X-Virus-Scanned: clean according to Sophos on Logan.com From: "Jeff Whaley" Received: from hub026-NJ-6.exch026.serverdata.net ([206.225.166.169] verified) by logan.com (CommuniGate Pro SMTP 6.0.9e) with ESMTPS id 6999515 for flyrotary@lancaironline.net; Fri, 25 Jul 2014 15:48:45 -0400 Received-SPF: none receiver=logan.com; client-ip=206.225.166.169; envelope-from=jwhaley@datacast.com Received: from MBX026-E1-NJ-2.exch026.domain.local ([10.240.14.52]) by HUB026-NJ-6.exch026.domain.local ([10.240.14.230]) with mapi id 14.03.0174.001; Fri, 25 Jul 2014 12:48:10 -0700 To: Rotary motors in aircraft Subject: [FlyRotary] RE: Rv7 renises p port Thread-Topic: [FlyRotary] RE: Rv7 renises p port Thread-Index: Ac+oQVvDYJEQl5V2TM2WcrzzPny2ww== Date: Fri, 25 Jul 2014 19:48:09 +0000 Message-ID: <234B758DE9A64449BEB6BB5D1345CA7A361AC3B3@mbx026-e1-nj-2.exch026.domain.local> Accept-Language: en-CA, en-US Content-Language: en-US X-MS-Has-Attach: yes X-MS-TNEF-Correlator: x-originating-ip: [209.87.232.162] Content-Type: multipart/related; boundary="_005_234B758DE9A64449BEB6BB5D1345CA7A361AC3B3mbx026e1nj2exch_"; type="multipart/alternative" MIME-Version: 1.0 --_005_234B758DE9A64449BEB6BB5D1345CA7A361AC3B3mbx026e1nj2exch_ Content-Type: multipart/alternative; boundary="_000_234B758DE9A64449BEB6BB5D1345CA7A361AC3B3mbx026e1nj2exch_" --_000_234B758DE9A64449BEB6BB5D1345CA7A361AC3B3mbx026e1nj2exch_ Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: quoted-printable We have had this discussion before ... I'll go out on a limb here and agree= with both James and Charlie. That is to say the prop pitch is not the sam= e from one end to the other; therefore the "effective pitch" is an estimati= on; 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's example of 217 mph Vs 212 mph is an error = of only 2.5% (not accounting for the takeoff and climb), possibly as much a= s 5-10% if takeoff and climb considered ... is that too hard to believe? Y= ou need a pretty slippery ship to get near the "effective pitch" performanc= e; here's another example. My Tri-Pacer has a 57" pitch propeller that cru= ises at 2300 rpm - theoretically this is 124 mph, exactly what the owner's = manual says; however, I can tell you that the airplane hasn't seen 124 mph = since the last time it was dove in a loop-entry - it cruises at 90 knots (1= 04 mph) or 20% off the theoretical prop pitch - it is not a slippery ship .= .. 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 [Message Header] [Undecoded Message] 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 sta= nding start takeoff and climbout. Prop was a 74 x 88, RPM was 7250 with a = 2.85 : 1 drive. If you calculate that out it comes to 212 mph with zero 's= lippage'. 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 [Message Header] [Undecoded Message] 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 actual= ly 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 airfo= il (properly made, a *twisted* airfoil) that rotates. If you put the flat b= lades from a ceiling fan on the nose of an airplane, then 'pitch' might hav= e some actual meaning (but I don't think so). Consider that 'pitch' is usually measured somewhere between 2/3 & 3/4 out f= rom 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", be= gging 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 b= lade must be pitched to generate thrust at the relative speed it moves thro= ugh 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 w= ind for the prop is not aligned with the path of the plane. Depending on your religion, either Newton or Bernoulli makes the air go bac= k 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 sailb= oats 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 c= raft 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 so= me 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 th= rust (in cruise). In a descent, sure negative slippage is a fact. And sli= ppage has to increase greatly in a climb. I guess I am arguing that the on= ly 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 fa= ctors you outlined Charlie. Supposing that we have a typical non-symmetric blade cross section, an accu= rate pitch based on the chord line of the cross section, and I suppose a tw= ist that is correct for the cruise RPM. What then would be considered a go= od 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 effic= ient 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 f= aster than the wind. 'Negative slippage'. :-) A more significant point might be that pitch numbers are virtually meaningl= ess, unless you're comparing two props from the same prop maker with the sa= me blade plan form. Even then, it just tells you which has a finer pitch th= an the other. Variables can be: whether the pitch is measured on the back side of the bla= de or through the chord line, where along the diameter the pitch angle is m= easured (due to blade twist), and no doubt others I'm not smart enough to t= hink of at the moment. Bottom line is that unless there's an identical airframe flying an identica= l 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 differen= ce between actual distance traveled and theoretical distance traveled if yo= ur propeller corkscrewed through the air with no thrust. I found a prop sl= ip 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 considere= d 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 th= e 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 equate= s to allot more hp at takeoff, just shy of 2300 prop rpm, I'm running a princ= e 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 This message, and the documents attached hereto, is intended only for the a= ddressee and may contain privileged or confidential information. Any unauth= orized disclosure is strictly prohibited. If you have received this message= in error, please notify us immediately so that we may correct our internal= records. Please then delete the original message. Thank you. --_000_234B758DE9A64449BEB6BB5D1345CA7A361AC3B3mbx026e1nj2exch_ Content-Type: text/html; charset="us-ascii" Content-Transfer-Encoding: quoted-printable

We have had this discussion before … I’l= l go out on a limb here and agree with both James and Charlie.  That i= s to say the prop pitch is not the same from one end to the other; therefor= e the “effective pitch” is an estimation; and that doing better than theoretical is simply an error in the pitch estimation.&= nbsp; It is impossible for any propeller to work at 100% or greater efficie= ncy.  Considering Tracy’s example of 217 mph Vs 212 mph is an er= ror of only 2.5% (not accounting for the takeoff and climb), possibly as much as 5-10% if takeoff and climb considered R= 30; is that too hard to believe?  You need a pretty slippery ship to g= et near the “effective pitch” performance; here’s another= example.  My Tri-Pacer has a 57” pitch propeller that cruises at 2300 rpm – theoretically this is 124 mph, exactly what the owner&= #8217;s manual says; however, I can tell you that the airplane hasn’t= seen 124 mph since the last time it was dove in a loop-entry – it cr= uises at 90 knots (104 mph) or 20% off the theoretical prop pitch – it is not a slippery ship …  Jeff

 

From:

"Tracy" <flyrotary@lanc= aironline.net>

Subject:

Re: [FlyRotary] Re: Rv7 renises p po= rt

Date:

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

To:

Rotary motors in aircraft <flyrot= ary@lancaironline.net>

3D"Message

Charlie is right.  Consider tha= t an airplane flying with the wing at 0 deg. AOA does not fall out of the s= ky :-)

 

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

 

Tracy

Sent from my iPad

 

From:

"Charlie England" <flyrotary@lancaironl= ine.net>

Subject:=

Re: [FlyRotary] Re: Rv7 renises p port

Date:

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

To:

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

3D"Message

3D"Undecoded

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 ol= d, 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 airfo= il (properly made, a *twisted* airfoil) that rotates. If you put the flat b= lades from a ceiling fan on the nose of an airplane, then 'pitch' might hav= e some actual meaning (but I don't think so).

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 r= oot, you'd probably get something like 120-140 inches of pitch on a prop fo= r 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 'pitc= h'. It doesn't mean that almost half the prop is 'slipping' and almost to h= alf 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 mov= ing through the air. Just like a sailing vessel, it's not moving directly w= ith the wind; it's moving at an angle. The relative wind for the prop is no= t aligned with the path of the plane.

Depending on your religion, either Newton or Bernoulli makes the air go bac= k 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 s= lippage'. :-)

 

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 no= t while pointing straight into it!  By definition to cruise in an airplane, the prop has to be generating some kind of thrus= t 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 slippag= e in cruise is if either your blade cross section is asymmetrical (it usual= ly is right?) or if the pitch number used in the calculation is not really right based on the kinds of factors you o= utlined Charlie.

 

Supposing that we have a typical non-symmetric blade= cross section, an accurate pitch based on the chord line of the cross sect= ion, and I suppose a twist that is correct for the cruise RPM.  What t= hen would be considered a good or reasonable slippage in cruise?  I saw 3% thrown out there.  And if your pro= p selection is good for all those conditions (in other words as efficient a= s 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 cla= ss sailboats can sail faster than the wind. 'Negative slippage'. :-)

A more significant point might be that pitch numbers are virtually meaningl= ess, unless you're comparing two props from the same prop maker with the sa= me blade plan form. Even then, it just tells you which has a finer pitch th= an the other.

Variables can be: whether the pitch is measured on the back side of the bla= de or through the chord line, where along the diameter the pitch angle is m= easured (due to blade twist), and no doubt others I'm not smart enough to t= hink of at the moment.

Bottom line is that unless there's an identical airframe flying an identica= l 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 thr= ust.  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 t= hink it matters as long as the units are correct.  There has to be som= e slip because there would be no thrust otherwise.  So what is considered a reasonable or good amount of slip?  Usi= ng 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 <<= a href=3D"mailto:flyrotary@lancaironline.net" target=3D"_blank">flyrotary@l= ancaironline.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 th= e
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 equate= s
to allot more hp at takeoff, just shy of 2300 prop rpm, I'm running a princ= e
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<= br> mph on previous tests, . So next plan is bigger prop and less pitch to let<= br> it rev to 7500 in strait and level.

Cheers
Christian
Rv7 renises Aus


Sent from my iPad
--
Homepage:  htt= p://www.flyrotary.com/
Archive and UnSub:
http://mail.lancaironline.net:81/lists/flyrotary/List.html


--
Homepage:  htt= p://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 a= ddressee and may contain privileged or confidential information. Any unauth= orized disclosure is strictly prohibited. If you have received this message= in error, please notify us immediately so that we may correct our internal records. Please then delete the origin= al message. Thank you. --_000_234B758DE9A64449BEB6BB5D1345CA7A361AC3B3mbx026e1nj2exch_-- --_005_234B758DE9A64449BEB6BB5D1345CA7A361AC3B3mbx026e1nj2exch_ Content-Type: image/gif; name="image001.gif" Content-Description: image001.gif Content-Disposition: inline; filename="image001.gif"; size=849; creation-date="Fri, 25 Jul 2014 19:48:09 GMT"; modification-date="Fri, 25 Jul 2014 19:48:09 GMT" Content-ID: Content-Transfer-Encoding: base64 R0lGODdhCgANAPcAAAAAAIAAAACAAICAAAAAgIAAgACAgMDAwMDcwKbK8AAAAAAAKgAAVQAAfwAA qgAA1AAqAAAqKgAqVQAqfwAqqgAq1ABVAABVKgBVVQBVfwBVqgBV1AB/AAB/KgB/VQB/fwB/qgB/ 1ACqAACqKgCqVQCqfwCqqgCq1ADUAADUKgDUVQDUfwDUqgDU1CoAACoAKioAVSoAfyoAqioA1Coq ACoqKioqVSoqfyoqqioq1CpVACpVKipVVSpVfypVqipV1Cp/ACp/Kip/VSp/fyp/qip/1CqqACqq KiqqVSqqfyqqqiqq1CrUACrUKirUVSrUfyrUqirU1FUAAFUAKlUAVVUAf1UAqlUA1FUqAFUqKlUq VVUqf1UqqlUq1FVVAFVVKlVVVVVVf1VVqlVV1FV/AFV/KlV/VVV/f1V/qlV/1FWqAFWqKlWqVVWq f1WqqlWq1FXUAFXUKlXUVVXUf1XUqlXU1H8AAH8AKn8AVX8Af38Aqn8A1H8qAH8qKn8qVX8qf38q qn8q1H9VAH9VKn9VVX9Vf39Vqn9V1H9/AH9/Kn9/VX9/f39/qn9/1H+qAH+qKn+qVX+qf3+qqn+q 1H/UAH/UKn/UVX/Uf3/Uqn/U1KoAAKoAKqoAVaoAf6oAqqoA1KoqAKoqKqoqVaoqf6oqqqoq1KpV AKpVKqpVVapVf6pVqqpV1Kp/AKp/Kqp/Vap/f6p/qqp/1KqqAKqqKqqqVaqqf6qqqqqq1KrUAKrU KqrUVarUf6rUqqrU1NQAANQAKtQAVdQAf9QAqtQA1NQqANQqKtQqVdQqf9QqqtQq1NRVANRVKtRV VdRVf9RVqtRV1NR/ANR/KtR/VdR/f9R/qtR/1NSqANSqKtSqVdSqf9SqqtSq1NTUANTUKtTUVdTU f9TUqtTU1AAAAAwMDBkZGSYmJjMzMz8/P0xMTFlZWWZmZnJycn9/f4yMjJmZmaWlpbKysr+/v8zM zNjY2OXl5fLy8v/78KCgpICAgP8AAAD/AP//AAAA//8A/wD//////ywAAAAACgANAAAINgDxCRw4 EAC+fwgTIjT4DwCAAwIXHlSYkKHDhxEZUpTYEONEjRstOoR4ECRFkSQ5bux4saXDgAA7 --_005_234B758DE9A64449BEB6BB5D1345CA7A361AC3B3mbx026e1nj2exch_ Content-Type: image/gif; name="image002.gif" Content-Description: image002.gif Content-Disposition: inline; filename="image002.gif"; size=857; creation-date="Fri, 25 Jul 2014 19:48:09 GMT"; modification-date="Fri, 25 Jul 2014 19:48:09 GMT" Content-ID: Content-Transfer-Encoding: base64 R0lGODdhCgANAPcAAAAAAIAAAACAAICAAAAAgIAAgACAgMDAwMDcwKbK8AAAAAAAKgAAVQAAfwAA qgAA1AAqAAAqKgAqVQAqfwAqqgAq1ABVAABVKgBVVQBVfwBVqgBV1AB/AAB/KgB/VQB/fwB/qgB/ 1ACqAACqKgCqVQCqfwCqqgCq1ADUAADUKgDUVQDUfwDUqgDU1CoAACoAKioAVSoAfyoAqioA1Coq ACoqKioqVSoqfyoqqioq1CpVACpVKipVVSpVfypVqipV1Cp/ACp/Kip/VSp/fyp/qip/1CqqACqq KiqqVSqqfyqqqiqq1CrUACrUKirUVSrUfyrUqirU1FUAAFUAKlUAVVUAf1UAqlUA1FUqAFUqKlUq VVUqf1UqqlUq1FVVAFVVKlVVVVVVf1VVqlVV1FV/AFV/KlV/VVV/f1V/qlV/1FWqAFWqKlWqVVWq f1WqqlWq1FXUAFXUKlXUVVXUf1XUqlXU1H8AAH8AKn8AVX8Af38Aqn8A1H8qAH8qKn8qVX8qf38q qn8q1H9VAH9VKn9VVX9Vf39Vqn9V1H9/AH9/Kn9/VX9/f39/qn9/1H+qAH+qKn+qVX+qf3+qqn+q 1H/UAH/UKn/UVX/Uf3/Uqn/U1KoAAKoAKqoAVaoAf6oAqqoA1KoqAKoqKqoqVaoqf6oqqqoq1KpV AKpVKqpVVapVf6pVqqpV1Kp/AKp/Kqp/Vap/f6p/qqp/1KqqAKqqKqqqVaqqf6qqqqqq1KrUAKrU KqrUVarUf6rUqqrU1NQAANQAKtQAVdQAf9QAqtQA1NQqANQqKtQqVdQqf9QqqtQq1NRVANRVKtRV VdRVf9RVqtRV1NR/ANR/KtR/VdR/f9R/qtR/1NSqANSqKtSqVdSqf9SqqtSq1NTUANTUKtTUVdTU f9TUqtTU1AAAAAwMDBkZGSYmJjMzMz8/P0xMTFlZWWZmZnJycn9/f4yMjJmZmaWlpbKysr+/v8zM zNjY2OXl5fLy8v/78KCgpICAgP8AAAD/AP//AAAA//8A/wD//////ywAAAAACgANAAAIPgDxCRw4 EAC+fwcQKjxgMCHDhAD+GUQYEcDDhhIzVjwIkSJDfA8zQjxYUWRDixVHmnSI0aHCiS5ZAphJs2ZA ADs= --_005_234B758DE9A64449BEB6BB5D1345CA7A361AC3B3mbx026e1nj2exch_--