Mooney Aerodynamic Curves (Nerd Alert)

[rahill]

22 minutes ago, KSMooniac said:

XFOIL is a 2-D airfoil code that is useful for some work, but not for 3-D/finite wing comparisons, or whole aircraft performance predictions.

Understood, she already told me that "xfoil doesn’t account for vortices, wing-body join, interface between the two airfoils, etc. so there are probably a lot of things contributing to the differences".  I think she gets it.

Edited December 11, 2018 by rahill

[KSMooniac]

Understood, she already told me that "xfoil doesn’t account for vortices, wing-body join, interface between the two airfoils, etc. so there are probably a lot of things contributing to the differences".  I think she gets it.

I'd say she gets it! Look into Benchmark for the next step. It is a great package from my brief intro to it.

I'm a structural analyst in my career but still wish to dabble in aero stuff as I enjoyed it in school. Anything airplane related is fun!

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[Chuyg]

20 hours ago, teejayevans said:

I’m surprised the lift continues past the critical AOA (16°) to 18°, I thought laminar flow wings stop producing lift entirely?

The whole wing doesn't completely stall in a moment, flow separates at the root first and then continues separating all the way to the tip, at which point no lift is being produced.

[aviatoreb]

2 hours ago, Chuyg said:

The whole wing doesn't completely stall in a moment, flow separates at the root first and then continues separating all the way to the tip, at which point no lift is being produced.

Right!  What we call a stall is when the lifting are, the part of the flow in front of the separation, shrinks to the point that the lifting force it can create is smaller than the weight of the aircraft.

[ArtVandelay]

The whole wing doesn't completely stall in a moment, flow separates at the root first and then continues separating all the way to the tip, at which point no lift is being produced.

I understand that.

Actually it depends on the wing shape, some go tip to root, some root to tip. It doesn’t happen naturally with Rectangular wings, which is why Mooney has stall strips.

I thought that once reaching stall, laminar flow wings lost all lift with greater AOA and that’s why some use vortex generators which effectively turns the wing into a non laminar flow wing.

BTW, this is a plot of a jet laminar flow wing, notice the quick reduction in lift, loosing all lift in less than 1° of additional AOA.

 

 

After research, it depends how laminar flow the wings are and where the separation occurs. Some airfoils are worst then others ( 5-digit NACA series are described as nasty).

 

[Bob - S50]

It was my understanding that wings stall from root to tip on purpose in order to maintain roll authority for the ailerons at the onset of the stall.  To do that they design the plane with washout.  That is, they design them so the angle of attack at the wingtip is less than that at the root.

And I don't think wings stop making lift at the stall, the coefficient of lift just starts to decrease at the stall.  It isn't a vertical line to the right of the stall.  I think the slope of the curve after the stall is steeper for a laminar flow airfoil, but it still isn't vertical.

And it was also my understanding that the stall strips are installed to get both wings to stall at essentially the same time.  If not for them, due to slight differences in the manufacturing of the wings, one wing might stall earlier than the other leading to a wing drop rather than a straight ahead stall.

[EricJ]

1 hour ago, Bob - S50 said:

It was my understanding that wings stall from root to tip on purpose in order to maintain roll authority for the ailerons at the onset of the stall.  To do that they design the plane with washout.  That is, they design them so the angle of attack at the wingtip is less than that at the root.

Not all wings have washout.   If you stand at the tip and look down a wing you can usually easily see whether it does or not.   C172s and C150s have a lot of visible washout, so it's easy to see how the wing stalls a lot earlier at the root.   Most Mooneys have no washout, hence the stall strips.   IIRC there were a couple years where the wings were made with washout, but only for a short period (around 67 IIRC). 

 

[KSMooniac]

There are also two types of washout...geometrical where you can see the twist, and aerodynamic where you might not since the airfoil changes from root to tip, with the tip airfoil stalling at a higher angle of attack.

The M20 wing has a different airfoil at the tip, but I don't know if it was done as a washout choice or for some other reason.

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[jaylw314]

1 hour ago, EricJ said:

Not all wings have washout.   If you stand at the tip and look down a wing you can usually easily see whether it does or not.   C172s and C150s have a lot of visible washout, so it's easy to see how the wing stalls a lot earlier at the root.   Most Mooneys have no washout, hence the stall strips.   IIRC there were a couple years where the wings were made with washout, but only for a short period (around 67 IIRC). 

 

IIRC, you can get the same effect as washout with camber, so a wing that goes from low camber at the root to high camber at the tip should stall from root to tip as well.

[ArtVandelay]

And it was also my understanding that the stall strips are installed to get both wings to stall at essentially the same time.  If not for them, due to slight differences in the manufacturing of the wings, one wing might stall earlier than the other leading to a wing drop rather than a straight ahead stall.

If that were true I would expect the stall strips to be
1. Unequal length or
2. Only 1 would be required, on the wing that stalls last.

The fact that my are equal and position the same location tells me Mooney knew of the stall characteristics and that was the fix to try to make more docile.
I’ve discuss this before, but I believe the J and F are the least docile of all models, they extended the bodies without changing the wings position or adding weight to the front (longer, heavier engines), this resulting in them being less nose heavy and instead just dropping the nose, they could drop the wing if the stall was severe enough.

[ArtVandelay]

IIRC, you can get the same effect as washout with camber, so a wing that goes from low camber at the root to high camber at the tip should stall from root to tip as well.

Don’t you have that backwards, more camber would result in earlier separation of the flow, like a car in a tighter turn?
I’d expect the tips to have less camber, ie be flatter.

[jaylw314]

2 minutes ago, teejayevans said:

Don’t you have that backwards, more camber would result in earlier separation of the flow, like a car in a tighter turn?
I’d expect the tips to have less camber, ie be flatter.

Camber allows for slower stall speed, so a high-camber airfoil is more resistant to stalls.  "Thick" wings like on a Cherokee are actually higher camber wings than we have, even though their bottom is also flat (top is more curved since the wing is thicker), so camber is not always obvious just looking at one surface of the wing.

In fact, the old myth of "flat on the bottom and curved on the top airfoils produce lift" is a bunch of malarkey.  The only real reason for camber is to lower stall speed, not to create lift (that comes from AOA).  I might be oversimplifying that, so any actually engineers please correct me!

[KSMooniac]

Camber creates lift at lower angles of attack. A flat plate needs AOA to generate lift, but a cambered airfoil can create lift at 0 AOA, or even negative AOA.

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[jaylw314]

17 minutes ago, teejayevans said:

If that were true I would expect the stall strips to be
1. Unequal length or
2. Only 1 would be required, on the wing that stalls last.

The fact that my are equal and position the same location tells me Mooney knew of the stall characteristics and that was the fix to try to make more docile.
I’ve discuss this before, but I believe the J and F are the least docile of all models, they extended the bodies without changing the wings position or adding weight to the front (longer, heavier engines), this resulting in them being less nose heavy and instead just dropping the nose, they could drop the wing if the stall was severe enough.

IIRC, the stall strips are there to ensure the part of the wing root stalls early so that you get stall buffet as a warning before the ailerons lose effectiveness.  They can adjust them up or down so that the buffet occurs simultaneously, so they don't need to be different sizes

[jaylw314]

9 minutes ago, KSMooniac said:

Camber creates lift at lower angles of attack. A flat plate needs AOA to generate lift, but a cambered airfoil can create lift at 0 AOA, or even negative AOA.

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I always thought that was cheating a bit, though, because a cambered airfoil actually has a positive AOA when it's sitting "flat", but you're right, it can create lift even to small negative AOA's.  I have to imagine, though that in typical flight, the majority of lift comes from AOA and not from camber, but I have no data

[KSMooniac]

I always thought that was cheating a bit, though, because a cambered airfoil actually has a positive AOA when it's sitting "flat", but you're right, it can create lift even to small negative AOA's.  I have to imagine, though that in typical flight, the majority of lift comes from AOA and not from camber, but I have no data

It's a combo of all of the parameters... Camber, thickness, AOA, and of course airspeed. What is most important varies plane to plane, and even between phases of flight.

For purity's sake, AOA is the angle between the chord line (leading edge to trailing edge) and relative airflow. A cambered airfoil at 0 AOA will produce lift. You can look at it's lift curve and typically 0 coefficient of lift will be at a negative angle of attack. A symmetric airfoil or a flat plate will have 0 Cl at 0 AOA.

For a Mooney designed to be an efficient cruiser, a lower drag laminar flow airfoil was chosen to minimize drag by operating at a low AOA in the "drag bucket" for typical cruise conditions. Flaps were added to add drag and lift at lower speeds (and AOA) for better landing performance. A STOL utility plane would be designed to maximize lift at low speed first with a high lift airfoil, very effective flaps and slats, etc while not worrying about cruise performance. Camber, thickness, high lift devices all play a part in the gumbo that is a complete wing design.

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[0TreeLemur]

On 12/9/2018 at 2:41 PM, Bob - S50 said:

Are you just curious about the lift and drag coefficients?  You don't need them to calibrate the AOA.

I think seeing the data would be great 'cause I really like that stuff.

 

[0TreeLemur]

6 minutes ago, Fred_2O said:

I think seeing the data would be great 'cause I really like that stuff.

 

Haven't checked this thread since I started it because I'm at a conference.  When I replied I thought there were three replies not a whole bunch.  This is really cool.  There is factory rep. here at the AGU conference- I visited with him briefly today- tomorrow I'll ask about curves for the short body.

[PT20J]

21 hours ago, Chuyg said:

The whole wing doesn't completely stall in a moment, flow separates at the root first and then continues separating all the way to the tip, at which point no lift is being produced.

Actually,  stall is the point where C_L begins to decrease with increasing alpha. If C_L actually went to zero, the plane would be in free fall.

[carusoam]

Stuff we believe to be true...

1) The Mooney wing varies from the root to the tip.  Pretty visible in the drawings in the POH.... otherwise, it would be called a Hershey bar wing. (Remember Hershey bars?)

2) The wing tips are the last place to stall, allowing roll control for the pilots with locked feet during stalls...  a pretty wise engineering selection during the design process...

3) Stall strips are placed on the leading edge of the wing... located in a way that when the AOA has the airflow split line below the strips... there is going to be a noticeable change caused by that disturbed airflow over the width of the strips...  early on in Mooney history, they were adjusted similar to a stall vane (position wise).  Only, the stall vane, doesn’t get permanently tacked in place...

4) To visualize how the stall strips work, would require doing some really slow flight in the rain, with a camera aimed at the wing... somebody captured a similar airflow study regarding how Mooney speed brakes work... water makes for some excellent tracer studies...  at the time when buffet is being felt, the stall strips have become involved in messing up airflow around that portion of wing...

5) So this is just a PP thinking out loud... not a mechanic, or CFI.... but, when you go out to the plane.... take a look at the the leading edge observe where the stall vane is, and where the stall strips are vertically placed on the rounded leading edge...Bet they are pretty close to the same horizontal line allowing the stall horn to activate a few AOA° before the buffet occurs...

6) Somebody decided that you are going to get a few fair warnings before a Mooney can no longer fly for you...

• You get an odd sight picture as the AOA goes pretty high... or bank angle if you are in an accelerated mood...
• You start getting some really low airspeed...
• You hear an unfamiliar tone... that you are programmed to react to first, then figure out what that noise really was...
• If you pushed on the yoke, and the sound went away... you did well.  You figured out what the tone was and fixed the AOA problem at the same time.... 
• if it was an accelerated stall... did you try to level the plane with the airlerons, while pushing on the yoke?

7) Somebody also noticed human beings can get distracted, or cognitively overloaded in the traffic pattern. That stall horn is an important tool to check on the pre-flight walk around.  Make sure it is working...

PP thoughts only, not a CFI...

Best regards,

-a-

[PT20J]

18 hours ago, teejayevans said:

Actually it depends on the wing shape, some go tip to root, some root to tip. It doesn’t happen naturally with Rectangular wings, which is why Mooney has stall strips.

Rectangular planform (Hershey bar) wings naturally stall at the root first -- one of the reasons for choosing that planform for RVs and Cherokees.

The Mooney wing has aerodynamic washout to accomplish the same thing: NACA 63-215 @ root, NACA 64-412 @ tip. Due to manufacturing tolerances, the left and right wing generally do not reach stall at exactly the same time and the stall strips are added to adjust for that. Not sure how it is done today, but in the early 90's during each airplane's factory test flights, the stall strips were attached with duct tape and adjusted until the roll at stall met specification and then they were permanently installed. At least that's what Rob McDonnell who was VP of Engineering at the time told me.

[0TreeLemur]

9 minutes ago, carusoam said:

Stuff we believe to be true...

1) The Mooney wing varies from the root to the tip.  Pretty visible in the drawings in the POH.... otherwise, it would be called a Hershey bar wing. (Remember Hershey bars?)

-a-

Interesting discussion here from April of this year by @Igor_U and what those numbers entail.

[PT20J]

9 hours ago, jaylw314 said:

Thick" wings like on a Cherokee are actually higher camber wings than we have, even though their bottom is also flat (top is more curved since the wing is thicker), so camber is not always obvious just looking at one surface of the wing.

In fact, the old myth of "flat on the bottom and curved on the top airfoils produce lift" is a bunch of malarkey.  The only real reason for camber is to lower stall speed, not to create lift (that comes from AOA).  I might be oversimplifying that, so any actually engineers please correct me!

Actually the "thick" Cherokee airfoil is also laminar flow: NACA 65-415.

The purpose of camber and other design parameters is the efficient generation of lift. A flat plate will generate lift at positive alpha, but it also generates a lot of drag. Remember, you only need to generate enough lift to equal the weight (and the tail down force, +/- any effects from any thrust line angle). The trick is to do it efficiently (minimum drag).

[PT20J]

25 minutes ago, carusoam said:

1) The Mooney wing varies from the root to the tip.  Pretty visible in the drawings in the POH.... otherwise, it would be called a Hershey bar wing. (Remember Hershey bars?)

2) The wing tips are the last place to stall, allowing roll control for the pilots with locked feet during stalls...  a pretty wise engineering selection during the design process...

3) Stall strips are placed on the leading edge of the wing... located in a way that when the AOA has the airflow split line below the strips... there is going to be a noticeable change caused by that disturbed airflow over the width of the strips...  early on in Mooney history, they were adjusted similar to a stall vane (position wise).  Only, the stall vane, doesn’t get permanently tacked in place...

The Mooney wing planform is a forward swept tapered wing. Without some form of washout, the stall would likely be uniform over the span.

Stall strips disrupt the boundary layer at high angles of attack causing flow detachment (stall). 

[carusoam]

PT,

which part of the wing is forward swept?

I get the feeling I am going to learn something new tonight!  

Going on old fuzzy memory, the leading edges are 90° to the cabin... but I didn’t measure them to see if there is some forward sweep there...

The part of the wing I stand on getting in the plane is really wide compared to the fancy wing tips... There is no obvious change to the taper along the way...

Private Pilot asking a question, trying to learn something new about Mooney wings...

Best regards,

-a-