<... 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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