Interesting point you make ( on throttle
openings) this is the same for a slide throttle, depending upon the shape of
the slide. For instance a half round cut out in the slide is more linear that
a flat slide over a round opening.
A piece of useless information for those who
don't make carbies.
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.