One simple question...

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MrManH

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MrManH
...with a complicated answer.

Hi everyone,

In short, what actually causes air to accelerate over the top of a wing?

In long, the often taught "equal transit time" theory that airflow at the top travels a longer distance and therefore must accelerate to reconnect to the bottom flow is incorrect. This is easily disproven by the fact that symmetrical airfoils with no camber do generate lift (although not all lift is the result of Bernoulli's principle), and also this video:

As the airflow splits at the stagnation point, it accelerates over the top surface up to the minimum pressure point which is usually found near the wing's maximum thickness/curvature point at low AOAs. There is definitely a relationship between the geometry of the airfoil and the velocity of the air traveling over it, but why?

The internet and books are full of contradicting information and yet this is one of the fundamentals of lift.

Thanks!
 
MrManH said:
...with a complicated answer.

Hi everyone,

In short, what actually causes air to accelerate over the top of a wing?

In long, the often taught "equal transit time" theory that airflow at the top travels a longer distance and therefore must accelerate to reconnect to the bottom flow is incorrect. This is easily disproven by the fact that symmetrical airfoils with no camber do generate lift (although not all lift is the result of Bernoulli's principle), and also this video:

As the airflow splits at the stagnation point, it accelerates over the top surface up to the minimum pressure point which is usually found near the wing's maximum thickness/curvature point at low AOAs. There is definitely a relationship between the geometry of the airfoil and the velocity of the air traveling over it, but why?

The internet and books are full of contradicting information and yet this is one of the fundamentals of lift.

Thanks!
Click to expand...

No contradicting information here. I dunno, but someone will be along soon with the actual answer:D
 
I searched this forum and came across some good threads about the boundary layer, laminar vs turbulent airflow etc but the information that I've failed to find is why air begins accelerating in the first place. I hope someone does come along with an answer :)
 
"Equal transit time" is bunk. When moving air hits a solid object, the pressure builds up, causing the air to accelerate.
 
Bullocks.

As long as your A&P sprinkles sufficient fairy dust in the wings every annual, you’ll be fine.

I ask him to put a touch extra on the leading edges of the propellor (a little known technique). Keeps efficiency high.

Keep your chem trail tank full.


Sent from my iPad using Tapatalk
 
Dana said:
"Equal transit time" is bunk. When moving air hits a solid object, the pressure builds up, causing the air to accelerate.
Click to expand...

Actually, its Newton's 3rd law, forward and backward (or top and bottom). Pressure builds under the wing, causing upward motion. Because pressure is higher below the wing, and because Bernoulli is a really smart guy, air molecules above the wing must speed up. Sounds like the chicken or egg scenario. But, its all Newton.
 
I agree with the Newton theory but have it heard this way:
The wing accelerates a mass of air downwards. The equal and opposite reaction is the acceleration of the wing (and attached airplane) upwards.
 
Difference in air pressures create lift.
 
Is the air on top moving faster or is the air on the bottom moving slower? To accelerate the air on top energy must be added but from what?
 
Dana said:
"Equal transit time" is bunk. When moving air hits a solid object, the pressure builds up, causing the air to accelerate.
Click to expand...

Maybe I'm mixing up some concepts here but the increase in pressure should slow the airflow as it increases skin friction drag. This is what happens in the adverse pressure gradient of the wing where as pressure increases, skin friction increases causing the airflow to slow down and potentially reverse causing airflow separation. This goes back to Bernoulli's principle that accelerating a fluid decreases its pressure and temperature, so conversely increasing its pressure should decrease its velocity.
 
It does, at the stagnation point, but the increased pressure pushed on the air farther away from the stagnation point, causing it to accelerate.
 
Let'sgoflying! said:
I agree with the Newton theory but have it heard this way:
The wing accelerates a mass of air downwards. The equal and opposite reaction is the acceleration of the wing (and attached airplane) upwards.
Click to expand...

This is correct but accounts for a different lift mechanism. My question isn't how lift is produced but what causes airflow to accelerate over an airfoil in the first place, as seen in the video I posted.
 
MrManH said:
This is correct but accounts for a different lift mechanism. My question isn't how lift is produced but what causes airflow to accelerate over an airfoil in the first place, as seen in the video I posted.
Click to expand...

Ive always heard it was the low pressure caused by the shape of the airfoil drawing, or sucking the air into the relative void. Different air pressures always seek to equalize so the air at the high pressure place moves, or accelerates to get to the low pressure place.
I posted the Newton theory because I have been told we need to rethink how lift is generated and the idea of the low pressure above/high below is not the major way lift is generated; it's the acceleration of a mass and the subsequent reaction.
 
Let'sgoflying! said:
Ive always heard it was the low pressure caused by the shape of the airfoil drawing, or sucking the air into the relative void. Different air pressures always seek to equalize so the air at the high pressure place moves, or accelerates to get to the low pressure place.
I posted the Newton theory because I have been told we need to rethink how lift is generated and the idea of the low pressure above/high below is not the major way lift is generated; it's the acceleration of a mass and the subsequent reaction.
Click to expand...

I agree, Bernoulli's principle has been overemphasized while "Newtonian" lift is a greater factor, at least according to some scientific opinions I've read.

It sounds like a chicken or egg kinda thing. Is the high speed causing the low pressure or the low pressure causing the high speed?

Dana said:
It does, at the stagnation point, but the increased pressure pushed on the air farther away from the stagnation point, causing it to accelerate.
Click to expand...

Is what you're saying related to Let'sgoflying's post in any way? How would you describe the forces and pressure changes involved in this timeline:
-Freestream air reaches the stagnation point
-Airflow streamlines are forced to flow over or below the airfoil
-Upper-air accelerates
-Upper-air reaches the minimum pressure point. It can't be a coincidence that this is located where the wing's geometry is the "thickest"
 
Funny you should ask.
Whatever you were taught by your flight instructor was probably a fairy tale.
Equal transit is total ********.You can use this fairy tale to prove that aircraft can't fly
looks like a venturi is ********. You can use this fairy tale to prove that many aircraft can't fly.
Curved on top vs. flat on the bottom is ********. This should be obvious from looking at actual aircraft wings.
Newton vs. Bernoulli is ********. Newton explains 100% of the lift, while Bernoulli explains 100% of the lift. Getting back and forth between Newtons second law and Bernoulli's equation only takes about a page of algebra (first video below). Newton's laws are very generic, Bernouilli is very specific.

Air flows along the surface - both top and bottom (unless you are stalled). This results in a net downwards velocity to the air flowing off the trailing edge (relative to the air well in front of the wing). That means a net acceleration downwards of the air that goes both above and below the wing. Per Newton, Force = Mass * acceleration - in order to have accelerated the air downwards, there had to be a net force - this means that there has to be a lower pressure on the top of the wing (on average) and a higher pressure on the bottom of the wing (on average) to get the air above and below to accelerate. Now, ain't no such thing as a free lunch. So, these changes in pressure result in changes in velocity above (on average, faster) and below (on average slower) per Bernoulli.
 
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Beats me. I dunno know how it works, just happy it does. Joking aside, understanding it in detail doesn't really matter much; just keep the AoA below the critical angle, and the wing will fly.
 
Let'sgoflying! said:
I have been told we need to rethink how lift is generated and the idea of the low pressure above/high below is not the major way lift is generated; it's the acceleration of a mass and the subsequent reaction.
Click to expand...
Two sides of the same coin. Can't have one without the other.
 
Bees can't fly. Planes can't either. Somewhere today, somebody did a spin in a plane, proving that planes can't fly. Oh, yes, the pilot recovered. But that is because he took the blue pill. Such discussions will lead to the complete death of GA. And, you'll only have yourselves to blame.

The fairy dust answer was nearest the truth.

You've been warned.
 
So how does Snoopy's Dog House fly ?
Not much of an air foil there.
Model aircraft pattern ships fly very well but not as stable as conventional wing.
 
Southpaw said:
So how does Snoopy's Dog House fly ?
Not much of an air foil there.
Model aircraft pattern ships fly very well but not as stable as conventional wing.
Click to expand...
I heard Snoopy screaming, "Curse you Bernoulli!" and "Darn you Newton!" (Snoopy is an inclusive soul) as his doghouse spiraled ever so gently to the ground, and he used his helicopter ears to drift slowly down next to it.

(True story)
 
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Flow turning. Fidkowski explains it eloquently as I have seen. I forgot about his vid, thanks Cap'n.
 
Southpaw said:
So how does Snoopy's Dog House fly ?
Not much of an air foil there.
Model aircraft pattern ships fly very well but not as stable as conventional wing.
Click to expand...

I thought the doghouse is just a box kite with propulsion mounted inside and a little exta decoration added to it. Still an airfoil.
 
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