Model Forum / Radio Controlled / Air Models / December 2009

Velocity stacks are very tricky on fixed wing aircraft. Unless you installed
it for a very specific reason, you will likely get better overall
performance without it installed. At least one of the problems is that if
you have a velocity stack with a angled cut on the inlet, pointing it into
the airflow creates extra pressure which varies with both air speed and
engine RPM. Turning the angle away for the prop blast/air speed creates a
vacuum which also varies with airspeed and RPM. Further compounding the
problem is that the reference pressure for the Walbro carb regulator is
mostly fixed and does not see the variation that the carb inlet sees with a
velocity stack installed. In fact, many people will solder a tube from the
carb regulator plate and route a tube to the inside of the fuse so that a
constant reference pressure will be maintained. Finally, changes in the air
due to humidity and temperature are amplified with a velocity stack. All of
this combined makes mixture adjustment problematic.

Helicopter and boat guys can get better performance from velocity stacks
because the carbs are normally located in a place that does not have varying
air velocity around it, and they run at fairly constant RPM. Similarly,
airplane racing guys get better performance because they are tuning their
engines for max RPM with a fairly constant air speed and don't much care
about idle, acceleration, deceleration, and the like. Some of the rear carb
engines, or engines that are fully enclosed in a cowl may also improve with
a velocity stack, but that is a crap shoot.

But, if you are looking for good overall performance from a G-62 a velocity
stack, especially one hanging out in the slipstream, may not be your answer.