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Robinson Safety Notice SN-25 Carburetor Ice - Clarification of the additional 1.5 in. HG additional MAP

Robinson R22 SN-25 Carburetor Ice Safety Notice

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#21 Mikemv

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Posted 17 September 2012 - 06:36

In response to MikeMV, I never noticed. I just know we started to drop and I pulled a little collective and watched the MP to make sure I was where I was supposed to be.


THIS IS HYPOTHETICAL BASED ON A DROP IN INDICATED MP

IF the MP indication DOES drop when full carb heat is applied, there's really no need to amend the POH because even if you do add that 1.5" (from its new indication), you're only bringing it back up to where it was before you added the heat and therefore not exceeding the limitation with the extra 1.5"

Pictures? Ok...

MP before carb heat = 20.0
MP WITH carb heat = 18.5 (this assumes a drop in indicated MP)
MP with extra 1.5" pulled in = 20.0

I think this was what Mike was getting at.

This was the point of my question exactly. Now at 20.0"MP, what happens when you remove carb heat? 21.5"?

Do not worry as iChris has addressed this as not busting a limitation! Do not read this as selectively disregarding Sect. 2. Limitations.

Mike

#22 lelebebbel

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Posted 17 September 2012 - 08:04

This was the point of my question exactly. Now at 20.0"MP, what happens when you remove carb heat? 21.5"?


Manifold pressure does not drop when carb heat is applied - it increases. Carb heat reduces engine power, which reduces engine RPM, causing the governor (or pilot) to open the throttle, thus increasing MAP. Manifold pressure increases by about 1-2" when full carb heat is applied. In your example, MAP would drop from 20" to around 18 or 19" when carb heat is removed.

iChris

The MAP limits and MAP charts in the RFM are for the convenience of the pilot in estimating horsepower output. MAP limits or MAP references are not listed in the TCDS; however, the RFM is not in contradiction since it does reflect, indirectly, the horsepower limits prescribed in the aircraft’s type certificate.



Hmmm.. so the RHC TCDS does not directly contain MAP limits, the limitations section of the flight manual - and the placards in the cockpit - do. The newest safety notice instructs us to ignore these limits under certain conditions, but so far no mention of this is made in section 2 or on the placards in the cockpit..
I am not a lawyer and I can't even begin to guess how all this would hold up before a judge - or not. Regardless, I still find this a rather unsatisfactory situation.

Edited by lelebebbel, 17 September 2012 - 08:16.

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#23 ridethisbike

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Posted 18 September 2012 - 03:13

Manifold pressure does not drop when carb heat is applied - it increases. Carb heat reduces engine power, which reduces engine RPM, causing the governor (or pilot) to open the throttle, thus increasing MAP. Manifold pressure increases by about 1-2" when full carb heat is applied. In your example, MAP would drop from 20" to around 18 or 19" when carb heat is removed.



You say that the MP actually increases with carb heat applied and then go on to say that it's the pilot causing this increase. If the pilot is the one CAUSING the increase, then logic points to a decrease from initial MP with the introduction of carb heat.


Reasons I agree that the MP decreases with the introduction of carb heat PRIOR to pilot input? Well what do we know about cold air vs hot air? Cold air is more dense and therefore has a higher pressure. Hot air is thinner and therefore has a lower pressure.

Now I know the first thing everyone is going to say is "nuh uh! When you warm air up it has a higher presure!" When you have a sealed container of air and heat it up, it will in fact exert a higher pressure on the container. This isn't because it actually has this kind of pressure, it's because the air is trying to EXPAND. This expansion is what exerts the pressure on the container.

None of the intake tract is a sealed container. The carb heat is being pulled from outside where there are no constraints on it's ability to expand so it isn't going to magically start building pressure. In fact, the opposite is true since we know that Bernoulli's Principal will affect that hot air as it passes through the carburetor. The velocity of the air will increase and according to said principal, the pressure will decrease even further. "But that's only at the venturi" Also true, but remember, a naturally aspirated engine pulls its air in via a vacuum in the cylinder. The air being pulled in by the cylinder increases its velocity through the intake tract, thereby decreasing it's pressure.

So, with a given MP and the introduction of carb heat PRIOR to pilot intervention, the MP indication should drop. Then it'll rise when the pilot pulls in more collective.


PS I hope that all makes sense... haha. I know it's broken down about as barney style as it gets, but it (hopefully) helped you understand what was going through my brain. It can be hard for me to explain things without breaking it all down sometimes...

#24 Lindsey

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Posted 18 September 2012 - 09:53

Ridethisbike--

Lelelebel's point (I believe) is that if you are in a hover with no carb heat applied, and then you apply carb heat, you will be seeing approximately a 1.5" increase in indicated MAP *to maintain that same hover*. Which, in my experience, is true.

#25 Mikemv

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Posted 18 September 2012 - 11:41

Ridethisbike--

Lelelebel's point (I believe) is that if you are in a hover with no carb heat applied, and then you apply carb heat, you will be seeing approximately a 1.5" increase in indicated MAP *to maintain that same hover*. Which, in my experience, is true.


I think everyone is in agreement that applying carb heat decreases MP. Lele's point was that after the decrease in engine RPM and (unstated but corresponding) MP that the pilot or governor will increase RPM & therefore MP.

I will ask what will happen to MP if the helo is at full throttle during a hover and carb heat is applied? (I know the answer but throwing it out for consideration) Let us make it any recip. helo just for understanding the principle of physics and operation. Now we can also be R22 specific.

Mike

#26 ridethisbike

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Posted 19 September 2012 - 01:20

Lindsey, I think the misunderstanding between lele and I is just a matter of two separate minds trying to view the same thing from different angles. I was merely trying to explain how I'm viewing it. That's all.

Well Mike, it stands to reason that if no intervention was made, the pressure would go up. Followed by a decrease once the pilot/governor made the appropriate corrections. Also, it would no longer be at full throttle since the increase in performance would warrant a reduction in throttle.

#27 lelebebbel

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Posted 19 September 2012 - 07:43

Reasons I agree that the MP decreases with the introduction of carb heat PRIOR to pilot input? Well what do we know about cold air vs hot air? Cold air is more dense and therefore has a higher pressure. Hot air is thinner and therefore has a lower pressure.



Sorry but this is not correct. Cold air has higher density, but not automatically higher pressure. These are two different physical properties. Pressure affects density, but density can also change completely independent of pressure.
For example, Sulfur-Hexafluride gas has an extremely high density, but if you filled it in a bath tub at home, it would have the same pressure as the ambient, much less dense air.
Or, for a more aviation-relevant example: During the average day, the temperature might change 10 or 20C between early morning and mid-day, but the altimeter setting might stay the same all day.


If there is no governor or pilot input, adding carb heat will not directly affect manifold pressure. It will, however, lead to a slight drop in engine RPM, which can in turn affect manifold pressure slightly, simply due to the change in suction.

Edited by lelebebbel, 19 September 2012 - 07:53.

When In Danger
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