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Rotary BSFC

 
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wdleonard(at)gmail.com
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PostPosted: Wed Sep 23, 2009 5:22 am    Post subject: Rotary BSFC Reply with 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][b]


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tedd(at)vansairforce.org
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PostPosted: Wed Sep 23, 2009 6:15 am    Post subject: Rotary BSFC Reply with quote

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
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PostPosted: Wed Sep 23, 2009 3:25 pm    Post subject: Rotary BSFC Reply with quote

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
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PostPosted: Wed Sep 23, 2009 4:17 pm    Post subject: Rotary BSFC Reply with quote

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
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PostPosted: Wed Sep 23, 2009 5:36 pm    Post subject: Rotary BSFC Reply with quote

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
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PostPosted: Wed Sep 23, 2009 7:33 pm    Post subject: Rotary BSFC Reply with quote

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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