gahorn wrote:There's no specification for amount of rpm drop to test carb heat that I know of. My airplane gets about 150 rpm drop typically....I think.
When my J3C-65 was upgraded from an A65 to a C90, the TDCS specified "Carburetor heat system must be modified to provide required heat rise." My recollection is that the required heat rise was 90 degrees F, but I don't know where that's specified. It's not in the data sheet. The modification I have was approved based on flight test data from a J3 with a carburetor temperature gauge. If you really want to know what's going on, there's no substitute for gauges.
Best Regards,
John
John Renwick
Minneapolis, MN
Former owner, '55 C-170B, N4401B
'42 J-3 Cub, N62088
'50 Swift GC-1B, N2431B, Oshkosh 2009 Outstanding Swift Award, 2016 Best Continuously Maintained Swift
I've got a carb heat gauge too, but darned if I can remember how much the temp rises with carb heat on - I just look for a rise and it's pretty significant. As far as the RPM drop goes, 300 sounds a skosh much - seems like 200 drop is more normal on mine. I may just have to find the time to run my craft outside over lunch and run it up so I'm not relying on my feeble memory. overdue for some bounce-n-go's anyway.
by the way, my suggestion on running without the filter was for a ground runup, shouldn't expose you to too much dirt during a quick test - especially since you're unfiltered when running carb heat anyway.
we've had Brackett filters ice up on us too, and not just from impact rain or snow - which is why I have that opinion about them. however, I'll take the bad with the more convenient good.
'56 "C170 and change"
'52 Packard 200
'68 Arctic Cat P12 Panther
"He's a menace to everything in the air. Yes, birds too." - Airplane
HA, I didn't mean to disparage your suggestion. Sorry if it seemed that way. You're correct about filters (any brand) icing up, but I think Bruce wasn't in airframe-icing conditions.
Carb heat system is of course, an alternate air system for such occasions. But when in airframe icing conditions....it is very unlikely to also be in carb-ice conditions. If visible moisture is already frozen....
then it's already too cold to form carb ice...UNLESS....partial carb heat is being used. (Then it can be melted back to liquid and re-frozen by a chilled carb body.)
BTW, ...what IS your actual name HA?
- George
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention. An originality nut (mostly) for the right reasons.
don't worry George, I'm not mad if anybody disagrees with me - opinions vary, and experience colors opinions. I'm happy if any thought I post here gives a nugget of help to anybody. Besides, this forum has been a wealth of useful info since I found it, even to this humble nosedragger driver
Hans
'56 "C170 and change"
'52 Packard 200
'68 Arctic Cat P12 Panther
"He's a menace to everything in the air. Yes, birds too." - Airplane
I'll just throw this out there for what it's worth. My plane has the Hanlon Wilson mufflers with one side running to the airbox the other to the heater. I get a huge reduction in RPM when I pull the CH, at least 200 RPM sometimes closer to three hundred. I initially felt like it was too much, but I've just gotten used to it.
However one of the things that the previous owner said comes to mind. He said that his mechanic had changed the carb ventury, and that made the drop in RPM "less" I guess the motor must have just about quit running when CH was pulled before.
The logs show a removal of an F12 vernturi and install of an F11. Obviously nobody changed your carb venturi in mid-flight or while you were at lunch. However, with as much time in the plane as you've had, that feeling that you aren't getting as much RPM reduction when you pull the CH is probably true. If the airbox and tubing are ok then perhaps something has happened in your carb?
I'm just saying trust your gut, it usually doesn't lie and something must have changed in your induction system.
My carb heat was installed as you describe directly to the one side and that is the way I have run the airplane the most. So perhaps the larger drop is ingrained in my head. I have not found anything wrong with the system. My friend who was in the same air with me in his 170 with the nearly the same system only gets about 100 rpm drop so my system is in line with others.
I'm sure it was carb ice at this point. My friend probably didn't get any because he was running a slightly higher RPM maybe creating more over all heat or like he says maybe his air box flap leaks and he get some heat all the time.
CAUTION - My forum posts may be worth what you paid for them!
Bruce Fenstermacher, Past President, TIC170A
Email: brucefenster at gmail.com
I don't know about how by-the-book it is, but my ragwing is set up with carb heat off one heat muff and cabin heat off the other. I like it, simple, no "carb heat union" or anything like that. I checked yesterday and saw about a 100-110 rpm drop, that's with pancake mufflers.
For what it is worth I had a few puzzling experiences with carb ice on my 170. I had carb ice on take 0ff with the temperature well above freezing and clear skies and after doing a normal carb heat check during run up with no sign of ice. The previous night was very damp, leaving moisture on surfaces. Normal take off RPM but at around 1000 feet the engine started loosing power so I pulled the carb heat and sure enough the engine went very rough for a few seconds before RPMs started going up again. After that episode there was no more carb ice for the whole flight.
My only explanation is there was probably some moisture inside the carburator at engine start and with the cooling effect during take off this moisture turned into ice. On subsequent flights under the same conditions I made sure to do a longer than normal run up to get rid of the moisture in the carburator. The interesting part is that there is no sign of ice at the first carb heat application an everything seems normal. But if I run the engine at 1700 RPMs for a bit longer and then apply carb heat again I clearly see the evidence of ice now.
