Newton's third law to explain lift

[sarm2005]

Ok so I just read this statement as part of an explanation of list using Newton's 3rd law

"The amazing thing about wings is that because they are flying through air which is a fluid, the top of the wing deflects air down as well as the bottom of the wing."

What I don't understand is how the top of the wing deflects air downwards. Anyone care to explain?

 

[Crazy Tosser]

I have no idea what that statement said.

Wings fly because their shape creates overpressure under and underpressure over them.
That's why racing cars have "inverted" wings, to keep them on the ground

http://wings.avkids.com/Book/Animals/Images/wing_diagram.gif

 

[sarm2005]

What you just said is an explanation of lift using Bernoulli's principle. Another explanation is using Newton's 3rd law which is what I'm talking about.

 

[rcgldr]

This website does a descent job of explaining it, with a lot of emphasis on Coanda effect, but towards the end of this web site, there's a diagram of a wind blowing over a roof, and although the air downwind of the roof is turbulent, it's also at lower pressure, due to what some call "void" effect: when a solid object passes through a fluid, or when a fluid passes around a solid object, low pressure "voids" are created because the solid object blocks or diverts the fluid flow away from these low pressure areas.

After visiting a large number of web sites, my conclusion is that lift is a combination of Coanda and "void" effects.

http://user.uni-frankfurt.de/~weltner/Mis6/mis6.html

 

[D H]

Wings fly because their shape creates overpressure under and underpressure over them.

That's a misapplication of Bernoulli's principle to explain lift. This misapplication is unfortunately used quite widely. The attached diagram is simply incorrect. If the plane is to go up (or even stay level), something else must go down. That something else is air: The wings deflect air downward. Lift results from Newton's third law.

How the bottom side of the wing accomplishes this downward deflection is obvious. How the top of the wing contributes to this deflection at all is a lot less obvious. Moreover, in a good wing, it is the top rather than the bottom that does the bulk of the work in deflecting air downward. The link Jeff Reid posted does a nice job of explaining the Coanda effect.

 

[russ_watters]

I would say that the sentence you quoted from Crazy Tosser is correct. The application of that sentence (the diagram he linked) is incorrect. The error is that the diagram shows the air straightening out after passing the wing instead of continuing downward as Newton's third law demands.

Also, just a little note - it is quite common for the lower surface of a wing to contribute nothing at all to the lift being generated by that wing.

 

[rcgldr]

Also, just a little note - it is quite common for the lower surface of a wing to contribute nothing at all to the lift being generated by that wing.

Actually this would be rare. Even in the case of flat bottom air foils which are less efficient, but easier to manufacture, than fully cambered airfoils, the aircraft that use them need a signficant angle of attack to fly. I recall an article mentioning that a Cessna would need a speed around 300mph instead of 150mph to fly with the flat bottom airfoil lower surface horizontal, instead of pitched upwards. Most commercial aircraft fly with a significant angle of attack, especially if they are near capacity, getting some amount of lift from the fuselage itself. The pitched up attitude is enough that you can feel it in the seats, and it's clear that the snack or meal carts need their brakes in order to keep from rolling to the back of the aircraft.

For normal air foils, most, but not all, of the lift force comes from above the wing, with a few exceptions like this lifting body prototype:

M2-F2 glider with F104 chase plane:
m2-f2.jpg

M2-F3 rocket powered model (reached a speed of Mach 1.6) with B52:
m2-f3.jpg

 

[mordechai9]

First of all, aircraft wings are usually fixed to the body at a slight positive angle. This is referred to as the wing's angle of attack. If you look at an aircraft sitting flat on the runway, with the cockpit facing left, you should be able to see this. The wing is actually turned clockwise a few degrees; the wing is pitched up, even when the aircraft body is flat. They do this because typical airfoil/wing geometries generate lift only when the wing sits at a positive angle of attack. Thus, by affixing the wing to the aircraft in this way, the aircraft will generate lift even when the body is sitting flat on the runway.

Now, imagine the aircraft on the runway again, with the cockpit facing to the left, as before. As the aircraft takes off, the airflow hits the wing, and "attaches" to it as it flows around it, assuming that the wing is properly designed and the flow remains laminar. Laminar flow basically just means that the airflow goes smoothly and continuously around the wing. From this, and considering how we just described the positive pitch of the wing, it's easy to see that the region of flow approaching the wing from the front turns downwards as it follows the shape of the wing. Hence, the top of the wing deflects the air downwards, but it's not like it's the top of the wing acting alone; it's really the entirety of the wing shape and geometric composition.

 

[russ_watters]

Actually this would be rare.

I'm not so sure. Here's the data for a 172: http://www.prism.gatech.edu/~gtg635r/Lift-Drag%20Ratio%20Optimization%20of%20Cessna%20172.html

Here's a picture of the airfoil: http://www.windmission.dk/workshop/BasicBladeDesign/naca.html

It's not flat on the bottom, it's curved, which means that at zero AoA the bottom surface has a negative contribution to the lift. By my calculation, with a .25 CL at 0 AoA, and a cruise weight of 2000 lb, it would need to be flying at 133 mph. Cruise speed for the 172 is 130mph. This is likely by design, as the minimum Cd tends to occur near 0 AoA.

Also: this link appears to show that at 6 degrees AoA, the bottom surface produces roughly zero lift (due to the velocity averaging 1): http://www.mh-aerotools.de/airfoils/velocitydistributions.htm

 

[rcgldr]

By my calculation, with a .25 CL at 0 AoA, and a cruise weight of 2000 lb, it would need to be flying at 133 mph.

I get:
((.25-.053)/4.56)*(180/pi) = 2.475 degrees AoA.

This link appears to show that at 6 degrees AoA, the bottom surface produces roughly zero lift (due to the velocity averaging 1): http://www.mh-aerotools.de/airfoils/velocitydistributions.htm

There's signifcant area below the 1. I updated the picture with a line going across v=1:

velo6.gif

 

[Cyrus]

I'm not so sure. Here's the data for a 172: http://www.prism.gatech.edu/~gtg635r/Lift-Drag%20Ratio%20Optimization%20of%20Cessna%20172.html

Here's a picture of the airfoil: http://www.windmission.dk/workshop/BasicBladeDesign/naca.html

It's not flat on the bottom, it's curved, which means that at zero AoA the bottom surface has a negative contribution to the lift. By my calculation, with a .25 CL at 0 AoA, and a cruise weight of 2000 lb, it would need to be flying at 133 mph. Cruise speed for the 172 is 130mph. This is likely by design, as the minimum Cd tends to occur near 0 AoA.

Also: this link appears to show that at 6 degrees AoA, the bottom surface produces roughly zero lift (due to the velocity averaging 1): http://www.mh-aerotools.de/airfoils/velocitydistributions.htm

That data is a bit off. A 172 cruises at 105KTS ~120mph, NOT 120KTS.

At 120Kts, your 7kts away from the maximum structural crusing speed. Use a max weight of 2550lbs for your calculation.

 

[Cyrus]

First of all, aircraft wings are usually fixed to the body at a slight positive angle. This is referred to as the wing's angle of attack. If you look at an aircraft sitting flat on the runway, with the cockpit facing left, you should be able to see this. The wing is actually turned clockwise a few degrees; the wing is pitched up, even when the aircraft body is flat. They do this because typical airfoil/wing geometries generate lift only when the wing sits at a positive angle of attack. Thus, by affixing the wing to the aircraft in this way, the aircraft will generate lift even when the body is sitting flat on the runway.

Now, imagine the aircraft on the runway again, with the cockpit facing to the left, as before. As the aircraft takes off, the airflow hits the wing, and "attaches" to it as it flows around it, assuming that the wing is properly designed and the flow remains laminar. Laminar flow basically just means that the airflow goes smoothly and continuously around the wing. From this, and considering how we just described the positive pitch of the wing, it's easy to see that the region of flow approaching the wing from the front turns downwards as it follows the shape of the wing. Hence, the top of the wing deflects the air downwards, but it's not like it's the top of the wing acting alone; it's really the entirety of the wing shape and geometric composition.

Careful, that's the angle of incidence. Not attack.

 

[rcgldr]

Since I didn't edit the post in time, I'm reposting:

By my calculation, with a .25 CL at 0 AoA, and a cruise weight of 2000 lb, it would need to be flying at 133 mph.

Using the formula from that link I get:
((.25-.053)/4.56)*(180/pi) = 2.475 degrees AoA.

A NACA 2412 wing with no washout would need less AoA to produce the same CL. The article also states the washout backwards, as it's the tips that get a negative AoA relative to the root. I don't know what the root incidence is.

This link appears to show that at 6 degrees AoA, the bottom surface produces roughly zero lift (due to the velocity averaging 1): http://www.mh-aerotools.de/airfoils/velocitydistributions.htm

There's signifcant area below the 1. I updated the picture with a line going across v=1:

velo6.gif

At 2.475 degrees, there would be very little lift generated below the wing. I'm not sure how much increase in AoA would be required to cruise at a more reasonable (than sea level) 5000 feet.

Commercial airliners cruise at over 30,000 feet, and at a pretty significant AoA, although I haven't been able to find exact numbers.

 

[russ_watters]

I get:
((.25-.053)/4.56)*(180/pi) = 2.475 degrees AoA.

What is that calculation? I'm using the lift equation to show how much lift you get at 0 AoA: Cl = Lift / ((1/2) * (rho) * ((velocity) ^ 2) * s)

.25=2000/(.5*rho*v^2*s)
.25=2000/(.5*.002377*v^2*175)
v^2= 196 fps = 134 mph

(I used a crusie weight of 2000lb, though Cyrus is saying I should use higher)

There's signifcant area below the 1. I updated the picture with a line going across v=1:

velo6.gif

You're right, I was thinking upside-down with that one.

 

[Cyrus]

To Russ: Your speed is too fast if your using 120kts. You have to use 105kts. (I would also use the density of air at 5000 feet, not sea level).


To Jeff: Angle of incidence at the root is what you have been mislabeling 'angle of attack'

http://www.centennialofflight.gov/essay/Dictionary/angle_of_incidence/DI6G1.jpg

 

[Cyrus]

What is that calculation? I'm using the lift equation to show how much lift you get at 0 AoA: Cl = Lift / ((1/2) * (rho) * ((velocity) ^ 2) * s)

.25=2000/(.5*rho*v^2*s)
.25=2000/(.5*.002377*v^2*175)
v^2= 196 fps = 134 mph

(I used a crusie weight of 2000lb, though Cyrus is saying I should use higher) You're right, I was thinking upside-down with that one.

Thats not going to be very accurate though. You are ignoring the fact that the fuselauge is creating lift, and that the tail is creating negative lift.

 

[rcgldr]

Angle of incidence at the root is what you have been mislabeling 'angle of attack'.

I don't know what the incidence is, only that the article states that there is 3 degrees of wash-out, and the conflicted itself by stating incidence was -3 at the root and 0 at the tips, which would be wash-in, so this is wrong.

 

[russ_watters]

Here's a link that discusses the issue and lists a plane with a 115kt cruise speed and a 4.5 degree AoA: http://www.av8n.com/how/htm/aoa.html

Note: this link uses the zero-lift definition of AoA, not the geometric definition. Using the previous link, the zero lift point is about -2 deg AoA for that wing, making the geometric AoA for the above link about 2.5 deg at cruise. Given cyrus's clarification of the performance, I'm willing to accept 2.5 deg instead of 0 for cruise.

Unfortunately all we've established is that somewhere above 0 deg and below 6 deg, the lower surface's contribution is 0, but some planes cruise with AoA's to the lower end of that.

In any case, we're getting a little specific about our scenario. The point that led to this little side argument just said that it was "common". I didn't specify cruise or max gross weight or anything. The point is that there are a lot of scenarios where a signficant amount of lift can be generated by the wing with no contribution by the lower surface.

 

[russ_watters]

Thats not going to be very accurate though. You are ignoring the fact that the fuselauge is creating lift, and that the tail is creating negative lift.

We're getting too bogged down in specifics here anyway. All I was trying to accomplish with the last sentence in post 6 was to point out that there are lot of scenarios where the lower surface of a wing could contribute nothing to the lift generated by the wing.

 

[Cyrus]

I don't know what the incidence is, only that the article states that there is 3 degrees of wash-out, and the conflicted itself by stating incidence was -3 at the root and 0 at the tips, which would be wash-in, so this is wrong.

incidence is the angle the wing makes with the fuselage.

 

[Cyrus]

That's a misapplication of Bernoulli's principle to explain lift. This misapplication is unfortunately used quite widely. The attached diagram is simply incorrect. If the plane is to go up (or even stay level), something else must go down. That something else is air: The wings deflect air downward. Lift results from Newton's third law.

How the bottom side of the wing accomplishes this downward deflection is obvious. How the top of the wing contributes to this deflection at all is a lot less obvious. Moreover, in a good wing, it is the top rather than the bottom that does the bulk of the work in deflecting air downward. The link Jeff Reid posted does a nice job of explaining the Coanda effect.

He simply said overpressure and underpressure, not Bernoulli. In addition, this is correct. Lift is the integral of the pressure forces over the wing in the -z direction of an Earth fixed coordinate system (N.E.D. -North,East,Down).

L=Ncos(\alpha)-Asin(\alpha)

N'=-\int^{TE}_{LE}(p_u cos(\theta)+\tau_u sin(\theta)ds_u+\int^{TE}_{LE}(p_lcos(\theta)-\tau_l sin(\theta)ds_l

A'=-\int^{TE}_{LE}(-p_u sin(\theta)+\tau_u cos(\theta)ds_u+\int^{TE}_{LE}(p_l sin(\theta)+\tau_l cos(\theta)ds_l

You said the flow turns down. How do you think that happens? It causes an increase in pressure at the bottom of the wing.

 

[Stan Butchart]

There are many foils with conditions that average zero pressure change below the wing. With this condition all of the FORCE that supports the wing comes from the ambient static pressure of the atmosphere. Pressure created by gravity. The objective for the flow above the wing is to reduce pressure force. There is no "pull".
I hope that someone can provide some meat for the implication that a downward deflection of air is required to give a lift force. For myself, while downflow is always the result of a passing wing, I cannot find a downflow within the "system" that is a reaction from any force that contibutes to the actual lift.

 

[Cyrus]

There are many foils with conditions that average zero pressure change below the wing. With this condition all of the FORCE that supports the wing comes from the ambient static pressure of the atmosphere. Pressure created by gravity. The objective for the flow above the wing is to reduce pressure force. There is no "pull".
I hope that someone can provide some meat for the implication that a downward deflection of air is required to give a lift force. For myself, while downflow is always the result of a passing wing, I cannot find a downflow within the "system" that is a reaction from any force that contibutes to the actual lift.

B1: Huh? Pressure created by gravity?

B2: Look at a control volume around the entire wing and use the Reyonlds Transport Theorem.

 

[Stan Butchart]

B1 -- The ambient prssure is derived from the force of gravity apon the mass of the atmosphere above.
B2 - Good this is the first hint that I have been given that such a relationship exists.
However you will have to help me out with Reyonlds Transport Theorem.

 

[rcgldr]

All I was trying to accomplish with the last sentence in post 6 was to point out that there are lot of scenarios where the lower surface of a wing could contribute nothing to the lift generated by the wing.

Russ is correct, after more research, this is not as rare as I thought.

Most of my "research" about aerodynamics is due to one of my hobbies, flying radio control gliders. There has been a lot of airfoil design work done for rc glider contest models (F3B, F3J), mostly because more new rc glider models are released per year than full scale aircraft.

Anyway, I keep forgetting that most powered civilian aircraft cruise much faster than best lift to drag ratio speeds, unlike gliders, and at these faster speeds, the AoA is smaller and depending on the airfoil, there are cases where virtually no lift is generated by higher pressure below a wing.

Commercial airliners seem to use a higher AoA than say a twin engine civilian aircraft, probably due to a combination of a relatively heavy load (full passenger load for maximum profit), and high altitudes where jet engine thrust versus drag versus fuel consumed for distances traveled is optimum.

 

[rcgldr]

I hope that someone can provide some meat for the implication that a downward deflection of air is required to give a lift force. For myself, while downflow is always the result of a passing wing, I cannot find a downflow within the "system" that is a reaction from any force that contibutes to the actual lift.

If there is a low pressure area above a wing, it causes air to accelerate towards it from all directions except upwards (and backwards) through the solid wing so the result is a net downwards (and forwards) acceleration of air. If there is a high pressure area below a wing, it causes air to accelerate away from it in all directions except upwards (and backwards) through the solid wing so the result also is a net downwards (and forwards) acceleration of air.

I refer to this link again:

"The physical cause of low or high pressure is the forced normal (perpendicular) acceleration of streaming air caused by obstacles or curved planes in combination with the Coanda-effect.":

http://user.uni-frankfurt.de/~weltner/Mis6/mis6.html

 

[rcgldr]

laminar airflow

A link to John Dreese's web site, parts 4 and 5 discuss how little air flow is laminar over many wings, and how "laminar" air foils increase laminar flow to 30% or more over the chord length of a wing.

In the case of gliders, laminar "bubbles" result in either more drag or less lift so the laminar air flow is deliberately broken up sooner than it normally would via rougher surfaces or turbulators (this is mentioned in the article).

http://www.dreesecode.com/primer/airfoil1.html

"The physical cause of low or high pressure is the forced normal (perpendicular) acceleration of streaming air caused by obstacles or curved planes in combination with the Coanda-effect.":

http://user.uni-frankfurt.de/~weltner/Mis6/mis6.html

This is true for lift related effects, but drag related pressures and accelerations are in the direction of travel, not perpendicular to it. As a simple example, a bus traveling down a highway, or a wing at zero effective angle of attack produce no lift, but generate drag, air is accelerated forwards, and the pressure behind the bus or wing is lower than the pressure in front of the bus or wing.

 

[Holocene]

That data is a bit off. A 172 cruises at 105KTS ~120mph, NOT 120KTS.

Really?

I fly a 172/S, and nearly always cruise at ~119KTS @ 2600RPM.

 

[Cyrus]

Really?

I fly a 172/S, and nearly always cruise at ~119KTS @ 2600RPM.

I fly a 172Q -180HP fixed propeller. Cruise speed is always 100-105KTS. What year is your airplane, and altitude you fly at?

 

[Holocene]

I fly a 172Q -180HP fixed propeller. Cruise speed is always 100-105KTS.

Interesting. The 172/S is 180HP as well.

Although I must admit, 2600RPM is what Cessna defines as a "maximum" cruise.

"Normal" is 112KTS @ 2500RPM.

 

[Holocene]

I fly a 172Q -180HP fixed propeller. Cruise speed is always 100-105KTS. What year is your airplane, and altitude you fly at?

The plane is a 2001 (not mine though). On XC flights, typically 7,500', unless of course I'm headed in a more westerly dirrection. (Only a VFR pilot at the moment).

 

[Cyrus]

Yeah, my plane is from 83'. I am not that high either. Usually around 4,500 feet. That would explain why your going faster.

 

[Holocene]

You're lucky to have your own plane. I would love to be able to afford one.

 

[Cyrus]

You're lucky to have your own plane. I would love to be able to afford one.

Bahahah. Own my own airplane. -sorry, couldn't help it.

 

[rcgldr]

Note: this link uses the zero-lift definition of AoA, not the geometric definition.

I prefer the term "effective angle of attack" since it means zero angle for zero lift, and is independent of the airfoil being used.

Given cyrus's clarification of the performance, I'm willing to accept 2.5 deg instead of 0 for cruise. The point is that there are a lot of scenarios where a signficant amount of lift can be generated by the wing with no contribution by the lower surface.

Which I've already posted that it was more common than I expected, since most of my knowledge on this stuff is glider oriented, where the speeds are near best lift to drag ratio, as opposed to the much faster cruise speeds of civilian aircraft.

I also mentioned the rare case where most of the lift is produced from under the airfoil with the m2-f2 type flying bodies.

Higher AoA becomes the norm for commercial airliners, because of their load factor (more passengers means more profit), and their high altitudes (best fuel milage with jet engines).

 

[Stan Butchart]

Re:downflow ( the following is contingent upon Cyrus not shooting me down!)
Newton only requires a ballance of forces for lift. The only directionality in fluid pressure is in the orientation of surfaces exposed to it.
We look at flow from the standpoint of the wing for purposes of calculation. In almost all cases what is happening must be seen from the remote still air. The basic displacement flow is essentually circular. The circular path of circulation directs most of the displaced air over the top. The flow following the top surface travels a 360deg path with a forward displacement.
The Bernoulli energy equation is from relative tangential acceleration between particles. Weltner and Graig prefer centrifugal pressure from the particles in normal acceleration. (Personnaly I could not get big enough numbers.) Both of these accelerations occur within the curving flow.
In the Bernoulli case all of the pressure reduction would be created where the flow has an upward component. Where the flow has a downward component the reduced pressure is being "destroyed", if you will. In the centrifugal case, half of the pressure reduction would occur with upwards and half with the downward components.
High pressure air cannot flow to low pressure areas unless the low pressure air has somewhere to go. If separation is not present, the air at the trailing edge will have a forward vector.
This represents my quandry.

 

[Cyrus]

Re:downflow ( the following is contingent upon Cyrus not shooting me down!)
Newton only requires a ballance of forces for lift. The only directionality in fluid pressure is in the orientation of surfaces exposed to it.
We look at flow from the standpoint of the wing for purposes of calculation. In almost all cases what is happening must be seen from the remote still air.

I don't understand this, and I don't agree with it if your saying what I think your saying.

The basic displacement flow is essentually circular. The circular path of circulation directs most of the displaced air over the top. The flow following the top surface travels a 360deg path with a forward displacement.

Again, I don't follow you. What do you mean circular?

The Bernoulli energy equation is from relative tangential acceleration between particles. Weltner and Graig prefer centrifugal pressure from the particles in normal acceleration. (Personnaly I could not get big enough numbers.) Both of these accelerations occur within the curving flow.

Again, I don't follow.

In the Bernoulli case all of the pressure reduction would be created where the flow has an upward component. Where the flow has a downward component the reduced pressure is being "destroyed", if you will. In the centrifugal case, half of the pressure reduction would occur with upwards and half with the downward components.
High pressure air cannot flow to low pressure areas unless the low pressure air has somewhere to go. If separation is not present, the air at the trailing edge will have a forward vector.
This represents my quandry.

Still not following you...

 

[Stan Butchart]

"In almost all cases, <to understand> what is happening it must be seen from the remote still air." Poor sentence. Pressure change comes from the relationship of air to air, not air to surface.

"What do you mean circular? " Let's take the 2d cylider as a model. We look at it from the remote still air in an ideal fluid so that separation is not present. The instantainious source/sink picture is made up of pure circles. As the cylinder moves forward thr air in front is accelerated forward. The air following the surface traces a cursive "e" path through space ,top and bottom. This is a full 360deg path with a cosiderable forward displacement.

"Both of these accelerations occur within the curving flow." Pressure changes occur within accelerations. Tangential relative accelerations occur because each air particle has its own individual flowpath. The normal accelerations are the "centrifical force" in the curving flow. I stick with tangential but you can pick your own poison.

"In the Bernoulli case all of the pressure reduction would be created where the flow has an upward co..." One half of wing lift occurs on the front half to third of the wing. Except for under the L.E. allof the flow here has an upwards component. This extends out for many,many chord lengths. Aft of here, where the flow all has a downward component, pressure is constantly increasing.

 

[Cyrus]

Im sorry, I still can't understand what your trying to say. Can you make it clearer please.

If your talking about flow circulation, that's a mathematical construct. The air is NOT doing circles in the actual flow field. All the air is deflected downstream and down to creat lift on the airfoil.

 

[Stan Butchart]

What I am trying to say is that I cannot find a dedicated downflow that is a reaction to lift force. This is not argument, I simply do not see it. The required pressures are produced within curving flows where the verticle components of acceleration are equally up and down and the horizontal components contribute as much as the verticle. Unless one doubts lifttig line the downflow of the vortex ribbon is a charactoristic of parallel vortexes and not a reaction to any lifting force.

If I find downflow, a specific momentum, it will be the product of a specific pressure force and I am back to ballancing weight with static pressures.

Like I say, this is only a search. For hundreds of statements about deflected air no one has ever offered the application to the forces against the wing.
No circulation is not a rotating flow but its effect applies equally around the whole wing.

Then there is always the balloon.

 

[rcgldr]

What I am trying to say is that I cannot find a dedicated downflow that is a reaction to lift force.

A wing (producing lift) accelerates air downwards (and forwards), and the air reacts to this acceleration with an upwards (and backwards) force. These accelerations coincide with pressure differentials, lower above and behind a wing, higher below and in front of a wing.

Above a wing, Coanda and what some call "void" effect, cause the air to (mostly) follow the convex upper surface of a wing (during the transition from laminar to turbulent flow, which occurs on almost all wings, there is a "bubble" of separation of flow from the wing). The curvature of the air above the wing results in a lower pressure above the wing and a net downwards acceleration of air, and the cross-section of the affected air extends well above the wing, it's not just a near surface effect. Below a wing, the air is simply deflected downwards (assuming a typical air foil), increasing the pressure and more net downwards acceleration of air. The drag related effects cause pressure differentials in front and behind a wing with a net forwards acceleration of air.

 

[Count Iblis]

So, given the Navier Stokes equations, what is the optimal shape of a glider if we fix the mass and minimize the descent angle?

 

[rcgldr]

So, given the Navier Stokes equations, what is the optimal shape of a glider if we fix the mass and minimize the descent angle?

Don't know about the equations, but the optimal shape is simply a huge wing span with narrow chord, in otherwords a huge aspect ratio (as pointed out by Fred Garvin in another thread), combined with good airfoils (as I pointed out in the same thread). The result is 60 to 1 glide ratios at around 60mph in open class cross country gliders. The wingspans are huge, 80 feet or more. The 15 meter (just under 50 feet) class gliders get up to 50 to 1 glide ratios. Aerobatic gliders are around 35 to 1.

Gliders with 60 to 1 glide ratios.

http://www.sailplanedirectory.com/PlaneDetails.cfm?planeID=28

http://www.sailplanedirectory.com/PlaneDetails.cfm?next=118

http://www.sailplanedirectory.com/PlaneDetails.cfm?next=274

http://www.sailplanedirectory.com/PlaneDetails.cfm?next=277

Wiki link to Nimbus 4
Schempp-Hirth Nimbus 4 Wiki.htm

Official site for Nimbus 4, with photos:
http://www.schempp-hirth.com/index.php?id=nimbus-4dm0&L=1

The ETA is a motorized prototype, with a 101 foot wingspan, quite a few photos here:
http://www.eta-aircraft.de

For radio control gliders the shape is basically a skinny pole with a wing and a tail:

jrartms.wmv

 

[Cyrus]

What I am trying to say is that I cannot find a dedicated downflow that is a reaction to lift force. This is not argument, I simply do not see it. The required pressures are produced within curving flows where the verticle components of acceleration are equally up and down and the horizontal components contribute as much as the verticle. Unless one doubts lifttig line the downflow of the vortex ribbon is a charactoristic of parallel vortexes and not a reaction to any lifting force.

If I find downflow, a specific momentum, it will be the product of a specific pressure force and I am back to ballancing weight with static pressures.

Like I say, this is only a search. For hundreds of statements about deflected air no one has ever offered the application to the forces against the wing.
No circulation is not a rotating flow but its effect applies equally around the whole wing.

Then there is always the balloon.

Im sorry, I still don't understand a word your saying. Its as if your throwing in physics words into a sentence and hoping something sensible will come out. Is english your first language?

I mentioned the raleigh transport theorem on directed flow and force analysis.

I don't have a clue what your talking about. Is your background aerospace engineering?

 

[Stan Butchart]

Yes, after rockets and spacecraft I spent twenty years configurating tactical combat aircraft.
Guys, I guess I should bail on this. I am looking for help IDENTIFYING A NET downflow (other than induced alpha) and am unable to get that across.

Is there a sight that might provide the gist of the raleigh transport theorem?

 

[Cyrus]

I would just grab any old fluid mechanics textbook.


Sorry, its Reynolds not raleigh.

 

[rcgldr]

One half of wing lift occurs on the front half to third of the wing. Except for under the L.E. allof the flow here has an upwards component. This extends out for many,many chord lengths.

This conflicts with every source of information about wings that I have found. In all the articles I've seen the air flow is downwards for the rear 1/2 to 2/3rds of an wing, both above and/or below the wing, depending on the airfoil.

Aft of here, where the flow all has a downward component, pressure is constantly increasing.

This transition occurs above a wing, not behind the wing, and produces a small bubble during the transition from laminar to turbulent flow. I've already posted links to web sites that mention this.

"All airfoils must have adverse pressure gradients on their aft end. The usual definition of a laminar flow airfoil is that the favorable pressure gradient ends somewhere between 30% and 75% of chord."

http://www.aviation-history.com/theory/lam-flow.htm

For non-laminar airfoils, this transition from acceleration+decreasing pressure to deceleration+increasing pressure occurs even sooner along the chord.

Here's a picture and link to another web site:

http://www.dreesecode.com/primer/p4_f003.jpg
http://www.dreesecode.com/primer/airfoil4.html

I cannot find a dedicated downflow that is a reaction to lift force.

I prefer to think of lift force as an inertial reaction to the downwards acceleratoin of air. Perhaps the links I just posted will help point out the fact that downward air flow occurs while still flowing along the chord of a wing, and is not delayed for "several" chord lengths as you suggested.

If the frame of reference is the air itself, then as a wing passes through a volume of air, there is downwards and forwards acceleration of air, corresponding to reactive lift and drag forces.

A model flying or gliding inside a sealed box is a good means to "prove" that wings are air pumps. As long as the center of mass of the sealed box, air, and model are not accelerating vertically, then the weight of this system remains constant, regardless if the model is resting at the bottom of the box, or if flying or gliding within the box. In the case that the model is resting in the box, then the weight of the air creates a pressure differential that decreases with height so that the net downforce of the pressure differential exactly equals the weight of the air, while the model exerts it's weight directly on the bottom of the box. In the case that the model is flying or gliding, then the model increases the pressure differential within the box so that the net downforce due to the pressure differential exactly equals the weight of the model and air inside the box; therefore the models lifting surfaces are effectively air pumps.

 

[Stan Butchart]

Cyrus - My Shames makes no reference.Do you have a more specific reference? Is this our old Reynolds frome Reynolds Number?.

Jeff - Note that Laminar/Terbulent flow is a different subject than a Laminar/Turbulent boundry layer. To describe the basic way in which lifting pressures are derived they consider the flow outside of the boundry layer. The downflow along the wing is not greater than the upflow at the front part of the wing. The upper pressure reduction is CREATED in the upflow.
I'll stuggle with the box.

 

[rcgldr]

The downflow along the wing is not greater than the upflow at the front part of the wing. The upper pressure reduction is CREATED in the upflow.

The downflow doesn't occur just along the wing, it occurs well above and/or below the wing as well (depending on the airfoil). The upper pressure reduction occurs because of Coanda and "void" effect, which cause pressure differentials and acceleration of air. Also, it's not the direction of the flow that counts, it's the direction of the acceleration, which is downwards, the flow curves downwards.

There's no way around the "model flying in a sealed box proof" that wings are "air pumps", as it's a closed system. The classic anecdotes for this are a closed truck full of birds and a guy banging on the truck to get the birds to fly so the truck "weighs" less, or the question about the weight of a plane if birds are flying inside the plane.

 

[Cyrus]

Cyrus - My Shames makes no reference.Do you have a more specific reference? Is this our old Reynolds frome Reynolds Number?.

Jeff - Note that Laminar/Terbulent flow is a different subject than a Laminar/Turbulent boundry layer. To describe the basic way in which lifting pressures are derived they consider the flow outside of the boundry layer. The downflow along the wing is not greater than the upflow at the front part of the wing. The upper pressure reduction is CREATED in the upflow.
I'll stuggle with the box.

Try fluid mechanics by Munson Young Okishi.