X-Virus-Scanned: clean according to Sophos on Logan.com From: "alex" Received: from mail-qa0-f42.google.com ([209.85.216.42] verified) by logan.com (CommuniGate Pro SMTP 6.0.9e) with ESMTPS id 7000305 for flyrotary@lancaironline.net; Sat, 26 Jul 2014 09:00:00 -0400 Received-SPF: pass receiver=logan.com; client-ip=209.85.216.42; envelope-from=alex.molteno@gmail.com Received: by mail-qa0-f42.google.com with SMTP id j15so5722806qaq.15 for ; Sat, 26 Jul 2014 05:59:25 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=mime-version:in-reply-to:references:date:message-id:subject:from:to :content-type; bh=9X9RSw++gw5KaWgtF+Hhdo3W/kMXVAJ2bi0fij3gtIs=; b=kRmKCdtzNkdfMBuDACEi5PA4T1dAALSXKkIvwRZXLDiAMFvlCXeu4J/YoX2tT7+33H xJuuR+ysucQaALnq+T1y1DtrcRfiYhm11d8AbqLYBEcmPvfbsKotFg5oK3B/rUiry8Oz Y0BGMvqFjcFqw/UT0MinKdDN2U8tPkz2+QHcOeyw66F2sTOBN7rNoItlIhfu9unuJEUY b6BHEya0N9JJldzqLUWF/EATLF7TUahPKSKcE1puldjmNi9++EHuFltcCBpO5ecsmUtc 1LOEbmKgVPKw4Xlg/p3pm9WaVzkOsdeEKKWpqQWP4pN5IAG0uYzqFaWMUD1g5zyBJuW7 TrOg== MIME-Version: 1.0 X-Received: by 10.224.89.10 with SMTP id c10mr38851391qam.51.1406379565120; Sat, 26 Jul 2014 05:59:25 -0700 (PDT) Received: by 10.96.206.194 with HTTP; Sat, 26 Jul 2014 05:59:24 -0700 (PDT) Received: by 10.96.206.194 with HTTP; Sat, 26 Jul 2014 05:59:24 -0700 (PDT) In-Reply-To: References: Date: Sat, 26 Jul 2014 16:59:24 +0400 Message-ID: Subject: Re: [FlyRotary] Re: Rv7 renises p port To: Rotary motors in aircraft Content-Type: multipart/alternative; boundary=001a11c3bf0066065604ff18423e --001a11c3bf0066065604ff18423e Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Hi Christian, do you have any info related to the whirlwind on vibration limits and harmonics on the engine/prop combination using a rotary instead of a lycoming. I know the whirlwind has rpm operating limits for a Lycoming but I'm not sure how that translates to a rotary, also any thoughts on how that harmonic might change with 1 rotor possibly misfiring Just curiosity,Cheers, Alex On Jul 26, 2014 9:37 AM, "Christamarmc" wrote= : > Hi Alex > Thanks, the whirlwind is ground adjustable > > Cheers > > Sent from my iPad > > On 26 Jul 2014, at 12:54 pm, "alex" wrote: > > Christian, > > On Vetterman's exhaust webpage he does a comparison of the whirlwind and > hartzell and flies both on his aircraft, how is the pitch controlled on t= he > whirlwind, electric? > > Alex > On Jul 26, 2014 6:17 AM, "Christamarmc" > wrote: > >> Hi Charlie >> That is excellent thanks, you can'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 the diameter that I can safely r= un, >> I'm currently only at 68" and can comfortably get 74' so I'll definitely >> think a larger 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 a= nd >> I'm currently running more pitch than a 200 hp rv7 ,but getting slightly >> less results. >> >> What is everyone's thoughts on ground adjustable, like the whirlwind ga >> 200 is what I'm looking at possibly, only comes in 72" diameter 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: >> >> 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 mak= es >> 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, a= nd >> 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 estima= te >> 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 reaso= n >> 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 decen= t >> takeoff performance is extra diameter, which increases low speed 'mass >> flow' & helps acceleration. The extra diameter doesn't seem to hurt at h= igh >> 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 ove= r >> 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: >> >> >> 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 pit= ch >> 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" >> wrote: >> >> We have had this discussion before =E2=80=A6 I=E2=80=99ll go out on a = limb here and >> agree with both James and Charlie. That is to say the prop pitch is not >> the same from one end to the other; therefore the =E2=80=9Ceffective pit= ch=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 mph V= s 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 t= hat too >> hard to believe? You need a pretty slippery ship to get near the >> =E2=80=9Ceffective pitch=E2=80=9D performance; here=E2=80=99s another ex= ample. My Tri-Pacer has a >> 57=E2=80=9D pitch propeller that cruises at 2300 rpm =E2=80=93 theoretic= ally this is 124 >> mph, exactly what the owner=E2=80=99s manual says; however, I can tell y= ou 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% off the the= oretical >> prop pitch =E2=80=93 it is not a slippery ship =E2=80=A6 Jeff >> >> >> >> From: >> >> "Tracy" >> >> Subject: >> >> Re: [FlyRotary] Re: Rv7 renises p port >> >> Date: >> >> Thu, 24 Jul 2014 12:09:11 -0400 >> >> To: >> >> Rotary motors in aircraft >> >> >> >> >> >> >> >> 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 w= ith >> a 2.85 : 1 drive. If you calculate that out it comes to 212 mph with ze= ro >> 'slippage'. Draw your own conclusions! >> >> >> >> Tracy >> >> Sent from my iPad >> >> >> >> From: >> >> "Charlie England" >> >> Subject: >> >> Re: [FlyRotary] Re: Rv7 renises p port >> >> Date: >> >> Thu, 24 Jul 2014 17:49:27 -0500 >> >> To: >> >> Rotary motors in aircraft >> >> >> >> >> >> >> >> 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 pr= op >> 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 th= e >> 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 ou= t >> 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 pro= p >> for RV's or EZ's, etc, and at the tip it would probably be something lik= e >> 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 hal= f >> 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 'stati= on' >> of the prop blade must be pitched to generate thrust at the relative spe= ed >> it moves through the air. The airfoil (BTW, I've never seen a symmetrica= l >> prop airfoil) is, or at least *can*, make the air move aft faster than t= he >> '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 a= n >> angle. The relative wind for the prop is not aligned with the path of th= e >> 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 int= o >> it! By definition to cruise in an airplane, the prop has to be generati= ng >> 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. An= d >> 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 th= e >> pitch number used in the calculation is not really right based on the ki= nds >> 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 suppo= se >> a twist that is correct for the cruise RPM. What then would be consider= ed >> a good or reasonable slippage in cruise? I saw 3% thrown out there. An= d >> 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 sai= l >> 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 angl= e >> is measured (due to blade twist), and no doubt others I'm not smart enou= gh >> 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 rat= io, >> 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 correc= t. >> 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 a= t >> 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 l= et >> 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 >> >> >> This message, and the documents attached hereto, is intended only for >> the addressee and may contain privileged or confidential information. An= y >> unauthorized disclosure is strictly prohibited. If you have received thi= s >> message in error, please notify us immediately so that we may correct ou= r >> internal records. Please then delete the original message. Thank you. >> >> >> --001a11c3bf0066065604ff18423e Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable

Hi Christian, do you have any info related to the whirlwind = on vibration limits and harmonics on the engine/prop combination using a ro= tary instead of a lycoming. I know the whirlwind has rpm operating limits f= or a Lycoming but I'm not sure how that translates to a rotary, also an= y thoughts on how that harmonic might change with 1 rotor possibly misfirin= g

Just curiosity,Cheers,

Alex

On Jul 26, 2014 9:37 AM, "Christamarmc"= ; <flyrotary@lancaironlin= e.net> wrote:
Hi Alex
Thanks, the whirlwind is ground ad= justable

Cheers

Sent from my iPad

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

Christian,

On Vetterman's exhaust webpage he does a comparison of t= he whirlwind and hartzell and flies both on his aircraft, how is the pitch = controlled on the whirlwind, electric?

Alex

On Jul 26, 2014 6:17 AM, "Christamarmc"= ; <flyr= otary@lancaironline.net> wrote:
Hi Charlie
That is excellent thanks, you c= an't beat real works experience, I think when the prop I have was order= ed was more on what the manufacturer was thinking May suit, I'm a fair = bit under the diameter that I can safely run, I'm currently only at 68&= quot; and can comfortably get 74' so I'll definitely think a larger= 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 r= esults.

What is everyone's thoughts on ground adjustable, l= ike the whirlwind ga 200 is what I'm looking at possibly, only comes in= 72" diameter though, but obvious advantage is the adjustable pitch,= =C2=A0
I'm gathering there may be a reason why not many run these at this= time just wondering in real world performance.

Ch= eers
Christian



Sent from my iPad=

On 26 Jul 2014, at 10:43 am, "Charlie England" <flyrotary@lanca= ironline.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 sy= stem 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 diamete= r 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@lancaironli= ne.net> 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 = and Charlie.=C2=A0 That is to say the prop pitch is not the same from one end to the other; therefore the =E2=80=9Ceffective p= itch=E2=80=9D is an estimation; and that doing better than theoretical is simply an error in the pitch estimation.=C2=A0 It is impossible for any propeller to work at 100% or greater efficiency.=C2=A0 Considering Tracy=E2=80=99s example of 217 = mph 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?= =C2=A0 You need a pretty slippery ship to get near the =E2=80=9Ceffectiv= e pitch=E2=80=9D performance; here=E2=80=99s another example.= =C2=A0 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= =99s manual says; however, I can tell you that the airplane hasn=E2=80=99t seen 124 mph since the last time it was dove i= n a loop-entry =E2=80=93 it cruises at 90 knots (104 mph) or 20% = off the theoretical prop pitch =E2=80=93 it is not a slippery shi= p =E2=80=A6=C2=A0 Jeff

=C2=A0

From:

"Trac= y" <flyrotary@lancaironline.net>

Subject:

Re: [FlyRotary] Re: Rv7 renises p port

Date:

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

To:=

Rotary motors in aircraft <flyrotary@lancaironlin= e.net>

<image001.gif><= span>

<image002.gif= >

Charlie is right. =C2=A0Consider that an airplane flying with the wing at 0 deg. AOA does not fall out of the sky :-)

=C2=A0

My average speed in the 2004 Sun 100 race was 217.58 mph which includes standing start takeoff and climbout. =C2=A0Prop was a 74 x 88, RPM was 7250 wi= th a 2.85 : 1 drive. =C2=A0If you calculate that out i= t comes to 212 mph with zero 'slippage'. =C2= =A0 Draw your own conclusions!

=C2=A0

Tracy

Sent from my iPad

=C2=A0

From:

"Charlie England" <flyrotary@lancaironline.net>

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= >

<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 actuall= y works.

(insert pause here to parse that awful sentence....)

'Pitch' implies that a prop is a screw. A pro= p 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 woul= d probably be something like 60", begging the question, which part of the prop is actually screwing through the air at a particular 'pitch&#= 39;. It doesn't mean that almost half the prop is 'slipping' and almost to half the prop is act= ually dragging, with a couple of inches moving the plane. It just means that each 'station' of t= he 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 s= crew 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 th= e 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:<= /u>

Charlie said: =C2=A0Ice sails, desert sails, and now, even unlimited class sailboats can sail faster than the wind. 'Negativ= e slippage'. :-)

=C2=A0

I don't think the analogy qu= ite applies here. =C2=A0For those types of crafts it is t= he wind that is doing the powering. =C2=A0While it is tr= ue these types of craft can sail faster than the wind, but not while pointing straight into it! =C2=A0= 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). =C2=A0In= a descent, sure negative slippage is a fact. =C2=A0And slippage has to increase greatly in a climb. =C2=A0I 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.<= /p>

=C2=A0

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. =C2=A0What then would be considered a good or reasonable slippage in cruise? =C2=A0I saw 3% thrown out there. =C2=A0And if= your prop selection is good for all those conditions (in other words as efficient as possible), is this the slippage you expect? =C2=A0I am just wondering if you can use a slippage calculation to judge efficiency (roughly).

=C2=A0

-= - 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. =C2=A0I found a prop slip calculator online and for 86 inch pitch, 2.85 gear ratio, 7000 rpm, 180 mph, I get 10% slip. =C2=A0Granted the calculator was for bo= at propellers, but I don't think it matters as long as the units are correct. =C2=A0There has to be some slip because there would be no thrust otherwise. =C2=A0So what is considered a reasonable or good amou= nt of slip? =C2=A0Using Bill's numbers 86 inch pitch= , 2.85 gear ratio, 7000 rpm, 200 mph, I get 0% slip. =C2=A0That can't be right!

<= u>=C2=A0

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. =C2=A0I 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: =C2=A0http://www.flyrotary.com/
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