Why does an airplane fly?

According to my instructor. In a climb it is mostly newton because of the AOA. The air is hitting the wing thus creating a force upward. But in Level or cruise, It is mostly bernouli which is the pressure difference. So its is a combo of both.
 
Sandesh said:
According to my instructor. In a climb it is mostly newton because of the AOA. The air is hitting the wing thus creating a force upward. But in Level or cruise, It is mostly bernouli which is the pressure difference. So its is a combo of both.
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Uh-oh...you're going to raise Seagull's blood pressure!

Your understanding is normal...and I was a CFI for 15 years before I started to take an interest and learn more about aerodynamics. Mostly...because Seagull called me out on my misunderstood and misapplied physics of the subject. Then I took a deeper look at the physics involved...instead of the skin deep gloss over that most pilots get in pilot training.

Most FAA publications, industry journals and pilot training material will re-inforce what you stated above. As a matter of fact, I reviewed my CFI certificate on line this summer...and that's exactly what the author stated about lift production (also what the FAA teaches). But truth be told....an airplane produces lift because of the pressure differential formed due to the shape of the airfoil and how it causes air to flow around it. Not by forces formed by air impacting the surface and bouncing off of it.

Newton's 3rd law is a misapplication and is largely insignificant in lift production until you reach very high altitude, hypersonic flight. Here the air particles hitting the surface and bouncing off become more significant in comparison with the extremely low air pressure at high altitude...and will become a player in force development.
 
Capt. Barry Schiff also says lift is due to a combo of both Newtonian and Bernoulli in his book, The Proficient Pilot.

It discusses in detail air striking the wing from underneath, air accelerating over the upper camber to produce a downward moment off the trailing edge, and the high/low pressure differential theory of static pressure and kinetic energy.
 
meritflyer said:
Capt. Barry Schiff also says lift is due to a combo of both Newtonian and Bernoulli in his book, The Proficient Pilot.

It discusses in detail air striking the wing from underneath, air accelerating over the upper camber to produce a downward moment off the trailing edge, and the high/low pressure differential theory of static pressure and kinetic energy.
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It's no wonder we're all so confused. :) There's an endless array of variations to the story.

I've been reading and asking pointed questions relating to fluid dynamics over the past couple of years...and those analysis discount the effect of bouncing air off the bottom of the wing as "statistically insignificant".
 
I am going to re-read that chapter and post my thoughts later tonight.

We'll get to the bottom of this..
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meritflyer said:
Capt. Barry Schiff also says lift is due to a combo of both Newtonian and Bernoulli in his book, The Proficient Pilot.
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Barry used to be a member of another online group that I participated in years ago. He struck me as being pretty non-technical. He would make statements such as you see in his book, but then couldn't explain or defend them. He said something to the effect that he had a friend that was a 'high-powered aerodynamicist", which led me to believe that much of what he wrote was his interpretation of what his friend said.

Anyway, what he writes on lift is silly. From reading his stuff, you'd think you'd have to add all the forces generated from the pressure differences, the downwash, and the bouncing off the bottom of the airfoil. He doesn't have the slightest idea of what he's talking about.

My recommendation, as always, is to seek out experts when you want to learn about a topic, and Barry isn't one on this subject. I recommend Skip Smith's "Illustrated Guide to Aerodynamics" as a starter book. Seagull likes Chris Carpenters "Flightwise" books. Those are the places to start, not anything written by pilots.
 
I can see your point but I give Schiff alot of credit. I dont think you're a 747 Captain with 30,000 hrs. and countless publications if you dont know what you're talking about.
 
meritflyer said:
I can see your point but I give Schiff alot of credit. I dont think you're a 747 Captain with 30,000 hrs. and countless publications if you dont know what you're talking about.
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I'm sure Barry is an expert on flying an airliner, but you don't need to know much about flight theory to do that. I've driven a car all my life, but I still don't know much about thermodynamics. :)
 
Well I just finished his chapter on lift and it is summed up to this -

Bernoulli and Newton.

Barry discusses lift as kinetic and static energy and their relation to Bernoulli's principle. He describes the Bernoulli effect about as text book as you get - an increase in velocity results a decrease in pressure and temperature - created on the upper camber of the wing.

--------------____________-------------
50 units .........................................50 units KE
kinetic energy ...... 75 units KE
............................25 units SP
50 units .........................................50 units SP
static pressure
________________--------------------_______________


Okay, so according to the laws of conservation, energy can not be destroyed, hence the drawing above. Taking 100 units of energy through the venturi tube, all must "pass through". Some of the air's static pressure is sacrificed into kinetic energy and explains Bernoulli.

As a result of this reaction, we achieve a downwash on from the accelerated air over the upper camber down off the trailing edge of the wing producing an equal and opposite reaction (Newtons Law of Inertia).

If we wanted to achieve "more lift", we must change the pressure differences that exist by "constricting" our make shift venturi tube. This is accomplished by changing our AoA which creates an increase of air velocity over the upper camber and a greater reduction in pressure. Larger quantities of air are now attracted over the wings leading edge and as a result, more downwash is produced.

Secondly (or maybe third), when air strikes the bottom of the wing in phases of flight with a high AoA, it too, is deflected downward and creates even more reaction and contributes to total wing lift. This reaction is similar to flying a kite. Air being deflected downward causes an upward reaction.

For anyone having a difficult time with this concept, Schiff says "The rotors of a helicopter create lift identically to the manner of a fixed wing creates lift. The only difference is that helicopter wings rotate to create relative wind without any movement of the helicopter. Fixed wings encounter relative wind only when the airplane is in motion."

Thoughts?
 
meritflyer said:
Secondly (or maybe third), when air strikes the bottom of the wing in phases of flight with a high AoA, it too, is deflected downward and creates even more reaction and contributes to total wing lift. This reaction is similar to flying a kite. Air being deflected downward causes an upward reaction.
Thoughts?
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Except that the air doesn't "strike" the bottom of the wing. It flows around it. About the only point it "strikes" is the stagnation point, creating a high pressure area on the leading edge. The presence of this high pressure area diverts the flow of air around the airfoil. Over the majority of the lower surface, the flow is moving parallel to the airfoil, yet it still contributes to lift due to its high pressure.

Look at figure 3 in Barry's book. Do you see air bouncing off the bottom of the airfoil? Look at how those streamlines divert around the stagnation point to flow parallel to the surface of the wing.

The only sense in which Barry is correct is that pressure itself is a momentum transfer....pressure is cause by air molecules bouncing off whatever it comes into contact with.


BTW, a kite works just like an airplane wing...creates pressure differences.
 
Okay, so after doing some thinking and research, I also cant completely agree with the wind striking the bottom theory for a few reasons.

1. It consideres the majority of lift is created by the lower surface of the wing.

I believe that at large AoA, there exists a higher pressure gradient and a larger amount of downwash, creating more lift by the Newtonian theory of flow turning.
 
meritflyer said:
I believe that at large AoA, there exists a higher pressure gradient and a larger amount of downwash, creating more lift by the Newtonian theory of flow turning.
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Ah, sanity returns. :) Mostly.

The pressure gradient alone is enough to explain lift. This will indeed turn the air, but the airfoil doesn't know and doesn't care about that, since all it feels is the pressure gradient.

Here's what John D. Anderson says in "Introduction to Flight":
No matter how complex the flow field, and no matter how complex the shape of the body, the only way nature has of communicating an aerodynamic force to a solid object or surface is through the pressure and shear stress distributions which exist on the surface. [p. 57, 3rd edition].
​
Take comfort in the fact that even if the striking of the bottom of the wing occured, what the wing would feel is a high pressure area, so lift would still be the result of a pressure differential.
 
meritflyer said:
Okay, so after doing some thinking and research, I also cant completely agree with the wind striking the bottom theory for a few reasons.

1. It consideres the majority of lift is created by the lower surface of the wing.

I believe that at large AoA, there exists a higher pressure gradient and a larger amount of downwash, creating more lift by the Newtonian theory of flow turning.
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Good, except the downwash doesn't CAUSE lift, it's a RESULT of lift!

Also, thanks to B767 and Tgrayson for their usual excellent replies to this topic.

As for Barry, I was also a member of that online group, and know him in real life as well. However, being an airline pilot with whatever hours doesn't, in itself, qualify him to be an expert on anything. He does write well, but not always accurate stuff.

Also, as B767 said, there is a time for the impact of molecules on the bottom of the wing to create lift, and that is hypersonic very high altitude. I'll have to look again, but I am fairly certain that is NOT the principle at work on a kite!
 
meritflyer said:
If it doesnt "strike" the bottom of the wing, where did this theory come from? There must be some substance to it??
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It has a great intuitive appeal. Newton came up with a formula for lift that used this concept, but it was grossly in error, as he knew.

Bernoulli's equation is derived (or can be) using Newton's laws, so it's all related.
 
tgrayson said:
Ah, sanity returns. :) Mostly.

The pressure gradient alone is enough to explain lift. This will indeed turn the air, but the airfoil doesn't know and doesn't care about that, since all it feels is the pressure gradient.

Here's what John D. Anderson says in "Introduction to Flight":
No matter how complex the flow field, and no matter how complex the shape of the body, the only way nature has of communicating an aerodynamic force to a solid object or surface is through the pressure and shear stress distributions which exist on the surface. [p. 57, 3rd edition].
​
Take comfort in the fact that even if the striking of the bottom of the wing occured, what the wing would feel is a high pressure area, so lift would still be the result of a pressure differential.
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Yes, one of my favorite books too. Or, to make it even more obvious. Things moved when they're PUSHED, and if you don't push them, they just continue doing what they were doing before (i.e, sitting at rest or moving in a straight line)! Come to think of it, people are much the same, this might be good for some strange evolutionary biology theory....;)
 
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