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

[Bob_Belville]

4 hours ago, takair said:

Bob

From CFR 14, 1.2:

VS   means the stalling speed or the minimum steady flight speed at which the airplane is controllable. 

VS0   means the stalling speed or the minimum steady flight speed in the landing configuration.

Key words for this discussion are or minimum steady flight speed.  I have started to notice this terminology in some definitions of these V speeds and with reference to the white arc.  The CAA regulations seemed to give some room in defining the bottom of the white arc.  It seems to me, the bottom of the Mooney white arc is minimum steady state flight and not stalling speed.  This might explain the disparity and seems like a logical way to mark an airspeed indicator, it actually gives some margin.  I wonder if this corresponds to the point where the stall horn is supposed to come on, maybe I’ll try some stalls this weekend to check.  If this theory is correct, I think the fault with the Mooney manual is simply that they didn’t define it this way, when other OEMs can and do use bottom of the white arc for stall speed.  

As a side note, I checked with one of our aero guys, but it was not obvious to him.  Of course, we tend to work on whirly wings, so we don’t look at a stall the same way.....

Thanks Rob, do you suppose there is a 5 kias difference between those concepts? Surely there are not 2 definitions of Vso. If I did not already have an idea of what "stall" meant I think I'd read that language as MSFS is a clarification of what we mean by stall speed. If 2 different speeds are intended  I suppose the other one would be holding the pitch up until the nose breaks which we used to be asked to do for (B)FR. 

At the PPP in Manchester NH last year my instructor had me flying for most of an hour at night with the stall horn blaring but maintaining control, perhaps even making very shallow turns. We did that clean and configured to land. Were we at MSFS?

I think we've changed our instruction and FR requirement for demonstrating slow flight a little since 1966 but the speeds we're trying to reconcile are both from 1966 documents. (And those documents do not use Vso.

(It's 3:30 here, I got up for a glass of water and I'm going back to bed, so this post might not make sense in the morning.) 

[takair]

3 hours ago, Bob_Belville said:

Thanks Rob, do you suppose there is a 5 kias difference between those concepts? Surely there are not 2 definitions of Vso. If I did not already have an idea of what "stall" meant I think I'd read that language as MSFS is a clarification of what we mean by stall speed. If 2 different speeds are intended  I suppose the other one would be holding the pitch up until the nose breaks which we used to be asked to do for (B)FR. 

At the PPP in Manchester NH last year my instructor had me flying for most of an hour at night with the stall horn blaring but maintaining control, perhaps even making very shallow turns. We did that clean and configured to land. Were we at MSFS?

I think we've changed our instruction and FR requirement for demonstrating slow flight a little since 1966 but the speeds we're trying to reconcile are both from 1966 documents. (And those documents do not use Vso.

(It's 3:30 here, I got up for a glass of water and I'm going back to bed, so this post might not make sense in the morning.) 

Bob

5kt is more than I would expect, but this could have been Mooney’s or the CAAs definition of “safe”.  After all, the stall horn is typically set to go of 5-7kt higher than actual stall.  Making the white arc correspond to the bottom of the white arc may make sense in this context.  Will keep thinking about this one.  I do suspect that Aspen would want the white arc to match the ASI, but I agree it would be good to understand.

[Yetti]

Stall speed vs "Flap operating range"   The white arc is where the flaps are operational and you can land the plane.  The stall speed is something different.  Apparently 5kt difference for the Mooney.   On another point.  Minimum controllable airspeed in a Cessna 172 is 0 knots since I know I have done that in slow flight.  

[mooniac15u]

49 minutes ago, Yetti said:

Stall speed vs "Flap operating range"   The white arc is where the flaps are operational and you can land the plane.  The stall speed is something different.  Apparently 5kt difference for the Mooney.   On another point.  Minimum controllable airspeed in a Cessna 172 is 0 knots since I know I have done that in slow flight.  

The M20J has a Vso of 55 KIAS and, in the corresponding chart of stall speed vs bank, a stall speed of 54 KIAS in landing configuration.  I guess it doesn't have a 5 knot uncontrollable "dead zone" like the M20E.

[Bob_Belville]

1 hour ago, takair said:

Bob

5kt is more than I would expect, but this could have been Mooney’s or the CAAs definition of “safe”.  After all, the stall horn is typically set to go of 5-7kt higher than actual stall.  Making the white arc correspond to the bottom of the white arc may make sense in this context.  Will keep thinking about this one.  I do suspect that Aspen would want the white arc to match the ASI, but I agree it would be good to understand.

Rob, I thought about another angle that might help us... does this same difference occur with other, especially later, models? I.e. does that "load Factor" chart exist for a J? If it does, does the "Vso" implied there agree with the bottom of the white arc speed or does it carry through with the difference we're seeing with early Es. 

Does anyone here have a really late Chaparrel '74, '75 or an even later Ranger which continued in production into '78?

Edit: @mooniac15u just posted that the J shows 54 kias on the "load factor" chart. The Vso is the same 55 kias as the bottom of my older white arc. I wonder when the chart got changed.

That begs the question, what are the Vso speeds for Cs, Fs, Gs, even Ks? 

[mooniac15u]

15 minutes ago, Bob_Belville said:

Rob, I thought about another angle that might help us... does this same difference occur with other, especially later, models? I.e. does that "load Factor" chart exist for a J? If it does, does the "Vso" implied there agree with the bottom of the white arc speed or does it carry through with the difference we're seeing with early Es. 

Does anyone here have a really late Chaparrel '74, '75 or an even later Ranger which continued in production into '78?

Edit: @mooniac15u just posted that the J shows 54 kias on the "load factor" chart. The Vso is the same 55 kias as the bottom of my older white arc. I wonder when the chart got changed.

That begs the question, what are the Vso speeds for Cs, Fs, Gs, even Ks? 

Here's the whole chart.

[takair]

44 minutes ago, mooniac15u said:

The M20J has a Vso of 55 KIAS and, in the corresponding chart of stall speed vs bank, a stall speed of 54 KIAS in landing configuration.  I guess it doesn't have a 5 knot uncontrollable "dead zone" like the M20E.

I don't disagree.  It is odd, but it comes down to someone's interpretation of "safe".  I think we can agree that the stall on approach is not safe.  So, with typical margins, perhaps Mooney or the FAA decided that (stall speed + 5kt= Vso (safe operating)) for the M20E in landing configuration....  We may never really know how Mooney got there.

[DXB]

56 minutes ago, Bob_Belville said:

Rob, I thought about another angle that might help us... does this same difference occur with other, especially later, models? I.e. does that "load Factor" chart exist for a J? If it does, does the "Vso" implied there agree with the bottom of the white arc speed or does it carry through with the difference we're seeing with early Es. 

Does anyone here have a really late Chaparrel '74, '75 or an even later Ranger which continued in production into '78?

Edit: @mooniac15u just posted that the J shows 54 kias on the "load factor" chart. The Vso is the same 55 kias as the bottom of my older white arc. I wonder when the chart got changed.

That begs the question, what are the Vso speeds for Cs, Fs, Gs, even Ks? 

I looked again in the '76-'78 POH for the M20C, which I posted previously on this site. It retains the identical disparity between white arc definition and the stall speed at 0-degrees bank, full flaps.   I wondered if other major manufacturers in the '60s added this buffer between bottom of white arc and stall speed or it was a Mooney specific idiosyncrasy, so I looked in the POH of a 1967 C172: https://www.redskyventures.org/doc/cessna-poh/Cessna_172_C172H_1967_POH_scanned.pdf.   It uses identical terminology to the old Mooney POHs and never mentions "Vso" anywhere.  The C172's white arc is described as the flap operating range and the bottom is at 52mph.  By contrast, the stall speed with no bank and full flaps is presented in identical table form as 49mph.   My tentative, non-expert conclusions:

-The current practice having the bottom of the white arc match Vso was not standardized in the 1960s, with more than one manufacturer including a buffer between bottom of white arc and true Vso on the ASI.  If Cessna was doing it also, it may have been the industry standard in that era.

-I  doubt the shift from including this buffer to having the numbers match reflects any intrinsic difference in  the J's stall characteristics.  I suspect they were following an industry shift toward having the ASI markings serve as a direct mnemonic for as many V speeds as possible.  Since this change isn't an esoteric Mooney-specific issue, it may not be too hard to find someone who recalls what really happened.

- Assuming the same flight test methodology was applied to the old Mooneys and the J to define stall speed in full flap landing config,  57mph should be used as Vso to set up the Aspen AOA.  As a modern instrument, their algorithm for calculating AOA likely uses the modern speed indication practices.  I've no idea how much difference the 5mph buffer makes in the AOA output, but  one would want it to use the best data possible.  

[Bob_Belville]

If I might do an Anthony and in the interest of simplicity bullet point the issue. (I have sent this to one of the PPP master instructors for his comment.)

• The AFM defines the bottom of the white arc on the ASI as 55 kias. 
• My Aspen was programmed with the 55 kias based upon the ASI marking.
• OTOH, there's a chart in the "Owners Manual" that has the stall speed in the landing configuration - full flaps, gross weight, power off at 50 kias.
• The Mooney Safety Foundation PPP manual uses the 50 knots speed: MAPA Proficiency Guide.pdfMAPA Proficiency Guide.pdf   
• This spec sheet was created by MAPA, I think, it uses 50 knots Spec sheet.pdfSpec sheet.pdf

While we do not fly near the stall speed, whatever that is, except for a FR, we do use that number. We target 1.3 x stall on final and perhaps 1.2 x stall crossing the threshold on a short runway w/o gusting winds. I use 65 kias as Vref (1.3 x 50). But perhaps 72 kias would be the correct reference speed.

[mooniac15u]

6 minutes ago, DXB said:

-I  doubt the shift from including this buffer to having the numbers match reflects any intrinsic difference in  the J's stall characteristics.  I suspect they were following an industry shift toward having the ASI markings serve as a direct mnemonic for as many V speeds as possible.  Since this change isn't an esoteric Mooney-specific issue, it may not be too hard to find someone who recalls what really happened.

 

It's just further evidence of the vast superiority of the J over earlier models.

[Yetti]

51 minutes ago, Bob_Belville said:

If I might do an Anthony and in the interest of simplicity bullet point the issue. (I have sent this to one of the PPP master instructors for his comment.)

• The AFM defines the bottom of the white arc on the ASI as 55 kias. 
• My Aspen was programmed with the 55 kias based upon the ASI marking.
• OTOH, there's a chart in the "Owners Manual" that has the stall speed in the landing configuration - full flaps, gross weight, power off at 50 kias.
• The Mooney Safety Foundation PPP manual uses the 50 knots speed: MAPA Proficiency Guide.pdfMAPA Proficiency Guide.pdf   
• This spec sheet was created by MAPA, I think, it uses 50 knots Spec sheet.pdfSpec sheet.pdf

While we do not fly near the stall speed, whatever that is, except for a FR, we do use that number. We target 1.3 x stall on final and perhaps 1.2 x stall crossing the threshold on a short runway w/o gusting winds. I use 65 kias as Vref (1.3 x 50). But perhaps 72 kias would be the correct reference speed.

If you stall an airplane on landing the nose would drop and prang a prop.  I believe there is a recent experience around here that did that.   If you land a plane with the stall horn just on chirp then only the outer part of the wing's airflow is separating.  The inner part is still flying so the wing is not fully stalled.   I am OK that there are two numbers.   I mostly use the ASI for take off and see how much the needle is into the white arc.

[Marauder]

8 hours ago, Bob_Belville said:

The Aspen does have an OAT sensor in it's "antenna". I would not think it uses the info to change the IAS but it would use it to display TAS.  

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).

[DXB]

45 minutes ago, Yetti said:

If you stall an airplane on landing the nose would drop and prang a prop.  I believe there is a recent experience around here that did that.   If you land a plane with the stall horn just on chirp then only the outer part of the wing's airflow is separating.  The inner part is still flying so the wing is not fully stalled.   I am OK that there are two numbers.   I mostly use the ASI for take off and see how much the needle is into the white arc.

It's actually the outer wing that is designed to stall later and thus retain aileron effectiveness - our inboard stall strips are placed to make the wing root stall early.  Also being at Vso should still allow stable flight with stall horn blaring if over the runway in ground effect.

[gsxrpilot]

I think I'm getting the AOA added to my Aspen this week... I'm not sure that's relevant to this thread, but I think I saw it mentioned a few pages up ;-)

[Andy95W]

1 hour ago, gsxrpilot said:

I think I'm getting the AOA added to my Aspen this week... I'm not sure that's relevant to this thread, but I think I saw it mentioned a few pages up ;-)

Plus it will give you a chance to post another picture of your instrument panel!  

(Written out of jealousy, not anger.)

[gsxrpilot]

2 minutes ago, Andy95W said:

Plus it will give you a chance to post another picture of your instrument panel!  

(Written out of jealousy, not anger.)

I'll really try to refrain... but I seem to be afflicted with the same psychosis as @Marauder and @Bob_Belville. Likely something to do with searching for approval and validation of our gross over "investment" in our vintage airplanes.

[Bob_Belville]

9 minutes ago, gsxrpilot said:

I'll really try to refrain... but I seem to be afflicted with the same psychosis as @Marauder and @Bob_Belville. Likely something to do with searching for approval and validation of our gross over "investment" in our vintage airplanes.

You're inserting yourself into pretty fast company... but your 252 does not qualify as vintage. 

[DXB]

@Bob_Belville and @gsxrpilot and other folks with single panel Aspen pfds - please  post your experience once you have AOA installed.  Are you comfortable relying on it in lieu of airspeed?  How long to get used to it?  I’m very interested but a bit put off by the tiny display size if you don’t have a second screen.  Also it’s unique as a purely calculated indicator without an external sensor. And it somehow accounts for your flap setting in the display too, which other indicators don’t. It sounds too good to be true.  

[mike_elliott]

5 hours ago, gsxrpilot said:

I'll really try to refrain... but I seem to be afflicted with the same psychosis as @Marauder and @Bob_Belville. Likely something to do with searching for approval and validation of our gross over "investment" in our vintage airplanes.

You guys do a lot of "panel wagging" for sure! And each of you do have some braggin' rights for them. They are like looking a nice pic of Jennifer Gardner..

 

[Bob_Belville]

1 hour ago, DXB said:

@Bob_Belville and @gsxrpilot and other folks with single panel Aspen pfds - please  post your experience once you have AOA installed.  Are you comfortable relying on it in lieu of airspeed?  How long to get used to it?  I’m very interested but a bit put off by the tiny display size if you don’t have a second screen.  Also it’s unique as a purely calculated indicator without an external sensor. And it somehow accounts for your flap setting in the display too, which other indicators don’t. It sounds too good to be true.  

Yeah, I'm going to be interested myself! And since I already have a CYA100 AOA with the display right beside the Aspen I will be able to comment on the Aspen AOA with something to compare it to. And vice versa. 

[carusoam]

Adding to the side discussion....

i have done a fair amount of slow flight with the stall horn blaring while watching the ASI... I have never watched the ASI during the actual stall...

I believe the external sensor for the Aspen AOAi is a pressure differential sensor on their specific pitot tube device... an actual measurement(?).  Check the Aspen site for an explanation of the pitot hardware...

The pilot has to know whether to look at the flaps up or the flaps down indicator... they use triangles to indicate the flap position, that are easy for me to confuse their meaning...

 

On other issues...  Many people go a long time without hearing their stall horn... Make sure it is still working.  (One year, Mine took a few months off without telling me...)

Reason to make sure it is still working... The one day you are really slow on final approach, you may be too distracted, tired, ill, CO'd, to realize what is happening.... the warning horn may be enough to motivate making an important change...

Best regards,

-a-

[takair]

8 hours ago, carusoam said:

Adding to the side discussion....

i have done a fair amount of slow flight with the stall horn blaring while watching the ASI... I have never watched the ASI during the actual stall...

I believe the external sensor for the Aspen AOAi is a pressure differential sensor on their specific pitot tube device... an actual measurement(?).  Check the Aspen site for an explanation of the pitot hardware...

The pilot has to know whether to look at the flaps up or the flaps down indicator... they use triangles to indicate the flap position, that are easy for me to confuse their meaning...

 

On other issues...  Many people go a long time without hearing their stall horn... Make sure it is still working.  (One year, Mine took a few months off without telling me...)

Reason to make sure it is still working... The one day you are really slow on final approach, you may be too distracted, tired, ill, CO'd, to realize what is happening.... the warning horn may be enough to motivate making an important change...

Best regards,

-a-

 

I think the Aspen is unique in that it does not use the differential pressure seen in other systems, but instead uses basic airdata and G load from its AHRS sensors.  I think the entire AOA is done in software, with no hardware change required.  My guess is that they then use a lookup table to approximate the bank angle tables from assumed loads to display the equivalent AOA.  It is basically like taking the bank angle chart and graphing it real time to the AOA display....or at least that is my simplistic interpretation.  It will be really interesting to see bob's side by side comparison.  My guess is that it will be close, not identical, but both will provide adequate information to make us safer.  Don't recall how they know the flap position, especially on our older Mooney's.

[Bob_Belville]

19 minutes ago, takair said:

 

I think the Aspen is unique in that it does not use the differential pressure seen in other systems, but instead uses basic airdata and G load from its AHRS sensors.  I think the entire AOA is done in software, with no hardware change required.  My guess is that they then use a lookup table to approximate the bank angle tables from assumed loads to display the equivalent AOA.  It is basically like taking the bank angle chart and graphing it real time to the AOA display....or at least that is my simplistic interpretation.  It will be really interesting to see bob's side by side comparison.  My guess is that it will be close, not identical, but both will provide adequate information to make us safer.  Don't recall how they know the flap position, especially on our older Mooney's.

The Aspen does not know what the flap position is. The calibration is done in both clean and landing configuration and the display has 2 pointers so the pilot has to know which pointer is appropriate to his current flap position.

 

[carusoam]

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-

 

[Marauder]

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.