jbhart(at)onlyinternet.ne
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 Posted: Mon Sep 01, 2008 9:57 am Post subject: Labor Day |
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Kolbers,
Two events mark today. The FireFly completed it's 250th in the air hour
during the 706th flight. It has taken me almost nine years.
The second event is that during the 706th flight, the air/fuel mixture was
controllable from the cockpit. The current setup is crude, but it enabled
me to lower the air pressure in the float bowl. I have been trying to do
this for years, but the problem has been trying to find a light weight
stable vacuum source. I tried various things, but discarded them as too
unstable or too complicated or too much weight.
Richard Pike gave me the idea when he sent me a little add on probe for the
carburetor inlet that allowed one to on the ground richen the mixture. I
liked the concept, but didn't like the idea of setting the carburetor lean
so that you could adjust it rich. If something failed, you could fry the
engine. It seemed safer to set the carburetor at its winter needle setting,
and then lean the mixture for higher air temperatures and humidities. That
way if the system failed the engine would go rich and one would not risk
engine damage.
The only change required to the carburetor is one hole must be drilled to
install a negative dynamic pressure probe. It is off set so that it does
not interfere with the air flow around the needle. Testing showed that at
wide open throttle one could peg a vacuum meter that had a 10 inches of H2O
scale.
The way the system works is that the carburetor inlet probe supplies the
vacuum. To keep from influencing carburetor inlet flow a critical orifice
is placed in the line from the probe. The hole through the orifice is 1/32
inch OD. After the orifice the flow is "Teed" so that one leg is connected
to a throttling flow valve and the other leg of the "T" is connected to the
float bowl vents. For a reference pressure, the output from the flow valve
is connected to an open ended tube that terminates at the root tube under
the wing. To see what is going on, a differential pressure gage is
connected across the throttle valve.
While watching the gage and in flight, one can slowly start to close the
throttle valve. Today, I found that at 1.4 inches of H2O, I could raise the
EGT to its upper operating limit at 5,200 rpm. The valve was not too
sensitive and the pressure and EGT readings remained stable for reasonable
rpm changes. Before the next flight I will move the carburetor needle to
it's cold weather position to see if again, I can raise the EGT to it's
upper limit. If so, I will go for a more permanent installation.
What I am doing is not new. It is similar to US Patent 4,040,399, Aug. 9,
1977, entitled "Back Draft Carburetor For Two Cycle Engines". The Bing does
provide this capability, but with the probe, critical orifice, flow
throttling valve, and connecting tubing, the "back drafting" components can
be externalized. If you would like to view this patent see below:
http://patft.uspto.gov/netahtml/PTO/srchnum.htm
Search for the patent number. When the patent comes up, click on the
"Image" button to view and print out the patent complete with all the text
and drawings.
Fly Safe!
Jack B. Hart FF004
Winchester, IN
do not archive
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