Why use only pitot-static system for altimeter/airspeed
But although the pitch values are easily remembered (remember the mnemonic "(DTD)585" for the B707?), the N1 or EPR settings are less so, unless you have noted the setting just before the IAS indications go AWOL. It can take a while to find the appropriate page in the QRH. On some a/c, the total fuel flow is a better tool than N1 or EPR, but it's not generally taught, and digital flowmeters don't seem to be as user-friendly as the old analogue ones.
This is all very elementary knowledge. It should be taught as part of the basic type rating; otherwise you should not be certified on your licence to fly the aircraft. Unfortunately the accent in type ratings is on flying the automatics when there should an equal emphasis placed on manual handling on instruments. There is a difference between becoming a flight deck "Manager" as most pilots are and being a professional fully qualified airman in every sense of the word "Airman".
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Shortly before the AF447 accident, a EU Commision supported project sponsored by Airbus, Dassault and Thales, and known as the NESLIE project was sufficiently advanced to be flight tested on a Citation II by the Nederland National Aerospace Laboratory (NLR). This TAS and AoA laser based system was deemed, following over 40 hours of flight tests in all climatic conditions and up to FL410, to be very promising and further technical work is underway to fine tune the results.
A technical paper detailing the NESLIE flight testing work in PDF format is available here.
The DANIELA project (also supported by the EU Commission) has taken the NESLIE project one stage further, i.e. the aim of the DANIELA (Demonstration of ANemometry InstrumEnt based on LAser) project is to prepare the operational use of a flush mounted ADS (Air Data System) built around a 3 axis Doppler LIDAR function as a primary air data channel on civil aircraft, and to assess optical concepts for the measurement of temperature and density to complete the ADS, in a second step.
Overall, these laser based projects are designed to acquire TAS, AOA, density and temperature, as well as being reliable in side-slip situations.
A technical paper detailing the NESLIE flight testing work in PDF format is available here.
The DANIELA project (also supported by the EU Commission) has taken the NESLIE project one stage further, i.e. the aim of the DANIELA (Demonstration of ANemometry InstrumEnt based on LAser) project is to prepare the operational use of a flush mounted ADS (Air Data System) built around a 3 axis Doppler LIDAR function as a primary air data channel on civil aircraft, and to assess optical concepts for the measurement of temperature and density to complete the ADS, in a second step.
Overall, these laser based projects are designed to acquire TAS, AOA, density and temperature, as well as being reliable in side-slip situations.
Last edited by mm43; 5th Feb 2014 at 23:12. Reason: spelling!
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@OK465: what i wanted to convey with the second statement you quoted was: use it at your own risk. I guess it doesn't really contravene the first one.
Now, the FPV is a very handy tool, but be aware of the interconnections in modern aircraft systems and wary of added complexity in case of partial or complete system failures. Always be sure to keep those basic flying skills up, despite all the nice displays and computers they are still very much the basis of our job.
Now, the FPV is a very handy tool, but be aware of the interconnections in modern aircraft systems and wary of added complexity in case of partial or complete system failures. Always be sure to keep those basic flying skills up, despite all the nice displays and computers they are still very much the basis of our job.
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The major issue I see with LDA (obviously meanwile called LIDAR) is that you need at least two reference points (in 3D at least 3) on the aircraft. Not easy on an object which is subjected to quite some deformation. An Aircraft is not a laboraty floor. The referenze points move realtive to each other with temperature, pressurisation, air loads, acceleration, thrust setting etc. I worked with LDA 20 years ago in a wind tunnel, and even there we had issues because auf relative movement due to temperature changes and related deformation. But this is nothing which could not be solved with a few more time and money.
And I still believe it would be a truly independent system, so the overall reliability would no longer be affected by common failure causes, to add this system would reduce the probability of total failure much more, than adding another pitot would.
And I still believe it would be a truly independent system, so the overall reliability would no longer be affected by common failure causes, to add this system would reduce the probability of total failure much more, than adding another pitot would.
One option that is already proven as an alternate airspeed measurement is to use the AoA vane. On the DH8-300, the AOA is used to feed a fast/slow indicator (looking like a glideslope indication, but on the L/H side of the horizon). It does not deliver numbers but it will be centered when at 1.3 Vs. So it allows to fly an approach at an appropriate speed.
Flap extension speeds need to be deducted via the speed booklet page - when at Va for the present flap setting, one is safe to extend the gear and also a further step of flaps. Apart from that, flying on this was fairly unspectacular. Programming such an indication into a contemporary airliner should be easily done. Unlike on the DH8, it would not necessarily need to be indicated at all times; besides manual selection eg. via a P/B on the EFIS control panel, it might be coupled with the ice detectors and pop up as soon as they are triggered.
Simple, easy to install and almost intuitive to use.
Flap extension speeds need to be deducted via the speed booklet page - when at Va for the present flap setting, one is safe to extend the gear and also a further step of flaps. Apart from that, flying on this was fairly unspectacular. Programming such an indication into a contemporary airliner should be easily done. Unlike on the DH8, it would not necessarily need to be indicated at all times; besides manual selection eg. via a P/B on the EFIS control panel, it might be coupled with the ice detectors and pop up as soon as they are triggered.
Simple, easy to install and almost intuitive to use.
Uplinker- that's exactly what the likes or early Dehaviland Moths were equipped with!!
Only problems I can foresee would be airflow turbulence affecting the plate and not giving a constant reading.
Also, if the heating fails or is overwhelmed by the amount of ice accreted, the bending moment will change and it becomes just as inaccurate or useless as an iced-up pitot tube.
What about a birdstrike - if a bird manages not to take off the plate but to bend it, the cosine of the bend angle will need to be calculated out of the measurement (if the system survives).
Also, a plate sticking out in the airflow will cause drag and might cause vibrations/tones at least in a certain speed range.
Well, the idea might be crazy - but why not harness this effect? An array of plates of increasing size and therefore frequency that will start to hum at different airspeeds might be installed in the airflow. Then add a microphone that catches the sound of this contraption and a computer that translates the frequency into an airspeed for indication.
Also, why not turn the pitot tube around? Then, Ptot will no longer be Pstat+Pdyn but Pstat-Pdyn - the much smaller values are likely harder to handle but at least there is no longer an opening facing forward and susceptible to icing.
What about a birdstrike - if a bird manages not to take off the plate but to bend it, the cosine of the bend angle will need to be calculated out of the measurement (if the system survives).
Also, a plate sticking out in the airflow will cause drag and might cause vibrations/tones at least in a certain speed range.
Well, the idea might be crazy - but why not harness this effect? An array of plates of increasing size and therefore frequency that will start to hum at different airspeeds might be installed in the airflow. Then add a microphone that catches the sound of this contraption and a computer that translates the frequency into an airspeed for indication.
Also, why not turn the pitot tube around? Then, Ptot will no longer be Pstat+Pdyn but Pstat-Pdyn - the much smaller values are likely harder to handle but at least there is no longer an opening facing forward and susceptible to icing.
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Originally Posted by Tu.114
Also, why not turn the pitot tube around? Then, Ptot will no longer be Pstat+Pdyn but Pstat-Pdyn - the much smaller values are likely harder to handle but at least there is no longer an opening facing forward and susceptible to icing.
Henry Crun, indeed, but could this not be used for airspeed indication by feeding the Pstat into the diaphragm and the Pstat-Pdyn into the instrument case (and adjusting the translating mechanism and the scale accordingly)?
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Tu.114 - sort of double doppler for each path, the sound pulses have to travel further forward than rearward
Sailplane variometers are well described (including math) in the book by Lorne Welch, alas I do not have current access to a copy. You need to express the quantites in algebra and differentiate.
A rearward facing variometer 'pitot' as used in sailplanes is carefully designed and mounted. It looks like a tube with crosswise slots embracing 120 degrees of the rear facing surface and is mounted with its axis vertical above the tailfin. Alternatively electronic computation from pitot and static is possible.
Airspeed sensing pitots are generally pointed forward and clear of the boundary layer. In protoype aircraft they are mounted way out ahead of the aircraft to get the cleanest possible air. Static vents are always a problem because they can only see already disturbed air. It is usual to calibrate them during a series of low-level test flights.
Your arithmetic would be correct if you could find a way of supplying your rear-facing orifice with undisturbed air.
Sailplane variometers are well described (including math) in the book by Lorne Welch, alas I do not have current access to a copy. You need to express the quantites in algebra and differentiate.
A rearward facing variometer 'pitot' as used in sailplanes is carefully designed and mounted. It looks like a tube with crosswise slots embracing 120 degrees of the rear facing surface and is mounted with its axis vertical above the tailfin. Alternatively electronic computation from pitot and static is possible.
Airspeed sensing pitots are generally pointed forward and clear of the boundary layer. In protoype aircraft they are mounted way out ahead of the aircraft to get the cleanest possible air. Static vents are always a problem because they can only see already disturbed air. It is usual to calibrate them during a series of low-level test flights.
Your arithmetic would be correct if you could find a way of supplying your rear-facing orifice with undisturbed air.
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henry crun
The Doppler-ish air speedometer you are suggesting... thinking about it.. I do wonder if it would work when in turbulence or clouds. Soundwaves would be somewhat absorbed or distorted, don't you think?
The Doppler-ish air speedometer you are suggesting... thinking about it.. I do wonder if it would work when in turbulence or clouds. Soundwaves would be somewhat absorbed or distorted, don't you think?
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henry crun
Please forgive me for being thick but does this not result in another radio device? I'm just trying to imagine how to construct such a thing. It is an interesting concept, and would solve the problems with pitots, but it is important to understand what new problems would be introduced with this kind of technology.
Please forgive me for being thick but does this not result in another radio device? I'm just trying to imagine how to construct such a thing. It is an interesting concept, and would solve the problems with pitots, but it is important to understand what new problems would be introduced with this kind of technology.
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Originally Posted by TU114
Also, why not turn the pitot tube around? Then, Ptot will no longer be Pstat+Pdyn but Pstat-Pdyn
AFAIK sailplanes use a venturi to produce a pressure of Pstat - Pdyn to drive a total-energy variometer.
Originally Posted by henry crun
Did nobody reconise it is a stand-alone direct-reading machmeter? No other air data required!
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Hazelnuts
It was only a five minute idea, and it does answer the OP question. I don't presume to have answered the prayers of all maidens everywhere, but I feel the device could have some merit.
Yes, you'd have to compute from air data, the inverse of mach computation, to get a more useful half rho v squared term. I acknowledge that any device which senses q directly would be the ideal solution.
I rather think reading the mach number local to the airfoil surfaces could be useful in some instances, for example where mach is the limiting factor, and of course the device could also be mounted remotely to get a true mach reading.
It also could monitor in three orthogonal directions to provide a full airspeed vector, providing a useful alternative or backup for aoa vanes and sideslip indicators.
It was only a five minute idea, and it does answer the OP question. I don't presume to have answered the prayers of all maidens everywhere, but I feel the device could have some merit.
Yes, you'd have to compute from air data, the inverse of mach computation, to get a more useful half rho v squared term. I acknowledge that any device which senses q directly would be the ideal solution.
I rather think reading the mach number local to the airfoil surfaces could be useful in some instances, for example where mach is the limiting factor, and of course the device could also be mounted remotely to get a true mach reading.
It also could monitor in three orthogonal directions to provide a full airspeed vector, providing a useful alternative or backup for aoa vanes and sideslip indicators.
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HN39
An aerodynamicist would say that you won't measure Pstat- Pdyn but Pstat-base pressure on a rearward facing step - say about -0.15*(0.5 rhoV^2).differential.
I fully agree, with the emphasis on local Mach, and close to the airplane. Unless it sticks well out into the flow it will measure the Mach Number in the boundary layer. If you stick it well out you may have a devil of a job calibrating it. The device, so far as I can see, measures Mach number along its "line of sight" which in general will not be aligned with the local flow (AoA or sidewash or both). I also wonder about sound waves "bounced" off the aircraft skin and how that might interfere with the received signal
Originally Posted by TU114
Also, why not turn the pitot tube around? Then, Ptot will no longer be Pstat+Pdyn but Pstat-Pdyn
Also, why not turn the pitot tube around? Then, Ptot will no longer be Pstat+Pdyn but Pstat-Pdyn
I wonder what an aerodynamicist would have to say about that assumption.
A machmeter indeed, measuring the local Mach close to the airplane. But staying airborne has to do with pressures, so you still need ambient pressure (Pstat) to have something that relates to lift.
Last edited by Owain Glyndwr; 8th Feb 2014 at 05:03. Reason: Minor correction
Let's keep in mind that in the case of AF447, the problem was not the general technology of pitot systems, but a specific model from a specific manufacturer that proved to be less-then-perfect when it came to certain icing conditions.
A few years back, an automaker shipped some SUVs with tires that were prone to tread separation and caused a number of crashes. No one took that as a sign that tires were an outmoded technology that needed an alternative.
The solution was not to back up tires with some other technology, but to refrain from building tires with engineering problems in the first place.
I certainly don't object to exploring what alternatives there are for additional backup to the pitot system, airspeed being as critical as it is. I'm just pointing out that well-designed, well-maintained pitot systems do this critical job very simply and effectively in 99.999999% of flights.
Sometimes new technology is a breakthrough in safety and effectiveness - and sometimes it is just a solution in search of a problem.
A few years back, an automaker shipped some SUVs with tires that were prone to tread separation and caused a number of crashes. No one took that as a sign that tires were an outmoded technology that needed an alternative.
The solution was not to back up tires with some other technology, but to refrain from building tires with engineering problems in the first place.
I certainly don't object to exploring what alternatives there are for additional backup to the pitot system, airspeed being as critical as it is. I'm just pointing out that well-designed, well-maintained pitot systems do this critical job very simply and effectively in 99.999999% of flights.
Sometimes new technology is a breakthrough in safety and effectiveness - and sometimes it is just a solution in search of a problem.