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On my 0-360 A1D Rayjay turbo powered constant speed prop M20B I get zero effect on idle (or any) RPM when applying carb heat on the ground.  Is that normal? Sorry I didn’t notice if there was an effect on manifold pressure.. it threw me to not see any effect on rpm.

Edited by Spike Kavalench

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I'm not sure what effect the turbo has...I don't have one,  but for a normally aspirated O-360, when you check your carb heat on the ground, go full rich.  Often the drop is minimal if you are doing lean ground ops.

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Gus is right. My understanding is that the post-engine-start Carb Heat pull is to make sure the engine continues to run smoothly. My procedure is crank engine, set 1000 RPM, look for Oil Pressure, pull Carb Heat then turn off, lean mixture. Not much of an effect . . . .

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Carb heat OK back to school-

When fuel is atomized in the throat of the carb the temperature drops. It can go many degrees lower enough to make the air below freezing. Any moisture in the air (even humidity) can and will freeze in the throat of the carb.  At low power settings it can completely close off the throat and kill the engine. 

Carb heat is used to keep the temperature above freezing and/or melt the ice already formed. Hot air comes from the muffler shroud to the carb by way of a duct. 

When carb heat is checked at run up one needs to see a corresponding drop in RPM showing that the hot air is in fact getting to the carb. Hotter air inducted causes the intake mixture to go rich because of the lower density of the hot air coming in thereby causing a loss of power (less RPM). 

If you don't see the drop in RPM on runup then that indicates that the hot air (carb heat) is not getting to the engine. If it doesn't get to the engine then you won't be able to clear any ice formed in the throat of the carb in flight. Not a place I would want to be. 

What does your flight manual say about Carb Heat use with the turbo?

 

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Cliffy, does your Manual say to check Carb Heat at runup? Here's what mine says.

In the Engine Start checklist:

Screenshot_20180213-211405.thumb.jpg.40e0ae563fddb00973c9b2e2ce7210a4.jpg

And for Runup:

Screenshot_20180213-211428.thumb.jpg.118a1a74bbb940b4a2c8f950620aae16.jpg

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Hank, I can understand where you are coming from on this. Not trying to say go against a check list.

My posting was more to educate on the operation and whys of carb heat rather than exactly when to test it as there seemed to be some confusion here on its use. Understand exactly why a system works or is required is paramount to understanding its use in flight. I'm VERY old school in that I was taught to check it on run up  along with a mental runup check list- controls, instruments, fuel, flaps, trim, seat belts, runup (mags and carb heat), rather than a "read and do" list. Even in Boeings we use a mental "flow" and then just check that stuff was done with the check list. I'm going back 60 years and it still sticks with me. 

How people are taught to fly today compared to back then has changed dramatically. We did FULL stalls straight and turning for the PP test back then. You don't do that today (I think we should).  New isn't always better. The FAA even got whacked for their ME VMC demo rules years ago after too many were killed following the FAA way. 

Carb heat won't check too well until the muffler gets heated up enough to run hot air to the carb heat system hence if it is checked right after start it might not show much of an RPM drop.  If you don't see the drop, how do you know its working? Its vital to safety in a carbed engine. 

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I know little about turbos and have a hard time understanding how the turbo would have an impact on the carb heat-induced rpm drop at runup.  The purpose of the check at runup per my understanding is simply to check if the system is functioning, as Cliffy notes, lest you really need it in flight. It may also clear any ice built up during the low power operations on the ground. If the rpm doesn't drop at runup RPM and full rich when pulling full carb heat, I think something is seriously wrong.  The basic system is not complex and can be partly checked by a quick look under the left side of the cowl by even a simpleton like me.  The cable attaches to a lever that opens and closes a large door facing the front intake in the airbox.  The lever position determines the relative amount of air that comes from the front intake vs the one at the back of the doghouse that is heated via the muffler shroud.  Don Maxwell does a beautiful job describing the system in the C, along with its variations over the production years, in this article:  http://www.donmaxwell.com/publications/MAPA_TEXT/Carb Heat Maintenance/CARB_HEAT_CARE.HTM

A quick personal anecdote that illustrates the utility of the runup procedure:  I was about to leave on my first long trip after getting my plane, and I pulled on carb heat at runup. The RPM dropped appropriately.  I pushed it back in, but the RPM didn't rise back to 1700.  Puzzled, I pulled it on again, and the control came out of the panel into my hand because the cable had snapped during the first pull.  This experience also illustrates that the default position for the door is closed (i.e. carb heat on) if the cable is not attached and pushing it open.  This design makes sense from a safety perspective. But if the OP's door is stuck in default position, it may also be robbing him of some power at takeoff that he would like to have back, as well as sucking in unfiltered air at all times.

I realize carb ice is rare in the engine-carb configuration in M20Cs, but I'm still glad I checked and paid attention to the abnormality (RPM not rising back to 1700) in the above instance.

 

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On 2018-02-13 at 6:56 PM, cliffy said:

Carb heat OK back to school-

When fuel is atomized in the throat of the carb the temperature drops. It can go many degrees lower enough to make the air below freezing. Any moisture in the air (even humidity) can and will freeze in the throat of the carb.  At low power settings it can completely close off the throat and kill the engine. 

Carb heat is used to keep the temperature above freezing and/or melt the ice already formed. Hot air comes from the muffler shroud to the carb by way of a duct. 

When carb heat is checked at run up one needs to see a corresponding drop in RPM showing that the hot air is in fact getting to the carb. Hotter air inducted causes the intake mixture to go rich because of the lower density of the hot air coming in thereby causing a loss of power (less RPM). 

If you don't see the drop in RPM on runup then that indicates that the hot air (carb heat) is not getting to the engine. If it doesn't get to the engine then you won't be able to clear any ice formed in the throat of the carb in flight. Not a place I would want to be. 

What does your flight manual say about Carb Heat use with the turbo?

 

Nice reply, Cliffy, thanks very much. That’s what I expected, same as every other plane I have flown, so I wanted to confirm. It’s back in the shop. The plenum between the filter and the carb looked pretty rough so I suspect that was the problem. The turbo was an add on and I don’t think I have a manual for it. Thanks again!

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@Spike Kavalench, while it's in the shop, have the flapper checked. One of the two hinges on mine broke off, flapping wide open. It did awful things to my climb rate leaving a short, thankfully downhill strip in the mountains. Even Atlanta Center asked if I was really a Mooney as I passed south of the Bravo. 

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Further to what Cliffs is saying I was taught that at run up power (and heat) to apply carb heat, watch for the drop, the to lean the mixture to see an increase in power caused by leaning the overly rich mixture caused by carb heat.

Clarence

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There are two intake options for the O360... (As there are for all planes)

  • Main filtered route...
  • Alternate air, routed through the heat muff, route.
  • The alternate air route is smaller than the filtered route and causes a slight MP drop on its own... before the heated air is applied... (this alternate air source is important, in the event of a filter blockage at the front of the cowl)

 

 

+1 on the plumbing check....

My C’s carb heat plumbing was so worn out it stopped working....

Turbo or not, the carb heat should be operational, and testable on the ground during the run-up...

Unless the turbo air intake has excluded the carb heat...  (check on how the plumbing is)

Can you notice any effect of the turbo during the run-up?  How much MP/rpm is needed to for the turbo to spool up?

 

wait a minute....

It is quite possible that the compressor is changing the RH of the air coming to the engine, and carb heat has been deemed not very helpful.

The plumbing of intake air to include carb heat, the heater muff, and a turbo... sounds way too complex to actually work this way.  But, the alternate air source still needs to be there...

 

I highly recomend you check your plumbing to see what the carb heat knob is actually attached to.

 

Let us know what you find.  There are not too many O360s with a turbo around...

PP thoughts only not a mechanic...

Best regards,

-a-

 

 

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To be really technical-

You are required to have the manual for the operation of the turbo with you in the airplane so that you and anyone else who flies the airplane knows how to operate it properly as it is add on equipment. You really do need to find a copy of it. If you got ramp checked they would look for it IF they  knew enough about it which they probably wouldn't.  Someone, somewhere has a copy of it. No, if you don't want to go to that trouble and just want to remove the turbo, call me, I'll take it off your hands :-) :-) :-)   Hope you get it figured out. 

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23 minutes ago, cliffy said:

To be really technical-

You are required to have the manual for the operation of the turbo with you in the airplane so that you and anyone else who flies the airplane knows how to operate it properly as it is add on equipment. You really do need to find a copy of it. If you got ramp checked they would look for it IF they  knew enough about it which they probably wouldn't.  Someone, somewhere has a copy of it. No, if you don't want to go to that trouble and just want to remove the turbo, call me, I'll take it off your hands :-) :-) :-)   Hope you get it figured out. 

Actually, somebody on this site already has a turbo for sale, you should look him up if you want one. As for a manual, everything you need to know is placarded on the panel and I have all the installation drawings and paperwork, so if an over zealous inspector asks, I have answers, but thanks for the tip! 

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YUP momentary brain fart time. Older Mooneys are "placard" airplanes as opposed to flight manual airplanes. 

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