop is at whatever AoA > is determined by >measuring the wind velocity vector > and the chord of the airfoil. It will be >very high > - basically 90' less the local pitch of the prop. > The > >drag will correspond to the airfoil drag at ?? AoA > (say 50'-80') depending >on where you measure along > the span. > > > Prop "freewheeling": (more snip). If our prop > is 80% efficient (is that >typical?), and our > airplane requires 2400 RPM to cruise at 80 kias, I > would >intuit that the terminal rpm of the > frictionless freewheeling prop would be >2400/0.8 or > about 3000 RPM. > > Hay, that's faster than the engine can turn it-me > thinks that if this is true, we are getting > something for nothing. But I do agree, a C-123 prop > in near FLAT pitch produces a whole lot of drag. > > > > Bob Darrah > -- Jim Sower ... Destiny's Plaything Crossville, TN; Chapter 5 Long-EZ N83RT, Velocity N4095T --------------8E55513BDE0B8AC6FF2BBA59 Content-Type: text/html; charset=us-ascii Content-Transfer-Encoding: 7bit <... that's faster than the engine can turn it - me thinks that if this is true, we are getting something for nothing ...>
2400 was a nice round number that fit 80%. Actually, I get 80 kts at about 2000 RPM which divided by .8 is 2500. In any case, you're not getting something for nothing. You're getting nothing and not paying very much for it. The prop is "in trail". It's "feathered" in the sense that it's at nearly zero AoA. It's just that, being a fixed pitch prop, it has to spin pretty fast to arrive at zero AoA.

Bob Darrah wrote:

Jim Sower wrote (snip)>uite often stalled, so it produces more drag and less lift). At cruise >airspeed, the forward velocity of the airplane becomes a component of the >AoA and effectively reduces it making for smaller Cl and Cd, but the >"airspeed" of the airfoil now has the airspeed of the airplane added to >rotational speed, so this higher speed increases total drag >(geometrically). High Cd at no airspeed and lower Cd at higher airspeed >tend to wash out and we end up with cruise RPM not too far removed from >static RPM.
>Power off: Let's assume a given airspeed (say 80 kias) on our fixed pitch >prop.
> Engine "seized": Our prop is at whatever AoA is determined by >measuring the wind velocity vector and the chord of the airfoil. It will be >very high - basically 90' less the local pitch of the prop. The
>drag will correspond to the airfoil drag at ?? AoA (say 50'-80') depending >on where you measure along the span.
> Prop "freewheeling": (more snip). If our prop is 80% efficient (is that >typical?), and our airplane requires 2400 RPM to cruise at 80 kias, I would >intuit that the terminal rpm of the frictionless freewheeling prop would be >2400/0.8 or about 3000 RPM.
Hay, that's faster than the engine can turn it-me thinks that if this is true, we are getting something for nothing. But I do agree, a C-123 prop in near FLAT pitch produces a whole lot of drag.

Bob Darrah

--
Jim Sower ... Destiny's Plaything
Crossville, TN; Chapter 5
Long-EZ N83RT, Velocity N4095T
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