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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. |
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