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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).
-- James
On Wed, Jul 23, 2014 at 7:59 PM, Charlie England <flyrotary@lancaironline.net> wrote:
Ice sails, desert sails, and now, even unlimited class sailboats can sail faster than the wind. 'Negative slippage'. :-)
A more significant point might be that pitch numbers are virtually meaningless, unless you're comparing two props from the same prop maker with the same blade plan form. Even then, it just tells you which has a finer pitch than the other.
Variables can be: whether the pitch is measured on the back side of the blade or through the chord line, where along the diameter the pitch angle is measured (due to blade twist), and no doubt others I'm not smart enough to think of at the moment.
Bottom line is that unless there's an identical airframe flying an identical prop, the pitch number isn't a reliable indicator of speed.
FWIW,
Charlie
On 7/23/2014 7:45 PM, James Osborn wrote:
I don't know jack about slippage, but I think it is the percentage difference between actual distance traveled and theoretical distance traveled if your propeller corkscrewed through the air with no thrust. I found a prop slip calculator
online and for 86 inch pitch, 2.85 gear ratio, 7000 rpm, 180 mph, I get 10% slip. Granted the calculator was for boat propellers, but I don't think it matters as long as the units are correct. There has to be some slip because there would be no thrust otherwise.
So what is considered a reasonable or good amount of slip? Using Bill's numbers 86 inch pitch, 2.85 gear ratio, 7000 rpm, 200 mph, I get 0% slip. That can't be right!
On Fri, Jul 4, 2014 at 7:19 AM, Bill Bradburry <flyrotary@lancaironline.net> wrote:
Christian,
It seems that you have a lot of prop slippage at cruise. I think that at
that prop rpm you should be getting 200mph if you had no slippage.
Bill
-----Original Message-----
From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net]
Sent: Thursday, July 03, 2014 4:01 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Rv7 renises p port
Well hi all
Just thought I'd throw out there the mods I,ve done to the renises in an
rv7'.
Well the p ported engine is now back in the plane and running well, over the
standard short manifold that was originally in the plane I have gained
around 400 static rpm, same prop and gearbox combo, 2.85 ratio, this equates
to allot more hp at takeoff, just shy of 2300 prop rpm, I'm running a prince
p tip prop at 68" x 86" pitch,
At 8000 ft it is turning 7000 at 180 mph tas which is an improvement of 25
mph on previous tests, . So next plan is bigger prop and less pitch to let
it rev to 7500 in strait and level.
Cheers
Christian
Rv7 renises Aus
Sent from my iPad
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