Static vent position

[markendee]

Can some one please explain why the static vent is placed on the outside of an unpressurised aircraft?

 

Why couldn't the instrument's static port just be open to cabin pressure?

 

Have attempted to find out using the net but cannot find the information I require.

 

Would rather not drill holes if I do not have to.

 

Regards and thanks,

 

Mark

 

 

[Guest pelorus32]

G'day Mark,

 

often the cabin of an aircraft is a low pressure zone - that is it's "static" pressure is lower than the ambient air pressure outside the aircraft.

 

Some manufacturers explicitly state that you must not use an "in cockpit" or "in aircraft" static source for this reason.

 

Regards

 

Mike

 

 

[slartibartfast]

Mine was accidently doing just that recently when the T-piece on the back of the VSI snapped off. I was trying to work out why the plane was stalling 15 knots faster and cruising so well. Eventually discovered the break and fixed it, then things were back to normal. The cabin on most aircraft will not usually be at the ambient pressure, it will either be higher because vents are open allowing air to get in faster than it can escape, or like mine have negative pressure because of the scoop I put under the cockpit floor to drag air through the cabin. Similar mechanisms can cause negative cabin pressure - airflow over the canopy, etc.

 

It gives fantastic airflow when you open the vents, but you don't want it for your static reference.

 

BTW - this was a great lesson in the value of flying by feel and not by the numbers. If I had been landing by indicated airspeed, I would have created a new crater somewhere along the line.

 

Cheers,

 

Ross

 

 

[Deskpilot]

What, the Cheetah falling apart already? :devil:

 

 

[slartibartfast]

Not a chance. It's too well-built for that. Must have knocked it during installation and it took a couple of weeks to snap through.

 

Didn't your mother tell you that if you go around casting nasturtiums you may end up pushing up daisies?

 

Neither did mine.

 

 

[markendee]

Thanks but there's more.

 

Thanks very much Mike and Ross - excellent answer, makes perfect sense.

 

While I'm asking, is there a best spot for the vent? eg front or rear of the cabin, high or low on the aircraft?

 

Mark

 

 

[Yenn]

Location depends upon design. The Cessnas I flew had a static port as a tiny hole both sides of the fuse. My homebuilt plane has the static just above the pitot head, extending beneath the wing. You could probably see different designs if you "Googled" Pitot.

 

 

[facthunter]

Static port location.

 

2 ports, one on either sideof the fuselage to average the reading, and interconnected and located where the motion (varying speed) in level flight, of the aircraft does not cause the pressure to vary. The exact position is found by a bit of good guesswork and experimentation. Invariably using the internal cabin pressure will result in an error because of the reduced pressure caused by the motion of the aircraft through the air. ( Higher speed indicated, particularly as you go faster where the error becomes greater). Nev..

 

 

[markendee]

Thanks to all