How a pitot tube works

[Don]

I'm reading Aerodynamics for naval aviators and I want to clarify how a pitot tube works. As a CFI I feel like I should already know, but its not explained like this in any FAA material. In any case; as I understand it:

The pitot tube creates a stagnation point. That is, it measures the pressure of the stagnant air in the tube. The pressure is then compared to the static pressure as read by the static port. The difference in the pressures being equal to the dynamic pressure of the moving airstream. This dynamic pressure (or the difference in pressures between the static port and the pitot tube) is then mechanically converted to airspeed, thus 305psf = 300knots.

Discounting compressability (I'm in a C-150 here) this all works untill the air density is non standard (density altitude) and as such its possible that the 305psf would really equal say 350knots as a result of a lower than standard atmospheric pressure.

Hope that makes sense, it does to me, tell me if I'm wrong.

 

[Timebuilder]

A pitot tube samples the ram air stream, and provides that pressure to an aneroid in the airspeed indicator. The expansion of the aneroid is modulated, or modified, by the STATIC port pressure to the area surrounding the aneroid in the ASI case.

Is that good enough?

 

[GeorgeTG]

Don, that's it...

Discounting compressability (I'm in a C-150 here) this all works untill the air density is non standard (density altitude) and as such its possible that the 305psf would really equal say 350knots as a result of a lower than standard atmospheric pressure.

you have just described the difference between CAS and TAS

I do hope you aren't trying to fly the C-150 at 350Kts ;-)

Cheers
George

 

[bunnyfufu]

you have just described the difference between CAS and TAS

i thought calibrated airspeed was true airspeed corrected for mechanical error in the instrument, not for deviations from standard temperature/pressure

he described the difference between TAS and IAS

 

[Singlecoil]

i thought calibrated airspeed was true airspeed corrected for mechanical error in the instrument, not for deviations from standard temperature/pressure

he described the difference between TAS and IAS

You're close. CAS is IAS corrected for installation and position error. Due to the change in airflow over the wing, and into the pitot tube, at different angles of attack, the pitot tube won't be able to take an accurate reading at every airspeed. They generally install it so that is the most accurate at approach speeds, where airspeed is more critical, and less accurate at cruise speeds. It is usually just a knot or two difference anyway. A 207 with a STOL kit has an 11 knot error at approach speed however, because the STOL kit greatly changes the airflow over the wing and into the pitot tube.

 

[Don]

Yikes I've learned a lot in the first 30 pages. I highly recommend this for anyone interested in learning the real details on lift. I feel a bit stupid as it’s taken me several readings to understand the math. At this rate I'll finish the book in one or two years.

New question:

Airfoils that have high lift coefficients (my C-150 wing as compared to an F-16 for example) produce "large twisting moments at higher speeds." I don't understand this. Anyone an engineer here?

I would have thought these wings less desirable on a high speed aircraft as at very high speeds and even very low angles of attack they would produce too much lift, but that is not mentioned. Only the twisting moments and low critical mach numbers. The critical mach numbers I understand as a result of compressibility at the stagnation point of a high lift wing, the twisting moments are a bit of mystery though.

Anyone?

 

[ksu_aviator]

I'll have to pass on the second question but I did want to comment on the first.

Airspeed measures air pressure. You have two types of pressure dynamic (moving) and static (not moving). Dynamic + Static is your total air pressure. We only want dynamic because it is directly related to airspeed.

So, to measure airspeed in simplistic terms, you point a pitot tube into the wind and watch the balloon blow up. But you still have static pressure.

Now the smarty pants engineers put the balloon in an air tight box and found out that the airspeed indications where wildly slow at altitude. Why? Because the box still had the high pressure from ground level trapped in it.

To compensate they ran the static port into the air tight box to rid the box of any excess pressure. Thus giving you a good indicated airspeed at all altitudes.

 

[Airline Pilot27]

Aero for Naval Aviators is an excellent book and one of the few that I kept after Graduation. Here is what I remember
Ias is read directly from the Pitot Static system
Cas is Ias corrected for position errors as was explained above
Eas (equivalent airspeed) is CAS corected for compressibility
TAS (True airspeed) is EAS corrected for density Alt and is usually about an extra 2 kts/1000ft

As for point 2 I think you may be comparing straight wings in your 150 vs swept wings. The F-16 is a totally different animal and I'm not sure if it has this engineering. If you read a little more forward in your book I remeber they explain this but it has to do with slow speed and stall characteristics. Swept back wings are designed with twist that gives the root a higher AOA. This is because pure swept wings have the undesirable characteristic of stalling tip first thereby negating aileron effectiveness. But by twisting the wings you get twisting moments on the wing especially at higher speeds IAS CAS EAS TAS or whatever.