Benelli lift theory under question

Discussion in 'All Things Boats & Boating' started by Frosty, Dec 5, 2012.

1. FrostyPrevious Member

This wing has no lift without angle of attack. I was dicusing this the other day with an airplane engineer who said the Benellis theory is becoming controversial and is being questioned as angle of attack is such that there is not lift without it.

Therefore it is angle of attack that gives lift and not the upper face having faster speed supposedly reducing pressure and lift

Fig 3 Common depiction of airflow over a wing. This wing has no lift.

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PARYacht Designer/Builder

An asymmetric wing, such as the one you've shown does produce lift at a zero angle of incidence, in spite of your concerns. BTW Benelli is not the principle you want, but is a semi automatic shotgun mechanism. As to the "Benellis theory", well this one's got me - I have no idea, but assuming a double typo what you may be referring to is Bernoulli's principle.

The real problems arise when trying to explain some of these principles to novices or those not well versed in physics. Several descriptions, describing lift generation can be offered (wings, props, etc.), some aren't entirely accurate and/or can be overly simplistic, if not out right incorrect. The debate has raged for generations. It often just boils down to what you "buy" into: Bernoulli's principle or Newton's laws of physics. In plain truth, both are relevant and appropriately describe lift generation. Some descriptions use the Bernoulli principle to direct motion mechanics (kinematics) to the flow-induced pressures. Most cases of incorrect (or partially correct) explanations relying on the Bernoulli principle, the mistakes or misunderstandings occur in assumptions on motion mechanics and how these are produced. It is not the Bernoulli principle that is questioned, because this principle is well established.

4. FrostyPrevious Member

Not anymore , new stuff coming all the time Like PVC exhausts.

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

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daiquiriEngineering and Design

Lift theory is well understood by those who are required to understand it. Much more difficult to explain to those who have little grasp in physics and math. Therefore some "light" versions are in use for the purpose of explaining the physical facts to the general public. Like the Bernoulli principle and the path length.
Hence, it is imo much more probable that you have misinterpreted the words of your friend aero engineer, who presumably knows the lift theories very well.

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murdomackNew Member

There is more than one principle at work here, surely. There is lift generated by the airflow over the wing form and there will be more lift generated from the deflection downwards of the air by the angle of attack.
Bernoulli's principle is used in many things. A regular bore ball valve is smaller than the pipe it is placed in and thus has less internal pressure acting at its critical parts,

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PARYacht Designer/Builder

This pretty much covers it, though (again) you do have to have a clue about the physics involved, or you're just going to have problems with the realities. This is way above your pay grade Frosty, you just don't have the equipment to grasp the mathematical models, of the differential equations invoked with Navier-Stokes or the perfect subroutine the Bernoulli principle preforms within.

8. FrostyPrevious Member

Bernoulli and Newton were well before flight , even the write bothers were aware that the principle was flawed

I certainly did not misunderstand my friends Idea that lift is entirely angle of attack ---try taking off without rotate.

How does a plane fly upside down is the question you should ask yourself.

stashes of info on the web

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TANSLSenior Member

Why do planes fly upside down?. Why not drop the moon up there, all alone, without any support?
Sorry for the joke, I could not keep quiet.

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PARYacht Designer/Builder

There's no sense attempting to discuss this with you Frosty, you haven't the foggiest. The only foil that requires incidence is an symmetric one. These can only generate lift with incidence angle. An asymmetric foil will generate lift at zero incidence, though offering some incidence will generate more lift.

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sottorfmember

PAR is absolutely correct. Bernoulli's law for flow is beyond question.

No use trying to explain mechanics of lift generation unless you have studied aerdynamics 101 and under the effect of profile shape, camber, thickness, trailing edge geomtery, Reynolds number etc. on the lift of foils and understand the principle of circulation - which is what generates lift.

12. FrostyPrevious Member

The whole point of the thread is to tell you of the dispute over this well accepted theory.

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daiquiriEngineering and Design

Frosty, in airfoil aerodynamics two basic angles of attack (AoA) are used - a geometric AoA and the aerodynamic AoA. Take a look at this drawing:

The green line shows a free-stream airflow. There are basically two ways of measuring the angle at which the airfoil is set with respect to the free-stream (and vice-versa):
1) The geometric AoA (red color) is measured relative to the chord line, which is the straight line connecting the extreme points of the mean chord line.
2) The aerodynamic angle of attack (blue color) is measured relative to the so-called zero-lift line.​

What is the zero-lift line? It is the line set at an angle relative to the airfoil's chord, such that when it coincides with the freestream, the airfoil lift is equal to zero.

For symmetric airfoils the lift is zero when the chord is parallel to the free stream, so the zero-lift AoA and the geometric AoA coincide for these airfoils.

For non-symmetric (or cambered) airfoils, they are not coincident, because a cambered airfoil will produce lift even when geometric AoA is zero. So the cambered airfoils have to be set at some negative angle to the incoming airflow, in order to have zero lift. This negative angle of attack (measured respect to the chord) is called zero-lift AoA.

So, a general airfoil (either cambered or non-cambered) requires a non-zero aerodynamic AoA in order to produce lift. It is not true for geometric AoA.

Therefore, it all depends on which definition of AoA you are using.

Cheers

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daiquiriEngineering and Design

Precisely, what theory are you talking about?

15. Number4Previous Member

Hi Frosty,
It is not Bernoulli's principle that is becoming controversial.
It is the longer path or equal transit theory. That is, you can not just use Bernoulli's Principle alone to explain a wing in flight.

http://www.av8n.com//how/htm/airfoils.html#fig-90flat03p
"•The wing produces lift “because” it is flying at an angle of attack,
•The wing produces lift “because” of circulation.
•The wing produces lift “because” of Bernoulli’s principle.
•The wing produces lift “because” of Newton’s law of action and reaction."

Some aerofoils will indeed generate lift at zero or even a negative angle of attack.

A carburretor is a classic example of Bernoulli in action.

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