Is this what they are teaching now??

[shuttlepilot]

I was at the dentist this morning and read this in the May 08 Popular Science magazine,pg10.

Question: ... stated that an airplane rises because air underneath the wing pushes it upwards. But I thought that it was because the wings shape creates low air pressure above it and the wing lifts...
PS reply:This is a common misconception. although the pressure difference you mention does exist (and is described by a physical law known as Bernouilli's principle) it is not nearly large enough to lift an airplane. Instead lift is achieved mostly by a process called "flow turning". As a wing moves forward, it pushes air downward. Per Newton's third law of motion-everey action has an equal and opposite reaction-that push generates a force in the opposite direction, which in this case is upward.

So what happened to the 4 forces of flight? Are there 5 now?

 

[scar1900]

I've heard this before and I would tend to agree. How often is your angle of attack 0?

 

[avbug]

Actually, the traditional explaination of lift has been off the mark for many years. Yes, the pressure differential between the upper surface and the lower surface of the wing accounts for lift, but not because of bernoulli. The reduction in pressure along the upper surface of the wing is fairly small with regard to it's curvature, and the traditional explaination that two molecules starting at the front edge ever meet at the back is completely in error...as is the suggestion that the one over the top must travel faster so that they can meet up at the trailing edge. Fantasy.

Dynamic pressure from beneath the wing, created by a positive angle of attack and upwash ahead of the wing (making a higher local AoA) accounts for lift, and is measured in the downwash; the amount of air displaced and deflected downward; the mass vector of the downwash equates to the lift of the airplane plus download on the horizontal stab.

You may have been taught that ground effect is a cushion of air beneath the wing...also a fallacy that's been popular for many years.

 

[VW Pilot]

Makes sense. Angle of attack may be at zero, but you'll notice most if not all planes have the angle of incidence....that small "installed" angle could or does account for this.

 

[Sig]

http://www.av8n.com/how/htm/airfoils.html

Pretty fascinating stuff, really. Considering the fact that the CFI that wrote this is probably one of the 10 smartest people alive, I'd say his work has some rigor behind it.

 

[VW Pilot]

Oh yeah and on the PPL exams they don' t get this technical....If it is wrong, why isn't the FAA or the companies like Jeppesen, ASA, Gleim doing anything about it? We all earned our certificates on false information.....what's next??

 

[avbug]

The FAA published wrong information for years; just a few years ago some of it was changed and corrected as publications were updated.

We all earned our certificates on false information.....what's next??

Your instructor probably taught you about best glide speed, but never mentioned minimum sink speed. Probably knew nothing about it. There's a lot in flight training that gets passed on incorrectly or not at all. Flight instructors and their students are often part of a heritage of inexperience; one individual who was taught by an inexperienced pilot who was taught by an inexperienced pilot who was taught by...

Too often incorrect information, or just not enough information, gets passed along. Nothing new here.

 

[Sig]

Your instructor probably taught you about best glide speed, but never mentioned minimum sink speed.

DING DING (and a lot of other dings here).

I was never taught this amazing concept. Then... a few years later, my ticket was in jeopardy based on the outcome of my students' ventures airward; I was a CFI all of a sudden. I started really thinking and really researching, and had one HECK of a time teaching my students thereafter. I, oddly, felt like I owed them at the very least that much.

 

[Colonel Savage]

More than a few aircraft have crashed on take off due to contaminated upper surfaces. Part of the pre departure check required by FAR's after deicing is to make sure all "critical surfaces" do not have any frost, snow, or ice adhereing to them. Critical surfaces are always the upper surfaces of wings, stabilizers, and even fuselage. No lower surfaces are ever mentioned. As a matter of fact, the lower surface of the wings are allowed to have frost adhereing to them prior to flight if less than 1/4 inch. This because fuel in the wing tanks will sometimes cool the lower surface of the wing to below dew point and if it's cold enough frost can form.

Seems logical to deduce from all this that there is something very important going on along the upper surface of the wing, whether it involves Bernouilli's or Boundary layer or molecules racing each other to the trailing edge or not. Leading edge slats are even slotted in most cases to help assure that flow.

Delta wing aircraft use high angle of attack to generate lift through something known as "flat plate effect", similar to what "avbug" described. Similar to putting your hand out of a car window at a decent speed and having it move up and down as you rotate your forearm. Delta winged aircraft rarely use slats, most do have flaps.

Ground effect was finally determined to be caused by wing tip vortices, the downward part of the vortex pushing against the ground, and the surviving upward part of the vortex pushing against the lower surface of the wing.

Hope this helps. Enjoy your flying. I sure did!

 

[avbug]

Ground effect is actually a reduction in induced drag owing to a decreased local angle of attack. Whereas downwash is preceded by upwash ahead of the wing, and upwash creates an increase in local angle of attack beyond that established by the free airstream, an aircraft operated close to the ground does not enable the same development of upwash ahead of the wing (and consequently from beneath it), meaning a reduced angle of attack, and reduced induced drag.