How do you teach lift?

[loopa]

Seems like more and more often I hear instructors teaching student's that lift is produced because of Bernoulli's theory. While this theory holds true somewhat, there's a great gap between the lift produced using Bernoulli's equation, and what actually goes on around a wing. When I approach and ask these guys why my little paper plane flies with no camber on its air foil then they draw a blank. I always thought the air turning theory was taught in instructor ratings these days?

There's a great link on nasa's website explaining lift.

How do you guys teach lift?

[DanJ]

Your paper airplane doesn't fly very far without thrust though. It's essentially a missile, and once it's energy is gone, it stops flying. I can take that same piece of paper, crunch it up into a ball and probably throw it as far as the airplane will go.

[Shiny Side Up]

I tend to agree with "H" in that Beroulli tends to get too much credit for lift creation and Newton not enough.

[tkdowell]

I tend to agree with "H" in that Beroulli tends to get too much credit for lift creation and Newton not enough.

+1

[iflyforpie]

From what I have read, Newtonian and Bernoullian lift are just to different ways of looking at the same thing. Newton pushing the air down is simply as a result of the difference in pressures created by Bernoulli.

What gets taught wrong is the 'equal time transit' and the requirement for the top of the wing to be curved and the bottom flat. We've all seen aircraft fly upside down and obviously you can get lift from a flat surface like a cheap balsa glider. Bernoulli never said such nonsense, he simply said that for a fluid at constant energy, if you increase the dynamic pressure (airflow) you will reduce the static pressure.

Air does indeed accelerate over the wing of the aircraft and actually results in the top molecule getting to the trailing edge before the bottom molecule, making up for the perceived shorfall (the top of a wing would have to be three times at long as the bottom for equal time transit to work).

All I say is that air is accelerated over the top of the wing, slightly decelerated under it, creating a large area of low pressure above the wing (yes, there is indeed a ton of low pressure, that's why we do rib stitching on fabric aircraft). It also creates a small area of sharp upwash in front of the wing (yes, I said that right, look at an electric stall vane and you'll get the idea) and a greater area of shallow downwash behind it.


For more on this, I suggest reading this.

http://www.av8n.com/how/htm/airfoils.html

[loopa]

Air does indeed accelerate over the wing of the aircraft and actually results in the top molecule getting to the trailing edge before the bottom molecule, making up for the perceived shorfall (the top of a wing would have to be three times at long as the bottom for equal time transit to work).

All I say is that air is accelerated over the top of the wing, slightly decelerated under it, creating a large area of low pressure above the wing (yes, there is indeed a ton of low pressure, that's why we do rib stitching on fabric aircraft). It also creates a small area of sharp upwash in front of the wing (yes, I said that right, look at an electric stall vane and you'll get the idea) and a greater area of shallow downwash behind it.


For more on this, I suggest reading this.

http://www.av8n.com/how/htm/airfoils.html

Bingo!

That website is a great read for all instructor's and aspiring pilot's in general. My class 1 had open ended exams on that website during my rating.

[Shiny Side Up]

What gets taught wrong is the 'equal time transit' and the requirement for the top of the wing to be curved and the bottom flat. We've all seen aircraft fly upside down and obviously you can get lift from a flat surface like a cheap balsa glider. Bernoulli never said such nonsense, he simply said that for a fluid at constant energy, if you increase the dynamic pressure (airflow) you will reduce the static pressure.

Indeed, hence the comment above about Bernoulli vs. Newton. I think if more instructors when explaining lift started with Newton's Laws rather than the starting point of Bernoulli's principle they would end up with a better explanation. Using Bernoulli as the starting point ends up with some confusing diagrams and doesn't get the point through to most new students. Its one of the learning factors - Relationship - that needs to be better used here. In this case effort needs to be made to start at the simple and move to the complex. Actions and reactions are easier to explain than fluid dynamics, though one leads to the other.

[SuperchargedRS]

For teaching newtunes law in regards to how a wing can fly I use the example of holding your hand out the window of a moving car. You angle your hand up and t goes up (this is something every kid has done). I usually lead into this with "could we make a flat board a flying wing"

[SkyWriter]

Seems like more and more often I hear instructors teaching student's that lift is produced because of Bernoulli's theory.

I should hope so !

While this theory holds true somewhat, there's a great gap between the lift produced using Bernoulli's equation, and what actually goes on around a wing.

Bernoulli alone only explains why the pressure changes in response to a velocity change. You also need continuity to explain the velocity chage in the first place. Together, they provide a very complete picture of what actually goes on around a wing.

When I approach and ask these guys why my little paper plane flies with no camber on its air foil then they draw a blank. I always thought the air turning theory was taught in instructor ratings these days?

That's unfortunate, as it means that these guys don't actually understand the material thay're teaching. Bernoulli's equation doesn't require camber in order to work any more than it forbids the possibility of inverted flight. Using continuity, wing shape and angle of attack, we can come up with a description of the airflow around a wing. Applying Bernoulli, we can come up with a description of the pressure distribution. The result of this pressure distribution is lift.

How do you guys teach lift?

As you can probably guess from the above, I go with continuity/Bernoulli. I also teach momentum change (what you call the "air turning theory") as a valid (but independent) description. Momentum change has the advantage of being easy to visualize, but it doesn't tell us anything about how a venturi works, how a pitot tube works, or why a wing stalls. For each of these, you need continuity/Bernoulli.

Some further reading here:
- http://www.flightwriter.com/2010/12/str ... -lift.html
- http://www.flightwriter.com/2010/12/whe ... art-2.html
- http://www.flightwriter.com/2010/12/goi ... art-3.html

Cheers!

[photofly]

If you teach a decent theory of lift (involving circulation) then you get wing tip and wake vortices "for free", so to speak.

[iflyforpie]

The problem with entering circulation into the equation is that its title is misleading (the air does not actually circulate around the wing) and is often completely foreign to anybody who has been taught a traditional lift theory.

In addition, you don't really need to know lift in that much detail to fly or even design an aircraft.

If I am going to relate to somebody circulation, I develop it like this.

First, air likes to flow cleanly off a sharp edge. Even if there is a strong low pressure area above the wing, the air has no desire to move up.

The angle of the air flowing off the wing is related to the angle of the chord/mean camber line as appropriate. The more we increase the angle of attack, the greater the downwash is (to a point). This phenomenon is known as the Kutta condition.

In order for the air to meet the requirements of the Kutta condition, it must--as I related above--substantially accelerate above the wing, decelerate (or not accelerate as much) on the bottom of the wing, and have an up wash in front of the wing to complement the down wash behind it. Kind of like the strong rowers on one side of the Viking ship, the faster moving air turns the airflow to form the Kutta condition (airflow turning).

If you subtract the free-stream vectors from the airflow around the wing, the net result would be the circulation, air traveling in a clockwise direction around a wing travelling left.

This is very easy to illustrate with a sheet of construction paper cut into a 1" strip and flicked away from you with a bit of back spin... there is circulation! On an aircraft we don't have that (hopefully) because our aircraft has longitudinal stability--but we must account for the pitch-up couple caused by the creation of lift. This is a consideration in propellers as well.

When the air spills off the wingtips, it is no longer bounded to the wing and trails off behind parallel to the airflow. The circulation vectors are still there, just turned 90 degrees. These become the wingtip vortices (you will note they go in the correct direction to what I described).


But the most important thing to realize is all the different mechanisms I described above are all links in a chain of producing lift. You destroy one, you destroy lift. A tiny spoiler (think gliders) on top of the wing looks like it wouldn't do anything to somebody who believes in pure Newtonian lift. But it is very effective at destroying lift. Put the same device on the bottom, well forward of the trailing edge, and it wouldn't hardly do anything.


FWIW, there is pure Newtonian Lift. It is called flat plate lift an is only pure momentum transfer from air bouncing off the bottom of a flat surface. It will achieve lift coefficients at around 45 degrees comparable to conventional lift, but at about ten times the drag coefficient...

[robertsailor1]

One of your early points really covers it. I have friends that could never answer that question in detail but they could fly the butt off of 90% of the pilots here. Hopefully student pilots spend there time on more important stuff. I remember a Doctor friend of mine when I asked him how Aspirin worked, his answer "who cares" it works that's all you need to know.

[photofly]

the air does not actually circulate around the wing

As a matter of fact it pretty much does, if you look at it in the frame of reference where the air is (initially) stationary, and the wing moves through it. A parcel of air just below the wing will move forward, up over the wing and then back down in the downwash all as the wing passes through the air.

I remember a Doctor friend of mine when I asked him how Aspirin worked, his answer "who cares" it works that's all you need to know.

Not the kind of doctor I ever want to be treated by, sorry.

[crazy_aviator]

We can have it both ways , lets put bernoullis and newtons bed together they both play a part in lift.

[photofly]

Your paper airplane doesn't fly very far without thrust though. It's essentially a missile, and once it's energy is gone, it stops flying. I can take that same piece of paper, crunch it up into a ball and probably throw it as far as the airplane will go.

And that totally misses the point; the ball follows a ballistic trajectory, and doesn't generate lift. The paper airplane, although unpowered, flies nearly horizontally and does generate significant lift. A glider is also unpowered, but you can't get your paper ball to do a 500km cross country trip.

Now if you spin your paper ball, then it's going to start generating lift...

[ktcanuck]

Teaching the theory behind lift is less important than teaching how alpha can make or break your day.

[Spokes]

Here is the NASA take on this subject

http://www.grc.nasa.gov/WWW/K-12/airplane/bernnew.html

[Bede]

There is a huge difference between how pilots are taught aerodynamics and engineers/physicists are taught aerodynamics.

I read in a book once that if Bernoulli's theory were the sole source for lift, the wing would need a thickness roughly equal to it's chord, which would of course cause more than an acceptable amount of drag.

[Shiny Side Up]

I read in a book once that if Bernoulli's theory were the sole source for lift, the wing would need a thickness roughly equal to it's chord, which would of course cause more than an acceptable amount of drag.

Albert Einstein tested that theory, using only Bernoulli's principle to design a wing.

http://www.wrightstories.com/aviators.html#Einstein’s Wing Flops

[iflyforpie]

But the thing is, it isn't Bernoulli's theory. Bernoulli died one year before even the first manned balloon flight. All Bernoulli says is that a package of air at constant total energy will lose static pressure when it is sped up. This alone explains how an aircraft lifts off the ground. But it comes part and parcel with circulation and downwash. It just depends on how you are looking at it.

But as SSU said earlier, the idea of momentum transfer is much easier to grasp. But you cannot have momentum transfer and downwash without Bernoulli unless you have 50,000 pounds of thrust, a 45 degree alpha, and a bunch of computers flying the aircraft.

[Shiny Side Up]

But as SSU said earlier, the idea of momentum transfer is much easier to grasp. But you cannot have momentum transfer and downwash without Bernoulli unless you have 50,000 pounds of thrust, a 45 degree alpha, and a bunch of computers flying the aircraft.

Sure you can, the 45 alpha is critical to get the maximum momentum transfer, but you don't need 50,000 pounds of thrust or the computers. All you need is a kite. Either that or use a different medium than air.

[iflyforpie]

Ahhh... touche!

[jump154]

Thanks guys! Reading this thread, and the links had me well prepared for my 14 year old Daughter asking me "So how come there is low pressure over the wing, I just don't get this lift thing - does the weight of the air change?" while watching the Dambusters documentary last night.

[Spokes]

Judging by the number of differing explanations, and assuming all here are pilots who don't crash too much, it seems that it is not all that important for a pilot to know how lift is generated. More, like just what affects the amount of lift generated. Just a thought.

[robertsailor1]

Kinda like who cares about why aspirin works. The thread was titled how you "teach" lift and certainly was interesting as I learned things I didn't know but its a complicated subject and if your actually "teaching" this way its no bloody wonder its taking so long to get students trained. The time would be much better spent on something important.