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

[Bob_Belville]

I've had a PPL since 1969 and IR since '77 with nearly 3000 hours in Mooneys and I guess I have always been able to ignore the difference between IAS and CAS without consequence.

I treat it like compressability, something the other guys may need to calculate...  

[carusoam]

These are the differences that are known to exist...

There is a note regarding the unknown unknowns that exist...

NOTE: The calibrated airspeeds shown only correct for...   ...airspeed indicators may have errors up to 2.5mph....

 

 

And that Note was referring to new ASIs.

Best regards,

-a-

[Hank]

Bob, do you not have this page in your Owners Manual? This is from my 1970 C model Owners Manual. It conveniently put the IAS to CAS conversion on the same page as the stall speed chart. Again, your E will be slightly different. Pay attention to the NOTE under the Airspeed Correction chart.

[takair]

The more I think about it, IAS is just that, what is on the airspeed indicator and should correlate to critical speeds.   CAS is a correction factor, usually left to a chart that Mooney did not readily provide.  Critical airspeed should correlate to the IAS, even if the engineering values (CAS) were different.  This stall speed situation is not making sense.  Will chat with a couple of the aero guys at work tomorrow.  That should really blow my mind.

[takair]

Just now, Hank said:

Bob, do you not have this page in your Owners Manual? This is from my 1970 C model Owners Manual. It conveniently put the IAS to CAS conversion on the same page as the stall speed chart. Again, your E will be slightly different. Pay attention to the NOTE under the Airspeed Correction chart.

Now I wish I didn’t post my last post.  I think Bob and I are looking at the same vintage books.  I know I don’t have that in my manuals, but I see I need to update. 

[Bob_Belville]

Just now, carusoam said:

It may take a CFI or instrument guy that really knows the details...

67/58

64/56

57/49

mias/kias

I believe all the markings are in CAS, accept Vne... which is IAS...?

The latest POH has both the knots and the mph in it.

Best regards,

-a-

Well, mine chart had 57 which I converted to 50 instead of your 49. (57/1.15 = 49.565) so either the 57 was rounded up or they truncated rather than rounded.

As I recall, Vne is actually a TAS.

[Hank]

2 minutes ago, carusoam said:

These are the differences that are known to exist...

There is a note regarding the unknown unknowns that exist...

NOTE: The calibrated airspeeds shown only correct for...   ...airspeed indicators may have errors up to 2.5mph....

 

 

And that hat Note was referring to new ASIs.

Best regards,

-a-

Apparently you were typing while i was screenshotting . . . You referenced the Note, I provided the text of the Note.

We must "be prepared for all known and unknown situations that may or may not exist."  

[Bob_Belville]

20 minutes ago, Hank said:

Bob, do you not have this page in your Owners Manual? This is from my 1970 C model Owners Manual. It conveniently put the IAS to CAS conversion on the same page as the stall speed chart. Again, your E will be slightly different. Pay attention to the NOTE under the Airspeed Correction chart.

Hank, my older Manuals do not have the top chart at all. But interestingly at full flaps, power off for the slowest speed on the chart the correction is only 1 mph (70 vs 69). And it might be in the wrong direction, I'm getting groggy. So that would not seem to be the explanation.

[Bob_Belville]

12 hours ago, takair said:

The more I think about it, IAS is just that, what is on the airspeed indicator and should correlate to critical speeds.   CAS is a correction factor, usually left to a chart that Mooney did not readily provide.  Critical airspeed should correlate to the IAS, even if the engineering values (CAS) were different.  This stall speed situation is not making sense.  Will chat with a couple of the aero guys at work tomorrow.  That should really blow my mind.

Now we're talking, Rob! Thanks for any insight you can come up with. I don't think any of our speculations and guesses to this point answer my question: Is Vso 50 or 55 KIAS (or KCAS)? 

[DXB]

I'm  glad Bob brought this issue up.  When I first got my '68C and pored over the POH, the  disparity between white arc and the Vso in the stall speed chart stood out.  My transition instructor was a DPE and a stickler for the regs and was also bothered by it.  I'm pretty sure my ASI is original (sorry for fuzzy pic below) and is marked in mph on the outer ring but is essentially the same as Bob's, as is the stall speed vs. bank angle chart in my POH. 

I also agree the difference is not trivial for setting up the Aspen AOA calculation - Perhaps the factory can provide some guidance?  It'll be nice to have a clear answer - I plan to add on the AOA to my Aspen as well at some point.

 

[ArtVandelay]

Maybe the best way is to go up and stall the plane? Make note of the airspeed indicator for future reference. No worries about instrument error, etc

[Bob_Belville]

18 minutes ago, teejayevans said:

Maybe the best way is to go up and stall the plane? Make note of the airspeed indicator for future reference. No worries about instrument error, etc

Even if I were a real test pilot I don't think I'd take my one, or several, datum point as being more valid than the AFM or Owner Manual. Vso varies with weight, CG, etc. And I'm sure the Avionics shop would advice me not to change the 55 kias they programmed into the Aspen at installation as Vso. This is a Certified Aircraft.

[N201MKTurbo]

I saw a factory test car in a parking lot reciently. It had human torso shaped water jugs strapped into the seats. They had the Nalgene logo on them. They would be great for adding weight to the plane for testing.

[steingar]

The aircraft were all built largely by hand, so I expect the stall speed in one to differ from the stall speed in another.  Moreover, most are approaching a half century in age with lots of aftermarket modifications.  As such, the question of the exact stall speed for any given airframe is anything but a simple question.

[Bob_Belville]

44 minutes ago, steingar said:

The aircraft were all built largely by hand, so I expect the stall speed in one to differ from the stall speed in another.  Moreover, most are approaching a half century in age with lots of aftermarket modifications.  As such, the question of the exact stall speed for any given airframe is anything but a simple question.

Yeah no argument on all that, but so far about the only guy here who seems to understand my question is Rob Takacs! I am not asking what my particular stall speed is. I am asking why there seems to be 2 very different Vso speeds in the old AFM and Owners Manual. It's about the documentation. For the record, I do not believe either number was intended to be specific to N943RW, ser. 929 based on a factory flight test. They are both intended to apply to all '66Es and I think the same documentation appears in other model year Es as well as Cs. 

FWIW, except for FR with a CFI beside me I never hear the stall horn sound. I just don't. I am routinely still at close to 90 KIAS turning base and not a lot slower on a 2 mile final. At 87 KIAS I can deploy full flaps and slow to 65 KIAS by the time I'm on short final. All of that is well above Vso whether that number is 50, 55 or some other speed unique to my old E. But my question is not about any of that. 

Thanks for all the help.  

[DXB]

Attached is a page from an old MAPA Pilot Proficiency Program guide that lists  the M20E stall speed in full flap landing configuration to be 57mph. A page from the LASAR Mooney Buyer's Guide is also attached and says the same.  Note the term Vso is not used here or in the POHs, and this speed does not appear in the TCDS. I suspect the reason is that neither the Vso terminology nor where to put the bottom of the white arc was  universally codified in the 1960s? Does anyone here know?  Using 57mph presently makes the most sense to me to calibrate the AOA indicator on the Aspen, but I would still like to hear from someone who really knows why the white arc speed on our planes was higher on the factory-installed ASIs.  

 

MAPA Proficiency Guide.pdfMAPA Proficiency Guide.pdf

Spec sheet.pdfSpec sheet.pdf

[Yetti]

I understand the question, but question the process.   You are wanting to know a number to put into a flight instrument to help you when you need it most ie: close to the ground.    I would want the most accurate number for your airframe, not what some clerk provided to the government.  There appears to be some inaccuracies in record keeping provided to the government to obtain certifications status, but that won't help you land the plane.

[Bob_Belville]

2 hours ago, DXB said:

Attached is a page from an old MAPA Pilot Proficiency Program guide that lists  the M20E stall speed in full flap landing configuration to be 57mph. A page from the LASAR Mooney Buyer's Guide is also attached and says the same.  Note the term Vso is not used here or in the POHs, and this speed does not appear in the TCDS. I suspect the reason is that neither the Vso terminology nor where to put the bottom of the white arc was  universally codified in the 1960s? Does anyone here know?  Using 57mph presently makes the most sense to me to calibrate the AOA indicator on the Aspen, but I would still like to hear from someone who really knows why the white arc speed on our planes was higher on the factory-installed ASIs.  

 

MAPA Proficiency Guide.pdfMAPA Proficiency Guide.pdf

Spec sheet.pdfSpec sheet.pdf

Dev, thanks for reminding me of that helpful page from the PPP manual. I have taken that training and have that page. 57 mph is 50 kias which matches the chart in the Owners Manual but is in conflict with the AFM which specifies that the bottom of the white arc is to be set at 63 mph = 55 kias. 

Do we know who is responsible for the (secondary) PPP document and what the basis is for their choice for Vso? (I'm using Vso as shorthand for stall speed in landing configuration at max gross weight, etc,. I suspect you might have a point about the use of that terminology 50 years ago but the concept itself, even without the label, surely goes back to the early year of powered flight.)  

[Bob_Belville]

1 hour ago, Yetti said:

I understand the question, but question the process.   You are wanting to know a number to put into a flight instrument to help you when you need it most ie: close to the ground.    I would want the most accurate number for your airframe, not what some clerk provided to the government.  There appears to be some inaccuracies in record keeping provided to the government to obtain certifications status, but that won't help you land the plane.

I don't think we've proven that this is a clerical error. And if it is an error we still have not determined which one, if either, is right.

I probably have 2000 plus landing as PIC of M20Es, all but one were good enough to allow me to taxi to the ramp and to take off again. (Speed was not the issue for the one exception.) I might need help but that is not what we're discussing here.

Not that it matters but for those of you who think this is about finding a technique for landing the plane, I have 55 kias programmed into the Aspen as bottom of the white arch per AFM and to agree with the back up ASI. I don't think I am allowed to do otherwise. OTOH, I can set any speed I want as Vref and I have that set at 65 kias which is 50 x 1.3. Go figure.

 

[mooniac15u]

2 hours ago, DXB said:

Attached is a page from an old MAPA Pilot Proficiency Program guide that lists  the M20E stall speed in full flap landing configuration to be 57mph. A page from the LASAR Mooney Buyer's Guide is also attached and says the same.  Note the term Vso is not used here or in the POHs, and this speed does not appear in the TCDS. I suspect the reason is that neither the Vso terminology nor where to put the bottom of the white arc was  universally codified in the 1960s? Does anyone here know?  Using 57mph presently makes the most sense to me to calibrate the AOA indicator on the Aspen, but I would still like to hear from someone who really knows why the white arc speed on our planes was higher on the factory-installed ASIs.  

 

MAPA Proficiency Guide.pdfMAPA Proficiency Guide.pdf

Spec sheet.pdfSpec sheet.pdf

Both Vso and the ASI white arc were well defined under CAR 3 and meant the same thing as they do today. You can find the definitions in sections 3.82 and 3.757. 

[takair]

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.....

 

[Yetti]

Does the Aspen ASI automatically do temperature correction?   I don't have that feature on my steam ASI

[DXB]

31 minutes ago, takair said:

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.  

Interesting.  I think we're getting closer to an answer, though having dual definitions seems like a bad thing in this context.  The question arises whether the stall speed or the minimum steady flight speed is optimal for the Aspen's AOA computation algorithm performance.

[Hank]

13 minutes ago, DXB said:

The question arises whether the stall speed or the minimum steady flight speed is optimal for the Aspen's AOA computation algorithm performance.

That's a question for Aspen . . . .

[Bob_Belville]

3 hours ago, Yetti said:

Does the Aspen ASI automatically do temperature correction?   I don't have that feature on my steam ASI

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.