carb heat

[flyer170]

I would appreciate some input: I'm going on a trip with my 170 and a friend is going along in his 90 hp Cub. Not a long trip but a couple of 300 mile legs. I know I can just meet him there or fly in circles but I would like to know your experiences flying at lower RPM settings. I have the operating charts showing all the numbers on RPM, HP, TAS, GAL/HR, range, etc. but at what point is carb heat necessary or recommended
What is the impact on the engine flying at lower rpm say 2000 rpm for 2 hours? I usually cruise between 2350 and 2450.
I'm talking about summer trip temps. but what about cold outside temps. like winter stuff
Any input would be appreciated.
Bob

[N1478D]

Hi Bob,

My instructor, who was a 170 guy, taught that anything outside of the green arc on the tach was full carb heat.

Over time, I have drifted somewhat away from that. On a hot, dry day, I don't apply carb heat, but I should. I think it got me in trouble going on a hot humid trip to eat good food east of Texas earlier this summer.

You might be able to put the flaps out if you want to keep your RPMs up a little bit higher. Logic says that if your oil pressure and temp is ok, a low RPM isn't going to hurt anything.

Have fun!

[GAHorn]

You can fly with any power setting you wish, including 2000 rpm. The green arc is the "normal" operational range for cruise. It isn't binding. If it were, then Cessna certainly would have boo-boo'ed publishing all that cruise performance data at 2100 rpm, and also notice that the seaplane actually has 2000 rpm performance charts listed in the Owner's Manual.
The green arc has nothing to do with carb heat use, although that was an "ad hoc" recommendation by many CFI's, as it was a convenient reminder to inexperienced students. (To avoid excessive carb heat usage and it's associated overly-rich fuel conditions, Joe typically just relies upon a stuck valve to reduce his rpm for slower cruise speeds, although he generally makes up for it on approach.)

[N1478D]

You can fly with any power setting you wish, including 2000 rpm. The green arc is the "normal" operational range for cruise. It isn't binding. If it were, then Cessna certainly would have boo-boo'ed publishing all that cruise performance data at 2100 rpm, and also notice that the seaplane actually has 2000 rpm performance charts listed in the Owner's Manual.
The green arc has nothing to do with carb heat use, although that was an "ad hoc" recommendation by many CFI's, as it was a convenient reminder to inexperienced students. (To avoid excessive carb heat usage and it's associated overly-rich fuel conditions, Joe typically just relies upon a stuck valve to reduce his rpm for slower cruise speeds, although he generally makes up for it on approach.)

Gee, I was hoping somebody would give me a hard time today!
I will be the first to admit that even after 500+ hours in 1478D I am still learning. Not sure where on the slow learning curve that I fit, but it is amazing how much there is to learn, and how much fun it is to do it. Asked for my first ever special VFR this morning. The front stalled over Dallas and the clouds where at 500 ft, boy was it fun flying thru Class B weaving thru the clouds - 3 miles and clear - and then tackling the 15 kt+ gusty crosswind at ADS. It is amazing to have a machine that allows you to move from a short grass strip on a lake with tall trees to navigating over Love Field with the Jumbos all in one day! George, did you ever get all of that oil stopped from dripping out of that old B of yours?

[GAHorn]

Only after I did the work myself! [/b]

[N2255D]

Hi Bob, Ive been flying formation with a friend who has a Cessna 140 for the last 4 years with no problems. I'm generally at 2100 to 2200 RPM. Most of the trips are 1 to 1 1/2 hrs. these trips have been winter and summer.

[Rudy Mantel]

As George pointed out, carb heat richens the mixture, which is the main drawback to using it unnecessarily. I tend not to use carb heat in the hot Florida weather. When it cools to 65 or so, especially if its humid, carb heat's a good idea at low power as in landing approaches. I usually lean the mixture when using carb heat.
But you never know when carb ice will strike. I've picked it up on a clear night at cruise power in a 180 in the tropics at 4,000 ft. The manifold pressure slowly dropped. When I pulled carb heat the power picked up-.
Rudy

[flyer170]

Thanks for the input, this site is fantastic.
Bob

[flyguy]

Now here is a bone to chew on! Putting "HOT AIR" through the induction system does richen the mixture but also will, if used for an extended period of time, increase the internal temperatures of the mill. Leaning this already hot fuel/air mixture could increase the internal temperatures even more.

Now operating the O-300 at 'low' rpm. It will increase cht and egt a little above normal but within specs. Long term effects may not show up before nearing run-out, but air-cooled engines work better with lots of cooling air going through them. One of the reasons the O-300G engine had less than the stellar sucess it deserved ( ) was operators were afraid of 3100 rpm at cruise and persisted in turning around 2550rpm which is in mid range for the C145/ O-300. This limited the horsepower of the "G" and caused overheating of cyl heads. Consequent cracking of the heads gave the engine a bad rap. Just try to avoid shock cooling those "hot heads" when operating at low rpm settings for a long period of time. (No, I ain't talking bout you Joe!)

[GAHorn]

Now here is a bone to chew on! Putting "HOT AIR" through the induction system does richen the mixture but also will, if used for an extended period of time, increase the internal temperatures of the mill. Leaning this already hot fuel/air mixture could increase the internal temperatures even more.

Now operating the O-300 at 'low' rpm. It will increase cht and egt a little above normal but within specs. Long term effects may not show up before nearing run-out, but air-cooled engines work better with lots of cooling air going through them. One of the reasons the O-300G engine had less than the stellar sucess it deserved ( ) was operators were afraid of 3100 rpm at cruise and persisted in turning around 2550rpm which is in mid range for the C145/ O-300. This limited the horsepower of the "G" and caused overheating of cyl heads. Consequent cracking of the heads gave the engine a bad rap. Just try to avoid shock cooling those "hot heads" when operating at low rpm settings for a long period of time. (No, I ain't talking bout you Joe!)

Just to be nitpickin'....
Ol' GAR means the GO-300 engine, ....as there was no such critter as an O-300-G. The most common reasons for the poor reliability of the GO series engine is the gearcase, which has that engine working overtime to manufacture an additional 25 hp. It basically always ran like a car stuck in 2nd gear. When the recommended 3100 rpm was run, the prop itself was only turning 3/4ths of that speed (which by the way, kept it within it's most efficient rpm range.) Running the engine slower didn't hurt the engine for the reasons commonly thought. It was commonly thought the engine was being "lugged" by being run slowler. In actuality, it's almost impossible to "lug" an aircraft engine using a fixed pitch prop. (Although it can be approximated in some constant-speed installations, but even then, only at high manifold pressure settings in combination with low rpm prop selections.) The reason the GO-300 was injured at low rpms was because in order to run at those low rpms, the throttle had to be retarded, which gave low MP, which gave low piston ring loading, and which caused ring flutter. Also, the aircraft was unsuitable for flight training due to the gyroscopic loads imposed on the gearcase, which caused pilot-shaft failures. Cylinder cracking rumors were almost always due to the incorrect part number cylinders being installed on this engine. Although O-200 and O-300 cylinders would fit, they were not correct, and were cosmetically almost indistinguishable from the correct part numbers. (Which by the way, were OK to use on the O200/300 series. The latest versions of cylinders meet the needs of both engines.)
Lastly, the GO-300 series, like all geared engines, did not like to be "driven" by the prop. Trying to slow down and descend simultaneously was almost impossible without abusing the engine with low power settings coupled with high speed. This caused excessive cooling of the cylinder heads, and also caused gearbox "chatter" due to the gears running in a backlash condition. (All the geared engines must be handled so as to make certain they are turning the prop,...not the prop turning the engine.) Large throttle excursions are death to geared engines. None of the geared engines have ever enjoyed the longevity of the direct drive engines. Current versions of GTSIO-520 and IGSO/TIGO-540/541's included.