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IMC groundschool - Static line question

10 Posts

Hi all,

I am just getting into my IMC ground school and I am working on the Static and Pitot line system.

They basically drew a pitot pipe with the static line coming out of it and also the pitot line, they put a hole in the static line and said the below....

Tt says a leak in the static pipeline makes the ASI over-read because the pressure surrounding the pipeline is lower then static because of aero dynamic suction, can anyone explain this to me in english please??!!

No complex answers please just simple logical ones!!! ;)

Also I believe a PA28 only has the Static and Pitot all in one, what effect would it have on things if the leak were in the side mounted Static line!?

Thanks

Mark

If the pitot/static was under the wing, then the air flow under the wing would create a suction effect, and the lower pressure reading would create a greater difference between the measured (but incorrect) static pressure and the dynamic pressure. The greater apparent pressure difference would cause the ASI to over-read.

The suction effect is similar to opening your DV window in the cockpit - there is a suction effect due to the slipstream.

If the leak was in the side-mounted static line then there wouldn't be a suction effect as there's no slipstream acting on the leak.

I expect this to be shot down...wasn't Bernoulli something to do with this phenomenon?

Swanborough Farm (UK), Shoreham EGKA, Soysambu (Kenya), Kenya

Mark - could you please post that diagram here?

I ask because the statement

because the pressure surrounding the pipeline is lower then static because of aero dynamic suction

is ambiguous. There may be an increase or there may be a decrease, or even no change; it all depends on where the hole it relative to the surrounding airflow.

For a quick way to post an image, stick it on Imgur and post the URL here using the Image button above your text entry box.

Administrator
Shoreham EGKA, United Kingdom

Generally, when the leak in static is in the case, it's assumed that pressure is lower then it should be - like when you activate "alternate static" in cabin. In that case ASI will over-read, VSI will briefly over-read in the moment when leak has been developed and altimeter will over-read as well.

Emir

LDZA LDVA, Croatia

Where does the suction effect come in and why??

Here is the picture Mark is referring to

Administrator
Shoreham EGKA, United Kingdom

IMHO, this question deserves a medal for obscurity, because that leak would actually be just inside the wing!

How is one supposed to guess what the pressure inside the wing cavity will be?

My guess is that it will be below ambient, but it will also depend on the AOA, the flap configuration, the wing construction, how much air is getting forced into the wing cavity through the gap around the stall warner, etc.

If I got this in the JAA IR, I would appeal it. Not that doing that ever does anybody any good...

Administrator
Shoreham EGKA, United Kingdom

Sigh...... The exams are just as stupid in Canada. Is it too much to ask the regulator to ask questions in a way that knowledge actually relevant to flying is tested ? Never mind I know I am dreaming in technicolour.....

The easy way to think about this is that accurate readings from the ASI depend on comparing true ram air pressure from the pitot tube with true static pressure ( ie perfectly still air)

If the static pressure was less then the true value then there would be a greater difference between ram and static pressures and the ASI will over read. The opposite will apply if the static pressure is higher then ambient.

From a practical view point, with respect to light GA aircraft static leaks don't matter because the error is proportional to the pressure differential between pitot and static. At low speeds the difference is less so the error is less and in practice not big enough to matter.

Wine, Women, and Airplanes = Happy
Canada

Thankyou very much and what a superb new forum this is.

Rgds

Mark

It is certainly true that the static pressure in some cockpits is below the outside pressure, when in flight.

This is true for the TB20 for example. So if you pull out the alternate static thingy, the altimeter reads a little higher - about 100ft I think (at 150kt).

I don't think the ASI reads significantly differently though.

So unless there is some other applicable text, the question seems completely bogus.

Administrator
Shoreham EGKA, United Kingdom
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