An airspeed question

[Don]

Tell me if this is all correct.

Airspeed is calculated by equating the difference between the total pressure of the moving air stream and the total pressure of the still air stream. The total pressure being the sum of the static and dynamic pressures.

Keep in mind I’m interested in the concepts, not the exact correct numbers!

Thus, if the free air stream has a total pressure of say 2000 lbs per square foot (measured at the static port) and the moving air stream has a total pressure of 2300 lbs per square foot then the difference between the two is 300 lbs per square foot and that is equated to an airspeed. Maybe 300 knots if 100 lbs per square foot equal 100 knots?

Are those concepts (not the exact numbers!) correct?

In following, if my static port iced over at altitude but my pitot tube did not (I've got the heat on) and I began a descent, then the differences in pressure would grow as I descended. Thus, If I were actually descending at a constant 200 knots, as I descended into the denser air with a static port still sensing the static pressure at altitude then my airspeed indicator would show an increase as I was going down.

 

[BrokeAssPilot]

WTF?

 

[TR4A]

A blocked static port will cause varying symptoms depending upon the degree of blockage and the altitude at which it occurs. Generally, however, static port blockages will cause the airspeed indicator to increase with decreasing altitudes and decrease during a climb. The static pressure will be compared against the non-varying pitot pressure and the result displayed as airspeed. You can see that if the static port is blocked at a relatively high altitude there will be an increasing difference between static and pitot pressure as the airplane descends.

 

[fr0g]

Don,
That sounds right - except I don't believe that the relationship between differential pressure and airspeed is linear - most airspeed indicators have a semi-logarithmic scale - probably because kinetic energy (hence pressure) increases with the square of the airspeed.
If your static port became completely clogged, and then you started to descend, you would have a greater pressure differential, and thus a higher airspeed indication than you were in fact flying.

 

[logolight]

Remeber the formula:

I - Pi
C - C
E - D
T - W
G -

Start with Indicated airspeed. Add Possition/instrument error to get Calibrated airpseed. Add Copressibility to get Equivilant airspeed. Add Density altitude to get True Airspeed. Add Wind direction to get ground speed.

Accronym: ICETG/PiCDW

 

[TonyC]

Are those concepts (not the exact numbers!) correct?

In following, if my static port iced over at altitude but my pitot tube did not (I've got the heat on) and I began a descent, then the differences in pressure would grow as I descended. Thus, If I were actually descending at a constant 200 knots, as I descended into the denser air with a static port still sensing the static pressure at altitude then my airspeed indicator would show an increase as I was going down.

YES.

YES.

YES.

 

[becket]

Man, you guys must be killer with the ladies. Next time you meet a girl in a bar, push your glasses back up to the top of your nose and let her have it with this conversation. She'll melt.