180 MODEL AVIATION
Focus on Competition
before the airplane is lost to either an offfield
landing or spin in case of a twin. The
problem occurs when the carburetor on a
gas engine is side mounted, which is the
normal position on a piston port type
engine.
The rotation of the carb in relation to
the front or upwind side can often cause
airflow problems over the intake side.
When air flows perpendicular to the intake
and its speed is greater than the intake
speed, the result is a vacuum or lowpressure
area that draws the fuel/air
mixture out of the carb instead of into the
cylinder.
If you’ve ever noticed raw fuel sprayed
down the fuselage side of your airplane,
this is what causes that. The end result of
this siphon effect is your engine running
lean soon after takeoff. It will often sound
fat or rich, but it’s actually going lean.
Check the plug if you’re in doubt, but
normally it will be lean.
The easiest cure for this scenario is to
install an air intake stack or extension to
the carb intake. They are available from
several vendors and are very simple to
install.
It’s a quick, easy fix to what most of the
time can be a baffling situation. Also
several engine manufacturers now often
include them in the engine package, so
check your box; you may already have one
and not know it (Dave!).
Another issue while we’re on the
subject of carbs that often causes problems
is the position of the diaphragm cover. If
SO FAR MOST of my tech talk has been
concerned mainly with the electronic
aspects of our hobby. This month I’m
going to hit on a problem I have faced
often, and I know it’s also problematic to
many others.
While test flying and working out the
bugs in my new Scale entry (Beech 18), I
ran into a situation that can catch you by
surprise, and hopefully you’ll figure it out
Technically Speaking ...
Greg Hahn
Technical Director
[email protected]
For information about how to become a part
of this exciting sport, contact:
Academy of Model Aeronautics
5161 E. Memorial Dr., Muncie IN 47302
(765) 287-1256 or (800) 435-9262 ([800] I-FLY AMA)
www.modelaircraft.org
America’s
Modeling
Adventure
you look at the diaphragm side of a Walbro
or Tillotson carb, you’ll notice a 3/16-inch
hole drilled directly over the center. This
hole allows air to flow both directions from
under the cover as the pumper operates the
diaphragm to pump fuel.
Often, rotating the carb to make throttle
linkages easier to connect puts this hole in
direct line with airflow from the propeller.
As the airflow increases with speed or
throttle, the pressure flows into the cover
hole and will eventually stop the diaphragm
from operating properly, causing the engine
to go lean again soon after takeoff.
There are numerous ways to combat this
problem, but all that’s really needed is to
stop the air from directly hitting the
diaphragm plate. My situation was solved
by my buddy Jeremy Arvin while in the
pits at Top Gun chewing the fat.
I, of course, was thinking of the most
difficult way to solve the problem, which
included the use of a soldering iron and
other scarce tools. He said to simply glue a
piece of plastic in between two of the
cylinders of my dummy engine to block the
air, and that should do the trick. Sure
enough, he was right. Problem solved in
less than five minutes. So all that’s needed
is to stop the air—nothing major.
Lesson: If you have problems with
engine performance changing while in
flight, most of the time it’s not mixture
related; it’s airflow related. Always go
there first.
See you at briefing. MA
07sig6.QXD 5/22/08 1:11 PM Page 180
Edition: Model Aviation - 2008/07
Page Numbers: 180