Aerodynamics - grump & rant

murphey

murphey

Touchdown! Greaser!
Joined
Aug 21, 2008
Messages
11,658
Location
Colorado
Display Name
Display name:
murphey
In between contracts, so I'm sitting in the Aerodynamics class at the local university (took it for real 15 yrs ago, thought a review might be fun). The target audience is pilots, not engineers or physics majors. In all fairness, the preface to FT4P states "This book is written for the typical pilots, who has no engineering background and no desire to become involved with mathematical solutions....".

So far, 3 weeks into the semester, it's been slides that simplify the book ("Flight Theory for Pilots"), which is already a simplification of "Aerodynamics for Naval Aviators" and working problems at the end of each chapter, which is exclusively plugging in numbers to the equations.

Day 1 the instructor asked for the definition of a vector. Only 1 other person knew what a vector was. Another student is terrified by the math needed in the class. Really? You can't do simple algebra or look up numbers in a table? How did you get into college? Ok, so you got admitted (this is an opportunity school, which means if you're a resident and graduated in the top 75% high school, you're in) but every student is required to take X number of math classes. And there's free math tutoring almost 24/7.

Should be interesting tomorrow when we cover Bernoulli's equation. Back in the day I could derive it. Should have taken Aircraft Systems instead, but that's turbine airplanes, which I'll never fly. Wanted to take Weather but class was full.

Funny, 15 years ago it was really interesting topic. Definitely dumbed down from back then. I was talking to the instructor after class - he remarked that Physics for Aviation used to be a prereq (and is still listed as a prereq), but it's not enforced, hence no one takes it. So I guess the only prereq now is breathing and using a calculator.

I'm bored. Incredibly bored.
 
I’m continually amazed at how many pilots are intimidated by a weight and balance problem.
 
I'd be curious to see whatever math the course has to do to supposedly explain how a wing generates lift. I imagine it's completely wrong, but I could be mistaken.
 
RingLaserGyroSandwich said:
I'd be curious to see whatever math the course has to do to supposedly explain how a wing generates lift.
Click to expand...
Dunno about the course, but if you are willing to just look at the big picture, simple algebra can be used. Newton's second law to Bernoulli's equation can be done with about a page of algebra (and the assumptions that make Bernoulli's equation valid).
 
LOL. These days it's not unusual to intimidate the staff at Starbucks by paying with cash. I remain convinced many of them couldn't figure out the change if the electronic till didn't do the calculation for them.
Algebra? Fergedaboudit.
 
RingLaserGyroSandwich said:
I'd be curious to see whatever math the course has to do to supposedly explain how a wing generates lift. I imagine it's completely wrong, but I could be mistaken.
Click to expand...

L = Cl x 1/2 rho x S x V2
 
Last edited:
That's "v-squared"
 
Aerodynamics is simple. Just remember -- the molecules on top of the wing have to run faster to catch their little friends on the bottom of the wing, so they don't have time to push as hard! (I learned that from PBS, so it must be true!)

bernoulli_02.jpg

:rolleyes: :confused:
 
Last edited:
Pilawt said:
Aerodynamics is simple. Just remember -- the molecules on top of the wing have to run faster to catch their little friends on the bottom of the wing, so they don't have time to push as hard! (I learned that from PBS, so it must be true!)

View attachment 77549

:rolleyes: :confused:
Click to expand...
I like this! I’m going to use it next time I teach Young Eagle ground school! I’ve ben trying to find a good demonstration for Bernoulli.
 
It amazes me how little most pilots know. Not that I am some aviation God sent to show everyone how little they know...but I am grateful I have a technical background and I at least feel it helps me process some of the information. I mean statics class is a semester long and is essentially just weight and balance problems.
 
I asked a student once “how does the rotor system generate lift?” Now, I didn’t expect anything along a mathematical explanation. I didn’t even care if he simply blurted out Bernoulli or Action Reaction. Anything would have been good except the reply I got of “well....it’s like a ceiling fan, it pushes the air down. (hand gestures) Whoosh...whoosh!”:confused:
 
Last edited:
Velocity173 said:
I asked a student once “how does the rotor system generate lift?” Now, I didn’t expect an expect anything along a mathematical explanation. I didn’t even care if he simply blurted out Bernoulli or Action Reaction. Anything would have been good except the reply I got of “well....it’s like a ceiling fan, it pushes the air down. (hand gestures) Whoosh...whoosh!”:confused:
Click to expand...

I like that reply. Surely your student was just breaking down newton into something easier for you to understand, and didn't want to trouble you with the depth of his/her technical knowledge? :D
 
GRG55 said:
LOL. These days it's not unusual to intimidate the staff at Starbucks by paying with cash. I remain convinced many of them couldn't figure out the change if the electronic till didn't do the calculation for them.
Algebra? Fergedaboudit.
Click to expand...
Buy one for the price of two and get an extra one absolutely free!!
 
Mtns2Skies said:
Not in engineering school.
Click to expand...

Perhaps not in some. But judging by the younger generation of "engineers" we have where I work the criteria for graduation from many engineering schools seems to be: pay tuition, fog a mirror, and show up for class.

Seriously, I can only think of three recent graduates I work with that have anything close to what I would consider a bright future.
 
Seriously? And how do the planes fly inverted? or with the symmetric profiles?
 
Wait until you ask how a sailboat sails. People point to the sail and describe how it's like an airplane wing. Well, yes and no. Then there's the interaction with the keel and water, not to mention that the wind can come from a variety of directions...
 
"The popular explanation of lift is common, quick, sounds logical and gives the correct answer, yet also introduces misconceptions, uses a nonsensical physical argument and misleadingly invokes Bernoulli's equation."

Professor Holger Babinsky, Cambridge University
 
The standard explanation of lift is problematic for another important reason as well: the air shooting over the wing doesn't have to stay in step with the air going underneath it, and nothing says it has to travel a bigger distance in the same time. Imagine two air molecules arriving at the front of the wing and separating, so one shoots up over the top and the other whistles straight under the bottom. There's no reason why those two molecules have to arrive at exactly the same time at the back end of the wing: they could meet up with other air molecules instead. This flaw in the standard explanation of an airfoil goes by the technical name of the "equal transit theory." That's just a fancy name for the (incorrect) idea that the air stream splits apart at the front of the airfoil and meets up neatly again at the back.

how-airfoil-wing-makes-lift.png


So what's the real explanation? As a curved airfoil wing flies through the sky, it deflects air and alters the air pressure above and below it. That's intuitively obvious. Think how it feels when you slowly walk through a swimming pool and feel the force of the water pushing against your body: your body is diverting the flow of water as it pushes through it, and an airfoil wing does the same thing (much more dramatically—because that's what it's designed to do). As a plane flies forward, the curved upper part of the wing lowers the air pressure directly above it, so it moves upward.

Why does this happen? As air flows over the curved upper surface, its natural inclination is to move in a straight line, but the curve of the wing pulls it around and back down. For this reason, the air is effectively stretched out into a bigger volume—the same number of air molecules forced to occupy more space—and this is what lowers its pressure. For exactly the opposite reason, the pressure of the air under the wing increases: the advancing wing squashes the air molecules in front of it into a smaller space. The difference in air pressure between the upper and lower surfaces causes a big difference in air speed (not the other way around, as in the traditional theory of a wing). The difference in speed (observed in actual wind tunnel experiments) is much bigger than you'd predict from the simple (equal transit) theory. So if our two air molecules separate at the front, the one going over the top arrives at the tail end of the wing much faster than the one going under the bottom. No matter when they arrive, both of those molecules will be speeding downward—and this helps to produce lift in a second important way.

How airfoil wings generate lift#1: An airfoil splits apart the incoming air, lowers the pressure of the upper air stream, and accelerates both air streams downward. As the air accelerates downward, the wing (and the plane) move upward. The more an airfoil diverts the path of the oncoming air, the more lift it generates.
 
sgenie said:
"The popular explanation of lift is common, quick, sounds logical and gives the correct answer, yet also introduces misconceptions, uses a nonsensical physical argument and misleadingly invokes Bernoulli's equation."

Professor Holger Babinsky, Cambridge University
Click to expand...
meh....he doesn't know anything. o_O
 
Velocity173 said:
L = Cl x 1/2 rho x S x V2
Click to expand...
Nothing wrong with this equation by itself (I think NASA has it right on their website) but it essentially takes the interesting part of "why do wings generate lift" and moves it into a black box called "Cl." Faster planes with bigger wings in denser air generate more lift. True, but only a small chunk of the story.

Velocity173 said:
I asked a student once “how does the rotor system generate lift?” Now, I didn’t expect anything along a mathematical explanation. I didn’t even care if he simply blurted out Bernoulli or Action Reaction. Anything would have been good except the reply I got of “well....it’s like a ceiling fan, it pushes the air down. (hand gestures) Whoosh...whoosh!”:confused:
Click to expand...
Actually I think "it pushes the air down" is about as good of an answer as "Bernoulli" lol... what does "Bernoulli" mean? "Action Reaction" is the best of these answers, but only if it can be explained.

sgenie said:
"The popular explanation of lift is common, quick, sounds logical and gives the correct answer, yet also introduces misconceptions, uses a nonsensical physical argument and misleadingly invokes Bernoulli's equation."

Professor Holger Babinsky, Cambridge University
Click to expand...
I trained under a professor once that said some very similar things. He also was a purist and refused to acknowledge that the Bernoulli Equation is applicable outside of venturis.

sgenie said:
The standard explanation of lift is problematic for another important reason as well: the air shooting over the wing doesn't have to stay in step with the air going underneath it, and nothing says it has to travel a bigger distance in the same time. Imagine two air molecules arriving at the front of the wing and separating, so one shoots up over the top and the other whistles straight under the bottom. There's no reason why those two molecules have to arrive at exactly the same time at the back end of the wing: they could meet up with other air molecules instead. This flaw in the standard explanation of an airfoil goes by the technical name of the "equal transit theory." That's just a fancy name for the (incorrect) idea that the air stream splits apart at the front of the airfoil and meets up neatly again at the back.

[edit - picture removed]

So what's the real explanation? As a curved airfoil wing flies through the sky, it deflects air and alters the air pressure above and below it. That's intuitively obvious. Think how it feels when you slowly walk through a swimming pool and feel the force of the water pushing against your body: your body is diverting the flow of water as it pushes through it, and an airfoil wing does the same thing (much more dramatically—because that's what it's designed to do). As a plane flies forward, the curved upper part of the wing lowers the air pressure directly above it, so it moves upward.

Why does this happen? As air flows over the curved upper surface, its natural inclination is to move in a straight line, but the curve of the wing pulls it around and back down. For this reason, the air is effectively stretched out into a bigger volume—the same number of air molecules forced to occupy more space—and this is what lowers its pressure. For exactly the opposite reason, the pressure of the air under the wing increases: the advancing wing squashes the air molecules in front of it into a smaller space. The difference in air pressure between the upper and lower surfaces causes a big difference in air speed (not the other way around, as in the traditional theory of a wing). The difference in speed (observed in actual wind tunnel experiments) is much bigger than you'd predict from the simple (equal transit) theory. So if our two air molecules separate at the front, the one going over the top arrives at the tail end of the wing much faster than the one going under the bottom. No matter when they arrive, both of those molecules will be speeding downward—and this helps to produce lift in a second important way.

How airfoil wings generate lift#1: An airfoil splits apart the incoming air, lowers the pressure of the upper air stream, and accelerates both air streams downward. As the air accelerates downward, the wing (and the plane) move upward. The more an airfoil diverts the path of the oncoming air, the more lift it generates.
Click to expand...
In addition to @Capt. Geoffrey Thorpe clarifying that the curve is not required for lift (although it is obviously used in many designs), I will also nitpick that the "stretching out" of air along the top of the curved wing surface does not lower pressure. The air is exchanging static pressure for dynamic pressure as it speeds up.

I just pulled a reference I like for this topic, Introduction to the Aerodynamics of Flight, NASA SP-367, 1975. It has some very interesting discussion. What if we tried to measure the pressure above and below a typical airplane wing like the ones pictured above? Given all the claims of pressure (i.e., "total pressure) being lower above the wing, let's say we measure total pressure. How do we do that? Per the author, Talay of the Langley Research Center, a pitot tube measures total pressure. Total pressure is the sum of dynamic pressure (1/2 rho v^2) and static pressure (p), neglecting altitude changes. As air dams up immediately at the tube entrance and comes to rest at the stagnation point, dynamic pressure goes to zero as it all converts into static pressure. The pressure measuring device measures the static pressure which is equal to the total pressure at that location within the pitot tube.

Now let's put pitot tubes above and below the wing in the air flow. Which one will read a higher pressure, neglecting second order effects? The mere acceleration of the airflow over the top of the curved wing does not decrease pressure but it does decrease static pressure. As I said, I was nitpicking.

Also, the airflow along the top of the curved wing bends to follow the surface because of the Coandă Effect, for anyone who cares but didn't already know.
 
sgenie said:
"The popular explanation of lift is common, quick, sounds logical and gives the correct answer, yet also introduces misconceptions, uses a nonsensical physical argument and misleadingly invokes Bernoulli's equation."

Professor Holger Babinsky, Cambridge University
Click to expand...

sgenie said:
The standard explanation of lift is problematic for another important reason as well: the air shooting over the wing doesn't have to stay in step with the air going underneath it, and nothing says it has to travel a bigger distance in the same time. Imagine two air molecules arriving at the front of the wing and separating, so one shoots up over the top and the other whistles straight under the bottom. There's no reason why those two molecules have to arrive at exactly the same time at the back end of the wing: they could meet up with other air molecules instead. This flaw in the standard explanation of an airfoil goes by the technical name of the "equal transit theory." That's just a fancy name for the (incorrect) idea that the air stream splits apart at the front of the airfoil and meets up neatly again at the back.

how-airfoil-wing-makes-lift.png


So what's the real explanation? As a curved airfoil wing flies through the sky, it deflects air and alters the air pressure above and below it. That's intuitively obvious. Think how it feels when you slowly walk through a swimming pool and feel the force of the water pushing against your body: your body is diverting the flow of water as it pushes through it, and an airfoil wing does the same thing (much more dramatically—because that's what it's designed to do). As a plane flies forward, the curved upper part of the wing lowers the air pressure directly above it, so it moves upward.

Why does this happen? As air flows over the curved upper surface, its natural inclination is to move in a straight line, but the curve of the wing pulls it around and back down. For this reason, the air is effectively stretched out into a bigger volume—the same number of air molecules forced to occupy more space—and this is what lowers its pressure. For exactly the opposite reason, the pressure of the air under the wing increases: the advancing wing squashes the air molecules in front of it into a smaller space. The difference in air pressure between the upper and lower surfaces causes a big difference in air speed (not the other way around, as in the traditional theory of a wing). The difference in speed (observed in actual wind tunnel experiments) is much bigger than you'd predict from the simple (equal transit) theory. So if our two air molecules separate at the front, the one going over the top arrives at the tail end of the wing much faster than the one going under the bottom. No matter when they arrive, both of those molecules will be speeding downward—and this helps to produce lift in a second important way.

How airfoil wings generate lift#1: An airfoil splits apart the incoming air, lowers the pressure of the upper air stream, and accelerates both air streams downward. As the air accelerates downward, the wing (and the plane) move upward. The more an airfoil diverts the path of the oncoming air, the more lift it generates.
Click to expand...

 
sgenie said:
The standard explanation of lift is problematic for another important reason as well: the air shooting over the wing doesn't have to stay in step with the air going underneath it, and nothing says it has to travel a bigger distance in the same time. Imagine two air molecules arriving at the front of the wing and separating, so one shoots up over the top and the other whistles straight under the bottom. There's no reason why those two molecules have to arrive at exactly the same time at the back end of the wing: they could meet up with other air molecules instead. This flaw in the standard explanation of an airfoil goes by the technical name of the "equal transit theory." That's just a fancy name for the (incorrect) idea that the air stream splits apart at the front of the airfoil and meets up neatly again at the back.

how-airfoil-wing-makes-lift.png


So what's the real explanation? As a curved airfoil wing flies through the sky, it deflects air and alters the air pressure above and below it. That's intuitively obvious. Think how it feels when you slowly walk through a swimming pool and feel the force of the water pushing against your body: your body is diverting the flow of water as it pushes through it, and an airfoil wing does the same thing (much more dramatically—because that's what it's designed to do). As a plane flies forward, the curved upper part of the wing lowers the air pressure directly above it, so it moves upward.

Why does this happen? As air flows over the curved upper surface, its natural inclination is to move in a straight line, but the curve of the wing pulls it around and back down. For this reason, the air is effectively stretched out into a bigger volume—the same number of air molecules forced to occupy more space—and this is what lowers its pressure. For exactly the opposite reason, the pressure of the air under the wing increases: the advancing wing squashes the air molecules in front of it into a smaller space. The difference in air pressure between the upper and lower surfaces causes a big difference in air speed (not the other way around, as in the traditional theory of a wing). The difference in speed (observed in actual wind tunnel experiments) is much bigger than you'd predict from the simple (equal transit) theory. So if our two air molecules separate at the front, the one going over the top arrives at the tail end of the wing much faster than the one going under the bottom. No matter when they arrive, both of those molecules will be speeding downward—and this helps to produce lift in a second important way.

How airfoil wings generate lift#1: An airfoil splits apart the incoming air, lowers the pressure of the upper air stream, and accelerates both air streams downward. As the air accelerates downward, the wing (and the plane) move upward. The more an airfoil diverts the path of the oncoming air, the more lift it generates.
Click to expand...

Yes. I'm amazed by how recently "equal transit" was being taught (and probably still is!)

Good graphic to show the acceleration of the upper vs. lower air masses:

fig2.jpg


http://www.aviation-history.com/theory/lift.htm
 
RingLaserGyroSandwich said:
Actually I think "it pushes the air down" is about as good of an answer as "Bernoulli" lol... what does "Bernoulli" mean? "Action Reaction" is the best of these answers, but only if it can be explained.
Click to expand...

Because if he said Bernoulli, that would at least mean he remembered something out of his million dollar taxpayer funded education. His “ceiling fan principle” isn’t taught in any of his classes, but if he expounded on that with Action Reaction, then I’d accept his answer. Instead I got “well I saw on Discovery Wings the other night that we basically beat the air into submission.” Another answer that isn’t referenced in any of his issued aerodynamics publications.
 
Looking at Cap'nJack's video, notice where the stagnation point is well under the lower leading edge of the airfoil. When doing this experiment with a symmetrical airfoil, you will see the same thing. Notice where your stall vane is located. This make the "upper surface" longer than the "lower surface" of even a symmetrical shape.
 
Velocity173 said:
I asked a student once “how does the rotor system generate lift?” Now, I didn’t expect anything along a mathematical explanation. I didn’t even care if he simply blurted out Bernoulli or Action Reaction. Anything would have been good except the reply I got of “well....it’s like a ceiling fan, it pushes the air down. (hand gestures) Whoosh...whoosh!”:confused:
Click to expand...

What? You mean it doesn't?

And of course, the sound is "whup, whup, whup!"
 
IK04 said:
What? You mean it doesn't?

And of course, the sound is "whup, whup, whup!"
Click to expand...

Nope, obviously helicopters are so ugly that the ground just repels them. He was a National Guard guy so he could’ve said f’ing magic and we would’ve passed him. Super nice guy though. Not very bright and couldn’t fly worth **** but super nice.
 
You must log in or register to reply here.
Back
Top