Carb heat with constant speed prop

Chief Captain

Chief Captain

Well-Known Member
Something occurred to me today. Why is it that carb heat causes a drop in RPM with a constant speed prop? When you're doing a runup and checking the carb heat, shouldn't the governor sense the change and return the prop to the commanded RPM?

I suspect I may be over thinking it and missing something simple here...
 
Chief Captain said:
Something occurred to me today. Why is it that carb heat causes a drop in RPM with a constant speed prop? When you're doing a runup and checking the carb heat, shouldn't the governor sense the change and return the prop to the commanded RPM?

I suspect I may be over thinking it and missing something simple here...
Click to expand...

Not enough power for the govenor to be usefull during a run-up.
 
The governor wants to allow more oil into the hub to decrease pitch and allow the prop to spin faster, but the prop is already at it's lowest pitch. As the pitch can't get any finer, the prop slows down.
 
Yeah, if you're doing a standard 1800-2000 rpm runup, the governor has the propeller all the way in low pitch because the engine isn't putting out enough power to spin the prop up to the commanded rpm (2700 on one airplane with which I am very familiar). You can make the governor cover for the carb heat 2 ways. One would be to go to 2000 rpm, then bring the prop control back until rpm drops off to 1800 and leave it there. Cycle the carb heat and you shouldn't get more than a momentary drop in rpm. The other would be to go to full throttle (assuming full throttle gets you up to full power rpm with your prop/engine combination, some don't) and then pull the carb heat. Again, you should get a momentary drop then back up as the governor catches up.
 
Yup. Run-up RPM is low enough that it reacts like a fixed pitch prop. Its not only useful for checking carb heat but also for ballpark leaning for high D-Alt takeoffs.
 
Chief Captain said:
Something occurred to me today. Why is it that carb heat causes a drop in RPM with a constant speed prop? When you're doing a runup and checking the carb heat, shouldn't the governor sense the change and return the prop to the commanded RPM?

I suspect I may be over thinking it and missing something simple here...
Click to expand...

That is a question I would expect a Chief Captain to know.......
 
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