Carb heat + Piper?
10-30-2008, 06:32 AM
#11
Super Moderator
Joined APC: Jun 2006
Posts: 3,945
The old rule of thumb is to keep your manifold pressure lower than your prop rpm such as crusing at 20" MAP and 2300 RPM. Also the same reason they say when you add power you go from right to left such as increasing your prop rpm to 2500 first and then adding throttle to get your MAP to 25" and the same with descent to ensure that you are always in that safe envelope they claim.
In actuality when you look into the POHs for most constant speed prop aircraft you will find many power settings where best performance and the factory reccomended setting requires an over square condition such as maybe having MAP at 25" and RPM at 2400.
In actuality when you look into the POHs for most constant speed prop aircraft you will find many power settings where best performance and the factory reccomended setting requires an over square condition such as maybe having MAP at 25" and RPM at 2400.
10-30-2008, 06:55 AM
#12
Blue Light Special
Joined APC: Jan 2007
Position: Learjet 45 & G-IV
Posts: 637
Yeah I always knew the rule, but I never understood why... the units are not even the same, so how could it be so convenient that squaring works in the first place?
And then there is the turbo thing too.
And then there is the turbo thing too.
10-30-2008, 08:31 AM
#13
Airport Hobo
Joined APC: Apr 2007
Posts: 844
Originally Posted by usmc-sgt
The old rule of thumb is to keep your manifold pressure lower than your prop rpm such as crusing at 20" MAP and 2300 RPM. Also the same reason they say when you add power you go from right to left such as increasing your prop rpm to 2500 first and then adding throttle to get your MAP to 25" and the same with descent to ensure that you are always in that safe envelope they claim.
In actuality when you look into the POHs for most constant speed prop aircraft you will find many power settings where best performance and the factory reccomended setting requires an over square condition such as maybe having MAP at 25" and RPM at 2400.
In actuality when you look into the POHs for most constant speed prop aircraft you will find many power settings where best performance and the factory reccomended setting requires an over square condition such as maybe having MAP at 25" and RPM at 2400.
On another thought, I would always use the best power setting on the mixture which was usually slightly rich of peak. However, I read somewhere, can't remember where (Pilot Magazine?) of someone who at low altitudes would use slightly rich of peak, then at high altitudes, slightly lean of peak. His philosophy being that in the thinner air you will have less BHP so you would not be doing the damage that you would be doing at high BHP values at low altitude, and in turn would be getting better fuel economy without sacrificing maintenance costs. Thoughts?
10-30-2008, 09:19 AM
#14
Flying Farmer
Joined APC: Jul 2006
Position: Turbo-props' and John Deere's
Posts: 3,160
250: Thank you for bringing that up, I got side-tracked in my writings. There's a wonderful chart out there that show's what temp/dewpoint combo's have the highest incident rate of carb-icing. I'm lazy and getting ready to depart, so I'm not finding it for you
USMC-sgt: Got it right on the head. When I flew the C310's, I'd fly them at 2100RPM, 25", and would get an equivelant KTAS as running 23/2300, with a slightly lower fuel burn
Mcartier: You're right, they are different units, but obviously related. Just remember that the Manifold is how much coals' you're putting to the fire, the Prop RPM is your transmission, no reason to be using low gear(IE 2500RPM+) when cruising. Lower RPM's also mean that the blade's AoA is more suited for efficiency also. Think of hte foward movement of the prop, vs RPM, and the faster you go, the faster RPM is actually less efficient, than it is as slower speeds such as climb/takeoff. This is regardless of turbo/non-turbo. Although obviously the turbo numbers will be higher. A piper Matrix for example will run 2500RPM/32" max cruise, or 2400/30" for a normal cruise!
FlyandDive: Good points on Mixture, honestly, unless you have a decent/good EGT monitor, it's hard to really tell exactly how close you are. Also, Lycomings have never recommended running LOP operations, just Continental's. I agree that you will be running lower power @ altitude, but we have to remember we are assuming non-turbo in this case. The exact situation would be any time you are running 65% or less in the Continentals if I remember right from the C310(Hence my 2100RPM/25" would be 60-65%, so I'd run it on peak-ish settings, at that point it couldn't produce enough heat to really damage anything anyways.)
USMC-sgt: Got it right on the head. When I flew the C310's, I'd fly them at 2100RPM, 25", and would get an equivelant KTAS as running 23/2300, with a slightly lower fuel burn
Mcartier: You're right, they are different units, but obviously related. Just remember that the Manifold is how much coals' you're putting to the fire, the Prop RPM is your transmission, no reason to be using low gear(IE 2500RPM+) when cruising. Lower RPM's also mean that the blade's AoA is more suited for efficiency also. Think of hte foward movement of the prop, vs RPM, and the faster you go, the faster RPM is actually less efficient, than it is as slower speeds such as climb/takeoff. This is regardless of turbo/non-turbo. Although obviously the turbo numbers will be higher. A piper Matrix for example will run 2500RPM/32" max cruise, or 2400/30" for a normal cruise!
FlyandDive: Good points on Mixture, honestly, unless you have a decent/good EGT monitor, it's hard to really tell exactly how close you are. Also, Lycomings have never recommended running LOP operations, just Continental's. I agree that you will be running lower power @ altitude, but we have to remember we are assuming non-turbo in this case. The exact situation would be any time you are running 65% or less in the Continentals if I remember right from the C310(Hence my 2100RPM/25" would be 60-65%, so I'd run it on peak-ish settings, at that point it couldn't produce enough heat to really damage anything anyways.)
10-30-2008, 09:42 AM
#15
First Rule of Fight Club
Joined APC: Jan 2007
Position: My seat smells like cat pee
Posts: 1,536
Originally Posted by 250 or point 65
Whoa guys. CARB HEAT HAS NOTHING TO DO WITH COLD OAT!
When it's cold outside, the air cannot hold much moisture. You worry most about it when its 50-70 degrees out because that temp air can hold much more moisture than 30 degree air. Also important to note is that carb ice does not have much to do with the venturi in the carb, it has a lot more to do with the heat robbed from fuel atomization. The liquid to gas cooling has a lot of effect on the temp of the air going through the carb.
We were also told a very important story in school about someone who did not understand carb ice. Before taking off in a twin on a very cold day, a guy decides he will use the carb heat before takeoff as a precaution. So what happens is he actually heats the air up to a temp that would cause carb ice, ices up his carb on one side and crashes after takeoff.
When it's cold outside, the air cannot hold much moisture. You worry most about it when its 50-70 degrees out because that temp air can hold much more moisture than 30 degree air. Also important to note is that carb ice does not have much to do with the venturi in the carb, it has a lot more to do with the heat robbed from fuel atomization. The liquid to gas cooling has a lot of effect on the temp of the air going through the carb.
We were also told a very important story in school about someone who did not understand carb ice. Before taking off in a twin on a very cold day, a guy decides he will use the carb heat before takeoff as a precaution. So what happens is he actually heats the air up to a temp that would cause carb ice, ices up his carb on one side and crashes after takeoff.
NO ONE SAID THAT IT DID. And in my opinion I suggest you tell your school to stop telling that story cause it's a crock. Carb ice is a function of moisture content of a particle of air. On a "very cold day" even if the air is warmed it doesnt add moisture to that particle of air.
10-30-2008, 11:20 AM
#16
Gets Weekends Off
Thread Starter
Joined APC: Jun 2008
Position: C402
Posts: 210
Originally Posted by 250 or point 65
Whoa guys. CARB HEAT HAS NOTHING TO DO WITH COLD OAT!
When it's cold outside, the air cannot hold much moisture. You worry most about it when its 50-70 degrees out because that temp air can hold much more moisture than 30 degree air. Also important to note is that carb ice does not have much to do with the venturi in the carb, it has a lot more to do with the heat robbed from fuel atomization. The liquid to gas cooling has a lot of effect on the temp of the air going through the carb.
We were also told a very important story in school about someone who did not understand carb ice. Before taking off in a twin on a very cold day, a guy decides he will use the carb heat before takeoff as a precaution. So what happens is he actually heats the air up to a temp that would cause carb ice, ices up his carb on one side and crashes after takeoff.
When it's cold outside, the air cannot hold much moisture. You worry most about it when its 50-70 degrees out because that temp air can hold much more moisture than 30 degree air. Also important to note is that carb ice does not have much to do with the venturi in the carb, it has a lot more to do with the heat robbed from fuel atomization. The liquid to gas cooling has a lot of effect on the temp of the air going through the carb.
We were also told a very important story in school about someone who did not understand carb ice. Before taking off in a twin on a very cold day, a guy decides he will use the carb heat before takeoff as a precaution. So what happens is he actually heats the air up to a temp that would cause carb ice, ices up his carb on one side and crashes after takeoff.
Was it below freezing or something???
10-30-2008, 11:59 AM
#17
Gets Weekends Off
Joined APC: Jun 2005
Position: Retired
Posts: 2,934
If the water vapor is already frozen, a drop in temp isn't going to affect it. It can't be "more frozen". It's also not going to be as likely to adhere. Or, the filter will take care of it if it's large enough. Much like flying in the clouds when it's very cold. You won't be as likely to pick up structural icing because the water vapor is already frozen and just bounces off the aircraft.
A carburator does two things that cause a lot of cooling. Atomization of the fuel, and the accelleration accompanied by the associated pressure drop of the fuel/air mixture going through the venturi.
Air that's heated to 50 degrees can hold much more water vapor than the same parcel if it's it's only 10 degrees. Ever notice how dry your skin gets in winter or the fact that every thing you touch results in a static discharge? Same reason. This is why it's much more likely to have carb icing on a day with a 50 degree temp than a 10 degree day.
I was never a fan of the twin story. Too much circumstantial crap. I liked to keep it much more simple. Does the AFM say use it on T.O.? If not, don't use it. What about landing? Are you saying you do something that is against what the AFM says in its procedures?
Nothing says you have to go flying. In the same respect, if you think you have carb ice, don't force a landing unless you absolutely have to. Always have yourself set up for a go around if you can. Clear the engine first, then come back and land if it's not bad enough that it's an emergency.
A carburator does two things that cause a lot of cooling. Atomization of the fuel, and the accelleration accompanied by the associated pressure drop of the fuel/air mixture going through the venturi.
Air that's heated to 50 degrees can hold much more water vapor than the same parcel if it's it's only 10 degrees. Ever notice how dry your skin gets in winter or the fact that every thing you touch results in a static discharge? Same reason. This is why it's much more likely to have carb icing on a day with a 50 degree temp than a 10 degree day.
I was never a fan of the twin story. Too much circumstantial crap. I liked to keep it much more simple. Does the AFM say use it on T.O.? If not, don't use it. What about landing? Are you saying you do something that is against what the AFM says in its procedures?
Nothing says you have to go flying. In the same respect, if you think you have carb ice, don't force a landing unless you absolutely have to. Always have yourself set up for a go around if you can. Clear the engine first, then come back and land if it's not bad enough that it's an emergency.
10-30-2008, 04:28 PM
#18
Gets Weekends Off
Joined APC: Aug 2008
Posts: 999
Originally Posted by BoredwLife
NO ONE SAID THAT IT DID. And in my opinion I suggest you tell your school to stop telling that story cause it's a crock. Carb ice is a function of moisture content of a particle of air. On a "very cold day" even if the air is warmed it doesnt add moisture to that particle of air.
and a bunch of people said that it did. the original question posed was framed by "its getting colder out and..."
10-31-2008, 02:03 PM
#19
Gets Weekends Off
Joined APC: Jul 2007
Position: C402, on the side with the switches!
Posts: 432
What good would carb heat be if it actually CAUSED carb ice? I agree, that story is a crock.
You're saying that the guy turned on the carb heat, and all of a sudden, the warmer air started sucking up moisture? Moisture from where??? If the air is too cold to hold enough moisture to cause carb ice, then there's also not any moisture to be picked up by warmer air! Just because warm air has the capability of holding more moisture, doesn't mean that it always does. The moisture would have to come from somewhere, and if the "colder" air couldn't provide, then where is it coming from?
You're saying that the guy turned on the carb heat, and all of a sudden, the warmer air started sucking up moisture? Moisture from where??? If the air is too cold to hold enough moisture to cause carb ice, then there's also not any moisture to be picked up by warmer air! Just because warm air has the capability of holding more moisture, doesn't mean that it always does. The moisture would have to come from somewhere, and if the "colder" air couldn't provide, then where is it coming from?
10-31-2008, 07:01 PM
#20
Gets Weekends Off
Joined APC: Oct 2005
Position: Trying not to crash
Posts: 1,260
Does the AFM say use it on T.O.? If not, don't use it. What about landing? Are you saying you do something that is against what the AFM says in its procedures?
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