B767Driver
New Member
How I love the lift discussion!
I'm a professional pilot with 9000 hours of flying time. Only within the last couple of years have I felt that I've begun to learn the true story about the production of lift (and even still, I can't claim to understand most of the physical actions at work in the production of this elusive force). Most aerodynamicists will tell you the story that Seagull points out in his posts...and that is to elimate Newton from the story.
Let me explain how my convulated and confused understanding of lift developed...and I believe it can be transferred to thousands of pilots just like me...who do not have an aero-engineering background. When I learned to fly in the 1980s the FAA published AC61-21A, the Flight Training Handbook. This was the authoritative source regarding all things surrounding primary flight training. (The AC is still around although has been superceeded...I don't have the latest copy.) My CFI taught out of this book religiously, I nearly memorized it, and by the time I became a CFI my copy was so beat up....well you get the point. This is the knowledge you had to bring before the FAA to pass any checkride.
The last chapter of AC61-21A pertained to the principles of flight and aircraft performance. The FAA described in seven paragraphs how Newtons 3rd law of motion was a major contributor in the production of lift. The FAA described that when the flat lower surface of the wing was inclined to the direction of flow, the air would deflect off of the surface and rebound downward causing a certain amount of lift by Newton's account. This AC, in my opinion, also contained verbage that led to the misunderstanding of the "equal time theory"...the one that theorizes that a parcel of air on the upper leading edge and one of the bottom leading edge of the wing must meet at the trailing edge at the same time.
This is all wrong...according to modern aerodynamicists...however, generations of pilots were trained in this manner...and probably still are today.
FWIW, here is what my reading and research of many engineering sources has led me to believe is a more accurate account of the production of lift.
1) Bernoulli is mostly responsible for the production of lift. However, the acceleration of air over the top of the wing is not due to the distance that air must travel over the top of the wing...however it is accelerated due to the ROTATION of air imparted by an increasing angle of attack made more efficient by the use of camber. (Hence an inverted aircraft can produce lift opposite weight by increasing the AOA...only in reverse of normal.)
2) Newton comes into play during WING DOWNWASH. As the air flowing over the top of the wing reaches the trailing edge, it departs the wing in a backward and downward manner. Newton's 3rd law responds with a forward and upward force. I believe a discussion of induced drag could begin here...but I'm not about to go there.
3) There is no truth to the "skipping stone" theory of lift. This is the one that states lift is produced by air deflected off the lower surface of the wing and produces an upward force. (Personally, production of lift here makes sense to me...but the aerodynamicists can prove and categorically discount this has any affect on lift.)
Seagull, you must be patient. Thousands of pilots have been trained improperly regarding the production of lift. While it can be frustrating for you...it's not really their fault. This is how most pilots have been trained their entire lives. I had been flying for 15 years before I really started to learn "the truth about lift". You would probably be a good candidate to write "The Rest of the Story" or some type of training piece about this topic.
I'm a professional pilot with 9000 hours of flying time. Only within the last couple of years have I felt that I've begun to learn the true story about the production of lift (and even still, I can't claim to understand most of the physical actions at work in the production of this elusive force). Most aerodynamicists will tell you the story that Seagull points out in his posts...and that is to elimate Newton from the story.
Let me explain how my convulated and confused understanding of lift developed...and I believe it can be transferred to thousands of pilots just like me...who do not have an aero-engineering background. When I learned to fly in the 1980s the FAA published AC61-21A, the Flight Training Handbook. This was the authoritative source regarding all things surrounding primary flight training. (The AC is still around although has been superceeded...I don't have the latest copy.) My CFI taught out of this book religiously, I nearly memorized it, and by the time I became a CFI my copy was so beat up....well you get the point. This is the knowledge you had to bring before the FAA to pass any checkride.
The last chapter of AC61-21A pertained to the principles of flight and aircraft performance. The FAA described in seven paragraphs how Newtons 3rd law of motion was a major contributor in the production of lift. The FAA described that when the flat lower surface of the wing was inclined to the direction of flow, the air would deflect off of the surface and rebound downward causing a certain amount of lift by Newton's account. This AC, in my opinion, also contained verbage that led to the misunderstanding of the "equal time theory"...the one that theorizes that a parcel of air on the upper leading edge and one of the bottom leading edge of the wing must meet at the trailing edge at the same time.
This is all wrong...according to modern aerodynamicists...however, generations of pilots were trained in this manner...and probably still are today.
FWIW, here is what my reading and research of many engineering sources has led me to believe is a more accurate account of the production of lift.
1) Bernoulli is mostly responsible for the production of lift. However, the acceleration of air over the top of the wing is not due to the distance that air must travel over the top of the wing...however it is accelerated due to the ROTATION of air imparted by an increasing angle of attack made more efficient by the use of camber. (Hence an inverted aircraft can produce lift opposite weight by increasing the AOA...only in reverse of normal.)
2) Newton comes into play during WING DOWNWASH. As the air flowing over the top of the wing reaches the trailing edge, it departs the wing in a backward and downward manner. Newton's 3rd law responds with a forward and upward force. I believe a discussion of induced drag could begin here...but I'm not about to go there.
3) There is no truth to the "skipping stone" theory of lift. This is the one that states lift is produced by air deflected off the lower surface of the wing and produces an upward force. (Personally, production of lift here makes sense to me...but the aerodynamicists can prove and categorically discount this has any affect on lift.)
Seagull, you must be patient. Thousands of pilots have been trained improperly regarding the production of lift. While it can be frustrating for you...it's not really their fault. This is how most pilots have been trained their entire lives. I had been flying for 15 years before I really started to learn "the truth about lift". You would probably be a good candidate to write "The Rest of the Story" or some type of training piece about this topic.