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Basic, basic question. What's the correct stall speed?


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8 minutes ago, carusoam said:

https://aspenavionics.com/aoa

A few things have changed over time...

  • the differential pressure, pitot system isn't used any longer(?) (could have been a memory I created...)
  • The hard to interprate triangles have labels in Bob's graphic above ^^^ (they are unlabeled on the Aspen website I linked here)
  • A bit surprising they don't integrate the flap position sensor with their sensor technology... (if desired)

Best regards,

-a-

 

Anthony, when I went looking for a graphic to attach to my post above the first one that popped up was the one with the pointers on both sides with the labels. I had seen the one you linked but I do not know what's going to appear when I do the calibration! (The AOA does not appear on my PFD until it has been calibrated.

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Just now, Marauder said:

Another thing to add to this topic is that Aspen's software derived AoA does use airspeed as part of the AHRS calculation. Be interesting to learn how it does the calculation.

Prior to all the discussions and the several rabbit holes explored on this thread I started a couple of days ago I was puzzled. Now I'm thinking that it might not be as black magic as I had imagined. The Aspen knows the bank angle. And the IAS. All it needs is Vs and Vso and the mathematical equivalent of the load factor chart we've been discussing. And that's just physics/aerodynamics(?), nothing specific to make/model. 

We'll see.

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21 minutes ago, carusoam said:
  • A bit surprising they don't integrate the flap position sensor with their sensor technology... (if desired)

 

Anthony,

The AoA calibration flight for the Aspen software upgrade is done in a variety of aircraft configurations, flaps up, down, gear up/down etc at different speeds... So the system learns the different attitudes and airspeeds associated with each. I don't see the need for a flap position sensor. 

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Just now, N6758N said:

Anthony,

The AoA calibration flight for the Aspen software upgrade is done in a variety of aircraft configurations, flaps up, down, gear up/down etc at different speeds... So the system learns the different attitudes and airspeeds associated with each. I don't see the need for a flap position sensor. 

Terry, I haven't done that flight yet but I've read the procedure a couple of time. I think it involves marking a stabilized straight and level and a stall speed in the 2 configurations - full and zero flaps. The position of the pointer is going to be an interpolation of those known data.

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8 minutes ago, Bob_Belville said:

Terry, I haven't done that flight yet but I've read the procedure a couple of time. I think it involves marking a stabilized straight and level and a stall speed in the 2 configurations - full and zero flaps. The position of the pointer is going to be an interpolation of those known data.

Yep, I haven't done a cal flight in a while, I used to do them for customers when the avionics shop I worked for was in business, so I'm a little fuzzy on the details. 

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1 hour ago, Marauder said:

Maybe you can do mine with me. :) 

Let's do it! It requires a lot of focus on the ASI because you have to maintain a specific airspeed for a set amount of time, so having someone to keep an eye out for traffic is a good idea! Although you guys have all that fancy traffic alerting boxes:D

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I think I have an answer for you.  Bear in mind I am not an aeronautical engineer, I am a damn lawyer, which only means I can maybe think and read, which is what I did.  Also, I only read the first couple of pages of this thread, so maybe someone gave this answer in the third or fourth page, if so I apologize for being redundant.

I went to three sources: Jeppesen's Commercial/Instrument textbook (big, thick hardcover), Gardner's The Complete Advanced Pilot, and Ecklabar's Flying High Performance Singles and Twins.  Jeppesen and Gardner say the same interesting thing.  KIAS and KCAS are virtually identical at higher aircraft speeds such as cruise, but separate in high angle of attack, low speed regimes of flight.  The pitot tube is measuring pressure, of course, not speed.  Ecklabar says this is: "dynamic pressure," i.e. the difference between static pressure and the total pressure the pitot sees.  What the pitot sees is essentially what a calibrated pitot measuring device sees ( a tube on an extended boom to avoid interference and compression effects near the airframe) at high speeds.  But at lower speeds, because the pitot is fixed, it also is moving through the air at a higher angle of attack, which I surmise reduces the cross section of the orifice in relation to the airflow, and therefore the pitot reads a lower pressure than if it we flat and away from the airframe.

So in low speed/high angle of attack situations, CAS becomes more important and IAS becomes less accurate.  IAS inaccurately reads the dynamic pressure as lower than it actually would be if the airplane were flat instead of at a high angle.  

This should tell you a couple of things.  One is that you cannot assume that at low speeds the difference between CAS and IAS is a fixed number, like "5 knots."  This is the regime of flight in which IAS is becoming less accurate and CAS is becoming more important and the difference is changing.

Now, the rest of this is my own thinking, so take it for what it is worth.  It is clear from my POH (I have a 231) that all of the arc speeds are KCAS, not KIAS.  We also know that the pitot can only measure what it "sees," in other words, knots indicated, and the ASI can only display what the pitot sees.  So my thinking is that the manufacturer, in creating a marked ASI, displays the arc speeds in KCAS, translated to KIAS, because that is the only thing the pitot can read and the ASI can show.  The manufacturer wants the pilot to be able to directly read from the arc markings, where the aircraft is in relation to the speed at which the aircraft is going to stall, so the CAS adjustment is made and the arc is marked so that at lower speeds, what you see on the airspeed indicator (an indicated airspeed) corresponds to the KCAS at which stall will occur.  In other words, to figure out stall the manufacturer goes out with the long boom and finds the KCAS at which the aircraft stalls in a specific configuration.  The knots indicated is, lets say, 2 knots lower, and that goes on the ASI, so that is part of the answer as to why the ASI arcs are marked as they are.

The other part is my own thinking also, so if you are really particular you should verify with the manufacturer.  My POH, and I gather those of others, has a stall speed table that shows stall speed for a given angle of bank, gross weight, and C.G., for a given flap angle.  My Airspeed Limitation chart has a "bottom of the white arc" which does not exactly correspond to any of the chart speeds.  Why would that be?  I think the answer is in the aircraft configuration.  The speeds in the table are for given flap angles only, they do not specify "landing configuration."  The arc table specifies "landing configuration."  What's the difference, if landing at full flaps then the number on the arc should be the same as the number for 33 degrees of flaps in the table.  The answer is that "landing configuration" as set out in the POH means THE GEAR IS DOWN, and I think the Stall/bank angle tables does not include the gear down. The gear being down will change the stall speed from the gear up stall speed, same flaps angle.  

So the bottom line is that the bottom of the white arc assumes full flaps, gear down, and is KCAS translated to what the ASI can display, which is an indicated airspeed.  

This was interesting, I knew there was a difference between these things but like everyone else, I just fly the plane.

YMMV

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43 minutes ago, jlunseth said:

I think I have an answer for you.  Bear in mind I am not an aeronautical engineer, I am a damn lawyer, which only means I can maybe think and read, which is what I did.  Also, I only read the first couple of pages of this thread, so maybe someone gave this answer in the third or fourth page, if so I apologize for being redundant.

I went to three sources: Jeppesen's Commercial/Instrument textbook (big, thick hardcover), Gardner's The Complete Advanced Pilot, and Ecklabar's Flying High Performance Singles and Twins.  Jeppesen and Gardner say the same interesting thing.  KIAS and KCAS are virtually identical at higher aircraft speeds such as cruise, but separate in high angle of attack, low speed regimes of flight.  The pitot tube is measuring pressure, of course, not speed.  Ecklabar says this is: "dynamic pressure," i.e. the difference between static pressure and the total pressure the pitot sees.  What the pitot sees is essentially what a calibrated pitot measuring device sees ( a tube on an extended boom to avoid interference and compression effects near the airframe) at high speeds.  But at lower speeds, because the pitot is fixed, it also is moving through the air at a higher angle of attack, which I surmise reduces the cross section of the orifice in relation to the airflow, and therefore the pitot reads a lower pressure than if it we flat and away from the airframe.

So in low speed/high angle of attack situations, CAS becomes more important and IAS becomes less accurate.  IAS inaccurately reads the dynamic pressure as lower than it actually would be if the airplane were flat instead of at a high angle.  

This should tell you a couple of things.  One is that you cannot assume that at low speeds the difference between CAS and IAS is a fixed number, like "5 knots."  This is the regime of flight in which IAS is becoming less accurate and CAS is becoming more important and the difference is changing.

Now, the rest of this is my own thinking, so take it for what it is worth.  It is clear from my POH (I have a 231) that all of the arc speeds are KCAS, not KIAS.  We also know that the pitot can only measure what it "sees," in other words, knots indicated, and the ASI can only display what the pitot sees.  So my thinking is that the manufacturer, in creating a marked ASI, displays the arc speeds in KCAS, translated to KIAS, because that is the only thing the pitot can read and the ASI can show.  The manufacturer wants the pilot to be able to directly read from the arc markings, where the aircraft is in relation to the speed at which the aircraft is going to stall, so the CAS adjustment is made and the arc is marked so that at lower speeds, what you see on the airspeed indicator (an indicated airspeed) corresponds to the KCAS at which stall will occur.  In other words, to figure out stall the manufacturer goes out with the long boom and finds the KCAS at which the aircraft stalls in a specific configuration.  The knots indicated is, lets say, 2 knots lower, and that goes on the ASI, so that is part of the answer as to why the ASI arcs are marked as they are.

The other part is my own thinking also, so if you are really particular you should verify with the manufacturer.  My POH, and I gather those of others, has a stall speed table that shows stall speed for a given angle of bank, gross weight, and C.G., for a given flap angle.  My Airspeed Limitation chart has a "bottom of the white arc" which does not exactly correspond to any of the chart speeds.  Why would that be?  I think the answer is in the aircraft configuration.  The speeds in the table are for given flap angles only, they do not specify "landing configuration."  The arc table specifies "landing configuration."  What's the difference, if landing at full flaps then the number on the arc should be the same as the number for 33 degrees of flaps in the table.  The answer is that "landing configuration" as set out in the POH means THE GEAR IS DOWN, and I think the Stall/bank angle tables does not include the gear down. The gear being down will change the stall speed from the gear up stall speed, same flaps angle.  

So the bottom line is that the bottom of the white arc assumes full flaps, gear down, and is KCAS translated to what the ASI can display, which is an indicated airspeed.  

This was interesting, I knew there was a difference between these things but like everyone else, I just fly the plane.

YMMV

Thanks for that! Lawyer, huh. Just like my JD kids... argue the law if it's on your side or argue the facts, logically of course, if that is where you need to come out. :rolleyes: 

  • Facts: on one of the pages you didn't get to (page 3, about 1/3 down) is a chart pasted by@mooniac15u that shows, for a J I think, that CAS and IAS were only 0.5 knot difference at the bottom of the speed chart, with full flaps, gear down. Obviously 0.5 knots difference between CAS and IAS does not explain the question in my OP. I Do not believe CAS/IAS is anything more than a rabbit hole in trying to understand the difference that occurs, as it turns out, with several (most/all?) models. 
  • Law: perhaps I missed it, but I don't think that anyone has found an official (read legal) definition of the bottom of the white arc and it's relationship to Vso.
  • Misdirection? You may well be right, but I'm not so sure the stall speed changes with gear position. I'd be interested in BCV. (The same question could be asked for speed brakes which logically might be expected to change the wing but I don't think they do.)

Keep those cards and letters coming. There's an answer out there somewhere - even though Mikey Miles has retired and Bill Wheat has gone on to his reward. (I asked a PPP CFI friend. He really didn't have an answer. I don't know if @donkaye has seen he thread. @mike_elliottmay have weighed in, I don't recall. Other Master CFIs, DPEs?

 

 

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The Aspen PFD does nothing to provide CAS. One of the factors that come into play calculating CAS is pitot placement and pitot/static dynamics. That is why CAS is different at different AoA (aka speed).

 

I think I have an answer for you.  Bear in mind I am not an aeronautical engineer, I am a damn lawyer, which only means I can maybe think and read, which is what I did.  Also, I only read the first couple of pages of this thread, so maybe someone gave this answer in the third or fourth page, if so I apologize for being redundant.

I went to three sources: Jeppesen's Commercial/Instrument textbook (big, thick hardcover), Gardner's The Complete Advanced Pilot, and Ecklabar's Flying High Performance Singles and Twins.  Jeppesen and Gardner say the same interesting thing.  KIAS and KCAS are virtually identical at higher aircraft speeds such as cruise, but separate in high angle of attack, low speed regimes of flight.  The pitot tube is measuring pressure, of course, not speed.  Ecklabar says this is: "dynamic pressure," i.e. the difference between static pressure and the total pressure the pitot sees.  What the pitot sees is essentially what a calibrated pitot measuring device sees ( a tube on an extended boom to avoid interference and compression effects near the airframe) at high speeds.  But at lower speeds, because the pitot is fixed, it also is moving through the air at a higher angle of attack, which I surmise reduces the cross section of the orifice in relation to the airflow, and therefore the pitot reads a lower pressure than if it we flat and away from the airframe.

So in low speed/high angle of attack situations, CAS becomes more important and IAS becomes less accurate.  IAS inaccurately reads the dynamic pressure as lower than it actually would be if the airplane were flat instead of at a high angle.  

This should tell you a couple of things.  One is that you cannot assume that at low speeds the difference between CAS and IAS is a fixed number, like "5 knots."  This is the regime of flight in which IAS is becoming less accurate and CAS is becoming more important and the difference is changing.

Now, the rest of this is my own thinking, so take it for what it is worth.  It is clear from my POH (I have a 231) that all of the arc speeds are KCAS, not KIAS.  We also know that the pitot can only measure what it "sees," in other words, knots indicated, and the ASI can only display what the pitot sees.  So my thinking is that the manufacturer, in creating a marked ASI, displays the arc speeds in KCAS, translated to KIAS, because that is the only thing the pitot can read and the ASI can show.  The manufacturer wants the pilot to be able to directly read from the arc markings, where the aircraft is in relation to the speed at which the aircraft is going to stall, so the CAS adjustment is made and the arc is marked so that at lower speeds, what you see on the airspeed indicator (an indicated airspeed) corresponds to the KCAS at which stall will occur.  In other words, to figure out stall the manufacturer goes out with the long boom and finds the KCAS at which the aircraft stalls in a specific configuration.  The knots indicated is, lets say, 2 knots lower, and that goes on the ASI, so that is part of the answer as to why the ASI arcs are marked as they are.

The other part is my own thinking also, so if you are really particular you should verify with the manufacturer.  My POH, and I gather those of others, has a stall speed table that shows stall speed for a given angle of bank, gross weight, and C.G., for a given flap angle.  My Airspeed Limitation chart has a "bottom of the white arc" which does not exactly correspond to any of the chart speeds.  Why would that be?  I think the answer is in the aircraft configuration.  The speeds in the table are for given flap angles only, they do not specify "landing configuration."  The arc table specifies "landing configuration."  What's the difference, if landing at full flaps then the number on the arc should be the same as the number for 33 degrees of flaps in the table.  The answer is that "landing configuration" as set out in the POH means THE GEAR IS DOWN, and I think the Stall/bank angle tables does not include the gear down. The gear being down will change the stall speed from the gear up stall speed, same flaps angle.  

So the bottom line is that the bottom of the white arc assumes full flaps, gear down, and is KCAS translated to what the ASI can display, which is an indicated airspeed.  

This was interesting, I knew there was a difference between these things but like everyone else, I just fly the plane.

YMMV

 

Working with legal teams daily, my answer was the “tell them only what they need to know”. Ironic that a lawyer went on to explain it.

 

 

Sent from my iPad using Tapatalk Pro

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1 hour ago, Bob_Belville said:
  • Misdirection? You may well be right, but I'm not so sure the stall speed changes with gear position. I'd be interested in BCV. (The same question could be asked for speed brakes which logically might be expected to change the wing but I don't think they do.)

 

I was taught that years ago during PP training - that gear down lowers stall speed.  I will have to go find it in a book for you.

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6 minutes ago, jlunseth said:

I was taught that years ago during PP training - that gear down lowers stall speed.  I will have to go find it in a book for you.

The chart for my 1970 C (posted somewhere above) gives stall speeds clean; Takeoff flaps, gear Down; and Landing flaps, gear Down. There is no information for gear Up vs. gear Down for any flap position. 

At 0° Angle of Bank, stall speed is 67, 64 and 57 mph (or 58, 56 and 49 knots). The white arc on my ASI ends at 63 mph, while the green arc begins at 70 mph.

Bob, I would expect your E to behave closer to my C than to any J, which is longer, heavier and has many aerodynamic improvements courtesy of Roy LoPresti.

The delta CAS-IAS is pretty small at the low end of the chart (70 mph), when power is OFF. it's pretty much 1 mph. Power ON, CAS can be 6mph lower than IAS, it is 3mph from Flaps 0-15°, and becomes 6mph at Flaps 33°.

As we used to say when working at a Japanese company, "more thinking need."

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On 11/12/2017 at 8:25 PM, Bob_Belville said:

I may have posted this before, it's been nagging at me for a very long time.

For my 1966 M20E: the bottom of the white arc on the ASI is 55 knots (63 mph). This agrees with the "FAA Approved Airplane Flight Manual" Issued Dec. 3, 1965.

But there's a chart in the "Owners Manual" that is labeled "stall speed vs. bank angle" (gross weight, IAS, power off).

That chart shows stall speed with zero bank and full flaps as 57 mph = 50 knots. 

What's the difference?

IMG_20171110_141410929[1].jpg

IMG_20171110_141524362[1].jpg

Best I can figure, they knew you were going to add bladders :) and upped the stall speed for you on the ASI in anticipation of the added weight :)

You might ping Stacy Ellis and see if he might have a clue as to why the diff.

 

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7 minutes ago, mike_elliott said:

Best I can figure, they knew you were going to add bladders :) and upped the stall speed for you on the ASI in anticipation of the added weight :)

You might ping Stacy Ellis and see if he might have a clue as to why the diff.

 

So much blather about bladders, and I thought Master Mooney CFIIs knew everything!  :P

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7 hours ago, Bob_Belville said:

 

  • Law: perhaps I missed it, but I don't think that anyone has found an official (read legal) definition of the bottom of the white arc and it's relationship to Vso

 

 

There is one.  I just don't know if it is the one that applies to your aircraft, or mine too for that matter.  They were born under the CARs, but I don't think the CAR's are significantly different than the FARs in these areas.  I noticed that someone posted part of the CARs earlier, perhaps someone has them who can verify whether they are the same as the FARs.

FAR 1.2 defines Vso as: "the stalling speed or the minimum steady flight speed in the landing configuration."  I think that shoots down the issue about minimum steady flight speed that was discussed earlier, I think the reg. says they are the same.  

FAR 23.1545 says the ASI must be marked:  (4) For the flap operating range, a white arc with the lower limit at VS0 at the maximum weight, and the upper limit at the flaps-extended speed VFE established under §23.1511.

The landing configuration is going to be defined in the AFM, in mine it is gear down, full flaps.  I don't know if that is in a table, it is in the "Normal Procedures" section that describes landing.

So the bottom of the white arc is stall speed in the landing configuration.

Now, there is a provision that says that the instrument must be accurate +/- 3% or 5 knots, whichever is greater.  (23.1323)  That would be 1.8 kts at 60kts, so perhaps that is part of the explanation.

But I still think it is the fact that landing configuration requires the gear to be deployed, which is different from the bank angle table, which does not say that.

 

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10 minutes ago, jlunseth said:

But I still think it is the fact that landing configuration requires the gear to be deployed, which is different from the bank angle table, which does not say that.

 

The bank angle chart that is pasted on page three of this thread includes an example at the bottom which is essential an aid to reading the chart. It in fact lists "gear down" as one of the conditions. The speeds - IAS and CAS, match the chart.

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On the subject of a legal requirement for the bottom of the white arc, I found CAR 3.82:

3.82 Definition of stalling speeds.

(a) Vso denotes the true indicated stalling speed, if obtainable, or the minimum steady flight speed at

which the airplane is controllable, in miles per hour, with:

(1) Engines idling, throttles closed (or not more than sufficient power for zero thrust),

(2) Propellers in position normally used for take-off,

(3) Landing gear extended,

(4) Wing flaps in the landing position,

(5) Cowl flaps closed,

(6) Center of gravity in the most unfavorable position within the allowable landing range,

(7) The weight of the airplane equal to the weight in connection with which Vso is being used as a factor to determine a required performance. 

Then CAR 3.757 says:

(4) The flap operating range—a white arc with the lower limit at Vso as determined in § 3.82 at the maximum weight, and the upper limit at the flaps-extended speed in § 3.742. 

I can't be certain I have the exactly right version of the CARs, just not possible to figure that out with what I have.

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More logic...

0) Partially stalled wing conditions don't seem to be well defined

1) The configurations in our POH are limited to the ones used/promoted in our POH...

2) Some caution comes to mind... things that cause disruption of airflow above and below the wing disrupt the lift in that area of the wing.

  • Landing configuration is throttle out, flaps down, gear down...
  • Not mentioning speed brakes... is kind of like ignoring the gear...
  • Speed brakes and gear cause localized airflow challenges. Losing that percentage of lift will increase descent rate... or raise the AOA. But not itself generate a full stalled condition.
  • kind of like the stall devices (name please, fences?) on the leading edges...
  • Wing design/construction changes specific parts of the wing to begin stalling first.

3) another thing left out of the conversation is the AOA...

4) Since The Stall begins near the root of the wing, an additional percentage of the wing's lift is being lost at milder AOAs...

5) There is definitely an Airspeed or AOA that makes the whole wing liftless....

6) Where There is some danger not discussed very much... on the go-around...

  • Landing configuration, prepped for go around.
  • Full Power is added in, quickly in most cases.
  • Nose tries to head skyward, if not actively controlled.
  • this configuration and power setting doesn't seem to make the chart...

7) What's the Stall speed of full power in the landing configuration?

8) the AOA on final approach and AOA on the go around seem pretty different to me...

9) charted Power On stalls seem to only be expected in the climb configurations.

10) charted Power off Stall seem to only be expected in the landing configuration.

This sounds like @201er's AOAi promotion threads...  

So much benefit, so little additional cost, when completing a panel make-over.

Thanks to jl for the additional legal eyes reading insight.

Our thin wing has an interesting span of working AOA.  The line where the airflow splits to go over or under the wing doesn't have as much room to move as other wings. words like burbling and airflow separation get used....

So....

Now approaching full circle....

Setting The AOAi based on Stall speeds has to take into account...

  • calibrated air speed 
  • actual instrument error
  • actual configuration combinations used
  • actual Wing construction and options
  • actual wing condition, including all damage, paint issues, leading edge tape, bird crap and paint scratches...
  • Speed brakes, fiki system, retreads or good year FCIIs....
  • yet, not get too complex to make it unusable...

Calibrating It seems to be a piece of cake compared to discussing these tiny details...

Testing its calibration at altitude seams to be the prudent catch-all method for all the variations listed here.  

Comparing the actual AOAi performance to the ASI and charted data, gains experience over many flights and weather conditions....

These important last steps seem to be done in kias...  

PP thoughts only not an instrument guru.  Our @CYA guy could probably give some good insight on this topic...?

Best regards,

-a-

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