Alternate Static Port

Systems_Operator · May 2nd, 2003, 22:23

Heya all!

I've been lurking here for about a week and a half now going through all this useful information, and I really have to share how impressed I am with the professionality of the users here!!

I am currently in the very early stages of going for my IFR ticket (after not having flown for like 7 years). Hopefully I should have my money together in a few months. Like many that I've read in the Changing Careers forum, I too have plans to leave the IT field, hence part of the decision to use this username. The other part of the reason is that I see a correlation between pilot and sysoperator....they both "operate systems", constantly fine tuning them to achieve a result. So there's that for my intro

To the question:

Both Gleim and Dogan indicate that when using an alternate static source, the air pressure inside the cockpit is usually lower than the airpressure outside the airplane, "due to the Venturi effect of the outside air flowing over the cockpit" (Gleim).

I suppose what's confusing me is: if air is flowing AROUND (and faster as compared to the air inside) a cockpit, shouldn't the air pressure OUTSIDE be less than the relative air pressure inside the cockpit?

E_Dawg · May 2nd, 2003, 22:33

Think of what happens when you are driving on the freeway and you roll the window down an inch or so. The air pressure drops a lot inside the car as soon as the window opens. Why? The lower atmospheric pressure immeaditely outside the car sucks out the air (the low pressure is caused by high speed air flowing around the vehicle: remember high speed = low pressure).

The same thing happens in non pressurized airplanes: small openings around the doors and windows allow cabin air to escape into the lower pressure, high velocity air flowing outside.

Systems_Operator · May 2nd, 2003, 22:51

Ahh! I understand now! The air inside the cabin gets sucked out via the cracks of the doors/other little openings, causing the pressure to drop. That makes complete sense to me now! heh, guess I just needed to bounce that off somebody. Thank you soo much for the quick reply!

Doug Taylor · May 3rd, 2003, 00:24

Remember the Bournolli (sp?) theory?

Fast air = lower air pressure?

The air in the cabin is relatively static, whereas the air outside is moving near whatever airspeed you're cruising at.

Systems_Operator · May 3rd, 2003, 00:35

Yah, that's what caused my confusion,...the books read "lower pressure inside the cabin as compared to the outside air". I was thinking that the faster air outside should be a lower pressure than inside the cabin, but I stand (sit) corrected....the air rushing outside the cabin "pulls" the air from the cabin, causing a lower density/pressure relatively.

btw...I don't mean to sound arrogant, I'm just a stickler for spelling (not typosesses)....it's Bernoulli

May 2nd, 2003, 22:23	#1
Systems_Operator Newbie Join Date: May 2003 Location: Arcata, California Posts: 29	Alternate Static Port Heya all! I've been lurking here for about a week and a half now going through all this useful information, and I really have to share how impressed I am with the professionality of the users here!! I am currently in the very early stages of going for my IFR ticket (after not having flown for like 7 years). Hopefully I should have my money together in a few months. Like many that I've read in the Changing Careers forum, I too have plans to leave the IT field, hence part of the decision to use this username. The other part of the reason is that I see a correlation between pilot and sysoperator....they both "operate systems", constantly fine tuning them to achieve a result. So there's that for my intro To the question: Both Gleim and Dogan indicate that when using an alternate static source, the air pressure inside the cockpit is usually lower than the airpressure outside the airplane, "due to the Venturi effect of the outside air flowing over the cockpit" (Gleim). I suppose what's confusing me is: if air is flowing AROUND (and faster as compared to the air inside) a cockpit, shouldn't the air pressure OUTSIDE be less than the relative air pressure inside the cockpit?

May 2nd, 2003, 22:33	#2
E_Dawg Moderator Join Date: Apr 2002 Location: chicago Posts: 4,132	Re: Alternate Static Port Think of what happens when you are driving on the freeway and you roll the window down an inch or so. The air pressure drops a lot inside the car as soon as the window opens. Why? The lower atmospheric pressure immeaditely outside the car sucks out the air (the low pressure is caused by high speed air flowing around the vehicle: remember high speed = low pressure). The same thing happens in non pressurized airplanes: small openings around the doors and windows allow cabin air to escape into the lower pressure, high velocity air flowing outside.

May 2nd, 2003, 22:51	#3
Systems_Operator Newbie Join Date: May 2003 Location: Arcata, California Posts: 29	Re: Alternate Static Port Ahh! I understand now! The air inside the cabin gets sucked out via the cracks of the doors/other little openings, causing the pressure to drop. That makes complete sense to me now! heh, guess I just needed to bounce that off somebody. Thank you soo much for the quick reply!

May 3rd, 2003, 00:24	#4
Doug Taylor Agent Smith Join Date: Dec 1969 Location: KSDL Posts: 39,396 Blog Entries: 1	Re: Alternate Static Port Remember the Bournolli (sp?) theory? Fast air = lower air pressure? The air in the cabin is relatively static, whereas the air outside is moving near whatever airspeed you're cruising at.

May 3rd, 2003, 00:35	#5
Systems_Operator Newbie Join Date: May 2003 Location: Arcata, California Posts: 29	Re: Alternate Static Port Yah, that's what caused my confusion,...the books read "lower pressure inside the cabin as compared to the outside air". I was thinking that the faster air outside should be a lower pressure than inside the cabin, but I stand (sit) corrected....the air rushing outside the cabin "pulls" the air from the cabin, causing a lower density/pressure relatively. btw...I don't mean to sound arrogant, I'm just a stickler for spelling (not typosesses)....it's Bernoulli

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