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

[ridethisbike]

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.

[lelebebbel]

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 September 19, 2012 by lelebebbel