Mooney Aerodynamics

[jlunseth]

Hmmh, there is a big difference between the car or semi or train moving through the air, and the wing (especially the well crafted Mooney wing), and that is laminar versus turbulent flow.  The passing car or semi pulls your car in because the flow around it is turbulent and therefore of lower pressure than ambient.  The laminar wing in ground effect does not pull the ground in, nor does the ground pull the wing down because the flow over neither is turbulent (assuming normal winds and not a hurricane). If the flow were turbulent, ground effect would be negative, the aircraft would be sucked into the ground, and we can’t have that.  To me, the obvious answer is that the wing, at a light angle of attack, causes high pressure on the under side, and as carusoam suggested, in ground effect that pressure has no place to go, it can’t move to an area of lower (ambient) pressure because the ground is there.  That in turn suggests that there is an area of high pressure under the wing in normal flight, caused by the shape of the wing and the fact that the wing is flying at an angle of attack, however slight, and that causes lift even in a piece of plywood.  So now who wins, Newton or Bernoulli?

[jlunseth]

10 minutes ago, PT20J said:

Here's one way to think about it. The flow around a finite-span wing far from the ground produces wingtip vortices and a net downwash behind the wing. As the wing nears the the ground, the ground interferes with the flow in such a way that vortices and downwash are reduced. It is as if the wing gains span. The result is that the aerodynamic force vector tilts forward. If we resolve the AF vector into a vertical component (lift) and rearward component (drag) the forward tilting increases lift and reduces drag.

Skip

Well, isn’t the theory of lift being discussed earlier, that lift is partially caused by downwash.  Every action has an equal an opposite reaction. So the wing creating downwash supposedly lifts the wing.  If downwash is reduced by the ground, then lift would be reduced would it not, and the plane would fall. I understand the theory about vortices, but were we not saying earier that vortices are a symptom, not a cause, the vortex occurs because there is drag caused by the flow under and over the wing meeting each other off the wingtip, so how does being near the ground change that flow? Or does it change that flow at all, or rather just causes the vortex to go nowhere because the bottom of it is blocked by the ground.

Edited December 24, 2019 by jlunseth

[0TreeLemur]

17 minutes ago, jlunseth said:

Well, isn’t the theory of lift being discussed earlier, that lift is partially caused by downwash.  Every action has an equal an opposite reaction. So the wing creating downwash supposedly lifts the wing.  If downwash is reduced by the ground, then lift would be reduced would it not, and the plane would fall. I understand the theory about vortices, but were we not saying earier that vortices are a symptom, not a cause, the vortex occurs because there is drag caused by the flow under and over the wing meeting each other off the wingtip, so how does being near the ground change that flow? Or does it change that flow at all, or rather just causes the vortex to go nowhere because the bottom of it is blocked by the ground.

The downwash component of lift is very small compared to the streamline curvature component created by a positive angle of attack.   Assuming SL air density of 1.2 kg/m3, and a wing planform area of 15.8 m2 (170 ft2), the downwash velocity to create 11,120 N (2500 lb) of lift is an amazing 24 m/s (79 ft/s or 54 mph).   Downwash, which is just the vertical component of the flow coming off the underside of the wing, so the total downwash speed would necessarily be considerably greater than this speed if that were the sole factor.  Also note that here I used the entire planform area, which is an overestimate of the cross-section of the downwash flow cross-sectional area, so the actual speed of downwash would necessarily be much larger than the calculated value, which it isn't.  I think it is safe to say that the downwash component of lift is much less than 10% of the lift.  

Somebody earlier in this thread said it well when they used the analogy of little kids (or not so little ) holding a curved hand out an open car window at highway speed.  The lift is due to the curvature.   There just isn't a high speed jet of air coming off the bottom of the kids' hand...

Your last sentence is pretty much right- the ground does interfere with vortex growth especially when the wing is near the ground.

[Ibra]

37 minutes ago, jlunseth said:

Hmmh, there is a big difference between the car or semi or train moving through the air, and the wing (especially the well crafted Mooney wing), and that is laminar versus turbulent flow.  The passing car or semi pulls your car in because the flow around it is turbulent and therefore of lower pressure than ambient

AFAIK ground effect manifest itself when wing generates highly turbulent airflow behind the wing and high wake on the wing sides, this is the case when ASI is at 55kts 

I don't land Mooney at 140kts where airflow is laminar but once I flew it at 100kts above threshold for fun then fully held flat, it took ages for it to touchdown and stop, roughly 2/3 runway something like 5000ft/8000ft 

[Austintatious]

20 minutes ago, Ibra said:

AFAIK ground effect manifest itself when wing generates highly turbulent airflow behind the wing and high wake on the wing sides, this is the case when ASI is at 55kts 

I don't land Mooney at 140kts where airflow is laminar but once I flew it at 100kts above threshold for fun then fully held flat, it took ages for it to touchdown and stop, roughly 2/3 runway something like 5000ft/8000ft 

Well, FWIW, I can feel ground effect in my glider, even at 140+ KIAS ,  not likely much  vorticies behind a glider wing at those speed!

[Ibra]

You can do that in jets as well, but surely not the ground effect where the aircraft can fly well bellow its stall speed 

In gliders, you have ridge or wave flying that also counts as sort of "ground effects" but they are there even when the aircraft is not flying 

Edited December 25, 2019 by Ibra

[PT20J]

3 hours ago, jlunseth said:

Well, isn’t the theory of lift being discussed earlier, that lift is partially caused by downwash.  Every action has an equal an opposite reaction. So the wing creating downwash supposedly lifts the wing.  If downwash is reduced by the ground, then lift would be reduced would it not, and the plane would fall. I understand the theory about vortices, but were we not saying earier that vortices are a symptom, not a cause, the vortex occurs because there is drag caused by the flow under and over the wing meeting each other off the wingtip, so how does being near the ground change that flow? Or does it change that flow at all, or rather just causes the vortex to go nowhere because the bottom of it is blocked by the ground.

Great question. That's where the "Newton third law" lift explanations lead us astray. Downwash is not necessary for lift. What is necessary is to cause the air to follow a curved path (Newton's second law as someone pointed out) which sets up the required pressure gradients. The idea, which appears in a number of aerodynamic texts, that lift is a product of the downward momentum induced by the wing turns out to be incorrect and based on a mathematical error (McLean, Understanding Aerodynamics, p. 433, and also discussed in his talk I posted earlier). 

Here's a pretty good article that explains lift without a lot of obscure concepts (such as circulation) and obfuscating math. The math is important if you need to calculate lift, but it's not very intuitive for explaining it.

howwingswork.pdf

Skip 

Edited December 25, 2019 by PT20J

[0TreeLemur]

4 minutes ago, PT20J said:

Here's a pretty good article that explains lift without a lot of obscure concepts (such as circulation) and obfuscating math.

howwingswork.pdf 356.14 kB · 0 downloads

Skip 

Is that a mic drop that I just heard???   

Streamline curvature causes pressure gradients in the direction opposite the curvature.  If you look at the streaklines on Fig 12(a) and 12(b), where they are the most curved the airfoil experiences the lowest pressure.  If that happens on the upper surface, then it is called lift.   Lift has virtually nothing to do with laminar flow (largely a myth), turbulent boundary layer, wingtip vortices, or Reindeer.  It's MAGIC as per the flight attendant in the video above.  The magic is the last equation in the paper that Skip just uploaded.

 

[jlunseth]

So, to sum it up in less magical terms, by flying a wing at an angle of attack, an area of high pressure is created under the wing where the curvature of the flow is toward the wing undersurface ( or in Newton’s words the wing is beating the air), and an area of low pressure above the wing where the curvature of the flow is away from the wing upper surface. Flaps work by increasing the curvature of the flow both above and below the wing. Ground effect is explained by the increase in pressure under the wing, because the high pressure induced by the flow curvature has nowhere to go, the ground is there.  Downwash, point of incidence, and the merger of flow at the wingtips that creates vortices are incidental.  That works for me.    

[Blue on Top]

22 hours ago, carusoam said:

Thanks for the extra details...

I was thinking parts of the wing were no longer generating lift... 

I think I made it more complex than possible...

Best regards,

-a-

@carusoam  Keep your thoughts simple.  It is the way Mother Nature works.  As I mentioned to someone (it may have been you), it often helps to look at air flow the way it is on an airplane.  The air is stagnate and the airplane passes through it.  Any net results after passage is lift and drag.  If one watches a tractor-trailer go by on a highway after/during a light rain, the flow is visualized (kind of … as water has more inertia).  As one sees the water moving forward this is drag.  Hope this helps. 

[Blue on Top]

We have voiced all of our opinions, but no one has changed their minds … typical pilot conversation (I am one, too).  BUT ...

1. We have good CFD models that predict lift, drag and pitching moment well … most of the time.  We pay good aerodynamicists to know when the CFD is right or wrong.  These CFD programs also include boundary layer (and millions of panels or nodes).  Typically, the equations involve full Navier-Stokes equations, which include Bernoulli and Newton.

2. If you believe a typical Mooney wing has laminar flow further aft than other airplanes, I have ocean-front property for you in Arizona.  Sorry.  Please note that there are basically 3 types of boundary layer flow (boundary layer is the flow close to the surface and measured perpendicular to the surface until the flow velocity becomes free stream.  It starts at 0" thickness at the stagnation point and grows until it is ~1/2" thick at the trailing edge.

3. The SAME airfoil cross section will perform differently at different Reynold numbers.  This is an area that separates the engineers from the pilots (innuendo intentional).  If the root chord is 72" and the tip chord is 36", the tip airfoil will stall first (the Rn of the root is 2X the tip).  If an airplane is flying at a higher speed and/or higher altitude, it will stall at a higher AOA because the Rn is hgher.  The Cd of a Pilates ball (smooth, large diameter) is lower than a ping pong ball (smooth small diameter because the Rn is higher on the big ball.  This is why smaller, round antenna produce more drag than a larger, streamlined antenna.

4. A golf ball is dimpled because … okay, I've gone to far.  

[N201MKTurbo]

25 minutes ago, Blue on Top said:

 

4. A golf ball is dimpled because … okay, I've gone to far.  

So, why do the dimples go in instead of out?

[Blue on Top]

18 minutes ago, N201MKTurbo said:

So, why do the dimples go in instead of out?

1. Less drag (good)

2. Easier to get the boundary layer to separate on the backside (good - less drag)

3. Forms a thicker, turbulent, boundary layer quicker (stops laminar flow earlier) so the ball flight can be tailored more by spin.

[brndiar]

2 hours ago, Blue on Top said:

@carusoam  Keep your thoughts simple.  It is the way Mother Nature works.  As I mentioned to someone (it may have been you), it often helps to look at air flow the way it is on an airplane.  The air is stagnate and the airplane passes through it.  Any net results after passage is lift and drag.  If one watches a tractor-trailer go by on a highway after/during a light rain, the flow is visualized (kind of … as water has more inertia).  As one sees the water moving forward this is drag.  Hope this helps. 

....Flying is the ART of batting the air down with wings of our birds where Angle of Attack is the most important principle. ART is that because not doing that right could break us our neks. ART is that, because where our FLYING WIND (relative wind) is comming from to meet the Wings of our Birds is not visible....

M

[Ibra]

6 hours ago, N201MKTurbo said:

So, why do the dimples go in instead of out?

First, that was discovred empirically first, it more a luck, actually it works for in as out as well 

For a rough surface to have less drag than a smooth one only works on specific speed/size/shape (Reynold numbers 10^5 for a sphere), at the Reynold numbers and shapes we fly I don't think a dented fat school PA28 wing will generates less drag than a slick clean M20K wing 

It is a good example of non-intuitive emprical result vs theory and survivor bias, which aircraft parts are strong enough and which ones you can let go?

https://www.boredpanda.com/world-war-2-aircraft-survivorship-bias-abraham-wald/

https://medium.com/@penguinpress/an-excerpt-from-how-not-to-be-wrong-by-jordan-ellenberg-664e708cfc3d

Edited December 25, 2019 by Ibra

[0TreeLemur]

 

17 hours ago, jlunseth said:

So, to sum it up in less magical terms, by flying a wing at an angle of attack, an area of high pressure is created under the wing where the curvature of the flow is toward the wing undersurface ( or in Newton’s words the wing is beating the air), and an area of low pressure above the wing where the curvature of the flow is away from the wing upper surface. Flaps work by increasing the curvature of the flow both above and below the wing. Ground effect is explained by the increase in pressure under the wing, because the high pressure induced by the flow curvature has nowhere to go, the ground is there.  Downwash, point of incidence, and the merger of flow at the wingtips that creates vortices are incidental.  That works for me.    

 Your statement about the effect of flaps is correct.  Leading edge slats on jets do the same thing.

For the sake of this discussion, I want to address THE MAGIC.  Here I make one last attempt to clarify some often repeated misperceptions.  To do this, I want to summarize some important observations using Figures 12(a) - (c) from the "howwingswork" pdf that Skip @PT20J uploaded and that clearly show it all.   Note that this figure uses a symmetrical airfoil.  I wish Babinsky would have included a photo with 0 angle of attack, you would see symmetrical streamline curvature around the airfoil.  A symmetrical streamline pattern would produce no lift because the pressure distributions on the top and bottom of the symmetrical airfoil would be the same.

Consider the following from "How Wings Work" by:

Consider these figures with the knowledge that:

K1)  Far (e.g. 50 ft above and below) from the wing, the ambient static pressure is unaffected by the motion of the wing

K2)  Curved streamlines cause a decrease the air pressure in the direction of the concave side of the curvature

You can deduce for yourself the following facts:

F1) There is a small region on the lower surface near the leading edge at where a streamline terminates and the pressure is greater than ambient static air pressure.  This "stagnation point" is natures way of telling the atmosphere that there is an object moving through the air, and "you better get outta my way". Stagnation points are very small- less than 1/2" (12mm) on our Mooney wings.

F2) other than very near the stagnation point the pressure on the lower surface of the wing is  less than ambient static air pressure because the streamlines are curved there towards the wing.  Look for yourself.  It is true.

F3) the pressure on the upper surface of the wing is everywhere less than ambient static air presure because the streamlines are curved there towards the wing, except right near the trailing edge.

F4) because of the positive angle of attack, the curvature of the streamlines is much greater above the wing than under it, meaning that the pressures on the upper surface are lower than those below it, despite the fact that the pressure on the lower surface of the wing is almost entirely less than ambient static pressure.

F5) the wing in Fig. (c) is not producing lift because the airflow is separated from the upper surface, destroying the lift producing streamline curvature seen in (a) and (b).

Conclusions:

C1)  Over most of the lower surface of a wing generating lift, the air pressure is less than ambient static.   Wings do not ride on a cushion of air that has a pressure greater than the ambient static air pressure.

C2) While the pressure over almost the entire wing surface is less than ambient air pressure, on average the pressure above the wing is less than below the wing.  That difference is the magic of lift.

<Disclaimer> I am an engineering professor with 23 years of experience teaching engineering fluid mechanics.   I am not a rocket scientist.

[amillet]

I find it fascinating that it was a couple of bicycle mechanics using trial and error, rather than engineers or physicists that figured out that machines really could be made to fly  Happy holidays to you all.

[0TreeLemur]

8 minutes ago, amillet said:

I find it fascinating that it was a couple of bicycle mechanics using trial and error, rather than engineers or physicists that figured out that machines really could be made to fly  Happy holidays to you all.

The Wright brothers were engineers in every sense of the word, and trained themselves in the scientific method.  You have probably heard that they built their own wind tunnel to take ideas they developed from reading the published work of Leonardo DaVinci, Otto Lillienthal, Octave Chanute, George Cayley and Samuel Langley, and test them.  They communicated regularly with some of these folks and shared ideas.   Although they were very careful to keep their work private because of the stigma associated with the pursuit of heaver-than-air flying machines, they did not work in a vacuum.

The Wrights used their considerable experience in machining and fabrication of bicycles to build excellent lightweight structures.  One might conclude that in the late 1800's the most likely profession to invent a working aeroplane would be  bicycle manufacturers.   Using their skills, experience, and self produced data to develop reliable flying machines, the Wright brothers took very few risks.   They set the standard for the entire certified airplane industry that we work within. 

Wishing ya'll the best in the new year!

[Blue on Top]

4 hours ago, Ibra said:

For a rough surface to have less drag than a smooth one only works on specific speed/size/shape (Reynold numbers 10^5 for a sphere), at the Reynold numbers and shapes we fly I don't think a dented fat school PA28 wing will generates less drag than a slick clean M20K wing 

A PA28 wing is not rotating … nor are any other airplanes wings (since the early 30s).  So, yes, dimpling an airplane wing will not reduce drag.  On the other hand …

A golf ball (or baseball or tennis ball or soccer ball, etc.) uses rotation to curve the flight path and reduce drag.  Due to a turbulent boundary layer and rotation of the golf ball, the flight path will change due to rotation.  For example with a duffer like me, if I top the ball, the ball will initially rise due to slope of the club face at impact.   Because I hit the top of the ball, it will rotate top down at the front.  Adding the velocity vectors, the top of the ball will have the velocity of the ball minus the velocity due to rotation, and the bottom will have the velocity of the ball plus the velocity due to rotation.  As a result, the velocity on the top is lower than the velocity on the bottom.  As a direct result (Bernoulli), the static pressure is higher on the top than the bottom, and the flight path of the ball curves dramatically downward.  This is the same for slices, hooks and skys … just rotation in different axes.  Note: Analyzing the aerodynamics of a golf ball while playing a round will not help your game (yes, personal experience).

As for drag, the air on the backside of the ball is trying to stay attached.  The dimples help make a clean separation from the ball.  Remember that the static pressure is low on the backside … creating drag.  

[EricJ]

11 hours ago, Blue on Top said:

We have voiced all of our opinions, but no one has changed their minds … typical pilot conversation (I am one, too).  BUT ...

This is the way of the world, especially enabled by social media platforms, but pretty much human nature regardless.

 

11 hours ago, Blue on Top said:

1. We have good CFD models that predict lift, drag and pitching moment well … most of the time.  We pay good aerodynamicists to know when the CFD is right or wrong.  These CFD programs also include boundary layer (and millions of panels or nodes).  Typically, the equations involve full Navier-Stokes equations, which include Bernoulli and Newton.

There are parallels in my profession where different mathematical points of view can be used to explain or characterize the same phenomena, and people will argue that one perspective is right or wrong*.    I've long argued that understanding as many points of view as possible well enough that you can use each effectively is the most useful and provides the highest degree of competence and likelihood of success in utilizing the theories.

I think it's similar here, where there are multiple points of view that are effective at characterizing wing behavior, and understanding the various points of view is beneficial.   Since few of us here actually design wings, just merely operate them, I don't think it's necessary to fully understand all of it.   Having a functional understanding, and that there may be different points of view, is sufficient for me.

*My favorite example from my own field is the nature of the periodicity of signals within the input window of a Discrete Fourier Transform and how it affects interpreting the output.   This causes nearly religious wars among practitioners.  This kind of thing isn't unusual across technical disciplines.  I still find it amusing. 

[PT20J]

13 hours ago, Blue on Top said:

3. The SAME airfoil cross section will perform differently at different Reynold numbers.  This is an area that separates the engineers from the pilots (innuendo intentional).  If the root chord is 72" and the tip chord is 36", the tip airfoil will stall first (the Rn of the root is 2X the tip). 

Now I can understand this old Peter Garrison article :

Rectangular Wings.docx

Skip

 

[Blue on Top]

@PT20J  Great article.  Manufacturing is definitely easier with a constant chord.  Roll rate is not as good (not mentioned).  Dead weight is heavier than the article eludes to.  Wonderful article..

[Blue on Top]

6 hours ago, 0TreeLemur said:

F5) the wing in Fig. (c) is not producing lift because the airflow is separated from the upper surface, destroying the lift producing streamline curvature seen in (a) and (b).

<Disclaimer> I am an engineering professor with 23 years of experience teaching engineering fluid mechanics.   I am not a rocket scientist.

I totally agree with all, except that 12(c) is not producing lift (may be a definition thing).  Even above stall AOA, wings (even symmetrical airfoils) produce lift (force perpendicular to the relative wind) until near an AOA of 180 degrees.  An airplane with a wing in a similar condition to 12(c) will produce ~0.80 G (Nz-stab ~= 0.80).  Great post! 

[Releew]

This is a great conversation.  Pretty deep!  I have an Engineering background, mostly is thermal so I do not claim to know anything about aeronautical engineering.  But....I think there ideas out there left to prove wrong!  The conversation regarding the cowl flaps is a good one. 

The problem to solve is obviously removing the heat for the engine, and to do that one must allow the static pressure built by thrust generated by the prop over the cylinder fins and reduce the D/P by opening up the cowl flaps....right?  Open them up and let the air flow...pretty simple?  Right?

So why have a mechanical mechanism (cowl flap) that opens.  Seems to me, the creation of a cowl flap that remained closed with an exit in the rear integrating a rounded louver system would create a pressure drop (using air flow) and literally suck the hot air out of the cowl.  

I'm sure this has been considered...right?

 

Rick

 

[PT20J]

Ron @Blue on Top, this is not a Mooney question, but perhaps you can explain how split flaps (DC-3, B-17, C-310) do so well at increasing lift coefficient (I believe they are somewhere between a plain flap and a single-slotted flap). The whole layout seems counterintuitive. And, doesn’t it violate the Kutta condition of circulation theory?

Skip