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  • Backup/alternate Airspeed Devices

    So, since you all know that I am not an engineer, pilot or anything close to knowing about these things in any technical manner read the following with the proverbial grain of salt.

    I have no idea how pitot tubes work, but i assume they operate on the basis of wind pressure. I assume since they are tubular, the infiltration of water and ice causes them to stop functioning, despite the presence of heaters.

    Since airspeed not ground speed is critical for maintaining flight, why not have a backup device that can be deployed by the crew in the event pitot tubes fail. something as simple as a windmill type anenometer.

    since the windmill type as larger external parts, as opposed to relatively tiny pitot inlets, i assume it would be easier to protect them from icing by using heat, which apparently does not work with pitots...at least in some circumstances and instances...

  • #2
    Originally posted by Fear_of_Flying
    There has been a fair bit of discussion on this topic in the AF447 thread...of course, one would have to spend some time sifting through it, but various alternatives were proposed.
    sorry, but you should go back and rad the thread if you think it answers my question. you jumped the gun by insinuating that i had not read the thread.

    perhaps an apology?

    Comment


    • #3
      jumpy much?
      Sam Rudge
      A 5D3, some Canon lenses, the Sigma L and a flash

      Comment


      • #4
        sonic is a very different technology. i did do a bit of reading before asking. apparently, there are several technologies but since the windmill type was the simplest, i thought that getting back to basics might be a possibility.

        often, the complex, technologically advanced is not the answer.

        these are just my thoughts and i have no formal training in these things...

        Comment


        • #5
          GPS readings or even the ATC could be helpful to get speed readings.
          "The real CEO of the 787 project is named Potemkin"

          Comment


          • #6
            Not GPS!

            Originally posted by Alessandro View Post
            GPS readings or even the ATC could be helpful to get speed readings.
            Sorry Alessandro, but GPS is not the solution. As thoroughly discussed and explained on the AF447 thread, GPS gives you ground speed which may vary vastly from airspeed. Even GPS altitude can only be used as a guide since aircraft performance depends on the air pressure (hence density) that the aircraft is flying through.

            Comment


            • #7
              Fly in formation

              One answer is for commercial aircraft to fly in formation ("Hi buddy - I've lost my airspeed reading -take over the lead position") - well of course I'm half joking here but the USAF is investigating this for their aircraft as a fuel saving option.
              http://www.af.mil/news/story.asp?id=123147709

              Comment


              • #8
                The windmill has an intrinsic limitation: The terminal angular velocity (that is, the RMP where the rotation-wise component of the blades' lift balances the counter-rotation wise component of the blades' drag) depends mostly on true airspeed, and not on dynamic pressure (what is what the airspeed indicator measures and what the airplane performance and safety relies on). The same holds true for the sonic device.

                In both cases, however, knowing the air temperature and static pressure (which are not affected by faulty pitots and AFAIK were not affected in the AF case) would let compute the indicated airspeed from the true airspeed.

                --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
                --- Defend what you say with arguments, not by imposing your credentials ---

                Comment


                • #9
                  Originally posted by Gabriel View Post
                  The windmill has an intrinsic limitation: The terminal angular velocity (that is, the RMP where the rotation-wise component of the blades' lift balances the counter-rotation wise component of the blades' drag) depends mostly on true airspeed, and not on dynamic pressure (what is what the airspeed indicator measures and what the airplane performance and safety relies on). The same holds true for the sonic device.

                  In both cases, however, knowing the air temperature and static pressure (which are not affected by faulty pitots and AFAIK were not affected in the AF case) would let compute the indicated airspeed from the true airspeed.
                  Gabe:

                  What is wrong with a calibrated trim wheel.

                  Sure, it won't be accurate +/- 1 knot, but so what?

                  Set the trim, descend away from coffin corner where +/- 1 kt does not cause you to break up and die, and fly through the sky fat, dumb and happy to your destination where you can get your pitot tube fixed.

                  I know, I'm missing something.
                  Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

                  Comment


                  • #10
                    Originally posted by Spectator View Post
                    Even GPS altitude can only be used as a guide since aircraft performance depends on the air pressure (hence density) that the aircraft is flying through.
                    The altitude you are as triangulated by the radio signals from however many satelites has absolutely nothing to do with air pressure. Provided there are at least 3 satelites in view the GPS should be capable of givving accurate groundspeed, direction and altitude. The only output that cannot be relied upon to fly the aircraft of those three is the groundspeed as it makes no calculation of windspeed and velocity, hence nothing on airspeed.

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                    • #11
                      Originally posted by SYDCBRWOD View Post
                      The altitude you are as triangulated by the radio signals from however many satelites has absolutely nothing to do with air pressure. Provided there are at least 3 satelites in view the GPS should be capable of givving accurate groundspeed, direction and altitude. The only output that cannot be relied upon to fly the aircraft of those three is the groundspeed as it makes no calculation of windspeed and velocity, hence nothing on airspeed.
                      GPS will give you absolute altitude (from which geometric altitude can be determined). However for flight purposes the pressure altitude is needed(to determine air density) which can differ from geometric altitude. GPS can't give you that - but the static port can if operative.

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                      • #12
                        Originally posted by 3WE View Post
                        Gabe:

                        What is wrong with a calibrated trim wheel.

                        Sure, it won't be accurate +/- 1 knot, but so what?

                        Set the trim, descend away from coffin corner where +/- 1 kt does not cause you to break up and die, and fly through the sky fat, dumb and happy to your destination where you can get your pitot tube fixed.

                        I know, I'm missing something.
                        Actually, I agree.

                        --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
                        --- Defend what you say with arguments, not by imposing your credentials ---

                        Comment


                        • #13
                          Originally posted by Highkeas View Post
                          GPS will give you absolute altitude (from which geometric altitude can be determined). However for flight purposes the pressure altitude is needed(to determine air density) which can differ from geometric altitude. GPS can't give you that - but the static port can if operative.
                          OK, gotcha. Apologies for correcting you Spectator.

                          Comment


                          • #14
                            Originally posted by Highkeas View Post
                            ...but the static port can if operative.
                            No flaming here- just pull back and chuckle, becuase that is a somewhat funny statement.

                            A static port does almost nothing so the thought of it "operating" is funny.

                            (Acknowledged- the little job it does do is important, and I would prefer that it was "operational" if I am riding the particular airplane )
                            Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

                            Comment


                            • #15
                              Originally posted by SYDCBRWOD View Post
                              OK, gotcha. Apologies for correcting you Spectator.
                              No prob, I assumed it was a misunderstanding of the point I was trying to make. Anyway, I never meant to go off on the altitude tangent. The OP's question was about airspeed, which is arguably more important than altitute, in the immediate short term at least.

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