Pitot tube
11-28-2006, 08:03 AM
#1
Che Guevara
Thread Starter
Joined APC: Aug 2005
Posts: 6,408
Pitot tube
I'm a little curious and this could be the dumbest question ever. So I'm ready to take whatever sarcasm comes with this....
Does the heating of the pitot tube cause any errors in the reading of airspeed? When I was younger I remember actually slightly burning myself on one of those things. So if you're at higher altitude and you have cold, less dense, air coming into the tube and it's then heated it would drop the density even more causing some form of incorrect reading no? I don't know how much it would heat the air entered and when reading about the errors all I've heard is compresibility and not this.
Just a curious question really.
Does the heating of the pitot tube cause any errors in the reading of airspeed? When I was younger I remember actually slightly burning myself on one of those things. So if you're at higher altitude and you have cold, less dense, air coming into the tube and it's then heated it would drop the density even more causing some form of incorrect reading no? I don't know how much it would heat the air entered and when reading about the errors all I've heard is compresibility and not this.
Just a curious question really.
11-28-2006, 08:28 AM
#2
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Joined APC: Apr 2006
Posts: 88
Huh... never thought about that. Good question 
Most people think that airspeed indicator reads the difference between the static air pressure and the dynamic air moving past the aircraft. In fact, it is impossible to directly measure the pressure of dynamic air. You have to convert it into static pressure before you can measure it. Therefore, the airspeed indicator actually measures the difference between the static air pressure, and static air pressure that was converted from the dynamic air, and is therefore the same pressure as that moving air.
Still with me? Basically what I'm saying is that because the airspeed indicator is measuring the difference between two static pressures, the air in the system isn't moving. Therefore, the air right in the pitot may heat up, but further down the system, in the actual instrument, it would still be the same temp.
Also, and probably far more relevant, you should take into account that just because the density of the air decreases, the pressure doesn't have to change. The density may be lower, but the pressure will still be that of the dynamic pressure moving around the tube.
Am I close? lol
Most people think that airspeed indicator reads the difference between the static air pressure and the dynamic air moving past the aircraft. In fact, it is impossible to directly measure the pressure of dynamic air. You have to convert it into static pressure before you can measure it. Therefore, the airspeed indicator actually measures the difference between the static air pressure, and static air pressure that was converted from the dynamic air, and is therefore the same pressure as that moving air.
Still with me? Basically what I'm saying is that because the airspeed indicator is measuring the difference between two static pressures, the air in the system isn't moving. Therefore, the air right in the pitot may heat up, but further down the system, in the actual instrument, it would still be the same temp.
Also, and probably far more relevant, you should take into account that just because the density of the air decreases, the pressure doesn't have to change. The density may be lower, but the pressure will still be that of the dynamic pressure moving around the tube.
Am I close? lol
11-28-2006, 09:21 AM
#3
Gets Weekends Off
Joined APC: Jan 2006
Position: 737 Captain
Posts: 777
As you know, the heating of the pitot tube is to eliminate icing up and impeding airflow to the pitot static system. There is no appreciable density difference in the air flowing through the heated tube. If the air were moving very slowly, you may be able to measure a miniscule density difference and maybe even a very small temperature difference, but at over 100 mph it makes absolutely no difference to aircraft systems.
BTW, The mass of air is proportional to the air density, which is proportional to the absolute pressure and inversely proportional to the absolute temperature. Changes to one will result in changes to others - It's the LAW
BTW, The mass of air is proportional to the air density, which is proportional to the absolute pressure and inversely proportional to the absolute temperature. Changes to one will result in changes to others - It's the LAW
11-28-2006, 09:56 AM
#4
Line Holder
Joined APC: Nov 2006
Posts: 25
Actually, there is an appriciable change in density, just don't ask me to prove it. Heat transfer sucks. However, the design of the pitot eliminates the problem. The fact that it is ram air at ambient temperature gives a certain pressure inside the tube. The pressure inside the tube is not related to the density of the fluid in there, it is related to the speed, angle of attack, density of outside air, and pitot nozzle design.
For practical purposes, the movement of air due to the drain should be negligible. There is a bit of change of flow out the drain as the density changes, but this is likely less than one knot of change over the temperature ranges that the air is likely to see.
Interesting note for those of you who fly the DC-9, one of my engineering profs designed the pitot static system for that aircraft. He says that the system is 5-6 knots off at approach angles. They just adjusted the speed charts accordingly so it flys right.
For practical purposes, the movement of air due to the drain should be negligible. There is a bit of change of flow out the drain as the density changes, but this is likely less than one knot of change over the temperature ranges that the air is likely to see.
Interesting note for those of you who fly the DC-9, one of my engineering profs designed the pitot static system for that aircraft. He says that the system is 5-6 knots off at approach angles. They just adjusted the speed charts accordingly so it flys right.
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