Langewiesche's Stick and Rudder

[iflyforpie]

There are no giant vaccum cleaners up in the
sky. Bernoulli is a side show at the most.

Nope.

Bernoulli is what makes the air turn downward. Without Bernoulli, there is no lift. There is suction at the top of the wing-ask anyone who has left their fuel cap off. This suction is far greater than any high pressure on the bottom of the wing-regardless of its shape.

What is wrong is believing that the reason the air accelerates over the wing (and lowers the pressure) is based only on camber.

Read up on the Kutta-Zhukovsky theorem if you want to learn more.

This site explains it the best IMHO:

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

[Rudder Bug]

This suction is far greater than any high pressure on the bottom of the wing-regardless of its shape.

So, how do you explain inverted flying then?

[MichaelP]

All wings create lift by accelerating air downwards.

Benouillis principle works not in creating lift but by ensuring the air that flows over the top of the wing stays nearly in contact with it.
His principle allows for streamlined flow.

The effect of both Benouilli and dynamic deflection is lift.
You might consider Benouilli to increase the efficiency of the wing by reducing the drag of cavitation.

The plywood board would fly better if it had a curve on top to 'prevent' the eddying effects of a corner and flat top.

Lift is displacing an amount of air downwards.

W = mg, L = W, so L = mg? But if we substitute the same equation F = ma then L = ma and L is equivalent to W.

A mass off air is accelerated downwards equivalent to the mass times gravity (g) or weight of the aircraft.

The angle of attack determines lift and when the aircraft stalls, once the streamlining effect of the curve on top is lost to cavitation the lift is reduced below the weight.... But given infinate power I bet you could drive this unstable wing enough to create enough deflection to stay in the air providing you don't flick out of it!

I write low speed: L = ma, high speed L = ma

[iflyforpie]

Bernoulli just applies the law of conservation of energy. It just says that static and dynamic pressure are interchangeable-faster airflow means lower static pressure and vice versa.

High velocity air over the wing means you get low pressure. Low pressure sucks the wing up. The low pressure also deflects the airflow down; but airflow that is already behind the aircraft can't be doing much to lift it, it is an effect only. If I had a cloaked ship and snuck up behind your plane and deflected the airflow up-provided I was far enough away to not change the pressures on the wing-you wouldn't know the difference.

On upside down aircraft, the bottom of the aircraft wing now has airflow accelerate over it-provided it is meeting the air at a positive angle of attack.

Camber is what allows the airflow to smoothly change direction at high angles of attack without cavitation-not Bernoulli.

An aircraft at a high angle of attack without flaps or cambered airfoil will create a very low pressure area above it, but it will be turbulent and this turbulence (unless it is organized like a vortex) will not allow the pressure to be reduced further.

An aircraft with a cambered airfoil and/or flaps will be able to produce lower pressures (read-coefficient of lift) since the air is more gradually accelerated and turned.

[COCO THE MONKEY]

Both the acceleration of air downward and the "suction" theory are correct just different ways of looking at the same thing.

We usually describe propellers and jets as accelerating air/gasses backwards and getting thrust from Newtons law of equal and opposite reactions. Rarely do we describe them as getting thrust from creating a low pressure at the intake and high pressure at the exhaust, the difference being the net thrust. But this is also correct, just another way of describing the same thing.

Accelerating air over the top of the wing is just a smooth and low drag way of getting the same lift as a flat plate that deflects air downward in a crude fashion.

I had the lift discussion with a TC inspector just after a ride and he got quite upset at me. I had just rediscovered Stick and Rudder after a few decades of it gathering dust. I think he was considering revoking my license. I apparently invaded his comfort zone on this.

Some more arguing points that are extensions of lift from this book:
- flaps don't increase lift, just the coefficient of lift.
- all other things (like fuel burn off ) being equal, ICE INCREASES LIFT by the weight of ice accumulated. For example, an A/C at 10,000 lbs clean picks up 100 pounds of ice. Assuming it is still in level unaccelerated flight, the lift has increased to 10,100 lbs. Assuming the 100 lbs is net after fuel burn, of course. So when answering multiple choice questions on icing exams, you must lie and check the option that says b. lift is decreased and drag is increased. We all know what we mean,but as Langewiesche points out, we pilots understand lift differently from aeronautical engineers.

Yes, GREAT BOOK!

[Hedley]

Sigh.

No wonder students have such a poor understanding
of the theory of flight.

[iflyforpie]

Both the acceleration of air downward and the "suction" theory are correct just different ways of looking at the same thing.

We usually describe propellers and jets as accelerating air/gasses backwards and getting thrust from Newtons law of equal and opposite reactions. Rarely do we describe them as getting thrust from creating a low pressure at the intake and high pressure at the exhaust, the difference being the net thrust. But this is also correct, just another way of describing the same thing.

Accelerating air over the top of the wing is just a smooth and low drag way of getting the same lift as a flat plate that deflects air downward in a crude fashion.

I had the lift discussion with a TC inspector just after a ride and he got quite upset at me. I had just rediscovered Stick and Rudder after a few decades of it gathering dust. I think he was considering revoking my license. I apparently invaded his comfort zone on this.

Some more arguing points that are extensions of lift from this book:
- flaps don't increase lift, just the coefficient of lift.
- all other things (like fuel burn off ) being equal, ICE INCREASES LIFT by the weight of ice accumulated. For example, an A/C at 10,000 lbs clean picks up 100 pounds of ice. Assuming it is still in level unaccelerated flight, the lift has increased to 10,100 lbs. Assuming the 100 lbs is net after fuel burn, of course. So when answering multiple choice questions on icing exams, you must lie and check the option that says b. lift is decreased and drag is increased. We all know what we mean,but as Langewiesche points out, we pilots understand lift differently from aeronautical engineers.

Yes, GREAT BOOK!

I agree with everything you said.

I don't agree with Hedley saying there is no suction on top of a wing (or bottom if you are upside down) and that Bernoulli has nothing to do with lift.

But I guess that's proof that pilots don't need to understand lift in detail to fly an airplane. As for myself I like to know what is going on more out of curiosity than anything else.

[Cat Driver]

I had the lift discussion with a TC inspector just after a ride and he got quite upset at me. I had just rediscovered Stick and Rudder after a few decades of it gathering dust. I think he was considering revoking my license. I apparently invaded his comfort zone on this.

Can a TC inspector Arbitrarily revoke a license?

[CID]

Sigh.

No wonder students have such a poor understanding
of the theory of flight.

Maybe your students.

Lift can't properly be described without including both true newtonian and bernoulli lift effects and not the incorrect "skipping stone" theory described by Hedley.

[Hedley]

... and now you're a flight instructor, too?

Where did I mention the words "skipping stone"?

I will gladly match my students against yours. Oh,
I forgot, you don't have any. Never have, and never
will.

Blow it out your ass, you ignorant, obnoxious internet coward.

[CID]

Flight instructor? Since when does a flight instructor need to intimately understand the theory of lift? Let's face it. It doesn't matter what you tell your students. You can tell them magic invisible unicorns lift the wings. They can still effectively control the airplane. I just thought an "engineer" like you would know better. I guess not.

"Skipping stone" is a term coined by NASA and other authorities in the matter for the incorrect description of newtonian lift. Much like the version you offered.

[Hedley]

I just thought an "engineer" like you

Graduate Engineer, Queen's University, class of 1986.

Blow it out your ass, you ignorant, obnoxious internet coward.

[CID]

Best way to teach theory of flight is to take a 4x8
sheet of plywood outside on a windy day.

How big was the sheet of plywood you learned to fly with Hedley?

[Hedley]

Blow it out your ass, you ignorant, obnoxious internet coward.

[CD]

clunckdriver got it right!

[Stearman]

Monkey and Micheal are the exact reasons why that book is so good.

Its a great read, go buy a copy if ya dont have it.

As for lift no bucks no buck rogers, eh.

[Louis]

http://www.grc.nasa.gov/WWW/K-12/airplane/lift1.html

But then again Clunk's hypothesis is pretty good too...

I found a copy and started reading it. Pretty interesting so far, and reminded me of the first homework I had to do in my theory of flight course: "Find three popular theories of lift and why they are wrong".

Goodbye,

Louis

[Hedley]

From the NASA link above:

Lift occurs when a moving flow of gas is turned by a solid object. The flow is turned in one direction, and the lift is generated in the opposite direction, according to Newton's Third Law of action and reaction. Because air is a gas and the molecules are free to move about, any solid surface can deflect a flow

[CID]

http://www.grc.nasa.gov/WWW/K-12/airplane/wrong2.html

This theory is concerned with only the interaction of the lower surface of the moving object and the air. It assumes that all of the flow turning (and therefore all the lift) is produced by the lower surface. But as we have seen in our experiment, the upper surface also turns the flow. In fact, when one considers the downwash produced by a lifting airfoil, the upper surface contributes more flow turning than the lower surface. This theory does not predict or explain this effect.

[Hedley]

If you were a flight instructor, you would know the
difference between "need to know", and "nice to know".

But since you aren't, you don't.

There are an awful lot of things a poor student pilot
needs to learn, to safely fly an airplane. Engine
mechanics. Gyroscopic theory. Coriolis force.
Weather systems. Radio theory. Airspace.
Regulations.

The list goes on, and on, and on.

Students only retain a fraction of the information
thrown at them. As a good instructor, you should
therefore be very choosy as to what information
you throw at them. This may on occasion result
in a weather expert having a tantrum, or an
avionics shop tech rolling his eyes, because the
level of knowledge that the student pilot has
does not approach their own.

A student pilot does not need to be an expert
on combustion chamber design to operate
and engine.

A student pilot does not need a Phd in fluid
dynamics to understand the production of lift.

I am happy when a student pilot can tell me
the basics of the lift equation ie

Lift is proportional to the coefficient of lift, and velocity squared.

If he can actually draw the Cl and Cd curves
on a piece of paper - and understand them -
I am ecstatic. Because that's what he
needs to know to operate an aircraft.

Not design one.

[Rockie]

From the NASA link above:

Lift occurs when a moving flow of gas is turned by a solid object. The flow is turned in one direction, and the lift is generated in the opposite direction, according to Newton's Third Law of action and reaction. Because air is a gas and the molecules are free to move about, any solid surface can deflect a flow

I always said with enough horsepower you can make a lawnmower fly. Seen it done actually, but it was made out of plywood which proves Hedley's theory. But it's not the only thing that creates lift. Your hand (symmetrical airfoil) out the window creates no lift until you give it an angle of attack, which is what you're talking about Hedley. But take an airfoil with camber across the top, place it in the airflow with 0 AOA and it still creates lift through that other guy's theory because the acceleration of airflow over the top creates an artificial low pressure across the top of the wing. High pressure below, low pressure above and voila...

Since I'm sure both you and CID know this I don't see what the argument is about.

[Hedley]

Yup.

Best I can figure is that CID is immensely bored and
picks fights for fun on the internet to pass the time.

[visual approach]

Yup.

Best I can figure is that CID is immensely bored and
picks fights for fun on the internet to pass the time.

and most of the time someone takes the bait.........

[CID]

Since I'm sure both you and CID know this I don't see what the argument is about.

Hedley originally stated:

Best way to teach theory of flight is to take a 4x8
sheet of plywood outside on a windy day.

All wings create lift by accelerating air downwards.

I disagree. So does NASA.

Then he went on to say:

Sigh.

No wonder students have such a poor understanding
of the theory of flight.

That was based on several informative (and correct) posts that disputed what he stated about the sheet of plywood demonstration.

I implied (rather sarcastically I admit) that a student pilot didn't need to know the theory of lift in detail (as Hedley implied) and that his plywood demonstration was actually misleading if we were in fact discussing the theory of lift in detail in techical terms not in operational terms.

Now Hedley did a "180" and agrees that student pilots don't need to understand the detailed theory of lift. I agree.

Best I can figure is that CID is immensely bored and
picks fights for fun on the internet to pass the time.

That's another area where your logic breaks down. I was only one of several that disagreed with your statement. You chose to concentrate your efforts on me rather than others who disagreed. Let's face it you'd rather concentrate on discrediting me than discussing the topic.

It's OK I can take it but if you have the desire to improve yourself and the qualtity of these discussions, I'm sure we can put our differences aside.

[Hedley]

Let's face it you'd rather concentrate on discrediting me than discussing the topic

It's hilarious when you accuse others of doing exactly what you do first.

A day for you, without assasinating someone's character, must
be like a day without sunshine.

I never did a 180. You wanted to make it as complicated
as possible. I was trying to keep it simple for the poor
students.