<... Anywhere close??? ...>
I don't think so.  A prop (we're discussing fixed pitch like we all use on our homebuilts) is a couple of sticks.  Contoured to be sure, but sticks all the same.  We can visualize them as a disc, but visualization doesn't make them any less sticks. :o((

Let's think this through:

A prop blade is a lifting body.  It's basically a high AR wing that rotates.  It has an airfoil that produces lift and in the process generates induced drag.  At zero lift it has some parasite drag, but it's the induced drag that dominates.

Power on:  At zero airspeed, the pitch of the prop is the AoA (very high, quite 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.
    Engine "windmilling":  Now, the AoA is reduced because the rotational speed vector is added to the forward speed vector and the angle will be substantially less than the "seized" AoA (although still quite high).  Cd will be less less at this smaller AoA, but still very substantial.
    Prop "freewheeling":  Here, there is no resistance to rotation (let's assume ZERO friction).  Lift produced by the AoA of the prop will accelerate the freewheeling prop to higher RPM until the rotational velocity combined with the forward velocity add up to the AoA for zero lift.  At that point, the prop will stop accelerating.  That speed will not be anything outrageous though.  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.

In order to quantify all of this, we would have to examine the lift curves of the prop airfoil(s) at very high negative angles of attack.  We are used to ignoring Cl - AoA curves much beyond stall AoA   That said, we do know that with increasing AoA, Cl declines steeply for a while after stall and then much more slowly until it reaches an AoA of about 90' from the zero lift AoA, and then starts to slowly rise again.  Cd, which has been increasing geometrically with increasing AoA up to stall, increases more slowly after stall AoA and then kind of stabilizes too.  As I said, the exact values are not easy to come by since there are not a lot of data available at those high AoA's, and the airfoil is constantly changing so you have to examine a LOT of different airfoils.

Qualitatively, it comes down pretty much as I've described.  No discs.  Just a couple sticks behaving like twisted wings with varying camber airfoils at various AoA's.

Does that make better sense?  .... Jim S.
--
Jim Sower ... Destiny's Plaything
Crossville, TN; Chapter 5
Long-EZ N83RT, Velocity N4095T
 

Ed Anderson wrote:

Ok, let me see if I understand what it was I thought I hear said about
props.

Case 1:  IF the prop is free (no engine connection -crankshaft broke, etc)
to spin without any drag restriction from the engine, it will continue to
spin faster and faster due to the air flow until reaching some equilibrium
point (or comes off the aircraft).  That in effect the entire area of the
prop disk appears to the airflow as a relative solid disk to the airflow and
greatly increases drag and rate of descent. I think I understood that the
prop pitch will probably determine the rpm of such a free spinning  prop and
therefore the "solidity" of the disk to airflow. Finer pitch = more RPM =
more solid disk resistance to air flow = more drag = faster rate of descent.
Coarse pitch = less rpm = less solidary to airflow = less drag = lesser rate
of descent.  Is this more or less approximately correct??

Case 2.  On the other hand, If the prop is windmilling (ie. connected to the
engine such that it has the pumping action as a drag) then while there is
some additional drag over a completely stopped prop, its not a great deal
more. For one the rpm will undoubtedly be considerably lower than a free
spinning prop and the disk less solid.  Again depending somewhat on pitch  -
but not as much drag as case 1.

Anywhere close???

Ed