Pitot Tubes and ASI Errors

[uwochris]

Hey guys,

I have a question in regards to a pitot tube blockage and how it affects your ASI.

I always thought that if the pitot tube was blocked, but the drain hole was not, the ASI would drop to 0. If both were blocked, the ASI would act like an altimeter. This is consistent with my textbooks. It was explained that if the drain hole was unobstructed, the dynamic pressure would simply vent through it, causing the ASI to read 0.

However, in normal flight with no obstructions, how come air pressure entering the pitot tube is not vented out the drain hole? It seems that air entering the pitot tube can also "escape" through the drain hole, causing false readings on the ASI.

Similarly, when the static port is blocked, the altimeter freezes at the current reading. Shouldn't the altitude readout be affected by air pressure from the static source "escaping" through the drain hole?

All comments and responses are welcome,

Chris.

 

[A Squared]

>>>>>>However, in normal flight with no obstructions, how come air pressure entering the pitot tube is not vented out the drain hole? It seems that air entering the pitot tube can also "escape" through the drain hole, causing false readings on the ASI.

It's because the drain hole is very small compared to the pitot tube opening. There is always air leaking out the drain hole, but it is a very small quantity so it doesn't significantly affect the pressure in the pitot line. If the drain hole was the same diameter as the pitot opening, or even half the diameter, it would affect the pitot pressure and thus hte airspeed reading. But it's small enough that it doesn't have an effect.


>>>>>Similarly, when the static port is blocked, the altimeter freezes at the current reading. Shouldn't the altitude readout be affected by air pressure from the static source "escaping" through the drain hole?

The altimeter has no connection to the pitot line, it is only connected to the static line, so the pitot drain has no effect the altimeter.

 

[Jedi_Cheese]

The reason you don't get errors with the pitot tube due to the drain hole is because they constrict the size of the tube going to the ASI after where the drain hole is. For instance, say the hole at the front is 2 square mm (I don't feel like working with a round hole due to the effects of squaring the radius) and the drain hole in the back is 1 square mm, then the tube going to the ASI is then 1 square mm. (realistically, it is a little more complicated than this but due to compressibility of air, bernuli's (sp?) principle, and g0d knows what else but this is the basic theory)

They also take into account the difference in air pressure due to the drain hole when they calibrate the ASI and any leftover errors become the calibrated airspeed.

I didn't know that there was a drain on the static system. What aircraft are you flying?

 

[minus273]

Hey there,

As said above the airpseed indicator is also calibrated to take into account the drain hole.

When I used to instruct we used to go outside during this part of the groundschool and I would breath into the pitot and would show how the airpseed would change.

I would then block the drain hole and show how it would increase indicated airspeed. I would then block both holes with my fingers and show that the airspeed indicator would hold the last asirpeed.

For me and alot of my students this visual demo helped more than reading the text in the book.

I recommend a C-152 as the pitot is the right height to do this, also best not to do it when it is freezing out as is embaressing getting your students to pull your frozen lips of the pitot.

-273

 

[fr0g]

hehehe.. just turn on the heater!!