The Pitot Tube... Explained???

[thezoolityre]

Hey Folks, I am getting ready for my CFI, so I am second guessing EVERYTHING . And in this case it has to do with the Pitot Tube.

Now, I have been studying up on all the instruments as a double checker to make sure I know in case he asks me to teach it, and I was running through a mock instruction of the pitot tube assembly with my CFI who is signing me off for my exam and when I was running through it saying it measures dynamic pressure he stopped me and asked if I could just blow into the pitot tube assembly on the ground and be okay. I responded with a faint memory of the fact that you can't or something would break.

Now he said the pitot tube would break, but two questions:

1) If the pitot tube would break just by blowing into it, then how come we can fly through air at a (hypothetically speaking) 120 knts TAS, and be fine?

2) Is it the pitot tube that really breaks, or is the diaphragm in the airspeed indicator that you mess up?

I know I should be more sure of this especially coming up as a CFI but it is one thing that seriously slipped my mind and now I am curious and second guessing on how the damn thing REALLY works!! (not the 10 lines the PHAK says either) and what would really break if you blew into it and why...

Thanks for the help, all!

 

[bnt83]

Hey Folks, I am getting ready for my CFI, so I am second guessing EVERYTHING . And in this case it has to do with the Pitot Tube.

Now, I have been studying up on all the instruments as a double checker to make sure I know in case he asks me to teach it, and I was running through a mock instruction of the pitot tube assembly with my CFI who is signing me off for my exam and when I was running through it saying it measures dynamic pressure he stopped me and asked if I could just blow into the pitot tube assembly on the ground and be okay. I responded with a faint memory of the fact that you can't or something would break.

Now he said the pitot tube would break, but two questions:

1) If the pitot tube would break just by blowing into it, then how come we can fly through air at a (hypothetically speaking) 120 knts TAS, and be fine?

2) Is it the pitot tube that really breaks, or is the diaphragm in the airspeed indicator that you mess up?

I know I should be more sure of this especially coming up as a CFI but it is one thing that seriously slipped my mind and now I am curious and second guessing on how the damn thing REALLY works!! (not the 10 lines the PHAK says either) and what would really break if you blew into it and why...

Thanks for the help, all!

Diaphram in the airspeed is very sensitive and is the only thing you may damage by blowing into the pitot tube. The tip of the pitot has a tiny hole in it which will equal a very small pressure acting on the diaphram at 120 knots.

 

[N801BH]

Doesn't take much pressure to kill the ASI...

http://www.rst-engr.com/rst/articles/KP89JUL.pdf

 

[flyingron]

If you've ever seen what happens when someone blows into an ASI you'd know why. The needle hits the high end limit and it doesn't take the wind of a trumpet player to do it. This is particulary true if someone wraps their lips around the thing like they were blowing up a balloon.

The pressures involved are tiny. Next time, sit through the whole altimeter/static test when you plane is getting it's IFR certs. It doesn't take much (and boy I was admonished not to knock into the static line while we had a vacuum pulled on the system if I wanted to keep my VSI as well.

 

[poadeleted20]

Now, I have been studying up on all the instruments as a double checker to make sure I know in case he asks me to teach it, and I was running through a mock instruction of the pitot tube assembly with my CFI who is signing me off for my exam and when I was running through it saying it measures dynamic pressure he stopped me and asked if I could just blow into the pitot tube assembly on the ground and be okay. I responded with a faint memory of the fact that you can't or something would break.


Now he said the pitot tube would break,

There are several possibilities:

• He doesn't understand the system.
• He's not good at explaining technical things.
• You misunderstood him.

I'm not going to try to guess which it is, but clearly you understand that what he said doesn't make sense, and further, you came up on your own with the answer which not only does make sense but is also correct. All in all, I'd say your more ready for this part of a CFI practical test than he is.
Carry on!

 

[Dan Thomas]

The pressures involved are tiny. Next time, sit through the whole altimeter/static test when you plane is getting it's IFR certs. It doesn't take much (and boy I was admonished not to knock into the static line while we had a vacuum pulled on the system if I wanted to keep my VSI as well.

When I tested pitot-static systems, a static pressure reduction of one inch Hg, or 1000 feet of altitude on the altimeter, would cause the ASI to indicate 162 MPH with just the ambient pressure on the pitot. That's about 1/2 psi. Very little.

The ASI doesn't have a diaphragm. It has a bellows made of extremely thin tempered brass. Anyone blowing into the pitot could easily overexpand the bellows and ruin the ASI. I had a cutaway ASI I used for training students so they could see just how fragile everything was in there. Gears with almost-invisible teeth, shafts riding in jewels like old watches, and so on.

Dan

 

[Dan Thomas]

Diaphram in the airspeed is very sensitive and is the only thing you may damage by blowing into the pitot tube. The tip of the pitot has a tiny hole in it which will equal a very small pressure acting on the diaphram at 120 knots.

The size of the hole doesn't matter. At a steady rate of speed, the air isn't moving in or out, so a small hole is enough and less likely to scoop up bugs. The dynamic pressure is per unit area; a large hole will give the same reading as a small one but will have more drag.

Dan

 

[N801BH]

Hey Folks, I am getting ready for my CFI, so I am second guessing EVERYTHING . And in this case it has to do with the Pitot Tube.

Now, I have been studying up on all the instruments as a double checker to make sure I know in case he asks me to teach it, and I was running through a mock instruction of the pitot tube assembly with my CFI who is signing me off for my exam and when I was running through it saying it measures dynamic pressure he stopped me and asked if I could just blow into the pitot tube assembly on the ground and be okay. I responded with a faint memory of the fact that you can't or something would break.

Now he said the pitot tube would break, but two questions:

1) If the pitot tube would break just by blowing into it, then how come we can fly through air at a (hypothetically speaking) 120 knts TAS, and be fine?

2) Is it the pitot tube that really breaks, or is the diaphragm in the airspeed indicator that you mess up?

I know I should be more sure of this especially coming up as a CFI but it is one thing that seriously slipped my mind and now I am curious and second guessing on how the damn thing REALLY works!! (not the 10 lines the PHAK says either) and what would really break if you blew into it and why...

Thanks for the help, all!

Hmmmm...

That is one scary CFI....................

How the hell did he even get that CFI rating??

Ps.. you are supposed to know that stuff during the private pilot training.

 

[thezoolityre]

Hmmmm...

That is one scary CFI....................

How the hell did he even get that CFI rating??

Ps.. you are supposed to know that stuff during the private pilot training.

SO you are saying I don't know this subject at all? Which clearly isn't the case, I am clarifying a discontinuity of what a CFI told me that didn't make sense in my training... I figured going out my way to swallow my pride, go on here, and be sure before I taught many students the wrong information would be better than holding back and teaching incorrectly, but apparently for YOU there are stupid questions, if you're a CFI you should know principle of primacy is first and foremost when it comes to introducing information to a student, but next time I will shut up and keep the wrong answers inside instead of fixing what I was taught to teach it right..... for you.....

 

[N801BH]

SO you are saying I don't know this subject at all? Which clearly isn't the case, I am clarifying a discontinuity of what a CFI told me that didn't make sense in my training... I figured going out my way to swallow my pride, go on here, and be sure before I taught many students the wrong information would be better than holding back and teaching incorrectly, but apparently for YOU there are stupid questions, if you're a CFI you should know principle of primacy is first and foremost when it comes to introducing information to a student, but next time I will shut up and keep the wrong answers inside instead of fixing what I was taught to teach it right..... for you.....

Nope................... you said in your first post :

"Now he said the pitot tube would break, but two questions:"

I read that to be your CFI said that... NOT you...

 

[thezoolityre]

Thanks (most of you) guys for some helpful clarity, just to summarize to make sure I've got it down... It is the small "bellows" or "Diaphragm (Page 7-8 PHAK)" that will bust if blowing directly into it. But one last clarification,it is the baffle plate/ size of the tube that helps slow the air down in the tube assembly, so the relative wind or ram air of the aircraft flying at 160 knts TAS won't bust the ASI?? Just that last clarification, and I can sleep well

 

[thezoolityre]

Nope................... you said in your first post :

"Now he said the pitot tube would break, but two questions:"

I read that to be your CFI said that... NOT you...

Understandable, but I didn't think my thread said "judge my CFI" It is easy to have stuff lost in translation, he could have said it right, I could have took it wrong... Either way I was asking for clarification of the pitot tube... that's it, you didn't leave a comment answering what was originally ask, you were judging my CFI, you don't think at my level of training, about to be a CFI myself, if I can disseminate between the good the bad and the ugly of the flight instructors at my 141?

 

[Capt. Geoffrey Thorpe]

Thanks (most of you) guys for some helpful clarity, just to summarize to make sure I've got it down... It is the small "bellows" or "Diaphragm (Page 7-8 PHAK)" that will bust if blowing directly into it. But one last clarification,it is the baffle plate/ size of the tube that helps slow the air down in the tube assembly, so the relative wind or ram air of the aircraft flying at 160 knts TAS won't bust the ASI?? Just that last clarification, and I can sleep well

Size is irrelevant. You get the same pressure from a 1/8 inch hole as you get from a 10 inch hole. Ths Pitot system is a dead end so there is no flow - just the pressure caused by decelerating the air from 160 to 0 (relative to the airplane). And that's ain't much pressure.

Don't confuse pressure with force. Force is pressure times area - so area would matter in determining how much force is being applied to the pitot tube assembly. But the ASI only sees pressure.

The ASI is easy to break because it has to react to very small pressures.

 

[thezoolityre]

Size is irrelevant. You get the same pressure from a 1/8 inch hole as you get from a 10 inch hole. Ths Pitot system is a dead end so there is no flow - just the pressure caused by decelerating the air from 160 to 0 (relative to the airplane). And that's ain't much pressure.

Don't confuse pressure with force. Force is pressure times area - so area would matter in determining how much force is being applied to the pitot tube assembly. But the ASI only sees pressure.

The ASI is easy to break because it has to react to very small pressures.

Okay I get what you are saying, but then how does changes in dynamic pressure get sensed by the airspeed indicator if the tube which catches it and decelerates it is a dead end?? what measures that deceleration? If that tube has a dead end what separates a pitot tube from a static port if they both function by measure of static pressure, where does that dynamic pressure in the pitot tube go to???

 

[\__[Ô]__/]

When I tested pitot-static systems, a static pressure reduction of one inch Hg, or 1000 feet of altitude on the altimeter, would cause the ASI to indicate 162 MPH with just the ambient pressure on the pitot. That's about 1/2 psi. Very little.

For reference... a quick, unscientific google search suggests the average person is capable of blowing 1 - 2 psi over ambient pressure.

 

[Capt. Geoffrey Thorpe]

Okay I get what you are saying, but then how does changes in dynamic pressure get sensed by the airspeed indicator if the tube which catches it and decelerates it is a dead end?? what measures that deceleration? If that tube has a dead end what separates a pitot tube from a static port if they both function by measure of static pressure, where does that dynamic pressure in the pitot tube go to???

The airflow comes to a halt at the inlet of the pitot tube (since there is no flow) - bringing it to a halt requires a force (F = M*A) and that force results in an increase in pressure at the inlet to the Pitot tube that is what we call the dynamic pressure. The tube is a dead end at the ASI so there is no flow. But the pressure in the tube is the same everywhere so the ASI sees the same pressure as the inlet to the Pitot. That is the pressure on the inside of the bellows in the ASI. The pressure on the outside of the bellows is static pressure (connected to the static port). The pressure difference between the inside and the outside of the bellows makes it distort and move the ASI indicator.

The static pressure is measured by a hole that allows the air to flow alongside without being accelerated or decelerated (in an ideal world) so it is not affected by the force / pressure required to bring the air to a stop like the Pitot tube.

 

[Silvaire]

... how does changes in dynamic pressure get sensed by the airspeed indicator if the tube which catches it and decelerates it is a dead end?????

Increased pressure causes the bellows to expand which in turn moves the indicator needle. The static side is ambient pressure, the pitot side is ram pressure.

 

[bnt83]

http://m.youtube.com/watch?v=HM7ZMPpbeDA

 

[Geico266]

Blowing in the pitot tube is like putting air in a tire. We use 120 psi to air up the tire, but don't dare take the tire all the way up to 120 psi or it may blow up. Using a very low pressure low volume pressure source ( like a syringe or manometer) is adding the lowest volume and pressure to see how it reacts. Blowing is just too powerful too quick, and too inaccurate to do any good other than watching the needle jump.


When checking the calibration of an airspeed indicator on the ground I have used a home made manometer.....a tube filled with colored water running in a loop to the pitot tube. After taping off the static air vents on the plane a little air pressure is added to the system and watch how high the water level goes past static levels in the loop. I used a calibrated scale and compare it to the AS indicator. Shutting off the air pressure off and watching the water level will reveal potential air leaks in the system. It is amazing how accurate this old school test is.

 

[slacktide]

For reference... a quick, unscientific google search suggests the average person is capable of blowing 1 - 2 psi over ambient pressure.

I cannot think of a response appropriate for a family friendly forum.

 

[thezoolityre]

Increased pressure causes the bellows to expand which in turn moves the indicator needle. The static side is ambient pressure, the pitot side is ram pressure.

This is good and I understand that, but how does the pitot system recognize that ram pressure if there is a dead end at the end of the tube??? THAT's my biggest conundrum about this topic now..

 

[Sleepingsquirrel]

I think the "dead end" you are refering to "is" the bellows. The bellows expands and contracts as the airspeed(ram pressure) increases or decreases. One side of the bellows is connected to the pitot by a tubing the other side of the bellows is connected to the static port by a tubing.

One could take a 6 foot piece of surgical rubber tubing and place it on the pitot inlet. Clamp the end shut. Gently , slowly roll the tubing while watching the airspeed indicator . This would give an idea just what pressure is required to get an indication. One could "tee" in a sensitive pressure guage and see what pressure is required for any airspeed.
The air speed indicator is a pressure guage with markings in Kts, MPH instead of PSI. (Pounds per square inch)

Over pressurizing a sensitive pressure guage could easily :
A. destroy its calibration
B. rupture the bellows
C. cause the needle gear to jump teeth on the sector gear

 

[Capt. Geoffrey Thorpe]

This is good and I understand that, but how does the pitot system recognize that ram pressure if there is a dead end at the end of the tube??? THAT's my biggest conundrum about this topic now..

It's a tube. There is no flow so there is no pressure drop from one end to the other. The "ram" pressure at the inlet is the same pressure seen at the other end.

It's like a long skinny baloon. As you blow air in the one end, the pressure is the same all through it.

 

[thezoolityre]

It's a tube. There is no flow so there is no pressure drop from one end to the other. The "ram" pressure at the inlet is the same pressure seen at the other end.

It's like a long skinny baloon. As you blow air in the one end, the pressure is the same all through it.

Okay well when I heard dead end and looked at the PHAK's figure... it looks like the ram air stops at the top of the tube and doesn't actually flow into the instrument, which I got confused about...

 

[ElPaso Pilot]

Okay well when I heard dead end and looked at the PHAK's figure...

OK, that's not the clearest pic with the red pressure line.

it looks like the ram air stops at the top of the tube and doesn't actually flow into the instrument, which I got confused about...

Look at this one instead.

The bellows is sealed in a can. The static line provides the proper baseline pressure to the inside of the can, and when the pitot tube isn't moving forward, the pressure on either side of the bellows is the same and the dial reads zero.

Note that there isn't any real "flow" through the pitot line. There are just enough additional molecules jammed into the tube to move the bellows ever so slightly.

 

[gismo]

This is good and I understand that, but how does the pitot system recognize that ram pressure if there is a dead end at the end of the tube??? THAT's my biggest conundrum about this topic now..

Let's make this as simple as possible. Consider an ordinary latex party balloon. To inflate it you apply air (or helium) pressure. As the gas flows in the volume and pressure inside the balloon rises (the pressure is opposed by the elasticity of the envelope). At about 1/2 PSI the balloon will probably be near it's "normal" size (e.g. about 6" diameter). Now put a ruler next to the balloon to measure it's diameter. If you increase the pressure in the balloon it gets bigger, reduce the pressure it gets smaller. Just like the bellows inside the ASI except that the change in volume of the bellows for a given pressure change is much smaller than the balloon. Both are "dead ends" to the applied pressure, both increase in size as the pressure increases. In both cases the airflow in or out ceases once the size has adjusted to the pressure.