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Newton's third law to explain lift

by sarm2005
Tags: explain, lift, newton
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russ_watters
#19
Apr22-08, 11:12 PM
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Quote Quote by Cyrus View Post
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
#20
Apr22-08, 11:25 PM
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Quote Quote by Jeff Reid View Post
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
#21
Apr22-08, 11:54 PM
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Quote Quote by D H View Post
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).

[tex]L=Ncos(\alpha)-Asin(\alpha)[/tex]

[tex]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[/tex]

[tex]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[/tex]

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
#22
Apr22-08, 11:55 PM
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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
#23
Apr23-08, 12:19 AM
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Quote Quote by Stan Butchart View Post
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
#24
Apr23-08, 12:30 PM
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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
#25
Apr23-08, 01:15 PM
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Quote Quote by russ_watters View Post
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
#26
Apr23-08, 01:29 PM
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Quote Quote by Stan Butchart View Post
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
#27
Apr23-08, 03:07 PM
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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
#28
Apr23-08, 06:37 PM
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Quote Quote by Cyrus View Post
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
#29
Apr23-08, 06:38 PM
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Quote Quote by Holocene View Post
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
#30
Apr23-08, 06:41 PM
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Quote Quote by Cyrus View Post
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
#31
Apr23-08, 06:48 PM
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Quote Quote by Cyrus View Post
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
#32
Apr23-08, 06:53 PM
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Yeah, my plane is from 83'. Im not that high either. Usually around 4,500 feet. That would explain why your going faster.
Holocene
#33
Apr23-08, 06:58 PM
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You're lucky to have your own plane. I would love to be able to afford one.
Cyrus
#34
Apr23-08, 06:59 PM
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Quote Quote by Holocene View Post
You're lucky to have your own plane. I would love to be able to afford one.
Bahahah. Own my own airplane. -sorry, couldnt help it.
rcgldr
#35
Apr23-08, 08:01 PM
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Quote Quote by russ_watters View Post
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
#36
Apr24-08, 01:36 PM
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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.


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