Very Basic Aerodynamics Question

[ThreeHoler]

I'm a beginner flight student, but like many here, I've been interested in aviation for as long as I can remember (I'm 19 now) and consider myself fairly savvy (for someone without a pilot's license yet) when it comes to flight.

That said, I've got an admittedly dumb question regarding exactly how the control surfaces work to affect roll, pitch, and yaw.

I've always understood the control surfaces to work in the context of Newton's Third Law of Motion (For every action there is an equal and opposite reaction yadda yadda yadda...). So for example, if the elevator is deflected downward, the air flowing under the horizontal stabilizer is deflected slightly downward when it hits the elevator. The downward deflected air particles thus exert an opposite force on the horizontal stabilizer, pushing it up, and therefore pitching the nose down.

Now, the textbook (The Pilot's Manual 2: Ground School) I recently starting reading in preperation for starting my flying lessons offered a somewhat different explanation for how the control surfaces change roll, pitch, and yaw. It basically said that the control surfaces change the shape of an airfoil to alter the lift force of that particular airfoil. So for example, if the elevator is deflected downward, the camber of the airfoil (horizontal stabilizer) is increased, thus it produces more lift and raises the rear of the aircraft, therefore pitching the nose down.

So which explanation is (more) correct? They both seem logical to me and my limited aerodynamic knowledge. Is it possible that neither explanation is totally wrong?

If the way I've always understood it is totally wrong, then I've been severely misinformed for a long time.

Thanks for any help!

[B767Driver]

My friend, you are asking very astute questions for someone of your age. As you continue to study basic aerodynamics, especially in books targeting pilots, you will find a myriad of different explanations concerning aerodynamic principles. Some of the explanations you will find in popular aviation training texts...are just plain wrong. Even the FAA, dating back to the 1960's, (as far as I can tell) has taught pilots inaccurate aeodynamic principles.

Why has this occured? Most likely to "dumb down" very complex subjects to more basic concepts...like taking a very complex electrical schematic and making it a block diagram.

Anyway...the reason an airplane produces lift with its wing, ailerons and elevator is because of the pressure differential produced. There is high pressure on one side of the wing and low pressure on the other side of the wing...and this pressure differential is the aerodynamic wonder that makes airplanes do the things they do. The whole "Newtonian" principles are at work...but they are secondary in response to the pressure differential created by the shape (camber) of the wing.

It sounds like the text you are reading is on the right track. If you have more questions, this is a good forum because they are several guys here who really know how to separate the wheat from the chaff on this difficult topic.

Now having said that, as you begin your flying lessons, understanding detailed aerodynamic principles won't be very important. Basic concepts will be just fine for now. If you don't feel you have a master's level understanding of aerodynamics...don't let it bother you. Over time you can pick up a better understanding. I had 8000 hrs of flying experience and was flying heavy jets...and still didn't 100% understand the more intricate details of aerodynamics. Even some aeronautical engineers have a difficult time explaining how lift is created. (They can run very complex formulas regarding fluid dynamics, but when you press them for the basics, some never seem to be able to communicate the basics in a nutshell.)

Tgrayson and Fish are contributors here...and both are very knowledgable in this area. More than likely they can help answer your questions if you post them here.

Good luck.

[nosehair]

Originally Posted by ThreeHoler

So which explanation is (more) correct? They both seem logical to me and my limited aerodynamic knowledge.

They are both working all the time. In high speed cruise level flight, very little air is pushing against the bottom of the wing as a result of the angle of attack, but mostly as a result of the upper camber of the wing which is increased/decreased when the (elevator/rudder/aileron) is deflected downward/upward.

At low speeds/high angle of attack the angle of attack lift (Newton's Law) contributes much more of the lift and is affected more by the control displacement.

At slow speeds, we fly more on angle of attack, like a kite. At high speeds, we fly more on camber lift, or Bournelli's theory.

[tgrayson]

Originally Posted by ThreeHoler

Is it possible that neither explanation is totally wrong?

Not only possible, but true. Notice that the second explanation is a bit vague about the "lift force". In one sense, the equal and opposite force of the deflected airflow you first mentioned IS the lift force. Changing the shape of the airfoils is merely the mechanism by which the airflow is deflected.

However, consider Newton's first law of motion; an object will move in a straight line unless acted on by a force. What force deflects the airflow? The answer is the pressure distribution around the airfoil.

So the best answer for how a wing works is that it changes the pressure distribution around the airfoil, creating a net force in the UP direction. This pressure distribution, in turn, will cause the airflow to deflect downwards in proportion to the lift being generated.

[sdfcvoh]

Your question is touching on some of the complex side of the aerodynamics. I'll try to chime in, and I'm sure Tgrayson will also have some explanations.

You are not completely incorrect in your thinking, but the much bigger contributor to the reason the nose goes down when you push down on the yoke is the answer you found while you were reading (which you didn't understand.) That said, don't be concerned that you felt the way you did. I'll step into the answer to your question below and try to explain it to you.

First: The Horizontal stabilizer is an airfoil. That by definition means that it creates an aerodynamic force (lift) when air flows over it. The wings are also airfoils. But, the Horizontal Stabilizer is upside down (in comparison to the wing.) It is creating downward lift.

Why is that needed? Balance. Picture a single engine airplane flying along in level flight without anything behind the wings (chop off the tail section). You've got the engine up front (very heavy) and you've got the passengers underneath the wings. The airplane is front heavy. If you tried to fly that way, the plane would be uncontrollable because the heavy nose would just point straight down.

Now, put the tail section back on the plane. We know that it is important to keep the weight of the plane as light as possible, but we need to stick something out on the tail to balance the weight of the front end of the plane so it will not go nose down as was described above. Instead of putting a big heavy weight back there, the engineers decided they would make the horizontal stabilizer create a "force" to hold the tail down as if it was weighted. They did that by making the horizontal stabilizer an "airfoil". Remember, an airfoil is an object that creates an aerodynamic "force" when air flows over it. But unlike the wings, which direct their aerodynamic force upward, the engineers designed the horizontal stabilizer to have a downward aerodynamic force. We call that "Tail Down Force".

The whole reason the tail was designed that way was to balance the nose down heavy front of the plane by putting an upside down wing (called the horizontal stabilizer) in the back of the plane.

So - now to answer your question.

Since the entire shape of an airfoil determines how much force (lift) it creates, go back to our picture of flying along in level flight. Heavy engine in front, wings lifting the weight of the plane, Horizontal stabilizer in the back making downward force and balancing the heavy nose, keeping the plane flying level. Push forward on the yoke..... The elevators move downward, and all of a sudden the shape of the airfoil (horizontal stabilizer) changes. And it changes in a way that destroys its ability to create enough "tail down force" to balance the plane's heavy front. So, the plane now does not have anything balancing the heavy nose (you've lost your tail down force) and what happens? The nose, being heavy, points down.

Hopefully my explanation helps you understand the question- and doesn't make it worse.

[subpilot]

very well explained!

[B767Driver]

Originally Posted by nosehair

They are both working all the time. In high speed cruise level flight, very little air is pushing against the bottom of the wing as a result of the angle of attack, but mostly as a result of the upper camber of the wing which is increased/decreased when the (elevator/rudder/aileron) is deflected downward/upward.

At low speeds/high angle of attack the angle of attack lift (Newton's Law) contributes much more of the lift and is affected more by the control displacement.

At slow speeds, we fly more on angle of attack, like a kite. At high speeds, we fly more on camber lift, or Bournelli's theory.

This is pretty much wrong.

[tobaknight]

As I understand it Newtons laws come into play, but the most applicable principle is Bernouli's that states that "as the speed of a moving fluid increases, the pressure within the fluid decreases."

The Camber of the wing is such that the air on the top of the wing has more surface to pass than does the air on the bottom. Like any other fluid the speed of the air increases to cover that larger surface, while the air passing under the wing has less surface area to cover and thus a slower speed is necessary to cover that area. In line with Bernouli's principle the difference in speeds is accompanied by a difference in resulting pressures, i.e. the faster air moving over the top of the wing has less pressure than the slower air moving under the wing. That difference in pressures is what lifts the wing.

There's a pretty cool website where you can kind of play with the concept. http://home.earthlink.net/~mmc1919/venturi.html

Thats a pretty basic explanation, it doesnt really explain how inverted flight is possible and the like, but its a good place to start, and as of yet I haven't really needed to be able to explain to any examiners in any more depth than that.

[sdfcvoh]

Originally Posted by sdfcvoh

I'll try to chime in, and I'm sure Tgrayson will also have some explanations..

Dang you're fast!

[tobaknight]

Can I rescind my first response and change it to Yeah what sdfvcoh said!

[B767Driver]

Originally Posted by tobaknight

The Camber of the wing is such that the air on the top of the wing has more surface to pass than does the air on the bottom. Like any other fluid the speed of the air increases to cover that larger surface, while the air passing under the wing has less surface area to cover and thus a slower speed is necessary to cover that area. In line with Bernouli's principle the difference in speeds is accompanied by a difference in resulting pressures, i.e. the faster air moving over the top of the wing has less pressure than the slower air moving under the wing. That difference in pressures is what lifts the wing.

.

This is getting into something commonly referred to as the equal transit theory...which is taught by the FAA actually...but has led to a convenient, but not quite accurate theory of how lift is produced.

Camber does play a significant role...along with the viscosity of the air...but the top and bottom of the wing are independent in this discussion and leads to some inaccuracies. However, camber alone doesn't produce lift...because symmetrical airfoils have zero camber and can sustain flight, right?

What really produces lift is the combination of the conservation of matter plus bernoulli's theorem.

As pilots, I think we need to get away from the Newtonion lift and Bernoulli lift.

I'd love to chime in more...but gotta run. I'm sure some other guys will run with this.

[tobaknight]

Originally Posted by B767Driver

I'd love to chime in more...but gotta run. I'm sure some other guys will run with this.

I hope so, my interest is officially peaked.

[tgrayson]

Circulation:

http://www.boundvortex.com/ReadArtic...x?ArticleID=53

[Matt13C]

Originally Posted by tgrayson

Circulation:

http://www.boundvortex.com/ReadArtic...x?ArticleID=53

Interesting read, thanks.

Just to clarify. It seems to me that it is saying that it is not the upper cambered surface being a greater distance than the lower less cambered surface. Instead it is the angle of attack that causes a rotational force that forces the air over the top of the wing at a greater speed and slows the air beneath the wing which actually causes the pressure variation?

This is different than it has been explained to me. I had it explained as a product of the barn door effect and camber, camber being the majority contributor. The upper cambered surface was a greater distance than the lower less cambered surface which caused the air to travel faster above the wing lowering the pressure. Does that still play a part?

[nosehair]

Originally Posted by Matt13C

This is different than it has been explained to me. I had it explained as a product of the barn door effect and camber, camber being the majority contributor. The upper cambered surface was a greater distance than the lower less cambered surface which caused the air to travel faster above the wing lowering the pressure. Does that still play a part?

Yes, it still plays it's part. Just as I described, and as you have read it. Popular current explanations tend to jump into more complications of this basic simplified-for-the-pilot explanation.

The camber is changed and the angle of attack is changed. This causes the pressure above to be less than before and greater lift is produced. The air striking the lower surface is pushing it up into the lowered air pressue above.

To all the astrophysiscists (sic) here, the above description does not make a picture, or give an explanation; it sells theory: Because I said so.

The explanation I previously gave makes a picture.

It is simplified. It is not 'pretty much wrong'. It is a simple easy to read description. But I do explain that it is simplified and if the student wants a more complete picture, then we go furthur into the real picture.

Primacy does not mean telling the whole picture at first. It means feeding the information in small enough bites to be able to digest it before giving a larger harder-to-chew chunk.

[B767Driver]

Originally Posted by nosehair

The air striking the lower surface is pushing it up into the lowered air pressue above.

.

The air striking the lower surface has a negligent effect on lift...until you get into high speed high altitude flying. The dynamic pressure of the air at subsonic speeds is too puny to have any meaningful contribution on the lower surface of the wing.

At high altitudes, once the air pressure gets very, very low...and circulation theory becomes less effective on lift production...then the "skipping stone" theory comes into play. But you pretty much need to be flying an SR-71 for the relative free stream to have lifting effect below the wing.

For subsonic flight...to say that air impacting the lower surface of the wing pushes the wing up for some type of lift...is inaccurate.

[B767Driver]

Originally Posted by Matt13C

Interesting read, thanks.

Just to clarify. It seems to me that it is saying that it is not the upper cambered surface being a greater distance than the lower less cambered surface. Instead it is the angle of attack that causes a rotational force that forces the air over the top of the wing at a greater speed and slows the air beneath the wing which actually causes the pressure variation?

?

Bingo....circulation theory, baby.

Actually, the more research you will do into lift...you will find that the trailing edge of the wing is more important to lift production than the leading edge!!

[B767Driver]

Originally Posted by nosehair

It is simplified. It is not 'pretty much wrong'. It is a simple easy to read description. But I do explain that it is simplified and if the student wants a more complete picture, then we go furthur into the real picture.

Primacy does not mean telling the whole picture at first. It means feeding the information in small enough bites to be able to digest it before giving a larger harder-to-chew chunk.

In your simplified picture, you are giving inaccurate information. The FAA does this too...and as a consequence we have the majority of pilots who are trained improperly. I don't see why it's necessary to give a "barn door" theory of lift. There's no truth to it and it's not necessary (until you get to hypersonic flight.)

[Matt13C]

Originally Posted by B767Driver

Bingo....circulation theory, baby.

Actually, the more research you will do into lift...you will find that the trailing edge of the wing is more important to lift production than the leading edge!!

Thanks!

If you have any articles you can point me towards that go a bit more into detail that would be excellent.

[B767Driver]

Originally Posted by Matt13C

Thanks!

If you have any articles you can point me towards that go a bit more into detail that would be excellent.

At some point in time, I realized that I had been trained on the FAA's view of lift production that contained glaring deficiencies and wanted to learn more. I read everything I could get my hands on over a couple year period. This led to more confusion.

One of the members here referred me to "Flightwise" by Chris Carpenter. He was head of aerodynamics at the Royal Air Force College in the 1980's and his book tackles the principles of flight like none I've found. He goes beyond telling how lift is created and proves it with formulas and data.

After reading this book you'll be able to mathematically prove how it is impossible for the barn door theory of lift to make an airplane fly. You'll also wonder how some of the other books got published!

I don't believe you can find it in the US....you might have to get it from Amazon's international unit.

Some guys here are a good reference also. Some guys will get too convulated...some guys do a good job of getting to the point. But there are some very knowledgeable guys here as well.

[Polar742]

Originally Posted by B767Driver

At some point in time, I realized that I had been trained on the FAA's view of lift production that contained glaring deficiencies and wanted to learn more. I read everything I could get my hands on over a couple year period. This led to more confusion.

One of the members here referred me to "Flightwise" by Chris Carpenter. He was head of aerodynamics at the Royal Air Force College in the 1980's and his book tackles the principles of flight like none I've found. He goes beyond telling how lift is created and proves it with formulas and data.

After reading this book you'll be able to mathematically prove how it is impossible for the barn door theory of lift to make an airplane fly. You'll also wonder how some of the other books got published!

I don't believe you can find it in the US....you might have to get it from Amazon's international unit.

Some guys here are a good reference also. Some guys will get too convulated...some guys do a good job of getting to the point. But there are some very knowledgeable guys here as well.

I'm a complete theoretical idiot (proud-ish of it). How do you rate Aerodynamics for Naval Aviators?

[tgrayson]

Originally Posted by Matt13C

Just to clarify. It seems to me that it is saying that it is not the upper cambered surface being a greater distance than the lower less cambered surface. Instead it is the angle of attack that causes a rotational force that forces the air over the top of the wing at a greater speed and slows the air beneath the wing which actually causes the pressure variation?

The necessity of being at an AOA is true for a symmetrical airfoil; a cambered airfoil will generate lift even at a zero AOA. There's no fundamental difference between the two situations.

This is different than it has been explained to me. I had it explained as a product of the barn door effect and camber, camber being the majority contributor. The upper cambered surface was a greater distance than the lower less cambered surface which caused the air to travel faster above the wing lowering the pressure. Does that still play a part?

First, let me stress that circulation theory is about the most fundamental concept in aerodynamics. It's not a fringe theory or something I just made up. Its discovery revolutionized the study of aerodynamics. You won't find it in any pilot literature, although Aerodynamics for Naval Aviators touches on it. But you see vague references to it for people selling things that reduce the stall speed, which are often called "circulation control" technology.

The mechanism you mentioned is often pooh-poohed by aerodynamics authors when they write towards pilots, because there is no physics which says that the air must speed up when it encounters a longer distance to the trailing edge.

However, there's something that's close: when you obstruct the flow of air, it speeds up. The problem with that as the *sole* explanation of lift is that studies show that the top and bottom of the air speed up the same, generating no lift. You have to somehow explain why the upper airspeed ups *more* than the lower. Circulation theory does that.

[tgrayson]

Originally Posted by Polar742

How do you rate Aerodynamics for Naval Aviators?

Technically accurate, but very light on theory. The presentation is poor, obviously. But it also has a lot of interesting factoids scattered about in the text that you'd have a hard time finding elsewhere. Math is algebra level. While it wouldn't be one of my top recommendations, it won't lead you astray.

There's a book out there "Flight Theory for Pilots" by Charles Cole that appears to consist almost solely of AFNA material, but prettied up. And more expensive.

[ThreeHoler]

Thanks for all the interesting and well thought-out replies, everyone.

I figured my original question had a pretty complex answer, but for now, I'll settle with the basic answer that both of my initial "explanations" (though my understanding was not completely accurate; thanks for the clearifications) for how the control surfaces work come into play, as was indicated in the initial replies of this thread.

Originally Posted by B767Driver

Now having said that, as you begin your flying lessons, understanding detailed aerodynamic principles won't be very important. Basic concepts will be just fine for now. If you don't feel you have a master's level understanding of aerodynamics...don't let it bother you.

That sounds like good advice and I don't plan to get too infatuated with the in-depth aerodynamics theories too soon. I do find it all rather interesting though. But as I said, I'll settle for basic explanations for now.

This thread seemed to have evolved into a discussion of the theories of how lift is produced, which is fine with me because I find it all really interesting. Another question that had been brewing in my mind was brought up at some point in the discussion: why does the air over the upper curved surface of a cambered wing have to travel faster than the air flowing underneath the flatter surface of the wing? Furthermore, how is lift produced from a symmetrical airfoil? I guess circulation theory sort of answers those questions.

Thanks for satisfying some of my curiosity everyone.

[cfii2007]

So if a train leaves the station at noon travelling 80 MPH heading North, what time will it meet the Westbound train that departed at 11AM?