Received-SPF: pass
 receiver=logan.com; client-ip=64.12.137.2; envelope-from=Lehanover@aol.com
From: Lehanover@aol.com
Message-ID: <1d0.2dc2f58e.2ef8bfb1@aol.com>
Date: Mon, 20 Dec 2004 18:52:17 EST
Subject: Re: [FlyRotary] Re: undo powder coating
To: flyrotary@lancaironline.net
MIME-Version: 1.0
Content-Type: text/plain; charset="US-ASCII"
Content-Transfer-Encoding: 7bit

In a message dated 12/20/2004 1:04:26 PM Central Standard Time, 
tomtugan@yahoo.com writes:

<< I changed my mind on how they might differ with the "I'm glad I only went 
as
 far as saying they might differ." post.  I can see where a small very light
 balsa model with a wide-blade prop would do better to let the prop free-spin
 where a heavier full-scale craft might do better to have the prop fully 
stalled
 and not autorotating.    And I know at least one person who switched from a
 3-bladed prop to a 2-bladed prop so the next time he has a power-failure he
 might get a reasonable glide ratio. 
  >>


The rubber band airplane has the prop turn free when the rubber has run out, 
because the prop is more than 1/3 of the wing span. (Not unlike WWII fighters).
If the prop is not free to turn, it becomes a very powerful full right 
aileron input. 
Making for an unhappy child, of 62 years in my case. 

Way back when we wound the rubber (Perelli)  with a hand drill from the rear 
of the plane, The prop was huge, nearly half the wingspan. The prop blades 
were hinged, and when the rubber ran down enough the blades folded up alongside 
of the fuselage.

When you transfer energy from the airstream to the propeller, the propeller 
is accellerated and the airstream is slowed (relative to the plane). The faster 
the propeller turns, the more energy has been removed from the airstream. A 
propeller that is turning an engine is limited as to how fast it can turn, and 
therefore how much energy it can extract from the airstream. If you have ever 
been coming down final fumbling to get Kilo because you have Juliet, and 
pulled off a bit too much prop, and find yourself hanging in the straps as the 
engine rushes to the red line, knows that there is ample energy to be extracted 
from the airstream with the prop being driven instread of driving. Note also 
that the plane as been arrested, with  a big loss of airspeed. How now can this 
be explained?  

The higher the tip speed of the propeller, the more energy being extracted. 
The truly free wheeling propeller would turn up RPM based on its poor (driven) 
shape and effective pitch. So, depending on air speed it might well reach the 
RPM that would be required to go that speed under power less some RPM figure 
to account for the poor shape. 

So, the fixed pitch prop would try (in effect) to stay just below the RPM the 
engine would have been running to maintain that same speed. So for a given 
speed there would be an additional nose down component required to account for 
that drag. 

Now that amount of energy lost may not amount to much. But it is slowing the 
air going through the propeller disc, and that is slowing the plane to some 
extent. 
It may not be true autorotation, but it is there. 

Put a model airplane prop on a big nail. Drive the car 60 MPH. Hold your 
thumb against the hub of the prop so it cannot turn.  Stick it out the window into 
the airstream. No big deal right?  Now move your thumb to the back of the hub 
and let the prop spin up on the nail.

Now park the car and go back and at least find the nail. You don't want 
anyone running over that. Was there any drag at all? Was there more drag with the 
prop spinning or stationary? I better put in "wear a leather glove" the next 
time. Better get that thumb looked at.

But I run on as usual.

Lynn E. Hanover