dl773

Hello all,

I am a little confused with these concepts now that I moved from a normally-aspirated Cessna 182 to a Turbo 182.

Clearly, I get a boost in power from both models when I turn off carb heat. However, I can get "take off, go around" manifold pressure indication on the turbocharged aircraft without turning off carb heat. This will not happen in the normally aspirated 182.

This is a bit of a problem on go arounds. If I simply apply the required 31 inches of MAP, then shut off carb heat, I overboost the engine.

Question is - for going around, do I need to remove that carb heat right away or can I just focus on MAP?

Thanks!

CaseTractor

What year/engine configuration do you have? It's tough to give sound advice without looking at your POH, so I'll speak generally and include a POH from a later model 182 with the Lycoming Turbo.

Also, what is "overboost" in your circumstance, an inch or many inches? This may be more of a controller issue not dealing well with power changes if it is more than a couple.

I cannot answer, your POH and qualified CFI will have to do that.

My recommendation would be to turn alternate air off first, then apply power, but I don't think you are hurting anything doing it the other way around and turn off alternate air off later and it is rising by an more than an inch. Getting power up and flying the Go Around should always take priority over fiddling for a small lever w/o looking or having time to look.

Also, I'd recommend not doing go arounds unless operationally necessary. Turbos are more happy at one temperature and not changing abruptly.

If you care for details, feel free to see my logic.
http://www.redskyventures.org/doc/cessna-poh/Cessna_182_T182T-2007-NavIII-G1000+GFC700-POH-PIM-fromCessna.pdf

Page 7-40 provides some good info.

Your 182 w/o turbo, if it was carbureted, would have been prone to icing, requiring carb heat in humid conditions with so much airflow through a venturi. Pressure change, fuel vaporization, and temperature drop associated with this causes the ice to form. So you need carb heat.

Fuel injected engines do not have this issue since the throttle bore is straight. That being said, they don't have carb heat, but an alternate air door/source. This air is not heated but drawn from engine compartment should primary path become blocked. Alternate air may have a lever in cockpit or be automatic with spring pressure keeping it closed unless required. Or Both.

If your Turbo 182 is injected (most likely) I doubt you the POH would call for alternate air on approach.

Also, Turbos will slowly "hunt" to reach equilibrium, so any configuration change (air intake) may cause it to overboost. Also, larger power changes (such as a go around) can cause an over boost. If it is momentary and only an inch, it is not considered a problem (see POH 7-45).

I think what is happening in your case is when you switch the alternate air off, ram effect increases manifold pressure since air being pushed into the air filter has a higher pressure than air drawn from engine compartment.

In the other plane, you were getting more power because the air was becoming cooler (denser) when you switched carb heat off. This didn't increase power due to manifold pressure change, but temperature change.

So basically the planes are increasing power for totally different reasons since they work quite differently.

dl773

1979 Cessna TR-182 with carburetor.

235HP @ 2400 RPM and MP 31 inches. This is just below the redline. If I apply throttle and hold at 31 inches, then close the carb heat, the MP will shoot well past the redline. I am unsure of how many inches it adds, but I'd say around 5 inches easily.

Unfortunately I have been unable to find the POH for this exact model online and I don't have a copy of it with me at the moment.

edit: I believe it may be a Lycoming O-540-L3C5D. Will double check tomorrow.

dl773

Picture of the engine diagram:

https://dl.dropboxusercontent.com/u/70008773/TR182.jpg

Type Certificate:

http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/0/084b5675182d8714862575f6006d1bf4/$FILE/3A13%20Rev%2069.pdf

See page 15.

Lycoming O-540-L3C5D, turbocharged

dl773

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CaseTractor

You'll have to disregard most of what I said in the previous post as I was referencing a completely different Turbo System. I've not flown one of these, so I can't say what is normal. You'll need to find a POH or someone very familiar.

I can see that when carb heat goes cold, the engine will produce more power (with denser air just like an natural engine) and thus produce more heat, which drives the turbine faster, producing more manifold pressure at a given throttle setting. I wouldn't expect it to be as high as 5 inches, but I have no frame of reference.

Ensure the throttle and wastegate linkages are adjusted to the required settings. I'd guess this is a pretty sensitive setup and all power changes should be as smooth and slow as possible. Also, ensure the pressure relief valves is adjusted properly.

Good luck, I'm sure the answer is simple and someone out there knows what is normal in this case. POH should be directive on sequence as well.

yimke

Why not just go-around then at 500 AGL just reduce 5" mp then turn off carb heat as part of the after takeoff checklist? Adjust your prop as normal.

I'm confused if you are asking if you have a mechanical problem or operational question?

Also, there is a pressure relief valve before the carburetor that "should" prevent over-boosting of the manifold if you turn carb heat off.

dl773

Thanks all for the help.

I was finally able to check the POH, and it indeed says not to worry about turning carb heat off until stabilized and after the power has been reduced to 25 MP.

The POH further states that temporarily exceeding the redline in the manifold pressure gauge is not harmful, if promptly corrected.

JohnBurke

I find that the subjects of manifold pressure, mixture setting, and carburetor use tend to be very misunderstood by most pilots who fly that equipment, and even many of the mechanics who work on it.

Your engine is a suction machine. Your throttle is a plate that covers the air inlet into that suction machine. Think in terms of a vacuum cleaner. Put your hand over the end of the hose, and the pressure in the vacuum hose drops. Same for closing the throttle in a normally aspirated aircraft engine. Pressure drops, as you're closing the induction inlet.

If you shut down the engine, you'll read ambient barometric pressure on the manifold pressure gauge. At sea level, shut down and look at the gauge, and you'll see just shy of thirty inches. Do the same thing at five thousand feet, and you'll see about twenty five inches. The pressure in your manifold is barometric, or ambient air pressure. When you open the throttle, you're not increasing pressure so much as letting it in. If you leave the throttle setting alone but increase the temperature of the air, the density of that air decreases. Decrease density, and you've got a decrease in pressure.

If you set manifold pressure while carb heat is in use, then shut off carb heat (decreasing temperature and increasing density), you'll see a rise in manifold pressure.

A normally aspirated engine can't get more than barometric pressure, even at full throttle (a slight rise above that is possible, due to ram effect or a well designed induction intake and propeller combination). A turbocharged engine, however, uses an exhaust-powered turbine to increase manifold pressure above barometric.