Newton's third law to explain lift

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

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

incidence is the angle the wing makes with the fuselage.

Cyrus

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

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

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

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

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

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

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

Really?

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

Cyrus

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

Holocene

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

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'. Im 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

Bahahah. Own my own airplane. -sorry, couldnt help it.