Injector carbs and inductions system icing

SOME promoters of injector carbs (POSA, Lake, Revmaster, AeroCarb and
Ellison fall into this category) claim that carb heat is un-necessary.
I have always felt, based on my analysis of the physics involved, that
this is an un-necessarily risky strategy. Ellison agrees; they suggest
carb heat with their carbs.

This gentleman, involved in the initial flight testing of his Teenie
Two, would appear to agree with me:

"Well I couldn't stand it so I went back out yesterday morning and did
it again. Now I am steady on my diet and hear that I am overloading the
little airplane but my 230 lbs is still flying stable as far as I can
tell. I am now finally getting confortable at 30 feet and have the wing
rock down pretty good too. Noticed that with left rudder to stay lined
up with the runway if u add a little left aleron the teenie lines up
better and stops its slip. The fbo boys came on the raido on the 6th
pass and suggested I go around the patch. Well as I applied the
throttle to 2500 the engine droped to 1000 and seemed to nearly stop
the prop spinning. I channged the rich a little and pumped the throttle
and it came back too life. Did not go around and when I pulled up to
the hanger and shut down I noticed water dripping from under the front
on the front tire. On inspection the y flange behind the posa was
dripping wet back 3' from the flange. I now believe this was frozen
with Ice... So I am fabracating a heat box and will let ya no how it
comes about.. Happy Thanksgiving to all my fellow flying buddies., Roy"

I have preserved the mis-spellings and possible errors (I think he
means 3 inches from the flange, not 3 feet, for instance) of this
message, and you can read the original message yourself he

http://www.network54.com/Forum/40291.../Ice+%21%21%21

This is not the first account I've heard of this sort of thing (at
least one other account has come my way concerning a POSA on a
Sonerai), but it's the only one I've got handy at the moment.

There are slight differences in physics between the way the various
injector carbs work, but the basic mass flow, and thermodynamics are
very, very close. The air/fuel mixture downstream of one of these
jobbers is colder than the inlet air, and that can lead to icing.

To be fair, there is another explanation besides ice to explain Roy's
RPM drop. The POSA does not have an Accelerator Pump, and the typical
VW intake is long. The vaporization of gas by the carb is far from
complete, and a film of liquid gasoline builds up on the inside of the
manifold, and driven by heat from the engine compartment, becomes part
of the carburation process. In cruise, this is OK; the rate the carb
deposits liquid on the walls of the intake reaches an equilibrium with
the rate evaporation and airflow removes it, and the mixture stays
pretty consistent.

But roll the throttle open suddenly, and equilibrium goes out the
window. The mixture leans out pretty dramatically. Note that Roy
richened the mixture and the problem resolved. If the intake was truly
blocked, this would have made little, if any difference.

Still, water dripping from the intake? That says SOMETHING was mighty
cold up in the cowling of this airplane....

Make your own mind up. Mine is.