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static line

bobnoffs

Well Known Member
hi all,
anyone found inaccuracies in airspeed etc. by just putting a filter on the static port of an instrument and let it sample air from the avionics bay?
i am a vfr only pilot. seems like less places for the static line to plug up etc. i could never see any inaccuracy the last time i did this but am wondering if others have noticed the same.
 
hi all,
anyone found inaccuracies in airspeed etc. by just putting a filter on the static port of an instrument and let it sample air from the avionics bay?
i am a vfr only pilot. seems like less places for the static line to plug up etc. i could never see any inaccuracy the last time i did this but am wondering if others have noticed the same.

Air inside the aircraft is not necessarily "static". Open and closing outside vents will change the "static" inside the fuselage.
On low speed aircraft like Cubs & such this is sometimes acceptable, but not in an aircraft like the RV.
 
Mel's advise is as usual valid. I failed to secure a static connection one time after messing with an autopilot. opening or closing a cabin vent would make the autopilot change altitude because it was reading cabin static.
 
My Alternate static source valve switch from Stein is cockpit air behind the panel.

Open the valve in cruise with vents open or closed on the autopilot and the plane descends 30 to 50'.
 
Lots of posts with reported airspeed errors due to static port issues, recommend you go with the stock van's setup.
 
Question about static air

I was wondering why cabin air cannot be used as a static air source. If it could be used, there would be no need for static plumbing. I assume the reason is that cabin air pressure varies due to aerodynamic effects of the aircraft?s speed, e.g. inlet ram air from ventilation ducts, suction from Venturi effect of airflow around the fuselage, etc.

Yet I am wondering how large these effects are in changing cabin static air pressure. For example, imagine an altimeter connected to the normal static system, but accompanied by an altimeter using only ambient cabin air as its static source. How large of discrepancy between these altimeters would be expected? Also how do we know the airflow at the static port itself is of neutral effect ?
 
Depending on your definition, the error(s) could be quite large. I can tell you from personal experience that an RV-4 with static ports in the recommended location, but simply flush on the fuselage sides (without the recommended pop rivet size bump), will cause airspeed errors of around 10 kts (low) at cruise, and altimeter errors of as much as 75 feet (low) at speed. I'd expect errors due to cabin static to be even larger.

Charlie
 
Depending on your definition, the error(s) could be quite large. I can tell you from personal experience that an RV-4 with static ports in the recommended location, but simply flush on the fuselage sides (without the recommended pop rivet size bump), will cause airspeed errors of around 10 kts (low) at cruise, and altimeter errors of as much as 75 feet (low) at speed. I'd expect errors due to cabin static to be even larger.

Charlie

Why would you expect that? I ask only because I've often wondered about this myself.

The one report so far in this thread shows about half that error.

What about putting it waaaaay in the aft portion of the fuselage? Or inside of a wing? It's an interesting engineering question, I think...
 
I didn't expect it. :)

But that doesn't change the facts. Airspeed was ~10 kts slow, and during a high speed pass down my home field runway would show my altitude as around 50 feet below field elevation. Gluing a rivet head over the static ports corrected both.

edit: I should mention that while I have no credentials & can speak with no authority, Kevin Horton is the one that cued me in on the cause (and cure) of/for the errors. Thanks, Kevin!
 
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I didn't expect it. :)

But that doesn't change the facts. Airspeed was ~10 kts slow, and during a high speed pass down my home field runway would show my altitude as around 50 feet below field elevation. Gluing a rivet head over the static ports corrected both.

edit: I should mention that while I have no credentials & can speak with no authority, Kevin Horton is the one that cued me in on the cause (and cure) of/for the errors. Thanks, Kevin!

I know and understand why the flush static ports on the outside cause errors (generally speaking, involves the boundary layer)...I'm asking about the effect of a static port *inside* the fuselage or elsewhere. There's no boundary layer in the cockpit :)
 
FYI

Here you go:

Flight deck static pressure is lower than outside static pressure. The reason for this error with alternate static source is that the air flowing around the airframe creates a slight vacuum inside the airframe, thereby lowering the barometric pressure and generating erroneous readings in the pitot-static system.

From the Pilots Handbook of Aeronautical Knowledge Pages 7-12
 
Here you go:

Flight deck static pressure is lower than outside static pressure. The reason for this error with alternate static source is that the air flowing around the airframe creates a slight vacuum inside the airframe, thereby lowering the barometric pressure and generating erroneous readings in the pitot-static system.

From the Pilots Handbook of Aeronautical Knowledge Pages 7-12

Excellent! Thanks...that's the answer I was looking for...
 
The altitude hold on my autopilot sense the cabin air pressure. The first time I pulled cabin heat on while using it, I got a big surprise! It will do the same opening or closing vents. I've never measure the actual difference, but suffice to say that there are several variables that could give erroneous readings while using cabin air for static pressure.
 
I bought an Aerostar many years ago and suffered a dose of buyer?s remorse on the flight home, all due to an open alternate static under the panel! Settled in the cruise at 8000 I couldn?t get more than 185 knots. Very disappointed. Finally found and closed the open static and immediately had a TAS nudging the magic 200. There can be a big difference between actual and cabin static especially if vents are open.

Jack
 
Real meaning

So does the static ports further back towards the tail mean the fuselage has lower drag? The way I see it, the static ports are located where the flow around the fuselage returns to free stream pressure. If this is true, and I have no training that says it is true, than a static port further back means less drag. It might follow then that a less draggy cooling airflow mod may mean static pressure error is slightly higher unless the ports are moved.
But what do I know, I am not any aero guy , just an arcy sparky.
 
So I was confused while studying for the instrument exam when the correct answer was to break the face of the vertical speed indicator in the event of a static air blockage.

I told my CFII that seems a little harsh on the passenger don't you think.

Get it, screaming, passenger during maneuvers, oh never mind...
 
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