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mcjon77

Joined: 17 May 2008
Posts: 55
Location: Chicago

Posted: Tue Oct 21, 2008 3:58 pm Post subject: Which Of Two STOL CH7XX Building Choices?

Hi guys,



I was planing on building a 701 after my Sonex is completed (probably a good year and a half away).  The engine I was planing on putting in it is the 2300cc VW with Redrive from Great Plains/Valley Engineering.  it pumps out 105 hp and swings a pretty big prop.  I have heard many good reports from this combination.  I was planing on increasing the gross to 1200 lbs to give me close to 600 lbs useful load.  (not the best idea going over recommended gross, but many have done it with the 701 successfully).



Here is the thing.  I'm a big guy and while I fit in the 701, the 750 has MUCH more room.  So that got me thinking of going with the 750/Corvair combination.  Simple enough, but the useful load drops to 545 (if I am lucky and build light).  So that got me thinking ab out using the same GPAS/Valley Engineering redrive combo for the 750, since it pumps out more than the 100 hp of the Continental.  It seems to be a pretty decent direct replacement for the 912ULS (like the Corvair is for the 0-200).  I figure that I would get at least 40 lbs useful load from it.  I am not concerned if my cruise drops a little, if I want to go fast I will use the sonex.



What do you guys think?  Would love to hear opinions on this matter.



Jon

Joe Kidd

Joined: 01 Aug 2008
Posts: 15
Location: TN

Posted: Tue Oct 21, 2008 5:47 pm Post subject: Which Of Two STOL CH7XX Building Choices?

I¢d reconsider using the 2300 VW w/Redrive package you¢ve mentioned due to cylinder head temp and service life issues. The 105 HP you¢ve mentioned is at take off only, you then drop back to a significantly less HP level at cruise. Using added power to climb or maintain a higher cruise speed or when at or near GW will result in a significant thermal heat transfer to the cylinder heads that they are not designed to carry. This of course will shorten TBO or more to the point require you to replace cylinder heads very rapidly.  

<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />  

While I think this engine package is a doable one in the 701 as long as you know how to take care of it I don¢t think it¢s a viable package for the 750. I¢d be more inclined to use a Corvair, C-90 or 0200 in either of these aircraft instead of the 2300 VW w/Redrive. Hope I haven¢t offended you, just my .02 worth after a lot of research. FWIW I am a tremendous fan and advocate of the 2180 and 2300 VW aircraft engines and plan on using a 2180 on a build.  

Joe S. 

  

Do not archive please 



 

   From: mcjon77 <mcjon77(at)yahoo.com>

To: zenith-list(at)matronics.com

Sent: Tuesday, October 21, 2008 6:58:41 PM

Subject: Which Of Two STOL CH7XX Building Choices?



--> Zenith-List message posted by: "mcjon77" <mcjon77(at)yahoo.com (mcjon77(at)yahoo.com)>



Hi guys,



I was planing on building a 701 after my Sonex is completed (probably a good year and a half away).  The engine I was planing on putting in it is the 2300cc VW with Redrive from Great Plains/Valley Engineering.  it pumps out 105 hp and swings a pretty big prop.  I have heard many good reports from this combination.  I was planing on increasing the gross to 1200 lbs to give me close to 600  lbs useful load.  (not the best idea going over recommended gross, but many have done it with the 701 successfully).



Here is the thing.  I'm a big guy and while I fit in the 701, the 750 has MUCH more room.  So that got me thinking of going with the 750/Corvair combination.  Simple enough, but the useful load drops to 545 (if I am lucky and build light).  So that got me thinking ab out using the same GPAS/Valley Engineering redrive combo for the 750, since it pumps out more than the 100 hp of the Continental.  It seems to be a pretty decent direct replacement for the 912ULS (like the Corvair is for the 0-200).  I figure that I would get at least 40 lbs useful load from it.  I am not concerned if my cruise drops a little, if I want to go fast I will use the sonex.



What do you guys think?  Would love to hear opinions on this matter.



Jon





Read this topic online  here:



http://forums.matronics.com/viewtopic.php?p=20igator?Zenith-List" target=_blank>http://www.matronics.com/Navigator?Zeni="http://forums.matronics.com/" =                    &n-> htt



         [quote][b]

hills(at)sunflower.com
Guest

Posted: Tue Oct 21, 2008 6:03 pm Post subject: Which Of Two STOL CH7XX Building Choices?

Jon;  

   

Ya, Joes right, I have trouble just cooling a 2180 cc engine, the 105HP is only available for a few minutes. But…..  

Why not use the Great  Plains 2300 CC VW engine with aluminum cylinders? It puts out 85 HP and weighs a lot less than a corvair engine (about 60 pounds) and you can  

get true duel ignition system (not available on corvairs).    

   

Roger  

   

         

  

From: owner-zenith-list-server(at)matronics.com [mailto:owner-zenith-list-server(at)matronics.com] On Behalf Of Joe Stevenson

 Sent: Tuesday, October 21, 2008 8:46 PM

 To: zenith-list(at)matronics.com

 Subject: Re: Which Of Two STOL CH7XX Building Choices?  

  

       

I¢d reconsider using the 2300 VW w/Redrive package you¢ve mentioned due to cylinder head temp and service life issues. The 105 HP you¢ve mentioned is at take off only, you then drop back to a significantly less HP level at cruise. Using added power to climb or maintain a higher cruise speed or when at or near GW will result in a significant thermal heat transfer to the cylinder heads that they are not designed to carry. This of course will shorten TBO or more to the point require you to replace cylinder heads very rapidly.   

   

While I think this engine package is a doable one in the 701 as long as you know how to take care of it I don¢t think it¢s a viable package for the 750. I¢d be more inclined to use a Corvair, C-90 or 0200 in either of these aircraft instead of the 2300 VW w/Redrive. Hope I haven¢t offended you, just my .02 worth after a lot of research. FWIW I am a tremendous fan and advocate of the 2180 and 2300 VW aircraft engines and plan on using a 2180 on a build.   

Joe S.  

   

Do not archive please   

   

    

         

  

From: mcjon77 <mcjon77(at)yahoo.com>

 To: zenith-list(at)matronics.com

 Sent: Tuesday, October 21, 2008 6:58:41 PM

 Subject: Which Of Two STOL CH7XX Building Choices?

 

 --> Zenith-List message posted by: "mcjon77" <mcjon77(at)yahoo.com (mcjon77(at)yahoo.com)>

 

 Hi guys,

 

 I was planing on building a 701 after my Sonex is completed (probably a good year and a half away).  The engine I was planing on putting in it is the 2300cc VW with Redrive from Great Plains/Valley Engineering.  it pumps out 105 hp and swings a pretty big prop.  I have heard many good reports from this combination.  I was planing on increasing the gross to 1200 lbs to give me close to 600 lbs useful load.  (not the best idea going over recommended gross, but many have done it with the 701 successfully).

 

 Here is the thing.  I'm a big guy and while I fit in the 701, the 750 has MUCH more room.  So that got me thinking of going with the 750/Corvair combination.  Simple enough, but the useful load drops to 545 (if I am lucky and build light).  So that got me thinking ab out using the same GPAS/Valley Engineering redrive combo for the 750, since it pumps out more than the 100 hp of the Continental.  It seems to be a pretty decent direct replacement for the 912ULS (like the Corvair is for the 0-200).  I figure that I would get at least 40 lbs useful load from it.  I am not concerned if my cruise drops a little, if I want to go fast I will use the sonex.

 

 What do you guys think?  Would love to hear opinions on this matter.

 

 Jon

 

 

 

 

 Read this topic online here:

 

 http://forums.matronics.com/viewtopic.php?p=20igator?Zenith-List" target=_blank>http://www.matronics.com/Navigator?Zeni="http://forums.matronics.com/" =                    &n-> htt

 

 

 

 

   

  

  

  



 

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        [quote][b]

mcjon77

Joined: 17 May 2008
Posts: 55
Location: Chicago

Posted: Tue Oct 21, 2008 7:21 pm Post subject: Re: Which Of Two STOL CH7XX Building Choices?

Great info guys.  Roger, you have a 2180/Valley Engineering Redrive?  What type of aircraft do you have it in?  Can you give more detail about your experience?



The only problem with a direct drive 2300cc vw from great plains is the prop length.  With its high RPM, I doubt I will be able to swing a big enough prop on that engine to take advantage of the CH701's STOL capabilities.  I plan on using a 2180cc on my sonex and I was hoping to use another VW in the 701.



 	  hills(at)sunflower.com wrote: 	
		  Jon;  

   

Ya, Joes right, I have trouble just cooling a 2180 cc engine, the 105HP is only available for a few minutes. Butâ€¦..  

Why not use the Great  Plains 2300 CC VW engine with aluminum cylinders? It puts out 85 HP and weighs a lot less than a corvair engine (about 60 pounds) and you can  

get true duel ignition system (not available on corvairs).ï¿½   

   

Roger

hills(at)sunflower.com
Guest

Posted: Tue Oct 21, 2008 10:01 pm Post subject: Which Of Two STOL CH7XX Building Choices?

Jon;

Na, I have a water cooled Great plains VW now, it's 100 HP but the heads

are no longer made, so that options off the board for you. I got my redrive data from Great plains VW, they sell the redrives, but warn against continues use at high power,  thus the 5 minutes estimate (from there literature). There just is not enough cooling fin area on the engine to 

get rid of the waste heat generated about 80 HP, without water cooling anyway.



The 701's gross weight does not justify an engine over 65 HP really, any

more than that is just an increase in climb, and a bit more cruise speed, 

unless you plan to put it on floats of course.



A little HP is good, but a lot is not necessarily very good.



Roger





--

mcjon77

Joined: 17 May 2008
Posts: 55
Location: Chicago

Posted: Wed Oct 22, 2008 12:29 am Post subject: Re: Which Of Two STOL CH7XX Building Choices?

This really is fascinating.  I never noticed it before, but the continuous hp of the 2300 redrive is the same as the 2300 direct drive.  So, basically, the redrive at best gives a quick 20-25hp boost for a few minutes, but after that I am flying with the same hp that I would have with a direct drive engine of the same size, I am just able to swing a bigger prop.



So Joe, when you warned about using the  max hp for redrive VW at gross continuously (how I would wind up burning through cylinder heads quickly) it is really no different than running the 2180cc direct drive VW that I plan on putting in my sonex at the same abnormally high hp.  It will cause additional stress on the engine and create early wear.  I always wondered how a redrive could just magically create additional hp, seemed like getting something for nothing, now I (hopefully) have a better understanding.



Roger, I agree about not needing more than 65hp.  IIRC, for several years the factory demo CH701 was run on a 2 stroke rotax 582 and did just fine.  The redrive may still be a good option if only by letting me use a larger prop.  Both the 701 and my future aerovee (2180 VW)powered sonex have the same gross, so there shouldn't be much of a problem staying in the air with only 70hp continuous.   good info.  It definitely answers some questions.



Jon

bryanmmartin

Joined: 10 Jan 2006
Posts: 1018

Posted: Wed Oct 22, 2008 7:44 am Post subject: Which Of Two STOL CH7XX Building Choices?

The maximum continuous power an engine can deliver depends on the  

engine itself and its cooling system, the presence or absence of a  

redrive is irrelevant. Whether or not it can actually be operated at  

that power level MAY depend on whether or not it has a redrive.



A redrive doesn't allow any engine to produce more power at a  

particular RPM and torque, but you may not be able to operate a direct  

drive engine at a that RPM because the propeller would have to be too  

small to be practical. A redrive may allow you to use more of the  

engine's available power by running it at the higher RPM while turning  

a suitable prop at a lower RPM. Or a redrive can allow you to use a  

larger, more efficient prop at the same engine RPM and thereby get  

more thrust at the same power setting.



The main effect of a redrive is to give you more thrust, not more  

power; and thrust is, ultimately, what drives the airplane through the  

air.



On Oct 22, 2008, at 4:29 AM, mcjon77 wrote:



 	  Quote: 	
		   



 This really is fascinating.  I never noticed it before, but the  

 continuous hp of the 2300 redrive is the same as the 2300 direct  

 drive.  So, basically, the redrive at best gives a quick 20-25hp  

 boost for a few minutes, but after that I am flying with the same hp  

 that I would have with a direct drive engine of the same size, I am  

 just able to swing a bigger prop.



 I always wondered how a redrive could just magically create  

 additional hp, seemed like getting something for nothing, now I  

 (hopefully) have a better understanding.




	




-- 

Bryan Martin

N61BM, CH 601 XL,

RAM Subaru, Stratus redrive.

_________________
--
Bryan Martin
N61BM, CH 601 XL, Stratus Subaru.
do not archive.

ideaz1(at)sbcglobal.net
Guest

Posted: Fri Oct 24, 2008 6:35 am Post subject: Which Of Two STOL CH7XX Building Choices?

The use of a re-drive to allow a longer prop at lower speeds is really 

critical not to get more horsepower but to use what you have to the best 

efficiency. This is just a proven fact. If not, nobody  but nobody would 

bother dragging around all that extra hardware.

Z

pete(at)usjabiru.com
Guest

Posted: Fri Oct 24, 2008 7:53 am Post subject: Which Of Two STOL CH7XX Building Choices?

I don't think it is a fact when applied to a light aircraft like a 701.

We've just completed testing with our 85 hp Jabiru 2200 powered CH701 using

a 62 inch Sensenich ground adjustable prop and a Tennessee Props 62 inch

wood prop.  We are off the ground in less than 100 ft and climb at 800+ fpm.

No redrive is used and prop turns about 3000 rpm on takeoff and climb out.

At cruise we see 87 mph (verified by GPS several times) at normal cruise rpm

of 2950.



Maybe for big heavy slow aircraft a longer slower turning prop may be needed

but for a 701 it just doesn't make much difference.  Those who saw our 701

at the Zenith open house can attest that is does get off the ground in a

hurry and climbs very steeply at 50 mph.

Pete Krotje

Jabiru USA Sport Aircraft, LLC

931-680-2800

www.usjabiru.com



--

psm(at)att.net
Guest

Posted: Fri Oct 24, 2008 8:44 am Post subject: Which Of Two STOL CH7XX Building Choices?

Hi Guys,



I agree with both of you to some degree, but my reasons are different 

from the ones you wrote.



I think there are some limits on propeller length that must be considered.



First, if a propeller is too long the tips will hit the ground and 

the prop will be damaged or destroyed.  This gives a maximum prop 

length controlled by the airplane's landing gear and engine position.



On the other end, the portion of the propeller's rotation disk that 

is blocked by the fuselage doesn't provide much thrust.  For the kind 

of planes we are discussing this is perhaps the first 24 inches of 

prop diameter.



The third limiting factor is the speed of sound.  If the propeller 

tips go too fast (around .8 mach) then energy is wasted forming sonic 

booms and related things rather than producing thrust.  The prop tip 

speed is a direct function of the prop's length and the operating RPM.



This all leaves a rather narrow range for a propeller to use on any 

given airplane design.  The limit includes both the length and 

maximum propeller RPM.  If you choose an engine that produces its 

best power at a higher RPM then you need to install a PSRU to allow 

both the engine and propeller to operate efficiently.  The PSRU 

consumes a considerable amount of engine power and also introduces a 

significant new point of failure for the power plant.  You would only 

want one if needed to get sufficient power from the engine converted 

into thrust to make the whole plane work well.



Engines designed for light plane use will not need a PSRU.  They are 

designed to operate at appropriately low RPM.  Sometimes, engines 

designed for other purposes like running automobiles or snowmobiles 

will require a PSRU to be compatible with propellers.



Paul

XL getting close

At 08:52 AM 10/24/2008, you wrote:



[quote]



I don't think it is a fact when applied to a light aircraft like a 701.

We've just completed testing with our 85 hp Jabiru 2200 powered CH701 using

a 62 inch Sensenich ground adjustable prop and a Tennessee Props 62 inch

wood prop.  We are off the ground in less than 100 ft and climb at 800+ fpm.

No redrive is used and prop turns about 3000 rpm on takeoff and climb out.

At cruise we see 87 mph (verified by GPS several times) at normal cruise rpm

of 2950.



Maybe for big heavy slow aircraft a longer slower turning prop may be needed

but for a 701 it just doesn't make much difference.  Those who saw our 701

at the Zenith open house can attest that is does get off the ground in a

hurry and climbs very steeply at 50 mph.

Pete Krotje

Jabiru USA Sport Aircraft, LLC

931-680-2800

www.usjabiru.com



--

jimandmandy(at)yahoo.com
Guest

Posted: Fri Oct 24, 2008 11:24 am Post subject: Which Of Two STOL CH7XX Building Choices?

Do you happen to know what factors dictate number of blades? Ive seen some three bladed props on Rotax 912 installations, so it cant be hp alone. It made some sense to be when looking at large transport aircraft with reduction geared radials, where maximum diameter was limited by ground clearance. Two blade props were generally used under 1000hp, three up to about 2000hp and four blades above that.



--- On Fri, 10/24/08, Paul Mulwitz <psm(at)att.net> wrote:

[quote]From: Paul Mulwitz <psm(at)att.net>

Subject: RE: Re: Which Of Two STOL CH7XX Building Choices?

To: zenith-list(at)matronics.com

Date: Friday, October 24, 2008, 9:43 AM



[quote]--> Zenith-List message posted by: Paul Mulwitz <psm(at)att.net>



Hi  Guys,



I agree with both of you to some degree, but my reasons are different 

from the ones you wrote.



I think there are some limits on propeller length that must be considered.



First, if a propeller is too long the tips will hit the ground and 

the prop will be damaged or destroyed.  This gives a maximum prop 

length controlled by the airplane's landing gear and engine position.



On the other end, the portion of the propeller's rotation disk that 

is blocked by the fuselage doesn't provide much thrust.  For the kind 

of planes we are discussing this is perhaps the first 24 inches of 

prop diameter.



The third limiting factor is the speed of sound.  If the propeller 

tips go too fast (around .8 mach) then energy is wasted forming sonic 

booms and related things rather than producing thrust.  The prop tip 

speed is a direct function of the prop's length and the operating RPM.



This all  leaves a rather narrow range for a propeller to use on any 

given airplane design.  The limit includes both the length and 

maximum propeller RPM.  If you choose an engine that produces its 

best power at a higher RPM then you need to install a PSRU to allow 

both the engine and propeller to operate efficiently.  The PSRU 

consumes a considerable amount of engine power and also introduces a 

significant new point of failure for the power plant.  You would only 

want one if needed to get sufficient power from the engine converted 

into thrust to make the whole plane work well.



Engines designed for light plane use will not need a PSRU.  They are 

designed to operate at appropriately low RPM.  Sometimes, engines 

designed for other purposes like running automobiles or snowmobiles 

will require a PSRU to be compatible with propellers.



Paul

XL getting close

At 08:52 AM 10/24/2008, you  wrote:



 	  Quote: 	
		  --> Zenith-List message posted by: "Pete Krotje"

<pete(at)usjabiru.com>
	


 	  Quote: 	
		  

I don't think it is a fact when applied to a light aircraft like a 701.

We've just completed testing with our 85 hp Jabiru 2200 powered CH701

using
	


 	  Quote: 	
		  a 62 inch Sensenich ground adjustable prop and a Tennessee Props 62 inch

wood prop.  We are off the ground in less than 100 ft and climb at 800+

fpm.
	


 	  Quote: 	
		  No redrive is used and prop turns about 3000 rpm on takeoff and climb out.

At cruise we see 87 mph (verified by GPS several times) at normal cruise

rpm
	


 	  Quote: 	
		  of 2950.



Maybe for big heavy slow aircraft a longer slower turning prop may be

needed
	


 	  Quote: 	
		  but for a 701 it just doesn't make much difference.  Those who saw our

701
	


[quote]at the Zenith open house can attest that is does get off the ground in a

hurry and climbs very steeply at 50 mph.

Pete [quote][b]

Gig Giacona

Joined: 10 Jan 2006
Posts: 1416
Location: El Dorado Arkansas USA

Posted: Fri Oct 24, 2008 12:56 pm Post subject: Re: Which Of Two STOL CH7XX Building Choices?

Some of the 3-blade installations I've seen on small planes would have been better off with 2 blades but the builder wanted that faster LOOK that you get with more blades.



Remember all things being equal the more effeceint prop is the one with the fewer blades right down to 1.

 	  jimandmandy(at)yahoo.com wrote: 	
		  Do you happen to know what factors dictate number of blades? Ive seen some three bladed props on Rotax 912 installations, so it cant be hp alone. It made some sense to be when looking at large transport aircraft with reduction geared radials, where maximum diameter was limited by ground clearance. Two blade props were generally used under 1000hp, three up to about 2000hp and four blades above that.

_________________
W.R. "Gig" Giacona
601XL Under Construction
See my progress at www.peoamerica.net/N601WR

jaybannist(at)cs.com
Guest

Posted: Fri Oct 24, 2008 1:22 pm Post subject: Which Of Two STOL CH7XX Building Choices?

Yes, it has been proved that a single, counterbalanced blade is the most efficient.  It is simply not acceptable because of the LOOK.

 

 Jay in Dallas

 Do not archive

 

   

 

   

 

 --

craig(at)craigandjean.com
Guest

Posted: Fri Oct 24, 2008 1:28 pm Post subject: Which Of Two STOL CH7XX Building Choices?

 	  Quote: 	
		   62 inch Sensenich ground adjustable prop and a Tennessee Props 62 inch

wood prop
	




So which prop performed better? I know which one is less expensive.



-- Craig



--

bryanmmartin

Joined: 10 Jan 2006
Posts: 1018

Posted: Fri Oct 24, 2008 1:32 pm Post subject: Which Of Two STOL CH7XX Building Choices?

Efficiency, ground clearance, noise and power all factor in to the number of blades used on a prop.



The most efficient propellor is a single blade prop with a counter balance. This arrangement has actually been used on a few motor gliders but it isn't very practical for high power applications because of the unbalanced moment it applies to the end of the prop shaft. In general, the more blades a propellor has the more aerodynamic interference there is between the blades, which hurts efficiency.

The most common prop has two blades because this gives the highest efficiency for most applications. The more power your engine produces, the more power the propellor has to absorb. Three ways to increase the amount of power a prop can absorb are to increase the pitch, chord or length of the prop. There is a practical limit to how much you can do any combination of these things. You may run out of ground clearance or start getting too much sonic losses if the blade get too long, you may start to exceed the maximum angle of attack if the pitch gets too high, etc. At some point, the only way to absorb more power may be to start adding blades.

Another factor is noise, in at least one case I know of, the original two blade prop had such high tip speeds on take-off that it was extremely noisy, so a shorter, three blade prop was substituted to reduce the noise level. Another point to consider is that a three blade prop produces a higher pitched sound which generally dissipates more rapidly over distance than a lower pitched sound. You give up a little bit of efficiency but keep the neighbors happy.



On Oct 24, 2008, at 3:24 PM, Jimbo wrote:

 	  Quote: 	
		  Do you happen to know what factors dictate number of blades? Ive seen some three bladed props on Rotax 912 installations, so it cant be hp alone. It made some sense to be when looking at large transport aircraft with reduction geared radials, where maximum diameter was limited by ground clearance. Two blade props were generally used under 1000hp, three up to about 2000hp and four blades above that.



--- 	




 -- 

Bryan Martin

N61BM, CH 601 XL,

RAM Subaru, Stratus redrive.

do not archive.



 



 [quote][b]

_________________
--
Bryan Martin
N61BM, CH 601 XL, Stratus Subaru.
do not archive.

bryanmmartin

Joined: 10 Jan 2006
Posts: 1018

Posted: Fri Oct 24, 2008 1:36 pm Post subject: Which Of Two STOL CH7XX Building Choices?

Yes, looks play a factor also. The swept back vertical tails on many  

light aircraft actually hurt the performance of the airplane, but they  

look good, so that's why they're there.



On Oct 24, 2008, at 4:56 PM, Gig Giacona wrote:



 	  Quote: 	
		   



 Some of the 3-blade installations I've seen on small planes would  

 have been better off with 2 blades but the builder wanted that  

 faster LOOK that you get with more blades.



 Remember all things being equal the more effeceint prop is the one  

 with the fewer blades right down to 1.


	


-- 

Bryan Martin

N61BM, CH 601 XL,

RAM Subaru, Stratus redrive.

do not archive.

_________________
--
Bryan Martin
N61BM, CH 601 XL, Stratus Subaru.
do not archive.

pete(at)usjabiru.com
Guest

Posted: Fri Oct 24, 2008 1:46 pm Post subject: Which Of Two STOL CH7XX Building Choices?

Performance was pretty much the same.  We had the Sensenich pitched to give

about 80 rpm more than the TN Prop so it climbed just a bit faster but I was

quite happy with both.



Next week we will test the latest Sensenich wood design that was made

specifically for the 701.  Once that testing is complete we will offer that

701 for sale.

Pete Krotje

Jabiru USA Sport Aircraft, LLC

931-680-2800

www.usjabiru.com





--

kmccune

Joined: 22 Sep 2007
Posts: 577
Location: Wisconsin, USA

Posted: Fri Oct 24, 2008 2:25 pm Post subject: Re: Which Of Two STOL CH7XX Building Choices?

Don't forget that a redrive multiples torque to the prop. This does give a 80hp redrive an advantage over a 80 hp direct drive of the same engine in swinging a prop at the same RPM and engine torque. The redrive engine has a greater latitude of pitch because of the higher prop torque available, as well as bland length options.



And don't forget that the 912S is limited to 5 min on top too. 



Kevin

_________________
â€œAlways do what you are afraid to do.â€
R.W. Emerson (1803-1882)

"Real freedom is the sustained act of being an individual." WW - 2009

"Life is a good deal...it's worth it" Feb 1969
Dorothy McCune

psm(at)att.net
Guest

Posted: Fri Oct 24, 2008 3:26 pm Post subject: Which Of Two STOL CH7XX Building Choices?

Hi Jimbo,



 I'm just guessing, but I think you have the most important issue when you mention power.  There is some limit to the amount of power, i.e. thrust, you can get from one blade.  Adding blades makes sense when your power is more than a two blade prop can handle.



 One other issue that applies to our size planes.  The ultralight community led to a lot of products for low power engines.  Perhaps some of them exist simply because people with money thought they were cool.  This may well explain why there are so many fancy multi-blade propellers available for engines up to 100 HP.



 Paul

 XL getting close



 

 At 12:24 PM 10/24/2008, you wrote:



 [quote]Do you happen to know what factors dictate number of blades? Ive seen some three bladed props on Rotax 912 installations, so it cant be hp alone. It made some sense to be when looking at large transport aircraft with reduction geared radials, where maximum diameter was limited by ground clearance. Two blade props were generally used under 1000hp, three up to about 2000hp and four blades above that.



 --- On Fri, 10/24/08, Paul Mulwitz <psm(at)att.net> wrote:

   From: Paul Mulwitz <psm(at)att.net>

  Subject: RE: Re: Which Of Two STOL CH7XX Building Choices?

  To: zenith-list(at)matronics.com

  Date: Friday, October 24, 2008, 9:43 AM



 

   	  Quote: 	
		  --> Zenith-List message posted by: Paul Mulwitz

<psm(at)att.net>

Hi



 Guys,

I agree with both of you to some degree, but my reasons are different 



from the ones you wrote.

I think there are some limits on propeller length that must be

considered.

First, if a propeller is too long the tips will hit the ground and 



the prop will be damaged or destroyed.  This gives a maximum

prop 



length controlled by the airplane's landing gear and engine

position.

On the other end, the portion of the propeller's rotation disk that 



is blocked by the fuselage doesn't provide much thrust.  For the

kind 



of planes we are discussing this is perhaps the first 24 inches of 



prop diameter.

The third limiting factor is the speed of sound.  If the

propeller 



tips go too fast (around .8 mach) then energy is wasted forming sonic 



booms and related things rather than producing thrust.  The prop

tip 



speed is a direct function of the prop's length and the operating

RPM.

This all



 leaves a rather narrow range for a propeller to use on any 



given airplane design.  The limit includes both the length and 



maximum propeller RPM.  If you choose an engine that produces

its 



best power at a higher RPM then you need to install a PSRU to allow 



both the engine and propeller to operate efficiently.  The PSRU 



consumes a considerable amount of engine power and also introduces a 



significant new point of failure for the power plant.  You would

only 



want one if needed to get sufficient power from the engine converted 



into thrust to make the whole plane work well.

Engines designed for light plane use will not need a PSRU.  They

are 



designed to operate at appropriately low RPM.  Sometimes,

engines 



designed for other purposes like running automobiles or snowmobiles 



will require a PSRU to be compatible with propellers.

Paul



XL getting close

	
 [b]

ideaz1(at)sbcglobal.net
Guest

Posted: Fri Oct 24, 2008 7:02 pm Post subject: Which Of Two STOL CH7XX Building Choices?

Bryan gives an excellent explanation of prop  physics. In a prior plane I built I had an IVO 3 blade electric adjust prop.  Because of the size and weight of the plane and engine many factors don't  applly. I did however have real world experience as I could in a few minutes  switch from 3 to 2 blades. I also could adjust the pitch in flight to get max  efficiency at any time, i.e. climbing or cruise. The results were that 3  blades were a bit quieter and smoother (less vibration) and climb was improved.  With 2 blades top level speed was about 3-4% higher, but of course climb  suffered a bit. Remember I was able to adjust the pitch at will so as to get the  most of the prop in any conditions, so theres no question of making a bad choice  of pitch.

  

 I preferred the 3 blades for my use as the climb  and smoothness was more important to me than a slight speed  improvement.

 The differences were subtle but  noticeable.

  

 Dirk Z

   [quote][b]

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