Static line theory

Granted this may be a non-issue but, I came across an RV web site where the owner changed his static line routing from having the "tee" part of his static line in the top center to back along the side of the fuselage as is shown in Van's static kit. He stated that he was told by a couple of people that putting the tee exactly in the middle of the two ports really screwed up the measurement of the system. I am confused as to why this would be so.

My Piper Tomahawk (I have crawled back in its fuselage before) has ports on both sides of the aft fuselage with the tee in the center top of the fuselage.
It also has not only an alternate air source valve under the panel to open in case of blockage, but it has a small plunger valve by the pilots arm rest that you can push to temporarily open the system and let any water that might have collected near the static ports to drain out. It is spring loaded and closes when you release it. It is in the lowest portion of the static system. Pushing it is part of the preflight checklist.
Dave A.
RV-6A

Aircraft with rear fuselage mounted static ports usually have one on each side, connected via a T fitting. If the aircraft is subjected to small amounts of sideslip, the pressure at one static port will increase, and the pressure at the other one will decrease. If the pressure at the two ports is not the same, there will be airflow through the line connecting the two ports. The pressure at the mid point of that line will be average of the pressure at the two static ports. If the T is in the middle of that line, it will see the average of the two pressures, and this will help minimize the effect of small variations in sideslip on the sensed static pressure.

So, ideally, the T would be halfway between the two static ports. Will the static system still work if the T is more to one side than the other? Yes. But, you should expect to have changes in the airspeed system error in sideslips.

And it should be noted that physical geography has nothing whatsoever to do with the balance point of the pressure - i.e. it does not matter WHERE physically the tee is located (right, left, top, bottom, etc) in the fuselage - what matters is that the plumbing lines are of equal distance on both sides of the tee, to their respective static ports.

You can mount the tee right next to one static port, as long as you loop enough line so that both plumbing runs are of equal length, you'll still have the average of the two ports present on the tee fitting.

Makes sense, but does it matter.

You will almost always get a varience in airspeed in slips between left or right. Your Pitot is on one side subjected to differing exposures in either right or left slip, at varying degrees of the slip. The variance of ram pressure error vs static error I would think would make the static pressure variant a non factor. If it was critical, Van's no dummy, and the standard kit would have addressed this a long time ago I would think.
Regardless, I was taught to never count on my airspeed as being anything but a generality in a slip. My L3 and Bucker, like many aircraft, have the static and pitot bundled together, I believe to keep them in the same postion subject to the same variances? Still, is Airspeed not a general indicator at best in a slip?

Maybe I am wrong?

JonJay said:

You will almost always get a varience in airspeed in slips between left or right. Your Pitot is on one side subjected to differing exposures in either right or left slip, at varying degrees of the slip. The variance of ram pressure error vs static error I would think would make the static pressure variant a non factor.

Click to expand...

According to the data in NASA Reference Publication 1046, Measurement of Aircraft Speed and Altitude, the pressure sensed by a pitot tube at RV type speeds should be unaffected by variations in angle, as long as the angle between the local airflow and the pitot tube is less than 15 to 20 degrees. I believe that this should be the case for all but quite aggresive side slips.

I do agree that airspeed should not be trusted in a sideslip. There is an effect on the static source at even small sideslip angles. Having ports on each side of the fuselage, with a tee half way between the two ports does reduce the effect of sideslip, but there is still an effect. At larger sideslip angles the pitot is also affected. And, the stall speed almost certainly increases, so even if you airspeed error did not change, the margin to the stall at a given IAS will decrease.

I can remember quite clearly during my primary training, my instructor drilled that point into me. When I would do slips he would tell me to keep the airspeed above 80 knots indicated (in a 172) and later after I got PPL I did some testing at altitude in that same aircraft, the airspeed indicator was WAY off even in modest slips. I got used to doing slips into a field at Va so I wouldn't have to worry about a stall, and to keep the drag high - that's the primary use of a slip anyway, to lose energy.

Try a hard full-rudder slip in a 172 while maintaining Va with no power - it results in a pretty decent nose-down angle and a good descent rate.

Airspeed / Static Correlation

I just did a static test per the "manometer method" to check for leaks, and got the corresponding rise in airspeed. Given that I can't get a commercial pitot/static check for some time, is there any additional information I can glean from the airspeed rise?

FWIW I read 175Kts at approx. 1350', 30.04 baro... and no leaks. Also, I checked the pitot with the "Calibrated Manometer Method" and it's spot on.

Gsuit said:

I just did a static test per the "manometer method" to check for leaks, and got the corresponding rise in airspeed. Given that I can't get a commercial pitot/static check for some time, is there any additional information I can glean from the airspeed rise?

FWIW I read 175Kts at approx. 1350', 30.04 baro... and no leaks. Also, I checked the pitot with the "Calibrated Manometer Method" and it's spot on.

Click to expand...

Yeah, there is some info to be gleaned, if you also know what the altimeter was reading before you applied the suction. For example, if the altimeter was reading 0' before you applied the suction:

0' on the altimeter with 30.04 altimeter setting equals -110 ft press altitude

The static pressure at -110 ft pressure altitude = 30.04 in HG. This is the ambient pressure.

1350' on the altimeter with 30.04 altimeter setting equals 1240 ft press altitude.

The static pressure at 1240 ft pressure altitude = 28.60 in HG.

The pitot pressure = the ambient pressure of 30.04 in HG, so the difference between pitot and static pressure = 1.44 in HG.

The correct ASI reading for a delta pressure of 1.44 in HG = 172 kt. But, note that this calculation is based on the altimeter reading zero before you started. If it was actually reading something else, then we would get a different answer.