Stalls and Leading edge question

[exM20K]

18 minutes ago, Blue on Top said:

In Alaska, airplanes are allowed to fly 15% over gross weight

121 and 135 operators in certain circumstances. Not 91 operations.....

-dan

[PT20J]

Just to be clear: I don’t fly the 201 the same way I flew the draggy Beaver on floats. But, I don’t spend a lot of time at Vx or Vy in the Mooney either. I start the takeoff roll with TO flaps and some back pressure and let the nose start up when it’s ready. After lift off, I reduce the back pressure to hold about 10 deg pitch until clear of obstacles and passing 86 KIAS (Vy) at which point I raise the flaps and lower the nose to about 7.5 deg and accelerate to a Vz climb. I use 2700 rpm and WOT for the entire climb.

I’m interested to learn how others do it. 

Skip

 

[Blue on Top]

1 hour ago, PT20J said:

Just to be clear: I don’t fly the 201 the same way I flew the draggy Beaver on floats. But, I don’t spend a lot of time at Vx or Vy in the Mooney either. I start the takeoff roll with TO flaps and some back pressure and let the nose start up when it’s ready. After lift off, I reduce the back pressure to hold about 10 deg pitch until clear of obstacles and passing 86 KIAS (Vy) at which point I raise the flaps and lower the nose to about 7.5 deg and accelerate to a Vz climb. I use 2700 rpm and WOT for the entire climb.

@PT20J Skip:  I was afraid my comments would come across poorly, but I know you, and know your great attention to detail which makes you a great pilot.  Your technique stated above is how I fly also.

The point I would like to make is the two parts of the newer AFM/POH,  The AFM sections (takeoff and landing data, emergency procedures) are the best procedures to get the highest performance out of your aircraft.  Those sections are also FAA-approved.  All remaining data (POH) is OEM data and recommended. 

Your owner's manual (trying to use one title for all the different manuals over the decades) should tell the operator HOW the data in that manual was collected.  For example, it might say  something like: 1) Hold brakes 2) Advance throttle to full 3) Release brakes 4) Hold slight back pressure on yoke 5) Rotate at 70 knots 6) Climb at 77 knots until clear of obstacles.

Edited December 14, 2020 by Blue on Top
Added "is" to last sentence in first paragraph.

[mike_elliott]

12 hours ago, Blue on Top said:

4) Yes, VGs can lower stall speeds (I have not personally tested an M20 ... except for tufting and airplane ... which are little VGs).  Certification testing alone (if done with an FAA ACO that actually makes the applicant meet the regulations) will cost more than one can ever think about making on selling them ... and VGs are really, really inexpensive.

5) The Mooney TKS installation is a scab on over the base airfoil.  It modifies the airfoil significantly.  Airfoil designers look at 0.010" as significant; TKS is closer to 0.250"

I have flown and stalled N242PT, SN 33-0005 which had both TKS and VG's. While I didnt do any high altitude Cruise speed tests, my Mark I sphincter gauge says it was a tad slower in cruise than other Ultra Acclaims, especially N218TA SN 33-0012. It also stalled lower at NEAR gross weight than the others, (maybe 100 under) at 52Kts. I wish I could have had more time to really see what the cruise penalties of these 2 mods added to, but the bennies of the stall reduction and FIKI were well worth it for the mission this plane was to take on. Perhaps now that it is back in the States Ill get my chance soon. Granted, this was not an instrumented test flight, but a real world experience.

[Petehdgs]

On 12/8/2020 at 12:52 AM, carusoam said:

So... how do you feel about some decent modern flaps for your bird?

Best regards,

-a-

 

Based on the research I have done, the softness of the stall will not be positively improved by changing the flaps system.  According to the performance curves in TOWS the stall is sharper with flaps applied, changing the flap system is not likely to improve that.  I am not interested in changing the flaps.  

[Petehdgs]

12 hours ago, MikeOH said:

Thinking this reaction sounds ridiculous is, I think, EXACTLY the insidious reasoning that is responsible for why it IS the predominant reaction!  Low altitude, unexpected stall, ground rushing up,....

This is not ridiculous at all if you have read Stick and Rudder.   WL discusses this reaction at length and explains why it is the wrong thing to do.  

Edited December 14, 2020 by Petehdgs
spelling

[Petehdgs]

In 1988 Harry Riblett wrote:

"In my studies I have identified a critical airfoil parameter that I call the Initial camber angle", or initial slope of the mean line, that is, perhaps the most important feature of the airfoil affecting stall performance. Unfortunately, in the NACA airfoils, the mean lines (-2xx, -4xx, -6xx) are all exact multiples of each other, and the initial camber angles are uncontrolled, and usually too small (flat).

For instance, this angle in the 64-212 is only 3 degrees, whereas I have identified the optimum angle to be about 12 degrees for a typical 12% thick laminar flow airfoil, up to about 15 degrees for thicker and urbulent airfoils.  If the angle exceeds these values, there will be excessive drag at high speed; less than this, and the stall performance is not optimized. It is more than a coincidence that this angle approximates the stall angle of attack for the airfoil. When this angle is correct, stall performance is optimized, with no performance penalty at high speed. In effect, this puts an optimum "leading edge cuff on the airfoil, and it also improves performance with flaps."

I wanted to see what that actually looks like on a wing, but I have been unable to find a description of how to measure it.  So I drew the 64-412 airfoil used on the outer portion of our Mooneys by using the leading edge circle and forming the upper and lower curves from the ordinate tables for the first 10% of wing chord.  I used the NACA slope and radius method as described by HR in a 1992 article of Sport Aviation.  This method has the undesirable effect of raising the true leading edge and de-cambering airfoil from the intended camber line, but this should be the way the airfoil was constructed at the factory. 

To determine the LE droop angle I chose to connect a line between the upper and lower surfaces where they intersect the LE circle, then measure that angle by taking the inverse SIN of the rise/run.  Obviously I could be off by a bit but I confirmed 3 degrees (or less) on the 64-412 airfoil.  I then added the LE Cuff by dropping the nose 0.70% of Chord from the actual raised nose position, then moving that point forward by 0.50% Chord.  I then drew the LE Circle and the upper and lower surface curves.  This puts the LE Cuff angle at about 12 degrees, exactly where HR recommends that it be for optimum soft stall and cruise. 

I also added the perpendicular thickness of the proposed GA cuff.  It will be less than 5/8 thick at the thickest point (Chord=67 inches).  That makes it very light indeed, less than 8 lbs total weight, maybe as low as 6. 

Riblett 1992_12_20_.pdf

Edited December 14, 2020 by Petehdgs
clarification

[PT20J]

8 hours ago, Blue on Top said:

@PT20J Skip:  I was afraid my comments would come across poorly, but I know you, and know your great attention to detail which makes you a great pilot.  Your technique stated above is how I fly also.

The point I would like to make is the two parts of the newer AFM/POH,  The AFM sections (takeoff and landing data, emergency procedures) are the best procedures to get the highest performance out of your aircraft.  Those sections are also FAA-approved.  All remaining data (POH) is OEM data and recommended. 

Your owner's manual (trying to use one title for all the different manuals over the decades) should tell the operator HOW the data in that manual was collected.  For example, it might say  something like: 1) Hold brakes 2) Advance throttle to full 3) Release brakes 4) Hold slight back pressure on yoke 5) Rotate at 70 knots 6) Climb at 77 knots until clear of obstacles.

No, I think I was unclear in my original post and I just wanted to better explain my thinking. I am more concerned about the rapid decay of airspeed, and significant pitch change required in a scary direction (down) near the ground to maintain control after a power loss in a nose high and low airspeed condition, than I am about maximizing altitude by what turns out to be a relatively small amount. 

[Petehdgs]

How do I intend to proceed going forward?  That is, if there is a forward place to go to from here.  I have thought about that a lot.  It appears from my research that there are airfoil errors that can be corrected and that those corrections can improve the stall characteristics (and safety) of our birds in flight.  ( I understand most of you do not agree) It also appears crystal clear from this discussion that the only person interested in pursuing this improvement is me.   If I had 10 or 15 other people who might be interested in improving the low speed handling of their birds then I might consider applying for an STC for multiple aircraft, but since it is only me that changes things quite a bit. 

The first step is to determine how the cuff will be applied.  I think I have a good plan for that so the next step is to discuss this with my AI and seek his advice about securing a 337 modification approval.  That discussion will be a rather lengthy on and based on the outcome of that discussion with my AI and the FAA, that may put the stops on the project right there.  If approval is given and the scope of testing is understood and acceptable then I would proceed to fabricate a LE cuff out of Epoxy resin, fiberglass, and 2lb foam core.  I have a plan for that.  Once completed and inspected a flight test will be performed based on the FAA requirements for the 337.  That would leave me with a one-off modification and improvement to my bird. 

On the plus side, there would then be one Mooney with an improved airfoil in the fleet.  At least then real numbers can be obtained as to weather the process warrants further investigation by other MSers.  Of course, in the mean time I am going to do my best to fly what I have as well as I possibly can. 

One Miracle at a time. 

[PT20J]

1 hour ago, Petehdgs said:

How do I intend to proceed going forward?  That is, if there is a forward place to go to from here.  I have thought about that a lot.  It appears from my research that there are airfoil errors that can be corrected and that those corrections can improve the stall characteristics (and safety) of our birds in flight.  ( I understand most of you do not agree) It also appears crystal clear from this discussion that the only person interested in pursuing this improvement is me.   If I had 10 or 15 other people who might be interested in improving the low speed handling of their birds then I might consider applying for an STC for multiple aircraft, but since it is only me that changes things quite a bit. 

The first step is to determine how the cuff will be applied.  I think I have a good plan for that so the next step is to discuss this with my AI and seek his advice about securing a 337 modification approval.  That discussion will be a rather lengthy on and based on the outcome of that discussion with my AI and the FAA, that may put the stops on the project right there.  If approval is given and the scope of testing is understood and acceptable then I would proceed to fabricate a LE cuff out of Epoxy resin, fiberglass, and 2lb foam core.  I have a plan for that.  Once completed and inspected a flight test will be performed based on the FAA requirements for the 337.  That would leave me with a one-off modification and improvement to my bird. 

On the plus side, there would then be one Mooney with an improved airfoil in the fleet.  At least then real numbers can be obtained as to weather the process warrants further investigation by other MSers.  Of course, in the mean time I am going to do my best to fly what I have as well as I possibly can. 

One Miracle at a time. 

It sounds like an interesting project. Please keep us posted on your progress. If no one ever tried out their ideas, we would never learn anything. 

Skip

[PT20J]

Hey Ron, @Blue on Top, here's a question for you. In another thread someone said that they believed that the stall speed was lower in ground effect. My understanding has always been that the primary effect of a nearby ground plane was to reduce the tip vortices which makes the wing appear as if it had a higher aspect ratio and increases the lift curve slope resulting in a lower AoA required to generate the same lift (with less induced drag). But I never thought about whether it affects stall speed. I guess the question is whether CLmax changes. (Probably an article on ground effect on the offing )

Skip

[Blue on Top]

3 hours ago, Petehdgs said:

On the plus side, there would then be one Mooney with an improved airfoil in the fleet.  At least then real numbers can be obtained as to weather the process warrants further investigation by other MSers.  Of course, in the mean time I am going to do my best to fly what I have as well as I possibly can. 

@Petehdgs  I appreciate your enthusiasm and desire to make your aircraft better.  I doubt that you will be able to complete a wing leading edge modification without FAA approval.  If allowed, your modification will be classified by the 337 as a major modification of the wing, both structurally and aerodynamically.  If it were me as a DER, the airplane will need to go into the "experimental - R&D, show compliance and/or market survey" category.  This allows a year for testing and to show compliance to the regulations before it must be removed.

This is why VG manufacturers don't change (and have the FAA approve) the reduced stall speeds.  They are simply claims.  Changing the stall speeds will require a major change to the Pilot Operating Handbook (POH), including stall speeds and performance (takeoff and landing) data.

Why do you think the M20 has bad stall characteristics and landing performance?

[Blue on Top]

11 minutes ago, PT20J said:

Hey Ron, @Blue on Top, here's a question for you. In another thread someone said that they believed that the stall speed was lower in ground effect. My understanding has always been that the primary effect of a nearby ground plane was to reduce the tip vortices which makes the wing appear as if it had a higher aspect ratio and increases the lift curve slope resulting in a lower AoA required to generate the same lift (with less induced drag). But I never thought about whether it affects stall speed. I guess the question is whether CLmax changes. (Probably an article on ground effect on the offing )

Skip

@PT20J Skip: Everything you stated above is exactly correct ... just continue your thoughts along those lines.  Because the wing has gotten more efficient (higher effective aspect ratio), C_L will get closer to C_l.  This results in a slightly lower stall speed ... and the reason Mooney aircraft are so good at floating the length of the entire runway  .

With today's instrumentation, a little work and 1-G stall criteria (not currently allowed in 14CFR23 airplanes), we could calculate the lower stall speed, but it has no practical value for small airplanes.  We do look at it indirectly during V_MU (minimum unstick velocity) takeoff tests, where the airplane, in ground effect, is made airborne at the slowest speed possible.  Btw, V_MU testing is very high risk, and all (FAA and Applicants) are trying to find ways to eliminate the testing).  Hope this helps.

[Petehdgs]

8 minutes ago, Blue on Top said:

@Petehdgs  

This is why VG manufacturers don't change (and have the FAA approve) the reduced stall speeds.  They are simply claims.  Changing the stall speeds will require a major change to the Pilot Operating Handbook (POH), including stall speeds and performance (takeoff and landing) data.

Why do you think the M20 has bad stall characteristics and landing performance?

 If the VG mfrs don't get FAA approval for their equipment then how do they leave them on the wing and how do they get approval for others to install them?

[Marcopolo]

1 minute ago, Petehdgs said:

If the VG mfrs don't get FAA approval for their equipment then how do they leave them on the wing and how do they get approval for others to install them?

  If I understand correctly, they do get approval, they just don't get that approval based on the "claim" of lowering the stall speed.

 

Ron

[Blue on Top]

16 minutes ago, Petehdgs said:

 If the VG mfrs don't get FAA approval for their equipment then how do they leave them on the wing and how do they get approval for others to install them?

@Marcopolo Bingo!  You hit the nail on the head perfectly.  No credit. 

@Petehdgs IF you are able to get an STC and claim lower stall speeds and better stall characteristics, you will also get to have your name included on all the lawsuits.  On a good note, Mooney has more money and will get to pay the majority of those bills.  Although I know you have a long, hard road ahead of you, I am rooting for you all the way!   

[PT20J]

50 minutes ago, Blue on Top said:

Btw, V_MU testing is very high risk, and all (FAA and Applicants) are trying to find ways to eliminate the testing)

Is that the test where they strap on a piece of oak to protect the tail at the high deck angle? 

Seems the max sink rate tests are pretty dicey, too.
 

[Petehdgs]

26 minutes ago, Blue on Top said:

@Marcopolo Bingo!  You hit the nail on the head perfectly.  No credit. 

@Petehdgs IF you are able to get an STC and claim lower stall speeds and better stall characteristics, you will also get to have your name included on all the lawsuits.  On a good note, Mooney has more money and will get to pay the majority of those bills.  Although I know you have a long, hard road ahead of you, I am rooting for you all the way!   

Blue on Top, if the VG folks aren't getting their approval based on performance, what are they getting it on? What are the STOL folks doing when they add leading edge cuffs, stall fences and VGs?  I mean, this type of stuff is done on Cessnas all the time.  What is the catch that they use?  

[Blue on Top]

1 hour ago, PT20J said:

Is that the test where they strap on a piece of oak to protect the tail at the high deck angle? 
 

@PT20J  Yep!  That's for the geometry-limited airplanes (airliners).  The smoke and flames during those tests are just way freakin' cool!

For me, it's been the only test I have seen a negative value on the radar altimeter with the gear in the well.  Although still technically a "Blue on Top" test, it was recorded as a "Brown on Bottom" test.  

[Blue on Top]

1 hour ago, Petehdgs said:

Blue on Top, if the VG folks aren't getting their approval based on performance, what are they getting it on? What are the STOL folks doing when they add leading edge cuffs, stall fences and VGs?  I mean, this type of stuff is done on Cessnas all the time.  What is the catch that they use?  

If they are not changing the POH through the STC process, they are not taking credit for the stall speed reduction.  The Robertson STOL kit is an STC.  The typical STOL people now are using experimental airplanes (with a 1-year limit),  They are also using power to lower the stall speed (not allowed for certification ... at the moment ... electrics might change that ... long story ... I sit on the ASTM committee that is writing the compliance to the "new" (3 years ago) regulations).

If an OEM took on the project that you are looking at, they would be required to re-calibrate the airspeed system (via 1 or 2 wing tip-mounted booms, redo all the stall testing and (my opinion) redo all the spin testing, too.  How do you plan to make sure that every installation is identical? (that's conformity).  For example, the stall strip adjustability (slight up and down location only) on the current airplanes is to allow for variations in wing build with production tooling.  If this leading edge droop is slightly misaligned, will the airplane go over on its back.  In addition, with higher leading edge camber, the odds of leading edge separation and stall are much higher.  There is a business jet that has VGs on the leading edge because of this.  A journey of 1000 miles starts with a single step!

I want to help you.

[midlifeflyer]

On 12/14/2020 at 12:40 AM, PT20J said:

Just to be clear: I don’t fly the 201 the same way I flew the draggy Beaver on floats. But, I don’t spend a lot of time at Vx or Vy in the Mooney either. I start the takeoff roll with TO flaps and some back pressure and let the nose start up when it’s ready. After lift off, I reduce the back pressure to hold about 10 deg pitch until clear of obstacles and passing 86 KIAS (Vy) at which point I raise the flaps and lower the nose to about 7.5 deg and accelerate to a Vz climb. I use 2700 rpm and WOT for the entire climb.

I’m interested to learn how others do it. 

Skip

 

Similar.  Vx for obstacles, Vy to a "safe" altitude, and transition to enroute climb. I just recently began adding Vz to my climb profile in airplanes where it matters.

[Petehdgs]

On 12/14/2020 at 6:26 PM, Blue on Top said:

If they are not changing the POH through the STC process, they are not taking credit for the stall speed reduction.  The Robertson STOL kit is an STC.  The typical STOL people now are using experimental airplanes (with a 1-year limit),  They are also using power to lower the stall speed (not allowed for certification ... at the moment ... electrics might change that ... long story ... I sit on the ASTM committee that is writing the compliance to the "new" (3 years ago) regulations).

If an OEM took on the project that you are looking at, they would be required to re-calibrate the airspeed system (via 1 or 2 wing tip-mounted booms, redo all the stall testing and (my opinion) redo all the spin testing, too.  How do you plan to make sure that every installation is identical? (that's conformity).  For example, the stall strip adjustability (slight up and down location only) on the current airplanes is to allow for variations in wing build with production tooling.  If this leading edge droop is slightly misaligned, will the airplane go over on its back.  In addition, with higher leading edge camber, the odds of leading edge separation and stall are much higher.  There is a business jet that has VGs on the leading edge because of this.  A journey of 1000 miles starts with a single step!

I want to help you.

I sent you a detailed IM.  Call me anytime 8am-6pm eastern