Stalls in a Mooney Ovation - do speed brakes affect stall speed? Does configuration make a difference?

[Blue on Top]

You guys are all totally awesome!!!  ... and all you say is correct, too!

I'll add a couple thoughts, but as I mentioned above, you're all on the right track.

As @PT20J mentioned, aerodynamicists don't change the chord line with flap, slat, aileron, etc. deflection.  Changing the chord angle would only account for one of many changes (and it's not an accurate method).  In other words, the chord length also changes (more on that shortly), the mean camber line changes, a slot may or may not open up, etc.  Bottom line is that it is just a different airfoil.  As a funny note for those that want to redefine the chord line, where is the new chord line/AOA when a split flap is deployed?  The upper surface is still the upper surface (and its trailing edge) but the lower surface has a new lower surface trailing edge. I'll get back to AOA in just a second.

One can call flap action whatever they want, as the flap movement isn't any one type of movement.  They are simply just definitions.  Even a simple flap changes with the hinge position: at the leading edge, in the middle of the leading edge radius (if it even has one), the lower surface, the upper surface, etc.  Personally, I would call the Mooney flap a single-slotted flap with some Fowler action.  Fowler action just means that it has some aft movement ... which increases the chord and wing area ... which we also don't account for

There is a difference between C_l and C_L.  WHAT???  C_l is the section coefficient of lift (2D, 2-dimensional, infinite wingspan) and C_L is the wing coefficient of lift (3D, 3-dimensional, real world).  These values are significantly different.  This gets us to, "How does an aerodynamicist deal with all these differences?"

Simply put, we don't.  There is one chord (normally the mean aerodynamic chord), one AOA (the calculations above ignored the fact that the wing is twisted), one wing area (some include the area within the fuselage - and how, some don't; some include wing tips/winglets, some don't, etc.).  All of these values are simply reference values.  Yes, if we change the reference values, the absolute, numeric values of the other parameters would change, but we are simply trying to duplicate the real world (which doesn't change with our definitions.  So, how do we do it?

Aircraft AOA is defined as the angle between the longitudinal axis of the airplane and the relative wind well out in front of the airplane.  We calibrate this using angle of pitch (AOP) from the AHRS while performing level flight speed sweeps.  It doesn't matter if the AOA is 3.5792 degrees or 5.0 degrees.  It's just a number, and it's all relative.  Yes, we try to mate the wind tunnel and CFD as best we can.  Now for the elephant.

Aerodynamicists relate everything to aircraft AOA.  Each spanwise location stalls at a certain aircraft AOA.  We tailor the stall progression to get good handling characteristics.  The wing doesn't stall all at one time; it progresses.  Even at stall, the wing is still producing a minimum of 80% lift.  I need to stop  

Ask away.

@Fly_M20R and @Oscar Avalle, I'm serious about doing some tufting tests if you're up for it. 

[Will.iam]

On 1/18/2022 at 8:01 AM, Jeff_S said:

2) Does the type of flap really matter? All flaps tend to promote the pitch down effect to some degree, so I'm curious if it's important that the Mooney's slotted flaps make a difference. Also, would be fun to take my Ovation out with the newer speed brakes to see if there's a measurable difference in effect.

As with all things in life there are sometimes exceptions to the rule. A DC-8 Will pitch up when you add flaps. I’ll never forget an instructor that would get tired of students ballooning up and busting altitude because they would not trim after calling for flaps. Finally he said look the the flaps go down the nose goes up every damn time just add 2 seconds of down trim when you call for the flaps and you will stay in trim and not have to push forward to keep from gaining altitude. 

[Fly_M20R]

4 hours ago, Oscar Avalle said:

Chris,

For work I have been interviewed on TV several times and I had to deliver statements etc. You are absolutely right the best thing it to be yourself. Next video I am going to try to speak and comment what is going on... let's see how it goes.

The flight was close to 3.5 hours. My main problem was always battery life for the outside cameras. I tried the wifi option but somehow it never really worked. I found some cheap, very simple, cameras that have a battery life of about 4 hours. No thrills, but decent video quality at 4K. The brand is "Drift". I turn them on before I get into the plane and then I leave them on until I arrive. Tons of video footage. 

Good advice on not accelerating landings. You are absolutely right. I always enjoy watching others land their planes. 

Oscar

Hi Oscar,

Ahhh.... I could not imagine fishing a USB power cable to the tail or wing!! Will look into those cameras. Thanks!!

Look forward to your next video.

Chris

[Fly_M20R]

2 hours ago, Blue on Top said:

You guys are all totally awesome!!!  ... and all you say is correct, too!

I'll add a couple thoughts, but as I mentioned above, you're all on the right track.

As @PT20J mentioned, aerodynamicists don't change the chord line with flap, slat, aileron, etc. deflection.  Changing the chord angle would only account for one of many changes (and it's not an accurate method).  In other words, the chord length also changes (more on that shortly), the mean camber line changes, a slot may or may not open up, etc.  Bottom line is that it is just a different airfoil.  As a funny note for those that want to redefine the chord line, where is the new chord line/AOA when a split flap is deployed?  The upper surface is still the upper surface (and its trailing edge) but the lower surface has a new lower surface trailing edge. I'll get back to AOA in just a second.

One can call flap action whatever they want, as the flap movement isn't any one type of movement.  They are simply just definitions.  Even a simple flap changes with the hinge position: at the leading edge, in the middle of the leading edge radius (if it even has one), the lower surface, the upper surface, etc.  Personally, I would call the Mooney flap a single-slotted flap with some Fowler action.  Fowler action just means that it has some aft movement ... which increases the chord and wing area ... which we also don't account for

There is a difference between C_l and C_L.  WHAT???  C_l is the section coefficient of lift (2D, 2-dimensional, infinite wingspan) and C_L is the wing coefficient of lift (3D, 3-dimensional, real world).  These values are significantly different.  This gets us to, "How does an aerodynamicist deal with all these differences?"

Simply put, we don't.  There is one chord (normally the mean aerodynamic chord), one AOA (the calculations above ignored the fact that the wing is twisted), one wing area (some include the area within the fuselage - and how, some don't; some include wing tips/winglets, some don't, etc.).  All of these values are simply reference values.  Yes, if we change the reference values, the absolute, numeric values of the other parameters would change, but we are simply trying to duplicate the real world (which doesn't change with our definitions.  So, how do we do it?

Aircraft AOA is defined as the angle between the longitudinal axis of the airplane and the relative wind well out in front of the airplane.  We calibrate this using angle of pitch (AOP) from the AHRS while performing level flight speed sweeps.  It doesn't matter if the AOA is 3.5792 degrees or 5.0 degrees.  It's just a number, and it's all relative.  Yes, we try to mate the wind tunnel and CFD as best we can.  Now for the elephant.

Aerodynamicists relate everything to aircraft AOA.  Each spanwise location stalls at a certain aircraft AOA.  We tailor the stall progression to get good handling characteristics.  The wing doesn't stall all at one time; it progresses.  Even at stall, the wing is still producing a minimum of 80% lift.  I need to stop  

Ask away.

@Fly_M20R and @Oscar Avalle, I'm serious about doing some tufting tests if you're up for it. 

Hi Blue on Top,

Thank you for your comments. They add to the material I need to read about so I can internalize it better. 

My limited knowledge base did not include a Ci. What is it?

I see what you are saying about just using the mean chord with flaps up for all configurations. I also understand what you mean by Aircraft AOA. Not sure why aerodynamicists refer to the aircraft AOA though and not to the actual mean wing AOA which would simply need adding the "mean" angle of incidence. Everywhere I read says that the typical wing we have in our planes stalls around 14 to 16 deg wing AOA (cannot remember exact figure). So why not use the actual wing AOA if it is easy to calculate based on pitch (aircraft AOA) plus angle of incidence?

Hopefully we get to meet at Sun n Fun so you can demo your buffing.

Chris

[Blue on Top]

5 hours ago, Fly_M20R said:

My limited knowledge base did not include a Ci. What is it?

I see what you are saying about just using the mean chord with flaps up for all configurations. I also understand what you mean by Aircraft AOA. Not sure why aerodynamicists refer to the aircraft AOA though and not to the actual mean wing AOA which would simply need adding the "mean" angle of incidence. Everywhere I read says that the typical wing we have in our planes stalls around 14 to 16 deg wing AOA (cannot remember exact figure). So why not use the actual wing AOA if it is easy to calculate based on pitch (aircraft AOA) plus angle of incidence?   -Chris

@Fly_M20R Chris:  Your aero knowledge is good!

C_l (lower case el) is the wing section lift coefficient.  This is the value that one will find in an airfoil book, and it is a 2D (infinite wingspan) value.  On the other hand, C_L (capital el) is the wing, as a whole, lift coefficient.  It is 3D and includes all the effects of the wing having a finite span.  Even the Mooney wing, which is much more simple than we would design today, uses at least a couple different airfoils and has twist (different local AOA at every spanwise location).  A wing designed today will not only have at least two different airfoils -root and tip (some have 5 or 6) defined airfoil sections, but the airfoils between those defined airfoils are transition airfoils (not defined per se).  In addition, wing twist used to be linear.  In other words and if a wing is twisted 3 degrees, at 1/3rd span it would be twisted down 1 degree, at 2/3rds span 2 degrees and at the tip 3 degrees.  Tod7 ay, though, we design exactly what we want.1

For example, the new M10 POC had no twist in the flapped section and 3 degrees in the aileron section.  This gave us an almost perfect elliptical lift distribution and kept aileron full deflection from stalling the outboard section.

We use aircraft AOA because every section (little spanwise slice of the wing) can be related to it.  Here a picture would be worth more than 1,000 words, but ...  Say the inboard airfoils starts to stall at an aircraft AOA of 14 degrees and is fully stalled at 16 degrees.  Midspan at 15 and 18, respectively.  Wingtip at 17 and 22, respectively.  So, if the aircraft AOA is 16, the very inboard airfoil is fully separated, midspan is beginning to separate, and the wingtip is still flying. Most likely, the aircraft is still flying at this point (but the tail should be buffeting).   If aircraft AOA is increased to 17, the inboard section is completely separated, midspan is halfway separated and the wingtip is still flying.  An educated guess is this condition is when the pilot says the airplane stalled.  Most of the wing is still flying.  Rarely does the full wing separate.

[Blue on Top]

PS. @Fly_M20R My buffing skills are really poor, but I don't need practice.  They will never improve.  Also, as an aerodynamicist, I know exactly how much wetted area (surface area) there is on an airplane  

[skykrawler]

Instrumentation ...... no mention here of indicated -vs- calibrated airspeed and probably doesn't matter that much.   I wonder what affect the speed brakes have on the static ports?  I haven't flown with a 275 but I know for certain that the G5 has filters applied to the IAS and altitude values displayed.  The ias filter seems to have a time constant that is several seconds.  These filters make the displays as smooth as they are and less responsive to gusts.

[Blue on Top]

4 hours ago, skykrawler said:

Instrumentation ...... no mention here of indicated -vs- calibrated airspeed and probably doesn't matter that much.   I wonder what affect the speed brakes have on the static ports?  I haven't flown with a 275 but I know for certain that the G5 has filters applied to the IAS and altitude values displayed.  The ias filter seems to have a time constant that is several seconds.  These filters make the displays as smooth as they are and less responsive to gusts.

@skykrawler  You are the perfect person for me to ask this question, but to address your comments first ...

Indicated versus calibrated is kind of taken care of by using the same reference for both the clean and speed brake out stalls.  Speed brakes should not affect the static ports at all.

Since your airplane has a G5, I believe that the pneumatic airspeed indicator is still required to be installed.  Which airspeed do you use to fly the takeoff (rotation speeds, 50' speeds)?  Thanks!

[Fly_M20R]

21 hours ago, Blue on Top said:

@Fly_M20R Chris:  Your aero knowledge is good!

C_l (lower case el) is the wing section lift coefficient.  This is the value that one will find in an airfoil book, and it is a 2D (infinite wingspan) value.  On the other hand, C_L (capital el) is the wing, as a whole, lift coefficient.  It is 3D and includes all the effects of the wing having a finite span.  Even the Mooney wing, which is much more simple than we would design today, uses at least a couple different airfoils and has twist (different local AOA at every spanwise location).  A wing designed today will not only have at least two different airfoils -root and tip (some have 5 or 6) defined airfoil sections, but the airfoils between those defined airfoils are transition airfoils (not defined per se).  In addition, wing twist used to be linear.  In other words and if a wing is twisted 3 degrees, at 1/3rd span it would be twisted down 1 degree, at 2/3rds span 2 degrees and at the tip 3 degrees.  Tod7 ay, though, we design exactly what we want.1

For example, the new M10 POC had no twist in the flapped section and 3 degrees in the aileron section.  This gave us an almost perfect elliptical lift distribution and kept aileron full deflection from stalling the outboard section.

We use aircraft AOA because every section (little spanwise slice of the wing) can be related to it.  Here a picture would be worth more than 1,000 words, but ...  Say the inboard airfoils starts to stall at an aircraft AOA of 14 degrees and is fully stalled at 16 degrees.  Midspan at 15 and 18, respectively.  Wingtip at 17 and 22, respectively.  So, if the aircraft AOA is 16, the very inboard airfoil is fully separated, midspan is beginning to separate, and the wingtip is still flying. Most likely, the aircraft is still flying at this point (but the tail should be buffeting).   If aircraft AOA is increased to 17, the inboard section is completely separated, midspan is halfway separated and the wingtip is still flying.  An educated guess is this condition is when the pilot says the airplane stalled.  Most of the wing is still flying.  Rarely does the full wing separate.

Perfect explanation @Blue on Top!!

I have a better understanding of how to view aerodynamics in a way that relates to our aircraft and keep the "blue side up"!  Will also be able to do some other videos with that added knowledge in mind.

Thank you!

Chris

[carusoam]

15 hours ago, skykrawler said:

Instrumentation ...... no mention here of indicated -vs- calibrated airspeed and probably doesn't matter that much.   I wonder what affect the speed brakes have on the static ports?  I haven't flown with a 275 but I know for certain that the G5 has filters applied to the IAS and altitude values displayed.  The ias filter seems to have a time constant that is several seconds.  These filters make the displays as smooth as they are and less responsive to gusts.

SK,

Which Mooney do you have?

You can edit your avatar, and add your airframe to the data….

 

Wait a second…

If i smooth my data out over several seconds…. My plane can stall before I know I have approached the stall speed…

My airspeed indications are pretty smooth…

Would I want to have really smooth data, that is less accurate?

VSI data with the traditional lag is pretty goofy… great back in the day, but an IVSI took care of the lag….

 

See what I’m asking here?

Some data smoothing is better than others… Cies fuel level gauges smooth things out…. But, fuel level isn’t time critical….

Air Speed in the traffic pattern… is critical.  
 

Blue wrote a paragraph above about characterizing a stall entry…  see what he says about smoothing data out in those situations…

 

PP wondering only, not a judgmental flame thrower…

Best regards,

-a- 

 

[Blue on Top]

2 hours ago, carusoam said:

Wait a second…

1) If i smooth my data out over several seconds…. My plane can stall before I know I have approached the stall speed…

2) My airspeed indications are pretty smooth…

3) Would I want to have really smooth data, that is less accurate?

4) VSI data with the traditional lag is pretty goofy… great back in the day, but an IVSI took care of the lag….

5) See what I’m asking here?

6) Some data smoothing is better than others… Cies fuel level gauges smooth things out…. But, fuel level isn’t time critical….

7) Air Speed in the traffic pattern… is critical.  

8) Blue wrote a paragraph above about characterizing a stall entry…  see what he says about smoothing data out in those situations…

-a- 

 

Bingo!  (hopefully not on fuel)

1) No.  Your stall warning horn/audio should be > 5 knots prior to stall.  It is AOA-based (when the stagnation point goes below/aft of the little tab on the wing).  @carusoam you bring up a good point, though.  If the signal goes through the avionics, is there a delay?  Good reason to do a check during preflight with the avionics turned on.

2) I bet your flying is too.

3) Good question.  I put out a request on Facebook (and I invite people here ... like @Fly_M20R) for video during takeoff with both the round dial and an electronic airspeed indicator in the same frame.  I'm curious what it will show (I know the answer.  I've done this on other airplanes, including jets).  Here's the short story.  If the airplane is accelerating at 6 knots/second and the data takes one second to get to the display, the pilot will rotate 6 knots late.  I'm curious, too.

4) Yes.

5) Exactly.

6) Yes.  Regretfully, there are still way too many fuel exhaustion (running out of fuel) and fuel starvation (fuel on-board but not getting to the engine, empty tank selected, etc.) accidents that claim airplanes and lives every year.

7) Yes, buy should not be changing a lot (except on takeoff).

8) It's a compromise.  Depends on the unit.  I need to not say more on this ... until I get more data  (hint, hint)

8') There is another active thread on takeoff performance deltas between the earlier and later J models.   How much longer do those distance become if the airplane needs to accelerate another X knots?

Way more than my two cents, Ron

[Fly_M20R]

20 hours ago, Blue on Top said:

 

1) No.  Your stall warning horn/audio should be > 5 knots prior to stall.  It is AOA-based (when the stagnation point goes below/aft of the little tab on the wing).  @carusoam you bring up a good point, though.  If the signal goes through the avionics, is there a delay?  Good reason to do a check during preflight with the avionics turned on.

 

Very informative reply and post on all points @Blue on Top!!   

I have only quoted point #1 since it explains very well the findings in a previous video I did on stalls.   I did not publish it though because I could not explain appropriately the reason for the results. Basically it was the same flight profile (experiment) with speed brakes on or off in clean or landing configuration. During that flight my stall warning never went off for stalls in clean configuration whereas I did get a stall warning around 3 - 4 kts prior to stall in the landing configuration. It did not matter whether the speed brakes were deployed or not. I concluded that the critical AOA was less in clean configuration vs landing configuration. However, a CFI friend of mine said it did not make sense since "flaps lower AOA". That was his mantra. I did not publish the video since I did not understand the aerodynamics that well at the time and therefore explain them cogently. However, now I gather that flaps allow lower aircraft AOA for any airspeed but also increase the critical AOA. Of course, I may be wrong or there is a better explanation.

Turns out my tab was not at the right angle to deflect upwards in clean configuration. I had it corrected since then and was at the right angle for the video I published. The critical AOA with flaps up probably has the stagnation point just at or slightly aft of the stall tab whereas it will be further aft (therefore higher AOA) with flaps down. If the tab is not "right" then it will not deflect up on clean and possibly not even on landing configuration.

At least that is my "layman's" explanation.

I am sure that you may have some further comments re my post and thank you again for your great input!

Chris

[Blue on Top]

2 hours ago, Fly_M20R said:

Very informative reply and post on all points @Blue on Top!!   

I have only quoted point #1 since it explains very well the findings in a previous video I did on stalls.   I did not publish it though because I could not explain appropriately the reason for the results. Basically it was the same flight profile (experiment) with speed brakes on or off in clean or landing configuration. During that flight my stall warning never went off for stalls in clean configuration whereas I did get a stall warning around 3 - 4 kts prior to stall in the landing configuration. It did not matter whether the speed brakes were deployed or not. I concluded that the critical AOA was less in clean configuration vs landing configuration. However, a CFI friend of mine said it did not make sense since "flaps lower AOA". That was his mantra. I did not publish the video since I did not understand the aerodynamics that well at the time and therefore explain them cogently. However, now I gather that flaps allow lower aircraft AOA for any airspeed but also increase the critical AOA. Of course, I may be wrong or there is a better explanation.

Turns out my tab was not at the right angle to deflect upwards in clean configuration. I had it corrected since then and was at the right angle for the video I published. The critical AOA with flaps up probably has the stagnation point just at or slightly aft of the stall tab whereas it will be further aft (therefore higher AOA) with flaps down. If the tab is not "right" then it will not deflect up on clean and possibly not even on landing configuration.

At least th

at is my "layman's" explanation.

I am sure that you may have some further comments re my post and thank you again for your great input!

Chris

@Fly_M20R  Love your post and thank you!  I believe you truly understand your wing now.  One down, 999,999 to go :))) 

You are very correct in that flaps will always move the stagnation point aft (it's probably not much on the Mooney airfoils with that sharp leading edge).  Bonus information for advance aero: It is not only the mean camber line that dictates the upper surface being a longer distance, local AOA and flaps moving the stagnation point aft add to the difference in distances.

When I first started reading your post, I thought, "How do I tell Chris in a PC manner that his stall warning tab needs to be moved up/forward?"  And then OMG the Heavens opened up and that is what you you did!  I am so proud of you!  Awesome job!

[PT20J]

I found this in my files. Some may find it interesting.

Stall Characteristics.pdf

[skykrawler]

On 1/24/2022 at 10:59 AM, Blue on Top said:

@skykrawler  You are the perfect person for me to ask this question, but to address your comments first ...

Indicated versus calibrated is kind of taken care of by using the same reference for both the clean and speed brake out stalls.  Speed brakes should not affect the static ports at all.

Since your airplane has a G5, I believe that the pneumatic airspeed indicator is still required to be installed.  Which airspeed do you use to fly the takeoff (rotation speeds, 50' speeds)?  Thanks!

SOMEthing causes the of IAS to CAS conversion chart to be a curve.   A combination of pitot/static port locations and aero affects changing due to speed.  Isn't there a different curve for flaps down?  I defer to your expertise, which I am certain you have.

My experience is the M20J 'rotation speed' is very sensitive to conditions:  weight in particular and temperature.   The G5 was installed as a backup and I pretty much use the dial as primary.   If just make sure the  needle gets in the white arc and the airplane is ready to fly.   I quit dialing the trim for light controls on take-off because I didn't like the pitch up and immediate need to trim nose down as the airplane accelerates.

Back to instrumentation.   All this analysis of when the airplane stalls wrt speed.  Shouldn't the question be 'does my stall angle of attack change when my speed brakes are deployed.'    Doesn't this eliminate affects of weight differences? 

Filters on sensors....very common.   It is a fact that EFIS on jet transports uses filtered data.  IMHO: If you are stalling your airplane in the pattern it is not the last 2 knots of airspeed that you lost - you shouldn't have been that close to stall to start with and you should reconsider your skills.   So not a valid objection to sensor filtering.

Research Services Directorate, Simulation Development and Analysis Branch, Langley Research Center.

[Blue on Top]

1 hour ago, skykrawler said:

SOMEthing causes the of IAS to CAS conversion chart to be a curve.   A combination of pitot/static port locations and aero affects changing due to speed.  Isn't there a different curve for flaps down?  I defer to your expertise, which I am certain you have.

My experience is the M20J 'rotation speed' is very sensitive to conditions:  weight in particular and temperature.   The G5 was installed as a backup and I pretty much use the dial as primary.   If just make sure the  needle gets in the white arc and the airplane is ready to fly.   I quit dialing the trim for light controls on take-off because I didn't like the pitch up and immediate need to trim nose down as the airplane accelerates.

Back to instrumentation.   All this analysis of when the airplane stalls wrt speed.  Shouldn't the question be 'does my stall angle of attack change when my speed brakes are deployed.'    Doesn't this eliminate affects of weight differences? 

Filters on sensors....very common.   It is a fact that EFIS on jet transports uses filtered data.  IMHO: If you are stalling your airplane in the pattern it is not the last 2 knots of airspeed that you lost - you shouldn't have been that close to stall to start with and you should reconsider your skills.   So not a valid objection to sensor filtering.

Research Services Directorate, Simulation Development and Analysis Branch, Langley Research Center.

The early Js had 4 calibration curves: flaps up, flaps 15, flaps 33 idle power and flaps 33 full power.  Flaps and power are significant (about 3 knots each).  To my knowledge, no one calibrates on the ground (airflow is different).  It would be easy to do today but useless to a pilot.  Except during takeoff, those calibrations are multi-dimensional, including weights, accelerations, time constants, etc.  In the AFM we give the pilot the indicated airspeed to rotate the airplane at the correct calibrated airspeed to get the published performance.   Indicated rotation speed is significantly below the real airspeed (5-10 knots).

The aircraft stall AOA might be slightly different with the speed brakes out.  If the speed delta at gross weight is more than 3 knots, it must be published (or operation of the speed brakes may be limited to XX feet above the runway).  Certificated AOA systems will account for all of these configurations: flaps, icing, speed brakes, etc.

I am not against airspeed filtering, but it will increase takeoff distances (if not accounted for).