X-Virus-Scanned: clean according to Sophos on Logan.com From: "Christamarmc" Received: from mail-pd0-f173.google.com ([209.85.192.173] verified) by logan.com (CommuniGate Pro SMTP 6.0.9e) with ESMTPS id 7000010 for flyrotary@lancaironline.net; Sat, 26 Jul 2014 01:37:24 -0400 Received-SPF: pass receiver=logan.com; client-ip=209.85.192.173; envelope-from=christamarmc@gmail.com Received: by mail-pd0-f173.google.com with SMTP id w10so6683615pde.4 for ; Fri, 25 Jul 2014 22:36:49 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=subject:references:from:content-type:in-reply-to:message-id:date:to :content-transfer-encoding:mime-version; bh=q/bXtphd6Alusnv+qXWBJECtOwZPRcVDJ4a94dP6tdA=; b=ofhG5wnL7uJ+Y2QzZI5e6jnCEeWbNl1MqEac1Btjfs1RBPf1xRbvQ/4yle/xyAYudW grHezIo4BwhXFKqqXiLJoAiQOUBI5fWxsgUiS1CDIVysv/QD05RB1d+M1SEhmLbXFHSx 6+iaCOb9+dOEJO2XYRVZj+NIYhoLXLjk2TM63heSPJA0C4auV0ixXTz1RmO1falXIRi8 6XTMFaOa+hyzJpVgBuTdMPGzD33iZdyUDRS6NhCVlx1+x5kL4otIth3UywjSlSfCa4ja RQuj3EPhSCUHWFvigdylVR42hQyyPrAR80nj22+mPTV9KI8kioTidctRSvvxNhNCzYjU QhRQ== X-Received: by 10.68.125.226 with SMTP id mt2mr23912261pbb.6.1406353009206; Fri, 25 Jul 2014 22:36:49 -0700 (PDT) Return-Path: Received: from [192.168.0.3] (210.88.215.218.dyn.commander.net.au. [218.215.88.210]) by mx.google.com with ESMTPSA id t12sm14215734pdj.12.2014.07.25.22.36.45 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Fri, 25 Jul 2014 22:36:48 -0700 (PDT) Subject: Re: [FlyRotary] Re: Rv7 renises p port References: Content-Type: multipart/alternative; boundary=Apple-Mail-30C3D694-970D-4FDC-A596-5308EF519314 X-Mailer: iPad Mail (11D201) In-Reply-To: Message-Id: Date: Sat, 26 Jul 2014 15:36:46 +1000 To: Rotary motors in aircraft Content-Transfer-Encoding: 7bit Mime-Version: 1.0 (1.0) --Apple-Mail-30C3D694-970D-4FDC-A596-5308EF519314 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Hi Alex Thanks, the whirlwind is ground adjustable Cheers Sent from my iPad > On 26 Jul 2014, at 12:54 pm, "alex" wrote: >=20 > Christian, >=20 > On Vetterman's exhaust webpage he does a comparison of the whirlwind and h= artzell and flies both on his aircraft, how is the pitch controlled on the w= hirlwind, electric? >=20 > Alex >=20 > On Jul 26, 2014 6:17 AM, "Christamarmc" wrot= e: >> Hi Charlie >> That is excellent thanks, you can't beat real works experience, I think w= hen the prop I have was ordered was more on what the manufacturer was thinki= ng May suit, I'm a fair bit under the diameter that I can safely run, I'm cu= rrently only at 68" and can comfortably get 74' so I'll definitely think a l= arger is going to help, I've only been able to compare the same manufacturer= that I'm running with the same airframe but with lycoming and I'm currently= running more pitch than a 200 hp rv7 ,but getting slightly less results. >>=20 >> What is everyone's thoughts on ground adjustable, like the whirlwind ga 2= 00 is what I'm looking at possibly, only comes in 72" diameter though, but o= bvious advantage is the adjustable pitch,=20 >> I'm gathering there may be a reason why not many run these at this time j= ust wondering in real world performance. >>=20 >> Cheers >> Christian >>=20 >>=20 >>=20 >> Sent from my iPad >>=20 >>> On 26 Jul 2014, at 10:43 am, "Charlie England" wrote: >>>=20 >>> If you're picking the prop yourself, it really is trial and error (with e= ither lots of luck or lots of error :-) ). Using an alternative engine that t= urns the prop at a non-typical rpm for the a/c you're building makes it even= harder. >>>=20 >>> However, most of the better known prop makers can come really close if y= ou can give them accurate info on which airframe we're building, the plane's= cooling drag, max allowable prop diameter, honest HP estimate, and the prop= rpm at that HP. I would think that the hardest thing for us to estimate is c= ooling drag, compared to a Lyc/Cont on the same airframe. I firmly believe t= hat liquid cooling can have lower drag than air cooling, but I also believe t= hat it rarely happens for us 'shade tree' cooling system designers, trying t= o graft a liquid system onto a plane designed for air cooling. >>>=20 >>> The Lyc guys are always asking the same kinds of questions about props, w= anting to know what pitch to order. I believe that the best answer for a fix= ed pitch prop is to ask for the largest diameter that can be safely swung on= the plane (usually somewhat larger than the conservative estimate by the de= signer), and then give the designer all the other info listed above. Then le= t them pitch and twist the prop as they see fit. The reason for asking for m= ax diameter is for takeoff/low speed performance. With fixed (high) pitch fo= r a high speed airframe, the best way to keep decent takeoff performance is e= xtra diameter, which increases low speed 'mass flow' & helps acceleration. T= he extra diameter doesn't seem to hurt at high speed until you get up in the= >200mph range. >>>=20 >>> I should say that I'm not an engineer or aero guy by training, or abilit= y, for that matter. But I have played with fast homebuilts for over 20 years= , and I've been through quite a few different props on several different pla= nes. What I wrote above has been my experience, and matches the engineering a= rticles I've read on the subject. >>>=20 >>> Hope this was useful.... >>>=20 >>> Charlie >>>=20 >>>=20 >>>> On 7/25/2014 6:50 PM, Christamarmc wrote: >>>>=20 >>>> I understand a bit more about the prop design now, theoretically speaki= ng it would be nice in a perfect world for all to use the same pitch measure= ment like inches from the hub etc, but I guess it's not a perfect ideah as t= here are many differing designs of blades and tips etc >>>> Also we are never quite sure where the manufacturer has measured there p= itch on all there differing props so I guess it is a starting point for whic= h prop to start with and then trial and error after that. >>>>=20 >>>> Thanks all for your input >>>> Cheers >>>> Christian >>>>=20 >>>> Sent from my iPad >>>>=20 >>>> On 26 Jul 2014, at 5:48 am, "Jeff Whaley" = wrote: >>>>=20 >>>>> We have had this discussion before =E2=80=A6 I=E2=80=99ll go out on a l= imb here and agree with both James and Charlie. That is to say the prop pit= ch is not the same from one end to the other; therefore the =E2=80=9Ceffecti= ve pitch=E2=80=9D is an estimation; and that doing better than theoretical i= s simply an error in the pitch estimation. It is impossible for any propell= er to work at 100% or greater efficiency. Considering Tracy=E2=80=99s examp= le of 217 mph Vs 212 mph is an error of only 2.5% (not accounting for the ta= keoff and climb), possibly as much as 5-10% if takeoff and climb considered =E2= =80=A6 is that too hard to believe? You need a pretty slippery ship to get n= ear the =E2=80=9Ceffective pitch=E2=80=9D performance; here=E2=80=99s anothe= r example. My Tri-Pacer has a 57=E2=80=9D pitch propeller that cruises at 2= 300 rpm =E2=80=93 theoretically this is 124 mph, exactly what the owner=E2=80= =99s manual says; however, I can tell you that the airplane hasn=E2=80=99t s= een 124 mph since the last time it was dove in a loop-entry =E2=80=93 it cru= ises at 90 knots (104 mph) or 20% off the theoretical prop pitch =E2=80=93 i= t is not a slippery ship =E2=80=A6 Jeff >>>>>=20 >>>>> =20 >>>>>=20 >>>>> From: >>>>>=20 >>>>> "Tracy" >>>>>=20 >>>>> Subject: >>>>>=20 >>>>> Re: [FlyRotary] Re: Rv7 renises p port >>>>>=20 >>>>> Date: >>>>>=20 >>>>> Thu, 24 Jul 2014 12:09:11 -0400 >>>>>=20 >>>>> To: >>>>>=20 >>>>> Rotary motors in aircraft >>>>>=20 >>>>> >>>>>=20 >>>>> >>>>>=20 >>>>> Charlie is right. Consider that an airplane flying with the wing at 0= deg. AOA does not fall out of the sky :-) >>>>>=20 >>>>> =20 >>>>>=20 >>>>> My average speed in the 2004 Sun 100 race was 217.58 mph which include= s standing start takeoff and climbout. Prop was a 74 x 88, RPM was 7250 wit= h a 2.85 : 1 drive. If you calculate that out it comes to 212 mph with zero= 'slippage'. Draw your own conclusions! >>>>>=20 >>>>> =20 >>>>>=20 >>>>> Tracy >>>>>=20 >>>>> Sent from my iPad >>>>>=20 >>>>> =20 >>>>>=20 >>>>> From: >>>>>=20 >>>>> "Charlie England" >>>>>=20 >>>>> Subject: >>>>>=20 >>>>> Re: [FlyRotary] Re: Rv7 renises p port >>>>>=20 >>>>> Date: >>>>>=20 >>>>> Thu, 24 Jul 2014 17:49:27 -0500 >>>>>=20 >>>>> To: >>>>>=20 >>>>> Rotary motors in aircraft >>>>>=20 >>>>> >>>>>=20 >>>>> >>>>>=20 >>>>> The 'negative slippage' was a tongue in cheek verbal concoction intend= ed to hint at the fact that 'slippage' is an artifact of old hangar tails ba= sed on lack of understanding, even among 'experts' of o= ld, of how a prop actually works.=20 >>>>>=20 >>>>> (insert pause here to parse that awful sentence....) >>>>>=20 >>>>> 'Pitch' implies that a prop is a screw. A prop isn't a screw. It's an a= irfoil (properly made, a *twisted* airfoil) that rotat= es. 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).=20 >>>>>=20 >>>>> Consider that 'pitch' is usually measured somewhere between 2/3 & 3/4 o= ut from the center to the tip of a blade. If you measure 'pitch' near the ro= ot, you'd probably get something like 120-140 inches of pitch on a prop for R= V's or EZ's, etc, and at the tip it would probably be something like 60", be= gging the question, which part of the prop is actually screwing through the a= ir at a particular 'pitch'. It doesn't mean that almost half the prop is 'sl= ipping' and almost to half the prop is actually dragging, with a couple of i= nches moving the plane. It just means that each 'station' of the prop blade m= ust 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 a= t 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 movin= g 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.=20 >>>>>=20 >>>>> Depending on your religion, either Newton or Bernoulli makes the air g= o back when the prop spins. Polytheists like me believe in both.=20 >>>>>=20 >>>>> I love talking religion. Can we talk about politics next? >>>>>=20 >>>>> Charlie >>>>> ;-) >>>>>=20 >>>>> On 7/24/2014 12:18 PM, James Osborn wrote: >>>>>=20 >>>>> Charlie said: Ice sails, desert sails, and now, even unlimited class s= ailboats can sail faster than the wind. 'Negative slippage'. :-) >>>>>=20 >>>>> =20 >>>>>=20 >>>>> I don't think the analogy quite applies here. For those types of craf= ts it is the wind that is doing the powering. While it is true these types o= f craft can sail faster than the wind, but not while pointing straight into i= t! By definition to cruise in an airplane, the prop has to be generating so= me kind of thrust and therefore could never go faster than "the wind" - the o= nly wind it sees is the relative wind that is generated due to its own thrus= t (in cruise). In a descent, sure negative slippage is a fact. And slippag= e has to increase greatly in a climb. I guess I am arguing that the only wa= y you could see zero or negative slippage in cruise is if either your blade c= ross section is asymmetrical (it usually is right?) or if the pitch number u= sed in the calculation is not really right based on the kinds of factors you= outlined Charlie. >>>>>=20 >>>>> =20 >>>>>=20 >>>>> 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 g= ood or reasonable slippage in cruise? I saw 3% thrown out there. And if yo= ur prop selection is good for all those conditions (in other words as effici= ent as possible), is this the slippage you expect? I am just wondering if y= ou can use a slippage calculation to judge efficiency (roughly). >>>>>=20 >>>>> =20 >>>>>=20 >>>>> -- James >>>>>=20 >>>>> On Wed, Jul 23, 2014 at 7:59 PM, Charlie England wrote: >>>>>=20 >>>>> Ice sails, desert sails, and now, even unlimited class sailboats can s= ail faster than the wind. 'Negative slippage'. :-) >>>>>=20 >>>>> A more significant point might be that pitch numbers are virtually mea= ningless, unless you're comparing two props from the same prop maker with th= e same blade plan form. Even then, it just tells you which has a finer pitch= than the other. >>>>>=20 >>>>> Variables can be: whether the pitch is measured on the back side of th= e blade or through the chord line, where along the diameter the pitch angle i= s measured (due to blade twist), and no doubt others I'm not smart enough to= think of at the moment.=20 >>>>>=20 >>>>> Bottom line is that unless there's an identical airframe flying an ide= ntical prop, the pitch number isn't a reliable indicator of speed. >>>>>=20 >>>>> FWIW, >>>>>=20 >>>>> Charlie >>>>>=20 >>>>>=20 >>>>>=20 >>>>>=20 >>>>>=20 >>>>> On 7/23/2014 7:45 PM, James Osborn wrote: >>>>>=20 >>>>> I don't know jack about slippage, but I think it is the percentage dif= ference between actual distance traveled and theoretical distance traveled i= f 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, 18= 0 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 s= ome 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, 2.8= 5 gear ratio, 7000 rpm, 200 mph, I get 0% slip. That can't be right! >>>>>=20 >>>>> =20 >>>>>=20 >>>>> On Fri, Jul 4, 2014 at 7:19 AM, Bill Bradburry wrote: >>>>>=20 >>>>> Christian, >>>>>=20 >>>>> 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. >>>>>=20 >>>>> Bill >>>>>=20 >>>>>=20 >>>>> -----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 >>>>>=20 >>>>> Well hi all >>>>> Just thought I'd throw out there the mods I,ve done to the renises in a= n >>>>> rv7'. >>>>> Well the p ported engine is now back in the plane and running well, ov= er 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 e= quates >>>>> to allot more hp at takeoff, just shy of 2300 prop rpm, I'm running a p= rince >>>>> p tip prop at 68" x 86" pitch, >>>>> At 8000 ft it is turning 7000 at 180 mph tas which is an improvement o= f 25 >>>>> mph on previous tests, . So next plan is bigger prop and less pitch to= let >>>>> it rev to 7500 in strait and level. >>>>>=20 >>>>> Cheers >>>>> Christian >>>>> Rv7 renises Aus >>>>>=20 >>>>>=20 >>>>> Sent from my iPad >>>>> -- >>>>> Homepage: http://www.flyrotary.com/ >>>>> Archive and UnSub: >>>>> http://mail.lancaironline.net:81/lists/flyrotary/List.html >>>>>=20 >>>>>=20 >>>>> -- >>>>> Homepage: http://www.flyrotary.com/ >>>>> Archive and UnSub: http://mail.lancaironline.net:81/lists/flyrotary/= List.html >>>>>=20 >>>>> =20 >>>>>=20 >>>>> This message, and the documents attached hereto, is intended only for t= he addressee and may contain privileged or confidential information. Any una= uthorized disclosure is strictly prohibited. If you have received this messa= ge in error, please notify us immediately so that we may correct our interna= l records. Please then delete the original message. Thank you. --Apple-Mail-30C3D694-970D-4FDC-A596-5308EF519314 Content-Type: text/html; charset=utf-8 Content-Transfer-Encoding: quoted-printable
Hi Alex
Thanks, the whirlwin= d is ground adjustable

Cheers

Sent fr= om my iPad

On 26 Jul 2014, at 12:54 pm, "alex" <flyrotary@lancaironline.net> wrot= e:

Christian,

=

On Vetterman's exhaust webpage he does a comparison of the wh= irlwind and hartzell and flies both on his aircraft, how is the pitch contro= lled on the whirlwind, electric?

Alex

On Jul 26, 2014 6:17 AM, "Christamarmc" <flyrotary@lancaironline.net>= ; wrote:
Hi Charlie
That is excellent thanks, you ca= n't beat real works experience, I think when the prop I have was ordered was= more on what the manufacturer was thinking May suit, I'm a fair bit under t= he diameter that I can safely run, I'm currently only at 68" and can comfort= ably get 74' so I'll definitely think a larger is going to help, I've only b= een able to compare the same manufacturer that I'm running with the same air= frame but with lycoming and I'm currently running more pitch than a 200 hp r= v7 ,but getting slightly less results.

What is everyone's thoughts on ground adjustable, like t= he whirlwind ga 200 is what I'm looking at possibly, only comes in 72" diame= ter though, but obvious advantage is the adjustable pitch, 
I'm gathering there may be a reason why not many run these at this time= just wondering in real world performance.

Cheers
Christian



Sent from my iPad

On 26 Jul 2014, at 10:43 am, "Charlie England" <flyrotary@lancaironline.net= > wrote:

=20 =20 =20 =20
If you're picking the prop yourself, it really is trial and error (with either lots of luck or lots of error :-) ). Using an alternative engine that turns the prop at a non-typical rpm for the a/c you're building makes it even harder.

However, most of the better known prop makers can come really close if you can give them accurate info on which airframe we're building, the plane's cooling drag, max allowable prop diameter, honest HP estimate, and the prop rpm at that HP. I would think that the hardest thing for us to estimate is cooling drag, compared to a Lyc/Cont on the same airframe. I firmly believe that liquid cooling can have lower drag than air cooling, but I also believe that it rarely happens for us 'shade tree' cooling system designers, trying to graft a liquid system onto a plane designed for air cooling.

The Lyc guys are always asking the same kinds of questions about props, wanting to know what pitch to order. I believe that the best answer for a fixed pitch prop is to ask for the largest diameter that can be safely swung on the plane (usually somewhat larger than the conservative estimate by the designer), and then give the designer all the other info listed above. Then let them pitch and twist the prop as they see fit. The reason for asking for max diameter is for takeoff/low speed performance. With fixed (high) pitch for a high speed airframe, the best way to keep decent takeoff performance is extra diameter, which increases low speed 'mass flow' & helps acceleration. The extra diameter doesn't seem to hurt at high speed until you get up in the >200mph range.

I should say that I'm not an engineer or aero guy by training, or ability, for that matter. But I have played with fast homebuilts for over 20 years, and I've been through quite a few different props on several different planes. What I wrote above has been my experience, and matches the engineering articles I've read on the subject.

Hope this was useful....

Charlie


On 7/25/2014 6:50 PM, Christamarmc wrote:
=20

I understand a bit more about the prop design now, theoretically speaking it would be nice in a perfect world for all to use the same pitch measurement like inches from the hub etc, but I guess it's not a perfect ideah as there are many differing designs of blades and tips etc
Also we are never quite sure where the manufacturer has measured there pitch on all there differing props so I guess it is a starting point for which prop to start with and then trial and error after that.

Thanks all for your input
Cheers
Christian

Sent from my iPad

On 26 Jul 2014, at 5:48 am, "Jeff Whaley" <flyrotary@lancaironline.net&g= t; wrote:

=20 =20 =20 =20

We have had this discussion before =E2=80= =A6 I=E2=80=99ll go out on a limb here and agree with both James a= nd Charlie.  That is to say the prop pitch is not the same from one end to the other; therefore the =E2=80=9Ceffective pi= tch=E2=80=9D is an estimation; and that doing better than theoretical is simply an error in the pitch estimation.  It is impossible for any propeller to work at 100% or greater efficiency.  Considering Tracy=E2=80=99s example of 217 m= ph Vs 212 mph is an error of only 2.5% (not accounting for the takeoff and climb), possibly as much as 5-10% if takeoff and climb considered =E2=80=A6 is that too hard to believe?&nb= sp; You need a pretty slippery ship to get near the =E2=80=9Ceffective= pitch=E2=80=9D performance; here=E2=80=99s another example.&nb= sp; My Tri-Pacer has a 57=E2=80=9D pitch propeller that cruises at 2300 rpm =E2= =80=93 theoretically this is 124 mph, exactly what the owner=E2=80=99= s manual says; however, I can tell you that the airplane hasn=E2=80=99t seen 124 mph since the last time it was dove in= a loop-entry =E2=80=93 it cruises at 90 knots (104 mph) or 20% o= ff the theoretical prop pitch =E2=80=93 it is not a slippery ship= =E2=80=A6  Jeff

 

From:

"Tracy" <flyrotary@lancaironline.net><= u>

Subject:=

Re: [FlyRotary] Re: Rv7 renises p port

Date:

Thu, 24 Jul 2014 12:09:11 -0400=

To:<= u>

Rotary motors in aircraft <flyrotary@lancaironline.= net>

<image001.gif>

<image002.gif&g= t;

Charlie is right.  Consider that an airplane flying with the wing at 0 deg. AOA does not fall out of the sky :-)

 =

My average speed in the 2004 Sun 100 race was 217.58 mph which includes standing start takeoff and climbout.  Prop was a 74 x 88, RPM was 7250 wit= h a 2.85 : 1 drive.  If you calculate that out it= comes to 212 mph with zero 'slippage'.   Draw your own conclusions!

 =

Tracy

Sent from my iPad

 

From:

"Charlie England" <flyrotary@lancaironline.net><= span style=3D"font-size:12.0pt">

Subject:

Re: [FlyRotary] Re: Rv7 renises p port

Date:

Thu, 24 Jul 2014 17:49:27 -0500

To:

Rotary motors in aircraft <flyrotary@lancaironline.net&g= t;

<image001.gif>

<image002.gif>

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 th= e wind that is doing the powering.  While it is tru= e these types of craft can sail faster than the wind, but not while pointing straight into it!  B= y 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 y= our 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 boa= t 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 amoun= t of slip?  Using Bill's numbers 86 inch pitch, 2.8= 5 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.n= et]
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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