X-Virus-Scanned: clean according to Sophos on Logan.com From: "Charlie England" Received: from mail-yk0-f173.google.com ([209.85.160.173] verified) by logan.com (CommuniGate Pro SMTP 6.0.9e) with ESMTPS id 6998004 for flyrotary@lancaironline.net; Thu, 24 Jul 2014 18:49:10 -0400 Received-SPF: pass receiver=logan.com; client-ip=209.85.160.173; envelope-from=ceengland7@gmail.com Received: by mail-yk0-f173.google.com with SMTP id 131so2258523ykp.18 for ; Thu, 24 Jul 2014 15:48:34 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=message-id:date:from:user-agent:mime-version:to:subject:references :in-reply-to:content-type; bh=OvKQcEzFduuEDc1YcgXxlmfaWepvDdcNkDS0elZEwag=; b=UQYOxO6j9erfhMs0n3mfsDpgX0m5Lh/XQ+3TnQzqmrWjgy567JK80cClp6cDOUEic4 tRqRYItBmYd3WZ0CwTI0MlXBzaR4qnisnAuyBiKzMRe/Hg6bbcF/bRI4hV0VRoKybKUx g8uAL5zP4pINC1swHeCsz6tzNdnvb6eJmsi1LlBBO8FEy9PTCkohmwdVOsP6gMLGaUVb NvagXgPkh5tSWCM97yHwxt85Qeh2Z0q3O4mMXh9gQbwrDV4zkUD/EGBdhjlQP4w9Ruef SpKzkzVcyCs4Ay06bA6Me/F5XPV93dMMrYQ3kI25znZtLYz+yRNHKlySBD1A46pIhSE+ CrZw== X-Received: by 10.236.220.100 with SMTP id n94mr16958272yhp.77.1406242114420; Thu, 24 Jul 2014 15:48:34 -0700 (PDT) Return-Path: Received: from ?IPv6:2602:306:25fa:d179:d836:6bed:c6c9:7de7? ([2602:306:25fa:d179:d836:6bed:c6c9:7de7]) by mx.google.com with ESMTPSA id c72sm16815594yhb.51.2014.07.24.15.48.32 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Thu, 24 Jul 2014 15:48:33 -0700 (PDT) Message-ID: <53D18D77.30006@gmail.com> Date: Thu, 24 Jul 2014 17:49:27 -0500 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:24.0) Gecko/20100101 Thunderbird/24.6.0 MIME-Version: 1.0 To: Rotary motors in aircraft Subject: Re: [FlyRotary] Re: Rv7 renises p port References: In-Reply-To: Content-Type: multipart/alternative; boundary="------------020004070005090904050007" This is a multi-part message in MIME format. --------------020004070005090904050007 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit 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 > > 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 >> > > 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 >> -- >> Homepage: http://www.flyrotary.com/ >> Archive and UnSub: >> http://mail.lancaironline.net:81/lists/flyrotary/List.html >> >> >> -- >> Homepage: http://www.flyrotary.com/ >> Archive and UnSub: >> http://mail.lancaironline.net:81/lists/flyrotary/List.html >> >> > > --------------020004070005090904050007 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: 8bit
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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