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Rotary BSFC

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wdleonard(at)gmail.com
Guest

Posted: Wed Sep 23, 2009 5:22 am Post subject: Rotary BSFC

 	  Quote: 	
		  



 	  Quote: 	
		   On his forum/sandbox someone sent in two dyno sheets for a rotary.

 The BSFC numbers were in the .65 range... Funny, not one person

 > spoke up about that, not even the "wizzard".
	


 	  Quote: 	
		  

Some people in the rotary community are sensitive about that.  I got
	


smacked down on the RV List a few years ago for suggesting (based on

engineering references) that it was an accepted fact that rotaries

 have a lower BSFC than piston engines, generally speaking.  Being two-

strokes, effectively, and having a very poor surface-volume ratio in

the combustion chamber (compared to a piston engine) it's not

surprising that they would have low BSFC.  I found it odd that anyone

 even challenged the statement, but there you go.



Tedd McHenry

Surrey, BC



	


 

 Tedd,

 Rightly so that you got smacked down...  lower BSFC is good, higher is bad.  so 2-strokes have HIGH BSFC.

  

 That being said, the rotary is in NO WAY (not effectively or otherwise) a 2-stroke.  

  

 -- 

David Leonard



Turbo Rotary RV-6 N4VY

http://N4VY.RotaryRoster.net

http://RotaryRoster.net



  [quote][b]

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tedd(at)vansairforce.org
Guest

Posted: Wed Sep 23, 2009 6:15 am Post subject: Rotary BSFC

On 23-Sep-09, at 6:19AM, David Leonard wrote:



 	  Quote: 	
		   Rightly so that you got smacked down...  lower BSFC is good, higher  

 is bad.  so 2-strokes have HIGH BSFC.


	


Good point.  I meant "worse," not "lower," as you obviously realized,  

but thanks for clarifying.



Tedd

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tracy(at)rotaryaviation.c
Guest

Posted: Wed Sep 23, 2009 3:25 pm Post subject: Rotary BSFC

A rotary tuned for all out best power production (especially if ported for racing etc) might indeed clock in at .65 BSFC.  That has little to do with what you can expect at cruise conditions in an aircraft application. 

 

 Using Van's Aircraft HP required vs airspeed chart published some time ago,  I computed my BSFC at .45 - .47 BSFC at cruise conditions.  That's a tiny bit worse than Lycoming numbers when run LOP but not enough to matter.  Especially considering it happily burns 87 octane auto fuel.

 

Tracy Crook



On Wed, Sep 23, 2009 at 9:19 AM, David Leonard <wdleonard(at)gmail.com (wdleonard(at)gmail.com)> wrote:

[quote] 

   	  Quote: 	
		  



 	  Quote: 	
		   On his forum/sandbox someone sent in two dyno sheets for a rotary.

 The BSFC numbers were in the .65 range... Funny, not one person

  > spoke up about that, not even the "wizzard".
	


 	  Quote: 	
		  

Some people in the rotary community are sensitive about that.  I got
	


smacked down on the RV List a few years ago for suggesting (based on

engineering references) that it was an accepted fact that rotaries

  have a lower BSFC than piston engines, generally speaking.  Being two-

strokes, effectively, and having a very poor surface-volume ratio in

the combustion chamber (compared to a piston engine) it's not

surprising that they would have low BSFC.  I found it odd that anyone

  even challenged the statement, but there you go.



Tedd McHenry

Surrey, BC



	


 

 Tedd,

 Rightly so that you got smacked down...  lower BSFC is good, higher is bad.  so 2-strokes have HIGH BSFC.

  

 That being said, the rotary is in NO WAY (not effectively or otherwise) a 2-stroke.  

  

 -- 

David Leonard



Turbo Rotary RV-6 N4VY

http://N4VY.RotaryRoster.net

http://RotaryRoster.net

 

   	  Quote: 	
		  



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tedd(at)vansairforce.org
Guest

Posted: Wed Sep 23, 2009 4:17 pm Post subject: Rotary BSFC

 	  Quote: 	
		   Using Van's Aircraft HP required vs airspeed chart published some  

 time ago,  I computed my BSFC at .45 - .47 BSFC at cruise conditions.


	


Tracy:



If your airframe has lower drag than Van assumed -- as it likely does,  

due to your lower cooling drag -- then Van's power-required numbers  

would have been too high, and the BSFC you calculated would have been  

low.  Any conservatism on Van's part (and he is famous for it) would  

also have lowered your BSFC result.



Tedd McHenry

Surrey, BC

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ceengland(at)bellsouth.ne
Guest

Posted: Wed Sep 23, 2009 5:36 pm Post subject: Rotary BSFC

Tedd McHenry wrote:

 	  Quote: 	

> Using Van's Aircraft HP required vs airspeed chart published some 

> time ago,  I computed my BSFC at .45 - .47 BSFC at cruise conditions.

 Tracy:

 If your airframe has lower drag than Van assumed -- as it likely does, 

 due to your lower cooling drag -- then Van's power-required numbers 

 would have been too high, and the BSFC you calculated would have been 

 low.  Any conservatism on Van's part (and he is famous for it) would 

 also have lowered your BSFC result.

 Tedd McHenry

 Surrey, BC 

Possible, but not likely. It's pretty rare (do-able, but rare) for 

homebuilt liquid cooled a/c to get cooling drag lower than a Lyc.

I've flown two 400+ mile cross countries in my 160 hp Lyc powered -4 

with Tracy (he burns 87mogas, I burn 93mogas), flying the same flight 

profiles for each flight, and each time he burned ~1.5-2.5 gal more than 

me. My oil burn probably offset that difference.

It should also be noted that I lean my stock Lyc *far* more aggressively 

than most Lyc drivers are willing to risk.

Charlie

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tedd(at)vansairforce.org
Guest

Posted: Wed Sep 23, 2009 7:33 pm Post subject: Rotary BSFC

 	  Quote: 	
		   

 >



 Possible, but not likely. It's pretty rare (do-able, but rare) for  

 homebuilt liquid cooled a/c to get cooling drag lower than a Lyc.


	


I'm not familiar with Tracy's installation, specifically, but cooling  

drag is one of the key advantages of liquid-cooled engines.  You're  

probably right that most homebuilts don't exploit that advantage very  

well.  You only have to look at the installations to see that.  But,  

given that Tracy's a pretty smart cookie, I'm giving him the benefit  

of the doubt and suggesting that is why his BSFC estimate is optimistic.



The numbers Tracy published would be quite good for an automotive  

piston engine.  When I see numbers like that for a 13B-based rotary I  

begin to look for alternate explanations.



Tedd McHenry

Surrey, BC

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