Airfoil lifting force misconception

[Asimov]

The textbook description of how airfoils lift is somewhat flawed. From grade school to pilot schools we are told the Airfoil- shape or path-length explanation of airfoil lift. That is the top of the wing being curved more so air must go farther and faster to meet at the back of the wing resulting in the wing being sucked up a la Bernoulli.Largely incorrect. There is no suction and the parcels of air divided by the leading edge dont meet as such.I assume that a little lift comes from this effect though it isnt the major factor..

The Newtonian or Attack angle explanation is a better description.Air is deflected downward and Newton's 3rd law comes into play.The airfoil is tilted and greates lift by deflection.With elevators the pilot assumes the angle of attack needed to get the desired lift.

Stunt planes have symmetrical airfoils that provide all the lift by deflection. I made model airplanes with wings that were flat on both sides and they flew as well as my friend's stunt planes with airfoils.You can fly upside down with airfoils curved more on the top. I've done that on purpose and accidentally.The math is said to work for both explanations and it is a matter of interpretation.Actually the classical explanation seems O.K. for cambered airfoils with no angle of attack.

 

IMO, however the curved top surface serves more to smooth the air flow over the top by delaying separation of the boundery layer.No reference,, my thoughts.

 

Some supersonic fighters have relatively flat airfoils.

 

Klein-Fogleman airfoils get tremendous lift by means of a parasitic bounderary layer as I recall.There is also the 3-D Vortex shedding

explanation and a coupla others. IMO most paper planes get their lift by deflection and the Klein Fogleman effect.

 

http://.amasci.com/wing/airfoil.html

[Endy0816]

Link is broken, but yeah, this has been discussed a few times here.

[studiot]

Did you have a question in this?

 

All the normal laws of Physics are obeyed in the mechanics of flight.

You can't exclude one.

 

In addition to Newton's and Bernoulli's laws you should also consider

 

The continuity equation

The circulation theorem

The momentum theorem.

[swansont]

The misconception is that the air "packets" meet up, not that Bernoulli's equation is wrong (if it applies). As for "better", it depends on the individual.

 

The main problem as I see it is that for some reason, people get worked up by examples that aren't universally applicable when it comes to explaining lift, but not in general areas of other explanations within physics. And because they can think of situations where the example doesn't work, then it must be wrong. But apparently they're fine with that as long as it doesn't involve airfoils.

[Enthalpy]

Fully agree with you Asimov. The explanation based on different length is wrong from begining to end and goes against most observations, wing profiles and so on.

 

To this, you could add that for the air to push the plane upwards, the plane has to push the air downwards. This is more basic, and safer to apply than for instance Bernouilli which can become tricky.

 

Be reassured that more people have understood it. The real interrogation is why so many people still propagate an obvious nonsense, but hey, that's common in science too.

 

Nasa has a full webpage to tell exactly that: not a path difference but air deflection.

[swansont]

Bernoulli is a statement about conservation of energy. So, if you are within the constraints where the equation is valid, how does it not apply?

[studiot]

It's a while since I said +1 swansont, but that comment on Bernoulli was short and bang on the button.

 

With regard to deflection, this takes place before the airfoil, since air cannot pass through the airfoil itself.

[Asimov]

Studiot: Thanks, a more detailed perspective. Sorta explains Coanda and Bernoulli. IMO what was bad about the old classical theory in school books was the implication all lift was due to Bournelli My scratch built models with thin zero airfoil wings flew pretty well.

 

Familiar with Cole's Law?

 

 

 

Thinly sliced cabbage!

[studiot]

 

Familiar with Cole's Law?

 

 

 

Nat King?

 

If you have studied at flight school you should have been taught that the lift force is often not vertical and the vector diagrams for the different flying conditions (level flight, banking, diving, ascending etc).

 

The sail on a boat generates drive by the same mechanism turned sideways.

Edited August 10, 2015 by studiot

[Asimov]

Studiot: Flight school teaches the essentials you really need to know. Most young men in WW-2 survived flight school in very hard to fly high powered fighters.Ask a pilot about vectors and he may wonder what that has to do with anything. Buy rounds. Flying by the seat of the pants is the favored expression.This can be misleading in bad visibility though.

You need know little about light to turn on a flashlight.

[studiot]

Here is an image from the textbook I was referring to, written by a Fellow of the Royal Aeronautical Society.

 

 

There is also a simple explanation of lift using Bernoulli, but not relying on fluid 'coming back together' on the following pages.

Edited August 11, 2015 by studiot

[Enthalpy]

Bernoulli is a statement about conservation of energy. So, if you are within the constraints where the equation is valid, how does it not apply?

 

I have nothing against Bernoulli. It's just that because it's so easy to misuse, one should be careful with it.

 

As already pointed out by the OP, symmetric and flat profiles do lift, planes fly on their back, air has no reason to spend the same time around the upper and lower faces of the foil, and so on and so forth.

 

So while air is faster at the succion face, this does not result from the profile being convex there.

 

What I wonder about is why misconceptions live for many decades despite simple observation and logic disprove them.

 

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For the people who need an external advice to make their opinion, and need for that a reputable source, I've found again Nasa's webpage:

https://www.grc.nasa.gov/www/k-12/airplane/wrong1.html

it has a "next" button at the bottom to the next theories, one per page.