Lehanover@aol.com wrote:

The rubber band airplane has the prop turn free when the rubber has run out, 
  

That would be "freewheeling".  The only resistance to the prop turning is the friction (microscopic?) between the prop and the shaft it's spinning on.

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 
  

OK as far as it goes (which is not all that far).  Energy removed from the airstream depends on how much work is done.  Work involves force.  If it only takes ounces of force to spin the prop, there will be very little energy extracted from the airstream.

propeller that is turning an engine is limited as to how fast it can turn, and 
  

If freewheeling, it's limited by the combination of aircraft speed and prop pitch.  Absent significant resistance to turning, the prop will increase faster and faster until it's aerodynamically feathered - that is, its AoA is just above zero - it's producing nearly zero lift (and, importantly, zero drag) because zero lift is nearly all that's required to spin a prop that's not attached to the powerplant.

<>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,

You situation you are describing is a windmilling engine, NOT a freewheeling prop.  Your prop is trying to accelerate to the speed described above, but the [failed] engine is producing HUGE resistance.  The prop is at a large AoA producing lots of lift (and lots of drag) because lots of lift is required to rotate the [practically] dead engine.  The faster the engine rotates, the more energy it's absorbing from the airstream.  The prop will turn faster and faster (reducing the prop AoA) until the "lift" produced by the prop equals the "drag" produced by the engine, and that will be your terminal rpm.  It will be much lower than with a freewheeling prop.

<>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?

Fairly easily.  In the interest of brevity, let's stipulate that the engine is running at idle:
In the landing pattern, you are carrying low (flat) pitch which means very large AoA.  You are presenting the maximum blade area to the airstream, and the prop will be pretty thoroughly stalled (HUGE drag not a lot of lift).  Also (but less important) engine rpm required to lower that AoA (and prop lift) to where it balances the engine "drag" will be really high.  You'll never get near there if the engine isn't running way above idle.  You'll always be at high AoA (max drag). 

If you forgot your prop and left it at cruise pitch, you are presenting a much smaller portion of blade area to the airstream.  AoA is less (perhaps enough less to unstall the prop) so drag is MUCH less and you are more able to generate enough lift to get the rpm to where AoA (and therefore drag) is reduced a good bit.

Go a little farther, and the prop is feathered.  It has minimum blade area exposed to airstream (minimal AoA and therefore minimal drag).  If it's slightly off feather (as they typically are) it might spin the engine slowly.  In any event, it's the best drag situation because it's the least prop AoA you can get with the prop connected to the engine.

Now, visualize suddenly breaking the crankshaft under TO or cruise pitch conditions.  The prop will immediately spin up to whatever rpm it's set for.  Cruise pitch will cause somewhat less drag than TO pitch since cruise rpm is lower and therefore speed of air over the prop is lower.  Prop AoA is very close to the same in either case (unlike when the engine is attached).

<>
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.

Actually, the prop AoA of the freewheeling prop will cause lift which will spin the prop faster and faster until it arrives at a very low AoA that produces just enough lift to overcome the modest drag at this low AoA plus the minute friction of between the prop and the shaft.

<>
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.

Yes.  In either the windmilling engine situation OR the freewheeling prop situation, the prop will  increase rpm until prop lift balances resistance to rotation.  That "balance" rpm will be higher for a freewheeling prop since it has to spin up to practically zero prop AoA, and much lower for a windmilling engine which has so much resistance that it will balance out at a MUCH HIGHER prop AoA in order to produce sufficient lift to spin the whole engine.

<>So for a given aircraft indicated air-
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.

Good idea.  But fashion some sort of device to actually measure the force generated by the prop.  To be more accurate, run three tests:  the two you describe, and a third in which the prop can still turn, but has enough resistance to only turn at 1/3 or 1/2 speed.

<>
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

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