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AOA indicator poll.



 
 
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  #11  
Old May 20th 20, 09:08 AM posted to rec.aviation.soaring
Luc Job[_2_]
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Posts: 22
Default AOA indicator poll.

Hi!

The basic principle comes from this reference:

http://www.nar-associates.com/techni...LowCostAoA.pdf

In order to be accurate for any atmospheric pressure (altitude), you can't make a straight differential measurement of your dynamic pressures. You need a reference pressure.

The thing is relatively tricky as you need to calculate a ratio of pressures to get the AOA... And at low speed the dynamic pressure is low... And the difference between the 2 dynamic pressures is equally low. And you must compute an accurate ratio of these 2 low values...

That's why my system uses 2 $35 pressure gauges: They are internally compensated for temperature and other factors, and outputs a 14 bits signal, with a full range corresponding to a 250 km/h dynamic pressure.

That's the price to pay to be below 1% at 70 km/h... And in practice it works...

The pitch string is corresponding to the instrument... But the string is very sensitive to sideslip... Not the instrument... That makes the side string useful only for calibration, when you can concentrate on keeping the middle string absolutely straight...
  #12  
Old May 20th 20, 09:14 AM posted to rec.aviation.soaring
Luc Job[_2_]
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Posts: 22
Default AOA indicator poll.

Hi!

The basic principle comes from this reference:

http://www.nar-associates.com/techni...LowCostAoA.pdf

In order to be accurate for any atmospheric pressure (altitude), you can't make a straight differential measurement of your dynamic pressures. You need a reference pressure.

The thing is relatively tricky as you need to calculate a ratio of pressures to get the AOA... And at low speed the dynamic pressure is low... And the difference between the 2 dynamic pressures is equally low. And you must compute an accurate ratio of these 2 low values...

That's why my system uses 2 $35 pressure gauges: They are internally compensated for temperature and other factors, and outputs a 14 bits signal, with a full range corresponding to a 250 km/h dynamic pressure.

That's the price to pay to be below 1% at 70 km/h... And in practice it works...

The pitch string is corresponding to the instrument... But the string is very sensitive to sideslip... Not the instrument... That makes the side string useful only for calibration, when you can concentrate on keeping the middle string absolutely straight...


  #13  
Old May 20th 20, 09:14 AM posted to rec.aviation.soaring
Luc Job[_2_]
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Posts: 22
Default AOA indicator poll.

Hi Mike!

I fly also with gasoline... And I think that the problem is different.

-GA AOA are relatively big... Gliders panels are small... And I prefer to have as much visibility as possible in the glider, So I won't add something directly in my eyesight, even a HUD... I fly in mountains, so I look at the rocks and other gliders, and it seldom stays straight ahead in front of the nose when it needs attention...

-GA AOA are not progressive, each bar is basically on-off... And it will focus your attention on the instrument each time that one of this bar will change... This is excessively disturbing in my point of view. A progressive change of color and position of the indicator can be caught by the peripheral vision... You can stay focused on what's important without missing the information.

-An added aggressive sound is useless in a stress situation... Look at all the planes that made belly landings with the gear warning shouting... A spoken warning is more explicit, but if it is Ok in a GA aircraft and for something that has time to be corrected, it takes too much time on a glider entering stall in a rough thermal... by the time the system says 'Pull' + interpretation of the message by your brain + action on the stick, it's too late...

Altering the normal sound progressively looks more efficient and less disturbing... I had a Zander 940 (It died for 2 years, very good vario), and it did exactly that: distorting the audio when approaching stall... Once you get used to that, your reaction will be instant and automatic. On my system, adding distortion means analog processing of the audio and that's more complex than hashing the signal, and has the same effect... I must tune that but presently the timebase is about 1/10 seconds: If you are at stall AOA, the audio is cut 100%, and if your AOA is half way between Cz max and stall, the signal will be cut 5/100 seconds and audible the remaining 5/100...

And it doesn't take many training hours to push the stick automatically when the sound looks strange... And push it full forward if it becomes dead silent.
  #14  
Old May 20th 20, 09:32 AM posted to rec.aviation.soaring
Luc Job[_2_]
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Posts: 22
Default AOA indicator poll.

One last point to finalize the design:

4) Would you like a remote display bar that you can glue over your panel, and place the instrument box somewhere else?

The display bar should be about 60x10x4 mm, including the calibration button and light sensor.

The present box is 60x70x15 mm and as you can see on the pictures it fits to the side of a 57 mm instrument, in a 80 mm hole.

I think I will design the new PCB to allow a detachable display bar anyway.

I'm still open to any proposition...
  #15  
Old May 20th 20, 01:48 PM posted to rec.aviation.soaring
[email protected]
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Posts: 281
Default AOA indicator poll.



That's why my system uses 2 $35 pressure gauges: They are internally compensated for temperature and other factors, and outputs a 14 bits signal, with a full range corresponding to a 250 km/h dynamic pressure.

That's the price to pay to be below 1% at 70 km/h... And in practice it works...

The pitch string is corresponding to the instrument... But the string is very sensitive to sideslip...


Excellent, looks like a great plan and paper to read.

Are you going to put the design up on github?

Put me down for #2.


  #16  
Old May 20th 20, 02:46 PM posted to rec.aviation.soaring
Luc Job[_2_]
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Posts: 22
Default AOA indicator poll.

Before I publish it, the code needs some cleaning and complements... I need to test the light sensor functions... It was left aside as it proved to be unnecessary until now.

And now I'm working on the PCB design for the 'production' version.

Basically the idea of the 5 leds is to have an indication for the 5 important AOA's to fly a glider:

1)Blinking red is stall.
2)Red (nonblinking) to amber is below Cz max, or below minimum sink rate if you prefer...So on the bad side of the flight envelope, also indicated by the audio perturbation.
3)amber is Cz max or minimum sink rate.
4)green is approach AOA... Logical color for this part of flight.
5)white is best glide angle AOA
6)blue is lower than best glide angle

With an unflapped glider it is in the right order... With a flapped glider, the approach AOA with the flaps in landing position at the recommended approach speed of 1.3 Vs is lower than the best glide angle... So technically the green led should be between the white and the blue... Assuming that you set the blue at the AOA near the Vne...

But on a flapped glider you can also decide that green is best glide angle and white is approach with landing flaps (white arc on ASI)... That's what I have done on my DG... And on some gliders you have more than 1 landing flaps position... So it's up to you to decide what is the best setting for you and your glider...

On a flapped glider it makes also sense to have the green for the ideal flaps position, increase the flap angle when the color turns towards amber and lower it when it turns white...

  #17  
Old May 20th 20, 04:56 PM posted to rec.aviation.soaring
jfitch
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Posts: 1,134
Default AOA indicator poll.

On Wednesday, May 20, 2020 at 6:46:19 AM UTC-7, Luc Job wrote:
Before I publish it, the code needs some cleaning and complements... I need to test the light sensor functions... It was left aside as it proved to be unnecessary until now.

And now I'm working on the PCB design for the 'production' version.

Basically the idea of the 5 leds is to have an indication for the 5 important AOA's to fly a glider:

1)Blinking red is stall.
2)Red (nonblinking) to amber is below Cz max, or below minimum sink rate if you prefer...So on the bad side of the flight envelope, also indicated by the audio perturbation.
3)amber is Cz max or minimum sink rate.
4)green is approach AOA... Logical color for this part of flight.
5)white is best glide angle AOA
6)blue is lower than best glide angle

With an unflapped glider it is in the right order... With a flapped glider, the approach AOA with the flaps in landing position at the recommended approach speed of 1.3 Vs is lower than the best glide angle... So technically the green led should be between the white and the blue... Assuming that you set the blue at the AOA near the Vne...

But on a flapped glider you can also decide that green is best glide angle and white is approach with landing flaps (white arc on ASI)... That's what I have done on my DG... And on some gliders you have more than 1 landing flaps position... So it's up to you to decide what is the best setting for you and your glider...

On a flapped glider it makes also sense to have the green for the ideal flaps position, increase the flap angle when the color turns towards amber and lower it when it turns white...


I am struggling to understand why I would need one, but a couple of technical questions: How well does it work in accelerated flight such as a 50 deg bank? Will you need an adjustment to account for different nose shapes and/or different pressure tube installation locations? What does it indicate in a spin or spin transition to spiral dive?
  #18  
Old May 20th 20, 08:50 PM posted to rec.aviation.soaring
Luc Job[_2_]
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Posts: 22
Default AOA indicator poll.

You need one precisely to know where your are, aerodynamically speaking, whatever would be the bank angle, load factor, weight (including ballast), flaps position or airbrake extension... On any case it will show the real position of your wing in the surrounding air, and how it changes, and this is the only thing that matters to fly...

Sorry if you learn that speed is life... AOA gives a much more straightforward information of your real safety margin, or 'lift reserve' before you fall from the sky...

And it indicates directly if you fly at the lowest possible rate of descent or at the best glide angle as these angles are purely aerodynamic parameters that does not depend on anything else.

The only thing that will have a slight influence is a modification of the airfoil in front of the separation point, like the extension of a leading edge slat.

Flaps have no influence on stall AOA, as the separation point is before the flap along the airfoil, but as it changes the camber of the airfoil, it changes its maximum lift... reducing the stall speed when positive, and allowing to keep the best angle through an extended speed range when negative... One more reason why speed isn't safety...

Of course, you can compute the stall or best speed form the mass of the glider and actual load factor, flaps position, airbrake position, and compare this computed speed with what your ASI indicates, but presently no instrument does this for you accurately and transparently.

The AOA indicator will be accurate as long as the measured pressures are significant. In a spin, It will indicate that you are above stall... Maybe the exact angle can't be determined as the dynamic pressure is likely to be inaccurate due to the airflow angle Vs the probe, but as soon as you recover the indication will be back instantly...

And this is precisely a situation where it is useful: When you recover from a dive, you will pull at least 2-3 g... You only have your feeling to pull enough to recover and stay below the Vne, but not too much or you risk a high speed stall... With an AOA, you can adjust precisely how much you pull the stick to maintain the indicator in the good range. This will guarantee that you will lose the minimum of altitude to recover from the dive...

The instrument is based on a pressure ratio. It's easy to understand that if the angle difference is low, the pressures will have less difference and the ratio begins to be inaccurate. So the ideal angle is 45°... But this is not common on gliders. AOA probes on fighters have about a 20° angle and they work even in supersonic airflow... So I would say that 20° is a reasonable minimum, and this can be found under the nose of any glider... And this is a place where the flow has no reason to be disturbed or modified within the allowed speed range, whatever will be the camber and shape of the nose.

Then as you calibrate the instrument for the different angles in flight, by pushing a button when you are stabilized at each reference angle, there is no further adjustment needed...

Any other question?
  #19  
Old May 20th 20, 11:55 PM posted to rec.aviation.soaring
jfitch
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Posts: 1,134
Default AOA indicator poll.

On Wednesday, May 20, 2020 at 12:50:35 PM UTC-7, Luc Job wrote:
You need one precisely to know where your are, aerodynamically speaking, whatever would be the bank angle, load factor, weight (including ballast), flaps position or airbrake extension... On any case it will show the real position of your wing in the surrounding air, and how it changes, and this is the only thing that matters to fly...

Sorry if you learn that speed is life... AOA gives a much more straightforward information of your real safety margin, or 'lift reserve' before you fall from the sky...

And it indicates directly if you fly at the lowest possible rate of descent or at the best glide angle as these angles are purely aerodynamic parameters that does not depend on anything else.

The only thing that will have a slight influence is a modification of the airfoil in front of the separation point, like the extension of a leading edge slat.

Flaps have no influence on stall AOA, as the separation point is before the flap along the airfoil, but as it changes the camber of the airfoil, it changes its maximum lift... reducing the stall speed when positive, and allowing to keep the best angle through an extended speed range when negative.... One more reason why speed isn't safety...

Of course, you can compute the stall or best speed form the mass of the glider and actual load factor, flaps position, airbrake position, and compare this computed speed with what your ASI indicates, but presently no instrument does this for you accurately and transparently.

The AOA indicator will be accurate as long as the measured pressures are significant. In a spin, It will indicate that you are above stall... Maybe the exact angle can't be determined as the dynamic pressure is likely to be inaccurate due to the airflow angle Vs the probe, but as soon as you recover the indication will be back instantly...

And this is precisely a situation where it is useful: When you recover from a dive, you will pull at least 2-3 g... You only have your feeling to pull enough to recover and stay below the Vne, but not too much or you risk a high speed stall... With an AOA, you can adjust precisely how much you pull the stick to maintain the indicator in the good range. This will guarantee that you will lose the minimum of altitude to recover from the dive...

The instrument is based on a pressure ratio. It's easy to understand that if the angle difference is low, the pressures will have less difference and the ratio begins to be inaccurate. So the ideal angle is 45°... But this is not common on gliders. AOA probes on fighters have about a 20° angle and they work even in supersonic airflow... So I would say that 20° is a reasonable minimum, and this can be found under the nose of any glider... And this is a place where the flow has no reason to be disturbed or modified within the allowed speed range, whatever will be the camber and shape of the nose.

Then as you calibrate the instrument for the different angles in flight, by pushing a button when you are stabilized at each reference angle, there is no further adjustment needed...

Any other question?


I should have phrased that differently, I'm pretty familiar with AOA and what it does. To me it is generally communicated by numerous other cues but that may be from a lifetime of hang gliding, sailing, soaring, etc. A high speed stall it is harder to feel from these things, but a high speed stall isn't a normal occurrence in a sailplane.

I'd think that flap setting would definitely affect it, as you are really measuring the AOA of the fuselage and inferring AOA of the wing assuming fixed angle of incidence? Increasing flap changes the angle of incidence, increasing the AOA of the wing compared to the fuselage?

Anyway, it sounds like you have the calibration tools to adapt to installation differences. One suggestion I have is to make the display as narrow as possible, panel space is very tight with today's gliders. You might even consider an option to mount discrete panel mount LEDs as these could be fit in-between instruments or at the edge of a panel in many situations where your current display could not.
  #20  
Old May 21st 20, 07:19 PM posted to rec.aviation.soaring
Luc Job[_2_]
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Posts: 22
Default AOA indicator poll.

When I say that flaps have non influence on stall AOA, I shoud have said 'simple flaps'... For the same reasons as leading edge slats, slotted flaps are another story... But the only glider I'm aware of with slotted flaps was the Breguet 904... That's history but I will never regret the day I had one at disposal...

Basically it is a question of definition:

If you consider that the AOA is the angle between the tip of the leading edge and the trailing edge, then the critical AOA will decrease with flaps down... But that's not what we measure... We measure the angle of the fixed part of the wing and unless you have a variable incidence this angle is the same on the fuselage or on the wing.

When I say that flaps position has no influence on the best glide angle or maximum lift angle, it assumes also that I consider low flaps deflection, say +- 15°... This might not be true for 45° flap deflection or more as this will modify the airflow regime considerably, create a lot of turbulence on top and new stagnation points under the profile... Indeed it will lower the critical AOA (as measured).

Same for critical stall angle... Stick a sidestring on a flapped glider and check for any flaps position, and stick some colored tape at stall angle! Wind tunnel experiments or calculation shows that we are within 1-2°.... Over a 25° range. This is sufficient to say that it will make no difference in flight...compared for example to stall speed in various configurations. And if we suppose that the pilot is sufficiently skilled to keep the right flaps settings it makes no sense anyway... And the AOA indicator is a precious instrument to set the flaps at the right position anytime.

As I said on a previous mail, I'm designing a detachable indicator... Dimension will be about 10x50x8 mm with a back connector and it might be integrated in the panel or simply stuck over the surface... I think to integrate it into a 80 mm ASI... I have a spare... And don't forget that the data will be available in any suitable format for your calculator to have something displayed there... Next step is to integrate this into XcSoar... Next winter?

And to anticipate the next remark: My system is not designed to work in inverted flight... But it is possible to setup a second one with a top pressure port for this purpose.
 




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