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The 'negative slippage' was a tongue in
cheek verbal concoction intended to hint at the fact that
'slippage' is an artifact of old hangar tails based on lack of
understanding, even among 'experts' of old, of how a prop actually
works.
(insert pause here to parse that awful sentence....)
'Pitch' implies that a prop is a screw. A prop isn't a screw. It's
an airfoil (properly made, a *twisted* airfoil) that rotates. If
you put the flat blades from a ceiling fan on the nose of an
airplane, then 'pitch' might have some actual meaning (but I don't
think so).
Consider that 'pitch' is usually measured somewhere between 2/3
& 3/4 out from the center to the tip of a blade. If you
measure 'pitch' near the root, you'd probably get something like
120-140 inches of pitch on a prop for RV's or EZ's, etc, and at
the tip it would probably be something like 60", begging the
question, which part of the prop is actually screwing through the
air at a particular 'pitch'. It doesn't mean that almost half the
prop is 'slipping' and almost to half the prop is actually
dragging, with a couple of inches moving the plane. It just means
that each 'station' of the prop blade must be pitched to generate
thrust at the relative speed it moves through the air. The airfoil
(BTW, I've never seen a symmetrical prop airfoil) is, or at least
*can*, make the air move aft faster than the 'pitch' of the
imagined screw is moving through the air. Just like a sailing
vessel, it's not moving directly with the wind; it's moving at an
angle. The relative wind for the prop is not aligned with the path
of the plane.
Depending on your religion, either Newton or Bernoulli makes the
air go back when the prop spins. Polytheists like me believe in
both.
I love talking religion. Can we talk about politics next?
Charlie
;-)
On 7/24/2014 12:18 PM, James Osborn wrote:
Charlie said: Ice sails, desert sails, and now,
even unlimited class sailboats can sail faster than the wind.
'Negative slippage'. :-)
I don't think the analogy quite applies here. For those
types of crafts it is the wind that is doing the powering.
While it is true these types of craft can sail faster than
the wind, but not while pointing straight into it! By
definition to cruise in an airplane, the prop has to be
generating some kind of thrust and therefore could never go
faster than "the wind" - the only wind it sees is the relative
wind that is generated due to its own thrust (in cruise). In
a descent, sure negative slippage is a fact. And slippage has
to increase greatly in a climb. I guess I am arguing that the
only way you could see zero or negative slippage in cruise is
if either your blade cross section is asymmetrical (it usually
is right?) or if the pitch number used in the calculation is
not really right based on the kinds of factors you outlined
Charlie.
Supposing that we have a typical non-symmetric blade cross
section, an accurate pitch based on the chord line of the
cross section, and I suppose a twist that is correct for the
cruise RPM. What then would be considered a good or
reasonable slippage in cruise? I saw 3% thrown out there.
And if your prop selection is good for all those conditions
(in other words as efficient as possible), is this the
slippage you expect? I am just wondering if you can use a
slippage calculation to judge efficiency (roughly).
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