Folks with digital carb temp - how do you use it?

[Mooneymite]

4 minutes ago, N1395W said:

I wonder if it changed for later models.  1967 and before definitely had heat from the muffler shroud ducted to the carburetor air box.

My '74 also has ducting from the muffler shroud to the carb air box.  My carb heat is definitely not just "cowl heat".  It's as hot as the air provided to cabin heat (minus the longer ducting heat loss).

In all these discussions of carb heat, I'm always perplexed by the resistance of some pilots to use the factory installed carb heat in accordance with the POH.  It doesn't cost a penny and at altitude, the danger to the engine of unfiltered air is miniscule.  My POH is pretty clear:  Warning:  Apply full carburetor heat when reducing power for descent, or landing.  Pretty clear to me.

Anyone who's left the carb heat on for take-off (NOT ME!!!!  ) knows that takeoff performance is terrible, but in a descent.....it is not noticeable, particularly if you lean the mixture properly.  So, my question is:  why not use carb heat for every descent like the POH says?

Hank, I realize our POH makes certain exceptions for a carb temp gage, but it does not say to not use it.  Those with a carb temp gage may use "partial" carb heat.  Unless you have a warning light, or watch that gage constantly, there's no assurance that carb ice won't sneak up on you while you're busy flying.

It's free.  Use it.

 

 

[Hank]

I don't use carb heat when I descend to land because I don't reduce speed or throttle. Coming down from 9-10K, I'll hold 20" and my cruise EGT until it's time to slow for pattern entry.

[carusoam]

M20F,

In fact, pressure does have an effect on freezing point.  'Water's melting point falls slightly when pressure is increased'.    for more details ask Siri about the Clausius-Clapeyron equation.  Water is a bit quirky because it's frozen density is lower than it's liquid density. 'Slightly' is one or two degrees °C according to the source.

Turns out I'm not familiar with pressure carb's either... But if that is like a turbocharger feeding a carb....The act of compressing air generates heat and increases the air temperature. Hence the reason for inter coolers and intake air temp gauges.

Other observations:

It's definitely a good idea to add the instrumentation if you don't  have it.  My carb had the position for a thermocouple with a plug in it.

The small butterfly valve  was opened and closed by some control wires.  The system had significant wear.  The original wires were not stainless. Under pressure they bent.  It looks like a hose can be mount to the body of the butterfly valve.

if there was supposed to be a tube coming from the heater muff, mine was missing.  I didn't recognize it's absence!. 

Great discussion,

-a-

 

[Mooneymite]

9 minutes ago, Hank said:

I don't use carb heat when I descend to land because I don't reduce speed or throttle. Coming down from 9-10K, I'll hold 20" and my cruise EGT until it's time to slow for pattern entry.

I guess the POH assumes that you will retard the throttle at some point.  It just says "for descent".....obviously you reduce your throttle in preparation for landing.

Unless you really like "long landings"!  

Edited February 8, 2016 by Mooneymite

[BDPetersen]

Did the IO-360 engines have alternate air source? That is the only system I can think of not involving a heat muff. 

The old C.A.R. certification requirement for "sea level engines with conventional venturi carburetors" was for an induction heat system capable of a 90F temp rise unless there were design features which contributed to raising intake temps. Then 60F was the requirement. Perhaps the Lycoming method of mounting the carb proximate to the oil sump qualified for that.

In and of itself, the MA4-5 carb is a wonderful ice machine. Hang it under a Franklin or big Continental to prove that. On the Lycoming, not so much.

[carusoam]

Air intakes have evolved a lot.  Getting the air filter off the front of the cowl was a step to keep snow and ice from simply blocking the filter.  Some modern air intakes include an alt air door that automatically opens when it overpowers a magnet...

Best regards,

-a-

[DXB]

1 hour ago, N1395W said:

I wonder if it changed for later models.  1967 and before definitely had heat from the muffler shroud ducted to the carburetor air box.

I'm pretty sure, at least per the Dmax article, that the air gets heated via same muffler shroud that heats the cabin in all carb'd Mooneys.  The article was very useful to my understanding after multiple A&Ps, including an MSC owner, puzzled over and misunderstood how the carb heat system in my '68C was rigged. To summarize:

1. Pre-'62 - No bypass valve on the carb heat box, so when carb heat and cabin heat are both off, the hot air in the shroud just sits there and overheats the muffler. 

2. '62-'67 - Turning off the carb heat simultaneously  opens a bypass valve on the carb heat box.  It solved the problem above, but the bypass valve mechanism was finicky.   

3. '68 on - No more bypass valve on carb heat box. Instead the bypass valve is moved to the cabin heat control on the right firewall.  When cabin heat is off, some air is always exiting the shroud to the cabin heat control, where it gets dumped via a scat hose ending near the right cowl flap opening.  When carb heat is opened with cabin heat off, I wonder if airflow out this hose slows or even reverses to prevent loss of carb heating capacity.  

Seasoned Mooney mechanics have told me that some unscrupulous mechanic previously "welded shut the bypass valve" on the carb heat box of my '68. In reality the factory probably welded it shut on their stock of older style carb heat boxes left over from the previous year when building my plane, which was the second '68C built.

In any of these systems, I would think that using cabin heat would decrease carb heating capacity, and vice versa, but I don't know if it's enough to matter.  Still, I wonder if it would be worth closing cabin heat if one got into a bad carb ice situation?  

[Andy95W]

Nice description, Dev.

I guess we need to get Hank to check out the effect cabin heat has on carb heat application, i.e., how much difference is there in carburetor temperature with cabin heat on then off.

[carusoam]

Include what heat is delivered to the inside of the windshield in your description.  You will probably be running that as well.

Best regards,

-a-

[takair]

6 hours ago, BDPetersen said:

Did the IO-360 engines have alternate air source? That is the only system I can think of not involving a heat muff. 

The old C.A.R. certification requirement for "sea level engines with conventional venturi carburetors" was for an induction heat system capable of a 90F temp rise unless there were design features which contributed to raising intake temps. Then 60F was the requirement. Perhaps the Lycoming method of mounting the carb proximate to the oil sump qualified for that.

In and of itself, the MA4-5 carb is a wonderful ice machine. Hang it under a Franklin or big Continental to prove that. On the Lycoming, not so much.

On the E there is the ram air door (not really an alternate) and there is an automatic alternate air valve that takes warm cowl air into the air box if the filter clogs.  It is spring loaded and low pressure in the air box causes it to unseat and allow air to flow to the fuel injection servo.  It must be properly maintained to be sure it does not jam shut.