X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Received: from mail-fx0-f225.google.com ([209.85.220.225] verified) by logan.com (CommuniGate Pro SMTP 5.3.3) with ESMTP id 4152716 for flyrotary@lancaironline.net; Thu, 04 Mar 2010 19:40:38 -0500 Received-SPF: pass receiver=logan.com; client-ip=209.85.220.225; envelope-from=lehanover@gmail.com Received: by fxm25 with SMTP id 25so3539495fxm.27 for ; Thu, 04 Mar 2010 16:40:03 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=gamma; h=domainkey-signature:mime-version:received:date:message-id:subject :from:to:content-type; bh=H2EFrAsAGLTrZUBf2VL3tMCGJpbDdsL5cn1sFVs0IuI=; b=ITIpV/D944Wc/LEtSgEzvAbwL9lrNcV7WWmB7uBT8WzgG88PKkvQVMQZQm+Q87tPZU /ZO77QTtUtENho023CiAEJMBXrAPmknYZB1GKCksh4jyiZRolM6Ka8TLDwTqgnhbM3xx BqlqfJnyBqFSJezAzwcH8TMUGSnOlu2/hQeIw= DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=mime-version:date:message-id:subject:from:to:content-type; b=FPUPJzCQzxUf19nB+z/jGa1sZrRn9N4T0fbbfGrwuvFEuhv9DQyn3ayjCXFgSHsGqr y7GgY6duJnVl7h2vF9xe3FdyseyLdvDJ0i48IhoKK/dnK/gJbbGrQ8sKDjZBOuZPEJqo 3WmPneAEDDBiSaU1BgkNRgsnNCFcAoLMemXVM= MIME-Version: 1.0 Received: by 10.239.187.146 with SMTP id l18mr236676hbh.29.1267749603385; Thu, 04 Mar 2010 16:40:03 -0800 (PST) Date: Thu, 4 Mar 2010 19:40:03 -0500 Message-ID: <1ab24f411003041640q722bae32qb8be3a3a7088ed51@mail.gmail.com> Subject: The stalled propeller blade From: Lynn Hanover To: flyrotary@lancaironline.net Content-Type: multipart/alternative; boundary=001485f6c706c62130048102f4dd --001485f6c706c62130048102f4dd Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: quoted-printable Veeeerrrrrry interesting indeed. I think the key difference is, not so muc= h the greater HP, but the greater torque (albeit via HP) to turn that larger prop. One thing I want to avoid is that accident you referred to with too fine pitch. For now I have an electric MT with auto pitch control. That should help me avoid problems like this. I have a lot of jet time, but little prop time. So when it comes to AOA, I think in terms of the wing producing max lift. The AOA changes with elevator input. A prop is not unlike a wing, but I still don=92t have a gr= asp of what is causing AOA change in a fixed pitch prop. But, usually I have t= o read things three or four times in order to =93get it=94. What gave rise to seeing a need for clipping the larger prop? Was the torque not there? I=92m sure you answered this a few days ago, I=92ll look= over the old posts. And yeah, it=92s possible to overthink this thing. Fun nonetheless. BW The propeller blade is a wing, but used in an impossible situation. To stall or not to stall that is the question. A stalled airfoil has some of the smooth flow near the trailing edge separating from the surface and rolling. This reduces the mass of air that is changing direction (Vector) and the thrust of the blade or the lift of the wing is reduced. This is a function of Angle of attack to the RELATIVE wind or airflow. When the plane is tied to a tree with a spring scale, or just sitting still, the flow through the propeller disc is nearly at 90 degrees to the zero lift line of the prop. This flow changes for the worst as the propeller RPM goes up. The cylinder of air going through the prop begins to accelerate reducin= g the (Effective) angle of attack of the blades, however the last bit of the airflow as it enters the disc begins to spin or, follow the blades around a bit. This increases the Angle of attack. The actual blade angle relative to the crankshaft never changes. The blades path through an ever faster moving column of air produces a lengthening spiral through the column. If you look at the angular difference between th= e spiral path and the zero lift line of the blade, you see the effective angl= e of attack. As the aircraft and, or the air column accelerates the distance between the points on this spiral will be further apart and the effective angle of attack will be further reduced. So the fixed pitch prop changes angle of attack to the relative wind as a function of the aircrafts foward velocity. So your RV whatever may not take full throttle from stopped. But once it is moving 15 or 20 MPH can take all you have. Because the effective angle of attack of the blades has been reduced, unstalling part of the prop. In the stationary aircraft: Also the cylinder of air is moving into the prop because the prop has generated a low pressure area just in front of the disc. So local air outside of this moving column is at a higher pressure than the air in the column, and begins to compress the column right in front of the disc. Now the prop tips may not be seeing a column of air moving as fast as the mid blade sees, and the tips may be stalled and nothing else is. The air leaving the back side of the blades is accelerated violently and Bernoulli using more local unmoving air chokes the fleeing cylinder of fast moving low pressure air tight against the fuselage. The cylinder of air is now spinning (P factor) and its low pressure is causing slow moving local air to mix with it, so it is gaining mass, And raising the speed of the slower air. So if the prop is small in diameter and turning very fast it may produce a higher velocity column, but of reduced mass. Conversely, a large diameter prop when the same torque is applied will turn more slowly, but move more mass per second, and attract more local air and accelerate that as well. Disc area is the easy way to express all of this. More is better. More HP is better. More torque is better. More diameter (Disc area) is better. More blades are better. Wider blades are better. Lighter blades are better. But not for everything. This should get us down to about 1,000 additional factors to think about. Static thrust. Not just a good idea. Ask the guy in the black biplane hovering 200 feet above the concrete at Sun&Fun. That's over 1,000 pounds of static thrust. Lynn E. Hanover --001485f6c706c62130048102f4dd Content-Type: text/html; charset=windows-1252 Content-Transfer-Encoding: quoted-printable

Veeeerrrrrry in= teresting indeed.=A0 I think the key difference is, not so much the greater= HP, but the greater torque (albeit via HP) to turn that larger prop.

One thing I wan= t to avoid is that accident you referred to with too fine pitch. =A0For now= I have an electric MT with auto pitch control. =A0That should help me avoi= d problems like this.

I have a lot of= jet time, but little prop time.=A0 So when it comes to AOA, I think in ter= ms of the wing producing max lift.=A0 The AOA changes with elevator input.= =A0 A prop is not unlike a wing, but I still don=92t have a grasp of what i= s causing AOA change in a fixed pitch prop.=A0 But, usually I have to read = things three or four times in order to =93get it=94.

<= /p>

What gave rise = to seeing a need for clipping the larger prop?=A0 Was the torque not there?= =A0 I=92m sure you answered this a few days ago, I=92ll look over the old p= osts.

<= /p>

And yeah, it=92= s possible to overthink this thing.

<= /p>

Fun nonetheless= .

<= /p>

BW
=A0
=A0
The propeller= blade is a wing, but used in an impossible situation.
=A0
To stall or n= ot to stall that is the question.
=A0
A stalled air= foil has some of the smooth flow near the trailing edge separating from the= surface and rolling.
=A0
This reduces = the mass of air that is changing direction (Vector) and the thrust of the b= lade or the lift of the wing is reduced.
=A0
This is a fun= ction of Angle of attack to the RELATIVE wind or airflow. When the plane is= tied to a tree with a spring scale, or just sitting still, the flow throug= h the propeller disc is nearly at 90 degrees to the zero lift line of the p= rop. This flow changes for the worst as the propeller RPM goes up. The cyli= nder of air going through the prop begins to accelerate reducing the (Effec= tive) angle of attack of the blades, however the last bit of the airflow as= it enters the disc begins to spin or, follow the blades around a bit. This= increases the Angle of attack.=A0
=A0
The actual bl= ade angle relative to the crankshaft never changes. The blades path through= an ever faster moving column of air produces a lengthening spiral through = the column. If you look at the angular difference between the spiral path a= nd the zero lift line of the blade, you see the effective angle of attack.<= /span>
=A0
As the aircra= ft and, or the air column accelerates the distance between the points on th= is spiral will be further apart and the effective angle of attack will be f= urther reduced. So the fixed pitch prop changes angle of attack to the rela= tive wind as a function of the aircrafts foward velocity. So your RV whatev= er may not take full throttle from stopped. But once it is moving 15 or 20 = MPH can take all you have.=A0
Because the e= ffective angle of attack of the blades has been reduced, unstalling part of= the prop.=A0
=A0
In the statio= nary aircraft:
=A0
Also the cyli= nder of air is moving into the prop because the prop has generated a low pr= essure area just in front of the disc.=A0
=A0
So local air = outside of this moving column is at a higher pressure than the air in the c= olumn, and begins to compress the column right in front of the disc.=A0
=A0
Now the prop = tips may not be seeing a column of air moving as fast as the mid blade sees= , and the tips may be stalled and nothing else is.
=A0
The air leavi= ng the back side of the blades is accelerated violently and Bernoulli using= more local unmoving air chokes the fleeing cylinder of fast moving low pre= ssure air tight=A0against the fuselage. The cylinder of air is now spinning= (P factor) and its low pressure is causing slow moving local air to mix wi= th it, so it is gaining mass, And raising the speed of the slower air.=A0
=A0
So if the pro= p is small in diameter and turning very fast it may produce a higher veloci= ty column, but of reduced mass. Conversely, a large diameter prop when the = same torque is applied will turn more slowly, but move more mass per second= , and attract more local air and accelerate that as well. Disc area is the = easy way to=A0express all of this.=A0
=A0
More is bette= r. More HP is better. More torque is better. More diameter (Disc area) is b= etter. More blades are better. Wider blades are better. Lighter blades are = better. But not for everything.
=A0
This should g= et us down to about 1,000 additional factors to think about. =
=A0
Static thrust= . Not just a good idea.
=A0
Ask the guy i= n the black biplane hovering 200 feet above the concrete at Sun&Fun. Th= at's over 1,000 pounds of static thrust.=A0
=A0
Lynn E. Hanov= er=A0
--001485f6c706c62130048102f4dd--