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Pitot-static issue

My airspeed reads normal in takeoff roll but falls to erroneously low values during climb done at a normal angle of attack and normal GPS ground speed. In cruise it catches up and goes higher than it should be. In descent it reads much higher than it should. Both the round gauge and the G-5 are doing this. The altimeter is normal but the VSI lags level changes in a sluggish way. Before I start chasing this, who has experienced the same and where should I start? I have blown out the tube itself with no positive results.
 
My airspeed reads normal in takeoff roll but falls to erroneously low values during climb done at a normal angle of attack and normal GPS ground speed. In cruise it catches up and goes higher than it should be. In descent it reads much higher than it should. Both the round gauge and the G-5 are doing this. The altimeter is normal but the VSI lags level changes in a sluggish way. Before I start chasing this, who has experienced the same and where should I start? I have blown out the tube itself with no positive results.

You have a blockage in the STATIC line. Check to make sure your lines are installed properly in the tailcone area with no low spots in whcih water can collect. That is usually the primary culprit.

If that's OK. Then you have a blockage or perhaps a kinked STATIC line.
If you happened to put an alternate static port on your panel, activate it and all will be well (except airspeed will read a little high and so will altitude due to the lower pressure of the cabin area).

Here's a little primer to help you understand why you are seeing your erroneous numbers:

With a blocked static line the pressure inside the static line actually increases as you climb, so it puts HIGH PRESSURE on the static side of the airspeed, causing it to read low. ONce it equalizes in cruise (due to a slow leak usually) it will read close to normal.
On descent, the reverse is happening. The pressure inside the static line now decreases, so the airspeed reads high.

For those who might be confused about the pressure increasing in a climb, and decreasing in the descent, keep in mind that this is with a BLOCKED line.

Vic
 
Minor point - the static pressure is not increasing in the climb, it's failing to drop, as it normally would.
 
Bugs

Alan - FYI - the Citabria parked directly across from you had a static line plugged by bugs only a couple of weeks ago. Moontown critters. Just say'in . . . . . . .
 
Minor point - the static pressure is not increasing in the climb, it's failing to drop, as it normally would.

Wrong. Actually, pressure is increasing within the static line. Outside atmospheric pressure is dropping as you climb. Due to the static line being a closed loop system due to the blockage, the pressure within the line increases.

Vic
 
Wrong. Actually, pressure is increasing within the static line. Outside atmospheric pressure is dropping as you climb. Due to the static line being a closed loop system due to the blockage, the pressure within the line increases.

Vic

Actually the plugged static is plugged so the pressure inside the static line remains constant at what is was on the ground. As you climb, the ambient static pressure drops and there is a relative pressure difference between the ambient static pressure and the pressure in the plugged static line with the static line being higher pressure than ambient.

Airspeed is a function of total pressure (pitot), minus static pressure (a differential pressure). With the plugged line, the airspeed reference is to the higher, ( but constant) pressure in the static line instead of the lower ambient pressure, generating the error.
 
when the pressure goes up, it is increasing. when the pressure goes down, it is decreasing. when the pressure is static, it is the same. also, if the pressure is going up and down equally at the same time, it is static.

it this is the case, I would look for bugs in the line. :)
 
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Wrong. Actually, pressure is increasing within the static line. Outside atmospheric pressure is dropping as you climb. Due to the static line being a closed loop system due to the blockage, the pressure within the line increases.

Vic

This logic only applies with a flexible walled material, like a diaphragm or a bag of chips that pops open in the climb. It does not apply to a rigid walled tube, like a static line, that eliminates the influence of ambient pressure. The pressure will remain constant from the time it was blocked and will not be influenced by the ambient pressure. It does increase on a "relative" basis compared to a reducing ambient pressure, but it does not increase per se.

relative to the point I think you were trying to make (pressure differential) - As the ambien pressure decreases, the higher pressure in the blocked line will exhibit an ever increasing force against any diaphragm (flexible material) in the blocked segment that has a pressure of an increasingly lower value on the opposite side. This is due to the increased pressure differential across the diaphragm and NOT an increase of the blocked pressure level.
 
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Wrong. Actually, pressure is increasing within the static line. Outside atmospheric pressure is dropping as you climb. Due to the static line being a closed loop system due to the blockage, the pressure within the line increases.

Vic

Hmmm. Other than due to temperature increase, or shrinking container, how can pressure rise in a closed container?
 
The on-going arguement is whether you are referring to absolute pressure in the line (most are) or ?gauge pressure?, what a standard gauge would read. Most gauges read the pressure difference between where they?re hooked to, and the outside world (e.g. a tire pressure gauge attached to a deflated tire reads zero, not 14.7 psi). This confusion tripped up a well-known theoretical physicist during the NFL ?deflate-gate?)
 
Hmmm. Other than due to temperature increase, or shrinking container, how can pressure rise in a closed container?

Now don't you go bringing your ideal gas equations in here, mister. We don't do Boyle's Law. :mad: Heck, in VAF we don't even do Cole's Law, except on barbecue.

What about carbonation? Did you think of that? Carbonated beverage trapped in the static line could make the pressure increase. Or Alka-Seltzer. Lots of possibilities, Charlie.

;)

Like I said (and I'm sorry I mentioned it now) "Minor point." Vic is absolutely right about the effect of a blocked static line on instrument readings. We just disagree on some semantics, more or less.
 
I appreciate the suggestions you guys made. It turned out to be simply a tube fallen off of a port. The ports were the cherry rivet type. The inside of the stem was so short I couldn?t make it stay on so I drilled the old ports out and installed Cleveland SPF140 ports and new tubing and connectors. On the flight test on the wing of another RV the airspeed and altitude numbers matched precisely. Thanks to Eric for recommending the Cleveland ports.
 
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