Question about alternate static system

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tankibenc

Filing Flight Plan
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tankibenc
When you use the alternate static air in the C172S, why is the pressure in the flight deck lower compared to regular static port pressure?
 
Airflow around the cabin creates a low pressure. Static ports are placed to be more accurate.
 
Bernoulli is actually relevant for this particular scenario. Air has to accelerate to get around the fuselage and Bernoulli tells us pressure has to go down (conservation of energy along a streamline).
 
Around the static port, there is fast moving air, and fast moving air means low pressure. That means the pressure inside the flight deck must be lower than that. But how is that possible if there is no airflow in the flight deck? Flight deck logically speaking would have higher pressure.
 
tankibenc said:
Around the static port, there is fast moving air, and fast moving air means low pressure. That means the pressure inside the flight deck must be lower than that. But how is that possible if there is no airflow in the flight deck? Flight deck logically speaking would have higher pressure.
Click to expand...
Let get a cigarette and see where the smoke goes…you’ll find that the pressure inside the cabin is higher than it is where the smoke is going (there is airflow), but that also lowers the pressure in the cabin below static.
 
It depends where your alternate air is vented. The description above hold true for the 172S. Cabin airflow is pretty bizarre at times. In the Navion, the air in the cabin gets sucked in from the tail and is blown out along the canopy rails. The static ports are both on the forward tail cone so I suspect they are in a fairly neutral pressure area.

Twice I've had static blockages, once in my Navion when evacuating my plane from VKX right after 9/11, the other ferrying a club Arrow. The arrow had an alternate static. For jollies, I closed it on approach to the maintenance shop so I could taxi in showing the airspeed at 40 knots or so while parked. The other was in the Navion (no alternate), but in VFR conditions, it's really kind of a non event. I use the reported winds and the GPS groundspeed to guess when I was below 87 knots to drop the gear. We've also landed the 172N with the pitot plugged. You should be attuned to what how your plane sounds/behaves when near the stall anyhow.

Rod Machado points out that when students are given the choice of an instrument to break, they usually pick the hobbs meter.
 
tankibenc said:
Around the static port, there is fast moving air, and fast moving air means low pressure. That means the pressure inside the flight deck must be lower than that. But how is that possible if there is no airflow in the flight deck? Flight deck logically speaking would have higher pressure.
Click to expand...
Faster than what? Lower than what?

The air in the hose coming from my air compressor is moving pretty fast - does that mean that the pressure inside the hose is lower than the still air around the hose? Do my air tools work by sucking air from around the tool and putting it in the hose?

Bernoulli's equation (which is simply the conservation of energy applied to a fluid) applies only along a streamline. Of an ideal gas. With no addition or loss of energy. Thus, blowing over a piece of paper has nothing to do with demonstrating Bernoulli's equation.

The Flight deck is close to the static pressures wherever the most air leaks happen.

The static port is usually placed somewhere where the air swooshing around the fuselage has had enough distance to settle down and get closer to the ambient pressure.
 
Why do my altimeter and VSI register a change when I check alt static during preflight? Engine off. I guess wind could be a factor... Perhaps heat too.
 
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