Carb heat question

[Li'lJugs]

I get it that I need to put on carb heat when reducing throttle, but I've been wondering ever since I got this plane, when I reduce throttle by how much? Just when reducing to idle, or anytime the throttle is reduced, even by 100 RPM, or what, exactly?

Silly, of course, but I really don't want to damage my virtual engine.

[Markoz]

I'm sure I saw a post where someone said that it is best to turn the carb heat on before reducing the throttle. Because the engine is hotter, the carb heat works better. I always do it that way if I'm reducing the throttle so that the engine RPMs drop below 2000.

Someone will surely correct me if I'm off a bit!

Mark

[Li'lJugs]

Hi, I went to a .pdf listed here (original Cub manual), and it didn't have any hard and fast rules, just basically "apply as needed to keep engine running smoothly", with a canard to to be very careful when ambient temperature is high to avoid overheating the engine.

Seems it's like everything else with the Cub, "seat of your pants", LOL.

[Alan_A]

The manual (page 53) recommends using carb heat when you "substantially reduce the throttle," for example on reduced power descents and landing approaches.

I usually turn carb heat on 30 seconds before reducing power below 2000 rpm. I could swear I'd read somewhere about using 2000 rpm as the threshold... but unfortunately I can't remember where.

Still seat-of-the-pants, but I guess there's some advice and experience behind it...

[BPL]

I too heard that 2000 RPM rule somewhere. That's what I use as my guideline.

[HercFan]

Count me in the "2000 RPM" club.

[skunker]

Use Carb Heat any time you are below 2000rpm. I sometimes fly cruise at 2000rpm because I get better endurance out of the airplane that way (although the recommended cruise rpm is 2150). I'm a real J3 pilot, so this is actually what I use.

The reason that we need to be very diligent on our carb heat usage is because these old continentals are very prone to carb ice, more so than almost any other engine out there.

[bushman]

Just a quick note when you have a set of 1320 floats hanging under you 2000 rpm cruise may drag you down for keeping your altitude, 2300 2250 rpm also depending on a head wind. I agree 2000 rpm for carb heat on. I fly with a 90 hp Continental. In winter on ski's can cruise nice at 2100 rpm. My 2 cents tossed in.

Bushman

[JWiley]

When I pull the carb heat on the engine seems to run roughly, almost as though it is too rich. Should I try to reduce it untill the engine smooths out? Or am I miss-hearing things?

[VulcanB2]

Hi,

Pull the carb heat first, then throttle back.

Carb heat heats the air, reduces the air density, and enriches the mixture. It will also result in reduced power, and a drop in RPM. If you throttle back before setting carb heat on, you may find you kill the engine completely.

You won't harm it if you pull the carb heat at high power.

Carb heat is required when:



Remember that once ice starts to form, it forms rapidly. Keeping it clear is the goal.

If you do get carb icing and it won't clear with carb heat, increase power and descend to warmer air. Fast airflow will help slow the rate ice accumulation. Don't throttle back - you'll make it worse!

I think this is correct for Lycoming engines: with carb heat ON, the air into the engine is *UNfiltered*, whilst the air with carb heat is OFF is filtered.

A symptom of carb icing is requiring to slowly increase the throttle to maintain RPM. Be aware of what is normal so you know when you've got a particularly high power setting for a given RPM. A gotcha is whilst waiting to takeoff.

Best regards,
Robin.

[Killratio]

As far as I know Robin, correct, in the Lycomings unfiltered air is introduced when carb heat is set on...the main reason for the "Carb heat Cold" check at 300' AGL on approach..otherwise you can suck cr4p into the engine.

The rule in Cessnas and other lights is *always* use Carb heat any time you run below the "green arc". At other times "as required" and picking those situations that may lead to ice is not really straight forward until you have had experience I know it took me ages...but I am t'ick!!.

FWIW

Darryl