Mike;
As others have pointed out there are
valid reasons for throttle position not being proportional to engine
power. The throttle plates are only one factor in the over flow air.
I’ll just add that even if it were the only restriction, rotating a
circular butterfly valve is not linear with flow – the first part of the
rotation increases flow much more than the last. Sizing of the TB is obviously
a big factor. With a giant TB you may be at full power with it just
cracked open. And aside from other factors with the induction system, at
some point the restricting factor is the engine porting. So, if you are
happy with the power you get, I wouldn’t make too much of it. OTOH;
at some point you may want to look at ways improving the system that might get
you more power.
On mine I’d say the last 20 –
25% of throttle travel makes little difference.
Al G
-----Original Message-----
From: Rotary motors in aircraft
[mailto:flyrotary@lancaironline.net] On
Behalf Of Mike Wills
Sent: Saturday, October 10, 2009
7:28 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] where's the
missing power?
I noticed quite a while ago and have
mentioned several times here that my engine does not have a linear response to
throttle. It reaches its max power before it reaches fully open throttle. I
havent worried too much about this up until now because the airplane has
sufficient power as is, has slightly better performance than my previous 160HP
Lyc powered -6A, and my wood prop is actually a pretty good match for the
current power level.
But I would like to understand
what's going on here and eventually address it. I was flying yesterday, my
usual boring holes in the sky directly over the airport. Decided to investigate
just a little so leveled at 5,000 feet at full throttle. Started reducing
throttle until I noticed a slight reduction in RPM and fuel flow. Then looked
down at where the throttle was actually set and was shocked to see it slightly
below half open. I dont have a regular manifold pressure gauge, just an
industrial type vacuum gauge ( I really gotta get an MP gauge). Anyway, the
vacuum gauge was indicating 4" of vacuum.
So I suspect I am giving up a
substantial amount of HP. I think the most likely suspect is my throttle body.
For a throttle body I copied Tracy's original design. Started with a stock
late 80s Mazda TB and hacked off the third port and all of the extra stuff. The
cowl in this area is very tight and I am unable to install any sort of air
filter or any sort of bell mouth on the TB. And I'm sure the cowl's close
proximity to the TB influences airflow into it as well.
Any comments on this or ways to test
it are welcome.