Helicopter Lift Theory - Equal Transit, Skipping Stone & 1/2 a Venturi - all wrong...

[Jay Bunning]

OK so here is a frustrating situation for a new Helicopter CFI working on how to teach lift theory:

 

The 2012 FAA Helicopter Flying Handbook explains the creation of lift as a combination of half a venturi (the bottom half is the airfoil) and air pushing up on the bottom of the airfoil (like a water skier).

 

The 2012 FAA Helicopter Instructor's Handbook explains lift as the upper surface having a longer distance for the air molecules to travel than the lower surface and so causing the airflow above to speed up to make sure the molecules meet at the trailing edge.

 

Two different theories in books released within a month or two of each other!!! To make things even more frustrating, both explanations are completely wrong according to NASA:

 

http://www.grc.nasa.gov/WWW/k-12/airplane/wrong1.html

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

http://www.grc.nasa.gov/WWW/k-12/airplane/wrong3.html

 

So apart from a moan at the FAA, the question is what do you teach a new helicopter student:

1. What the Helicopter Instructor's Handbook says you should teach - 'Equal Transit Time' theory
   
2. What the Helicopter Flying Handbook teaches - a combination of 'Skipping Stone' & '1/2 a Venturi' theories
   
3. What NASA (who put a man on the moon using a pocket calculator) says - ' Turning of a fluid'
   

I WANT to teach option 3, but will they suffer in written tests and at the hands of DPEs because of it?

 

Note to the FAA - the NASA theory has been established for a long time - even longer than the amount of time ADFs have NOT been used in the average cockpit... ;-)

[eagle5]

Show your student figures 2-9 and 2-10 (I'm using the old Rotorcraft Flying Handbook p.2-4). Then tell them as the air flows through the constricted portion of the tube it accelerates, reducing its pressure. Air flowing underneath is not accelerated and therefore its pressure remains the same. As you can see the bottom of the tube resembles an airfoil (rotorblade). Higher pressure below, lower pressure above,...bingo lift!

 

Just keep it simple! If you get too technical with a student (especially early on) you will lose their attention!

[kodoz]

I spend almost no time teaching lift theory...it comes down to rote memorization. Sure, you can ask some things that go beyond the rote, but practical application for a pilot? Mostly it's a warm-up for getting into aerodynamics. I spend more time (at least at the commercial level) on the lift equation.

 

Regardless, I always offer to clarify Magnus effect and it's role, and point them in the direction of the right info for really understanding lift.

[heloidaho]

I read through all those NASA articles. Parts of every one of those theories are correct, even according to the website, such as differential pressure causing lift. It even states that the skipping stone theory is accurate depending on the conditions.

 

The complaint wasn't about differential pressure; it was about equal transit time. The molecules on top of an air foil don't meet back up with the ones on the bottom. They actually move much faster.

 

It seemed more to me like NASA was saying, "Hey, these are halfway right, but the reality is more complex depending on more variables."

 

Seems to me like you could just keep is simple. Teaching differential pressure isn't wrong, and then tie it into the life equation like kodoz recommends.

[iChris]

So apart from a moan at the FAA, the question is what do you teach a new helicopter student:

1. What the Helicopter Instructor's Handbook says you should teach - 'Equal Transit Time' theory
   
2. What the Helicopter Flying Handbook teaches - a combination of 'Skipping Stone' & '1/2 a Venturi' theories
   
3. What NASA (who put a man on the moon using a pocket calculator) says - ' Turning of a fluid'
   

 

Teach the basic concepts, then fill-in were needed.

 

Lets look at both schools of teaching this subject;

 

The basic concept of lift is simple. However, the details of how the relative movement of air and airfoil interact to produce the turning action that generates lift are complex. In any case causing lift, an angled fat plate, revolving cylinder, airfoil, etc., the flow meeting the leading edge of the object is forced to split over and under the object. REF: FAA-H-8083-21A, (NEW) Helicopter Flying Handbook, page 2-2

 

"All that is necessary to create lift is to turn a flow of air. An aerodynamic, curved airfoil will turn a flow. But so will a simple flat plate, if it is inclined to the flow." REF: NASA

 

This is all really parts of the same explanation. It really doesn't matter which school you teach from, as long as you relay the basic concepts.

 

Most of what you've been learning about in aerodynamics is approximations. The heart of the theory requires a fairly high level of mathematical sophistication. Therefore, most aerodynamic books are written on a professional level, yet comprehensible to the student pilot. For simplicity and clarity, some details were omitted were it was considered unnecessary to effective flight operations.

 

As an example in this case, the Helicopter Flying Handbook (other textbooks included) left out the contribution of both sides of the airfoil. Sides, top and bottom, are essential to the production of lift. Moreover, some textbooks because of over simplification were unintentionally misleading in their explanations that the origin of lift was solely based on Bernoulli’s theorem.

 

There are three common factors that affect the turning of the flow, which creates lift.

 

1. The object, airfoil inclined to turn the flow of air.

 

2. The motion of the airfoil, its relative wind, as the sum of airspeed and the rotors rotational velocity.

 

3. The air mass, its density and viscosity.

 

The lift equation is also an approximation; however it expresses the basic concepts of an airfoils interaction with airflow to produce the turning action that generates lift.

 

L = [1/2p V² ] [C_L]

 

1. The object is representative of an airfoil with a given surface area, inclined [C_L] to turn the airflow.

 

2. Airflow is expressed by the dynamic pressure part of the equation as [1/2p V² ].

 

3. The density of the air mass expressed by rho p.

 

Helicopter aerodynamicists explain it by two basic theories, the "Momentum Theory" and the "Blade Element Theory." Again, both are two parts of the same explanation.

 

The Momentum Theory follows Newton's third law, in the case of the helicopter in hovering flight; the action is the development of a rotor thrust equal to gross weight. The reaction is represented by the acceleration of a mass of air from a stagnant condition far above the rotor to a condition with finite velocity in the wake below the rotor.

 

The Blade Element Theory follows along with those airfoil vector diagrams you know so well in your textbooks. This theory looks at what is actually happening at the blade that results in producing the Total Aerodynamic Force (TAF).

 

Lookup these two theories (Momentum & Blade Element) and that should give you what you need to teach from any of these textbooks and then be able to fill in the gaps.

 

"It is the mark of an educated mind to rest satisfied with the degree of precision which the nature of the subject admits and not to seek exactness where only an approximation is possible."

 

Aristotle,

 

Aristotle was a Greek philosopher and polymath, a student of Plato and teacher of Alexander the Great.

Edited November 21, 2012 by iChris

[ridethisbike]

Seems to me like whoever wrote that article was doing so just to poke holes where none needed to be poked. In the very end they even admitted that both theories were correct and that to truly get into it would be far too complex.

 

 

If you teach option #3, you'll be teaching both of the other choices anyways.

 

 

 

Stick with the basics and fill in the gaps, as stated by others. This method of teaching isn't broken, so why try to fix it?

[nightsta1ker]

Agreed. I was taught bernoulli's theorem as the explanation for lift when I learned to fly fixed wing and didn't even learn the other aspects until well into my CFI training (just doing some digging I found all this stuff). When I brought it up to the chief pilot/examiner. He gave me a funny look and said "We don't teach physics, we teach people how to fly helicopters." Message received. The best thing you can do as an instructor is have a simple easy way of explaining things, and if the student starts asking hard questions (some of my students are engineers and physics majors), then know enough to go toe to toe, or at least know where to point them for the right answer. Hell, I routinely look stuff up with my students. That way we can both learn. Your students will respect you a lot more if you answer a tough question with "You know... I have no idea! Lets find the answer!" rather than try and bluff through it or brush it off.

Edited November 21, 2012 by nightsta1ker

[eagle5]

"We don't teach physics, we teach people how to fly helicopters." Message received. The best thing you can do as an instructor is have a simple easy way of explaining things

 

My thoughts exactly! If CFIs were supposed to know EVERYTHING they'd have to go to college like real teachers!

[Rupert]

Venturi, skipping and turning all have an element of truth to them, but, due to the inability of language to represent reality, each explanation also falls far short of reality.

 

Yes, we have a low pressure area over the moving wing, and a high pressure area beneath the moving wing; and, yes, the wing effectively skips or screws itself up through the fluid, and, yes, by moving a mass of air downward the wing conforms to Newton's third law and travels upward.

 

Does this matter to the student?

 

It should.

 

A pilot builds the understanding he receives through experience on top of the first foundation he receives as a student.

 

Give the student the best first foundation you can; and, this includes all three explanations plus a fourth for passing the oral and written.

 

If you prepare your presentation adequately, this takes less than five minutes and lasts a lifetime.

[pilot#476398]

Why do I have to know this?

 

What am I supposed to do with this information?

 

How can knowing this make me a better pilot?

 

Questions I asked myself far too often during training! How much information is TOO MUCH information?

[aeroscout]

Why do I have to know this?

 

What am I supposed to do with this information?

 

How can knowing this make me a better pilot?

 

Questions I asked myself far too often during training! How much information is TOO MUCH information?

 

This is where the law of diminishing returns begins to take effect. TMI for me happens at the point where the initial subject of what and why I was studying becomes fuzzy in my memory.

[Counterrotate]

Venturi, skipping and turning all have an element of truth to them, but, due to the inability of language to represent reality, each explanation also falls far short of reality.

 

Yes, we have a low pressure area over the moving wing, and a high pressure area beneath the moving wing; and, yes, the wing effectively skips or screws itself up through the fluid, and, yes, by moving a mass of air downward the wing conforms to Newton's third law and travels upward.

 

Does this matter to the student?

 

It should.

 

A pilot builds the understanding he receives through experience on top of the first foundation he receives as a student.

 

Give the student the best first foundation you can; and, this includes all three explanations plus a fourth for passing the oral and written.

 

If you prepare your presentation adequately, this takes less than five minutes and lasts a lifetime.

 

I would love to see your 5 minute presentation on aerodynamics.

[schrocks01`]

I look at it the same way as if my five year old approached me and asked where babies come from.

 

1.) Give enough information that it satisfies his curiosity. Understand that each student will have different reasons for asking the question and it is our job to decipher "Where is he going with this?"

 

2.) Give accurate information. Don't play the games of "beat the air into submission" cliches.

 

3.) Don't overteach. Most of the time they only wanted a specific portion of that answer.

[Tom22]

One of my flight instructors a long time ago told me that the lift and drag coefficients are dimensionless numbers. How are these numbers determined to be dimensionless when you have seven physical quantities (air velocity, air density, size of the lifting surface, angle of attack, airfoil shape, air viscosity on the lifting surface, compressibility) acting on a wing all at the same time?

Edited December 17, 2012 by Tom22

[Counterrotate]

One of my flight instructors a long time ago told me that the lift and drag coefficients are dimensionless numbers. How are these numbers determined to be dimensionless when you have seven physical quantities (air velocity, air density, size of the lifting surface, angle of attack, airfoil shape, air viscosity on the lifting surface, compressibility) acting on a wing all at the same time?

 

Simple answer, your flight instructor didn't know what they were talking about, or you misunderstood what they said.

[Tom22]

He told me the derivation had something to do with Buckingham Pi Theorem.

Edited December 17, 2012 by Tom22

[Counterrotate]

Buckingham Pi Theorem is a way to calculate parameters when the form of the equation is unknown. When talking about coefficients it's important to remember that the term "dimensionless" basically a means that a quantity has no associated physical dimension. That does not mean that it cannot be calculated or measured. It just cannot be quantified into terms of size or shape.

[Tom22]

He also said that a good explanation for lift is that it is the integrated (from the Calculus) distribution of pressure and shear stress (friction). The aforementioned seems somewhat terse; do you think it is a good explanation?

 

Thanks for playing along.

[pilot#476398]

He also said that a good explanation for lift is that it is the integrated (from the Calculus) distribution of pressure and shear stress (friction). The aforementioned seems somewhat terse; do you think it is a good explanation?

 

If you're going for that "deer in the headlights" gaze from your student then,...yes!

[Tom22]

I understand that the purpose of this discussion is teaching lift in a manner that is appropriate for every student.

 

However, the simple explanation, in my opinion, is that lift is the result of pressure differential and friction on whatever object is in the free stream. In fact, Anderson presents this in the first sections of his books; for example, a passage from Fundamentals of Aerodynamics is quoted as follows:

 

At first glance, the generation of aerodynamic force on a giant Boeing 747 may seem complex, especially in light of the complicated three-dimensional flow over the wings, fuselage, engine nacelles, tail, etc. However, in these and all other cases, the aerodynamic forces and moments on the body are due to only two basic sources.

 

Pressure distribution over the body surface

 

Shear stress distribution over the body surface

 

It does not matter how complex the flow is, aerodynamic forces and moments are “entirely” due to the aforementioned basic sources.

 

The beauty of all this is that from those two basic physical source aerodynamics branches into many theorems that seek to explain lift.

 

What I’m trying to say? I don’t know I’m just kind of having fun on the internet. However, stick your arm out of your car window; that’s aerodynamic force. It doesn’t have to be complicated but if you want to go that direction it can be. It’s only pressure distribution and friction!

 

Anderson, J. (2007). Fundamentals of aerodynamics . (4 ed.). New York New York: McGraw Hill

[Eric Hunt]

Been a while since this was dragged up, let's have some fun.

 

The problem with the venturi idea is that the airflow stays in the tube, or in the half venturi theory, it stays flat.

 

Rubbish. The air is deflected downwards, which in turn causes the "thing" pushing it down to be pushed up. Stick your arm out the car window at 60mph, have your hand flat downwards, no incidence. Then rotate your hand to give an angle of attack. Your arm gets pushed up and back.

 

This is the Total Reaction, and it can be split into as many vectors as you like, but the usual ones are Lift (at 90 degrees to the relative airflow) and Drag (parallel to the airflow.) You deflect moving air down, your arm is pushed back by the drag and up by the reaction to the downgoing air.

 

If no air goes down, no lift is produced, only drag.

 

Look at windtunnel pictures of wings, see the smoke go over the top and off the back with a downwash velocity added. The downwash happens at the wingtips too, and it curls into a vortex which is trailed behind in a closed system. When the wing first moves it sheds the starting vortex, drags the bound tip vortices with it, until it stops and sheds one which moves forward.

 

No vortex, no lift.

 

Easy.

 

Lights blue touch paper, stands clear.

[Wally]

They are the same. The higher pressure is lift (skipping stone), the lower pressure (venturi) turns the mass (Coanda). The question is why- venturi vs coanda?

An analogy- lights illuminate by emitting photons or sucking darkness? Another- value is Implicit or labor used? How much argument do you really want? Unless you're doing quantum physics or theoretical economics, you take the test, read the question, and supply the requested (rote) answer.

 

A cram course instructor told me once that questions are ambiguous and sometimes all the answers are wrong, but one is less wrong in the opinion of the individual grading the test. Which makes, I think, the answer "venturi". And the more checklists and forms, the better- this week.

[Eric Hunt]

For the Venturi theorists only:

 

So if there is higher pressure below the wing than above the wing, why is there not an overall UPwash behind a wing instead of downwash? Hmmm? Doesn't air go from high to low?

[ridethisbike]

For the Venturi theorists only:

 

So if there is higher pressure below the wing than above the wing, why is there not an overall UPwash behind a wing instead of downwash? Hmmm? Doesn't air go from high to low?

 

And that's exactly why it's not just one theory that applies. It's a combination of them all.

[Wally]

For the Venturi theorists only:

 

So if there is higher pressure below the wing than above the wing, why is there not an overall UPwash behind a wing instead of downwash? Hmmm? Doesn't air go from high to low?

That's exactly what happens at the wing tip, isn't it?