FD,
As Dale says, there is no way an engine at 12" manifold is going to keep a specified RPM. The prop will be fully fine on the pich stops. This usually happens at around 19-20" inches as you reduce power. The prop will then act like a fixed unit, because essentially that is what it has become.
Alternate air and carb heat are NOT the same thing. I don't know exact figures, but I would doubt that the power differential between 'normal' and alternate air would be the same as with carb heat hot or cold.
FD if you think about the way a CSU operates, then you will see what I am saying HAS to be true. As I stated before, all mechanical devices have some lag in them so when the power is reduced the prop has to become finer to compensate and keep the same RPM, this takes a finite amount of time so the RPM initally will drop and then recover to the original setting. This may only take 1/4-1/2 of a second, but it is noticable.
Drauk, usually you won't have a problem with non filtered air in flight unless you fly through a dust storm! The problem usually comes with taxiing with carb heat on. I would stick to the principle that when in cloud I would use carb heat more often, but not constantly, just be very aware that it can creep up on you. Generally an engine will only stop due to carb ice when the throttle is closed, as Whirly has already alluded to, but obviously it can happen at other times. At large throttle openings it is less likely for carb ice to form due to the speed of the airflow and the fact that there isn't as much of a pressure drop behind the throttle butterfly. (I still don't understand why we don't use flat-slide carbs on a/c, any engineers out there know the reason? Or is it just a 'certifcation issue' as per usual?)
BRL, shouldn't some of the replies to this thread be moved across to the carb heat in the circuit thread? If this is possible of course!
Last edited by Say again s l o w l y; 27th October 2003 at 18:34.