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Static Port - or not

R

rvbldr3170

Active Member
Hi all,
I was talking to a local mechanic on Sunday, and I was talking about running the static line on my -4 per Van's reccommedation, and he asked why I was running a static line at all ?!
I didn't have an answer other than "because they told me to". He said that the requirements are only for static air, and the air inside the cockpit is "static", and said to simply leave the port open to the air inside the cockpit.
Has anyone else not installed a static line? If so, did the instruments read correctly?
 
Instrument Error, that's why

Because the static air inside the aircraft will be at a lower pressure that the ambient air (outside). Because of the venturi effect of the flow of air over the cockpit, this static pressure in the cabin is usually lower than the pressure provided by the normal static air source.

When the alternate static source (like in the cockpit) is used, the following differences in the instrument indications usually occur: the altimeter will indicate higher than the actual altitude, the airspeed will indicate greater than the actual airspeed, and the vertical speed will indicate a climb while in level flight.

That's why....
 
Also, the "static" air inside the cockpit will change drastically with the opening and closing of the vents.
This woks OK on a 100 mph airplane with a VERY leaky cabin, NOT on an RV!
 
Just to throw out some numbers, when I've had a static leak behind the panel, my airspeed read almost 30 knots high at cruise power - quite a bit of error!

As Mel said - cabin static might be Ok on a slow leaky plane, but not on an RV.

Paul
 
I have an alternate static port "T" in my system that I used for testing during Phase I. Much error that increases with speed in the RV's when on cockpit static. This error can really mess up your Altimeter as well so be careful.
 
Before the panel upgrade in my RV4 my static was vented to the cabin and it took me a while to figure out why my instruments were so far off. As other have said, higher airspeed and altitude indications. It was a huge difference when I added in the external static ports for my new avionics. It is so easy to add the static system in, why would you even consider not doing it. If you are considering any type of IFR flying or fly in, under, or around Bravo airspace, not having the static ports on the outside of the plane is a bad thing. A good example is Paul's discussion on the OSH Notam which states to fly in at 90kts. How would you know how fast you are really flying?
 
Cockpit static worked well in my Stinson.

Oh, waitaminute----slow, leaky airplane.

Never mind.
 
rvbldr3170 said:
Hi all,
I was talking to a local mechanic on Sunday, and I was talking about running the static line on my -4 per Van's reccommedation, and he asked why I was running a static line at all ?!
Click to expand...

Mel, Mike, and Paul are spot on with this. The 1946 J-3 that I flew worked just fine without a static port. Of course the only time that the A/S & altimeter indicated correctly was passing through as the needles were always in motion!
You will notice that any aircraft that has an alternate static source (vented to the cabin) will also have limitations on it's use. There is a very good reason for this!
Whomever this "local mechanic" is, really needs to get a little more savvy before he gives out any more advice. Stuff like that can lead to a serious consequence for the pilot and pax.
 
Instead of "hanging" the mechanic, someone should take the opportunity to take him/her flying and show them how systems really work while in flight. Way too many mechanics are not pilots, and don't have the inate understanding of some aircraft systems that are rarely worked on, such that they have forgotten what perhaps they once learned in school or have not even had the chance to work on particular systems since they left school. A well-educated mechanic can be our best friend, and with all of the RV's being sold we just know there are mechanics out there working on them everyday. :)
I bet there are things we can even learn from expereinced mechanics, too. :)

Vic
 
Beg to differ on the VSI

ncarmon said:
Because the static air inside the aircraft will be at a lower pressure that the ambient air (outside). Because of the venturi effect of the flow of air over the cockpit, this static pressure in the cabin is usually lower than the pressure provided by the normal static air source.

When the alternate static source (like in the cockpit) is used, the following differences in the instrument indications usually occur: the altimeter will indicate higher than the actual altitude, the airspeed will indicate greater than the actual airspeed, and the vertical speed will indicate a climb while in level flight.

That's why....
Click to expand...

Beg to differ........ IMHO the VSI indicates a climb/descent with a CHANGE in static pressure, as that decreasing/increasing pressure leaks through the calibrated leak orifice thingy. I think the VSI would work properly even with the static error induced by the cabin pressure. Maybe if you sped up or slowed down, the static pressure might change and indicate a climb/descent in error, but at a steady speed the VSI should be correct.

Correct me if I'm yet-again wrong.....:p
 
static port

Alrighty then, a static port it is.
But seriously thanks for all the input. That's what I love about this site !

In defense of the mechanic, he is an old WWII guy, and mostly all the work he does is for the low and slow crowd, so I suppose for those guys an unported static source is acceptable.
 
PCHunt, You are correct, my bad, shouldn't do anything from memory here, too many people that need to be technically correct, even when the issues was that there were instrument errors, not what the instrument errors were....:)

Looked it up in the Pilot's Handbook of Aeronautical Knowledge: When the alternate static source pressure is used, the following instrument indications are observed:
1) The altimeter indicates a slightly higher altitude than actual
2) The ASI indicates an airspeed than the actual airspeed.
3) The VSI shows a momentary climb and then stabilizes if the altitude is held constant.

So if you speed up or slowed down, you may get some differential pressure. In fact it would change if you climbed or descended...:)
 
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