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Technical Alternatives for Pitot Tubes?

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Technical Alternatives for Pitot Tubes?

Old 9th Jun 2009, 18:12
  #21 (permalink)  
 
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How about just sticking with the good old reliable double(or triple) pitot system, but coupled with an airplane that is designed from the outset to be safely flyable (in weather) under pilot manual control, in the event of A/P failure.
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Old 9th Jun 2009, 19:03
  #22 (permalink)  
 
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And how do you think Boeing airplanes know what their airspeed is?
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Old 9th Jun 2009, 19:54
  #23 (permalink)  
 
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Optical Air Data Systems

I actually did some research (layman research) on air data systems a few months ago and discovered that [email protected] based optical air data systems had been developed.

Here are a few interesting links:
Optical air data systems
Michigan Aerospace Corporation - Molecular Optical Air Data System (MOADS) - FAQ
Michigan Aerospace Corporation - Molecular Optical Air Data System (MOADS)
http://www.michiganaerospace.com/pdf...orne_lidar.pdf

I hope this helps
Obi
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Old 9th Jun 2009, 20:08
  #24 (permalink)  
 
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laser [email protected] [email protected] [email protected] [email protected] [email protected]

funny...

Is it because [email protected] is a trademark?
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Old 9th Jun 2009, 20:25
  #25 (permalink)  
 
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Way back in the late 1960s when I was instructing on the Varsity, one exercise practiced by instructors was to make an approach to land without an ASI (simulating pitot icing or some other failure). You would be under the “hood” while the safety pilot had full visual and full instruments. You flew by attitude, power, etc, and if all was in the grove by around 200 feet, you were allowed to look up and land.

Frankly, it wasn’t difficult given adequate practice, but a very useful exercise nonetheless. The Varsity was a primitive design (direct development from the WW2 Wellington) so failures were not unknown. However, only once did I have to carry out a “no ASI” landing for real.

I guess such training exercises are totally alien to the modern generation of pilots.

Jack
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Old 9th Jun 2009, 21:11
  #26 (permalink)  
 
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LOL. Dani I don't no whats happening, but I can't seem to type [email protected] with an 'a' on this board for some reason.
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Old 9th Jun 2009, 21:51
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Airbus actually developed a very expensive and heavy [email protected] airspeed measuring system for use on their flight test aircraft. Initial flight testing is a special case in airspeed measurement, as you don't yet know what the errors are in the aircraft's pitot and static pressure systems. The determination of the errors in the pitot and static pressure systems is one of the test phases that must be done early in the flight test program.

The system that Airbus developed is a very large unit mounted next to a cabin window. It shoots three orthogonal [email protected] beams out the cabin window. Some of the [email protected] light bounces off dust particles, and is reflected back to the device. It is able to measure the aircraft velocity on each of the three orthogonal axes, and thus determine the true airspeed, angle of attack and angle of sideslip. They combine the TAS with static pressure from the static ports and outside air temperature and then calculate calibrated airspeed.

I don't know if they are still using this system, but it was used back in the days of the A340 flight testing. It was found to be superior to the usual trailing cone + pitot pressure system in some parts of the flight envelope.

It would theoretically be possible to develop a lighter, smaller version of this system, but it would be fairly expensive.
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Old 9th Jun 2009, 23:01
  #28 (permalink)  
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Off topic: On Light Amplification

For those wondering about why laser gets changed to [email protected], the bulletin board software behind this site has the capability to replace certain terms, which the site's administrators have taken advantage of.

This would have probably been done to stop people Googling for laser pointers finding this site.
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Old 9th Jun 2009, 23:20
  #29 (permalink)  
 
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life in the corners is rarely that simple..

Icing is a tough taskmaster.

In a prior life I worked beside a group that did way-excellent ultrasonic anemometers .. but they'd be useless iced over.. yes, they can be heated, but so can pitot tubes..

I have not spent a lot of time in the technology of [email protected] anemometers, but I can see how things might get challenging with particles of varying aerodynamic parameters flying every which way in an MCS..

one needs to be careful when looking for technological silver bullets.. PCBs, anyone?

..tom
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Old 10th Jun 2009, 00:20
  #30 (permalink)  
 
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Originally Posted by Old Smokey View Post
The AoA indicator is the only suggestion that bears some merit.
Of course those ice up too, if things aren't going well. And it's hard to provide accurate airspeed to AOA relationships at high speed, because the change in AOA for even moderate speed changes is small, and you start getting into instrument accuracy ... it's hard to get much under 0.5 deg total AOA system accuracy, but 0.5 deg out at Vmo-type speeds is a LOT of speed shift. AOA isn't bad at low speeds, but its pretty useless at cruise speeds. Especially if you need accurate data for other systems, like rudder limiters and the like.

AOA alone at low speed has its own problems too - with just AOA it's harder to know how oversped one might be - so landing distances might become more problematic. Also, failure cases where we today routinely apply speed increments become more interesting when we have to think in AOA increments. Not saying thats impossible, but its certainly different.
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Old 10th Jun 2009, 00:28
  #31 (permalink)  
 
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MS. Not saying thats impossible, but its certainly different.
Airbus BUSS (Buck Up Speed System) simply relies on AOA.

As I mentioned before, we used o fly final App with AOA on T38.

This system has been in use in Military Airplanes for many years.
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Old 10th Jun 2009, 00:41
  #32 (permalink)  
 
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It's one thing for a back-up to rely on AOA. You've already got system degradation and you're now in the "continued Safe Flight and Landing" element of certification, where the assumption is that we're simply trying to recover the a/c to any safe base. It's a whole other thing to plan flight on that basis. How are you going to respect MMO/VMO based on AOA? It's almost impossible, because depending on weight, VMO will be a different value.

Again, your T-38 LANDINGS were on AOA. How did you do a descent from cruise alt? I'm willing to guess, not by reference to AOA.
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Old 10th Jun 2009, 01:01
  #33 (permalink)  
 
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MS,

Airbus BUSS, as it named it is a back up system, only displays in case of triple failure of ADRs; and better than nothing.

I respect definitely your posts in this forum, however I disagree with the comment "Vmo/Mmo is dependent on weight." No it is not. it is a fixed speed regardless of weight. Below crossover altitude where dynamic pressure is dominating it is given as KEAS; above crossover altitude where shock waves are dominating it is given as Mach No.

It was old days I do not remember all details about T 38, but another thing I remmeber, if Flaps are Up, the green light was on at L/D max. We never used for descent but in the worst case L/D max speed can be used as descent speed safely, takes too much time though.

I agree with you, for the accuracy of AOA. In one of the bulletins of A 320, it mentions 0.1 degree AOA error causes around 3t weight calculation error
(some systems are fed with calculated weight depending on AOA, CG, THS and N1)
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Old 10th Jun 2009, 01:21
  #34 (permalink)  
 
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Originally Posted by JABBARA View Post
however I disagree with the comment "Vmo/Mmo is dependent on weight." No it is not. it is a fixed speed regardless of weight. Below crossover altitude where dynamic pressure is dominating it is given as KEAS; above crossover altitude where shock waves are dominating it is given as Mach No.
Afraid you've misunderstood me.

My point is that VMO and MMO are, as you say fixed absolute speeds.

But if I'm flying on AOA I don't know my speed, just my AOA.

Lets say I'm actually right at VMO (or MMO). If I'm heavy, my AOA may be +1 deg. If very light, maybe -1 deg? So, what is my AOA limit for VMO/MMO? It ends up being a function of weight - alphaMO=f(weight). I dont think anyone would appreciate having to know their weight and look up a chart to know if they had a speed exceedance.

Basically, its very hard to respect a defined speed limit in Knots or Mach if all you have is AOA.
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Old 10th Jun 2009, 01:37
  #35 (permalink)  
 
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MS,

Apology,

Now I understand what you mean.

In that sense, I have no idea to determine Vmo/Mmo by AOA.

Meanwhile, from my previous observations for A 330-200: F3 landing, at Vapp AOA is around 7.5 degree regardless of weight. This can be read from one maintenance page of FMGEC.

I was keeping this in memory for the worst case scenario, even before the BUSS is invented

Last edited by JABBARA; 10th Jun 2009 at 13:23.
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Old 10th Jun 2009, 13:13
  #36 (permalink)  
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Many thanks for your answers guys

What about using some piezzoelectric device which was mentioned in the very beginning
of this thread? Would that make sense or be applicable.
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Old 10th Jun 2009, 13:37
  #37 (permalink)  
 
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no need to reinvent the wheel..

personally think, that pitots as they are , are good enough, or let's better say, should be good enough..

it worries me a bit that we do find in today's highly sophisticated airplanes pitot / statics sensors, maybe also TAT probes that may not conform to specs..

what I do not find ideal is that such a vital system component, considering that a lot of the airplane's flight control system and pilot interface depends on correct pitot / static / tat probe data, that, first, the most modern designs seem at times to be defect by design ( i mean that technology has been out there for a long time, what happened in the engineering departments of the suppliers and OEMs??,) and second, it may not be ideal that OEMs use more than one supplier and obviously design for such an important subsystem..

seems on the A330 for instance, and I am not fluent on that aircraft, so please correct me if I'm wrong, there are two suppliers of those probes. Only one of the suppliers probes obviously may have a problem at times, that's why upgrades are available,...

So while its fine if an airline can chose powerplants, if I were an aircraft OEM, I would limit myself to one supplier and one model of such probes.

Why?, simply stated, I would like my statistical sample rate as big as possible, to find out rather sooner than later if there may be a glitch..

I mean considering how far reaching the consequences are to my flight control system, if that basic probe goes south ( by design..ice, humidity whatever, you name it)..

and, OK, in hindsight, it surprises me a bit, that the upgrades had not been implemented by a "mandatory" SB, reviewing the history of actual and possible malfunctions..and considering what it does to the flight control system, and what difficulties a crew may face by being forced to fly a plane in alternate law in the flight levels....( probably already highly demanding in smooth air, but may be near to impossible in weather conditiions which actually may "fail" those probes..)

So from a total system integrity point of view, that may not be OK..

any opinions on that??
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Old 10th Jun 2009, 13:56
  #38 (permalink)  
 
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Laser anemometers

It shoots three orthogonal [email protected] beams out the cabin window. Some of the [email protected] light bounces off dust particles, and is reflected back to the device. It is able to measure the aircraft velocity on each of the three orthogonal axes, and thus determine the true airspeed, angle of attack and angle of sideslip. They combine the TAS with static pressure from the static ports and outside air temperature and then calculate calibrated airspeed.
Laser anemometers can become unusable in very clean air (such as high at high altitude) due to the lack of particulate matter to scatter off. Having said that, water droplets in clouds would do nicely. And the fact that such a device can operate from behind glass could remove the problems of icing faced by a pitot or ultrasonic (or direct pressure) solution.

The appeal of the time proven pitot static method is that it is a direct physical measurement of the very dynamic forces that are keeping the aircraft flying. If the only real problem is icing, then that suggests simple redesign rather than technology replacement.
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Old 10th Jun 2009, 14:16
  #39 (permalink)  
 
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greek freak

piezo elec can measure pressure.but it still must take a reference pressure from outside the aircraft in the immediate flt environs.

There is often a yawning gap between ground speed and airspeed.

The airbus (and boeing) air data modules do something just like that.
A pitot head externally and a short length of tube within the fuselage transfer a dynamic air pressure to a small electronic box (the ADM or similar) that converts the info to an elec signal for processing at the adiru or similar for use and distribution to a variety of other computers and indicators.

Last edited by ampclamp; 11th Jun 2009 at 01:03.
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Old 10th Jun 2009, 15:53
  #40 (permalink)  
 
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Originally Posted by Dont Hang Up
Laser anemometers can become unusable in very clean air (such as high at high altitude) due to the lack of particulate matter to scatter off. Having said that, water droplets in clouds would do nicely. And the fact that such a device can operate from behind glass could remove the problems of icing faced by a pitot or ultrasonic (or direct pressure) solution.
Hi Dont hang Up

There are [email protected] devices that can provide air data in clean air, without the presence of particulate matter in the atmosphere. I think these are perhaps cutting edge technologies?
In terms of costs, I'd be very surprised if the initial installations prove to be cheaper that the current systems but over the life of an airframe it could be?

Originally Posted by falconer1
personally think, that pitots as they are , are good enough, or let's better say, should be good enough..
what I do not find ideal is that such a vital system component, considering that a lot of the airplane's flight control system and pilot interface depends on correct pitot / static / tat probe data, that, first, the most modern designs seem at times to be defect by design ( i mean that technology has been out there for a long time, what happened in the engineering departments of the suppliers and OEMs??,) and second, it may not be ideal that OEMs use more than one
I think this situation is a little like the mechanical vs FBW issue, they both have their advantages and disadvantages but for larger/expensive airframes FBW seems to be the way to go.

As you mentioned air data systems are critical components of an aircraft. The only thing that can be done is to make them as reliable as possible and install redundancies.

BTW how are posters able to use 'a' instead of '@' when typing [email protected]?

Cheers
Obi

Last edited by Obi Offiah; 10th Jun 2009 at 16:05.
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